September 2016

Plasma Science and Fusion Center Massachusetts Institute of Technology

Cambridge MA 02139 USA

PSFC/JA-16-31

DRIVE Tokamak: A student run experiment in high-field-side heating and current drive

A.E. White, S. J. Wukitch, J. Irby, P. Bonoli

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DRIVE Tokamak: A student run experiment in high-field-side heating and current drive

A.E. White, S. J. Wukitch, J. Irby, P. Bonoli Synopsis

The proposed DRIVE tokamak is a student-run, small-scale facility designed for entirely new, foundational experiments in RF current and flow drive for fusion research. It will focus on high-field-side-launch of lower hybrid current drive (LHCD) and ion cyclotron range of frequency (ICRF) wave physics. The DRIVE tokamak will be part of international research in support of Advanced Tokamak (AT) physics for next generation fusion devices. High-field-side (HFS) launch LHCD and ICRF, used for mode conversion flow drive (MFCD) and plasma heating, have never been done before in a diverted plasma. Therefore the DRIVE tokamak will establish for the first time the feasibility of high-field-side LHCD and ICRF launch for use in a wide range of next-step AT devices, .e.g the proposed ADX tokamak [LaBombard NF 2015], the steady-state PMI tokamak Vulcan [Olynyk FED 2012], the ARC pilot fusion device [Sorbom

FED 2015] and FSNF-AT [Chan NF 2011]. The unique DRIVE experimental data set will be used to validate RF wave simulation codes and integrated models, supporting directly the development of predictive capability for current drive, current profile control (off axis and central current drive), and flow drive in ITER and other burning plasma experiments.

AT physics involves four elements: operation at high power density; with high confinement time; in steady-state with high bootstrap fraction; with adequate divertor heat removal, particle and impurity control [Burrell NF 2003]. Advanced tokamak physics is studied at DIII-D, AUG, JET via an integrated approach. For a small-scale device, an integrated approach is not feasible or appropriate, therefore the proposed DRIVE tokamak will focus its efforts in one area of AT physics – understanding the fundamental RF physics underpinning the use of LHCD and ICRF actuators used for steady-state operation with high bootstrap fraction (Figure 1). High Field Side (HFS) launch is predicted to be extremely favorable for LHCD and ICRF wave physics in a reactor.

High-field-side-launch LHCD and ICRF systems are potential game changers, promising higher current drive efficiency and improved launcher survivability – but have never been used before in a diverted tokamak plasma. This is a fertile area for innovative, discovery science at a small-scale device. While it could be possible to retrofit devices such as DIII-D, EAST, etc. with HFS launch systems, these are international user facilities with broad scientific programs and complex arrays of stake-holders. To retrofit such tokamaks with HFS RF systems will be costly and sometimes not possible due to lack of space at the HFS. Given competition for run-time at major facilities, detailed and dedicated HFS RF physics experiments would require many calendar years, at high cost.

Figure 1: ARIES-AT current profile shows a typical off axis peaked current profile dominated by bootstrap current and augmented by off axis and central current drive. Figure adapted from Najmabadi FED 2006.  

DRIVE is a small-scale experiment, dedicated to study of HFS launch RF physics, supporting AT physics via foundational experiments.    

HFS launch LHCD and ICRF are potential game changers.  

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The primary objective of the DRIVE tokamak will be to test predictions of how HFS launch LHCD and ICRF compare/contrast with LFS launch, in a smaller, faster, affordable platform, while also providing an optimum environment for graduate and undergraduate student education. The technical approach will be to design, install, operate, and qualify HFS launch RF systems on a small-scale test-facility. Predicted benefits of HFS Lower Hybrid Current Drive: HFS current drive with LH waves exploits the higher magnetic field on inboard side, where the accessibility condition and wave propagation physics allows the LH waves to rapidly penetrate to the core. Recent modeling studies found that current drive efficiency increases significantly for LHCD waves launched from

the HFS compared to the LFS. In addition to improved conditions from the RF physics perspective, the use of HFS launchers should minimize their plasma-material interaction (PMI) issues, a critical concern for any reactor. The HFS SOL is comparatively quiescent, which allows the launcher to be located very close to the LCFS without PMI issues. With the decreased physical distance and improved wave-plasma coupling, non-linear loss-mechanisms would be greatly reduced. Simulations also show improved wave penetration to the core when the launcher location is near active X-points.

Predicted benefits of HFS ICRF heating and flow drive: HFS launch ICRF can greatly improve the utility of mode conversion flow drive, for suppression of turbulence and improved profile control in AT scenarios. With HFS launch, fast waves will mode convert to ion Bernstein waves (IBW) and ion cyclotron wave (ICW), leading to strong single-pass absorption and direct electron heating (Figure 2). With mode conversion, the ions will not be heated directly, so there is no generation of energetic ion tails that can lead to instabilities and enhanced fast ion losses.

Thus HFS launch ICRF mode conversion heating will greatly reduce chances for fast ion-induced sputtering of first-wall components, a major PMI issue for reactors.

The MIT PSFC is ideally suited to host the DRIVE tokamak experiment. The former C-Mod facility provides a large vacuum vessel and TF and PF coils. Existing hardware, power supplies, RF sources (some on-site, some loaned from collaborators), and in-place infrastructure support all mean that DRIVE can be built with minimal capital investment (~$250k for hardware/year). Second, the team includes world-famous experts in RF physics and engineering at MIT, which will allow for rapid development of the HFS launchers for the DRIVE tokamak – elements can be designed quickly and built in-house. The proposal will also leverage existing collaborations with ORNL for RF measurements and modeling. Third, the Theory Group at the PSFC is deeply involved in cutting edge theory and numerical simulation of LHCD and ICRF waves – DRIVE

data will provide ample opportunity for collaboration, using the new HFS experiments to test and validate predictive RF models and codes. Fourth, Drive will be an excellent tool to attract and train high-quality students at MIT.

 Figure   2:   Illustrative   TORIC   high-­‐field-­‐side   launch   ICRF   simulation  showing  fast  wave  mode  conversion  to   ion   cyclotron   wave   where   60%  power   is   deposited   onto   thermal  ions   [Wukitch   SOFE   2015  Proceedings]      

Funded at <$3M/year, DRIVE could study HFS launch RF with on-axis Bt = 2.0T, ne = 2e19 m-3, Te = 1keV, for longer than 0.2sec, with >1MW of RF power, in diverted plasmas (DN, USN, LSN) with R = 0.9m, a = 0.3m, b=0.5m.        

DRIVE does not compete with DIII-D or ADX or NSTX-U.  

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DRIVE Tokamak: a student run experiment in high-field-side heating and current drive

Section 1. Project Objectives, p.3

Section 2. Background, p.4

Section 2.1 Advanced Tokamak (AT) Physics and Current Drive, p.4

Section 2.2 Relevance of DRIVE tokamak to AT physics studies at large-scale, p.5 tokamak facilities

Section 2.3 Physics of HFS launch for heating, current drive, and flow drive, p.6

Section 2.4 Summary of HFS Launch Benefits, p.9

Section 3. Proposed Research and Methods, p.10

Section 3.1 Hypotheses and general set-up for RF experiments, p.10

Section 3.2 List of diagnostics used in RF experiments, p.13

Section 3.3 Commissioning the DRIVE tokamak, p.15

Section 3.4 Design and construction of the HFS LHCD and ICRF Launchers, p.19

Section 3.5 Comparison with theory and simulation, p.21

Section 3.6 Anticipated challenges and alternative strategies, p.21

Section 4. Summary and Conclusions p.25

Bibliography & References Cited, p.25

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Section 1. Project Objectives

One of the most critical issues for the viability of a tokamak fusion reactor is developing efficient, robust current drive systems that can survive the harsh reactor environment. The proposed DRIVE (not an acronym) experiment is a student-run, small-scale tokamak focused entirely on performing foundational experiments in radio frequency current and flow drive. DRIVE will be able to answer key physics questions concerning high-field-side-launch of lower hybrid current drive and ion cyclotron range of frequency (ICRF) waves. The DRIVE experiment will be part of a larger international research program in support of Advanced Tokamak (AT) physics for next generation fusion devices.

Lower hybrid range of frequency current drive is among the most promising for off-axis current drive with high efficiency and the proper radial deposition profile to augment the bootstrap current, and ion cyclotron range of frequency (ICRF) power provides efficient bulk heating and promising flow drive. RF (radiofrequency) actuators allow for continuous current drive, heating, and flow drive and have very high system efficiency (~70% wall-plug to plasma). Because of this, RF actuators have are essential ingredients for steady state, advanced tokamak operation [Greenwald 2007]. Design studies for FSNF-AT [Chan 2010, Chan 2011, Garafalo 2014] and ARIES-AT [Najmabadi 2006], and ideas for DEMO all include lower hybrid current drive (LHCD) and ICRF to achieve the current and pressure profiles needed for efficient fusion electricity production.

However, the harsh reactor environment poses challenges for lower hybrid and ICRF launching structures, which must be placed near to the plasma. The known challenge of achieving efficient coupling and load tolerance, with little impurity contamination, becomes even more difficult to surmount in a reactor environment. For the RF launchers, the plasma material interaction (PMI) issues are similar to the first wall, and this is a potential show-stopper for using these highly efficient actuators in a reactor [Zinkle 2012]. This problem must be solved, since without current drive, the tokamak concept as the basis for a steady-state, electricity producing power plant is likely unachievable. A tantalizing, and apparently simple, solution to this complex problem has been identified: launch the RF waves from the high-field-side instead of the low-field-side. Low-field-side launch is done now in all present-day experiments, and very little work has been done to investigate effects of varying the poloidal location of the launcher in tokamaks [Bernabei 1986], and such experiments have never been performed in a diverted tokamak.

Re-locating the RF launchers to the high-field-side (inboard side) of the tokamak is predicted to dramatically improve launcher robustness in a reactor environment. The reduction in the PMI issues is due to several effects: In near-double null plasmas, a low heat-flux, quiescent boundary layer naturally forms on the high field side: (1) no heat/particle pulses reach there from ELMs [Petrie 2003], essentially zero fluctuation-induced fluxes and no ‘blobs’ [Smick 2013]; (2) local plasma recycling fluxes are low, which minimizes neutral pressures in the vicinity of antenna/waveguide structures, leading to improved RF voltage handling [Wukitch 2004] (3) the flux of energetic ion orbit loss on the high-field side is virtually nonexistent; (4) there is no impact from runaway electrons at this location; (5) impurity ions produced from the launcher are expected to be very well screened (factor of ~10 compared to low-field side), based on results from impurity transport experiments [McCracken 1997] (6) RF driven convective cells that lead to RF enhanced heat loads and impurity sources at the antenna are half compared to those on the low field side [D’Ippolito 1993]; (7) local density at high-field side launch structures can be precisely controlled via upper/lower X-point flux balance due to steep SOL density profile [Boswell 2004] and/or distance from the last-closed flux surface (LCFS) to launcher.

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High-field-side launch also improves several aspects of RF actuator capabilities due to differences in the wave physics on the high-field-side. For lower hybrid current drive, wave penetration is vastly improved resulting in driven current at mid-minor radius [Podpaly 2012], precisely the location where current needs to be driven to enable flat or reverse magnetic shear [Bonoli 1997]. The current drive efficiency is also improved by up to ~50% [Bonoli 2014]. For ICRF, high-field launch leads to efficient electron heating via direct mode conversion of fast waves. Moreover, mode conversion could have significant power to bulk ions. Mode converted waves transferring power to the ions has been demonstrated to drive plasma flow [Lin 2009, Phillips 2000] – a potentially powerful tool to enhance core plasma confinement via flow shear in a reactor. Theory predicts that high-field-side-launch LHCD and ICRF systems are potential game changers, promising higher current drive efficiency and improved launcher survivability. But they have never been used before in a diverted tokamak plasma. Existing tokamaks utilizing LHCD and ICRF, such as EAST [Ding 2013], WEST and KSTAR, would only implement high-field-side launch after a working demonstration on a smaller, relevant facility. The timing is perfect and the physics is compelling: this is a fertile area for innovative, high-impact, discovery science in a small-scale device. Motivated by the state of the field and the need for reactor-compatible RF actuators for FNSF and DEMO, we propose an accelerated, inexpensive, proof-of-principle experiment to establish the feasibility of high-field-side launch of lower hybrid waves for current drive and ICRF for heating and flow drive. The primary objective of the DRIVE experiment will be to test predictions for how well high-field-side launch LHCD and ICRF will perform. The technical approach will be to design, install, operate, and qualify high-field-side launch RF systems on a small-scale, largely student-run experiment. By re-using existing infrastructure at MIT-PSFC, the DRIVE experiment can be brought on-line very quickly and at relatively low-cost. Starting in late Summer 2016, the Alcator C-Mod tokamak will be shut down. This means that the decommissioned tokamak will be available for use in new experiments. By re-purposing the vacuum vessel, full coil sets, power systems, cryogenic systems, control systems and existing set of RF relevant diagnostics into the DRIVE experiment, critical questions regarding high-field-side launch RF physics can be answered relatively quickly. The DRIVE experiment is a small-scale experiment that can be built and operated for between $2-3M a year, depending on scope of physics and engineering tasks. Section 2. Background Section 2.1 Advanced Tokamak (AT) Physics and Current Drive

The need to develop robust methods of driving current is essential for fusion energy development. To generate electricity in a fusion power plant based on the tokamak concept will require efficient and robust steady state current drive. Furthermore, the input power required for current sustainment is a major constraint upon plant efficiency. “Advanced Tokamak” or AT physics involves four elements: operation at high power density; with high confinement time; in steady-state with high bootstrap fraction; and with adequate divertor heat removal, particle and impurity control [Burrell 2003].

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Figure 1 shows a current profile from the ARIES-AT (where AT refers to “Advanced Tokamak”) design [Najmabadi 2006], which illustrates typical current profiles foreseen for reactor concepts. The AIRES-AT and FNSF-AT design studies have shown that the optimum current profile is one that is peaked off axis, which aids in achieving the desired plasma stability and confinement [Chan 2010, Chan 2011, Garafalo 2014]. A current profile with its peak off-axis also aligns well with the plasma generated bootstrap current [Zarnstoff 1988]. As can be seen in Figure 1, the total current profile that is desired requires both central and off-axis current drive to augment the bootstrap current.

Section 2.2 Relevance of DRIVE tokamak to AT physics studies at large-scale tokamak facilities

Lower hybrid range of frequency current drive is among the most promising for off axis current drive with high efficiency and the proper radial deposition profile to augment the bootstrap current in AT scenarios, and ion cyclotron range of frequency (ICRF) power provides efficient bulk heating and promising flow drive. RF (radiofrequency) actuators allow for continuous current drive, heating, and flow drive and have very high system efficiency (~70% wall-plug to plasma). Because of this, RF actuators have are essential ingredients for steady state, advanced tokamak operation [Greenwald 2007]. Design studies for FSNF-AT [Chan 2010, Chan 2011, Garafalo 2014] and ARIES-AT [Najmabadi 2006], and ideas for DEMO all include lower hybrid current drive (LHCD) and ICRF to achieve the current and pressure profiles needed for efficient fusion electricity production.

However, the harsh reactor environment poses challenges for lower hybrid and ICRF launching structures, which must be placed near to the plasma. The known challenge of achieving efficient coupling and load tolerance, with little impurity contamination, becomes even more difficult to surmount in a reactor environment. For the RF launchers, the plasma material interaction (PMI) issues are similar to the first wall, and this is a potential show-stopper for using these highly efficient actuators in a reactor [Zinkle 2012]. This problem must be solved, since without current drive, a steady-state, electricity producing tokamak power plant is likely unachievable.

A tantalizing, and apparently simple, solution to this complex problem has been identified: launch the RF waves from the high-field-side instead of the low-field-side. Low-field-side launch is done now in all present-day experiments, and very little work has been done to investigate effects of varying the poloidal location of the launcher in tokamaks [Bernabei 1986], and such experiments have never been performed in a diverted tokamak. The LHCD and ICRF wave physics for high field side (HFS) launch are predicted to be extremely favorable for LHCD and ICRF wave physics in a reactor [Sorbom 2015].

Figure 1: ARIES-AT current profile shows a typical off axis peaked current profile dominated by bootstrap current and augmented by off axis and central current drive. Figure adapted from [Najmabadi 2006].  

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While it could be possible to retro-fit devices such as DIII-D, AUG, EAST, KSTAR etc. with HFS launch RF systems, these are major international user facilities with very broad scientific programs and a complex array of stake-holders. Given natural limitations on run-time at such facilities, it is unlikely that detailed and dedicated HFS launch RF physics experiments could be conducted at a level needed to drive the field forward. In contrast, the DRIVE tokamak is a small-scale experiment with a focused mission. With an operating budget of $2M-3M/year proposed here, DRIVE cannot compete with large-scale tokamak facilities, where advanced tokamak physics is studied via an integrated approach. Also, DRIVE does not compete with proposed new large-scale tokamak facilities at MIT, such as ADX [LaBombard 2015], but rather is complementary. The proposed DRIVE tokamak will be focused on understanding the fundamental RF physics of HFS launch LHCD and ICRF actuators. Demonstration of HFS launch LHCD waves and ICRF waves in diverted tokamak plasmas at DRIVE will help validate predictions for the improved RF physics and PMI effects of HFS launching systems for next-step devices. As described next in Section 2.3, the benefits of high field side launch are a significant from both physics and engineering perspectives.

Section 2.3 Physics of HFS launch for heating, current drive, and flow drive

Section 2.3.1 Core Wave Physics For any actuator, the launcher solution needs to be compatible the core physics, in this case wave penetration and absorption. For the LHCD, the HFS launch improves the LH wave accessibility, penetration and current drive efficiency [Podpaly2012, Wallace2015a, Wallace2015b]. The LH wave accessibility [Brambilla1979] is determined by avoiding coalesce of the fast and slow wave modes in LH range of frequencies. This can be expressed as

                                                       𝑛||,!"" > 1 − 𝜔!"2 𝜔2 + 𝜔!"2 𝜔!"2 + 𝜔!" |𝜔!"|~!!!

(1)

n||≡ ck||/w is the parallel index of refraction, ωpi is ion plasma frequency, ωpe is the electron plasma frequency, ωce is electron cyclotron frequency, ne is plasma density and B the magnetic field. The accessibility criterion roughly scales inversely with magnetic field. Thus, accessibility can be improved for a given density by increasing the magnetic field and/or utilizing HFS launch, since the magnetic field in the tokamak is larger on the inboard side/high-field-side. The LH wave penetration depth is determined by the condition for strong electron Landau damping and is given as

                                                                                                                           𝑛||,!"# < 30 𝑇! (2)

where Te is the electron temperature [Brambilla1981] For hot reactor grade plasmas, this tends to limit the radial penetration of the waves to the outer region of the plasma core. Finally, the current drive efficiency, ηCD, scale as                                                                                                                                    𝜂!"    ∝ 1

𝑛||!   (3)

Note that the efficiency improves primarily because the wave damps on faster, less collisional electrons [Fisch1978]. Additional improvement in current drive efficiency arises from damping at higher electron temperature [Karney1985].

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From these equations, one can readily deduce that higher magnetic field improves wave accessibility at higher density and allows launch of lower n|| waves. This improves wave penetration into the plasma and current drive efficiency. To illustrate, we consider driven current

profiles for LFS and HFS LHCD launch for FNFS [Chan2010]. In Figure 2 at left, the HFS launch has a driven current profile peaked broadly at r/a~0.8-0.9 (red) and 40% higher efficiency compared to the LFS launch profile (black). The HFS driven current is better aligned with that required for an ARIES-AT. HFS current drive with LH waves exploits the increased magnetic field on the inboard side of the tokamak, which through the accessibility condition and wave propagation physics, allows the LH waves to better penetrate to the core [Podpaly2012]. Recent modeling studies [Sorbom2015, Bonoli2014] also show that the poloidal location of the launcher is extremely important to wave penetration to the core, with launcher location near active X-points favored.

For ICRF, the wave penetration and damping are modified but the improvement from a physics perspective is less dramatic than the LHCD case. The primary change is that a fast wave launched from the HFS can propagate to the mode conversion layer without encountering a cutoff layer and thus have 100% single pass absorption nearly independent of deuterium-tritium concentrations. This avoids having fast wave in the scrape-off-layer (SOL) and concomitant RF enhanced plasma material interactions [Myra1994]. With on-axis HFS launch, fast waves will mode convert to ion Bernstein waves (IBW) and ion cyclotron wave (ICW). This leads to strong single-pass absorption and direct electron heating [Nelson2003] so that there is no generation of energetic ion tails that can lead to instabilities and enhanced fast ion losses. HFS launch ICRF mode conversion heating will greatly reduce chances for fast ion-induced sputtering of first-wall components, a major PMI issue for reactors. With off axis launch, a majority of the incident wave mode converts to ICW and damps on thermal ions. This could lead to significant flow drive, which can significantly improve confinement and stability [Lin 2011]. To illustrate the off-axis flow drive with HFS ICRF, a TORIC simulation (2D full wave code) [Brambilla 1999, Wright 2004] of a C-Mod 15% hydrogen minority deuterium plasma with off mid-plane high field side launch is shown in Figure 3. The incident fast wave mode converts primarily to ICW wave and 60% of the power is deposited onto ions.

 

   Figure   3:   Illustrative  TORIC   high-­‐field-­‐side  launch   ICRF   simulation  showing   fast   wave   mode  conversion   to   ion  cyclotron   wave   where  60%   power   is   deposited  onto  thermal  ions]      

Figure 2: For FNSF profiles, the driven current profiles for high field side (red) and low field side LHCD launch is peaked at a location consistent with ARIES-AT and has 40% higher efficiency.  

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Section 2.3.2 Improved Wave Coupling

One of the primary requirements for a reactor device is steady state operation, and is enabled by robust coupling of RF power to the plasma. For typical tokamaks, the heating and current drive have been launched from ports on the out-board (or low-field-side, LFS) mid-plane as shown in Figure 24 (a). LFS launch at these ports is chosen in most tokamaks in part due to convenience access to the launcher for maintenance. This location however is complicated by plasma physics  considerations. Due to bad magnetic curvature, turbulence is much larger in amplitude on the LFS compared to the HFS, as illustrated in Figure 4 (b). This results in the strong ballooning-like  poloidal transport asymmetry peaking the heat and particle flux is peaked on the low field side

about +/-30° of the mid-plane [Gunn2007, LaBombard2004, Garcia2007]. The result is that the RF launcher coupling power is facing into a high heat exhaust and turbulent plasma. On the LFS of the tokamak, the SOL is very turbulent, with rapid and bursty cross-field transport that leads to PMI issues for wave launching structures – forcing the LHCD launcher to be placed far from the LCFS, resulting in poorer wave-plasma coupling. In contrast, the HFS SOL is comparatively quiescent, which allows the LHCD launcher to be located very close to the LCFS without PMI issues. Locating the RF launcher on the HFS has significant beneficial impact on the wave coupling. Due to the smaller SOL scale lengths on the high field side, the distance to the last closed flux surface (LCFS) is significantly reduced. In reactor studies, the distance to the LCFS is ~3-5 cm whereas the distance to the LCFS is ~20 cm on the low field side. Thus, the need for long distance coupling is eliminated and it minimizes the RF power in the SOL. The steep

         

 Figure 4: (a) Cartoon of a single null plasma equilibrium showing the conventional location of the RF launcher on the LFS and high field side innovative RF launcher positions. (b) Turbulent potential fluctuation intensity color-map (green) is adapted from XGC1 simulation results [Diallo2013] and illustrates the differences between HFS and LFS turbulence. Here red is higher fluctuation level, blue is lower fluctuation level. Note that for the conventional LFS position, the RF launcher faces the region of high turbulent flux (red dashed box) whereas the high field side RF launchers face more quiescent regions.

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density profiles potentially could allow for optimized coupling through small changes in plasma position allowing for more robust power delivery. Furthermore, the high field side SOL has been observed to be relatively quiescent in a number of experiments [Smick 2013], which is consistent with results from gyrokinetic turbulence simulations [Diallo2013]. The reduced turbulence on the HFS is expected due to good curvature of the magnetic field lines. The lack of density fluctuations should also minimize coupling variations and reduce wave scattering. Hence, because the HFS SOL is comparatively quiescent, the LHCD launcher can be located very close to the LCFS without PMI issues. With the decreased physical distance between the LHCD launcher and the plasma, one expects improved wave-plasma coupling, and also a beneficial reduction of non-linear loss-mechanisms.

Section 2.3.3 Reduced Plasma Material Interaction  Perhaps the most challenging requirement for RF actuators in a reactor environment is the plasma material interaction. For LHCD and ICRF, the launchers need to located near the plasma edge. The launchers are subject to the same   requirements,   <1  mm/year   erosion,   as   the   first  wall  with   the   additional   issue   of   RF   enhanced   heat   and   particle   flux   and requiring complex, conductive structures. For low field side launch, the erosion rate particularly for LHCD has been estimated at >1 mm/year [Wallace2015] and no viable solution to reduce the launcher erosion rate at the LFS mid-plane while maintaining coupling is yet proposed.

With the launchers on the high field, there is potential for dramatic reduction in the heat flux and erosion due to the shallower angle of incidence. Heat flux and erosion are dependent on the mapping of parallel heat flux to perpendicular heat flux. On the low field side, the field lines are concave relative to the launcher, so the grazing angle is larger. On the high field side, the field lines are convex relative to the launcher and have a shallower grazing angle. For a JET size device, the reduction in perpendicular heat flux is about 5 times due to the different in grazing angle. Furthermore, measurements of the high field side SOL temperature profiles shows that the electron temperature scale length is ~1/10 that of the low field side. Thus, sputtering is expected to be lower at the high field side launcher due to reduced temperature. Finally, the RF enhanced heat flux and impurity source arising from RF driven convective cells will likely be mitigated. In the near field of the antenna, a radial electric field arises due to sheath rectification of parallel RF electric fields. The RF driven convective cell scales as 1/B and this drive is reduced by ~1/2 on the high field side compared to the low field side. From these considerations, the high field side launcher is expected to have lower erosion and lower impurity production.

Section 2.4 Summary of HFS Launch Benefits

• HFS current drive with LH waves exploits the increased magnetic field on the inboard side of the tokamak, which through the accessibility condition and wave propagation physics, allows the LH waves to better penetrate to the core [Podpaly2012].

• Recent modeling studies [Sorbom2015, Bonoli2014] found that current drive efficiency increases significantly for LHCD waves launched from the HFS compared to the LFS.

• Simulations also show that the poloidal location of the launcher is extremely important to wave penetration to the core, with launcher location near active X-points favored.

• With HFS launch, fast waves will mode convert to ion Bernstein waves (IBW) and ion cyclotron wave (ICW), leading to strong single-pass absorption and direct electron heating.

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• In addition to improved conditions from the RF physics perspective, the use of HFS launchers should minimize their plasma-material interaction issues. • On the LFS of the tokamak, the SOL is very turbulent, with rapid and bursty cross-

field transport that leads to PMI issues for wave launching structures – forcing the LHCD launcher to be placed far from the LCFS, resulting in poorer wave-plasma coupling.

• In contrast, the HFS SOL is comparatively quiescent, which allows the LHCD launcher to be located very close to the LCFS without PMI issues.

• With the decreased physical distance between the LHCD launcher and the plasma, one expects improved wave-plasma coupling, and also a beneficial reduction of non-linear loss-mechanisms.

• With ICRF mode conversion, the ions will not be heated directly, so there is no generation of fast ions. HFS launch ICRF mode conversion heating will greatly reduce fast ion-induced sputtering of first-wall components, a major PMI issue for reactors.

Section 3 Proposed Research and Methods

The mission of the DRIVE experiment is to investigate the effects of high-field-side launch on plasma-wave interactions relevant for current drive, flow drive, and heating in diverted tokamak plasmas. Primarily, we are interested in comparing current drive and heating efficiency between HFS and LFS launching systems in order to determine whether HFS launch offers improved performance compared LFS launch. The DRIVE team will consist of graduate and undergraduate students, engineers, and technicians, and PSFC scientists. DRIVE can be brought on-line quickly and at relative low cost, because we propose to re-purpose existing infrastructure, hardware and equipment available at MIT. In Summer 2016, the large tokamak facility at MIT, Alcator C-Mod, will shut down. We propose to create the DRIVE experiment from the unused vacuum vessel, magnet coils, control system, cryogenic system, several power supplies, and a small subset of diagnostics that were once part of the C-Mod facility. Obviously, the new DRIVE experiment cannot possibly utilize all of C-Mod’s capabilities (since the DRIVE annual budget is an order of magnitude smaller than the C-Mod annual budget), but the essential elements needed to operate the DRIVE experiment for focused RF physics studies would be available. It is important to emphasize that DRIVE is not a continuation of C-Mod. DRIVE is a new experiment, with a new mission.

Section 3.1 Hypotheses and general set-up for RF experiments

In this section we present several hypotheses that we would like to test using the DRIVE experiment. Simple descriptions of the experiments and measurements are provided. During the course of DRIVE operations new ideas will be developed, but testing these hypotheses would form a new foundation for physics understanding of LFS launch RF physics.

1. The LHCD “density limit” in diverted tokamak plasmas is different for LFS and

HFS launch.

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• The “density limit” in diverted tokamak plasmas has been observed at C-Mod [Wallace 2010] and at EAST [Ding 2013]. At high densities, the lower hybrid current drive efficiency drops much faster than predicted by theories based on core wave physics. It is speculated that the reduced current drive efficiency is caused by interactions of the lower hybrid wave with the SOL plasma.

• This hypothesis can be probed by running plasmas with HFS LHCD launch and LFS LHCD launch and performing density scans, to determine if the current drive efficiency drops off at a different density for HFS launch compared to LFS launch.

• The only diagnostics required to test this hypothesis are loop voltage measurements and a hard x-ray camera (which both can provide measurements of current drive efficiency), and an interferometer to measure line-averaged density. Measurements with edge plasma diagnostics are desired to provide information on the SOL.

2. HFS launch for lower hybrid waves results in an expanded wave accessibility

window and will result in modified “spectral gap” physics • There is a predicted asymmetry in the wave accessibility for lower hybrid waves, as

illustrated in Figure 5. In the top panel assumed temperature and density profiles are shown. In the middle panel, the accessibility window for the HFS launch is shown. In this case, a wave with n|| = 2 launched from the HFS will penetrate to r/a = 0.2 (stay out of the red region). In contrast, the bottom panel shows the accessibility window for the LFS, where the same wave penetrates only to r/a = 0.6. HFS launch offers a large improvement to allow for both on and off-axis current drive in FNSF where in most scenarios the LH wave cannot access the core if launched only from the LFS [Garafalo 2014].

• This hypothesis can be probed by alternately running plasmas with HFS and LFS LHCD launch, and launching the same n|| in sister discharges. If the accessibility window is larger on the HFS than the LFS, then given the same launched power and same target plasma, we expect higher current drive efficiency for the HFS launch.

• The only diagnostics required to test this hypothesis are loop voltage measurements and a hard x-ray camera (which both can provide measurements of current drive efficiency), and an interferometer to measure line-averaged density. Measurements of the temperature profiles during LHCD are desired, and some temperature information could be available from an ECE radiometer, though non-thermal emission can complicate the measurement. However, a Thomson Scattering system for Te and spectroscopic Ti measurements are too expensive and are outside the scope of this project.

 Figure 5: The asymmetry in lower hybrid wave accessibility for LFS and HFS launched waves for a model case at B = 2 T in DRIVE (circular plasma is assumed in this simple illustration, and the effect is expected to be more pronounced in a diverted plasma)

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• This hypothesis also addresses the “spectral gap” observed in present-day LHCD experiments, where a wave launched at an n|| that should not damp does in fact damp, and efficient current drive is observed. For example, in Figure 5, according to linear wave theory a wave with n|| between 2-4 will not damp, only n|| > 6 is predicted to damp. However, from experiments, we know the spectral gap is bridged and the lower n|| waves will damp. We will be able to study the spectral gap physics in detail, and probe differences between LFS and HFS. Since scattering of the LH wave off of edge/SOL turbulence is believed to be a cause of the spectral gap bridging, there are expected differences between the HFS and the LFS launch due to the in /out asymmetries in the edge/SOL turbulence.

3. Varying the poloidal location of the HFS LHCD launcher will change current drive

efficiency. • It is predicted that an off-midplane HFS launcher will improve current drive

efficiency by modifying the wave penetration. If true, this would benefit FNSF designs, where the LH wave does not have access to the core plasma from the LFS side [Garafalo 2014].

• This hypothesis can be probed by varying the poloidal position of the HFS LH launcher. We expect to measure different current drive efficiency for the off-midplane HFS launch compared to the HFS midplane launch.

• The only diagnostics required to test this hypothesis are loop voltage measurements and a hard x-ray camera (which both can provide measurements of current drive efficiency), and an interferometer to measure line-averaged density. Measurements of the temperature profiles during LHCD are desired, and some Te information will be available from an ECE radiometer, though non-thermal emission can complicate the measurement. However, a Thomson Scattering system for Te and spectroscopic Ti measurements are too expensive and are outside the scope of this project.

4. HFS launch Mode Conversion ICRF should heat the electrons more effectively than

LFS launch Mode Conversion ICRF. • There are two general scenarios for ICRF heating in tokamaks: minority heating or

mode conversion. In the minority heating scenario, a D plasma is seeded with a small amount of hydrogen. The ICRF ion cyclotron wave damps on the protons, generating a fast ion tail, which can effectively heat electrons or ions via collisions depending on the target plasma parameters. In the mode conversion scenario, a larger hydrogen fraction is used to promote mode conversion of the ion cyclotron wave to the Ion Bernstein Wave, which heats electrons directly by Landau damping, and does not generate fast ions.

• Fast ions can be lost, and can damage the first wall and the launching structures themselves, hence the mode conversion scenario is favored in a reactor. However, the mode conversion heating with LFS ICRF launch can be very inefficient because the launched fast wave has to tunnel through a gap in order to reach the mode conversion layer. In contrast, the fast wave launched from the HFS will reach the mode conversion layer directly, and does not need to tunnel through the gap. See Figure xx for an illustration. Hence, the HFS launch MC heating should be much more efficient than the LFS launch.

• This hypothesis can be probed by running plasmas with HFS ICRF launch, and modulating the ICRF power and measuring the break in slope of the electron temperature, a standard technique [Lin 2003]. The temperature should respond

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immediately to the HFS launch ICRF in the MC scenario, but not in the LFS launch in the MC scenario, given the same launched power.

• The only diagnostic required to test this hypothesis is an ECE profile radiometer. If LH is not used in the experiment (which it wouldn't be) the EC emission will remain thermal and straightforward measurements are possible

5. HFS launch of the fast wave for MC flow drive should be more effective than LFS

launch. • The basis for this hypothesis is the same as for number 4: the MC layer is directly

accessible from the HFS (whereas the fast wave must tunnel to the layer if launched from the LFS).

• This hypothesis can be probed by running plasmas with HFS and LFS ICRF launch for mode conversion flow drive, comparing the changes in plasma rotation with and without the ICRF in both cases.

• The only diagnostic required to test this hypothesis would provide a measurement of the plasma rotation. Passive x-ray spectroscopic measurements and active NBI-based charge exchange are too costly for this project, so instead, a Doppler reflectometer would be implemented. In addition, the sawtooth precursor frequency can be used to estimate the core rotation.

6. HFS RF launchers should suffer less severe PMI effects than LFS launchers • Due the much more quiescient SOL and edge plasma conditions on the HFS

compared to the LFS, there should be less plasma interaction with the HFS launchers, which will reduce PMI effects.

• This hypothesis can be probed by measuring the SOL/edge plasma effects using probes mounted on the active HFS RF launchers.

• The diagnostics required are surface probes, Retarding Field Analyzers, and thermocouples implemented on the launchers themselves. In addition, spectroscopic measurements provide monitoring of impurity generation during RF operation. Also, ex-situ measurements to compare the surfaces of the HFS and LFS structures will be performed.

Section 3.2 List of diagnostics used in RF experiments

A series of experiments to study HFS launch LHCD and ICRF will be conducted on the new DRIVE tokamak. In this section, we describe the diagnostics used in RF experiments, provide descriptions of how the measurements are used to study RF physics, and we list the availability of existing diagnostics for use at DRIVE. The tokamak itself is also already available at MIT, as described in Section 3.3, with more details in the Appendix on Equipment. Design of the HFS launchers is summarized in Section 3.4. Section 3.2.1 List of diagnostics used for RF experiments This section described the diagnostics needed to perform the RF experiments to test the seven hypotheses. We have ranked these measurements, and we note that the majority of the DRIVE mission could be completed with just the required high priority diagnostics. However, we have budgeted for implementation of all required diagnostics. It is envisioned that graduate students will be responsible for one or more related diagnostics for their PhD thesis research. With six

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graduate students and several undergraduates on the project, we have manpower to run all the required diagnostics:

Required High Priority • Loop voltage measurements • Hard x-ray camera for detecting fast (non-thermal) electrons • Multi-channel Interferometer for line averaged density and density profile

reconstruction • Midplane (standard) ECE radiometer for Te profile measurements and detection of

non-thermal electrons during LHCD. • Spectroscopy for monitoring of impurity generation during RF operation

Required Medium Priority • Doppler Reflectometer • Surface Langmuir probes mounted on RF launching structure • Emissive probes for plasma potential measurements at launcher • Surface thermocouples mounted on RF launcher for heat flux • Retarding field analyzer at the RF launcher for measurement of the ion energy Desired Low Priority • Neutral Particle Analyzer (NPA) • Fast Ion Loss Detectors (FILD)

Out of scope, but desired and will be pursued if resources became available.

• Multiple hard x-ray cameras for tomographic reconstruction of the emission • Thomson Scattering for Te profile measurements during LHCD • Phase contrast imaging for measurements of mode converted wave • X-ray spectroscopy for core rotation measurements (e.g. x-ray imaging crystal

spectroscopy for Argon) • NBI-based Charge Exchange for Ion temperature rotation measurements • NBI-based MSE or polarimeter for current profile measurements

Section 3.2.2 Details of measurements needed for RF experiments Testing hypotheses regarding LH current drive require experimental measures of the current drive efficiency. Current drive efficiency will be assessed on DRIVE in several ways. First, LHCD power scan experiments can be performed and measurements of the loop voltage (from magnetics) can be used to determine the LHCD efficiency. In these experiments, some discharges will be fully non-inductive, and others will have some ohmically driven plasma current. We can use the following equation to estimate current drive efficiency from loop voltage measurements, plasma current, and knowledge of the launched lower hybrid wave power. Where PLHW is the LH power (known), ne is the line averaged density (measured with an interferometer), The plasma current is Ip = Irf + Ioh, Irf = (- ∆V/Voh)Ip, and ∆V = VLHW – Voh.

                                                   (4)  

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Non-thermal resistivity corrections to the simple Vloop measure of LHCD efficiency [Giruzzi 1997] can also be applied. In addition to loop voltage measurements, we will also employ hard x-ray cameras. In general, the count rate of the hard x-rays is proportional to the fast electrons. More efficient current drive will result in more fast electrons and more counts on the hard x-ray camera. In high performance plasmas, with substantial neutron fluxes, hard x-ray camera data can be polluted by the neutrons impinging on the imaging chips. However, in the DRIVE tokamak, which can run H and He, will avoid neutron production. But even when running with D, we expect small neutron fluxes are expected, so the hard x-ray camera should be a very robust diagnostic. We also are considering implementing vertical and oblique ECE to diagnose the fast electrons, but this will be limited by costs (more on cost reduction via diagnostic loans from C-Mod will be discussed in section xxx). It is straightforward to test the LHCD hypotheses by measuring the current drive efficiency (e.g. Eqn. 4) and using the hard x-ray camera, but to examine the asymmetry, it will be important to have accurate density and temperature profile measurements to construct the theoretical accessibility condition. Prior to and after LHCD, the standard ECE radiometer will be used for temperature profile measurements, but ECE would become polluted by non-thermal emission during LHCD. There is no low-cost alternative for temperature profile measurements, thus in the out years, we would propose a Thomson Scattering system. Testing hypotheses about the mode conversion ICRF heating requires measurements of the electron temperature profile with high time and spatial resolution as well as at least partial profile measurements of the plasma rotation. To measure direct electron heating by ICRF, we will conduct standard power modulation experiments [cite] to compare electron heating when using the LFS and the HFS ICRF antennas. We will measure the changes in the electron temperature profile with a high-resolution, high time resolution ECE profile radiometer. To measure fast ions generated by LFS ICRF, and to confirm that no fast ions are generated with MC heating via HFS ICRF, we could deploy two types of fast ion diagnostics: a neutral particle analyzer and a fast ion loss detectors. To measure the rotation in the plasma and to measure the changes in plasma rotation during MC flow drive experiments, we could deploy a Doppler Reflectometer. Testing hypothesis about the PMI effects on the HFS RF launchers requires measurements of the plasma-surface-interaction region very near the launcher structures. Typically, surface probes and thermocouples are implemented on the launchers themselves. In addition, spectroscopic measurements provide monitoring of impurity generation during RF operation. We will employ the same techniques at DRIVE. Also, it will be possible ex-situ after experiments to compare the surfaces of the HFS and LFS structures. Section 3.3 Commissioning the DRIVE tokamak

Section 3.3.1 Unique Collection of Existing Equipment, Facilities and Infrastructure The DRIVE tokamak can be commissioned rapidly and with little hardware investment because we will re-purpose an existing facility (formerly C-Mod, for details of available equipment, please see Appendix 5). By re-purposing the C-Mod facility, we have a vacuum vessel, magnetic field coils, and LN2 cooling system already in place, and the control systems used for operating DRIVE will be adapted from the existing systems. The RF sources for LH waves and ICRF are already available, as are the LFS RF launchers, which can be re-used at DRIVE. We can utilize a subset of the diagnostics already installed. The power infrastructure at PSFC provides a wide range of capability for the DRIVE tokamak. Several 13.8 kV lines from our utility power MIT

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and the PSFC. Approximately 42 MVA of power is available from the utility, and pulsed power is available from the MIT motor/generator/flywheel at the 300 MVA level for ~1 s flat-tops (This is not planned to be used for DRIVE because this requires a much larger budget for operations). The list of on-site power supplies can provide currents from hundreds of Amps up to approximately 260 kA. The appropriate power supplies from among those listed will be selected for use at DRIVE. By selecting power supplies that can run 55-75kA current in the TF coils, the DRIVE tokamak can be operated between 1.0-2.7T. A table of DRIVE parameters is given here. Bt Ip R a PICRF PLH ne Te

1.0-2.7T 0.2-0.5 kA 0.67m 0.18m 2MW 1MW 2-6 x10 19 m-3 1-2 keV

Table 1: A table of DRIVE parameters (re-purpose of C-Mod vessel and magnets). Note that due to modifications of the inner wall to accommodate the HFS RF launchers the size of the plasma is smaller in DRIVE than it was in C-Mod. Section 3.3.2 List of diagnostics that are already available at MIT-PSFC

In this section we list all the diagnostics available for use at the DRIVE. Costs for this project are kept low because diagnostics are already available because they were used for the C-Mod project and can be re-used for DRIVE with little to no modification. If a diagnostic is listed as “already available” this means we are going to use the existing diagnostic from the C-Mod project. In other cases, hardware is available and can be loaned to this project from other groups at MIT at no cost. The funds in this proposal will pay for students to operate the diagnostics and perform physics research at DRIVE. We have allocated some funds for anticipated repairs or required upgrades of existing systems, but no major diagnostics must be developed/purchased for this proposal.

Basic diagnostics for DRIVE Tokamak Operation • Full magnetic diagnostic set for equilibrium reconstruction ($0 already available) • Multi-channel Interferometer for line averaged density and density profile

reconstruction ($0, already available) • RGA ($0, already available)

List of diagnostics used for RF experiments • Loop voltage measurements ($0, already available) • Hard x-ray camera for detecting fast (non-thermal) electrons ($0, already available) • Midplane (standard) W-band ECE radiometer for Te profile measurements and

detection of non-thermal electrons (on loan from Prof. White’s group) • Multi-channel Interferometer for line averaged density and density profile

reconstruction ($0, already available) • Doppler Reflectometer ($0, already available) • Neutral Particle Analyzer (NPA) ($0, already available) • Fast Ion Loss Detectors (FILD) ($0, already available) • Surface Langmuir probes mounted on RF launching structure ($0, already available) • Emissive probes for plasma potential measurements at launcher ($0, already

available) • Surface thermocouples mounted on RF launcher for heat flux ($0, already available) • Retarding field analyzer at the RF launcher for measurement of the ion energy ($0,

already available)

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• Visible, VUV, XUV spectroscopy for monitoring of impurity generation during RF operation ($0, already available)

Section 3.3.3 Decision to re-purpose C-Mod as the new DRIVE experiment instead of the VTF facility. After extensive scoping studies and discussions with the engineers and technicians, we decided to propose in this Narrative that we would re-purpose C-Mod as the DRIVE experiment. In this section, we discuss the background behind this decision and present an alternative option (for comparison) so that the reader understands why we have made this decision for our proposal. When the DRIVE experiment was initially conceived, we thought it best to re-use the C-Mod facility. C-Mod would be shutting down in 2016, around the same time that DRIVE (if funded in by DOE in FY17) would be coming on-line. From a scientific and technical perspective, re-commission of C-Mod made the most sense. In this scenario, we could bring DRIVE on-line much faster, and would be able to use a suite of already existing and installed diagnostics, as well as re-using the LFS launchers for LHCD and ICRF already in place at C-Mod. There is also another old tokamak vessel and coil set at MIT, called VTF, which we considered as well for use in DRIVE. Cost estimates for both options are provided here. VTF had been used in recent years by Jan Egedal’s group for reconnection experiments. The device can be operated as a tokamak, but this had not been done for nearly 20 years. When we completed our VTF scoping study, we estimated the funds needed to re-start VTF and run it as a tokamak (Table 2, next page). With this plan, it would take us about a year to make plasmas, and we would expect to have built three new RF launchers needed for the experiment in year 1, and installed them/commissioned in year 2 and run the RF experiments in year 3. The personnel needed for the VTF plan would be two engineers and three technicians, in addition to the six graduate students and several undergrads who would work on the project. The large number of expert personnel is required to “revive” a device, since and initial tests and inspections revealed that significant repairs would likely be needed. In addition, a completely new control system and magnetic probe system would need to be built, and a large amount of effort would be put into moving equipment from the C-Mod cell to the VTF cell, and adapting power systems and diagnostics for use at VTF. According to our estimates for re-commissioning the VTF device as the DRIVE experiment (Table 2, page 17), the cost for hardware and operations (and personnel) would be outside the scope of the budget of this proposal. To build DRIVE using VTF would require a total budget of more than $7M over 3 years instead of $6M. In contrast, re-purposing C-Mod for the DRIVE experiment could be done for $6M over 3 years, as shown in Table 3 for comparison. The scoping study concluded that re-purposing C-Mod for DRIVE would fit in the budget of $2M / year, with the anticipated hardware and operating costs summarized in Table 3. Fitting the proposed operations and experiments for DRIVE within budget by re-purposing the C-Mod coils and vacuum vessel is made possible by four major results of our scoping studies.

• First, the engineering studies that uncovered that the C-Mod coils can be operated at low magnetic fields (1.0T-2.7T) safely at room temperature or using cooling from LN2, and at these fields, the amount of LN2 needed is roughly an order of magnitude less than at the full field operation of the former C-Mod device (5.4 T).

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• Second, the engineering studies found that the C-Mod coil set can be operated without

the alternator at the low magnetic fields envisioned for DRIVE, making it possible to run the DRIVE experiment using standard power supplies (already available) in the experimental cell. This results in lower electric costs and lower maintenance costs.

• Third, well-established wave-physics predicts that the same LHCD source at 4.6GHz

(and the LFS launcher) used in C-Mod formerly will drive current efficiently at low magnetic fields used in DRIVE, and experiments run at C-Mod demonstrate that the existing ICRF source at 57MHz can be launched with existing LFS ICRF antenna. This means that we save nearly $1M in cost by re-using the existing LFS RF launchers; we only need to build the HFS RF launchers. And designs are already available that we can use, and can get to work immediately.

• Fourth, and probably most important: the DRIVE experiment is a focused and dedicated

RF physics experiment. DRIVE will not be a full tokamak facility (DRIVE will not, it cannot, operate as a competitor to DIII-D, NSTX-U, etc. as C-Mod did). There will be no other experiments performed at DRIVE outside of the scope of this proposal. For this

 System Cost Comments

Year of proposal

Tokamak control system $200k Includes electronics, computers Year 1

Magnetics $125k Includes hardware, electronics and digitizers

Year1

Two LHCD launchers $1M Launchers only, the 4.6GHz source and PS are available

Year 1, 2, and 3

One ICRF antenna $400k Includes antenna and transformer, sources and PS already available

Year 1 and 2

Repairs/upgrades to existing diagnostics, power supplies, electronics

$300k Miscellaneous repairs anticipated. More for VTF re-commission than C-Mod re-commission, because VTF has not operated as a tokamak in ~20 years

$100k per year

LN2 for cooling $0k Coils are water cooled up to 1.8T $0k Moving PS, transformers, diagnostics and other equipment from C-Mod cell to VTF cell, and adapting for use on VTF

$150k New electronics, new hardware, must be made in order to adapt existing C-Mod equipment for use at VTF

$75k in year 1 and year 2

Diagnostics $0k Borrow set from C-Mod. Does require funds to adapt for use on VTF, see line item above.

n/a

Total costs (equipment and hardware, operations and maintenance).

$2.175 Does not include personnel. Over three years

 Table 2: Estimated hardware and operations costs if the VTF vacuum vessel and coils are re-commissioned as the DRIVE Experiment. Re-purposing the C-Mod vacuum vessel and coils makes more sense financially (and scientifically); see Table 3 for comparison of costs.  

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reason, only a subset of the C-Mod diagnostics and small subset of C-Mod capabilities will be retained for use in the DRIVE experiment.

We must emphasize to our internal colleagues, to external observers, and to the reader that DRIVE in not a continuation of C-Mod. It is a new experiment, with a new mission and new leadership, and a much smaller budget (order of magnitude smaller). We believe the best science for DRIVE can be done more quickly and at lower cost by re-purposing the C-Mod coil sets and vacuum vessel after that facility shuts down. We would be leveraging a large base of unique infrastructure, facilities and equipment available at MIT-PSFC. Section 3.4 Design and construction of the HFS LHCD and ICRF Launchers

The RF sources and LFS launchers already available at C-Mod can be repurposed for use on DRIVE. Here we discuss the plans for the HFS RF launchers.

 System Cost Comments

Year of proposal

Tokamak control system $0 Existing from former C-Mod tokamak

n/a

Magnetics $0 Existing from former C-Mod tokamak

n/a

One LHCD launcher on HFS $500k Launcher, the 4.6GHz source and PS are available

Year 1

One ICRF antenna on the HFS $400k Includes antenna and transformer, sources and PS already available

Spread over Year 1 and Year 2

Repairs/upgrades to existing diagnostics, power supplies, electronics

$150k Miscellaneous repairs anticipated. $50k per year

LN2 for cooling $120k Required to cool TF coils for operation > 2T

$40k per year

Diagnostics $0k Use set existing from C-Mod tokamak

n/a

Total costs (equipment and hardware, operations and maintenance).

$1.17M Does not include personnel Over three years

Table 3: Estimated hardware and operations costs if the C-Mod vacuum vessel and coils are re-purposed as the DRIVE Experiment. This is the option we have selected for this proposal. It is less expensive to re-purpose C-Mod for DRIVE than it is to revive VTF, and the existing RF systems and diagnostics systems will allow for faster ramp up to scientific experiments starting in year 1 of the proposal.  

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The HFS LHCD launcher planned for DRIVE is based upon LFS designs already successfully utilized in Alcator C-Mod [Shiraiwa2011]. The launcher utilizes a novel splitter concept, which evenly distributes the microwave power in the poloidal direction and a representative subsection is shown in Figure 6. The dimensions of the output waveguides are 60 mm x 7 mm with 1.5 mm septum. To minimize reflections at the end waveguides, passive waveguides (~λg/4, where λg is the wavelength in the waveguide) are located on both sides of the array.

A unique feature is that if one of the splitter outputs is shorted the power is evenly redistributed to the other outputs. This design will allow relatively easy variation of the poloidal launch position through installation of shorts at the output. This design also allows flexibility to vary the peak launched toroidal index of refraction, Nφ, from -4 to 4. Each poloidal array is powered by one klystron and this design allows for conventional WR187 waveguide up to a matching transformer, minimizing transmission line losses. Each klystron has a maximum output power of 250 kW and we expect to couple 125 kW per klystron using 4 poloidal outputs or 500 kW total from the HFS. This is consistent with the operational experience on C-Mod where 1 MW is coupled out of 2 MW source. The primary downside of the four toroidal waveguide array is the relatively low directivity compared to an 8 or 12 toroidal waveguide array [Porkolab1984]. A four waveguide toroidal array was first used in Alcator C to investigate current drive. For the low field side (LFS) LHCD launcher, DRIVE will make use of the existing C-Mod LHCD launcher [Shiraiwa2011]. This launcher has operated up to approximately 1.1 MW and has flexible Nφ spectrum.

The HFS ICRF antenna is based on the low field side antenna design used in C-Mod. The antenna is an end fed center grounded with long shielded strip lines to transmit power from the coaxial feedthroughs at the top and bottom of the machine in a manner reminiscent of the LH. At the HFS mid plane, we expect to have low flux coupling due to limited space, ~40 mm between the strap and ground plane. This is about 40% less than the LFS ICRF antenna. The LFS ICRF is proposed as the primary heating system for DRIVE. Alcator C-Mod has demonstrated efficient ICRF heating up to 6 MW (80 MHz, hydrogen minority) with power input combined from three antennas. It is envisioned that DRIVE would make use of one of the existing antennas on the low-field side. The heating scenario is flexible. With modification only to the transmission line network, second harmonic minority hydrogen absorption at 60 MHz will heat effectively. This is similar to C-Mod operation at 2.7 T which uses 2nd harmonic minority hydrogen absorption at 80 MHz and routinely heats effectively to produce H-modes. For the mode conversion experiments, the antenna will be operated at 40 MHz and compare RF core absorption for H minority scenario (~20% H) at 2.7 T.

Figure 6: Representative sub-section (in brown/bronze color) of the HFS LHCD launcher planned for DRIVE. The launcher utilizes a novel splitter concept, which evenly distributes the microwave power in the poloidal direction. The dimensions of the output waveguides are 60 mm x 7 mm with 1.5 mm septum.

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Section 3.5 Comparisons with theory and simulation

As described in Section 3.1, there are six central hypotheses we will probe with the DRIVE experiment. These hypotheses have been developed through discussions theorists at the PFSC, notably Dr. Abhay Ram and Dr. John Wright. Through collaborations with the MIT-PSFC theory group and the RF SciDac center, we anticipate making our data available to theorists in the RF community, and we will pursue active collaborations with RF scientists in order to use our unique data set from HFS launch RF experiments on drive to constrain and validate RF codes.

Section 3.6 Anticipated challenges and alternative strategies Several challenges have been discussed by the team, including:

1. Target plasma parameters (high enough density and temperature) to pursue “interesting” LHCD physics: We must achieve target plasmas where the wave will be strongly absorbed on the first pass. High densities (2-5 x 1019 m-3) are desirable to probe the studies of the density limit for LHCD. We are concerned about attaining such plasmas at low field (1T - 2.7T) in the converted C-Mod device (which supports a small-size plasma). One key thing will likely be H-mode operation. We think that DRIVE will be able to achieve H-mode (based on experience from JFT-2M [Ida 1990], a similar small plasma, with low plasma current 250kA and less than 1MW input power). But we can also use biased probes at the edge to stimulate a L-H transition. A benefit of re-purposing C-Mod for DRIVE is that the LFS ICRF is already available and functioning and can be used to achieve the target plasma parameters needed to study the LHCD physics. In addition, we plan to conduct experiments at C-Mod in FY15 and FY16 (operating at 2T and below) to develop target plasmas for DRIVE experiments.

2. Limitations in resources due to implementing HFS LHCD and HFS ICRF simultaneously. While the mission of DRIVE is to study both HFS LHCD and ICRF physics, our priority is on the LHCD physics. We believe this to be the most directly applicable to current research in Advanced Tokamak research at large-scale facilities and also will be most directly applicable for informing FNSF. If significant issues arose in the design, construction or performance of the ICRF, we would skip it and focus on the LHCD.

3. Challenges installing HFS lower hybrid launchers. Retrofitting challenges involve

removal of inner wall tiles, which will be replaced with discrete limiters, that will be vertical strips and will not go all the way toroidally around the center stack. This will shrink the size of the plasma, removing 3.5 cm from the diameter of the plasma at the midplane. This will reduce performance, so we return to issues/solutions discussion for point 1 above. Coupling issues for LHCD can be surmounted because the gap between the waveguide and the plasma will be smaller on the HFS, and easier to control (e.g. due to flexibility in the magnetic field geometry (SN, DN, etc) and gas puffing capabilities).

 4. Costs of electricity and LN2 for cooling. Unlike C-Mod, DRIVE will use a small

amount of electricity that will be covered under the MIT cover (we do not need to pay for electricity out of proposal funds, it is covered in the standard overhead). With LN2 cooling, at 80K the stored energy in the TF magnets at 2T is nearly an order of magnitude lower than at 5.4T (35MJ compared to 350MJ), so we need roughly an order of magnitude less LN2. We have performed several engineering studies to determine if LN2

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cooling is necessary can show that the magnets can be operated at room temperature if needed. However, while we can operate at 300K, the resistive losses are significant and in this case electric costs can outweigh the LN2 costs, so operation above 80K but below 300K can keep costs down.

5. Operations and scenario development in a new tokamak experiment. Operating the

DRIVE experiment will require development of breakdown scenarios, with equilibrium and density control. Many of this report’s authors have been trained as Physics Operators at the C-Mod tokamak and are familiar with the operations and control systems, however, we anticipate some challenges with breakdown/start-up at low-field and will work with on-site experts like Dr. Steve Wolfe and Prof. Ian Hutchinson to ensure development of a “basic DRIVE operations” manual. In addition, experiments at C-Mod in FY15 and FY16 (operating at 2.7T) help to develop target plasmas for DRIVE experiments (similar to addressing challenge #1)

6. Calibrating the ECE profile radiometer. Since the only temperature profile

measurement will be a ECE radiometer loaned from White’s mm-wave group, we must calibrate the system. Direct calibration of a radiometer is challenging, and typically (e.g. at DIII-D and C-Mod) a Michelson interferometer to cross-calibrate the other ECE systems. The Michelson system will be available from C-Mod and could be used to cross-calibrate the ECE radiometer for DRIVE, but this proposal does not support manpower to do this routinely. Fortunately, calibrations are found to be very robust, so we could cross-calibrate the ECE radiometer once a year (at the most) or, given limited resources, could cross-calibrate it at the beginning of the DRIVE experiment and then use lab tests of the IF section to maintain the calibration year to year, as is used in similar systems for turbulence measurements [White 2008].  

 7. Availability of C-Mod equipment may change in the future. Under current guidance

from US DOE, C-Mod operations end September 30, 2016. However, if current guidance changed and C-Mod kept running, we would reassess the option to re-purpose the VTF equipment for DRIVE (with different scope). Re-purposing VTF is Option B.

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Table 4: Timetable of activities, milestones and deliverables for three-year effort, e.g. funded on a normal DOE type cycle. We can naturally extend this to a 5 year plan as needed, which would still be compatible with changeover at PSFC to building ADX / DTT by repurposing C-Mod/DRIVE.  Year 1 Activities Comment Develop operating scenarios with LFS ICRF launch on DRIVE to achieve high temperature (1-2keV) target plasmas for LHCD experiments

Commissioning DRIVE, bringing high and medium priority diagnostics on-line, developing operating scenarios, developing experience with the control system at low-field (1-2 Tesla) with the new power supplies and cryogenic system, developing experience with 50MHz ICRF coupling at low field.

Milestone

Design and install the HFS LHCD launcher

Design, approval and installation of new structures on the inboard wall of the tokamak

Deliverables

Demonstrate HFS launch of LH waves for efficient current drive

Discharge and scenario development, initial HFS LHCD experiments

Milestone

First preliminary comparisons of HFS LHCD experiments with theory

Develop data workflow to allow for comparisons with theory and simulations; Perform experiments to address hypotheses, present at conferences, write papers

Deliverables

Complete design of the HFS ICRF antenna

Design and approval of new structures on the inboard wall of the tokamak

Deliverable

Year 2 Activity Comment Address LHCD Hypotheses 1,2 and 3. Begin work on PMI Hypothesis 6

Perform experiments to address hypotheses, present at conferences, write papers

Deliverables

Installation of the HFS ICRF antenna

Installation of new structures on the inboard wall of the tokamak

Milestone

Demonstrate HFS launch of ICRF waves for efficient heating and flow drive

Discharge and scenario development, initial HFS ICRF experiments

Milestone

Year 3 Activity Comment Address Hypotheses 4 and 5 write papers. Continue work with Hypothesis 6.

Perform experiments to address hypotheses, present at conferences, write papers

Deliverables

Develop new experimental hypotheses, continue comparisons with RF codes.

Perform experiments to address hypotheses, present at conferences, write papers; Bring low priority and desired diagnostics on-line as budget permits.

Milestones

OUTYEARS Activity Comment Develop new experimental hypotheses, continue comparisons with RF codes.

Perform experiments to address hypotheses, present at conferences, write papers; Implement a Phase Contrast Imaging (PCI) diagnostic for mode conversion wave physics; Bring new diagnostics such as Thomson Scattering on-line to allow for better profile measurements of density and electron temperature; Continue to write papers/give presentations on DRIVE data

Milestones/Deliverables

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Section 4. Summary and conclusions

The primary objective of the DRIVE tokamak will be to test predictions of how HFS launch LHCD and ICRF compare/contrast with LFS launch, in a smaller, faster, affordable platform, while also providing an optimum environment for graduate and undergraduate student education. The technical approach will be to design, install, operate, and qualify HFS launch RF systems on a small-scale test-facility, to help retire risk for deployment of HFS RF on ADX [LaBombard 2015] and future high-field devices, such as ARC [Sorbom 2015].

The MIT PSFC is ideally suited to host the DRIVE tokamak experiment.

• First, repurposing the C-Mod facility provides a large vacuum vessel and TF and PF coils. Existing hardware, power supplies, RF sources (some on-site, some loaned from collaborators), and in-place infrastructure support all mean that DRIVE can be built with minimal capital investment (~$250k for hardware/year). Funded at <$3M/year, DRIVE could study HFS launch RF with on-axis Bt = 2.0T, ne = 2e19 m-3, Te = 1keV, for longer than 0.2sec, with >1MW of RF power, in diverted plasmas (DN, USN, LSN) with R = 0.9m, a = 0.3m, b=0.5m.  

• Second, the team at MIT is perfect for DRIVE. In addition to the authors on this report, there are several other scientists at PSFC who work with LHCD and ICRF, and with data processing and experimental analysis, who could help make DRIVE a reality. Knowledge for operating the systems for DRIVE will come from the engineers and technicians: RF engineer, a power systems engineer, and an engineer for cryogenics; and technician support is critical as well for vacuum, for RF and for general electronics. Given our expertise, we can make rapid development of the HFS launchers for the DRIVE tokamak – elements can be designed quickly and built in-house.

• Third, the Theory Group at the PSFC is deeply involved in cutting edge theory and

numerical simulation of LHCD and ICRF waves – DRIVE data will provide ample opportunity for collaboration, using the new HFS experiments to test and validate predictive RF models and codes. The proposal will also leverage existing collaborations with ORNL for RF measurements and modeling. And new off-site / non-MIT collaborators will be welcomed; and supported as well as limited resources will allow.

• Fourth, Drive will be an excellent tool to attract and train high-quality students at MIT.

All the diagnostics and RF experiments at DRIVE will be run by graduate and undergraduate students, who will be supervised by faculty and scientists at PSFC in the same manner as has been done at C-Mod so successfully over the years. Undergraduates, UROPs, are welcome  

   

Acknowledgements:

AEW is grateful to Drs S. Shiraiwa and G. Wallace for numerous, insightful discussions; and special thanks to G. Wallace for the accessibility calculations in Figure 5.

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