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FULLY NON-INDUCTIVE 2ND HARMONIC ELECTRON CYCLOTRON CURRENT RAMP-UP WITH POLARIZED FOCUSING-BEAM IN THE QUEST SPHERICAL TOKAMAK

H. IDEI Research Institute for Applied Mechanics, Kyusyu University Kasuga, Japan Email: idei@triam.kyushu-u.ac.jp

T. ONCHI, M. HASEGAWA, K. NAKAMURA, R. IKEZOE, O. WATANABE, K. KURODA, K. HANADA, Y. NAGASHIMA, A. HIGASHIJIMA, T. NAGATA, S. SHIMABUKURO Research Institute for Applied Mechanics, Kyusyu University Kasuga, Japan T. KARIYA, T. IMAI Plasma Research Center, University of Tsukuba

Tsukuba, Japan T.I. TSUJIMURA, S. KUBO, S. KOBAYASHI National Institute for Fusion Science TOKI, Japan M. FUKUYAMA, M. YUNOKI, S. KOJIMA, R. YONEDA Interdisciplinary Graduate School of Engineering Sciences, Kyushu University Kasuga, Japan A. EJIRI, N. MATSUMOTO, Y. TAKASE Department of Complexity Science and Engineering, University of Tokyo Kashiwa, Japan A. FUKUYAMA, S. MURAKAMI Department of Nuclear Engineering, Kyoto University K. MISHRA Institute for Plasma Research Gandhinagar, India G. TAYLOR, N. BERTELLI, M. ONO Princeton University Plasma Physics Laboratory (PPPL) Princeton, USA Abstract

A transmission line and a launcher system have been newly developed to conduct the second (2nd) harmonic electron cyclotron (EC) plasma ramp-up with an eXtra-ordinary (X) mode wave in the QUEST spherical tokamak (ST). The incident elliptical polarizations could be controlled with two corrugated (quarter/one-eighth wavelength l) polarizers. The launcher system with two quasi-optical (QO) mirrors produced a sharply focused incident beam with a waist size w ~ 0.05 m at the 2nd electron cyclotron resonance (ECR) layer. The obtained electron density was one order of magnitude higher, compared to the previous experiments with no polarized focusing-beam. As a new record of non-inductive plasma ramp-up with EC-waves, a highest plasma current Ip of 86 kA was achieved with a focused 230 kW 28 GHz-beam even when the incident polarization was not optimized. The record Ip ramp-up efficiency on the incident power in the 2nd harmonic EC scenario was also achieved. Optimization of parallel refractive index N// to the magnetic field and incident polarization were explored to attain significant single absorption due to highly energetic electrons carrying the ramped Ip. Linearly polarized vertical- and horizontal-field waves were excited in N// and incident polarization scan experiments.. Single-pass absorption effects are also discussed with ray tracing analysis results.

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1. INTRODUCTION

The Q-shu University experiment with steady-state spherical tokamak (QUEST) project was proposed and constructed at Kyushu University [1]. The QUEST project has focused to study the plasma-wall interaction at steady state discharges that will be required for reactor operations. The long-duration discharges for 1 hour and 55 minutes has been achieved using electron cyclotron (EC) 40 kW 8.2 GHz-wave, and the hydrogen recycling and its modification by a hot first-wall have been investigated [2]. The radio frequency (RF) plasma current drive method has been used for the long-duration discharges in the QUEST.

The RF non-inductive plasma current ramp-up has been also explored in the QUEST as an attractive tokamak start-up method. The non-inductive plasma ramp-up either without or with a small central solenoid (CS) gives distinct advantages in reactor engineering design and cost considerations as compared to conventional tokamak. Advanced tokamak reactor designs with either zero or small CS fluxes have been proposed. [3,4]. In spherical tokamaks (STs), the CS fluxes are essentially small due to the limited center stack area. Non-inductive plasma ramp-up experiments using EC waves have been conducted in STs [5-9] as well as tokamaks. [10-12].

A 28 GHz electron cyclotron heating and current drive (ECHCD) system has been developed to conduct non-conductive plasma ramp-up in the QUEST. The 28 GHz gyrotron with a maximum output power of over 1 MW has been developed at University of Tsukuba [13]. The output power from the 28 GHz gyrotron was led to the QUEST with the simplest transmission, composed of circular-corrugated 63.5 mm diameter waveguides (WGs) following a miter bend. One WG was inserted to the mid-plane of the QUEST device as a launcher through a sapphire single-disk window with electrical isolation to the WG line. Since the output electric-field polarization of the gyrotron is in the horizontal direction, the incident 28 GHz-wave was an ordinary (O) mode. Although significant single path absorption of the incident O-mode wave was not expected in the second (2nd) EC plasma ramp-up scenario, a high plasma current of Ip = 54 kA was attained and be stably sustained for 0.9 sec. The higher Ip of 66 kA was achieved with a 270 kW 28 GHz-wave with a slow ramp-up of the vertical field [14].

New transmission line (TL) and launcher systems have been developed to conduct the local 2nd harmonic EC plasma ramp-up experiments in the QUEST [15]. There are two important aspects of conducting the EC Ip ramp-up experiments. One is a beam focusing, and the other is control of incident polarization. All elliptical polarization states can be controlled in combination with rotation angles of the two corrugated (quarter/one-eighth wavelength l) plates with respect to incident planes of the waves. The two-mirror launcher system has been developed and assembled to obtain a narrow beam size of w ~ 0.05 m in the QUEST. The incident beam can be steered from perpendicular to tangential directions. The steering capability with focusing property was confirmed at the low power test facilities. The TL with the two polarizers and the two-mirror launcher for strong focusing were installed on the QUEST.

The transmission power gradually increased as an aging process of the TL. The 140 kW power could be transmitted with no serious arcing problems, and the relatively high Ip of a 70 kA was ramped up by the 28 GHz 140 kW wave with parallel refractive index to magnetic field N// of 0.78 [16]. However, a number of arcing events were detected at the polarizer section with the higher transmitted power though the TL was evacuated. Gaps between the rotating polarizer-plate and the miter-bend box were reduced to block the RF leakage into O-ring sealing of the rotator-rod damaged by the arcing, but the arcing occurred at the reduced gaps in turn. A new quasi-optical (QO) concept of the polarizer system was proposed and developed to avoid the arcing events [17].

The paper is organized as follows. Section 2 describes the developed TL with the QO polarizer system. The TL route was modified to change the launcher port of the QUEST device. The polarization states were measured inside the QUEST device at a low power level. The low power test results are also shown in the section. Section 4 shows the 2nd harmonic EC plasma ramp-up experimental results with no optimized process of the incident polarization. Experimental results of N// and polarization scans with linearly polarized vertical- and horizontal-field waves are also shown in the section. Section 5 discusses single-pass absorption effects based on the ray tracing analyses, and the summary is finally given in Section 6.

2. DEVELOPED TRANSMISSION LINE

2.1. Transmission line for local 2nd ECHCD and EC plasma ramp-up experiments

Figure 1 shows the developed TL with the QO polarizer and launcher systems for the local 2nd ECHCD and EC plasma ramp-up experiments. The major and minor radii of the QUEST, R0 and a, are 0.64 m and 0.36 m,

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respectively. The TL was mainly composed of circular-corrugated (CC) 63.5 mm diameter WGs of ~ 15 m length as used before. A sapphire double-disk window was also installed in the TL, together with an arc-sensor miter bend to look arcing lights at the polarizers and the window. The QO polarizer system with l/8 and l/4 polarizer plates was installed, following a miter bend. Figure 2(a) shows the QO polarizer system. A QO vacuum chamber was prepared with two l/4 and l/8 polarizer plates and a QO mirror for coupling between the input and output HE11 modes. The QO coupling mirror was designed to have a phase reversal surface. The complex field profile of Erad (x, y, z) of the irradiant HE11 mode from a CC waveguide at the mirror surface coordinates of (x, y, z) was evaluated using a developed Kirchhoff integral code [18] expressed as,

where E0 (xWG, yWG) was a field distribution at the HE11 (xWG, yWG) aperture, and k was a wavenumber of the operating 28 GHz frequency. The TiO2 absorber with a cooling water channel was installed at the chamber wall to remove the stray power there. As a first trial, the polarizer plate angles were fixed at the pre-setting to clear the gap. After installing the QO polarizer system to the TL system, the 250 kW 28GHz-wave could be transmitted to the QUEST sequentially. The 2nd harmonic non-inductive plasma ramp-up experiments with the fixed polarizer angles were conducted with no optimization processes of the incident polarization. Despite non-optimized incident polarization, the highest Ip of the 80 kA level was non-inductively ramped up using the TL shown in Fig.1.

Erad(x, y, z) =1

4π

∫ ∫E0(xWG, yWG)

exp(−jkr)

r(jk +

1

r)z

rdxWGdyWG,

r =√

(x− xWG)2 + (y−WG)2 + z2,

1

FIG.1. Developed transmission line （TL） with a quasi-optical (QO) polarizer and launcher systems for local 2nd ECHCD, mainly composed of circular-corrugated 63.5 mm diameter waveguides of ~ 15 m length. A sapphire double-disk window is installed in the TL, together with an arc-sensor miter bend to look arcing lights at the polarizers and the window. The QO polarizer system with l/8 and l/4 polarizer plates is also installed, following a miter bend.

28 GHz Gyrotron

ArcSensor

QOPolarizer

Miterbend

Waveguide switchand Dummy load

Sapphire double-disk window

Two-mirrorLauncher

QUEST

(1)

FIG.2. Developed quasi-optical polarizer with l/4 and l/8 polarizer plates. (a): As a first trial, the plates were fixed to clear gaps between the stage and the chamber wall. (b): An actuator system with rotator rods are developed to change the incident polarization. Before rotating the plates, the rotator stages are pushed to keep the gap between the stage and the chamber wall, and then the stages are pulled to the wall to clear the gap after the polarizer settings.

QO PolarizerSystem

Coupling MirrorHE11óHE11

l /8 polarizer

l/4 polarizer

Ti02 CoatingAbsorber

(a) (b)

CircularCorrugatedWaveguide

Motor

Pumping

ActuatorCylinder

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The 2nd harmonic EC resonance (ECR) layer is located at a major radius of R 2nd res = 0.32 m where is close to the inboard vacuum-vessel boundary of R lim in = 0.22 m, whereas the fundamental ECR position R = 0.16 m is outside the boundary of R lim in [14]. A narrow beam should be considered for non-disturbed beam-path at the inboard boundary in the tangential injection. The required mirror radius for strong focusing was evaluated in a reverse propagation from the ECR layer to the focusing mirror position of R~1.5 m in Gaussian optics. In the reverse propagation process, a small waist size of w0 ~ 0.05 m was assumed at the ECR layer. The small-sized beam was so expanded along the propagation. A large-sized focusing mirror of a diameter D = 0.37 m was required for strong focusing to the narrow beam at the ECR layer, because the mirror should cover a 1% intensity edge of the beam. The first mirror was designed as a convex mirror to obtain the large-sized beam, and the 2nd mirror was a concave focusing-mirror [15]. In the mirror design, the complex propagating and reflected field-profiles were evaluated using the developed Kirchhoff integral in Eq.1.

2.2. Improvement and modification of transmission line

2.2.1. Development of configurable quasi-optical polarizer system

A configurable QO polarizer needs for optimization of incident polarization states. An actuator system with the rotator rods has been developed to change or to optimize the incident polarization as shown in the Fig. 2(b). Before rotating the polarizer plates, the rotator stages are pushed to keep the gap between the stage and the chamber wall, and then the stages are pulled to the wall to clear the gap after the polarizer settings. Detailed QO-polarizer design and its performance are reported in ref. [17].

2.2.2. Modification of transmission line route

A high voltage power supply (HVPS) of 75 kA / 25 A has been shared for operations of the 28 GHz gyrotron and 8.56 GHz klystron systems. Since simultaneous operations were impossible using one HVPS in principle, the launcher box was also shared. Figure 3 shows the launchers for the 8.56 GHz and 28 GHz systems. A focusing mirror for the 8.5 GHz launcher was rotated for 90 degree, and then was used as the 2nd focus mirror for the 28GHz launcher. The focusing mirror had different mirror surfaces at both sides of the mirror. In the 8.5 GHz system, irradiant TE10 modes from fundamental waveguides were focused to a relatively small-sized beam of w ~ 0.3 m for fundamental or 2nd harmonic EC experiments with the ECR layers at R = 0.55 m.

FIG.4. Modified route of the 28 GHz transmission line. The parts marked by red lines shows the 28 GHz system.

C

�

�

�

�� � � � � � � � � � � � �� � � � � � � �

� �

���

���� � ����

��� �

���

D

FIG.3. (a): 8.56 GHz and (b): 28 GHz launcher systems. A focusing mirror for the 8.5 GHz launcher was rotated for 90 degree, and then is used as a 2nd focus mirror of the 28GHz launcher. The launcher box is shared for both systems because simultaneous operations of the both systems were impossible using one shared high voltage power supply.

FocusingMirror

Fundamental Waveguides

(a)Circular Corrugated

WaveguideFirst ConvexMirror

SecondFocusingMirror

(b)

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A HVPS of 53 kV / 14 A of the 8.56 GHz system has been developed for the simultaneous injections using the both systems [19]. The 28 GHz TL route was modified to inject the 28 GHz wave from a prepared launcher box at the other port of the QUEST device. Figure 4 shows the modified 28 GHz TL route. The polarization states changed at the TL. It should be noted that the incident fields into a couple of miter-bends became to be not in parallel and perpendicular to the incident planes. The polarization states were measured inside the QUEST device at a low power level to confirm the polarization controllability by the QO polarizer system in the EC plasma ramp-up experiments. The N// and incident polarization scan experiments were conducted using the modified TL shown in Fig. 4 with the configurable QO polarizer.

2.3. Measurements of polarization states inside the QUEST

To obtain significant single-pass absorption effects in the ECHCD or EC plasma ramp-up experiments, the incident polarization has to be controlled properly. The polarization states can be expressed as a semi-major axis direction (azimuth angle) of a and a ratio of a semi-minor to the semi-major axis components (ellipticity) of b of the polarization ellipse in an Ex-Ey plane, as shown in Fig.5(a). The polarization parameters a and b can be determined by the amplitude ratio and the phase delay between transverse Ex and Ey components to the propagation. The two transverse electric-fields of Ex and Ey were measured inside the QUEST device. A HE11 mode generator, composed of a circular corrugated horn antenna and a lens, was set at the TL in Fig.4 before the QO polarizer system. The incident polarizations could be controlled by rotations of the l/8 and l/4 polarizer plate angles using the actuator system. A 3-dimensional stage was set in front of the two-mirror launcher system properly as shown in Fig.5 (b). A horizontal and vertical fundamental WR28 waveguides were used to detect the Ex and Ey components as receiving antennae. The operating 28 GHz-wave was generated by using a 14 GHz synthesizer- output and a frequency doubler with an amplifier, and then it was measured as an intermediate frequency (IF) at super-heterodyne detection. The IF amplitude ratio and phase difference between the reference and the propagating-wave sides were measured with a network analyser [17]. Figure 5 (c) and (d) show measured polarization parameters a and b as functions of rotated l /8 and l /4 polarizer angles. In the N// and polarization scan experiments, linearly polarized vertical- and horizontal-fields, [a, b ]~ [+/-p/2,0] and [0,0], were excited.

3. EXPERIMENTAL RESULTS

3.1 Fully non-inductive plasma current ramp-up

As a first trial of the QO polarizer system, the l/8 and l/4 polarizer-plate angles were fixed at the pre-setting to avoid the arcing events there. Therefore, there were no optimization process of the incident polarization at the fixed pre-setting. Figure 6 shows time evolution of the plasma current Ip, the loop voltage, poloidal coil currents IPF, the Ha intensity, absolute extreme ultra violet (AXUV) signal, electron density (ne) and temperature (Te), and hard X-ray (HX) count and energy, with the 230 kW 28GHz-wave of N// = 0.78 at the 2nd harmonic ECR layer. The 20 kW 8.2GHz-wave was also used for the pre-ionization. The plasma current reached 80 kA, following the poloidal field ramp-up with zero or negative loop voltages. The ne and Te were measured by Thomson scattering (TS) diagnostics at a major radius R = 0.34 m which is near R 2nd res = 0.32 m, averaged from 6-shot data with high repeatability. The obtained electron density was one order of magnitude higher, compared to the previous 66 kA experiments. The right-hand cut-off density nc2nd,X of the 2nd harmonic 28GHz-wave is expressed as,

FIG.5. (a): Polarization ellipse in the transverse Ex - Ey field plane. A semi-major axis direction (azimuth angle) of a and a ratio of a semi-minor to the semi-major axis components (ellipticity) of b are illustrated. (b): Low power test setting for measurements of polarization parameters a and b inside the QUEST device. Measured (c): a and (d):b polarization parameter map for rotated angles of l/8 and l/4 polarizer plates.

focusingmirror

detector

(a) (b) (c) (d)

k : wavenumber of operating frequency

n2nd,Xc for the 2nd harmonic X mode injection is expressed for

various N// setting as,

n2nd,Xc =

ϵ0me2 [1 − N2

// (R)]ω(ω − ωce(R)), (3)

where ϵ0, m and e are the primitively in vacuum, electron massand elementary charge, respectively. The operating and elec-tron cyclotron angular-frequencies are described as ω and ωcehere. The refractive indexes N// can be expressed as a functionon R so as to satisfy RN// = const.. The cyclotron frequency ωceis also a function of R through the 1/R dependence of the mag-netic field strength B. The cutoff density n2nd,X

c decreases withdecreasing R along the beam propagation from the launcher atthe low field side. The cut-off densities should be properly con-sidered for developing various experimental scenarios.

4. 2nd harmonic EC plasma ramp-up experiments

After installing the QO polarizer system to the TL system,the 250 kW 28GHz-wave could be transmitted to the QUESTsequentially. The 2nd EC plasma ramp-up experiments wereconducted with no optimization of of the incident polarizationas a fist trial. Figure 8 shows time evolution of Ip, the Hα inten-sity, absolute extreme ultra violet (AXUV) bolometer signal,and electron density ne and temperature Te by Thomson scat-tering diagnostics, with 230 kW 28GHz-wave of N// = 0.78 atthe 2nd ECR layer. The 40 kW 8.2 GHz-wave was used for thepre-ionization. The plasma current Ip was ramped-up follow-ing the vertical field ramp-up by the poloidal field coil control.The temperature Te and density ne were measured by Thomsonscattering diagnostics at R = 0.34 m where is close to R2nd

res =0.32 m. The cut-off density n2nd,X

c at R = 0.34 m was evaluatedas 2.4 × 1018 m−3 for a pencil straight ray with a settingN// of0.78 at the R2nd

res = 0.32 m. The relatively high-density plasmaclosed to the cut-off density was obtained, compared to the pre-vious experiment with no beam focusing [11]. The hard X-rays(HXs) have been measured at forward (FWD) tangential view-ings for current-carrying energetic electrons at the mid-plane. ACdZnTe detector measured HXs in the energy range up to sev-eral hundred keV, and the energy spectrum was obtained by thepulse-height analysis [17]. The HXs count with the averagingenergy of ∼ 60 keV increased, following the ramped current. Itis very likely that energetic electrons have a key role to generatethe plasma current.

5. Conclusion

New transmission line (TL) and launcher systems have beendeveloped to conduct local ECCD or EC plasma ramp-up ex-periments in the QUEST spherical tokamak. A quasi-opticalpolarizer system was proposed to avoid the arcing events there.After installing the QO polarizer system to the TL system, the250 kW 28GHz-wave could be transmitted to the QUEST se-quentially. The incident beam was focused to small beam sizeof ∼0.05 m with toroidal steering-capability from the perpen-dicular to tangential injections. The high plasma current ofthe 80 kA level was non-inductively ramped up in the QUEST

Figure 8: Time evolution of plasma current Ip, poloidal coil currents, Hα inten-sity, and electron density ne and temperature Te by Thomson scattering diag-nostics, with 230 kW 28GHz-wave of N//= 0.78 at the 2nd ECR layer.

using the system despite non-optimized incident polarization.Hard X-ray emission from abundant energetic electron with the60 keV level was observed along the plasma current ramp-up.

Single path absorption due to the energetic electron shouldbe evaluated by a ray tracing code. Higher harmonic absorptiondue to the energetic electrons will be evaluated in the ray tracinganalysis also. The N// and polarization scan experiments wouldbe conducted to confirm the single path absorption effect due tothe energetic electrons.

Acknowledgments

This work was performed under the auspices and support ofthe NIFS Collaboration Research Programs. This research waspartially supported by Grant-in-Aid for Scientic Research (B)(JSPS KAKENHI Grant Number JP1504231).[1] Hanada K., et al 2010 Plasma Fusion Res. 5 S1007.[2] Hanada K., et al 2017 Nucl. Fusion 57 126061.[3] Forest, C.B., et al 1992 Phys. Rev. Lett. 68 3559.[4] Maekawa T., et al., 2005 Nucl. Fusion 45 1439.[5] Ejiri A., et al 2009 Nucl. Fusion 49 065010.[6] Shevchenko V.F., et al 2010 Nucl. Fusion 50 022004.[7] Idei H., et al 2012 Plasma and Fusion Research, 7 2402112.[8] Tanaka, S., et al 1993 Nucl. Fusion 33 505.[9] Forest, C.B., et al 1994 Phys. Plasmas 1 1568.

[10] Uchida, M., et al 2011 Nucl. Fusion 51 063031.[11] Idei, H., et al 2017 Nucl. Fusion 57 126045.[12] Shevchenko, V.F., et al 2015 EPJ Web. of Conference 87 02007.[13] Tsujimura, T.I., et al 2017 Fusion Eng. and Design 114 97-101.[14] Fukuyama, M., et al, in submission to 2018 Fusion Eng. and Design.[15] Idei H., et al 2015 J. of Infrared, Millimeter, and Terahertz Waves, 36

662.[16] Kubo S., et al 1995 Fusion Eng. and Design, 26 (1995) 319-324.[17] Tashima S., et al 2014 Nucl. Fusion 54 023010.

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(2)

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where e 0, m and e are the primitively in vacuum, electron mass and elementary charge, respectively. The operating and electron cyclotron angular-frequencies are w and w ce here. The refractive indexes N// can be expressed as a function on R so as to satisfy RN// = const.. The cyclotron frequency wce is also a function of R through the 1/R dependence of the magnetic field strength. The cutoff density nc2nd, X

decreases with decreasing R along the beam propagation from the launcher at the low field side. The cutoff density was evaluated as 2.4 x 1018 m-3 at R = 0.34 m for this obliquely injected ray. The density reached to be close to nc2nd, X in the Ip ramp-up phase at the inboard side. The electron temperature decreased near nc2nd, X, then hard-X ray (HX) count with the averaging energy of ∼ 60 keV increased, following the ramped current. The HXs have been measured at forward tangential viewings for current-carrying energetic electrons at the mid-plane. A CdZnTe detector measured HXs in the energy range up to several hundred keV, and the energy spectrum was obtained by the pulse-height analysis [20]. It is very likely that energetic electrons have a key role to generate the plasma current Ip .

The plasma current suddenly dropped around t ~ 3.097 s when the AXUV signal and the Ha intensity sharply increased. Figure 7 shows the visible plasma images and the AXUV poloidal fan-arrayed signals before and after the Ip drop. Here the impact parameter r in Fig. 6(c) is the normal distance to each line-of-sight of the AXUV array channel from the mid-plane. The +/- large r expressed upper and lower peripheral regions in the figure, respectively. The plasma peripheral region became bright, also supported by the AXUV arrayed measurement. The mechanism of the sudden Ip drop will be studied as a future work.

A highest plasma current Ip of 86 kA was finally achieved with a focused 230 kW 28 GHz-beam even when the incident polarization was not optimized. The record Ip ramp-up efficiency on the incident power in the 2nd harmonic EC scenario was also achieved.

3.2 Incident N// and polarization scan experiments

The N// scan experiments have been conducted to study relativistic Doppler-shifted resonance effects for the incident beam experimentally. Linearly polarized vertical-field waves of the X-mode-like were excited in the N// scan experiments. The gyrotron operation was limited at 180 kW for 0.4 sec due to the tube condition at the time. Figure 8 (a) shows time evolution of the ramped-up Ip with N// = 0.75, 0.43 and -0.22. The experiments with N// = 0.43 and -0.22 were conducted after a shot with default N// setting of 0.75. At a shot just after changing N//, the Ha intensity was relatively large while the ramped Ip was significantly small (< 20 kA), indicating effects of outgassing from the CS surface gotten hit by the incident beam. The time evolution after a few shots in the N// scanning are plotted in the figure. The larger plasma current Ip were obtained at larger N// due to the relativistic Doppler-shifted resonance effect for the incident beam. It should be noted that the excited vertical field with no ellipticity was not optimized polarization in the oblique injection. Even larger Ip is expected when the optimized elliptical-polarization wave is injected at larger N//. The HX counts and its averaging energy increased, following the ramped Ip. Figure 8 (b) and (c) shows dependencies of the Ip peak on the measured HX counts and the average

FIG.6. Time evolution of plasma current Ip , loop voltage (Vloop), poloidal coil currents IPF, Ha intensity, absolute extreme ultra violet (AXUV) signal, electron density (ne) and temperature (Te), and hard X-ray (HX) count and energy, with the 230 kW 28GHz-wave of N// = 0.78 at R 2nd

res = 0.32 m.

FIG.7. Visible images of EC ramped-up plasmas, (a): before and（b）：after the plasma current ( Ip ) drop around t ~ 3.097 s in Fig.6. (c): absolute extreme ultra violet (AXUV) poloidal fan-arrayed signals before and after the Ip drop.

8.2GHz 28 GHz

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energy, respectively. The shots with the significantly small Ip after changing N// are omitted in the plots. The larger HX counts and averaging energy were also attained at larger N//. The coupling of the incident beam to highly energetic electrons carrying Ip , due to the relativistic Doppler-shifted resonance, was suggested experimentally.

As a polarization scan, linearly polarized horizontal-field of the O-mode-like wave was excited in the Ip ramp-up experiments. The plasma currents Ip s were not started and ramped up in most of shots, but rarely they were ramped-up to a 30 kA level with the horizontal-field wave. The EC plasma ramp-up was definitely difficult with the O-mode-like wave in comparison to that with the X-mode-like waves, suggesting the effective single-pass absorption effect in the local 2nd EC ramped-up scenario.

4. DISCUSSION

The single-pass absorptions were evaluated using pencil ray-tracing analyses by the TASK-WR code [21]. The electron temperature and density profiles, Te (r) and ne (r), were assumed to be Te (r) = Te0 [ 1-(r/a) ]8 and ne (r) = ne0 [ 1-(r/a) ]2 with central temperature of Te0 = 30 eV and central density of ne0 = 2 x 1018 m-3 along the TS diagnostics measurements. The highly energetic (60 keV) electron density of the 3 % component for the bulk electron was considered here. Figure 9 (a) and (b) show evaluated ray trajectories and absorption along the ray propagation in the pure X-mode and O-mode injections with N// = 0.78 at R 2nd res, respectively. The pure elliptically polarized X- and O-mode waves were excited with finite ellipticity or b. As expected, the incident X-mode wave was significantly absorbed in the single-pass, compared to the O-mode injection. The ray started to be absorbed around fourth (4th) harmonic ECR layer of R 4th res = 0.64 m, where is close to the magnetic axis, due to the relativistic Doppler-shifted effect. The third (3rd) ECR layer is located at R 3rd res = 0.48 m. Since the density was rather near nc2nd,X at the inboard side, Ip the X-mode ray was refracted well, and then it wrapped around the CS post after passing R 2nd res, causing the effective single-pass absorption. The X-mode single-pass absorption rate was evaluated to be about 0.4 at N// = 0.78 as shown in the figure. The single-pass absorption was also evaluated with no energetic electron component, but it was quite small in comparison to that with the energetic electrons. The single-pass absorption property by the energetic electrons due to the relativistic Doppler-shifted resonance effect can explain the ramped Ip dependencies on the incident N//.

If the incident wave has the ellipticity or b , an inversion process of b should be considered at reflection of the CS post. Even if the pure O-mode wave with finite b is excited at the launcher for the oblique injection, the O-mode

FIG.8. (a): Time evolution of ramped-up plasma currents Ip s with parallel refractive indii of N// = 0.75, 0.43 and -0.22. (b): Dependence of ramped plasma current Ip on (b): measured hard-X ray (HX) counts and (c): on measured HX energy.

FIG.9. (a): Evaluated ray trajectories and (b): absorption along the ray propagations in the pure X-mode and O-mode injections with N// = 0.78 at R 2nd

res.

X-modeO-mode

2nd ECR3rd ECR4th ECR

2nd 3rd 4th ECR

X-mode

O-mode

R [m]

(a) (b)

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wave is partially converted to the X-mode wave at the reflection. The high field side (HFS) injection scenario with the different mode should be considered in this case. In the N// and polarization scan experiments, the linearly polarized vertical- and horizontal-waves with no ellipticity were excited. The rarely ramped Ip with the O-mode-like wave seemed to come from the multiple-wall reflection effect, not from the HFS injection effect.

5. SUMMARY

New transmission line (TL) and launcher systems have been developed to conduct local 2nd ECCD or EC plasma ramp-up experiments in the QUEST. A quasi-optical polarizer system was proposed to avoid the arcing events there. After installing the QO polarizer system to the TL system, the 250 kW 28GHz-wave could be transmitted to the QUEST sequentially. The incident beam was focused to small beam size of ~0.05 m with toroidal steering-capability from the perpendicular to tangential injections. A recode plasma current Ip of 86 kA was achieved with a focused 230 kW 28 GHz-beam despite non-optimized incident polarization with fixed polarizer setting. Hard X-ray emission from abundant energetic electron with the 60 keV level was observed along the plasma current Ip ramp-up.

The N// and polarization scan experiments have been conducted to study relativistic Doppler-shifted resonance effects for the incident beam experimentally. The larger Ip were obtained at larger N//. The larger HX counts and averaging energy were also observed at larger N//. The coupling of the incident beam to highly energetic electrons carrying the Ip , due to the relativistic Doppler-shifted resonance, was suggested experimentally. The single-pass absorptions in the 2nd EC plasma ramp-up experiments were evaluated using pencil ray-tracing analyses by the TASK-WR code. The evaluated large single-pass absorption by the energetic electrons due to the relativistic Doppler-shifted resonance effect was one of major candidates to explain the ramped Ip dependencies on the incident N// and polarization.

ACKNOWLEDGEMENTS

This work was performed under the auspices and support of the NIFS Collaboration Research Programs under Grants NIFS09KUTR046, NIFS10KUTR048, NIFS11KUTR059, and NIFS11KUTR069. This research was partially supported by Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Number JP1504231).

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