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C-Mod

C-Mod Advanced Tokamak ProgramProgram Advisory Committee Review

February 20, 2003

MIT PSFC

Presented by A. HubbardMIT Plasma Science and Fusion Center,

for the C-Mod team

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C-ModC-Mod Advanced Tokamak Program

• Introduction– What do we mean by “Advanced Tokamak”?– Why do we want it?– Why on C-Mod?– What tools will we use?

• Overview of Schedule and Goals.• Five-Year Plans and Recent Progress

– By topic, will focus on results since last PAC meeting.

• Summary of Goals and Program

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C-Mod“Advanced Tokamak”

• Attractiveness of the tokamak as an ultimate fusion reactor increases if it is steady state and produces power at lower cost.

• Steady state implies non-inductive current drive.

• Lower cost implies much of the current drive is self-generated, ie. High bootstrap fraction.(external drive is expensive, in $ and Watts!).

• Also want high confinement, and high ββββ for lower cost (all are inter-related).

• Research aimed at demonstrating these features and optimizing the tokamak configuration is a key near-term component of the US fusion development plan. Will they be achievable in CTF and/or DEMO?

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C-ModUnique Features of C-Mod

• AT scenarios look very attractive in design studies (eg. ARIES-RS,AT) and there are active experimental programs on several tokamaks (eg. DIIID, JT60U, AUG, JET, others).What will C-Mod contribute that is new?

• In physics terms, “Steady-state” current drive implies pulse lengths >> current relaxation time τCR. C-Mod can run 5 second pulses, τCR~ 0.2-1.4 s .

[τCR = 1.4 a2κTe3/2 /Zeff; Zeff=1.5; Te= 2-7.5 keV ]

• Most AT expts have Ti > Te, τe-i > τE and use NBI for core fuelling and rotation drive in ITBs. Reactor scenarios have τe-i << τE (Te >Ti), no core fuelling, RF heating and CD.C-Mod can test feasibility of AT scenarios with all these features simultaneously.

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C-ModProfile Control Tools

“The crucial distinguishing feature of an Advanced Tokamak over a conventional tokamak is …the use of active control of the

current or shear profile, and of the pressure profile or transport characteristics” (AT Workshop, GA, 1999)

Tools available or under development:• Current profile:

– Lower Hybrid Current Drive. (Phase I 2003. Phase II 2005).4 MW, 4.6 GHz, 2 launchers with independent phasing, N//.

– Mode Conversion Current Drive. (on-axis, tests 2003)– Bootstrap current drive via pressure profile control.

• Density profile.– Control of core transport, peaking.– Cryopump controls edge source. (2005)– D2 and Lithium pellet injectors.

• Temperature Profiles– 8 MW ICRH, 40-80 MHz, 2 independently variable deposition locations.– 4 MW LHCD.– Control of core transport via RF deposition, magnetic shear.

• Shear Flow - MC flow drive.

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C-ModC-Mod complements other AT programs

• Each program has different tools, features, emphasizes areas appropriate to these (table does not list all research!)

Long pulse,ITBs, L-mode edge. Circular plasmas.

tailored Ip ramp, LHCD (low n)

Tore-Supra

Internal transport barriersD-T operation. Current ‘holes’.

tailored Ip ramp, NBI, LHCD (low n)

JET

Hybrid H-mode/AT scenarios. High βN, NTMs

tailored Ip ramp, NBI, ECCDAUG

Full non-inductive CD, high βp,double-barrier regimes. Current ‘holes’

tailored Ip ramp, NBI, LHCD (low n), ECCD

JT60-U

High ββββN, active RWM stabilization, NTMs, ITBs.

tailored Ip ramp, ECCD, NBI, FWCD

DIIID-D

Steady state. High pressure.Double-barrier regimes; all RF driven, ττττe-i << ττττE. Transp. control.

LHCD (high n), MCCDC-Mod AT program emphasisj(r) control toolsTokamak

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C-Mod

Overview of 5-year schedule for Advanced Tokamak Program

Test MCCD

ACCOME, TRANSP + CQL3D

ITBs with ICRH

GS2 TRANSP add particle transp help develop, include first principles χ models

ITB control w flow drive?

2nd Active MHD

Core MHD instabilities, NTM stab.

LH target opt (L, H, ITB).

2003 2004 2005 2006 2007 2008

MSE, CXRS, HIREX up.

Strike pt sweep

LH coupling, eff

LHCD Phase I: 1 coupler, 3 MW

2003 2004 2005 2006 2007 2008

Current Profile

Control

Power handling

Density Control

MHD Stability

Core Flow, Transport

and Profile Control

LHCD Phase II: 2 couplers, 4 MW. Pulse to 5 secs

LH j(r) mod 2 N//s, increase CD

Bootstrap via ITBs

Full wave MCIBW + LHCD

increase non-inductive CD

Test MC flow drive

Influence of j(R) on ITBs ITBs with reversed shear

β to no-wall limit Optimize p(r), j(r), βN to 3.

Active stabilization? Active MHD

PEST MARS KINX Assess, design stabilization techniques

Cryopump

Active ne pumping ITB control long pulse density control

Outer divertor Advanced toroidal divertor Upper divertor

Dissipative divertor tests 3 sec pulses 5 sec pulses

Diagnostics New DNB

Hard x-ray camera Non-thermal ECE?

Polarimetry

New Hardware Research Topic Theory/modelling

Calendar Yr

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C-Mod

Main goals of the AT physics program

1. Demonstrate and model current profile control using LH and ICRF waves, at high densities (>1020 m-3).

2. Understanding, control and sustainment of Internal Transport Barriers, with coupled ions and electrons, τe-i << τE (Te~Ti ) and without momentum input (RF only). Global confinement H89P > 2.5

3. Achieve full non-inductive current drive (70% bootstrap) and extend pulse length to near steady state (5 sec, 4-6 ττττCR) - divertor power handling and wall particle issues.

4. Attain and optimize no-wall ββββ to MHD limit (ββββN > 3). Explore means of achieving higher values.

Program involves all physics areas (RF, transport, divertor, MHD)and has broad participation from the C-Mod team. More

details in each of topical science presentations.

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C-ModExample of an AT target scenario

• One of many optimized scenarios modelled with ACCOME.– ILH=240 kA– IBS=600 kA (70%)

J (M

A /

m2 )

r / a

Ip = 0.86 MA Ilh = 0.24 MA fbs = 0.7

r / a

Saf

ety

Fac

tor

- q(

r)

q(0) = 5.08

qmin = 3.30

q(95) = 5.98

• Double transport barrier • BT=4 T• ICRH: 5 MW• LHCD: 3 MW, N//0=3• ne(0)= 1.8e20 m-3

• Te(0)=6.5 keV (H=2.5)• Scenarios without barrier,

or only an ITB, have similar performance.

P. Bonoli, Nucl. Fus. 20(6) 2000.

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C-Mod

Current Profile Control:Lower Hybrid Current Drive system

• Designed for well controlled spectrum.

• Each antenna will have flexible N// , variable over range 2-4.• Variable between or during discharges using phase shifters.• 2 launchers can have different spectra.

Phase I Phase II

Frequency 4.6 GHz 4.6 GHz

Power 3 MW 4 MW

Antenna 4X24 WaveguideGrill (1)

4X24 WaveguideGrills (2)

N(Variable)

2-4 2-4

2003 2005

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C-Mod

• RF sources, power supplies, WG prepared by MIT.- 12 Klystrons (3 MW) are installed in the C-Mod cell.

LH Phase I nearing completion

• LH Coupler and splitter fabricated by PPPL- Grill modules tested. - Components all in-house.- Final assembly in progress.

• Delivery to MIT on schedule for late March 2003

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C-Mod2002: LHCD Preparatory Expts

• Ongoing experiment/modellingscenario optimization

Experimentaltarget profiles

Modellingw RF, LHCD

Sensitivity studiesHow should target

be improved?

New experiments,Optimized parameters

• 2002 experiments focused on producing suitable LH target plasmas for early operation:– Current ramps with early ICRH– Lower density EDA H-modes.

600 kA

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C-Mod2003: Begin LHCD Phase I

• Spring/summer 2003: Install Launcher, splitter, high P tests, controls.

• Fall 2003 Begin LH Experiments.– Assess power handling.

What is the limit for short, long pulses?– Focus on LH coupling, wave physics studies.– Measure coupling efficiency, reflectivity vs edge density,

launcher and limiter position.– Begin measurements of current drive profile and

efficiency.– Key diagnostics:

• MSE for j(r) (commissioned in 2002).• Hard X-ray camera – 2003

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C-Mod2004-05: j(r) control via LHCD

• Measure LHCD and heating efficiency and deposition profile vs density, N//. Both on and off-axis CD.

• Combine LHCD and ICRH.Raising Te will increase LH efficiency.

• Explore L-mode, H-mode and barrier regimes.

• Goal is 50% non-inductive with phase I LH.

Eg. ACCOME modelling of initial experiments with L-mode targets:

Te0 = 5 keV, ne0 = 1.5 x 1020 m-3 1.8 MW ICRH.PLH=2 MW, N//=2.75Ip = 690 kA

Predicts:ILH = 250 kA. (35%), at r/a~0.6.

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C-Mod2005-6: LHCD Phase II.

• Use flexibility of two launchers to create spectrum with two N//peaks.– Modelling and other

experiments (egASDEX) show that a high N//component increases off-axis absorption and localization.

0

2

4

6

8

10

0 5 10 15 20 25

power flux in the waveguides

f2b (GHz2 cm)

kW/cm2

breakdown weak

conditioning

waveguidedimensions

6.0x0.55 cm2

2.3 MW net power (12 Klystrons - 24 kW/guide)

1.5 MW net power (8 Klystrons - 15.5 kW/guide)

JET 2 secJET, TS long pulseJET, TS long pulsePBX-M 0.5 sec

�

• 2nd antenna allows 4 MW source,

• 3 MW coupled for modest power density, 5 sec pulse.

• Add new 4-strap ICRF antenna.

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C-ModCurrent profile control via MCCD

• Fast waves mode convert to IBW and/or ICW.

• MCCD demonstrated on TFTR. • 2002: Improved TORIC

modelling (Sci-Dac), good agreement with PCI, local heating.

• Predicted MCCD exceeds ohmicj(r) on-axis.– Good complement to LHCD,

may provide ‘seed current’.– Off-axis CD also possible

(lower efficiency).• 2003: Initial tests of MC with

current drive phasing.• 2004: If successful, combine

with LHCD. Synergism??

TORIC - Adjoint Code Prediction for On-axis MCCD in C-Mod(Nm=63, Nr=960) D/3He/H, 5.5 T, 50 MHz)

Ip=800 kA IRF = 70 kAPICRF = 3 MWne(o) = 2 × 1020 m-3

Te(o) = 3.5 keV

0.0 0.2 0.4 0.6 0.8 1.0r/a

0

10

20

30

40

J_icrf

J_icrf (No Trap)

J (M

A / m

2 / M

Win

c)

P. Bonoli, J. WrightAPS 2002

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C-Mod

Control of core transport, profiles(Internal Transport Barriers)

• ITB’s routinely triggered by off-axis ICRH, at r/a ~0.5.

• Core barriers co-exist with edge pedestal (EDA H-mode.)

• Also seen in ohmic H-mode.• Reversed shear not needed.

0.70 0.75 0.80 0.85 0.90Major radius (m)

ITB

0

2

4

6

8

n e (

10 2

0 m

-3) ETB

Stable conditions were reached for ~15 ττττE, through addition of modest on-axis ICRH.We can control the degree of transport within the barrier!

S. Wukitch, APS 2001, PoP 2002. t (sec)

keV

MW

kJ

10 20 m-3

mW / cm2 / sr

104 m / s

on-axis

off-axis

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C-ModSome of new ITB results in 2002

• Expanded range of scenarios to higher current, field.

• Barrier Location depends on B, not on heating location

• Still sharp B threshold (<0.1 T) for all scenarios.

Fiore and Wolfe, APS 2002

• Central ICRH into ohmic H-modes gave fine control of density peaking.

0.65 0.70 0.75 0.80 0.85 0.90 Major Radius (m)

0.0 MW0.2 MW

0.3 MW0.4 MW

0.6 MW

2

8

0

4

6

nxsqrt(Z

)x10

/me

eff

20

3

0.70 0.75 0.80 0.85R (m)

0

2

4

6

n e (10

20/m

3 )

0.2 0.4 0.6 0.8r/a

3.9 T

4.5 T5.5 T

6.3 T

70 MHz

80 MHz

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C-Mod

• Localized energy transport barrier is seen clearly in sawtooth heat pulse propagation, with new high res. X-ray (0.3 cm) and ECE

• Break-in-slope seen in Te(r)when on-axis ICRF is applied.

• Prior TRANSP analysis showeddecrease in core χ.

Clear energy transport barrier

3

2

1

2

1

0

Te [keV]

ne [x1020 m-3]

1021024018

GPC

radiometer

tp xrtp ece

t p [

mse

c]

Rmajor [m]

1010801027

BT=5.4 TIp = 0.8 MA

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C-Mod

Emerging understanding of ITB mechanism from GS2 simulations

• Formation starts with decrease in ITG mode (note low ηe , R/LT).– At transition time, ExB shear

does not appear dominant.• Ware pinch peaks ne, pe.

– ne gradient then further stabilizes ITG (positive feedback), but can drive weak TEM in barrier.

• When on-axis heating is applied, TEM increases (lower ν*).– Nonlinear simulations show

enough transport to balance Ware pinch, arrest peaking.

– Too much heating erodes the barrier.

• Preliminary Picture; need many tests in models, experiments!

M. Redi, TTF 2002, EPS 2002C. Fiore, PoP 2001, TTF 2002

TEM

ITG

ITB 4.5T 1001220016 at 1.34 sec

steep density gradient

drives TEM

Run with adiabatic e-

DR

E12060102

Run withT=0

∆

Run without collisions: TEM growth rate increases 3x

0.0 0.2 0.4 0.6 0.8 1.0

Max. Growth RateReal Frequency/10

During on-axisICRH

[Mra

d/s]

-0.1

0.0

0.1

0.2

0.3

D. Ernst, Sherwood 2002, APS 2002

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C-Mod

Transport and Pressure Profile Control

2003 (planned expts)• Improve understanding of ITBs

in ohmic plasmas and with off-axis ICRH.Threshold condition? Hysteresis? Role of rotation? Detailed profiles, time behaviour of χ, D .R/LT , η variation, BT ramps, heat and impurity pulses.

• Barrier location control.Want to expand for more attractiveAT scenario - lower B, f

• Improving performance. Maximize energy confinement, bootstrap current. Does regime extend to higher T, lower ν*?– Higher power, Ip rampdown.

Time-dependent TRANSP model of proposed expt:

Ip rampdown increases IBS.

V. Tang, R. Parker, APS 2002

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C-Mod

Transport and Pressure Profile Control

2003-7• Investigate influence of magnetic shear on ITB formation, location and

transport profiles.– Use LHCD and MCCD to control j(r).– Produce ITBs with reversed shear, central ICRH.

• Study effect of flow drive on barriers.– Depends on MCIBW flow drive tests.– Can it be an active barrier control tool?

• Adjust heating profile to modify Te(R)– Two ICRF frequencies.– LH Heating– Also need to modify transport; C-mod profiles are “stiff” without barriers.

• Optimize density, temperature, bootstrap profiles for compatibility with LHCD, maximum non-inductive CD scenario.– Goal is 50% non-inductive in 2004 (Phase I LHCD)– 100% non-inductive by 2007 (Phase II LHCD)

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C-Mod

TRANSP simulation of full non-inductive CD with ITB, L-mode edge.

• Input ne(ρ) with barrier at r/a=0.5.• Input χ(ρ) taken from analysis of C-Mod ITB experiments.• Te, current profiles evolve in time.

PICRF=3 MW

PLH=1.65 MW

N//=2.75

J. Liptac, APS 2002

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C-Mod

0.0 0.2 0.4 0.6 0.8r/a

-1

0

1

2

3

4

VT

or (

104 m

/s)

L-mode

H-mode

ITB

Flow Profile Control

• We observe large toroidal rotation, (up to 120 km/s), without momentum input (ohmicor RF).

• Magnitude and profile vary with confinement regime. (better radial coverage with upgraded HIREX)

• Shear flow is known to affect transport, barriers; an active RF control tool is of great interest to all experiments.

2003-4• Improving Vφ, Vθ diagnostics (X-ray and

CXRS).• Will look for evidence of localized

poloidal flow drive by mode converted IC waves.

• If flow drive proves significant, will later test influence on transport, ITBs.

Toroidal rotation profiles

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C-Mod

• Plans for cryopump:– Experiments in 2002 varied Ssep.

High upper neutral pressure (5-10 mTorr) when close to DN.

– Results used to for design ofupper cryopump.

– Plan to use for active density control in 2005.

• Transport control will be the best tool for density peaking.– Also have Li, D pellet injectors.

• Will assess impurity accumulation, wall saturation effects for long pulses.

Density Profile Control.

Density is critical for LHCD accessibility, efficiency and deposition profile!

E-Top

E-Bottom

-20 -10

SSEP (mm)

0.1

1

10

0.1

1

10

(mtorr)

1.01.41.61.82.0

NL04(1020 m-2)

(mtorr)

Neutral Pressures vs. SSEP

100

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C-ModPower Handling in Long Pulse AT

• In 2001, demonstrated 3 second pulses, but with PRF ~ 1 MW.• Divertor/SOL power handing will be a major challenge as power, pulse

length increase: Parallel flux is already up to 0.5 GW/m2 with 3-4 MW ICRH !

• Requirement for fairly low edge ne (1-2x1020 m-3) for LHCD makes radiativedivertor difficult.

• C-Mod experience will be highly relevant to AT scenarios on ITER.

• 2003-5:– 6 MW ICRH + 2 MW LH (coupled)– ~3 second pulses.– Add IR cameras to monitor LH antenna, hot spots– Try strike point sweep.

• 2005-7:– 6 MW ICRH+3 MW LH – 5 second pulses.– Upgrade outer divertor, plus other areas as required.

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C-ModMHD Stability of non-inductive plasmas

• Expect core MHD stability to be more important for C-Mod as power, β raised.

• Ideal no-wall limit βn~3.– With optimized p(r), j(r).– Strong shaping.

• New antennas for active core MHD spectroscopy can measure linear growth rates.– Plan to feedback on power,

profiles to avoid limit.• Study ELM, core MHD interaction.• Try stabilization of NTMs using

LHCD and/or MCCD.

ACCOME

• Plan to carry out a design/feasibility study of active stabilization methods to allow β > no-wall limit

• May install such a system ~ 2007.

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C-ModScenario Modelling.

• Modelling is critical to assess wave accessibility, damping, and CD efficiency, and guide target plasma development toward more optimal scenarios.

• Exploring several different regimes, in close connection with expts: – Rampup, L-mode, H-mode and Double-barrier.

• Currently available models include:– ACCOME: LH, ICRH and bootstrap Ip. Consistent MHD equilibria.

No transport; temperature, density specified.– CQL3D (R. Harvey): Self-consistent 2-D velocity space Fokker-Planck.

(20-30% higher LHCD than ACCOME).– TRANSP: Time dependent, predictive or simulation mode.

• Input either T profiles or transport coefficients.• Initially using χs obtained from analysis of C-Mod ITBs, simplified criteria for

barrier formation.• Limited particle transport model. • LSC for LHCD. Limitations in reverse N//, 1 poloidal ray.

– TORIC: Full wave field solver for ICRH, mode conversion.

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C-ModPlans for AT-Related Modelling

RF near term, longer term• Sensitivity studies of LHCD current profile using ACCOME (vary ne,

B, Te, N//) (Collaboration with R. Dumont, PPPL)

• 2-D (Vperp, V//) Fokker Planck simulations of LHCD; Couple CQL3D to ACCOME (R.W. Harvey – CompX, P. Bonoli)

• LHCD efficiency, distribution simulations, X-ray diagnostic design (Y. Peysson, Cadarache, A. Bers, J. Decker, MIT).

• Full Wave simulations of LHCD (1-D) and IBW (3-D) (R. Dumont, and C.K. Phillips, PPPL, P. Bonoli, J. Wright, MIT).

• Full Wave LH simulations in 2-D (R. Dumont and C.K. Phillips, PPPL, P. Bonoli, J. Wright, MIT;part of SciDac effort)

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C-ModAT-related Modelling (con’t)

TRANSPORT• Coupled current drive and transport modelling using TRANSP in

predictive mode: extend range of scenarios. (J. Liptac, P. Bonoli).

• Continue gyrokinetic analysis (GS2) of ITB discharges. (M. Redi, PPPL, D. Ernst)

• Couple LHCD model from ACCOME to TRANSP. (MIT, PPPL)• Use evolving capabilities of TRANSP for more theory-based

predictive modelling. Eg. assessing ωExB vs γITG.• Develop and incorporate improved particle transport modelling

(critical for ITB simulations).

MHD• Low n and ballooning stability analysis of modeled scenarios with

PEST-2, Keldysh code and MARS (J. Ramos, MIT PSFC)

.

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C-ModIntegrated Advanced Tokamak Goals

• A successful AT demonstration must combine all of the control tools and physics/technology areas discussed.– Eg. LHCD and high bootstrap and high β and long-pulse divertor.– Integration and parameter optimization will be an important part of

the program from the beginning. For example, tradeoffs necessary in Ip, density. Not possible to separate the various parts of the program (eg current, transport, density control).

– With so many tools, regimes to explore and exploit, increased run-weeks will be essential.

• Five-year goal is:– fully non-inductive current drive of 0.85 MA, from LHCD plus

bootstrap current, – ββββN=3.0 (or higher), for– 5 second pulse length (~6 τCR at 5 keV).– Core transport barrier with H89P > 2.5

• Intermediate objectives established to guide program, assess progress in both performance and scientific understanding (IPPA goals).

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C-ModGoal 1: Long Pulse Current Drive

Demonstrate, and develop predictive models for, full non-inductive CD using LH and ICRF waves, in a high n regime(> 1020 m-3) for pulse lengths >> tCR.

• Produce some optimized targets for LH studies 2003.• Commission LHCD Phase I, measure coupling, power handling 2003.• Measure profile of fast electrons with hard X-ray camera 2004.• Measure LHCD efficiency, localization in L, H, and ITB regimes

and determine effective upper density limit. 2005.• Demonstrate 50% CD, possibly at reduced ne, for 3 sec pulses. 2005.• Compare to ACCOME, CQL3D, and full wave models. 2004-06.• Commission LHCD Phase II, assess CD localization with multiple

N// spectra. 2006.• Maximize and document LH driven current vs target ne(r). 2006• Combine LHCD with bootstrap current from ITBs, to maximize

total non-inductive CD. 2006-08.• Extend pulse length to 5 seconds. 2007.

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C-ModGoal 2: Core Transport Barriers

Produce, understand and control core transport barriers in LH and ICRF driven regimes with strongly coupled electrons and ions.

• Measure core χ, D with off and on-axis ICRH, over a range of Ip, BT, and ne. 2003.

• Conduct gyrokinetic simulations of several cases, compare withexperiment. 2004.

• Assess influence of reduced or reversed shear (produced by LH) on ITB formation, location. 2005.

• Test whether barriers form spontaneously with reversed shear, with on-axis ICRH. Compare χe, χi and D and assess impurity accumulation.

2005.• Optimize profiles and transport for steady state, with maximum

off-axis bootstrap current (target 70%). 2004-06.

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C-ModGoal 3: Increase Beta

Attain and optimize no-wall ββββ limits, with ββββn of at least 3, and explore means of achieving higher values..

• Use ICRH power to increase β in inductive discharges, and measure MHD growth rates using active MHD antennas. 2003-04.

• Compare observed growth rates with stability codes. 2003.• Explore MHD properties of reversed shear discharges 2005.• Use control of current and pressure profiles to optimize no-wall

beta limits, guided by modelling. 2006-07.• Conduct feasibility and design studies of conformal wall and

active MHD stabilization coils. 2004-05.• If studies show feasibility and if warranted by demonstrated high β,

fabricate and install MHD control hardware. 2006-08.Full exploitation would likely extend to next five-year period.

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C-ModSummary

• Advanced Tokamak thrust will be an increasingly important part of the C-Mod program.

• Focusses on RF control of current, transport and pressure profiles in high density regime, for t >> ττττCR, to make uniquecontributions to the world AT program, and to the US fusion development program in configuration optimization.

• We have succeeded in modifying core transport without momentum input or reversed shear.

• LHCD is nearly complete - Phase I on schedule for March 2003 delivery, 2003 commissioning.

• Long term program leads progressively to a non-inductive, steady state, high confinement advanced tokamakdemonstration in a unique regime highly relevant to ITER and the steps beyond (CTF, DEMO).– All RF drive, BT = 4-8 T, Ti~Te, ne~1-5 x 1020 m-3.– Plenty of work, and exciting physics, for the next 7+ years!