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Plasma Start-up In NSTX Using Transient CHI

R. Raman, T.R. Jarboe1, D. Mueller2, B.A. Nelson1, M.G. Bell2, M. Ono2, T. Bigelow3, R. Kaita2, B. Leblanc2, R. Maqueda4,

J. Menard2, S. Paul2, L. Roquemore2 and the NSTX Research Team

1University of Washington, Seattle, USA2Princeton Plasma Physics Laboratory, USA

3Oak Ridge National Laboratory, Oak Ridge, TN, USA4Nova Photonics, USA

12th International ST Workshop11-13 October 2006

Chengdu, China

Supported byOffice ofScience

College W&MColorado Sch MinesColumbia UComp-XGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU MarylandU RochesterU WashingtonU Wisconsin

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAERIHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech Rep*Work supported by US DOE contracts DE-FG03-9ER54519 and DE-AC02-76CH03073.

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Outline

• Motivation for solenoid-free plasma startup

• Implementation of Coaxial Helicity Injection (CHI) in NSTX

• Requirements for Transient CHI

• Experimental results from NSTX– Brief summary of HIT-II results

• Summary and Conclusions

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Solenoid-free plasma startup is essential for the

viability of the Spherical Tokamak (ST) concept

• Elimination of the central solenoid simplifies the engineering design of tokamaks (Re: ARIES AT & RS)

• CHI is capable of both plasma start-up and edge current in a pre-established diverted discharge

- Edge current profile for high beta discharges

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Implementation of CHI in NSTX

Transient CHI: Expect axisymmetric reconnection at the injector to result in formation of closed flux surfaces

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Requirements for optimizing Transient CHI

• Bubble burst current*

• Volt-seconds to replace the toroidal flux– For 600 mWb,

at ~500V need ~1 ms just for current ramp-up

• Energy for peak toroidal current

• Energy for ionization of injected gas and

heating to 20eV (~50eV/D)– For 2 Torr.L injected, need ~2kJ

2 /inj toroidal

* T.R. Jarboe,"Formation and steady-state sustainment of a tokamak by coaxial helicity injection," Fusion Technology 15, 7 (1989).

toroidal

1 12 22 2CV LI

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Capacitor bank used in Transient CHI Experiments

• 50 mF (10 caps), 2 kV• Operated reliably at up to

1.75kV• Produced reliable breakdown

at ~ 1/10th the previous gas pressure (20 Torr.Liter used in 2004)

– Constant voltage application allowed more precise synchronization with gas injection

– EC-Pi and gas injection below divertor used for Pre-ionization assist

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Improved pre-ionization to a level that results in

injected gas 10 times less than in 2004

• Novel pre-ionization system

– Injects gas and 10-20kW of 18GHz ECH in a cavity below the lower divertor gap

– Successfully tested, achieved discharge generation at injected gas amount of < 2 Torr.Liter

• Fast Crowbar system

– Rapidly reduces the injector current after the CHI discharge has elongated into the vessel.

The small glow shown by the arrow is in the gap between the lower divertor plates and it is produced solely by EC-Preionization of the gas injected below the lower divertor plates. No voltage is applied.

Shot 116565

Shot 116570

EC-Pi glow along the center stack

Divertorgap

ECH: T. Bigelow (ORNL)

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Closed flux current

generation by Transient CHI

• Plasma current amplified many times over the injected current.

• The sequence of camera images shows a fish eye image of the interior of the NSTX vacuum vessel. The central column is the center stack, which contains the conventional induction solenoid. The lower bright region seen at 6ms is the injector region.

6 ms 8 ms 10 ms

12 ms 15 ms 17 ms

Hiroshima University (N. Nishino) Camera Images: R. Kaita (PPPL)

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Discharges without an absorber arc show high current multiplication ratios (Ip / Iinj) of 60

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Dramatic improvement in closed flux current generation from 2005

LRDFIT (J. Menard)

2006 discharges operated at higher capacitor bank voltage and higher toroidal field

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Electron temperature and density profiles become less hollow with time

120814: Black: 8ms, Red: 12ms

120842: Black: 8ms, Red: 10ms

Thomson (B. LeBlanc)

Profile becomes less hollow with time Plasma and Injector current

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• Thomson scattering data indicates Te drops to 50% in 3-5ms TauE ~ 4ms

• Zero-D estimates indicate 200kW ECH would increase Te ~ 60eV in 8ms and 100eV in 20ms, assuming TauE does not increase.

• Consistent with Radiated power levels of <100kW

• Consistent with low electron densities of ~2x1018m-3, for impurity burn through. Li a possibility for controlling Oxygen.

Data indicates that ~200kW of ECH would increase Te to ~100eV

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Some discharges persist for as long as the equilibrium coil currents are maintained

Fast camera: R. Maqueda

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Movie of a high current discharge

Fast Camera:R. Maqueda &L. Roquemore

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Favorable scaling with machine size

Attainable current multiplication is given as ,

For similar values of BT,

So current multiplication in NSTX should be 10x HIT-II, which is observed

Next step STs would have about 10x the toroidal flux in NSTX,

Which means current multiplication ratios in excess of 100 is not unrealistic in larger STs

Potential for high current multiplication in larger STs

( / )P inj T injI I

~ 10NSTX HIT IIT T

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Allowable injector currents determined by maximum voltage

Assuming constant ,

For similar values of , at the same voltage,

in HIT-II is about 10 times higher than in NSTX

Consistent with ~15-20kA on HIT-II vs ~2kA in NSTX

Also consistent with the bubble burst relation,

Which requires 10x more current in HIT-II than in NSTX

10x more injector flux of that in present NSTX 60kA experiments with 10x more

injector flux leads to >2MA startup currents with 20kA injector current in future

larger machines.

)/(2 222TFoinjinj IdI

injI

( )injinj inj

TI V

inj

injI

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Full 2kV capability in NSTX would increase Ip ~ 300kA

Best results from NSTX 2005 and 2006HIT-II data: R. Raman, T.R. Jarboe et al., Nuclear Fusion, 45, L15-L19 (2005)

Voltage, flux optimization allowed HIT-II to increase closed flux current as capacitor charging voltage was increased

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Record non-inductive plasma startup currents in a tokamak (160kA in NSTX) verifies high current feasibility of CHI for

plasma startup applications

The significance of these results are:

1) demonstration of the process in a vessel volume thirty times larger than HIT-II on a size scale more comparable to a reactor,

2) a remarkable multiplication factor of 60 between the injected current and the achieved toroidal current, compared to six in previous experiments,

3) results were obtained on a machine designed with mainly conventional components and systems,

4) indicate favorable scaling with machine size.

• NSTX high current discharges not yet optimized– Extension to ~300kA should be possible at 2kV

• Future experiments to explore coupling to OH– 200kW ECH to heat the CHI plasma– Coupling to RF and NBI