Plasma Startup In NSTX Using

1

• 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, USA
• 2Princeton Plasma Physics Laboratory, USA
• 3Oak Ridge National Laboratory, Oak Ridge, TN,
  USA
• 4Nova Photonics, USA

College WM Colorado Sch Mines Columbia
U Comp-X General Atomics INEL Johns Hopkins
U LANL LLNL Lodestar MIT Nova Photonics New York
U Old Dominion U ORNL PPPL PSI Princeton
U SNL Think Tank, Inc. UC Davis UC
Irvine UCLA UCSD U Colorado U Maryland U
Rochester U Washington U Wisconsin
Culham Sci Ctr U St. Andrews York U Chubu U Fukui
U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu
Tokai U NIFS Niigata U U Tokyo JAERI Hebrew
U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST
ENEA, Frascati CEA, Cadarache IPP, Jülich IPP,
Garching ASCR, Czech Rep
12th International ST Workshop 11-13 October
2006 Chengdu, China
Work supported by US DOE contracts
DE-FG03-9ER54519 and DE-AC02-76CH03073.
2
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

3
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

4
Implementation of CHI in NSTX
Transient CHI Expect axisymmetric reconnection
at the injector to result in formation of closed
flux surfaces
5
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

T.R. Jarboe,"Formation and steady-state
sustainment of a tokamak by coaxial helicity
injection," Fusion Technology 15, 7 (1989).
6
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

7
Improved pre-ionization to a level that results
in injected gas 10 times less than in 2004
Shot 116565
EC-Pi glow along the center stack

• 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 lt 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.
Divertor gap
Shot 116570
ECH T. Bigelow (ORNL)
8
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.

Hiroshima University (N. Nishino) Camera Images
R. Kaita (PPPL)
9
Discharges without an absorber arc show high
current multiplication ratios (Ip / Iinj) of 60
10
Dramatic improvement in closed flux current
generation from 2005
2006 discharges operated at higher capacitor bank
voltage and higher toroidal field
LRDFIT (J. Menard)
11
Electron temperature and density profiles become
less hollow with time
Profile becomes less hollow with time
Plasma and Injector current
120814 Black 8ms, Red 12ms
120842 Black 8ms, Red 10ms
Thomson (B. LeBlanc)
12
Data indicates that 200kW of ECH would increase
Te to 100eV

• 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 lt100kW
• Consistent with low electron densities of
  2x1018m-3, for impurity burn through. Li a
  possibility for controlling Oxygen.

13
Some discharges persist for as long as the
equilibrium coil currents are maintained
Fast camera R. Maqueda
14
Movie of a high current discharge
Fast Camera R. Maqueda L. Roquemore
15
(No Transcript)
16
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

17
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 gt2MA startup currents with
  20kA injector current in future
• larger machines.

18
Full 2kV capability in NSTX would increase Ip
300kA
Best results from NSTX 2005 and 2006
HIT-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
19
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
• demonstration of the process in a vessel volume
  thirty times larger than HIT-II on a size scale
  more comparable to a reactor,
• a remarkable multiplication factor of 60 between
  the injected current and the achieved toroidal
  current, compared to six in previous experiments,
  
• results were obtained on a machine designed with
  mainly conventional components and systems,
• 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