Comparison of MA and Vphi Example of asymmetry

1
NSTX
Supported by
Improved Coupling of a Transient CHI Started
Discharge to Induction
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 Purdue
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 JAEA Hebrew
U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST
POSTECH ASIPP ENEA, Frascati CEA, Cadarache IPP,
Jülich IPP, Garching ASCR, Czech Rep U Quebec
R. Raman, T.R. Jarboe, B.A. Nelson, D. Mueller,
M.G. Bell et al., University of Washington,
Seattle, WA, PPPL, ORNL, Nova Photonics etc.
NSTX Research Forum for FY2009 Research 8-10
December 2008
2
Transient CHI Axisymmetric Reconnection Leads to
Formation of Closed Flux Surfaces

• Demonstration of closed flux current generation
• Aided by gas and EC-Pre-ionization injection from
  below divertor plate region
• Demonstration of coupling to induction (2008)
• Aided by staged capacitor bank capability

CHI for an ST T.R. Jarboe, Fusion Technology, 15
(1989) 7 Transient CHI R. Raman, T.R. Jarboe,
B.A. Nelson, et al.,
PRL 90, (2003) 075005-1
3
CHI started discharge couples to induction and
transitions to an H-mode demonstrating
compatibility with high-performance plasma
operation
Te Ne from Thomson Ti from CHERS

• Discharge is under full plasma equilibrium
  position control
• Loop voltage is preprogrammed

• Projected plasma current for CTF gt2.5 MA
• Ip Iinj(?Tor??Pol)
• Based on 50 kA injector current (250kA equivalent
  achieved on HIT-II)
• Current multiplication of 50 (70 achieved in
  NSTX)

CHERS R. Bell Thomson B. LeBlanc
T.R. Jarboe, Fusion Technology, 15 (1989) 7
4
Experiments have now demonstrated the successful
coupling of a transient CHI started discharge to
induction in NSTX

• Made possible as a result of an effort to clean
  the lower divertor plates using high current
  discharges using the CHI capacitor bank power
  supply and by using a smaller sized capacitor
  bank for initiating the discharge.
• However, the total energy deposited by the
  capacitor bank was small and the lower divertor
  tiles were not cleaned to the extent that could
  have been with a larger power supply.
• In addition, the plasma radiated power increased
  soon after the occurrence of an absorber arc,
  because of ablated surface impurities entering
  the plasma after the CHI plasma contacted the
  upper absorber.

5
Need auxiliary heating or metal divertor plates
to compensate for increased radiated power with
more capacitors

• Low-z impurity radiation increases with more
  capacitors
• High Te in spheromaks (500eV) obtained with metal
  electrodes
• Test with partial metal outer divertor plates
  during FY10
• Reverse TF polarity to make outer vessel cathode
  (FY10)
• Upper divertor radiation also increases with more
  capacitors
• Need to reduce absorber arcs
• Absorber field nulling coils to be used during
  FY09
• Assess benefits of partial metal plates
  Absorber coils
• Discharge clean divertor with high current DC
  power supply
• Use 350kW ECH during FY11

Plasma Current
128400 5mF (7.6kJ) 128401 10mF (15.3kJ) 129402
15mF (22.8kJ)
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Further Control of the Influx of Impurities
Needed to Improve Coupling and to see Flux
Savings

• Prior to the start of CHI experiment, run high
  injector current discharges using the DC
  rectifier power supplies (200-400ms, 10-15kA), in
  a stuffed injector current mode to further clean
  the lower divertor electrodes.
• Reduce the gain on the filterscopes to be able to
  track the reduction in C and O line emission
• Use the divertor infrared camera to monitor the
  divertor tile temperature
• Then reconnect the CHI capacitor bank power
  supply and initially repeat the 160kA discharges,
  and by increasing the injector flux and toroidal
  field, determine the highest level of closed flux
  current that can be obtained.
• Initially repeat with 9 capacitor and allow
  absorber arcing as a way to clean the absorber
  electrode region
• Later repeat with the absorber PF coils turned on

7
Energize the Absorber PF coils to Reduce the
Interaction of the CHI Plasma with the Upper
Absorber.

• Determine the highest amount of current that can
  couple to induction using a solenoid with zero
  pre-charge.
• As in 2008, the Li evaporator would be used to
  support these experiments. Increase the amount of
  evaporated Li to see a benefit during the CHI
  startup phase.
• The Li-dropper may be tested in some discharges.
• Use DC preprogrammed currents (20-50ms in
  duration),based on updated (FY2006) calculations
  using XFLUXNSTX (use conditions from new
  dedicated XP)
•  
• After the Liquid-Li divertor plates are
  installed, switch the direction of TF to make the
  outer electrode the cathode and investigate the
  effect of reduced low-Z impurity influx from the
  largely metal outer electrode.
• For comparison conduct shots with the metal plate
  as the anode
• Use reference shots produced with a graphite
  divertor

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Summary

• Coupling of CHI to induction and the
  transitioning of a CHI started discharge to an
  H-mode demonstrates that CHI start works in a
  large ST
• Improvements to the level of coupling current
  requires further reduction in the level of low-z
  impurities (consistent with HIT-II results)
• During 2009 clean the graphite focus on cleaning
  the lower divertor tiles and use the absorber PF
  coils to the extent possible
• When the LLD plates become available repeat
  reference discharges for a comparison of the
  benefits of the metal target plate (then switch
  direction of TF)
• If the coupling current could be doubled,
  pronounced flux savings should happen naturally
• Run Time
• One day for conditioning
• Three days for the experiment