Enhanced Plasma Performance by ICRF Wall Conditioning

1
Enhanced Plasma Performance by ICRF Wall
Conditioning
ASIPP
HT-7

• B.N. Wan, J. Li, Y.P. Zhao, X.Z.Gong, X.M.
  Zhang, H.L. Ruan, L. Zeng, Z.W. Wu, J.R. Luo,
  W.W. Ye, X.M.Guo, C.F.Li, X.M.Wang, J.K. Xie and
  HT-7 Team
• Institute of Plasma Physics, Academia Sinica,
  230031, Hefei, China

2
Abstract
ASIPP
HT-7

• Coating of the walls of HT-7 super-conducting
  tokamak with boron and silicon by ICRF has led to
  improved plasma performance. Significant
  suppression of C, O, Mo and hence Zeff and
  radiating power fraction are achieved. The
  improved confinement both for particle and energy
  is observed in full operation parameters. Energy
  balance analysis shows that electron heat
  diffusion coefficient is strongly reduced in
  outer half region. Operation limit was extended
  both in low and high electron density regions
  after ICRF B-coating and further to low q region
  after Si-coating.

3
Introduction
ASIPP
HT-7

• HT-7 is a super-conducting Tokamak having
  permanent toroidal field.
• GDC is not suitable for HT-7 for wall cleaning
  and conditioning.
• Wall coating is necessary for HT-7 operation.
• New RF Boronization and Siliconization technique
  is developed.
• ICRF Boronization and Siliconization led to
  enhanced tokamak discharges.

4
ASIPP
HT-7
Experiments

• C2B10H12 powder is used for ICRF boronization.
• Applying ICRF cleaning with helium reduce
  hydrogen in the fresh film.
• Density control can be achieved after about ten
  shots.
• To ensure the fresh properties of the coating
  film, plasma behavior has been investigated
  within 100 shots after ICRF boronization.
• He SiH4 with the ratio of 955 is used for ICRF
  siliconization.
• There is no recycling problem at first shot after
  siliconization.
• Plasma behavior has been studied within 50 shots
  after ICRF siliconization for fresh coating film.
  
• Only ohmic heated discharges with deuterium gas
  fueling have been analyzed in this work

5
ASIPP
HT-7
HT-7 Superconducting Tokamak
R 1.22m a 0.285m (Mo Limiter) Ip 100250
kA (250) ne 18x1013cm-3 (6.5) BT
12.5T(2.5) Te 12 KeV (1.5) Ti 0.20.6K eV
(0.8) t 1 5s ( gt10s) ICRF f 1545MHz,
P 0.3MW, CW(0.25, 1.5s) LHCD f 2.45GHz,
P 1.2MW, 10s (0.85MW) Pellet injector up to
8 pellets /per shot Supersonic beam injection
lt1.0 km/s Pump limiter ( Mo head) Main Goal
Steady-state advanced operation and related
physics ( Ip gt 100kA, Negt1.0x13cm-3, t310s)
6
ASIPP
HT-7
Results

• Reduced Impurity influxes and radiating power
• Increased plasma current and decreased loop
  voltage (increased bootstrap current?)
• Improved energy/particle confinement times
• Reduced electron heat diffusion coefficients in
  entire plasma region for B-coating and in outer
  half radius region for Si-coating
• Extended operation limit, especially to low-q
  after Si-coating

7
Discharge after ICRF boronization
ASIPP
HT-7

• Close line averaged central electron density
• Increased plasma current and lower loop voltage
• Reduced impurity influxes and radiating power

8
Discharge after ICRF Siliconization
ASIPP
HT-7

• Close line averaged central electron density
• Increased plasma current and lower loop voltage
• Reduced impurity influxes and radiating power

9
Very good discharge was obtained after 30 minutes
siliconization
ASIPP
HT-7
10
ASIPP
HT-7
Zeff as fucntion of line averaged central
electron density
11
ASIPP
HT-7
C and O influxes as fucntion of line averaged
central electron density
12
ASIPP
HT-7
C and O line radiations as fucntion of shot number
13
Energy/particle confinement time as fuction of
line averaged central electron density
ASIPP
HT-7
14
ASIPP
HT-7
Energy balance analysis for two discharges before
and after ICRF boronization

• Close line averaged central density
• Reduced electron heat diffusion coefficients in
  entire plasma region
• Lower edge electron temperature

15
ASIPP
HT-7
Energy balance analysis for two discharges before
and after ICRF siliconization

• Close line averaged central density
• Reduced electron heat diffusion coefficients in
  outer half radius
• Higher Te(a) and radiating power than these after
  boronization

16
ASIPP
HT-7
Extended operation limit after ICRF coating

• Operation limit are extended both in low and high
  electron sensity
• ICRF siliconization extend operation limit
  further to low-q region

17
ASIPP
HT-7
Comparison of boronization and siliconization

• Boronization 24 hours
• Siliconization 0.51.0 hour
• Easy control of density and recycling for
  siliconization.
• Shorter life time for Si-coating.

After siliconization

• After boronization

18
Summary
ASIPP
HT-7

• RF condition technique is routinely used in HT-7
  experiments, which is very powerful for
  superconduting devices.
• RF conditioning have led to suppression of
  impurity level and radiating power.
• RF conditioning leads to enhanced plasma
  performance both for energy /particle and
  transport.
• RF conditioning leads to extension of operation
  limit of the machine.
• RF siliconization was effective within more than
  100 shots and boronization for about 1000 for
  good plasma performance.

19
Reference
ASIPP
HT-7

• 1. J.Winter et al., J. Nucl. Mater. 162-164
  (1989) 713.
• 2. J.Winter et al., Phys. Rev. Lett. 71 (1993)
  1549.
• 3. H.G.Esser et al., J. Nucl. Mater 241-243
  (1997) 861.
• 3 J K Xie, "RF wall conditioning A new
  technique for future larger Superconducting
  tokamak", this conferences
• 4 Jiangang Li, et al., Nuclear Fusion, 39(1999)
  973
• 5 ZHAO Yan-ping et al, CHIN. PHYS. LETT.
  Vol.14, No. 12 (1996)
• 6J.S.Hu et al. "Removal of particles by the
  ICRF cleaning in HT-7 superconducting tokamak",
  this conference.