Improvement of long distance ICRF coupling to ELMy Hmode plasmas by gas puffing at JET

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Topical Group on Heating and Current
Drive Coordinating Committee on ICRH (CCIC) April
10, 2008, Culham
Improvement of long distance ICRF coupling to
ELMy H-mode plasmas by gas puffing at JET
I. Monakhov, M.-L. Mayoral and P. Jacquet
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Issues for discussion
Lots of interesting stuff data not fully
analysed yet some extra time and effort required
to finalise the conclusions

• Relevance of JET ICRF coupling conditions to ITER
• Experimental set-up, discharge scenarios,
  measurement technique etc
• SOL parameters during gas injection in different
  scenarios
• Coupling improvement vs gas injection rate
• Coupling improvement vs position of gas injection
  inlets
• Between-ELM and during-ELM coupling vs
  antenna-plasma distance
• High-power gas-assisted long-distance operations
  in ELMy H-mode
• Gas-assisted coupling improvement vs core plasma
  confinement

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Relevance of JET ICRF coupling conditions to ITER
JET

ITER gt5 cm (behind limiter) ? Antenna strap
recess ? 4.5 cm (behind the first wall) 14
cm (min) ? Separatrix to wall/limiter
distance ? 12 cm (min) 14 cm (average)
2-3?1019 m-3 ?
Typical density at separatrix ?
3-4?1019 m-3 ?2 cm ? Near SOL
typical density decay length ? 2-4 cm
weak ? Far SOL density profile
flattening ? noticeable (?) ?
Cut-off density and spectrum ?
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Experimental set-up and plasma configurations

• ICRF
• f47 MHz ?- phasing antennae A,B,(C),D
• ?0.5MW (coupling studies) 5-8 MW (high-power
  shots)
• Plasma
• Deuterium ( few Hydrogen)
• ELMy (type-I) H-mode
• 10-14cm ROG (midplane separatrix-limiter gap)
• Two high-triangularity configurations
• ITER-AT
• BT3.1 T, Ip1.9 MA, dl0.50, du0.38, PNBI15
  MW,
• ??CH
• high recycling, high SOL density, good
  coupling
• HT3-mod
• BT1.55 T, Ip1.5 MA, dl0.35, du0.45 PNBI8
  MW,
• ?2?CH
• low recycling, low SOL density, poor coupling

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Typical discharge scenarios
ITER-AT, high recycling
HT3, low recycling
Injected power
Outer gap
Gas injection rate
Line-averaged density, Oct7
D? Oct1, vertical chord
B4 coupling resistance
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Gas inlets position and magnetic connection to
antennas
ITER-AT configuration

• GIM 6 magnetic connection
• antenna A strong
• antenna B strongest
• antenna C marginal
• antenna D no

Divertor ring GIMs should affect all antennas
in the same way
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Gas inlets position and magnetic connection to
antennas

• GIM 8
• A marginal
• B strong
• C marginal
• D no

HT-3 configuration, pulse 70680

• GIM 2
• A strong
• B marginal
• C no
• D no

• GIM 6
• A marginal
• B strong
• C marginal
• D no

GIM 9 A,B,C,D same
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SOL density vs gas injection rate
ITER-AT (high recycling)
HT3 (low recycling)
Line-averaged density at R3.73 m (near SOL)
vertical chord Octant 8 vs gas injection rate

• Linear dependence of SOL average density on gas
  injection rate
• Different efficiency of gas injection modules
  (GIMs)
• Higher gas injection efficiency in ITER-AT (high
  recycling) configuration particularly for
  divertor ring inlets (GIM9 and GIM10)

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SOL density profiles
Reliable data available for ITER-AT (high
recycling) configuration only
ITER-AT
1.4 1022 el/sec GIM9GIM10GIM6
no gas puffing
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Coupling analysis in ELMy plasmas
Not straightforward what is representative Rc
value average, max or min?

• Average not much sense due to short and
  strong ELM perturbations sensitive to ELM
  frequency
• Maximum relevant mainly to ELM tolerance
  studies
• Between-ELM most adequate for conservative
  estimates of RF system power injection
  capabilities

Our approach use smoothed running maximum and
minimum in 20ms sliding window of fast (10kHz)
Rc measurement to get temporal behaviour of
ELM-maximum and between-ELM coupling
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Between-ELM coupling improvement during gas
injection
Up to 5-fold Rc increase in both scenarios
(depending on GIMs and antennas)
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Coupling improvement vs GIM location
ITER-AT configuration
Introduction of gas injection from GIM6 clearly
have stronger effect on antennas A and B
(magnetically connected to GIM6), as compared
with toroidally symmetrical injection from
divertor ring (GIM9GIM10) However, antenna D
also benefits from GIM6 (no magnetic connection!)
in line with global SOL density increase during
gas injection.
Conclusion there are two contributing factors -
global and local - to ICRF antenna coupling
improvement
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Coupling improvement during gas injection
HT-3 configuration

• Coupling improves for all antennas including
  those which are not magnetically connected to
  GIMs
• Biggest improvement for antennas magnetically
  connected to GIMs
• Linear dependence on injection rate for all
  antennas

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Coupling improvement during gas injection
HT-3 configuration
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Dependence on antenna-plasma distance
HT-3 low recycling , pulse 70683

• In low recycling configuration both
  between-ELM and ELM-maximum coupling
  linearly decrease with ROG
• ELM-maximum (i.e. coupling perturbation during
  ELMs) seems to be decreasing faster

Antenna B GIM6 2 1022 l/sec
ELM-maximum
Between-ELMs
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Dependence on antenna-plasma distance
ITER-AT high recycling, four pulses with
different gas injection rates
Gas
ELM-maximum
Between-ELMs

• ELM-maximum coupling linearly decrease with
  ROG
• between-ELM coupling looses dependence on ROG
  during strong gas puff ?

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High-power long-distance operations in ELMy H-mode
ITER-AT up to 8 MW ICRF power injected
(trip-free) over 14cm ROG
1.8 1022 el/s from GIMs 6, 9 and 10
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High-power long-distance operations in ELMy H-mode
HT3 up to 5 MW ICRF power injected (trip-free)
over 14cm ROG
2 1022 el/s from GIM 8
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Conclusions

• ICRF coupling to ELMy H-mode plasma over
  ITER-relevant antenna-plasma distances studied in
  two scenarios with distinctly different SOL
  plasma behaviour high- and low- recycling
  regimes
• Linear decrease of ELM-maximum and
  between-ELM coupling with increasing
  antenna-plasma gap was recorded, the absolute
  values of coupling resistance being strongly
  dependant on the plasma scenario
• Gas injection was demonstrated as a powerful tool
  for between-ELM antenna coupling improvement
  with up to 5-fold coupling resistance increase in
  both plasma scenarios
• The impact of gas injection on coupling of
  individual antennas around the torus has both
  global and local component, the latter being
  related to magnetic connection of the gas inlets
  to antennas
• Gas injection allowed to inject up to 8MW ICRF
  power (trip-free) into ELMy plasma over 14cm
  plasma-limiter gap (19cm separatrix-strap
  distance)

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Supplementary slides
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Confinement vs gas injection
ITER-AT high recycling, four pulses with
different gas injection rates