Radiation divertor experiments in the HL2A tokamak

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Radiation divertor experiments in the HL-2A
tokamak

• L.W. Yan, W.Y. Hong, M.X. Wang, J. Cheng,
• J. Qian, Y.D. Pan, Y. Zhou, W. Li, K.J. Zhao,
• Z. Cao, Q.W. Yang, X.R. Duan and Y. Liu

Southwestern Institute of Physics, Chengdu, China
Presentation for 18th PSI conference in Toledo,
Spain, May 29, 2008
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Outline

• Objectives
• Introduction of HL-2A tokamak
• Diagnostics arrangement
• HL-2A divertor parameters simulated by SOLPS5.0
  code
• Experimental results
• Detached plasma fuelled at midplane
• Detached plasma fuelled in divertor
• Conclusion
• Discussion

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Objectives

• Develop radiation divertor experiments
• Understand the conditions for obtaining
  completely detached plasma
• Observe the detached plasma characteristics
  fuelled from midplane and divertor chamber
• Compare experimental results with modelling
  results by SOLPS5.0 code
• Explore an optimization method for attaining the
  detached discharge

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Introduction of HL-2A Tokamak

• The stable and reproducible discharges with LSN
  divertor configuration have been obtained by
  reliable feedback control and wall conditioning
  techniques.
• Significant progresses are achieved on natural
  PTB, ZFs, QMs, Electron fishbone etc. due to the
  hardware improvement.

• BT 2.8 T 2.7 T
• IP 480 kA 430 kA
• Duration 5 s 3.0 s
• Plasma density 6.0 x 1019 m-3
• Electron temperature 5 keV
• Ion temperature gt1 keV
• Fuelling system GP, SMBI, PI
• Heating system ECRH/2MW/68GHz
• Heating system NBI/1.5MW/45keV
• Heating systemLHCD/1MW/2.45GHz

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Diagnostics arrangement for radiation divertor
experiment

• Direct GP and SMBI fuelling at midplane
• Divertor fuelling with deuterium and inert gases
• Flush probes for Te and ne profiles at inner and
  outer target plates
• Two fast gauges for neutral particle pressures in
  divertor and main chamber
• Movable probes for Te and ne profiles in divertor
  through shot by shot
• An IR camera for the temperature rise at outer
  target

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Arrangement of flush probes at target plates

• Seven sets of triple probes on each plate
• Spatial resolution of 10 mm in vertical direction
  and 15 mm in Bt direction
• Each plate vertical to the midplane
• Fixed flush probes measured for Te, ne and Vf
  profiles
• Decay lengths of heat flux, temperature and
  density estimated

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HL-2A divertor parameters simulated by SOLPS5.0
code

• neu,m upper midplane ne
• net,in inner target ne
• net,out outer target ne
• Teu,m upper midplane Te
• Tet,in inner target Te
• Tetout outer target Te
• ( PSOL500kW)

• No linear regime exists
• No clearly high-recycling regime is observed
• Partial detachment appears with low density

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Partially detached plasma with strong GP at
midplane

• The compression ratio of neutral particle
  pressures (P0d/P0m) rises, radiation power in
  divertor (Pdiv) first rises and then drops
• Electron pressures (Pe,div) at inner and outer
  targets slightly decrease
• Electron temperatures (Te,div) at inner and outer
  targets gradually diminish
• Radiation power in main plasma (Prad) rises and
  plasma current (Ip) continues
• Line-averaged density (ne) rises and deuterium GP
  pulses gradually reduce

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The CDP discharge with SMBI fuelling at midplane

• The Te,div , Pe,div, Pdiv and the ratio P0d/P0m
  drop during the detachment
• Prad clearly increases
• Lowest Te,div lt 2.0 eV
• Most ratio P0d/P0m gt10
• The ne,max 4.6?1019 m-3, higher than Greenwald
  limit nG4?1019 m-3
• Target detachment is more difficult if the Grad-B
  drift is away from X-point

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The CDP discharge with deuterium GP in divertor

• The Te,div , Pe,div and P0d/P0m drop during the
  detachment
• Prad weakly rises
• Lowest Te,div lt 2.0 eV
• The ratio P0d/P0m lt10
• The ne,max 4.3?1019 m-3, higher than Greenwald
  limit nG4?1019 m-3

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The CDP discharge with helium GP in divertor

• The Te,div , Pe,div , Pdiv and P0d/P0m drop
  during detachment
• Prad increases quickly
• Lowest Te,div lt 2.0 eV
• Most ratio P0d/P0m lt6
• The ne,max 5.6?1019 m-3, higher than Greenwald
  limit nG4?1019 m-3

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The CDP discharge with a neon pulse in divertor

• The Te,div , Pe,div, Pdiv and P0d/P0m reduce
  during the detachment
• Prad rises rapidly
• Lowest Te,div lt 3.0 eV
• The ratio P0d/P0mlt10
• The ne,max 1.8?1019 m-3, much smaller than
  Greenwald limit nG4?1019 m-3
• No clearly linear and high recycling regimes are
  observed

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Ted and pressure profiles in divertor versus
major radius

• The peak Ted and Ped decrease a factor of 8.2 and
  8.8 after the SMBI fueling
• The measured decay lengths of power density and
  electron temperature are 0.6 cm and 2.0 cm in
  divertor
• Theoretic prediction results are 0.6 cm and 2.2
  cm at target plate

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Electron heat flux, pressure and particle flux
profiles vs. major radius

• The electron heat flux, pressure and particle
  flux in divertor decrease a factor of 75, 34 and
  11 after the helium fueling in divertor
• The detached discharge can dramatically reduce
  the heat flux to divertor plate

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Conclusion

• The CDP discharges have been performed in HL-2A
  using direct GP and SMBI fueling at midplane,
  deuterium, helium and neon injections in divertor
  chamber.
• The Te,div at inner and outer target plates can
  be decreased below 2 eV in the CDP discharges.
• The Pe,div, Pdiv and compassion ratio P0d/P0m
  gradually drop during target detachment.
• Partial detachment first appears at inner target
  plate even if plasma density is very low due to
  the specific geometry with narrow and transparent
  divertor fans in HL-2A.
• The detached discharge can dramatically reduces
  the heat flux to divertor plate (1/75).
• No clearly linear and high-recycling regimes are
  observed before target detachment, consistent
  with modeling results.

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Discussion

• Radiation power in divertor gradually drops
  during the complete detachment because main
  ionization processes can take place in more
  upstream region.
• It is difficult to precisely determine the decay
  lengths of electron temperature, density and
  pressure at divertor targets during the
  detachment because electron temperatures at the
  strike points are lower than the around region
  and bad spatial resolution.
• The inert gas injection in divertor is an
  effective method for obtaining completely
  detached plasma
• The electron temperature at inner target is
  higher than that at outer one and more difficult
  detachment when the Grad-B drift is away from
  X-point.

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Thank you for your attention !