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

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

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

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. B T :2.8 T 2.7 T I P :480 kA 430 kA Duration: 5 s 3.0 s Plasma density: 6.0 x m -3 Electron temperature: ~5 keV Ion temperature: >1 keV Fuelling system:GP, SMBI, PI Heating system ECRH/2MW/68GHz Heating system NBI/1.5MW/45keV Heating systemLHCD/1MW/2.45GHz

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

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 V f profiles Decay lengths of heat flux, temperature and density estimated

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 ( P SOL =500kW) No linear regime exists No clearly high-recycling regime is observed Partial detachment appears with low density

Partially detached plasma with strong GP at midplane (a)The compression ratio of neutral particle pressures (P 0d /P 0m ) rises, radiation power in divertor (P div ) first rises and then drops (b)Electron pressures (P e,div ) at inner and outer targets slightly decrease (c)Electron temperatures (T e,div ) at inner and outer targets gradually diminish (d)Radiation power in main plasma (P rad ) rises and plasma current (I p ) continues (e)Line-averaged density (n e ) rises and deuterium GP pulses gradually reduce

The CDP discharge with SMBI fuelling at midplane The T e,div, P e,div, P div and the ratio P 0d /P 0m drop during the detachment P rad clearly increases Lowest T e,div < 2.0 eV Most ratio P 0d /P 0m >10 The n e,max = 4.6  m -3, higher than Greenwald limit n G =4  m -3 Target detachment is more difficult if the Grad-B drift is away from X-point

The CDP discharge with deuterium GP in divertor The T e,div, P e,div and P 0d /P 0m drop during the detachment P rad weakly rises Lowest T e,div < 2.0 eV The ratio P 0d /P 0m <10 The n e,max = 4.3  m -3, higher than Greenwald limit n G =4  m -3

The CDP discharge with helium GP in divertor The T e,div, P e,div, P div and P 0d /P 0m drop during detachment P rad increases quickly Lowest T e,div < 2.0 eV Most ratio P 0d /P 0m <6 The n e,max = 5.6  m -3, higher than Greenwald limit n G =4  m -3

The CDP discharge with a neon pulse in divertor The T e,div, P e,div, P div and P 0d /P 0m reduce during the detachment P rad rises rapidly Lowest T e,div < 3.0 eV The ratio P 0d /P 0m <10 The n e,max = 1.8  m -3, much smaller than Greenwald limit n G =4  m -3 No clearly linear and high recycling regimes are observed

T ed and pressure profiles in divertor versus major radius The peak T ed and P ed 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

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

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 T e,div at inner and outer target plates can be decreased below 2 eV in the CDP discharges. The P e,div, P div and compassion ratio P 0d /P 0m 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.

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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