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 b

y 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 c

ompletely detached plasma• Observe the detached plasma characteristi

cs 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 TokamakIntroduction of HL-2A Tokamak• The stable and reproducible discharges with LSN divertor con

figuration 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: >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 Vf 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

( PSOL=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 mi

dplane(a) The compression ratio of neutral particle pressures (P0d/P0m) rises, radiation power in divertor (Pdiv) first rises and then drops

(b)Electron pressures (Pe,div) at inner and outer targets slightly decrease

(c ) Electron temperatures (Te,di

v) at inner and outer targets gradually diminish

(d) Radiation power in main plasma (Prad) rises and plasma current (Ip) continues

(e ) Line-averaged density (ne) rises and deuterium GP pulses gradually reduce200 400 600 800

0

2

4 (e)

t (ms)

ne

Gas puffing pulses in main chamber

80

160

Te,

div (

eV)

P0d

/P0m

Pe,

div (

Pa)

n e (10

19 m

-3)

Pra

d (kW

)

Shot 7243 with deterium puffingIp (kA)

Prad

(d)

0

8

16 (c)

Outer target

inner

0

8

16

(b)

Outer target

Inner

0

10

20 (a)

Pdiv (a. u.)

P0d/P0m

The CDP discharge with SMBI fuelling at midp

lane • The Te,div , Pe,div, Pdiv and t

he ratio P0d/P0m drop duri

ng the detachment• Prad clearly increases

• Lowest Te,div < 2.0 eV

• Most ratio P0d/P0m >10

• The ne,max= 4.61019 m-3, h

igher than Greenwald limit nG=41019 m-3

• Target detachment is more difficult if the Grad-B drift is away from X-point

200 400 600 8000

3

6

(e)

t (ms)

ne

50 SMBI pulses in main chamber

80

160

Te,

div (

eV)

P0d

/P0m

Pe,

div (

Pa)

n e (10

19 m

-3)

Pra

d (kW

)

Shot 7498 with SMBI fueling Ip (kA)

Prad

0

6

12

(d)

(c)

Outer target

inner

0

8

16(b)Outer target

Inner

0

8

16 (a)

Pdiv (a. u.)

P0d/P0m

The CDP discharge with deuterium GP in divertor

200 400 600 8000

3

6(e)

t (ms)

ne

10 deuterium pulses in divertor

80

160

Te,

div (

eV)

P0d

/P0m

Pe,

div (

Pa)

n e (10

19 m

-3)

Pra

d (kW

)

Shot 7254 with Divertor deuterium

Ip (kA)

Prad

0

5

10

(d)

(c)

Outer target

inner

0

8

16 (b)Outer target

Inner

0

5

10 (a)

Pdiv (a. u.)

P0d/P0m

• The Te,div , Pe,div and P0d

/P0m drop during the d

etachment• Prad weakly rises

• Lowest Te,div < 2.0 eV

• The ratio P0d/P0m <10

• The ne,max= 4.31019

m-3, higher than Greenwald limit nG=41019

m-3

The CDP discharge with helium GP in divertor

200 400 600 8000

3

6(e)

t (ms)

ne

5 Helium pulses in divertor

100

200

Te,

div (

eV)

P0d

/P0m

Pe,

div (

Pa)

n e (10

19 m

-3)

Pra

d (kW

) Shot 7118 with Divertor Helium

Ip (kA)

Prad

0

8

16

(d)

(c)

Outer target

inner

0

5

10 (b)Outer target

Inner

0

6

12 (a)

Pdiv (a. u.)

P0d/P0m

• The Te,div , Pe,div , Pdiv a

nd P0d/P0m drop durin

g detachment• Prad increases quickly

• Lowest Te,div < 2.0 eV

• Most ratio P0d/P0m <6

• The ne,max= 5.61019

m-3, higher than Greenwald limit nG=41019

m-3

The CDP discharge with a neon pulse in divertor

300 400 500 600 7000

2

4 (e)

t (ms)

ne

A neon pulse in divertor

100

200

300

Te,

div (

eV)

P0d

/P0m

Pe,

div (

Pa)

n e (10

19 m

-3)

Pra

d (kW

)

Shot 7332 with divertor neon

Ip (kA)

Prad

0

10

20

(d)

(c)

Outer target

inner

0

8

16(b)Outer target

Inner

0

8

16 (a)

Pdiv (a. u.)

P0d/P0m • The Te,div , Pe,div, Pdiv and P

0d/P0m reduce during the d

etachment• Prad rises rapidly

• Lowest Te,div < 3.0 eV

• The ratio P0d/P0m<10

• The ne,max= 1.81019 m-3,

much smaller than Greenwald limit nG=41019 m-3

• No clearly linear and high recycling regimes are observed

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

0

20

40

1.63 1.65 1.67 1.69 1.71R (m)

Ted

(eV

)

0

4

8

12

Ped

(Pa)

TedTed_SMBIPedPed_SMBI

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

0

100

200

300

400

500

1.63 1.65 1.67 1.69 1.71

R (m)

qed

(kW

m-2

)

0

10

20

30

40

Ped

(Pa)

, Qed

(10

20m

-2s-1

)

qedqed_HePedPed_HeQedQed_He

Conclusion• The CDP discharges have been performed in HL-2A usin

g 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.

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.

Thank you for your attention !