HT-7

ASIPP

Investigation on Zeff and impurities behavior with molybdenum

limiter in lithium coating experiments on HT-7 tokamak

Presented by Y.J.ChenPresented by Y.J.Chen

Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, ChinaInstitute of Plasma Physics, Chinese Academy of Sciences, Hefei, China

2011.7.192011.7.19

1

Workshop of 2011 Spring Experiment on HT-7

HT-7

ASIPP

Introduction -- Significance of Zeff and impurity control -- International development -- The principle of Zeff from V.B. and Define of averaged Zeff

Experimental Set-Up and major spectroscopy diagnostics

Lithium coating wall conditioning Experiment

Zeff and major impurities with lithium coating

Further more benefits for lithium coating

-- Lower particle recycling

-- Lower ratio of H/H+D Conclusion and Discussion

Outline

2

HT-7

ASIPP

Introduction

For magnetically confined plasmas, How to control of impurity level is a critical issue for the fusion reaction in the future. the Z_effective is a significant parameter with indication of the total impurity level. Visible bremsstrahlung measurement is a powerful tool approaching to derive this key data.

Determination Zeff via Bremsstrahlung was first applied by Kadota on JIPP T-II in 1980, and soon used worldwide on many devices ， such as DIII-D, JET, FTU,ASDEX Upgrade, LHD etc, for the regardlessness of the number of impurity species in the plasma. The best region to measure the radiation is in the visible.

3

HT-7

ASIPP

The Definition of an averaged Zeff

4

2

=i i

ieff

i ii

n ZZ

n Z

, 1

i ii

effi

i eff i ii

i ii i

ie

p ZZ

pZ p Z

n Zp and p

n

F̂F

Statistics

Quantum Mechanics

( )

( )

ii

i

p i AA

p i

HT-7

ASIPP

The principle of Zeff from V.B.

5

The emission coefficient of bremsstrahlung radiation2

1/26

2 3 1/2 2

16 exp3 6

e i i ffff i

e e e

n n Z gdP e hcd c m T T

1/2 1/2 22

6 3 2

3( )16 6 exp /

eeff

e e ff e

TcZ re m n g hc T

Zeff can be expressed as:

and the line averaged Zeff is defined as:

1/22 2

6 32 1/2

)316 6

( ) ( )exp /

a

aeff a

e ffe e e

a

r drcZe m g

n r T r hc T dr

（

HT-7

ASIPP

Introduction -- Significance of Zeff and impurity control -- International development -- The principle of Zeff from V.B. and Define of averaged Zeff

Experimental Set-Up and key spectroscopy diagnostics

Lithium coating wall conditioning Experiment

Zeff and major impurities with lithium coating

Further more benefits for lithium coating

-- Lower particle recycling

-- Lower ratio of H/H+D Conclusion and Discussion

Outline

6

HT-7

ASIPP

Experimental Set-up

The HT-7 superconducting tokamak has a major radius of R=1.22m and a minor radius of a =0.27m in the circular cross-section. From this spring the graphite limiters had been instead of molybdenum limiters in order to develop the lithium wall conditioning experiments Basically, the HT-7 tokamak with the limiter configuration is normally operated under the basic parameters: Ip=100-250kA, the toroidal magnetic field Bt=2T, the central line-averaged plasma density ne=0.5-6e19m-3 , central electron temperature Te=0.5-3.0keV and central ion temperature Ti =0.2-1.5keV

7

HT-7

ASIPP

Key spectroscopy diagnostics

8

1 Two 35-channel Da (λ = 656.1 nm) array2 The 6-channel Visible Bremsstrahlung emission3 The fiber optic spectrometer4 The 9-channel CIII (λ = 464.7 nm) array5 Single channel impurity emission: OII (λ= 441.5 nm), OV (λ = 278.1 nm), MoI (λ = 386.4 nm) and LiI (λ = 670.8 nm).

HT-7

ASIPP

Introduction -- Significance of Zeff and impurity control --international development --The principle of Zeff from V.B. and Define of averaged Zeff

Experimental Set-Up and key spectroscopy diagnostics

Lithium coating wall conditioning Experiment

Zeff and major impurities with lithium coating

Further more benefits for lithium coating

--lower particle recycling

--lower ratio of H/H+D Conclusion and Discussion

Outline

9

HT-7

ASIPP

Typical experiment of lithium coating

10#113140 before lithium coating, #113191 after lithium coating

HT-7

ASIPP

Introduction -- Significance of Zeff and impurity control --international development --The principle of Zeff from V.B. and Define of averaged Zeff

Experimental Set-Up and key spectroscopy diagnostics

Lithium coating wall conditioning Experiment

Zeff and major impurities with lithium coating

Further more benefits for lithium coating

--lower particle recycling

--lower ratio of H/H+D Conclusion and Discussion

Outline

11

HT-7

ASIPP

Zeff and major impurities with lithium coating

12

HT-7

ASIPP

The impurity amount varies with ne

13

After lithiation After boronization

boronization is further more effective to decrease the level of impurity

HT-7

ASIPP

Introduction -- Significance of Zeff and impurity control -- International development -- The principle of Zeff from V.B. and Define of averaged Zeff

Experimental Set-Up and key spectroscopy diagnostics

Lithium coating wall conditioning Experiment

Zeff and major impurities with lithium coating

Further more benefits for lithium coating

-- Lower particle recycling

-- Lower ratio of H/H+D Conclusion and Discussion

Outline

14

HT-7

ASIPP

Further more benefits for lithium coating

15

In despite of much more reduced Zeff after boronization, lithium coating can get a very lower recycling of hydrogen and isotopes in the plasma edge and can get much more lower ratio of H/H+D after many times operation. The value of H/H+D can be decreased below 10%. It is benefit for ICRF minority ion heating

Red --after boronizationBlue -- after lithium coating

HT-7

ASIPP

Introduction -- Significance of Zeff and impurity control --international development --The principle of Zeff from V.B. and Define of averaged Zeff

Experimental Set-Up and key spectroscopy diagnostics

Lithium coating wall conditioning Experiment

Zeff and major impurities with lithium coating

Further more benefits for lithium coating

--lower particle recycling

--lower ratio of H/H+D Conclusion and Discussion

Outline

16

HT-7

ASIPP

Conclusion

Impurities and Zeff are reduced obviously by lithium coating wall conditioningIt is indicated by statistics that reduced impurities and Zeff seems inverse proportional to the quantity of lithium coatingThe impurity amount varies approximately linearly with ne, which is useful for Zeff determination and also indicated that boronization is further more effective to decrease the level of impurityIn despite of much more reduced Zeff after boronization, lithium coating can get a very lower recycling of hydrogen and isotopes in the plasma edge and can get much more lower ratio of H/H+D after many times operation. The value of H/H+D can be decreased below 10%. It is benefit for ICRF minority ion heating

17

HT-7

ASIPP

Discussion

18

• Why does the Zeff much more larger with the molybdenum limiter than with the graphite limiter? It comes into being from higher Z molybdenum PFC and limiter configuration in lower density region(1-3e19m-3) on HT-7

• We are looking forward to doing the analysis of Zeff and impurities behavior on EAST with molybdenum PFC in lithium coating wall conditioning experiments in near future

HT-7

ASIPP

Thank you !

Workshop of 2011 Spring Experiment on HT-7

20