Study on Power Threshold of the L-I-H Transition on the EAST Superconducting Tokamak

L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 1

Study on Power Threshold of the L-I-H Transition on the EAST Superconducting Tokamak

L. Chen,1 G.S. Xu,1 H.Q. Wang,1 S. Y. Ding,1 N. Yan,1,2 L. M. Shao,1 S. C. Liu,1 R. Chen,1 W. Zhang,1 L. Wang,1 N. Zhao,1 Y.

L. Li,1 Y. L. Liu,1 J.C. Xu,1

1Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China2Association Euratom-RisÁ DTU, DK-4000 Roskilde, Denmark

E-mail: [email protected]


Outline

• Introduction• Power threshold of sharp L–H transition in EAST

with C wall versus Mo/C wall• L-H transition in EAST with Mo/C wall• Summary and conclusion


Outline




R [m] 1.88

a [m] 0.44

BT [T] ~1.9

auxiliary heating

LHW dominant (2010)

LHW 2 MW and/or ICRF 1.5MW (2012)

C wall in 2010

Mo/C wall in 2012

All carbon (C) wall in 2010, molybdenum (Mo) first wall and C divertor in 2012 reversed BT in 2010, normal or reversed BT in 2012 Auxiliary heating advanced One cryopump at lower divertor Intensive Lithium wall coating

Brief introduction of EAST in 2010 and 2012


All the H-mode discharges have sharp (single-step) L-H transition, including some discharges with small dithering (~3% in Dα signal), in EAST with C wall.

Besides these sharp L-H transitions, I-phase displays dithering cycles with a much larger amplitude (~30% in Dα signal) has been observed in EAST with Mo/C wall.

In order to make comparison, all the H-mode discharges are classified as sharp L-H or dithering L-H transitions.

The time history of (a) divertor D emission and (b) its zoom-in plot of sharp L-H (left figure) and dithering L-H transition (right figure).

EAST with C wall in 2010 EAST with Mo/C wall in 2012

Behavior on Dα signal for L-H transition


Outline


with C wall versus Mo/C wall• L-H transition in EAST with Mo/C wall• Summary and conclusions


dtdWPPPPP dia

radohmicICRFLHloss

941.0803.0717.00488.0 SBtnP ethr

Calculate the loss power (Ploss) through the separatrix and threshold power (Pthr) predicted by the international tokamak scaling[1]:

[1] Martin Y.R., Takizuka T. and the ITPA CDBM H-mode Threshold Database Working Group 2008 J. Phys.: Conf. Ser. 123 012033


Power threshold of sharp L–H in EAST with C wall versus Mo/C wall


Ploss is well consistent with Pthr in 2010 with marginal input heating power. Ploss and Pthr versus <ne> , shows that its lower boundary follows the scaling in 2012.


L–H power threshold in EAST with C wall versus Mo/C wall


There is no significant dependence of Ploss versus different divertor configurationsin EAST.



Ploss for sharp L-H transition which occurred during plasma current ramp-up is ~10% higher than that occurred during plasma current flat top in 2010.

There’s no clear dependence of Ploss versus Ip ramp in 2012.




Ploss increases with increasing plasma current (Ip) of sharp L-H transition in 2010. Ploss has no clear dependence on Ip during sharp L-H transition in 2012.



Outline




|dRsep|<0.01 , DN or biased DN configurationdRsep<-0.01 , LSN configurationdRsep>0.01 ,USN configurationReversed BT: USN is favorable configuration

EAST with Mo/C wall

Sharp L-H transitions are mostly obtainedin LSN configuration, a few are obtained in DN and USN configurations. In dithering L-H transitions with reversedBT, DN has the lowest power threshold. Powerthreshold for USN is higher than LSN.

Ploss versus dRsep


The dithering behavior in the D emission signals varies significantly with different divertor configurations The dithering cycles in the unfavorable configuration (normally LSN with reversed BT) are much irregular in frequency and amplitude. The dithering cycles in DN and biased DN configurations (|dRsep|<1 cm) are mostly regularlarge-amplitude oscillations, which typically exhibiting clear and sharp transition from theL-mode to the I-phase The duration of I-phase in the favorableconfiguration (normally USN with reversed BT) isusually very short in time, with small amplitudeof dithers.

The dithering behavior in the D emission


Ploss slightly decreases with increasing triangularity with DN configuration both in reversed BT (left figure) and normal BT (right figure). The dotted lines are linear fitting of the data respectively.

Ploss versus triangularity δ


dithering L-H transition versus sharp L-H transition :what is role of the neutral particle density near the divertor region?

In the graph, the dithering L-H transition and sharp L-H transition have been somewhat separatedby the maximum value of lower divertor Ha emission normalized by central line-averaged electron density.

Hamax


Dithering L-H transition and sharp L-H transition obtained in one shot

Marginal heating with LHCD and ICRF Similar density and plasma stored energy Divertor configuration has changed The divertor Da/Ha emission changed significantly

-dithering L-H-sharp L-H


38905 2.7s dithering L-H38905 4.5s sharp L-H

Cryopump system

Divertor corner

As calculated by EFIT code and divertor probe, the distance between the strike-point and the divertor corner has changed about 2 cm at the time of dithering L-H and sharp L-H transition.

The strike-point is more far away from divertor corner in sharp L-H transition


The duration of I-phase relates to the ramp rate of power in excess of threshold power

Study of the duration of I-phase occurring at the transition reveals that it somewhat decreases with the ramp rate γP of power in excess of L-I-H transition threshold power [2].

Ethr

thrlossP P

PP

[2] Zohm H et al 1994 Phys. Rev. Lett. 72 222


Dedicated experiments in EAST with Mo/C wall have shown that: duration of dithering cycles decreases with γP , which obtained by changing the auxiliary heating power at similar threshold power and energy confinement time.

Ethr

thrlossP P

PP


Outline

• Introduction• L–H power threshold in EAST with C wall versus

Mo/C wall• L–H transition in EAST with Mo/C wall• Summary and conclusion


A statistical study of low-to-high confinement transition (L–H transition) in EAST over a broad range of operation is presented.

Ploss for sharp L-H transition which occurred during plasma current ramp-up is ~10% higher than that for sharp L-H in EAST with C wall.

Ploss slightly decrease with increasing triangularity with DN configuration, both in normal and reversed BT in EAST with Mo/C wall.

It exhibits lower power threshold for the ion drift away from the dominant X-point in EAST, in striking contrast to other tokamaks.

Study on the I-phase has revealed,a) the estimated divertor neutral particle density of dithering L-H transition has lower value

than that of sharp L-H transition.b) the duration of I-phase decreases with the ramp rate γP of power in excess of threshold power.


Thank you for your attention!

Study on Power Threshold of the L-I-H Transition on the EAST Superconducting Tokamak

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