1. CUP collaboration 2007
10-15 Mar. 2008, ASIPP, Hefei
Avoidance of Current Termination using Runaway Electrons generated by Lower Hybrid Current Drive in HT-7
K. Toi (NIFS)

2. Outline Introduction & Motivation Objectives Experimental plan
Expected Results and future plan

3. Introduction & Motivation
Reliable reactor operation: ~1 disruption/year operation
Demonstration of reliable disruption avoidance and mitigation techniques is a key issue for ITER & DEMO operation.
Since initial phase of tokamak research, disruption is being studied from various aspects. However, any reliable techniques of disruption avoidance are not developed so far, although mitigation techniques such as noble gas jets have been developed to a certain extent.
Avoidance techniques are translated to stabilization of RWM and locked modes such as kink and tearing modes.

4. Mitigation of Disruption Effects
High pressure noble gas jets can mitigate: minimize the effect on
divertor thermal loading, halo current and runaway electrons.
Experimental results in DIII-D and C-Mod
Gas jets do not penetrate beyond edge, but it is not necessary
for disruption mitigation.
But, noble gas jets may lead to efficient generation of REs
in ITER.

5. Experimental Objectives for minimized Ip drop
(1) To study generation of relativistic runaway electrons (REs) during disruption
(2) To search a techniques to replace plasma current (Ip) with RE current with a small Ip-drop by application of LHCD/ECCD/ECH
- minimize halo current and minimize |dIp/dt|
issues: minimize thermal quench effect
and RE loss to PFC and divertor plates
Plasma sustained by RE with ECH
on JT-60U(Y. Kawano et al., EPS2005)
Plasma sustained by RE in JET
(R.D. Gill, NF 1993)
Plasma sustained by RE with LHCDon FTU(J.R. Martin-Solis et al., PRL 2006)

6. Experimental Objectives (cont.)
(3) To search for realization of RE-sustained tokamak plasma (Astron-Spherator configuration)
Large Shafranof shift- deep magnetic well
Peaked current profile route to high bN ( ~ 4li ),
High poloidal filed in the core  good alpha confinement
RE core rigid current profile with very long tR
Force free configuration of high energy electron beam “Astron-Spherator”
(S. Yoshikawa et al., PRL 1971)
Cross-section of Astron spherator
which is similar to a spherical tokamak.

7. First Trial of Avoidance of Ip-Disruption with REs Produced by LHCD
A stable plasma sustained by runaway current produced by LHCD + disruption-induced loop voltage
MHD activities are still active.
By Yong Jiang, 3 July 2007

8. Discharge Condition on 12 March.
●Bt= T; Ip= kA; gas: deuterium; <ne>=0.8-2x1019 m-3
Large Ip at low <ne> tends to be slide-away and
appreciable amount of REs will be present before
disruption. Ip/<n> should be small.
Candidates:
Bt=1.3T, Ip~100 kA, <ne>~0.8-2x1019 m-3(?)
Bt=1.7T, Ip~130kA, <ne>~0.8-2x1019 m-3(?)
( last year shots: Bt=1.94T, Ip=148kA, <ne>~1.8x1019 m-3)
●Feedback control of plasma position is critically important.
●Monitor of runaway electrons and plasma position
Experimental objectives:
(1) demonstration of RE-sustained plasma and reduction of |dIp/dt|
(2) study of loss process of REs
(3) Additional heating of a RE-sustained plasma to raise beta

9. 本提案以後の世界の研究動向（２） 日中拠点協力事業の枠組みで共同実験を開始：K. Toi (NIFS), L. Hu(ASIPP)
●ＨＴ－７（中国）におけるディスラプションフェイズへのLHCD印加の効果の研究
LHCDとディスラプション時の誘導電場を利用した逃走電子電流生成によりプラズマ電流の減衰率の低減を実証。
●逃走電子の損失過程の解明と定量化
●逃走電子電流保持トカマクの実現とICRFによる追加熱実験を計画
MHD的にかなり安定な逃走電子保持プラズマの生成
ＭＨＤ的不安定性がまだ活発なプラズマ

10. Additional Information

11. Experimental study of REs in HT-7(1)
Present status: development of various runaway electron diagnostics
experimental study of effects of LHCD on RE generation
Z.Y. Chen et al., RSI 2006, Z.Y. Chen et al., PPCF 2006
RE generation is determined by
Competition between remained
loop voltage and acceleration by
LHCD.
Experimental scenario
Experimental interests:
(1) Characteristics of disruption-produced REs
(2) How much fraction of Ip is replaced with RE current by disruption?
(3) ICRF heating of RE-sustained plasma