23P078-P
Study of Plasma Collapse in Plasma Current Ramp-Down Phaseof Tokamak and Helical Devices
トカマク・ヘリカル装置における電流ランプダウン時のプラズマコラプスの比較
Yoshihide Shibata, Kiyomasa Watanabe1, Noriyasu Ohno, Masaaki Okamoto2,Akihiko Isayama3, and Yasunori Kawano3
柴田欣秀, 渡邊清政1, 大野哲靖, 岡本征晃2, 諫山明彦3, 河野康則3
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan
名古屋大学工学研究科，〒464-8603	  名古屋市千種区不老町
1
National Institite for Fision Science, Oroshi-cho, Toki, Gihu 509-5292, Japan
自然科学研究機構 核融合科学研究所, 〒509-5292	  岐阜県土岐市下石町322-6
2Ishikawa National College Technology, Kitachujo, Tsubata, Kahoku-gun, Ishikawa, 929-0392 JAPAN
石川工業高等専門学校, 〒929-0392	  石川県河北郡津幡町北中条タ1
3
Japan Atomic Energy Agency, Mukouyama, Naka, Ibaraki 311-0193, Japan
日本原子力研究開発機構, 〒311-0193 茨城県那珂市向山801
We investigated the current behavior of JT-60U and LHD in the current ramp-down phase. In the current
ramp-down phase in the 2008 JT-60U experimental campaign, the positive spikes of plasma current wereobserved in the end of many current ramp-downs. The positive spike of plasma current was generated by
the rapid change of electron temperature profile. On the other hand, in the LHD discharges with the net
toroidal current by unbalanced Neutral Beam Injection (NBI), the positive spike of plasma current is notobserved at the end of current ramp-down.

1. Introduction
In tokamak device, the plasma current isnecessary to maintain MHD equilibrium, and theelectromagnetic energy of plasma in large tokamak
device such as ITER and demo-reactors is huge.
Therefore, the time change of plasma current has animpact into device such as vacuum vessel. Because
the damage generated in the device is large during
the disruption, the study of determination
mechanism and control method of plasma current
decay in disruption has been performed [1].
On the other hand, when the plasma discharge
terminates without the disruption, it is necessarythat plasma current decrease to zero. In the end ofcurrent ramp-down phase, it is expected thedestruction of the MHD equilibrium and rapid
release of electromagnetic energy. However, theearly research for the plasma behavior in the end ofcurrent ramp-down phase is poor. In this study, weinvestigate the behavior of plasma parameter during
the current ramp-down phase of JT-60U in order toclear up the influence of dissipation of MHD
equilibrium on the termination of plasma discharge.
And we investigated the behavior of plasma
parameter during the current ramp-down phase of
LHD in order to compare the results in tokamak andhelical devices because the plasma current is notnecessary to maintain the MHD equilibrium in thehelical device.

2. Current Ramp-Down Phase in the JT-60U
Fig. 1 shows the developmental rate of positive
spike of plasma current during current ramp-downin 2008 JT-60U experimental campaign. As shownin Fig. 1, it was found that the positive spike ofplasma current occurred in many current rampdown phases. However, the reason why that thepositive spike of plasma current occurred during thecurrent ramp-down is not understood.
In order to investigate the behavior of plasmaaround the on-set time of the positive spike of theplasma current, we paid attention to the end ofcurrent ramp-down phase. Time evolution of theplasma current, the electron temperature profile,and the safety factor of plasma surface in theFig. 1 Developmental rate of the positive spike ofplasma current during current ramp-down phase in
the 2008 JT-60U experimental campaign.

Fig. 2 Typical Waveforms of (a) Ip, (b) Te profile,(c) Te at plasma center, Te0, and (d) qsurf in the end ofcurrent ramp-down phase of the JT-60U.

Fig. 3 Time evolution of (a) Ip, (b) a radial profile ofRTe, (c) RTe at plasma center, an areal mean of RTe
profile, and the current decay rate normalized by Ip.

current ramp-down phase of the JT-60U are shownin Fig. 2. Te profile is measured from an ECE
diagnostic system. Because qsurf cannot be evaluatedby using the magnetic sensors' signals and theMHD equilibrium calculation code, the safety
factor of plasma surface is evaluated from ECE
measurement and the following equation [2]:qsurf = 2πa 2 Bt µ 0 Ip R0 .
(1)
Here Bt and a are the toroidal field and plasma
minor radius, respectively. Supposing that theplasma edge is the boundary of ECE measurement,€the S is evaluated. As shown in Fig. 2 (b), Te
gradually decreases before the positive spike ofplasma current. qsurf gradually decreases just Te
decreases as shown in Fig. 2 (c). The rapid decay of
Te occurs after Te at plasma center becomes about
200 eV, and the positive spike of plasma current isobserved. The rapid decay of Te profile is observedat the disruption phase in many devices before thepositive spike of plasma current. The above resultssuggest that the disruption occurs during the current
ramp-down phase in the JT-60U discharges. As
shown in Fig. 2 (d), qsurf decreases just before therapid decay of Te profiles. That means the plasma at
the time, when the positive spike is observed, ismore favorable for MHD equilibrium than that
before the appearance of the positive spike ofplasma current. This result suggests that disruption
is triggered by a growth of instability in the JT-60U.
3. Current Ramp-down Phase in the LHD      In the LHD, net toroidal current is not necessary
to confine the plasma because the helical devices
always keep magnetic surfaces only by the externalcoils. In this paper, we focus on the current
termination of the discharges maintained by theunbalance of NB injection. In order to investigate
the reason why the positive spike of plasma current
is not observed just before the rapid decay of theplasma current in the current ramp-down of theLHD, we evaluated the time evolution of radiation
temperature, RTe, profile by using ECE diagnosticsystem. It should be noted that RTe means theelectron temperature that is not considered about
the effect of optical thickness of ECE measurement
here. Fig. 3 shows the time evolution of Ip, a radial
profile of RTe, RTe at the plasma center, an areal
mean of RTe profile, and the current decay ratenormalized by Ip value, dIp/dt/Ip. In Fig. 3, the rapid
decay of the plasma current starts at t = 3.764 s
when RTe at the plasma center is less than 100 eV.
However, the positive spike of plasma current was
not observed before the rapid decay of plasma
current in the LHD. During the rapid decay ofplasma current, RTe and Ip slowly decrease
comparing with the rapid decay of plasma current
in the JT-60U. These results suggest that the rapid
decay of plasma current in LHD is not triggered bya strong MHD instability.
4. Summary
The behavior of plasma current during the current
ramp-down was investigated in the JT-60U and theLHD. The disruption triggered by a strong MHD
instability was observed in many current rampdowns of the JT-60U. On the other hand, in the endof the LHD discharge, the positive spike of plasma
current was not observed before the rapid decay ofplasma current occurred. These results suggest thatthe rapid decay of plasma current in LHD is not
triggered by a strong MHD instability despite theJT-60U. In future work, we need to investigate thebehavior of plasma ramp-downs in which thepositive spike of plasma current was not observed
to investigate the determination mechanism ofdisruption in the end of current ramp-down phase.
Reference[1] T.C. Hender, et al., Progress in the ITER PhysicalBasis Chapter 3, Nucl. Fusion, 47(2007) S128.
[2] J. Wasson, Tokamaks third edition (Oxford Science
Publications, Oxford, 2004) chapter 3.