S11-2
Development on Disruption Research with the Goal of DEMO Reactor Control

Status and Issues of Disruption Study
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Akihiko ISAYAMA/

Naka Fusion Institute, Japan Atomic Energy Agency
801-1, Mukoyama, Naka, Ibaraki 311-0193, Japan
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Disruption control in tokamaks is one of the most important issues in the present and future devices
because disruption has a large influence on the tokamak device through large heat loads andelectromagnetic forces. In addition, runaway electrons generated during the disruption need to be
suppressed because they damage the plasma-facing components through their high energy. In this paper,present status and issues on disruption study are briefly introduced by referring to recent topics discussed at
the MHD, Disruptions & Control Topical Group of the International Tokamak Plasma Activity.

1. Introduction
Disruptions in tokamaks release a large amountof plasma energy and magnetic energy in a shorttime. Heat load and electromagnetic force causedby the disruption can have a serious influence on
the plasma-facing components and tokamak
device structure. In addition, runaway electrons
can damage the plasma-facing components. For
this reason, control of disruption is an important
issue in ITER and DEMO.
Disruption study has a long history, and its
results toward ITER are reviewed in ITER
Physics Basis [1, 2]. Even with continuous efforts
and significant progress, disruption is still one of
the most important research topics in ITER andalso in DEMO. In this paper, recent topics
especially toward ITER are briefly reported; theywould also be a good reference to disruption
study for DEMO. The topics presented in thispaper are mainly the ones discussed in the MHD,
Disruptions & Control Topical Group (TG) of theInternational Tokamak Plasma Activity (ITPA),
which operates under the auspices of ITER.
2. Disruption Research Activity toward ITER
Typical temporal evolution of disruption isschematically depicted in Fig. 1. When plasma
collapse occurs due to MHD instabilities,
significant degradation of the stored energy, whichis called thermal quench (TQ), occurs. Then, adegradation of the plasma current, which is calledcurrent quench (CQ), occurs. It is considered in
ITER that the CQ time should be in the range of
50-150 ms, because too short CQ time causes large
electromagnetic force while halo current becomes aproblem for too long CQ time. During the CQ
phase, runaway electrons are generated.

Fig. 1. Time sequence of disruption.
The ITPA MHD TG, which consists of 42
members from the ITER parties and the ITER
Organization, discusses issues on MHD stability,
such as sawtooth oscillations, neoclassical tearing
modes and resistive wall modes. Issues on plasma
control and error field are also discussed. Amongthem, disruption is the most important issues
because a wide variety of complicated events occur
in a short time as seen in Fig. 1. Multi-machine
experiments and analyses on disruption are inprogress. Typical ongoing research topics are as
follows:- Disruption mitigation by massive gas
injection (MGI)- Disruption database development- Runaway electron generation, confinement
and loss- Disruption prediction- Radiation asymmetry during MGI- Halo current modelingIn these topics, asymmetry in the toroidal andpoloidal directions, i.e. 3D disruption physics, isalso an important research topic.

3. Selected Recent Topics on Disruption
3.1 Radiation during thermal quench
During the thermal quench, part of the plasmaenergy and magnetic energy is released as radiation.
It is widely observed that the radiation is
non-uniform both in the poloidal and toroidal
directions. Since such non-uniformity can cause
local heat spots exceeding the melting limit of theplasma-facing components, mitigation of thenon-uniformity is an important issue.
The effectiveness of massive gas injection fromseveral locations was investigated in severaldevices. In Alcator C-Mod, effects of the delay timeof MGI from two gas injection valves located ~180ºapart in the toroidal direction on radiation
asymmetry were investigated [3]. The asymmetry
was found to be reduced with decreasing delay inthe pre-TQ phase. On the other hand, theasymmetry became unclear in the TQ phase. Thissuggests other effects such as MHD instabilities
affect the asymmetry.
Radiation asymmetry was also investigated using
a 3D code NIMROD including MGI process [4]. Itwas found that radiation asymmetry appears evenfor toroidally symmetric gas injection due to theexistence of an m/n=1/1 mode (m and n are thepoloidal and toroidal mode numbers, respectively).
In addition, it was found that for gas injection froma localized location, the asymmetry is affected by
the relation between the phase of the 1/1 mode andthe gas injection location. Comparison with DIII-D
experiments indicated that although such 1/1 mode
structure was observed in radiation asymmetry, its
magnitude was much smaller than expected [5].
In JET, the material of the plasma-facing
components was modified to match those in ITER
(called ITER-like wall, ILW). It was found thatwhile the radiation fraction is 50-100% of the totalenergy for carbon tile cases, it is < 50% and also
down to 10% for ILW cases [6]. This indicates that
disruption mitigation is mandatory for ILW.
3.2 Halo current during current quench
When a disruption occurs and a verticaldisplacement event follows, a halo region is formed
outside the last closed flux surface. Since anelectrically closed path is formed through the haloregion, plasma-facing components, divertor andvacuum vessel, large electromagnetic forces arise inthese regions. Halo current distribution measured atdifferent toroidal and poloidal locations indicatedthat there is significant non-uniformity. In addition,
rotation of the asymmetric structure, which canaffect the tokamak device through the naturalperiod of the structure, was also observed [7].

Although the temperature profile at the halo region
affects the decay of the plasma current, it is difficult
to measure it in experiments. To investigate thetemperature profile at the halo region, halo currents
measured in several poloidal locations and thosefrom DINA code calculations were compared. It
was shown that by adjusting the temperature and its
profile at the halo region, measured and calculated
values agree well each other, in addition to theagreement of the plasma current evolution [8].
3.3 Runaway electrons
During the CQ phase, runaway electrons (REs)appear and grow. Since the REs can damage thefirst wall because of their high energy, it isnecessary to establish how to mitigate REs. Several
methods are proposed to mitigate and/or control
REs. One is to inject massive materials like gas andpellets to reach the critical density at which REs
decay through collisions. Experiments on several
devices such as ASDEX-U and JET indicated thatat present reachable electron density is far below
the critical density. Another method is to control therunaway beams until the plasma current becomes
low enough. Experiments on some normalconducting devices demonstrated that RE beam canbe controlled. DINA code simulations at ITER
parameters indicated that under some conditions,e.g. disruption detection ~1 s before TQ and plasma
position control, RE beams can be controlled until
the plasma current including RE component
decreases to ~2 MA [9]. Furthermore, the electricfield at which REs are generated was investigatedunder the ITPA Joint Experiment [10]. It was
shown that the electric field strength at which REs
are generated is larger (3-5 times or more) than thatpredicted by relativistic collisional theory,suggesting the existence of other RE loss
mechanisms.
References[1] ITER Physics Expert Group on Disruptions, PlasmaControl, and MHD, Nucl. Fusion 39 (1999) 2251.
[2] T.C. Hender and J.C Wesley et al., Nucl. Fusion 47(2007) S128.
[3] R.S. Granetz et al., Proc. 24th IAEA FEC (2012);
G.M. Olynyk et al., Nucl. Fusion 53 (2013) 092001.
[4] V. Izzo, Phys. Plasmas 20 (2013) 056107.
[5] N. Commaux et al., Phys. Plasmas 21 (2014)
102510.
[6] M. Lehnen et al., Nucl. Fusion 53 (2013) 093007.
[7] S.P. Gerhardt, Nucl. Fusion 53 (2013) 023005.
[8] Y. Nakamura et al., Proc. 37th EPS Conf. on Plasma
Physics, P2.184 (2010).
[9] V.E. Lukash et al., Proc. 40th EPS Conf. on Plasma
Physics, P5.167 (2013).
[10] R.S. Granetz et al., Phys. Plasmas 21 (2014) 072506.