J. Plasma Fusion Res. SERIES, Vol.2 (1999) 69-72

Gharacteristics of Low-q Discharge
in the lR-Tl (lran-Tokamak 1l
GHORANNEVISS Mahmood, MASNAVI Majid and KASHANI Mehrdad
Plasma Physics Research Center, Azad University, Tehran 14835-159, Iran(Received: 8 December 1998 / Accepted: 13 May 1999)

Abstract
This paper presents an experimental study on the hard disruption instability in low-4r(a) discharge
on the Iran-tokamak I (IR-Tl), that limits the accessible operation plasma parameters. Reproducible
major disruptions are triggered by contact of a large mln = 2ll magnetic island with the limiter. The
growth of m|n = 2ll mode is precipitated by a thermal collapse of the plasma periphery during the current
decay phase.

Keywords:
hard disruption instability, locked mode, critical error field

1. Introduction

instability on confinement. Diagnostics and operational

region of the IR-TI tokamak have already been

Operation at low-edge safety factor (qw@\ < 3) andthe corresponding magnetohydrodynamic (MHD)
characteristics have been investigated in the IR-Tltokamak. It is necessary, for a tokamak operating atpresented [3,4].

2. Description and Discussion of theExperimental Results
The IR-TI is a conventional tokamak with aspect

low-qr(a) values without a conductive shell and using amaterial limiter, to make a careful adjustment of theratio (R/a = 45 cmll2.5 cm) and a circular poloidal
limiter of minor radius 0.115 m. This has allowed
current operation (Ip<60 kA ) at low toroidal fields (Baplasma parameters and have an appropriate ramp-up rate

ofplasma current in the startup phase and low effective
ion charge (Z.fi.Inlower-qr(a) discharges (qr(a) < 4),locked modes are often followed by a disruption, andalso observed in many tokamaks [1,2]. Locked modes
are usually n = I and m = 2 or 3 modes which can either
grow in a stationary position in the torus or evolve froman oscillating mode whose rotation frequency slows
down to zero. Here m (n) is the poloidal (toroidal) mode

< 0.9 tesla). This work draws on previous experiences[4,5], over a significant range of parameters whichshowed that the MHD behavior can be characterized by

the single parameter ey@). The starting point for thisstudy occurs for 2 < qv@) < 3 and is typified by strong
oscillatory signals on both soft x-ray and magnetic coils
and dominatedby mln = 2/1 spatial harmonic. A typicalmajor disruption on the IR-TI tokamak is shown in Fig.
l, where toroidal field Ba = 0.64 tesla and mean electron
density fi"= 1.4 x l0l3cm-3. The characteristics of thedischarge are summarized in Table (I). As the current isnumber. Characteristics of discharges with locked
modes include cessation of soft x-ray (SXR) sawteeth,
reduced energy confinement and cessation of the plasmamode rotation at safety factor q = 2, etc. [1]. In thepresent work we have studied characteristics of such
fluctuations in lower and higher edge qr(a)-discharges.
In particular, we investigate effects of m = 2 modeincreased, instabilities occur which degrade confinement@1999 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author's e-mail: masnavi@ sinasoft.net
69

Ghoranneviss M. et al., Characteristics of Low-q Discharge in the IR-Tl (Iran-Tokamak 1)

thtatu)2I0l'',",r,-""Signd"ouS&ab.U.)
21o(D sft X-rry sgmt

oukrrdK\ab.U)H o(cm)
20I0G) Elahm cyclotronRadiation(c) Horironhl dbplacement

signilq,(a)[_-,(h) Srfcty Frcbr st the(d) Hydrcgen rrdoftn

Plsms Fig.

Slgnd

0 6 t2 18 24 300 6 12 18 24 30
Time(MS)

Time(MS)

Fig. 1 Time dependence of plasma parameters (Shot No. 151,96) in the major disruption event on the lR-Tl tokamak; (a)plasma current (kA), (b) loop voltage (volt), (c) horizontal displacement signal (cm), (d) Hcr signal (arb. units), (e)pick-up coil signal (arb. units). (fl soft x-ray signal (arb. units). (g) ECE signal (arb. units) and (h) time evolution of
the safety factor at the plasma edge qr(a). The disruptive instability is appeared close to 11.5 ms. Disruption
occurs during ramp-down of plasma current, that is caused by the decrease of the safety factor at the plasmaedge. Positive plasma current tips and negative loop voltage spikes are clearly observed.

Table ( | ) DISCHARGE CHARACTERISTICS
Before disruption

After disruption

Toroidal field (tesla)
Plasma current (kA)
Safety factor at the plasmaedge (q,(a))

0.67

o.62

54

54-+0

2.3

2.3 -->

Loop voltage (V)

3.5

18

Plasma energy content (kJ)

0.89

0.02

Ouantity

Main ion species

H.

Limiter radius (cm)
Major redius (cm)Minor radius (cm)1',t.545
12.570*Ghoranneviss M. et aI., Characteristics of Low-q Discharge in the IR-TI (Iran-Tokamak l )and restrict the range of useful operating conditions.
Instabilities have been observed on the plasma current,loop voltage, horizontal displacement, magnetic

pick-up coil is located. For this shot and m = 2 mode,r"coir = 18 cm, rq=2= 8-10 cm and (6B0 lBi = 0.007
corresponds to 6= 1.5 cm. So,the full island width ofcoil,electron cyclotron emission (ECE, 5-channels
heterodyne radiometer) t5l , Hd signal (,?, = 6563A, 32) and an array of soft x-ray detectors. Several
characteristic features of the disruptive instability arethe m = 2 mode is d = 3 cm, and therefore thedisruption could be the result of a contact of the m = 2
magnetic island with the limiter.

identified. The post-disruptive plasma is characterized

by a very high level of magnetic turbulence,corresponding to relative large fluctuation of thepoloidal field at location of the Mirnov coil, mainly at n
= I toroidal wavenumber. The thermal quench occurs
rapidly as evidenced in the sudden drop of the near
central soft x-ray amplitude. The soft x-ray emissivity
enhances suddenly before the disruption, and sawteeth
were evident from 4 to 7 ms. The stored energy of theplasma also decreases from a maximum of about 0.89 kJ
to 0.02 kJ by the end of the phase of the thermal

Mlnov Coll Stgnalwlth
Safcty F.ctor:

2qI-(a) : 2.5

olt1firDov Coll Slg[al wtth

Safefyquench. The increase of Ha radiation between one andq

2two orders of magnitude during the post-disruptive is1Factor:-(a) : 2.9oclearly associated with a thermal collapse of the plasmaboundary. As is seen on horizontal displacement signal,
the plasma moves inwards to interacts with the limiter,
which is evident by pulsed increases in the emission of
the Hc signal. This means a steadily increasing heatloading on the limiter, which could be the cause of theincrease in Ha radiation. During the post-disruptive
phase, the rate of current decrease is often -17 kA'ms-l
for typical shot considered. Analysis of MHD activity
by means of magnetic coils shows that the only modeseen on IR-TI during the plasma current plateau isusually specified by mln = 2/1. Although all steady state
low qr(a) discharges are sawtoothing, the mln = 2ll

15030Time(MS)Fig. 2 Comparison of discharge evolution during theOhmic heating phase for two discharges, one withqvla't = 2.5 (top panel), and one with gr(a)= 2.8(bottom panel) before the discharge disruptions.

mode grows to a large value before disruption as%(EB o/ B o)observed on many tokamaks. This phenomenon occurs
in the narrow window at high current limit in the Hugill
plot of the IR-Tl [4]. The relative amplitude (68stBo)

Iof the magnetic perturbations induced by this m = 20.8

IFmode is, under stable conditions, in the l0-a-10-3 range
and its frequency is in the 18 kHz < f < 22 kHz range.
This frequency is quit in agreement with a scaling lawof the critical relative intrinsic error field at the plasma
surface for locked modes / - R'etsB'tts [l], has beenobtained in Ohmic discharges with safety factor qr(a) <ileIF*rF

0.6
0.4

4. In this paper we have followed convention bydefining ev@) in its cylindrical approximationi ev@):nr)2nBra2lltolrR. The amplitude of the observed signal canbe related to the half-width (6) of the magnetic island atredius rn=2 through the formula [6]: 6 = [2rl=iry",, /m)(6BolBdl l/2, where rcou is the redius at which the2.6

3q,, (a)Fig. 3 Maximum MHD amplitude against eo(a). The flat

top value was varied from shot to shot and themaximum amplitude was measured in each case.

71

Ghoranneviss M. et al., Characteristics of Low-q Discharge in the IR-T1 (Iran-Tokamak l)

An overview of the two types of low-qfa\Institute of Plasma Physics (ASIPP) of China, fordisruption scenarios is shown in Fig. (2). In contrast tohelpful discussion concerning this tokamak. Thanks andthe higher-4v, (a) shot, the lower-q, (a) discharges

appreciation are also expressed to the National Institutefor Fusion Science (NIFS), Japan Society for Promotion
Sciences (JSPS) and Chinese Academy of Sciences(CAS) for supporting fully Mr. Majid Masnavi's trip toexhibited relatively low level of MHD activity up until

several of milliseconds before the disruption. Thisbehavior is quite similar to the result presented by Kayeet al. [7], on the Princeton Beta Experiment (PBX).
The essential features of the results presented here
are summarized graphically in Fig (3). This diagram isChina to participate in '96 Asian Science Seminar',
Frontier of Physics in Fusion Relevant Plasmas. Also,
the authors would like to thank the technical staffofthe

derived from data taken from twelve different

laboratory for their upkeep of the IR-TI and indischarges showing the relation between the amplitudeof the exterr.rally measured poloidal field fluctuationsagainst the value of the safety factor at the plasma edge.

particular, Yahya SADEGHI for his efforts in handling

3. Conclusion

lll R.J. La Haye et al.,Phys. Fluids B 4,2098 (1992).

the data acquisition system.

References
To summarize, low-qr(a) operation in IR-TI has

t2l J.T. Scoville et a/., Nuclear Fusion 31, 875 (1991).
t3l M. Ghoranneviss, M. Masnavi et al., Scientia

indicated that disruptions always occur for discharges
with q* (a) < 3.The major disruption studied here
consist of three phases. First, a thermal instability,
secondly, growth of the m = 2 island to a large size andthirdly, the trigger mechanism, which is the contact of
this large m = 2 island with the limiter. The otherconclusion from this study is that in contrast to higher4,r (a) shot, the lower-qr (a) discharges exhibited

Iranica 10,73 (1999).

t4l M. Ghoranneviss, M. Masnavi et al., J. Sciences
LR. Iran, in press.

t5l M. Ghoranneviss and M. Masnavi, J. Plasma
Fusion Res. Series 1, 281 (1998).

t6l A.D. Cheetman et aI., Aust. J. Physics 39, 35(1986).

relatively low level of MHD activity before the[7] S.M. Kaye et al., Nuclear Fusion 28,1963 (1988).

disruption.

Acknowledgments
The authors wish to thank the entire HT-6B Groupfor installation of the IR-Tl, particularly professor
Jikang XIE and Dr. Rong HUANG in Academia Sinica,72