J. Plasma Fusion Res. SERIES, Vol.4 (2001) 191-193

MHD Activity and Disruption in Low-g(a)in lran Tokamak 1 (lR-Tl)
HOJABRI Alireza* and GHORANNEVISS Mahmoodr
Physics Dept. Islamic Azad University, Karaj, Iran.
lPlasma Physics ResearchCenter, Azad Universiy, Tehran 14835-159, Iran(Received: 5 December 2000 / Accepted: 13 September 2001)

Abstract
In the present paper, an attempt is made to explain the behavior of the magneto-hydro-dynamic(MHD) instabilities observed at low values of the edge safety factor q(a) on the basis of the experimental
results on the Iran Tokamakl (IR-Tl). A Hugill diagram for IR-T1 is shown. In addition to the wellknown low 4 and high-density limits. It is found that the disruption activity occurs when value of thesafety factor in the plasma edge is less than 2.7.

Keywords:
IR-TI tokamak, low-edge safety factor, disruption, density limit, Hugill diagram

1. Introduction

Understanding in detail the various mechanisms that canlead to disruption is important for achieving tokamak

Tearing modes are known to play an important rolein confinement degradation and disruption in tokamaks[1]. They are resistive instabilities driven by the freeenergy contained in the poloidal magnetic field. Due totheir resonant character, they are localized around flux
surfaces and change the topology of the magnetic flux
distribution through the formation of magnetic islands.
At present, there are several distinct MHD models ofconfinement, and especially so with a view tosuppressing disruptions [5]. In this paper, we present
density behavior in IR-Tl tokamak, and systemicallyexamine ohmic data during different operatingconditions.

2. Experimental Results and Discussion

tokamak disruptions, a) the interaction of a single
magnetic island of helicity (m=2)/(n=1) with the limiter

The IR-TI is a conventional tokamak with a majorand minor radii of 45 cm and 12.5 cm, respectively, andor cold-gas region ruins the energy confinement [2], b)a circular cross section without a copper shell anddivertor and using a material limiter of minor radius

the disruptions are believed to be triggered by thenonlinear coupling of modes of different helicities. In11.5 cm. A typical shot including a major disruption is
illustrated in Fig. l. In this shot, where toroidal field 0.6

cases where both the (m=2)/(n=1) and the (m=3)/(n=2)

modes are unstable, a broad spectrum of waves is
excited which destroy magnetic surfaces and therefore
strongly enhance cross-field thermal collapse in thetesla and mean electron density l.2xl0t3 cm-3 for whiche@)=2.3, the relevant major disruption sets in at 12.2
ms. The main characteristics are: l) the major decreaseof the plasma current after two small vertex close toregion of the stochastic magnetic field [3]. c) A
nonlinear interaction between the (m=1)/(n=1) and(m=2)/(n=l) modes, taking place through theintermediary of the current profile, was proposed [4].

I2.2 ms and 12.8 ms (Fig. 1a), 2) negative spikes in theloop voltage signal corresponding to plasma current

C o r r e sp ondin g aut ho r' s e - mail : alire za _hoj ab r i @ y aho o. c omt9l@2001 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchHojabri A. et al., M}{D Activity and Disruption in Low-q(a) in Iran Tokamak I (IR-TI)

the plasma appear. The classical description of thescaling for the density limit is well known as theMurakami-Hugill limit [6]. The value of the density
limit is found to vary from machine to machine [7-9].
The result is shown in Fig. 2 in Hugill-diagram. Hugill

vertexes (Fig. 1b), 3) a displacement of the plasma
column towards the inner part of the torus due toinstability (Fig. 1c), 4) decrease of the electron
temperature is shown by a fast decrease in the electron
cyclotron emission (ECE) of the plasma (Fig. 1d) and 5)(m=2)/(n=l) mode has a relatively low amplitude up toten milliseconds before the disruption (Fig. 1e). In thefirst phase of a major disruption differed from a minor
disruption by a stronger decrease in the electron
temperature at the center of plasma (Fig. 1e). Before
disruption, the ECE radiation from the plasma center
presents sawtooth like oscillations, which affect thecentral part of the discharge and level the electron

diagram showing the boundaries of the IR-TI operating
parameter space. The region is bounded below by thehigh density limit at roughly D = 3.15 (where D - qn.RlB1), above by the high current limit at q=2, and to theleft by the low density locked mode limit. For low 4, ina clean machine the low density limit is D = 0.4 [10].

This figure shows that major disruptions most likely
occur in a fairly restricted region of the Hugill space,between curves A and B (which only delineate theexperimental points). Low q (2<q< 3) discharges wereobtained in the IR-TI tokamak. It is found that thetemperature. The ECE intensity decays with thedevelopment of the (m=2)/(n=1) instability andprecursors of disruptive energy loss in the central part ofsr{oTr#26153'l
PLASMACURRENT

Ip(kA)t-(b)MIRNOV SIGNAL(A.U.)

vr(v)LOOPVOLTAGE

ILD.(cm)

HORIZONTALDISPLACEMENT
outward%inward

Fig. 1 Time behavior of principal parameters during the'l-U--

major disruption event on the lR-Tl tokamak; (a)plasma current (kA), (b) loop voltage (volt), (c)horizontal displacement signal (cm), (d) ECE

signal (eV). (e) Mirnov coil signal (arb, units). The
disruptive instabiliW appears close to 12.2 ms192

Hojabri A. et al., MHD Activity and Disruption in Low-4(a) in Iran Tokamak I (IR-TI)

Acknowledgmentsl/q(a) 0.

The authors wish to thank the entire HT-6B Group0.

z.)

0.

5of the Academia Sinica, Institute of Plasma Physics(ASIPP), for installation and helpful discussion

ota )concerning this tokamak. The first author would like togreatly acknowledge Prof. N. Hashemi, President of
Karaj Islamic Azad University, and Mr. K. Bakht, headof faculty of sciences for their hospitality. The firstauthor is also grateful to Dr. H. Zadeh, head of physics
group (IAU) for his kind hospitality.

0.1

0("'fu')v'"*'''t
Fig. 2 Hugill diagram showing the boundaries of the lRT1 operating parameter space.

References

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3. Gonclusion
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Scientia Iranica, (2OOl) in press.

tearins mode.

r93