J. Plasma Fusion Res. SERIES, Vol.3 (2000) 214-217

Poloidal Magnetic Field Fluctuations in Low-q(al inlran-Tokamakl (lR-TlI
Mahmood Ghoranneviss, Alireza Hojabri and Davood Doranian*
Plasma Physics Research Center,Islamic Azad University, Tehran 14835-159, Iran.

(Received: 18 January 2000 / Accepted:22 June 2000)

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 on the basis of experimental results
obtained on the Iran Tokamak I (IR-TI). The major disruption studied here consist of three phases: l)thermal instability, 2) growth of the magnetic island, 3) trigger mechanism. Estimates are given for therateofislandgrowthandthecharacteristicgrowthtimeofthem=2/n=Imode,c'andthe
experimentally and theoretically deduced values are found to be in reasonable agreement with each other.

Keywords:
IR-TI Tokamak, low edge safety factor, poloidal magnetic-field fluctuations, rate of island growth.

1. Introduction

thermal collapse in the region of the stochastic magnetic
field [4]. In the third model of disruptions, a nonlinear
interaction between the m = l/n = I and the m = 2ln = |modes, taking place through the intermediary of thecurrent profile, was proposed [5,6]. Understanding indetail the various mechanisms that can lead to disruption

The present paper reports on detailed investigations

into the confinement properties of low-q(a) discharges(i.e., discharges with 2 < q(u) < 3) and the MHD
characteristics of these discharges. Tearing modes areknown to play an important role in confinement
degradation and disruptions in tokamaks [,2]. They are
resistive instabilities driven by the free energy contained

is important for achieving tokamak confinement, andespecially so with a view to suppressing disruptions [7].
The purpose of this paper is to further investigate thein the poloidal magnetic field. Due to their resonant
character, they are localized around flux surfaces andchange the topology of the magnetic flux distribution

major disruptions occurring in low-q(a) discharges in

IR-TI, and to compare the theoretical and experimental
results for the rate of island growth. Experimental
evidence has shown that disruptions are connected with
MHD activity and that the time scale involved isintermediate between the resistive and the ideal MHD
time scales [8]. Diagnostics and operational region ofthrough the formation of magnetic islands. At present,
there are several distinct MHD models of tokamak
disruptions. According to the first model, the interactionof a single magnetic island of helicity m = 2ln = I withthe limiter or cold-gas region ruins energy confinement[3]. In a second scenario, disruptions are believed to be

the IR-TI have already been presented [9,10].

triggered by the nonlinear coupling of modes ofdifferent helicities. In cases where both the m = 2ln = |and the m = 3ln = 2 modes are unstable, a broad
spectrum of waves is excited which destroy magnetic

2. Experimental Results and Discussion
The IR-TI is a conventional tokamak with a major
and minor radii of 45 cm and 12.5 cm, respectively, andsurfaces and therefore strongly enhance cross-field*@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author's e-mail: doran@ azad.ac.ir214

Mahmood G. et al., Poloidal Magnetic Field Fluctuations in Low-q(a) in lran- Tokamakl(IR-Tl)
PLAAMACUNNT(I.'I--)rI-z'-tI'II'-I\' ''A--

Hc ruDtaTtoN--tJr-'ttl'fl* llf r'(.-u.)ll*--[dI

TloN (.v)''"tt*rfft/f'6'"-l|LhL

Fig. 1 Time behavior of principal parameters (shot No.
132) during the major disruption event in the lR-

T1 tokamark; (a) plasma current (kA), (b) loop
voltage (volt), (c) horizontal displacement signal(cm), (d) Mirnov coil signal (arb. units), (e) H.
signal (arb. units), (f) ECE signal (arb. units). The
disruptive instability appears close to 13.5 ms.

a circular cross section without a copper shell anddivertor and using a material limiter of minor radius
11.5 cm. A typical shot including a major disruption isFig.2 Plasma current (a), loop voltage (b), horizontal

illustrated in Fig. l. In this shot, where toroidal field 0.6
tesla and mean electron density 1.1 x l0l3 cm-3, forwhich q(a) = 2.5, the relevant major disruption sets in at

displacement signal (c), Mirnov coil signal (d), H,signal (e), ECE signal (f) in expanded time-scale

12-17 ms before and during the disruptive
instability corresponding to the discharge shown

13.5 ms. The main characteristics are : the majorin Fig. 1.

decrease of the plasma current after vertex close to 13:5
ms (Fig. la), negative spikes in the loop voltage signal

high level of magnetic turbulence corresponding to a
relative large fluctuation of the poloidal field at location(Fig. 1b), a displacement of the plasma column towards
the inner part of the torus due to instability (Fig. 1c),
2r5

Mahmood G. et al., Poloidal Magnetic Field Fluctuations in Low-q(a) in lran- Tokamakl(IR-TI)

oilr

Mirnov signal(a.u) ,'Ir'0,

13Time(ms)Fig. 3 Poloidal magnetic field fluctuation for a Shot No.
132 on an expanded time-scale 12-13 ms. ThisTIME(ms)figure shows the final, approximately explosive
groMh of the disruption precursor mode. The
dashed line ashematically indicates the approximate explosive groth of the oscillatio envelope.

Fig. 4 The frequency and relative amplitude 64/8" as afunction of time.

measured by a Mirnov coil during the time interval l2-of the Mirnov coil (Fig. ld), loss at the edge (Fig. le)and decrease temperature that observed in electron

13 ms. It was observed to increase slowly at first and tothen undergo rapid growth. Figure (4) shows as

cyclotron emission (Fig. 10.
As shown, when the plasma is pushed inwards inmajor radius it causes a positive spike on the loopvoltage. The major disruption is preceded by a series ofminor disruptions not resolvable in Fig. l. In figure (2)we show the evolution of the plasma current, the loopvoltage, the horizontal displacement, the MHD activity
and the ECE signal in expanded time-scale 12-17 ms

measured by integrated Be and the measured frequency fof the m = 2 mode as a function of time at 12-13 ms.
This figure shows that fiBelBe grows and mode
frequency decreases.
The average time scale of the final growth is about
60-100 ps. The time scale for ideal-mode growth isgenerally bounded from below by the ideal time scale
and from above by the resistive time scale. Typical ideal
time scales are given by the poloidal Alfv6n time, z, =before and during the disruptive instability
corresponding to the discharge shown in Fig. l.
The approximately explosive growth of thealVl where Vo is the Alfvdn velocity, and the resistive

precursor activity is presented at time 12.6 ms andappears to be correlated with a decrease in temperatureobserved in the ECE signal as shown in Fig. 2f, andincrease ,FIo dradiation (Fig. 2e). The increase of I1oradiation between two and three orders of magnitude
during the pre-disruption phase is clearly associatedwith a thermal collapse of the plasma boundary. Thecollapse phase is punctuated by a series of minor
disruptions. The macroscopic effects of the minor

time scale [2],+aO.6v
!S =- !R
^aO.4!A(l)where ?o = poa"/q(O) is the resistive skin time, with theresistivity at the plasma center. In the IR-TI tokamak,
the Alfv€n velocity is about l.15 x 106 m/s and \(0) =6x 10-7 Qm so that tA = 0.1I ps and ?p = 3l ms. By using

formula (1) we find the resistive time scale to be about
200 1ts. These growth time values span the range between those expected from ideal and resistive MHD
time-scale.Therefore. we can estimate the rate of island
growth, dWldt, to lie in the range 3.6 - 6 x lOa cm/s. It
is instructive to compare these with the predication ofdisruptions are illustrated in Fig. 2. Each minor
disruption is accompanied by a positive voltage spike, aburst in both MHD activity and Horadiation and a dipin electron cyclotron harmonic emissions: however, thetotal plasma curent remains unaffected (apart from athe Rutherford non-linear theory :slight decrease accompanying the occurs minor

O* _Nxr1e)disruption) until the major disruption. We now consider

dt

Foa>

the characteristic time r, for the growth of the m = 2
mode from an amplitude 680180= 0.004 to a large value> 0.01 at the onset of disruption. Figure (3) shows thewhere 4 is the resistivity and A' is the jump in the logarithmic derivative of the perturbed helical flux function

oscillation in the poloidal magnetic field. Ii6, asof order I cm-r 1111 and take 4(0) = 6 x l0-7 Clm. If we

U/ across the resonant surface I I 1]. We assume that216/' isMahmood G. et al., Poloidal Magnetic Field Fluctuations in Low-q(a) in lran- Tokamakl(IR-TI)"t"(6Be/Be)

Mirnovsigml(r.u.)q(a)43Fig. 5 The measured amplitude 68r/8, ol the 2/1 mode
as a function of q(a).

make the further assumption that /'does not change significantly during the rapid growth phase then (2) pre-Tine(n$dicts dWdt = 5.2 x lOa cm,/s. Despite the considerable
uncertainties involved in both the experimentally andthe theoretically deduced values of dVdt, they appear to
be in reasonable agreement. We determined the ratio of
the amplitude of the poloidal magnetic field fluctuation
over the equilibrium poloidal magnetic field by using
Mirnov coil signals in several shots with different q(a),the results being plotted in Fig. (5) $2). It is clear fromthe figure that 586/Bs exhibits a peak as a function ofq(a). The peak in question corresponds to q(a) = 2.5 andis accompanied by a large amount of MHD activity. InFig. 6, a Mirnov oscillation is observed at e(a) = 2.1. 11Fig. 6 Mirnov oscillation at q(a) = 2.3 in another shot.

Acknowledgments
The authors wish to thank the entire HT-6B Groupfor installation of the IR-TI, particularly professor J.Xie
for helpful discussion concerning this tokamak.

References

FlWesson, J.A. et al., Nucl. Fusion 29,641(1989).

[2] Schuller, F.C., Plasma Phys. Contr. Fusion 37,A135 (1995).
[3] Sykes, A. and Wesson, J.A., Phys. Rev. Lett. 44,

contrast to the higher-q(a) shot, the lower-q(a) discharges exhibited a relatively low level of MHD activity
up to several milliseconds before the disruption. Thisbehavior is quite similar to the result presented by Kayeet al. |31for the Princeton Beta Experiment (PBX).

l2ls (1980).

[4] Hicks, H.R. e/ a/., Nucl. Fusion 22,ll7 (1982).
[5] Turner, M.B. and Wesson, J.A., Nucl. Fusion 22,1069 (1982).

[6] White, R.B. er al., Phys. Rev. Lett. 39, 1618
3. Gonclusion
In this work we have described detailed
measurements of disrupting hydrogen plasmas in the(re17).

[7] Vannucci, A. et c/., Nucl. Fusion 31,ll27 (1991).
[8] Jahns, G.L. et al., Nucl. Fusion,2E,881 (1988).
[9] Ghoranneviss, M., Masnavi, M. et al., Scientia

ohmic-heating regime. The major disruption studied
here consist of three phases: 1) thermal instability, 2)
growth of the magnetic island, 3) trigger mechanism.
The corresponding islands grow so rapidly, at rates

Iranica 4,71 (1997).
[0] Ghoranneviss, M., Masnavi, M. et al., J. Sciences
I.R.Iran, 10, 7 3 (1999).

consistent with the Rutherford non-linear tearing mode
theory, that they reach size sufficient to cause
disruption. The rate of island growth is about 3.6 - 6 x
104 cm/s, corresponding to a characteristic growth timez, of 60-100 ps. These values are in reasonableagreement with theory. The other conclusion from thisstudy is that the magnitude of z, poloidal fluctuations(Mirnov oscillations) depends on the value of the safety[11] Rutherford, R.H., Phys. Fluids 16, 1903 (1973).
[2] Ghoranneviss, M., Masnavi, M. et al., J. Plasma
Fusion Res., Series Vol. 2, 67 (1999).
ll3l Kaye, S.M. et a/., Nucl. Fusion, 2E,1963 (1988).

factor at the limiter, q(a).

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