J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)638–641
000–000

The Study of Fluctuations at the Edge of CT-6B Tokamak
KHORSHID Pejman1, WANG Long2, GHORANNEVISS Mahmoud, AGHAMIR Farzin M.3,
YANG Xuanzong2 and FENG Chunhua2
Plasma Physics Research Centre, Islamic Azad University, Tehran, Iran1
Dept. of Physics, Islamic Azad University, Mashhad, Iran
2
Institute of Physics, Chinese Academy of Sciences, Beijing, China
3
Dept. of Physics, University of Tehran, Tehran, Iran(Received: 9 December 2003 / Accepted: 3 October 2004)

Abstract
Edge plasma fluctuations of CT-6B tokamak based on Mirnov oscillations, a moveable limiter and Hα line
emission were investigated through the application of a positive limiter biasing, changes in gas pressure and plasma
displacement. The results show that when the vessel gas pressure increases, the initiation of magneto-hydrodynamic(MHD) activity occurs in high plasma current and the main frequency of magnetic oscillations which corresponds tothe maximum amplitude of the magnetic perturbation increases in direct correlation with increases in gas pressure.
Positive biasing reduces MHD oscillations frequencies, initially, but after a short delay time, the main frequency ofoscillations increases. Effects of positive biasing on the magnetic, electrostatic and Hα fluctuations are also presented.
Keywords:
magnetic fluctuation, electrostatic fluctuation, limiter biasing, Mirnov oscillations, tokamak

1. Introduction

poloidal limiter. The vacuum chamber was a stainless steel
welding structure with two toroidal breaks and a minor radius
b=0.15 m. An array of twelve Mirnov coils is used formeasuring the poloidal magnetic field oscillations and asegment poloidal limiter made of thin molybdenum plate,with 8cm height and 2 cm width, electrically isolated withthe chamber positioned at r=0.125 m on the outer side of theequatorial midplane of the plasma, was employed as thebiasing limiter, where r is the minor radius. The biasing
voltages was continuous. It was applied on the plasma at somemoment (starting time) during the plasma current plateau andended after the discharge termination. The biased voltage wasrestricted to –125 V≤Vbias ≤220 V. The array of Mirnov coilsand Hα spectrometer detectors were mounted at toroidally 75°
and 90°, respectively, from the limiter-biasing device. The
horizontal displacement of the plasma column, ∆x, wasmeasured with a set of cosine coils and saddle coils. The
value of ∆x can be controlled with a feedback system ofvertical magnetic field in each discharge.
The conditions of the experiment were as follows: The
toroidal magnetic field Bt =6.5–7.5 kG, plasma current Ip =17–25 kA, chord-averaged electron density n̄e=0.5–1.5×1019 m-3in hydrogen and the plasma discharge duration ~30 ms. The
data were digitized at 6.4 µsec resolution using a multichannel data acquisition system.

Plasma turbulence is considered one of the main causesof anomalous transport in toroidal magnetic confinement
devices. Edge biasing experiments have been found to beimportant in modifying edge turbulence and transport, but themechanism of biasing penetration in edge fluctuations and its
levels are different with respect to devices operation. The
magnetic and floating potential fluctuations and their levels
have been shown as addresses of the edge turbulence andparticle transport. [1-10]
In the present research, an experimental study for edge
fluctuation in the regimes with and without limiter biasing iscarried out. Without biasing, the effect of changing chamber
gas pressure, Pgas , and plasma horizontal displacement, ∆x,
on magneto-hydrodynamic (MHD) activities using Mirnov
coils are investigated. During biasing regime, the effects of apositive limiter biasing on the plasma floating potential,
poloidal magnetic field and Hα emission fluctuations are alsoexamined.
In the sections to follow a description of the experiment,
its findings and conclusion are presented.

2. Description of the experiment
The experiments were conducted on the ohmically heatediron core CT-6B tokamak, with a major radius R=0.45 m anda minor radius a=0.125 m defined by a fixed four-block©2004 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author’s e-mail: khorshid@mshdiau.ac.ir
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Khorshid P. et al., The Study of Fluctuations at the Edge of CT-6B Tokamak

3. Results and discussion
At the beginning of the experiment, a typical discharge
condition for observing the typical behavior of sinusoidal
Mirnov coils oscillations with clear and large MHD activity
was selected. In Fig. 1, the points show the conditions forgas pressure, plasma displacement and plasma current whena clear sinusoidal oscillation is seen in the Mirnov coils. Thepoints are from shot by shot experiments. By changing ∆x
via a feedback control vertical system to maintain the plasma
column at a given horizontal position, and Pgas , the thresholdof Ip about MHD activity changed, too. These experiments
are conducted under two different pressures, 7.0×10-5 torr and8.5×10-5 torr. As the vessel pressure increases, a clear MHD
behavior is initiated in high plasma current, 20 kA, andthrough a lowering of pressure the beginning of MHD
activities occurs in low plasma current, 17 kA.
In addition, the poloidal magnetic field fluctuations weredetected through shot-to-shot experimentation by varying
chamber’s gas pressure, Pgas =1.0×10-4 torr to Pgas =1.6×10-4
torr, on a fixed plasma displacement at a value of 1.4 cm.
Subsequent to the taking of Fast Fourier Transform (FFT)from Mirnov signals, it was observed that the frequency ofoscillations increased in direct relationship to increases in gas
pressure. Yet, in another set of experiments, the chamber gas
pressure was kept constant at value of 1.2×10-4 torr and theplasma displacement changed from ∆x=0.5 cm to ∆x=2.5 cm
outer-side of the chamber. The main frequency spectrum of
the Mirnov signals under this set of conditions shows that bymoving plasma displacement to outer-side, the mainfrequency of oscillations decreases. The results of theseexperiments are shown in Fig. 2. During limiter biasing
experiment we chose an optimum condition of gas pressure
and plasma displacement, because it is seen that after biasing,
the plasma moves 0.1–0.2 cm outward and it effects less than
0.5 kHz on reduction of main frequency of Mirnov
oscillations after positive biasing. In this set of experiments
we didn’t measured variation of plasma density for thepressure effects.
In the biasing experiments, subsequent to the applicationof a positive bias, a decrease followed by an increase in themain frequency of magnetic field fluctuations was observed.
To cite an example, after applying +180 V bias voltage in theplasma current plateau at 16.8 ms, the main frequency ofoscillations decreased by about 10–15 %. Then, after a shortdelay time of about td =2–2.5 ms from 16.8–18.5 ms, it
increased by about 20–25 % with respect to their values
without biasing. Temporal evolution of the magnetic
fluctuations spectrum is shown in Fig. 3. The result of thisexperiment for positive bias is different from other tokamaks.
For example, the MHD behavior of negative bias in theISTTOK [11] tokamak is similar to positive bias in the CT6B tokamak. The plasma current in CT-6B experiments isabout four times higher than in the ISTTOK one, and in theFig. 1, it has been shown that plasma current plays aneffective role in MHD activity. Also, in the CT-6B, theamplitude of high-frequency fluctuations (>25 kHz) decreased

Fig. 1

Relationship between plasma current and plasma
displacement, when the MHD activity began. Points arefrom shot by shot.

Fig. 2

The frequency dependence of MHD oscillations withchanging chamber gas pressure (left) and plasma
displacement (right). Points are from shot by shot.

following a positive bias application, as in the KT-5C [12],Thorello [13] and DIII-D [14] tokomaks. Figure 4 illustrates
the spectrum of magnetic fluctuations before biasing at t =15msec, after biasing at t =18 msec and ramp down of discharge
at t=23 msec for a 0.48 msec partial time interval.
A possible mechanism for the suppression of turbulenceis the strong shear of the E×B velocity flow, so that the resultsof [15] suggests the existence of a close link between theelectrostatic and magnetic behavior of plasma turbulence [15].
In the CT-6B Tokamak the FFT analysis of oscillations
detected by the Mirnov coil as magnetic behavior positioned
at equatorial outer-side midplane, the floating potential
fluctuation, Vf as electrostatic behavior, at r/a=1 and line
emission Hα intensity at r/a=0.8 investigated before and3msec after biasing (Vbias = +150 V) shows a clear coherency
between signals at the frequency of about 25 kHz. At many
shots the Hα intensity fluctuations in the area of q=2–3
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Khorshid P. et al., The Study of Fluctuations at the Edge of CT-6B TokamakFig. 3

Temporal evolution of the magnetic fluctuations
spectrum. The time of biasing is tbias=16.8 msec and thePgas=1.4×10-4 torr. The solid line indicates the reference
data without biasing and dash-line indicates the peak ofmain frequency in the biasing experiment.

correlates more with Mirnov signal fluctuations, which are
not shown here.
When the limiter is biased, the plasma potential floats toa value close to the bias potential. Following the comparisonof the potential fluctuations detected by the segment poloidal
limiter and the oscillations detected by Langmuir probe at thesame position, correlation is found in their behavior. In Figs.
5(c) and 5(f), it can be seen that the low frequency floating
potential fluctuations level < 20 kHz decreases after a positive
bias near the limiter. This reduction is similar to results ofthe ADITYA [16], KT-5C, ATF Torsatron [17] and Thorello.
Thus, our results may show that after the biasing, the outwardflowing current based upon the existence of the turbulence isquenched and an inward radial current is recovered. On theother hand, the floating potential fluctuation in TEXTOR [18]and STOR-M [19], through positive biasing, increases near
the limiter when outward flow decreases. The difference
between results can be due to the conditions of device
operation. Therefore, determining the penetration value ofmagnetic fluctuations and floating potential fluctuations levels
on the transport and turbulence requires further measurementsof plasma parameters.

Fig. 4The spectrum of magnetic fluctuations, before biasing
at t=15 msec, after biasing at t =18 msec and ramp
down of discharge at t =23 msec for a 0.48 msec partial
time interval. Time of bias application is 16.8 msec
from shot in Fig. 5.

Fig. 5The FFT analysis of oscillations detected by the Mirnov
coil positioned at equatorial outer-side midplane, thefloating potential fluctuation, Vf , at r /a=1 and lineemission Hα intensity at r /a=0.8; (a,b,c) before biasingand (d,e,f) at 3 msec after biasing (+150 V).

and the frequency spectrum of the Mirnov signals show thatby increasing plasma displacement to outer side, thefrequency of oscillations decreases. Positive biasing reduces
MHD oscillations frequency, at first, but after a short delay
time the frequency of oscillations increases. During thebiasing regime, it was found that the magnetic andelectrostatic fluctuations in the CT-6B tokamak have a close
coherency with Hα line emission fluctuations. The results formagnetic oscillation frequency and floating potential
fluctuations during positive biasing are different from otherdevices, which may be due to the conditions of device
operation. Therefore, the determination of the penetration
value of magnetic fluctuations and floating potential
fluctuations levels on the transport and turbulence require
further measurements of plasma parameters.

4. Conclusion
Behavior of plasma fluctuations with changing gas
pressure, horizontal displacement experiments, and a positive
limiter biasing experiment were studied on the CT-6B
Tokamak using an array of external Mirnov coils and an
isolated limiter, to get magnetic and potential fluctuations, andusing an optical system to measure the line emission of Hαintensity. The results prove that by changing plasma
conditions MHD activities varied. When vessel gas pressure
increases, the start of MHD activity occurs in high plasma
current and by decreasing pressure, the beginning of MHD
activity occurs in low plasma current. The frequency ofoscillations increases along with the increases in gas pressure
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