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CHATTOPADHYAY et al.: ANALYSIS OF DISRUPTIVE INSTABILITIES IN ADITYA TOKAMAK DISCHARGES 827
Many groups around the world have revisited these
phenomena and newer ideas are emerging.
Considering this point of view we report here the
plasma disruptions in the medium size, low β
tokamak, Aditya
18
. We have analysed a number ofdisruptive shots in ohmically heated plasma with the
plasma current, I p, in the range 73 kA≤ I p≤82kA and
safety factor q(a) range 3≤q(a)≤3.3.
2 Experimental Details
The basic parameters of Aditya tokamak 18 are:
major radius R0=75 cm, minor radius a=25 cm,
toroidal magnetic field BT ≈0.75 T, central temperature
T 0≈400 eV, plasma current I P lies between 65 and 85
kA and discharge duration up to 100 ms. The two
main diagnostics used in the analysis are soft X-ray
diagnostic system with an array of 12 surface barrierdetectors (ORTEC, active area 50 sq mm, thickness
100 micro metre) placed inside an imaging camera18
and a garland of 36 Mirnov coils19. SXR tomography
was done with the help of analytical method 20,
assuming rigid rotation21-23 of the mode.
3 Disruption
The plasma pulse shown in Fig. 1 is a typical
example of how internal and major disruption occur
in Aditya tokamak operated in the previously
mentioned current range. This corresponds to a
plasma current, I P≈75 KA [Fig. 1(a)] which disrupts
at 96.4 ms. The SXR emission [Fig. 1(b)] goes below
zero at 79.9 ms, far behind the total current disruption
time which indicates a rapid temperature drop at thistime. Growing amplitude in poloidal magnetic field
derivative (Mirnov oscillation) [Fig.1(c)], spikes in
Hα line emission [Fig. 1(d)] and negative loop voltage
[Fig. 1(e)] seem to be strongly correlated with the
cessation of soft X-ray emission.
3.1 Internal disruption
The presence of sawtooth event [Fig. 1(b)] was
observed in Aditya tokamak for the first time in
December, 1995, when the device was operated with
high plasma current. Since then these have been
observed in many experimental situations and under
different machine conditions24. The period and
amplitude of the sawteeth [Fig. 1(a)] are seen to
change slightly with time with an average frequency
of 0.98 kHz, and no transients were observed in loop
voltage [Fig. 1(e)]. Figure 1(f) is the evolution of hard
X-ray which indicates the good confinement and low
density of runaway electrons. Fig. 2 shows the
expanded version of SXR signals at tangent chord
radius at r =1.56 cm [Fig. 2(b)] and r =6.24 cm
[Fig. 2(a)]. An inverted sawtooth [Fig. 2(a)] is
Fig. 1 — Time history of plasma current (a), soft X-ray signal of the central chord (b), Mirnov oscillation (c), H α spectra (d), loop voltage
(e) and time evolution of hard X-ray (f) in a typical Aditya discharge
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INDIAN J PURE & APPL PHYS, VOL 44, NOVEMBER 2006828
observed for each sawtooth crash [Fig. 2(b)]. Figure
2(c) shows the signal as observed in Mirnov coil at
that particular time interval, showing fast increase in
amplitude associated with sawooth crashes. These
phenomena are associated with the energy transport
from inner to the outer region of the q=1 magnetic
surface. Loop voltage signal [Fig. 2(d)] has nonegative value in sawteeth region which negates any
minor disruption in this region. Not only that when
minor disruptions occur, which deteriorate the
confinement and increase the plasma Z eff value, the
crashes no longer reached a rapid and/or sharp decay
as shown in Fig. 2(a and b). In this situation, it has
been reported that decay time almost increases 3 fold
and m/n=1/1 oscillations are predominant during this
period 25. The average time for sawtooth crashes
measured in several discharges lies in the interval 40
μs ≤τc≤90 μs. Fig. 3(a) shows the expanded view of
the traces found from different detectors in the samedischarge. The oscillations are not pure sinusoidal
which indicate the mixture of modes. The horizontal
scale is time in millisecond whereas the vertical label
refers to the tangent chord radius, r, in centimeters
(0<r <7cm). There is obviously a node near 6 cm
because after that a phase reversal in SXR radial
mode structure occurs [Fig. 3(a)]. This radius is
related to the location of the q=1 surface and is called
inversion radius, r inv. Figure 3(b) shows the relative
sawtooth amplitude, Δ Ã/ A, of the sawtooth oscillation
Fig. 3 — Line integrated SXR emissivity at different radial
positions versus time is shown in (a). (b) shows the the relative
amplitude, Δ Ã/ A of sawteeth oscillation versus tangent chordradius , r with inversion radius at 6 cm
of different chords. The sawtooth amplitude becomes
zero at r =6 cm which is identified as r inv because
outside of this radius sawtooth is inverted. Calculating
from the empirical law26 r inv = 0.5a/√q(a) , we get theinversion radius as 6.3 cm for q(a)=3.3 and a=23 cm,
Fig. 2 — SXR signals of the detectors at tangent chord radius(a) r =6.24 cm and (b) r =1.56 cm. Mirnov oscillations and loopvoltage are shown in Fig (c) and Fig (d) respectively
(a)
(b)
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CHATTOPADHYAY et al.: ANALYSIS OF DISRUPTIVE INSTABILITIES IN ADITYA TOKAMAK DISCHARGES 829
which is in good agreement with experimentally
found r inv, taking into account the accuracy of such
measurement.
The most interesting phenomenon is the presence
of two modes during the internal disruptions; one at4.88 kHz for m=1 mode and another at 9.76 kHz.
Mode with frequency 9.76 kHz is observed in the
disruption region, which is identified as m=2 mode.
Figure 4 is the Fourier analysis of the signal in a
sawtooth which shows the existence of two modes,
one at 4.88 kHz and another at 9.76 kHz. Fourier
analysis of other soft X-ray signals, performed on the
whole or part of the sawtooth like relaxation also
yielded these two modes besides the other modes of
low intensities. The two prominent frequencies of the
precursor mode to sawtooth crashes might be the
harmonics of the m=1 mode. This only proves that the precursor mode is not 100% sinusoidal which is not
uncommon in a toroidal system. This m/n=2/1 mode
on the q=1 surface, therefore, easily couples to the
m/n=2/1 mode on the q=2 surface at the time of
disruption. It is not unusual because EQUIPE TFR 17
has also observed such two modes. They have
explained this as the growth of a new mode, with an
m=1 structure and the frequency of m=2 mode
localized in the vicinity of q=1 surface at the time of
the growth of the normal m=2 mode at q=2 surface.
Full explanation with strong experimental support of
this phenomenon is not possible at this stage.
Sequence of tomographic images depicting the time
evolution of the SXR emissivity contours are as
shown in Figs 5 and 6. Figure 5 is the contour plots of
SXR emissivity before sawtooth crash and Fig. 6 is
those for sawtooth crash. No prominent shift of the
central region is observed in internal disruption as
shown in Fig. 5. Some distortion in the structure may
be due to the presence of other higher m modes
besides the prominent m=1 mode and/or small
artifacts. A prominent m=1 mode rotation occurred atinternal disruption radius as shown in Fig. 6. Pushing
of the central region towards m=1 inversion region is
also obvious and indicates Kadomtsev like disruption.
Rotation of the perturbation is not sinusoidal, as the
motion of the peak of the emissivity is not uniform
from one frame to another. The cause of the island
rotation might be due to ω∗ diamagnetic effect (the so
called drift tearing mode27) or to radial electric field,
E r , giving rise to Er B drift, where B is the toroidal
magnetic field. Figure 7 shows a few 3D emissivity
plots at sawtooth crash region. The rotation of mode
and corresponding flat region of emissivity are verydistinct.
3.2 Major disruption.
The major disruptions, of ohmically heated plasma,
in Aditya tokamak in the range are observed by
noticeable precursor oscillations, sudden
disappearance of SXR signals much before the total
current disruption, burst of Mirnov oscillations and
negative spikes in the loop voltage. The duration of
major disruption event changes from shot to shot and
lies in the range 3.3-19.2 ms. Cessation of SXR
emission before the total current disruption also variesfrom shot to shot and the time lies in the range 1.9-
18.5 ms. This phenomenon indicates that due to
disruptive activity temperature drops abruptly below
100eV in the core to stop SXR emission. Fourieranalysis of the the MHD signals shows m/n=2/1 was
always the dominant and responsible mode for major
disruption in all the shots. Such long duration major
disruptive event is not uncommon and has already
been observed by Vannucci et al. in TEXT-U
tokamak 28. Figure 8 shows that the disruption starts
around the time 77.2 ms with current fall of ∼35%
[Fig. 8(a)] and stopping of sawtooth oscillation[Fig. 8(b)]. After a short while SXR emission stopstotally at 77.9 ms [Fig. 8(b)] followed by a lot of
phenomena like a small spike in current [Fig. 8(a)],
burst in Mirnov oscillation [Fig. 8(c)], spikes in H α
emission [(Fig. 8d)] and negative loop voltage
[Fig. 8(e)]. After that a lot of negative loop voltages
are seen at 79.4, 80.8, 92.9 and 94.4 ms in the loop
voltage with continuous current decay until current
ends at 96.4 ms. A burst in Mirnov oscillation, spikes
in H α spectrum and voltage surge in the loop voltage
Fig. 4 — Frequency spectra of the soft x-ray signal (a). The lower
trace (b) is the original SXR signal. 128 points (27) of the data are
considered. The sampling time is 8 μs
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INDIAN J PURE & APPL PHYS, VOL 44, NOVEMBER 2006830
Fig. 5 — Contour plots of soft X-ray emissivity before the sawtooth crash, showing not so prominent mode rotation and shifting ofcentral region towards sawtooth inversion radius
just before the total current disruption (Fig. 8) are the
common physical phenomena in tokamak plasma
disruption. Analysis of Mirnov coil oscillations at the
time of disruptive events shows the dominant m/n=2/1
mode oscillation19. Figure 9 shows that the frequency
of soft X-ray and Mirnov oscillation match well at the
time of disruptive events indicating the presence of
m/n=2/1 frequency in SXR signal. The presence ofnegative spikes in the loop voltage and the burst in
Mirnov oscillation prior to the disruptions rule out the
interaction between the plasma column and the wall
(limiter) of the vessel as the possible cause of
disruption. In these discharges position feedback
control was not operative. In the event of plasma
column disruption, due to contact with the limiter,
there would be no negative spikes in the loop voltage
or burst in Mirnov oscillations. The radiative power
stayed well below the input power (figure not shown)
which indicates that the discharge is far away from
the density limit. Hence, the contraction of the current
channel and consequently plasma detachment as a
result of edge cooling is not the main disruption
triggering mechanism in this case. Z eff values and the
H α spectra also indicate that impurities and/or
transportation of H atoms are not the main factor for
disruption. Judging all aspects, it seems that coupling between the m/n=2/1 and m/n=1/1 modes could be the
realistic mechanism for the cause of disruption. The
presence of m/n=2/1 mode at the time of sawtooth
oscillation favours the coupling of m/n=1/1 mode at
q=1 surface with m/n=2/1 mode at q=2 surface very
easily. Calculation regarding the q=2 resonant
magnetic surface localization and the width of the
corresponding island from the empirical relation
given by K Toi et al.29 and F Salzedas et al.30 showed
that there may be less chances of islands interacting
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CHATTOPADHYAY et al.: ANALYSIS OF DISRUPTIVE INSTABILITIES IN ADITYA TOKAMAK DISCHARGES 831
Fig. 6 — Contour plots of soft X-ray emissivity at sawtooth crash, showing prominent m=1 mode rotation and
shifting of central region towards sawtooth inversion radius
with the limiter because q=2 surface is well inside the plasma.
4 Discussion and Conclusion
The experimental sawteeth periods were found to
vary between 980 to 1050 μs and were compared with
the scaling laws. Reasonable agreement was found
using the expression of EQUIPE TFR 26, which gives
the value of 1135 μs. The internal disruption
(sawtooth relaxation) mentioned presently, is a very
interesting phenomenon. The dominant frequencies at
the time of internal disruption could be the harmonicsof the same mode which is very common in toroidal
system. The presence of such harmonics makes the
SXR signals non-sinusoidal and can couple in
resonance with the mode oscillations in higher
q-surfaces to accelerate the major disruption. The
observation of the internal disruption with growing
m/n=1/1 oscillation and the tomographic images
indicate that the sawtooth instabilities seem to be due
to the total reconnection model by Kadomtsev, but the
crash time according to Kadomtsev model does not
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INDIAN J PURE & APPL PHYS, VOL 44, NOVEMBER 2006832
Fig. 7 — 3D plots of soft X-ray emissivity at internal disruptive region (as shown in Fig. 6), showing clearly the mode rotation and flatemissivity region
Fig. 8 — Expanded time history of plasma current (a), SXR emission (b), Mirnov oscillation (c), H α spectra (d) and loop voltage (e) atdisruptive period. Disruption starts at 77.2 ms and ends with total current loss at 96.4 ms. Complete cessation of SXR emission occurs at
77.9 ms
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