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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
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ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 4, Issue 6, November - December (2013), pp. 24-32
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SURVEY ON PLASMA DISRUPTIONS, ITS CAUSES AND CLOSED LOOP
CONTROL IN TOKAMAK MACHINES
1

T.Thaj Mary Delsy,

2

N.M.Nandhitha

1

Research Scholar, Sathyabama University, Jeppiaar Nagar, Old Mamallapuram Road,
Chennai 119
2
Professor, Dept. of ECE, Sathyabama University, Jeppiaar Nagar, Old Mamallapuram
Road, Chennai 119

ABSTRACT
Tokamak means toroidal chamber with magnetic coils that uses controlled thermonuclear
fusion for power generation. Amount of power generated is proportional to the plasma confinement.
However due to unprecedented reasons, plasma disruptions occur leading to the sudden collapse of
the plasma. Hence it is necessary to identify the causes for plasma disruption and predict the same so
that it can be controlled. Considerable research is done to understand plasma disruptions in Tokamak
machines. This paper provides an extensive literature survey on the various techniques used for
modeling, predicting the disruptions and studying the impact of disruptions on the in vessel
components.
Keywords: Plasma Disruptions, MHD, Runaway Electrons, Modes, Tomography
INTRODUCTION
In a tokamak machine, controlled thermonuclear fusion is used for power generation.
Enormous power is required for generating plasma. Power generated is dependent on the amount of
time plasma is confined within the chamber. However due to unpredictable reasons, plasma
disruption is inevitable which leads to the eventual collapse of the plasma. There are two types of
plasma disruptions namely soft and hard disruptions. Soft disruptions do not result in plasma
collapse. So research is focused towards hard disruptions as it not only leads to plasma collapse but
also damages the in-vessel components. In most cases, runaway electrons damage the components
completely as large amount of energy is dissipated within a very short period of time. Hence it is
necessary to develop a circuit that prevents/controls disruptions. In order to develop closed loop
24


control, it is necessary to understand the causes responsible for disruption, impact of disruptions on
the in-vessel components. Considerable research is carried out in this area to identify the causes for
disruptions. This paper provides an extensive survey on different types of disruptions, its causes and
impact on the machine. This paper is organized as follows. Section II provides a detailed survey.
II. IMPACT OF RUNAWAY ELECTRONS DURING PLASMA DISRUPTIONS
Riemann et. al. (2012) found that part of energy contained in the poloidal magnetic field can
be converted into kinetic energy of the runaway electrons. They simulated the process numerically
and found that in large tokomak like ITER, runaway electrons can generate kinetic energies up to
about 70 MJ. When the plasma current is large, a major fraction of it can be converted into runaway
electrons. Initially the kinetic energy is 3% of the poloidal magnetic field energy ie, it around 20 MJ.
If the plasma current is affected by the wall and it produce the current loss and it accelerates the
plasma. In this phase the kinetic energy is 11% of the poloidal magnetic field energy ie, it around
70 MJ. It is clear that most of the growth of runaway kinetic energy occurs after the point where the
plasma first hits the vessel wall and thus it directly related to the subsequent loss of plasma current.
They modeled the vertical movement of the plasma, ignoring any horizontal displacement so as to
avoid solving a two-dimensional equation of motion.
Fulop et.al. (2009) found that the runaway beam stimulates whistler waves (a very low
frequency electromagnetic wave generated by lightning) that scatter the electrons in velocity space
to prevent the beam from growing. The growth rate of the unstable whistler waves are inversely
proportional to the magnetic field strength and the magnetic field dependence. In this paper they
studied the two possible reasons of the observed magnetic field. The first reason is associated with
the whistler wave instability (WWI) which can be excited by runaway electrons. The WWI causes a
rapid pitch-angle scattering of the runaways which may stop the runaway beam formation. The
second reason is related to the runway avalanche (CRASH), which can be derived from the coupled
dynamics of plasma current and runaway generation. They concluded that the whistler waves can
stop the runaway formation below a certain magnetic field.
Arena, et.al. (2005) proposed a new strategy for real time detection of plasma instabilities,
called MARFEs. The Frascati Tokamak Upgrade (FTU) is an experimental device in the field of
controlled nuclear fusion as a source of clean energy. A TV system for observing the tokamak
plasma has been installed in FTU, and provides movies of the discharge evolution, as well as
information on the status of the vacuum chamber and toroidal limiter from different locations.
Generally, most of the light comes from line emission of atomic and molecular hydrogen and ionized
impurities at the edge of the plasma where the electron temperature is less than 20 eV. However, it
often happens to observe edge phenomena like, flying debris as a consequence of runaways electrons
or plasma disruptions. In the presence of an applied electric field, a very small number of electrons in
the tail of the velocity distribution gain so much energy before encountering an ion, that they can
make only a partial collision. If the electric field is large enough then these runaway electrons never
make a collision. They form an accelerated electron beam that drifts along the major radius of the
torus until it is stopped by the external limiter. The power is deposited in a very narrow region. The
principal reason is due to line radiation from impurities. The line radiation that increases with
decreasing temperature. This instability appears at high density. After the onset of a MARFE,
further gas puffing does not lead to an increase of the electron density, moreover, strong gas puffing
into the discharge with a MARFE leads to detached plasma or to a hard disruption.
Salzedas et. al.(2002) used electron cyclotron emission (ECE), heterodyne radiometer and a
Thomson scattering (TS) system to measure the electron temperature evolution in plasmas of the
Rijnhuizen Tokamak Project (RTP). During the current quench, runaway electrons are generated due
25


to the increased toroidal electric field associated with the increased plasma resistance. These
electrons damage the plasma facing structures on which they collide. The initial stages of the energy
quench is called m/n=1/1 erosion of the electron temperature profile which is observed in all
tokamaks. The major Fourier component of the magnetic disturbances is the function of the poloidal
(toroidal) angle of the torus. Major disruptions were triggered by puffing Neon gas in Helium ohmic
plasma.
Helander (2004) quantitatively determined the runaway current during plasma disruptions.
The runaway current typically becomes more peaked on the magnetic axis than the pre-disruption
current. Also it was found that the runaway current easily becomes radially filamented due to the
high sensitivity of the runaway generation efficiency to plasma parameters. They modeled the
evolution of the toroidal electric field and plasma current (Ohmic + runaway) following the thermal
quench of a tokamak disruption. This is done by calculating the runaway current profile selfconsistently with the toroidal electric field E obtained from the induction equation. Due to peaked
temperature profile, runaway generation is most efficient in the centre of the plasma and the rising
runaway current therefore limits the electric field relatively early near the magnetic axis. The
additional electric field is diffuse into the center and more generation occur in this region. In the lowtemperature plasma after the thermal quench, the resistive diffusion time scale is comparable to the
growth time of the runaway avalanche, so that the electric field diffuses radially at the same time as
it generates runaways.
III. IMPACT OF DISRUPTIONS ON MAGNETIC FIELD INSIDE THE PLASMA
Kim, et.al, (2008) generated stress of the antenna cage by EM forces from the Faraday shield
tube. Initially the plasma is in normal operation and drifts downward. When the plasma makes
contact with the wall, the resistivity of plasma is rapidly increased by a rush of impurities, this
increase causes the plasma current to decay. In this event, the magnetic field inside the tokamak is
changed rapidly due to the decay of the plasma disruption current from 2 MA to 0 MA in 2 ms and
drifts vertically onto the position of the ICRF antenna near the outer wall of the tokamak. By varying
the magnetic field, the cavity current at the front wall of the antenna is induced. This induced-current
in the Faraday shield tube and the cavity wall leads to EM forces, Lorentz forces, by an interaction
with the magnetic field of the tokamak. The torque is generated by these EM forces at the contact
position between the Faraday shield tube and the cavity wall. They conclude that all of the forces act
at the edge of septum wall can be find by using Fleming's left hand rule, and the bending forces in
the front surface of the antenna will be downward in the right wall and upward in the left wall.
IV. METHODS AND MODELS TO REDUCE THE EFFECT OF DISRUPTION AND
EVALUATION OF RUNAWAY ELECTRON CURRENT
Smith, et.al. (2006) proposed that, after the thermal quench of a tokamak disruption, the
plasma current decays and is partly replaced by runaway electrons. They have developed and
explored a relatively simple model for the evolution of the current that carried by runaway electrons
in a tokamak disruption. The model consists of an equation for the generation of runaway electrons
coupled to a diffusion equation for the electric field. As the plasma cools down in the thermal quench
of the disruption, a large toroidal field is induced which accelerates runaways. The current carried by
these short circuits the plasma and reduces the electric field and it continues to generate new
runaways until it has spread out of the plasma. If no distribution occurred, the final plasma current
which is entirely carried by runaways would be almost the same as the predisruption current, and
their radial profiles would also be the same. The model is based on the diffusion of electric field that
26


enables the plasma current to change and it controls the specific fraction of the predisruption current
which is converted to runaway electrons.
Yoshimura and Maekawa (2011) used soft X-ray computer tomography to evaluate the
internal structure of plasma during ECH (electron cyclotron heating) by deliberately inducing a
locked-mode disruption. Locked modes are observed in many tokamaks and are followed by major
disruptions. The experiment is carried out in the WT-3 tokamak. The RF power generated with a
klystron (2 GHz) and is injected into the plasma via four waveguide launchers with the waveguide
phase of ∆φ = π/2. Microwaves for ECH from a gyrotron (56 GHz) are injected perpendicularly to
the toroidal field from the low field side. The excited wave, propagating in the extraordinary mode,
is absorbed at the second harmonic resonance layer. MHD activities can be observed on magnetic
probe signals and SXR detector signals. Fourier–Bessel expansion technique is used to reconstruct
the spatial structure of the SXR emissivity. They can resolve poloidal mode numbers up to m = 4
with this CT (computer tomography system). In WT-3, a stationary m = 2/n = 1 oscillation is
observed on magnetic probe signals in ohmic heating (OH) plasmas with a safety factor of q = 3 at
the limiter. When lower hybrid current drive (LHCD) (80 kW) is applied to an OH plasma with an
electron density of ne =0.8 × 1013 cm−3, the frequency of m = 2 mode begins to decrease. The
frequency becomes zero within 10 ms after LHCD is turned on. CT results show that the amplitude
of the locking mode signal increases continuously. A major disruption follows the mode locking. For
ECH (60 kW), it seems that the mode locking takes place and a major disruption does not take place
and a steady, m = 2 oscillation suddenly reappear. They have investigated the plasma internal
structure in the control experiment of a locked-mode disruption with SXR CT. They have found that
the mode locking and the growth of m = 1 and m = 2 modes are suppressed by ECH.
Combs, et.al. (2010) developed the model for injection of considerable quantities of noble
gases or D2 to mitigate some of the deleterious effects of disruptions in tokamaks. Experiments have
been carried out in DIII-D and other tokamaks to test the mitigation capability of the injection of
massive amounts of gas (most commonly, noble gases such as Ne or Ar). In this model, single or
multiple fast valves have typically been used to inject the gas via ports on the machine. For the
discharge of single-plasma required more than 1022 molecules of gas to injected in DIII-D tokamak.
Very large pellets (gas freezes in the freezing zone at room temperature) are needed to get the
density high enough to prevent the large runaway electron currents. The large pellets have the
potential to cause the damage themselves in the firstwall if they are not fully ablated by the plasma.
The reliable formation and acceleration of large (∼16 mm diameter) D2 and Ne pellets were
demonstrated in laboratory testing. In addition, a special target was developed and tested for
effective and reliable shattering of the pellets at a downstream position and that directed the debris in
a controlled direction. They prepared and installed the system on DIII-D and successfully tested into
plasmas. The system fires a large (∼15 mm diameter by ∼22 mm long,∼ 2.3 × 1023 electrons with
frozen D2) pellet onto a double impact target located inside the torus, which shatters the pellet and
directs the resulting debris toward the center of the plasma. These pellets are intended to inject deep
into the plasma to provide better absorption of the injected material than that produced by typical
massive gas injection via fast valves. In initial experiments on DIII-D, the plasmas were successfully
terminated by a shattered pellet. The system will be used for disruption mitigation experiments on
DIII-D.
Witvoet, el.al(2010) proposed control oriented model for the sawtooth instability with
current drive as input and sawtooth period as output. This model is numerically implemented and
combined with PI-controllers in a feedback loop. This model and its parameters are focussed on the
TEXTOR tokamak (Tokamak Experiment for Technology Oriented Research) which is equipped
with an Electron Cyclotron Current Drive installation(ECCD) and a steerable mirror to deposit this
ECCD very locally in the plasma. The model was embedded in a Simulink feedback control loop,
27


and several simulations with different PI controllers were performed. They have shown that the
sawtooth instability can be divided into three different regions, each having unique controller
demands. Each region could be controlled with 0.3 s settling time to any desired sawtooth period
inside that region. This illustrates the feasibility of the sawtooth control problem. Furthermore, they
have shown that time delays induced by detection algorithms can be deal successfully, as long as the
amount of time delay is reasonably small. Also non-linear control strategies instead of the standard
PI-controllers will be investigated.
Fitzpatrick(2007) developed a simple theoretical model which describes how current flows
are excited in the scrape-off layer (SOL- the plasma region characterized by open field lines ) of a
large aspect ratio, low-β, circular cross-section tokamak by time-varying magnetohydrodynamical
instabilities originating with in the plasma. In an axisymmetric SND (single null divertor) plasma, all
magnetic field lines in the SOL plasma have the same path length. However, this is not the case for
DND (double null divertor) plasmas. The inboard SOL in a DND plasma may be more strongly
coupled to MHD modes than the outboard SOL. This effect is likely to significantly modify the
interaction of the SOL with MHD modes in DND plasmas compared to SND plasmas. Thus, the
above discussion suggests that a possible experimental signature of SOL interaction with MHD
modes would be a significant difference between MHD behavior in otherwise similar SND and DND
plasmas.
Buzio and Walker (2006) developed methods to infer estimates of plasma parameters from
measurements of the dynamic response. The plasma is initially in a steady-state equilibrium position
during the plateau of the discharge, during which stage a vertical force acts on the vessel due to the
attraction between plasma and diverter coils. A vertical instability is then triggered, due to MHD
activity. The plasma accelerates vertically, inducing stabilizing toroidal eddy currents with top
bottom anti symmetry. The plasma then strikes upon the vessel, remaining for typically 10–20 ms in
a state of so-called attached equilibrium as induced currents exert their stabilizing effect and
eventually reverses the sign of the vertical motion. If the plasma current at this stage is still large,
substantial halo currents will flow along a hybrid circuit from the plasma to the vessel. A plasma
disruption finally occurs, causing the current to quench and inducing toroidal eddy currents in the
same direction as the plasma current over the entire vessel. During the quench, the plasma is pushed
inwards by the vertical field as it moves back towards the equatorial plane. At this stage, the
direction of the vessel force may be reversed with respect to the initial one. When the disruption
occurs before the start of the vertical displacement, the event is usually much less violent.
Distributed loads on the vessel are generated mainly by the interaction of eddy and halo currents with
the poloidal and toroidal field components, respectively.
Crocker, et.al. (2003) discussed the Magnetic field line reconnection, the breaking and
reconfiguration of magnetic-field lines in a plasma occurs in numerous natural settings and in many
magnetically confined plasma. They found that reconnection occurs predominantly in bursts,
providing clear experimental signatures. The bursts are cyclic, part of a relaxation process in which
many quantities have a sawtooth behavior in time. The bursts of reconnection occur during the crash
phase of the sawtooth oscillation. Finally, many of the key scale lengths, which potentially determine
the width of the current sheet, may be separable, such as resistive layer width, the ion and electron
skin depths, the ion acoustic gyro radius, and the magnetic-island width. It is resonant in that its
parallel wave number vanishes at the reversal surface. This current density perturbation oscillates in
time, along with the plasma and it rotates in the lab frame. Interestingly, the measured magnetic
fluctuations in this vicinity have dominant contributions from modes that are resonant in the core of
the plasma. This is consistent with the expectation from MHD theory that current density
fluctuations associated with reconnection are more spatially localized than the corresponding
magnetic fluctuations.
28


Hassan in and Konkashbaev (2000) found that the deposited plasma energy results in surface
erosion and structural failure. The surface erosion consists of vaporization, spallation, and liquid
splatter of metallic materials, while the structural damage includes large temperature increases in
structural materials and at the interfaces between surface coatings and structural members. Material
damage can evaluated using comprehensive models which contain the computer simulation package.
Splashing mechanisms occur as a result of volume bubble boiling and liquid hydrodynamic
instabilities and brittle destruction mechanisms of non-melting materials. The extent of such damage
depends on the detailed physics of the disrupting plasma. Plasma instabilities can cause both surface
and bulk damage to surface and structural materials. Surface damage includes high erosion losses
from surface vaporization, spallation, and melt-layer erosion. Bulk damage includes large
temperature increases that can cause high thermal stresses, possible melting, and material weakness
and failure. The vapor cloud quickly develops above the surface material during a disruption. This
vapor-shielding layer completely absorbs the incoming particle, therefore heating up to several tens
of eV. At such temperatures, the vapor plasma radiation becomes comparable with the incoming
power. The detailed vapor motion above the exposed surface is calculated by solving the vapor MHD
equations for conservation of mass, momentum, and energy under the influence of a strong magnetic
field. They conclude that erosion of plasma facing materials is governed by both the characteristics
and distribution of incident plasma particles from the SOL, as well as by processes resulting in vapor
and droplet formation and shielding. The use of a renewable material such as free-surface liquid
lithium may significantly extend the lifetime of PFMs.
V. ANALYSIS AND INVESTIGATION OF PLASMA DISRUPTION
Chattopadhyay et.al. (2009) found that sawtooth oscillations (internal disruptions) and major
disruptions are normally observed in ohmically heated Aditya tokamak discharges. The HXR (Hard
X-Ray) time evolution indicates good confinement and low density of runaway electrons. The
absence of negative value of loop voltage signal in the sawtooth region indicating no minor
disruption in this particular region. Fourier analysis of the signal in the sawtooth region shows the
existence of two prominent modes, one at 4.98 KHz and another at 9.96 KHz remaining other modes
having low intensities. The major disruption in ohmically heated Aditya tokamak is described by the
SXR( Soft X-Ray) signals which is disappear before the total current disruption, burst of Mirnov
oscillations, negative spikes in the loop voltage and spikes in Hα spectrum. The duration of major
disruption event and termination of SXR emission which occurs much before the disruption only
indicate the sudden drop in core temperature due to some MHD (Magneto hydrodynamics)
phenomena.
Himura, et.al. (2007) investigated disruptive phenomenon by measuring electron particle
flow into a probe tip inside the magnetic surfaces. Preliminary results indicated that the plasma
disruption is started near the top of the plasma and then propagate toward the center of the plasma.
Non neutral plasmas confined in toroidal magnetic surfaces without any electric fields have been just
Investigated. The method of confining electrically non neutral plasmas in completely closed
magnetic surfaces offers a possibility of re-proportioning electrons and ions inside magnetic
surfaces. This property may be applied to produce various mesmerizing plasmas such as two-fluid
plasmas or electron positron plasmas. Contrary to theoretical prediction, the helical non neutral
plasmas exhibit instability. The propagation speed of the disruption is in the order of 102 m/s, which
is very slow compared to all other typical speeds of CHS nonneutral plasmas. Also, the disruption
happens only in the upper side from the equatorial plane. This result strongly suggests that the
observed disruption is some local mode and driven from the region near the plasma edge. In fact,

29


although the detail is still under investigated, the observed disruption may be related with the
experimental finding of the non-uniformity.
Fitzpatrick (2009), calculated halo current force exerted on the vacuum vessel directly from
linear, marginally stable, ideal-magnetohydrodynamical (MHD) stability analysis. The basic premise
of their model is that the halo current force modifies pressure balance at the edge of the plasma, and
therefore also modifies ideal-MHD plasma stability. In order to prevent the ideal vertical instability,
the halo current force must adjust itself, such that the instability is marginally stable. This allows the
vertical instability to develop on a relatively long time scale determined by resistive diffusion of
magnetic flux-surfaces through the scape-off layer and vacuum vessel. Consequently, the plasma
remains in an approximate axisymmetric equilibrium state throughout the duration of the VDE. They
have used their model to perform a number of simple simulations of axisymmetric VDEs in
tokamaks. These simulations predict halo currents and halo current forces which are similar in
magnitude to those observed in experiments. In addition, the simulations indicate that the halo
current increases in magnitude as the edge-q decreases, and the kink mode is triggered if the edge-q
becomes too close to unity. These conclusions are also in broad agreement with experimental data.
They have generalized their model to deal with a non axisymmetric halo current induced by the
kink mode. This extended theory allows them to predict the peak halo current force exerted on the
vacuum vessel.
Vannucci, et.al (2000) identified that the coupling between the m = 1 and m = 2 MHD modes
has been usually accepted as the main triggering mechanism for the disruptive instabilities in
tokamaks. Therefore, both soft X-ray and Mirnov magnetic signals were chosen, alternatively, to be
used in a neural network to forecast disruptions. Using the data taken during the last 200 ms of
TEXT discharges. Disruptions could be predicted in 1.12 ms and 3.12 ms in advance, when magnetic
and soft X-ray signals were used, respectively. The neural network architecture has been tested and
which yielded much better results. The neural network used has one input layer, two hidden layers
and one output layer, but the number of neural units within each layer was chosen as to follow the
relation m:2m:m:1, where m is the embedding parameter of the system. Both minor and major
disruptions can be forecasted during a tokamak plasma discharge and even hectic time series can be
predicted using a proper architecture for the neural network.
CONCLUSION
This paper presents an extensive literature survey on the various techniques used for the
diagnosis and analysis of the disruption signals to identify the causes. This paper also provides
various models used for mathematically describing the plasma disruption currents and flux linkage.
From this paper, the following are the major conclusions. A model is yet to be developed for
accurately predicting the energy released due to runaway electrons. Currents sharing due to Halo and
Hiro currents in Tokamak machine must be analyzed to prevent damage in the vessel components.
Based on the embedding parameter, number of layers in the ANN can be determined to predict the
chaotic time series. By ergodising the magnetic field, high energy population can deplete which in
turn lowers runaway prediction. Hydrogen pallets can be used to prevent the large runaway electron
currents during disruptions, Non linear controllers based on soft computing tools can be used for
controlling disruption instead of conventional PI controllers.

30


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