Observations of high density disruption on HL2A

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Observations of high density disruption on HL-2A
Q.W. Yang, X.T. Ding, Z.B. Shi, Y.D. Pan, Z. Cao,
Y. Zhou, Yi. Liu, Z.Y. Cui, W. Li, B.B. Feng
Apr. 11, 2005, SWIP
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Introduction

• Major disruption has been considered as a serious
  problem for tokamak operation. How to avoid the
  disruption is an important issue on tokamak
  operation.
• To control and mitigate the major disruptions,
  its mechanism and the characters of disruption
  have to be well understood.
• Disruptions can be triggered by several reasons,
  for example, the low-q discharge, the MHD
  instability, the vertical displacement events
  (VDEs) and the high density operations.
• The main reason of density limit disruptions is
  due to the enhanced impurity radiation and
  anomalous transport in the plasma edge. The
  physical effects being considered include
  divertor power balance, MARFE, poloidally
  symmetric radiative instabilities, MHD
  instabilities, and transport.
• In this paper, the characters of the density
  limit disruption on HL-2A will be presented.

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Experiment arrangements

• The top-view of diagnostics and their positions.
• The position of poloidal mode Mirnov coils,
  pickup coils and the flux loops.
• the bolometer array and its alignments.
• the soft X ray cameras and their alignments.
• the ECE system, HCN interferometer, visible and
  vacuum ultra-violet (VUV) spectroscopy.
• The data sampling rates of different diagnostics
  are different (10200µs).
• Gas puffing are used.

• Plasma current IP 200 300kA
• Toroidal field BT 2.0 2.3T
• Plasma density ne 4.0 4.51019m-3
• Electron temperature Te0 600 800eV
• Plasma configuration limiter divertor

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Hugill diagram
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Features during density limit disruption

• As the last sawtooth of soft X ray emission
  collapses at the first dashed line (t 462ms),
  the normal plasma operation ends.
• From this time, the electron density increases
  rapidly from 3.71019m-3 to 4.51019m-3 within
  5.5ms. Simultaneously, the soft X-ray emission
  crashes, and the profile of plasma temperature
  begin to shrink.
• From t 470ms denoted by the second dashed line,
  the electron temperatures and the soft X ray
  emission begin to collapse. But the plasma
  current does not quench.
• From t 473ms, the electron temperature almost
  decreases to the half of the original value. At
  this time, the plasma current begins to quench,
  and the extreme burst of plasma radiation occurs.

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Plasma radiation
The strong radiation starts from plasma central
region when the electron temperature drops down
(t 472ms), and a kink-like formation of
radiation is observed. The kink-like radiation is
believed that it is with an m 1 formation and
occupies the most of central plasma (with the
radius of r 20cm). We can not associate this m
1 radiation with the MHD instabilities because
no MHD perturbations are detected in this period.
After the kink-like radiation, the plasma
radiation suddenly increases to a higher level
within about 400µs. the MARFE formation does not
be observed in our experiments. We suggest that
the impurity radiation is not the most important
reason of density limit disruption in this shot.
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Features during density limit disruption

• the all duration of plasma current damping down
  spends about 120ms, which accompanies a series of
  soft disruptions.
• Although the soft disruptions gradually lead to
  the losses of plasma store energy and the
  decrease of central electron temperature during t
  500 560ms (from the second to the third
  dotted line), but the contraction of electron
  temperature profile is not observed.
• The central electron temperature begins to drop
  down, and the plasma channel to shrink from
  560ms. During t 560 570ms (between the 2
  dashed lines) , the profile of Te become narrow.
  This typical contraction of plasma channel leads
  to the plasma current rapid quench at t 570 ms.

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Contraction of thermo-plasma
Although a series of soft disruptions and
electron temperature decrease can be found during
plasma current damping, but they can not stop the
discharges directly. The density limit disruption
always occurs after the electron temperature
shrinkage and collapse. The event of contraction
of plasma channel maybe play a key role in major
disruptions.
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Hard X ray during plasma current quench

• The loop voltage always has an obvious increase
  when the plasma current quenches at the
  disruption occurrence.
• The induction voltage causes the runaway
  electrons.
• The runaway electrons bombard the limiter or
  vessel and produce an amount of impurities.
• The plasma current tail corresponds to the traces
  of loop voltage, Ha and impurity emissions. But
  no relationship between current tail and hard X
  ray is found -- the current tail is not a runaway
  current.
• The great interaction between runaway electrons
  and limiter/surface occurs during disruption.

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Summaries and discussions

• The density limit disruption is always occur when
  the discharges approach/exceed the Greenwald
  limit in HL-2A Ohmic plasma.
• The density limit disruptions usually undergo two
  stages.
• The soft X ray emission decreases, and the
  profile of electron temperature begins to shrink
  and collapse. The impurity radiation and the edge
  plasma radiation enhance a little. The duration
  of this stage is about 812ms. In this stage, the
  plasma current doesnt quench.
• Sometimes kink-like plasma radiation appears in
  the central region of plasma. The huge of energy
  begins to lose, and the plasma current quenches.
  Consequently, the plasma radiation extreme burst,
  and the plasma current drops to nearly zero.
• The event of contraction of plasma channel maybe
  play a key role in major disruptions.
• The contraction of plasma channel is just before
  the burst of plasma radiation. We can not suggest
  that the density limit disruption is caused by
  the plasma radiation burst in HL-2A.
• The fast decay of plasma current can cause the
  high loop voltage and then drives the runaway
  electrons. The interaction between runaway
  electrons and surface is strong during this
  period and always lead to the amount of
  impurities.
• After the major disruption, the current tail is
  always observed. this current tail is not a
  runaway current.

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Future studies

• Density limit.
• Plasma displacement.
• MHD instabilities.
• Low-q.
• Runaway electrons.
• Disruption database.
• Halo current.
• Prediction.
• Mitigation and control.

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Thanks for your attention