Disruptions

1
Disruptions
cause

• operational limit in current and density
• large mechanical stress and intense heat load

2
Sequence of Disruptions
Current or density increase or none
Onset of MHD instability typically 10msec
Central temp. collapse typically 1msec Rapid
flattening of J negative voltage spike positive
current spike
Current decay typically 100MA/sec
3
Basic Causes of Disruptions
Operational limit

• Low-q disruptions
• Density limit disruptions

Hugill diagram

• Large amplitude tearing modes due to unstable
  current profiles
• lead to disruption with too rapid current rise
• Solid fragments into plasma
• Magnetic field errors
• Vertical instability

Murakami density limit
4
Low-q Disruptions

• qo is restricted by an m1 sawtooth instability
• sawtooth oscillations restrict qo by flattening
  central q profile
• increased current gradient in the outer region
  m2 tearing/kink instability
• calculated amplitude becomes very large as qa
  approaches 2

5
Density Limit Disruptions

• Increase in impurity radiation as the density is
  increased
• Increasing fraction of heat loss in the form of
  radiation and smaller fraction of heat conduction
  out of the plasma boundary
• Plasma contraction and unstable q value at the
  contracted edge

energy balance
100 radiation --gt no conduction at the edge
heat conducted from the plasma core
Energy balance for ohmic heating
Similar to Murakami parameter
Additional heating increase density limit
6
Contraction Instability Model
When 100 radiation condition is reached,
contraction follows

• Increase either in the electron or the impurity
  density
• Instability radiation in a narrow layer

Power balance
ohmic heating(1) radiation( ?) conduction (1-
?)
Linearizing at constant current yields the
stability equation
or
Energy confinement time
Criterion for contraction instability for and
resistance per length
Disconnected from the boundary (?1)
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Physics of Disruptions Sequences

• Tearing mode instability
• Non-linear growth of the tearing mode
• mode locking
• The fast phase (Thermal quench)
• Current decay (Current quench)
• Runaway electron current
• Vacuum vessel current (Halo currents)

8
Tearing Mode Instability

• m2 tearing mode due to increased current
  gradient in the outer region via
• sawtooth oscillations restrict qo by flattening
  central current profile
• increased resistivity from concentrated
  impurities in the central region

If the current is increased rapidly, skin
current forms and enhance the current gradient at
the edge of the plasma, destabilizing higher m
modes
9
Growth of the Tearing Mode
10
Thermal Quench and Fast Phase
Sudden energy loss from the central region and
the general loss of confinement --gt no
explanation yet, experimental evidences are as
following

• Flattening of the current profile negative
  voltage spike
• Rapid energy loss with m1 structure soft X-ray
  perturbations

11
Rapid flattening of J --gt negative voltage
spike positive current spike
Central temp. collapse typically msec
12
Current Decay and Runaway Electron Current

• Faster current decay low Te
• radiation cooling due to impurity influx
• Runaway electrons sometimes persists after the
  current decay phase

runaway criterion

• Low electron temperature is required
• Increase in the ohmic electric field overweigh
  the increased collisional drag --gt runaway

13
Vacuum Vessel Current Halo Current
Disruption cause large forces on the vacuum vessel

• the loss of plasma pressure leads to an increase
  in toroidal magnetic field pressure inside the
  plasma (diamagnetism)
• --gt poloidal current across the vessel

• rearrangement of toroidal field in a rapid
  current decay, the toroidal magnetic field
  pressure is transferred to the vacuum vessel

• toroidal current driven during current decay
  couples with the poloidal magnetic field

Plasma current decay time
14
Physics of Various Disruptions

• Mode locking
• Error field instability
• Vertical disruption event(VDE)
• Ergodicity

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Mode Locking
16
Error Field Instability
Small deviations from toroidal symmetry of the
magnetic field can lead to the growth of m2
tearing modes, resulting disruption

• Causes of field errors are
• the internal winding structure of the coils
• the connection to the coils
• misalignment of the coils

Toroidal plasma spin substantially diminish the
island size --gt spinning the plasma prevents
disruptions

• Without spinning, tearing modes rotate toroidally
  at a frequency of electron diamagnetic frequency.
• For a small error field, the rotation prevents
  large island growth
• Above the critical level of error field, the
  tearing mode locks to the frame of the error
  field and the island grows to a large size
• The critical level of error field depends on the
  plasma density

17
Vertical Instability

• Failure of the feedback control system
• Gross perturbation resulting from a disruption

• For large displacement, plasma makes substantial
  contact with the vessel, driving a large poloidal
  current in the vessel
• The outer flux surfaces intersect the vessel
  over a halo region

Overall force balances

• Force balance on the plasma

• Force balance on the vessel

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Ergodicity
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Ergodicity