Title: Disruptions
1Disruptions
cause
- operational limit in current and density
- large mechanical stress and intense heat load
2Sequence 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
3Basic 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
4Low-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
5Density 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
6Contraction 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)
7Physics 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)
8Tearing 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
9Growth of the Tearing Mode
10Thermal 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
11Rapid flattening of J --gt negative voltage
spike positive current spike
Central temp. collapse typically msec
12Current 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
13Vacuum 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
14Physics of Various Disruptions
- Mode locking
- Error field instability
- Vertical disruption event(VDE)
- Ergodicity
15Mode Locking
16Error 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
17Vertical 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
18Ergodicity
19Ergodicity