H-mode access on MAST

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H-mode access on MAST Presented by Andrew Kirk
UKAEA With thanks to Anthony Field, Hendrik
Meyer and Martin Valovic

• Effect of magnetic configuration
• Effect of gas puff location
• Effect of divertor leg length
• Effect of pellets

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Comparison of Pthr with Ps

• Pthr 0.53?0.03 MW

Ps0.061?ne0.62 ?BT0.69 ?S0.88 F.Ryter et.al.,
Plasma Phys. Contr. Fusion 44 (2002) A415-A421

• Pthrgt 1.8 Ps (Ps 0.29 MW)

Ps0.072?ne0.7 ?BouT0.7 ?S0.9 ?(Zeff/2)0.7F(A)g T.
Takizuka et.al., Plasma Phys. Contr. Fusion 46
(2004) A227-A233

• Pthr Ps

• Minimised by requiring
• Inboard fuelling
• DND configuration.

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Effect of magnetic configuration

• Pth reduced in DN (? rsep lt ?i/2) by more than
  factor 2

LDND Pthr 1.2?0.15 MW
This effect is also observed on AUG and NSTX
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Possible explanations

• In the L-mode phase
• No change of Te, ne or Ti for CDN, LSN, USN
• DEr - 1kV/m between CDN and LSN - B2SOLPS
  modelling also produces these changes

Similar effect observed on AUG
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Possible explanations

• In the L-mode phase
• No change of Te, ne or Ti for CDN, LSN, USN
• DEr - 1kV/m between CDN and LSN - B2SOLPS
  modelling also produces these changes
• SOL Flow patterns change similar to effects
  observed on C-MOD

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Effect of gas puff location
H-mode access easier using Inboard Gas puff
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Possible explanations

• HFS fuelling changes the toroidal rotation of the
  plasma due to
• Neoclassical toroidal viscosity (P. Helander
  et.al.)
• Flows driven by ?B drifts (V.A. Rozhansky
  et.al.)
• The Rozhansky explanation predicts an Increase in
  toroidal flow with HFS gas puff rate

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Possible explanations
Increase in toroidal flow with HFS gas puff rate
supports flows driven by ?B drifts interpretation
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Divertor leg length
Repetitive L-mode phases induced by change of the
connection length on MAST. Loss of bootstrap
current pulls leg inwards Shortening of leg ?
H-mode
Similar effects have been observed on JET with
the X-point height scan
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Fuelling Pellets can induce L-H transition
Similar to what has been observed in DIII-D

• Explained by increased density gradient due to a
  pellet

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Summary

• H-mode access on MAST is facilitated near to CDN
  and using HFS gas fuelling. Studies have been
  performed trying to understand these effects
• In addition the effect of
• Loop voltage and Density
• Error fields
• Effect of co vs cntr NBI
• have been studied.

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Backup material
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Possible explanations for gas puff location

• HFS ionisation source drives outward parallel
  flow.
• Net toroidal torque due to ?B-drift of ions.
• ? v? in counter-current direction
• observed in experiment.
• Radial transport of toroidal momentum from SOL.
• ? v? in co-current direction
• Balance of both toroidal torques determines
  toroidal rotation ? Er

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Possible explanations for gas puff location

• HFS ionisation source drives outward parallel
  flow.
• Net toroidal torque due to ?B-drift of ions.
• ? v? in counter-current direction
• observed in experiment.
• Radial transport of toroidal momentum from SOL.
• ? v? in co-current direction
• Balance of both toroidal torques determines
  toroidal rotation ? Er
• Predicts an Increase in toroidal flow with HFS
  gas puff rate

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Possible explanations for shorter divertor leg

• B2SOLPs Modelling shows that the shorter divertor
  leg leads to
• Lower edge temperature results in strong
  reduction of V
• More negative Er, and increased shear ? lower
  Pth(?)