Dust grain temperature in a tokamak plasma

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Dust grain temperature in a tokamak plasma

• J D Martin
• M Coppins
• G F Counsell

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Dusty Plasmas

• Individual Grains Collective phenomena

Morfill et al (1997)
Garscadden et al (1994)
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Importance of Dust in Tokamaks

• Modelling the transport of impurities in dust
• Hydrocarbons, tungsten, CFC
• Safety issues tritium retention, explosive
  limit, cracking of water
• Understanding dust will lead to more efficient
  discharges

Winter (1998)
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Outline

• Tokamak plasmas
• Plasma-surface interaction
• Dust grain charge
• Dust grain temperature
• Conclusions

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Tokamak Plasmas

• Interested in edge region
• Scrape-off layer (SOL)

http//www.physics.auburn.edu/plasma/fusion/fusio
n_lab/fusion.html
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Plasma-Surface Interaction

• Electrons react quickest colliding with the wall
  to produce a negative potential
• Ions accelerated towards wall causing a shock
  (presheath)
• Thin positive space charge region (sheath) with
  large potential drop
• Reach steady state with recycling of particles

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Dust Creation Mechanisms

• Sputtering
• Energy supplied by ions breaks bonds in the solid
• Arcing
• Driven by sheath potential
• Evaporation
• Agglomeration
• Flaking

MAST dust production
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Dust Grain Charging

• Dust generally ve
• Electrons Maxwellian
• Ions in attractive potential
• Use probe theory
• Find floating potential by equating currents
• Take into account secondary emission

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Heating Cooling Mechanisms

• Ion, electron and neutral bombardment
• Ion-electron recombination and molecule formation
• Backscattering
• Secondary electron emission
• Radiative cooling

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Steady-State Temp Carbon

• Independent of dust grain size

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Tungsten Complication

• For tungsten, the secondary emission yield can
  exceed 1
• Electron flux is -ve
• Dust grain charge flips sign
• Dust charges ve until secondaries are trapped
• Potential well found in kinetic simulations by G.
  Lapenta

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New Charging Heating

• Modal energy of secondaries around 3eV
• Ions are Maxwellian
• Electron flux balances ion flux, without
  secondary contribution
• Little energy lost to secondaries

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Steady-State Temp Tungsten
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Conclusions

• Dust grains can survive at high temperatures in
  tokamaks
• Tungsten is interesting
• What next?
• Sort out a better model for the positive charging
  process
• Use current model with tokamak plasma conditions
  simulated by code