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Dust grain temperature in a tokamak plasma

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Modelling the transport of impurities in dust. Hydrocarbons, tungsten, CFC. Safety issues tritium ... Interested in edge region. Scrape-off layer (SOL) ... – PowerPoint PPT presentation

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Title: Dust grain temperature in a tokamak plasma


1
Dust grain temperature in a tokamak plasma
  • J D Martin
  • M Coppins
  • G F Counsell

2
Dusty Plasmas
  • Individual Grains Collective phenomena

Morfill et al (1997)
Garscadden et al (1994)
3
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)
4
Outline
  • Tokamak plasmas
  • Plasma-surface interaction
  • Dust grain charge
  • Dust grain temperature
  • Conclusions

5
Tokamak Plasmas
  • Interested in edge region
  • Scrape-off layer (SOL)

http//www.physics.auburn.edu/plasma/fusion/fusio
n_lab/fusion.html
6
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

7
Dust Creation Mechanisms
  • Sputtering
  • Energy supplied by ions breaks bonds in the solid
  • Arcing
  • Driven by sheath potential
  • Evaporation
  • Agglomeration
  • Flaking

MAST dust production
8
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

9
Heating Cooling Mechanisms
  • Ion, electron and neutral bombardment
  • Ion-electron recombination and molecule formation
  • Backscattering
  • Secondary electron emission
  • Radiative cooling

10
Steady-State Temp Carbon
  • Independent of dust grain size

11
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

12
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

13
Steady-State Temp Tungsten
14
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
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