FESAC Priorities Panels Fusion Engineering Science Theme

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FESAC Priorities PanelsFusion Engineering
Science Theme Question 15c Long-pulse Power-
and Particle Extraction
Peter Mioduszewski Oak Ridge National Laboratory
April 30, 2004
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In a stationary fusion plasma, energy and
particles need to be removed continuously !

• Handling high power fluxes plasma solutions -
  engineering solutions
• plasma solutions
• e. g. high recycling detachment
• high recyling reduces Te and sheath potential
  -gt sputtering
• detachment reduces power and particle flux,
  but
• deteriorates core performance partial
  detachment acceptable
• engineering solutions
• materials choice low-Z (T-retention) vs.
  high-Z (melt-layer)
• cladding - bonding - substrate
• thermal stress issues
• thick cladding long erosion life/low power
• thin cladding short erosion life/high power
• plasma/engineering solutions
• SOL design e.g. stochastic layers, islands,
  etc.

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Normal operation and off-normal events

• Presently accepted values for steady state
  power fluxes are 5 MW/m2
• with temporary excursions up to 10 MW/m2
  (ITER).
• CFC and Be would have low Zeff, but short
  erosion life times
• W would have long erosion life times (excluding
  melt layer erosion),
• but needs to be well controlled in the
  divertor
• Critical issues for power handling in (future)
  tokamaks are
• Disruptions 100 MJ/m2 in 1 ms, 100 GW/m2
  (ITER50 x JET)
• dramatic erosion effects, but vapor shield
  will save the day
• -gt losses 30 mm of carbon 25-75 mm for
  metals
• ELMs 0.4 MJ/m2 (CFC) and 0.64 MJ/m2 (W)
  maximum tolerable
• repetitive pulsed heat load
• Todays pulsed experiments (t 10s) can
  handle higher power loads - at low duty cycle -
• through their thermal capacity

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Particle control and exhaust

• 1. A burning fusion plasma needs continuous
  exhaust of helium ash
• to prevent fuel dilution.
• This requires sufficient particle fluxes into
  the divertor and
• corresponding pumping speeds -gt divertor
  design operating regime
• 2. Particle (neutrals) control is an essential
  knob for optimizing
• plasma performance
• This is achieved by divertor design and
  operating regime
• 3. Power- and particle control are closely
  coupled through both
• plasma- and engineering solutions divertor
  regime, materials
• choice, geometry, design, etc.

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Power- and particle extraction issues

• High core performance and PFC design is marginal
  for ITER!
• Critical issues Disruptions mitigation,
  prevention
• Type I ELMs type II ELMs, ELM-free
  scenarios
• VDEs 1mm per event on FW in ITER
• Medium term (5 years)
• Find scenarios (experiment modelling) that
  simultaneously satisfy safe divertor operations
  and optimum plasma performance!
• 2. Longer term (10 years)
• Need to develop scenarios for power- and
  particle control with safe margins, low
  T-inventory, low erosion, low Zeff, coupled with
  optimum plasma performance. Need models and codes
  to support experiments developing integrated
  divertor and core scenarios.