Plasma turbulence in tokamaks: some basic facts

1
Plasma turbulence in tokamaks some basic facts

• W.Fundamenski
• UKAEA/JET

2
Controlled Nuclear Fusion

• Applied Science community with a mandate
• Goal Artificial Star on Earth, i.e. exo-thermal
  reactor
• No gravity Magnetic (MCF) vs. Inertial (ICF)
  Confinement
• MCF Closed B-field lines, toroidal geometry
• Tokamak strong toroidal field, with weaker
  poloidal transform, hence net Helical field
• Today JET, AUG, JT-60, DIII-D, MAST, NSTX,
• Tomorrow ITER, a burning plasma experiment (EU,
  Japan, Korea, China, Russia, Canada, US)
• Decision on site in final stages, expected by
  end of the year

3
Tokamaks I

• Excellent textbook by J.Wesson,
• eg. JET (Joint European Torus) near Oxford, UK
• Core fields Btor ? 3 T, Edge Bpol ? 1 T ? 10,000
  Gauss
• Core temperatures Ti,Te ? 10 keV, Edge Ti,Te ?
  100 eV
• Core density ni,ne ? 1e20 m-3, Edge ni,ne ?
  1e19m-3
• Low beta plasma, ß p_plasma / p_B lt 3
• Btor ? 1/R, Bpol(jtor), poloidal transform
  necessary to prevent free (hoop) expansion via
  ExB drift

4
Tokamaks II

• Core closed field lines, radial transport
• Edge or SOL open field lines, parallel gtgt
  radial transport
• Energy source neutral beams, ICRH, ECRH (in
  ITER and beyond fusion reactions themselves) heat
  the core
• Particle source edge recycling gtgt core fuelling
• Hence, mostly interested in power flow across
  B-field
• Due to high temperatures, the core is very
  difficult to diagnose for fluctuations
• Must rely on global transport reconstruction, to
  extract radial velocities, diffusivities,
  viscosities, conductivites, etc.
• In the colder edge, some measurements are
  possible

5
Tokamaks III

• Core transport found gtgt classical (Spitzer,
  Braginskii)
• Also gtgt neo-classical (corrections for toroidal
  geometry)
• Must infer the flow is not laminar but turbulent
• Electrostatic vs. Electromagnetic turbulence
• Comparison with theory suggests ion (critical
  gradient modes) dominate
• 3-D Numerical codes (gyro-fluid and
  gyro-kinetic) are reaching a stage where global
  turbulence resolved
• Transport barriers (local reduction of transport
  to laminar, neo-classical levels)
• Edge (ETB) vs. internal (ITB) transport barriers
• ETB L-H bifurcation, highly intermittent MHD
  bursts
• these edge localized modes (ELMs) eject
  particles and energy on transit time scales
  similarity to solar flares ?!