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Plasma turbulence in tokamaks: some basic facts…

Plasma turbulence in tokamaks: some basic facts…. W.Fundamenski UKAEA/JET. 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

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Plasma turbulence in tokamaks: some basic facts…

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  1. Plasma turbulence in tokamaks: some basic facts… W.Fundamenski UKAEA/JET Chalkidikhi Summer School

  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 Chalkidikhi Summer School

  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, β = p_plasma / p_B < 3 % • Btor  1/R, Bpol(jtor), poloidal transform necessary to prevent free (hoop) expansion via ExB drift Chalkidikhi Summer School

  4. Tokamaks II • Core: closed field lines, radial transport • Edge or SOL: open field lines, parallel >> radial transport • Energy source: neutral beams, ICRH, ECRH (in ITER and beyond fusion reactions themselves) heat the core • Particle source: edge recycling >> 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 Chalkidikhi Summer School

  5. Tokamaks III • Core transport found >> classical (Spitzer, Braginskii) • Also >> 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 ?! Chalkidikhi Summer School

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