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GOLEM operation based on some results from CASTOR J. Stockel, and the CASTOR team, Association EURATOM / IPP.CR, Czech Republic • • • • • Several historical picture Tokamak CASTOR/GOLEM Basic tokamak diagnostics Power supplies Basic plasma performance Marianská, 13-16 January 2009 CASTOR tokamak Major radius Minor radius Toroidal magnetic field Plasma current Pulse length 40 cm 8,5 cm < 1,5 T 5 - 20 kA < 50 ms Features: Small tokamak Small ripple (24 TF coils) Routine operation (100 shots/day) Flexible (a good plasma already 1 day after opening) Hardware required to have tokamak plasma? • A tokamak (CASTOR/GOLEM) • Vacuum and gas handling system • Power supplies • Basic diagnostics • Control and Data acquisition system Preparation of the GOLEM vessel for operation • Pumping of the vessel down to 10-6mBar (10-4 Pa) by turbo-molecular pumps is suficient • Baking of the vessel to 250o C (CASTOR) every day before shots • Glow discharge cleaning Vessel is filled by Hydrogen gas (~ 1 Pa) Electrode (anode) is inserted into the vessel an biased to ~ +500 V Glow discharge is burning at ~0.5 – 1 Amp Inner wall is bombarded by H ions Molecules are desorbed from the inner surface Basic diagnostics to operate GOLEM • • • • • • Toroidal magnetic field Plasma current Loop voltage Position of plasma column in the vessel Plasma density Plasma radiation in visible range (H_alpha, impurity lines, soft x-ray, …..) Basic power supplies of CASTOR/Golem schematically Grid Rectifier Capacitor bank 1 MJ, 2 kV TF COILS Grid Rectifier Capacitor bank 0.4 MJ, 0.5 kV Primary winding of transformer Toroidal magnetic field Breakdown and plasma current Loop voltage d/dt – magnetic flux Uloop = - d/dt The loop voltage is measured by a loop located in the proximity of the plasma column The toroidal electric field E tor = Uloop/2R accelerates charged particles in the toroidal direction (and drives plasma current) Toroidal magnetic field Signal of the loop has to be integrated • analogue integrator • numerical integration dne/dt = Sion * no*ne Plasma is fully ionized I halpha ~ no*ne ~ dne/dt ~ density of electons Maximum of I halpha is observed when no= ne ~ density of neutrals Time [ms] Plasma current Plasma current is measured by means of the Rogowski coil (the solenoid of a toroidal shape surrounding the plasma column) Start-up phase of a discharge on the CASTOR/GOLEM Loop voltage [V] Uloop Toroidal current [kA] I_plasma+ I_vessel Plasma density ne [1019 m-3] 0 2 4 Time [ms] 6 I_vessel = I_plasma – Uloop/Rvessel Maximum plasma current - Safety factor q Stability of plasma requires the safety factor at the edge q(a)MIN > 3 aB 2 a B TOR TOR Imax RB Rq POL 0 min 2 Dimensions of a tokamak and the value of the toroidal magnetic field determine the maximum plasma current which can be driven in a tokamak: CASTOR JET a = 0.085 m, R = 0.40 m, Bt = 1.0 T a = 1.250 m, R = 2.96 m, Bt = 3.45 T GOLEM Reality ~ 15 kA max => IpMax ~ 0.033 MA => IpMax ~ 3.2 MA Safety factor 2 aB 2 a B B TOR TO q(a) TOR 100 RB RI I POL 0 plasma plas Temporal evolution of a discharge on the CASTOR tokamak Vessel is filled by working gas Toroidal mag. field Bt Loop voltage Uloop Plasma current Ip Plasma density ne -20 0 Time [ms] 20 40 Plasma radiation Photomultipliers equipped by interference filters or scintillators puffs of H2 Plasma density ne [1019 m-3] Gas puffing H_alpha [a.u.] Recycling H_alpha [a.u.] Impurities CIII line [a.u.] Additional gas (Hydrogen) is injected into the plasma to keep plasma density at a “constant” value. Hard x-ray E>40 keV [a.u.] A piezoelectric valve is used Time [ms] Ohmic heating in tokamaks Plasma – secondary winding of the transformer (a single loop) with a finite conductivity Ohmic power dissipated in the plasma column P Iplasma U OH loop 3 / 2 plasma loop plasma e R U/ I T Electron temperature can be estimated from plasma resistivity (Spitzer formula) Resistivity of plasma column and ohmic power R = Uloop/Iplasma Poh = Uloop*Iplasma Global confinement of energy (1) dW W P dt τE Heating Energy losses E Balance equation for energy E Energy confinement time !! globally characterizes losses of energy Total kinetic energy in the torus W = 3/2 (ne*Te + ni*Ti) * V = 3 ne Te = W/ P Electron temperature and confinement time Te ~ R-3/2 e = (ne*Te*V)/ POH Conclusions •GOLEM is the oldest tokamak in the word •It will be operational soon (hopefully) •Its education potential is huge! •Its scientific potential is not definitely exhausted •We are looking forward to meeting you in the control room of GOLEM