3st EPS Conference on Plasma Physics 28th June 2nd July, 24, Imperial College, London Plasma Breakdown Analysis in without the Use of Center Solenoid H. Tsutsui, S. Tsuji-Iio, R. Shimada, M. Sato, K. Tsuzuki, Y. Kusama, H. Kimura Research Laboratory for Nuclear Reactors Tokyo Institute of Technology Naka Fusion Research Establishment Japan Atomic Energy Research Institute
Introduction In order to demonstrate compatibility between the low activation ferritic steel (such as F82H) and plasma, performed the Advanced Material Tokamak EXperiment (AMTEX) program. Measurements of permeability in a perpendicular direction of magnetic field Measurements of a breakdown characteristic (required loop voltage and neutral gas pressure) with the ferritic steel. A numerical investigation of the breakdown characteristic
Measurement of F82H Magnetization magnetization of the ferritic steel is measured. B = µ H + M B M ( H ) ( H = µ + ) p H p Tokamak Toroidal Field Poloidal Field Coil Vertical Field(<5T) Toroidal Field Equivalent permeability µ p
Measurement of Magnetization (Results) Magnetic Flux Density, Bp[T] 2 - µ H T-eff [T].2.27.45.53-2 - -5 5 Manetic Field Intensity, Hp[A/m] B ext = T µ P /µ 6 4 2 Measurement Calculation 2 3 4 5 µ H T-eff [T] It was found that the equivalent permeability µ p in the poloidal direction follows the theory.
Breakdown Experiment Using only TF coils and S coils, relations of a neutral gas pressure and a required voltage for a breakdown were measured. OH coil does not work!! Toroidal field was changed to check an effect of the ferritic steel. (µ Cross Section of
Breakdown Experiment (Results) Breakdown characteristics were measured aginst the toroidal field strengths of.6t.6t.3t. Succeeded and failed cases are marked with and, respectively. The minimum neutral gas pressure required for breakdown exists. Under the pressure, the required voltage increases abruptly or a breakdown does not occur. This result qualitatively agrees with the experiments of JFT-2 and a conventional breakdown theory. V B D (V) 4 3 2 B t =.6 T B t =.3 T B t =.6 T -4-3 -2 - P(Pa)
Breakdown and Plasma Formation Breakdown Plasma Formation Current Ramp Up n e V Loop I P Breakdown Electron-neutral gas collisions are dominant (Avalanche ionization<. model by Townsend) Plasma Formation Tim e Electron-Ion collisions have to be considered including the thermal motion of electrons Electromagnetic field by plasma current is negligible
Simulation Collision Ionization Model collision Electron-neutral gas collisions are dominant when ionization >., breakdown is succeeded. Electron-Ion collisions are included Electromagnetic field by plasma current is negligible E [V/m] <v>=v d <v>= P [Pa] L=3[m] - -4-3 -2 - Application for JT-6U A insulation in the toroidal direction is introduced. An effect of iron core is included by a modification of inductance.
Basic Equations n t e eµ v α = d µ = + ( n v ) = αv νi v d = e( E + v B) e m ν e p n e d d e Townsend Coefficient d Mobility Drift Approximation ψ + d = V t E l E = η j = j en µ e 2 ψ ( x) = G( x, x') j ( x') d x' ϕ nw e t 3 2 e + ( nwv ) = j E n ν W kt = W mv 2 2 B e e d e e d e Shifted MaxwelIian i i ν ν i p i = = n target i ( T, v ) ν i= elastic < σ v i + > i elastic e e d Wiν i W + ν I
Elementary Process
Dependence of Ve, α on E/P in Static Model 6 He V e [m/s] 5 4 H2 - -2-3 3. E/P [V/m/Pa] -4. E/P [V/m/Pa] Solid Lines: Static Model Dashed Lines: Experiments with electrodes
2-D Simulation Code PF Coils Plasma 2 Vacuum Vessel
Insulation in the toroidal direction obtains a loop resistance by a toroidal insulation. I I 2 Vacuum Vessel R G In order to simulate the toroidal insulation, a gap resistance R G is inserted among coils which simulate a vacuum vessel. I G = n I n V V = R G I G
An iron core scarcely affects a poloidal field in a vacuum vessel. r It is assumed that a iron core, which is sufficiently fine and long, exists on the Z axis. r 2 A inductance is modified to include a magnetic flux in the iron core. M ( r, r ) 2 = µ χ 2 m πa r r 3 2 a χ m relative permeability
Results of Simulations Using the 2D-simulation code with the modifications for without the ferritic steel, time evolutions of ionization are computed. Effects of the toroidal gap and the iron core are investigated.... n e /n e-6 e-8 e- B T =.6T, W/O core, cut B T =.6T, W core, W/O cut B T =.6T, W core, cut B T =.6T, W core, cut..2 Time [s] The iron core and toroidal gap are required for the breakdown.
Results of Simulations (2) Using the modified simulation code, we calculated the relations of breakdown voltages and gas pressures. Comparing the simulation with the experiments, the range of neutral gas pressure for the breakdown is narrow, and the voltages become low. Effects of toroidal field has not yet obtained. 2 5 V loop [V] 5 B T =.3T(Exp) B T =.6T(Exp) B T =.6T(Exp) B T =.6T(Sim) -4-3 -2 - P[Pa] It is needed that more detail investigation of calculation results and a modification of a simulation model.
Configurations at Breakdown B T =.6T P=5x -3 Electron Density Current Density Poloidal Flux 2 Closed magnetic surfaces are produced and a breakdown happens there. Distributions of electron and current densities are not overlapped.
Time Evolution in Breakdown Failure B T =.6T P=5x -3 Pa t =.6x -3 ms t =.2x -3 ms t =.8x -3 ms t =.24x -3 ms 2 2 2 2 A breakdown was failed in spite of the production of closed magnetic surfaces.
Conclusion The equivalent permeability in the poloidal direction was measured. The usual theory is confirmed. Breakdown characteristics (voltage and pressure) were experimentally investigated. Dependences on the toroidal fields B T =.6T,.6T,.3T are obtained. Breakdown voltage decreases with the toroidal field strength. Modifying the simulation code for, computation results are compared with the experiments. The toroidal gap and the iron core are required for the breakdown. Quantitative agreements between the experiments and the simulation are not obtained. More modification of the code is needed.