Kinetic Analysis of the Characteristics of Electron Cyclotron Heating Assisted Ohmic Start-Up in the Trapped Particle Configuration of a Tokamak

Abstract

Electron cyclotron heating (ECH) assisted start-up is considered to be necessary for reliable start-up of tokamaks with superconducting central solenoid (CS) because of the low loop voltage. Pure Ohmic start-up using the CS requires the field-null configuration to minimize electron loss. For ECH assisted Ohmic start-up, the trapped particle configuration (TPC) was found to have wider operational parameter space than the field-null configuration experimentally. In this work, we have analyzed electron transport under the TPC using the orbitaveraged kinetic equation. The global electron distribution function was simulated by solving for the steady-state distribution function. The parameter boundary for successful pre-ionization was estimated by evaluating the net particle number growth rate from the total ionization rate and the particle flux out of the limiter boundary. Upper limit of the ECH power was predicted as that for the net particle number to grow. In the absence of the inductive electric field, the simulated high ECH power limit increased with neutral pressure and vertical field strength, consistently with the experimental results. Application of loop voltage did not change this behavior qualitatively up to the inductive electric field of 0.48 V/m, which is the typical range of low voltage start-up experiments. c