Non-inductive build-up of a hot plasma by X-wave ECH/ECCD to remove initial high loop-voltage from start-up of fusion tokamaks

The possibility of non-inductive build-up of a hot plasma by X-wave ECH/ECCD to remove initial high loop-voltage stage from start-up of fusion tokamaks is theoretically investigated, using ITER parameters. There are two injection methods for X-wave ECH/ECCD in ITER start-up plasmas. The first is oblique X-wave injection from the low-field side, referred to as the DX method. For the low density start-up plasma at n e ≈ 1.0 × 10 19 m − 3 , the wave with ω / 2 π = 200 GHz and N ‖ ∼ 0.6 is damped away via cyclotron resonance absorption before encountering the cyclotron cutoff layer when the electron temperature is as high as T e > 170 eV. The wave power is deposited into the resonance electrons of which parallel velocity is as fast as v ‖ > 2 2 T e / m , which enables an efficient ECCD. The second method is oblique O-wave injection from the low-field side, referred to as the OX method. The waves pass through the ECR layer with negligible power deposition when T e < 400 eV and are mode-converted into oblique X-waves upon reflection on the inboard wall. The X-waves propagate toward the ECR layer without cutoff, and are cyclotron-damped away even when the temperature is as low as T e ∼ 7 eV. The OX method is appropriate for the initial open field stage and transition stage of the discharge, while the DX method is appropriate for the final tokamak stage, where T e is high. Discussions are given on the choice of appropriate B V and location of ECR layer for initiation of closed flux surfaces. Nine model plasmas from an initial open field plasma to a final hot tokamak plasma, consisting of a series of snapshots of a build-up discharge, are generated using an equilibrium theory, enabling estimation of required RF power to drive the current. These investigations suggest that an injection power P r f ∼ 4 MW could build up a hot plasma with I p = 205 kA, n e 0 = 1.2 × 10 19 m − 3 , T e 0 = 2 keV, a = 1 m and R 0 = 5.1 m.