First experiments of plasma start-up assisted by ECRH on Tore Supra

Abstract

ECRH pre-ionization and assisted start-up will be necessary in ITER due to the low electric field available (⩽0.3 V m⁻¹). This paper reports on the results recently obtained on assisted start-up at the fundamental EC resonance on the tokamak Tore Supra (B_T= 3.85 T,R= 2.4 m,a= 0.72). The Tore Supra ECRH/ECCD system is based on two 118 GHz gyrotrons (EC resonance atR∼ 2.17 m). The power is injected into the plasma as Gaussian beams by an antenna located on the low field side which, using actively cooled mirrors inside the vacuum vessel, allows extensive control of both poloidal and toroidal injection angles. During the last experimental campaign, the ECRH system has been used for the first time to assist the plasma start-up. The usual start-up voltage on Tore Supra is 25 V (1.7 V m⁻¹) with deuterium prefill pressure of around 20 mPa. The minimum stray field value (B_⊥) in most of the vacuum vessel is lower than 5 mT, which gives toroidal connection lengths (a₀B_T/B_⊥) longer than 770 m. In these conditions, attempts have been made to lower the start-up voltage. Sustained breakdowns were obtained down to ∼0.4 V m⁻¹without external assistance at 5 mPa. Besides, ∼300 kW of ECRH has been applied during 50 ms (one gyrotron in O-mode). Pre-ionization was successfully achieved at the resonance location and ∼30 kA of current was established in 10 ms. Successful plasma initiation has been obtained down to ∼0.28 V m⁻¹at 5 mPa.