Tokamak start-up with electron-cyclotron heating

Experiments are described in which the start-up voltage in a tokamak is reduced by about a factor of two by the use of a modest amount of electron cyclotron resonance heating power for pre-ionization. The solution of the zero-dimensional start-up equations indicates that the effect is due to the high initial density which increases the rate at which the conductivity increases in the neutral-dominated initial plasma. The effect extrapolates favourably to larger tokamaks. A 50% reduction in the start-up volt-second requirement and impurity reflux is also observed. One important technological difficulty with a tokamak fusion reactor is the high toroidal voltage required to initiate the discharge when the temperature, density and conductivity are low and energy losses are dominated by line radiation from low-Z, partially-stripped impurities [ 1 ]. One possible means for reducing the loop voltage is the use of electron cyclotron resonance heating (ECRH) to produce a plasma of modest conductivity before the onset of Ohmic heating [2]. ECRH has the added advantage of initiating the discharge in a local region away from the walls so that one might expect reduced impurity reflux from the walls during the early stages of the discharge when the plasma is poorly confined. ECRH has been used previously for plasma production in a purely toroidal field. Anisimov et al. [3] injected microwave power from the high-magnetic-field side and observed absorption at the upper hybrid layer. ECRH bulk heating in tokamaks with high power (100 kW) gyrotrons has also been performed [4, 5]. A comparison of pre-ionization techniques was performed by Bulyginskii et al. [6] in FT-1. Main-electric-field breakdown and initial-reverse-field breakdown were carried out at roughly twice the loop voltage which was recorded for the 50 kW of the ECRH pre-ionization discharge. It is noteworthy that the 50 kW were launched from the low-field side. Cho et al. [7] launched about 20 kW into WT-1 in the vertical plane. WT-1 exhibited an initial-loop-voltage reduction of about 40% with this ECRH. Gilgenbach et al. [8] used 80 kW from the high-field side of ISX-B and observed pre-ionized densities of 5 X 10 cm" and electron temperatures of about 10 eV. It was suggested that the 40% reduction in loop voltage was due to the initially low resistivity of the pre-ionized plasma. Our work differs from other published work in that the voltage reduction is achieved with lower ECRH power (10 kW) and hence lower initial densities. In addition, we have injected both from the low-field side and the vertical plane with similar results. Furthermore, we provide a zero-dimensional start-up code which indicates that the effect (1) arises NUCLEAR FUSION, Vol.21, No. l l (1981) 1483