Injection of a relativismo electron beam into a tokamak for current drive: Experimental results

Initial results of an experiment to inject an IREB (450–900 kV; 80–170 kA; 60 ns) into the Macrotor tokamak at UCLA (R = 95 cm; a = 45 cm, BT = 1–3 kG) with pre-existing plasma (np= 10¹²–10¹³ cm⁻³) and plasma current (Ip ≈10–30 kA) are described. Use of an efficient injector consisting of a 1.5 m long MITL and a foilless diode allowed for injected energies of up to 4 kJ at 900 kV with power fluxes in excess of 10¹⁰ W/cm². With co-injection, the beam (nb ≈ 1−2 × 10¹¹ cm⁻³) appears to follow single particle orbits with most of it being re-incident onto the back of the diode after 1, possibly 2, poloidal transits, representing typically 3 to 6 toroidal passes of the machine. Despite substantial loss of the beam to the back of the injector, the toroidal plasma current was enhanced by up to 5 kA. Calorimeter signals show some energy appearing on the minor axis.