Current drive by beat waves generated with collinear high frequency electromagnetic waves

The paper discusses the feasibility of a current drive technique based on the collinear excitation of plasma waves by two high frequency electromagnetic pump waves. Particle trapping into the intense plasma wave creates hot electrons which sustain a slowly decaying current. Momentum and energy transfer to electrons can be enhanced by pump wave cascading. The method offers possibilities to localize the generation region and to control the current profile. With a CO2 laser, the maximum current density rises to approximately 50 kA?cm?2 at 1012 W?cm?2 in a 1015 cm?3 plasma. A current density of 1.0 kA?cm?2 in a 1014 cm?3 plasma is predicted for a 100 ?m free electron laser operating at 4 ? 1010 W?cm?2. The total induced current is about Jtot (MA) R(m)ne(1014cm?3)/Pavg(MW)0.5 (R is the major radius, ne is the plasma density and Pavg is. the average laser power) for the 10-200 ?m cases considered; the actual value depends on the hot electron collision rate and the amount of cascading taking place. The main problems and the basic parameter scalings are discussed. Also, a novel method for amplifying the induced current by the bootstrap effect is suggested. A seed current of 0.2-0.4 MA in a spot of 30-50 cm diameter would be sufficient for a tokamak reactor; this appears achievable as far as laser requirements are concerned.