High frequency Raman current drive

Current drive in reactor grade tokamak plasmas with a pulsed, high power free electron laser (FEL) is investigated. The stimulated Raman process generates fast, longitudinal electron plasma waves which accelerate electrons to relativistic energies. The fast electrons with vparallel much greater than vperpendicular are not trapped toroidally and the resulting current decays slowly. The paper presents a systematic study on different combinations of Raman forward and backward scattering and FEL polarization. The optimal FEL frequencies fall within the range 2.2-4.0ωp (left circularly polarized case) and 3.8-7.0ωp (right circularly polarized case) in a typical tokamak plasma with Ωe ≅ 1.5ωp. In a reactor grade plasma the best current drive efficiency, 5x1019A/W·m2, is obtained by using the stimulated Raman forward process and a left circularly polarized FEL beam. The FEL driven current can be localized to the plasma centre and amplified by the bootstrap effect to further enhance the overall efficiency. Raman current drive in high temperature reactor plasmas is quite different from small scale experiments with Te ∼ 1 keV, where the Raman backward process dominates and the current drive efficiency remains rather low. (author). 43 refs, 10 figs, 1 tab