Effect of Energetic Particle Absorption on Fast-Wave Current Drive

Abstract

Fast-wave current drive presents a promising scheme for steady-state operation of reactor tokamaks. This scheme is being studied for application in the International Thermonuclear Experimental Reactor (ITER), the Joint European Torus (JET), and the Doublet III-D reactor (DIII-D). There are two regimes that appear to be promising, the low-frequency range 0 {lt} {omega} {lt} 2{Omega}{sub D} and the lower hybrid frequency range {Omega}{sub D} {much lt} {omega} {lt} {omega}{sub LH}. In the latter scheme, the wavelength of the fast wave becomes much shorter than the alpha particle gyroradius and alpha-particle absorption can become significant. An analytic formula for alpha-particle absorption of fast waves for the standard slowing down distribution has been derived and compared with electron absorption at ITER parameters. It has been found that at t{sub D} {gt} 30 keV and n{sub e} {approximately} 10{sup 14} cm{sup {minus}3}, the alpha-particle absorption is large and can greatly decrease the current drive efficiency. However, without sacrificing the fusion reactivity rate, by increasing the density and decreasing the temperature 15 keV {lt} T{sub D} {lt} 25 keV, the alpha-particle absorption can become small at a sufficiently high frequency. In this paper is suggested that a simulation of the alpha-particle absorption effect onmore » fast-wave current drive can be made in DIII-D by using a lower frequency source ({approximately}30 MHz) to create a minority tail and a high-frequency source (200 MHz) to drive the current. Results of minority absorption are presented. Effects that can improve current drive efficiency are discussed.« less