High harmonic fast waves in high beta plasmas

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Abstract
High harmonic fast magnetosonic waves in high beta plasmas are investigated. In the high beta regime, a combination of reduced group velocity and a high beta enhancement of magnetic pumping lead to such large values of electron absorption that one can expect strong (≊100%) single pass absorption. In particular, by controlling the wave spectrum, the prospect of localized electron heating and current drive appears to be feasible in high beta low‐aspect‐ratio tokamak regimes. Inclusion of finite‐Larmor‐radius terms reveals an accessibility limit in the high ion beta regime (βi≊50% for a deuterium plasma) due to mode conversion into an ion‐Bernstein‐wave‐like mode. No similar beta limit is expected for electrons. With increasing ion beta, the ion damping can increase significantly, particularly near the mode‐conversion accessibility limit. The presence of an energetic ion component expected during intense neutral beam injection and α heating does not appear to modify the accessibility condition nor cause excessive wave absorption by the energetic ions.