Characteristics of electron-cyclotron-resonance-heated tokamak power reactors

A set of constraints is derived for the operating characteristics of tokamak power reactors which are bulk-heated by electron cyclotron resonance heating (ECRH). Four heating modes are considered: ordinary wave heating at the electron cyclotron frequency, Ω, and at the second-harmonic frequency, 2Ω, and extraordinary-wave heating at Ω and at 2Ω. For ordinary-wave heating at Ω, which appears to be the most promising method, the wave frequency ω ≈ Ω must exceed the plasma frequency, ωp, for wave penetration into the plasma. The authors' main conclusion is that the need for high-density operation (n0 > 4 × 1020m−3) in moderate-size tokamak reactors, coupled with the wave accessibility condition Ω > ωp, leads to the requirement of frequencies in the 200-GHz range for ECRH of reactor plasmas. A further condition on the heating frequency may be derived by consideration of the ignition condition using empirical scaling laws for the energy confinement time. This latter condition does not increase the heating frequency requirement unless impurities are present or the energy confinement degrades with increasing temperature. It is also found that, for ordinary-wave heating at Ω, the average plasma β is limited to less than 0.039 for a central temperature below 15 keV, assuming parabolic density and temperature profiles. The use of extraordinary heating at Ω might lower the frequency requirement for ECRH of a reactor. However, it appears to be unattractive for reactor operation because, in order for the wave to penetrate to the centre of the plasma, the heating ports must be located on the inside of the torus. High-beta, lower field reactors can be heated from the outboard side of the torus with second-harmonic radiation. However, these devices will have to be heated at frequencies which are as high as or higher than those needed for devices which are heated with the ordinary wave at the fundamental frequency. Extrapolation of cyclotron resonance maser (gyrotron) technology to provide modular heating systems in the 200-GHz range is discussed.