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Slow-wave coupling to the lower-hybrid resonance

Previously, it has been shown that in a cold, inhomogeneous, magnetized plasma half-space the lower-hybrid resonance is accessible to the transverse-magnetic (TM) plane waves incident on the vacuum-plasma interface at an approximately grazing incidence, provided that at the hybrid layer ωpe/ωce ≲ 0.4. In this paper, these results are extended to the slow-wave case when nz, the refractive index in the static magnetic field direction, exceeds unity. It is found that the plasma is indeed accessible to the slow waves if Golant's accessibility criterion nz > 1 + (ωpe/ωce)2 is satisfied. The following recommendations can be made for coupling r.f. energy to the lower-hybrid resonance: (i) if ωpe/ωce ≲ 0.4, efficient coupling is possible by launching TEM-like waves on the plasma column, (ii) if ωpe/ωce ≳ 0.4 and if the transverse machine dimensions exceed the r.f. vacuum wavelength, it is possible to couple TM waves using passive slow-wave structures inside the machine walls, (iii) if ωpe/ωce ≳ 0.4, but for smaller machine dimensions, recourse must be taken to transverse-electric slow-wave coupling with current-carrying coils of appropriate periodicity . If, as was pointed out by Glagolev, propagation from the plasma edge to the hybrid layer is not materially affected by the inclusion of finite-temperature effects, by far the most elegant solution (with potential application to thermonuclear plasmas) for coupling r.f. energy from the second to the twentieth ion-cyclotron harmonic waves is by launching TEM-like waves in the coaxial waveguide formed by the plasma columnand the containing walls.