Resonances,Absorption and Linear Excitation of Cyclotron Harmonic Waves in a Discharge Plasma

Standing cyclotron harmonic waves below the second harmonic are observed using the noise radiation of a mercury discharge tube at 1465 MHz. From the positions of the Buchsbaum-Hasegawa resonances the density profile and the electron temperature are derived, these parameters agree quite well with the behaviour of the collision-free, magnetized plasma column. A onedimensional model applicable to our waveguide geometry is developed taking into account linear excitation of cyclotron harmonic waves by a quasistationary external field in a bounded plasma. Absorption and excitation are controlled by the scale length L₁ at the hybrid point and a coupling length l_c, introduced by Eq. (20); the electron gyration radius r_c, scale length L₁, coupling length l_c, and slab thickness 2a obeying the relation r_c?l_c?L₁?2a. The main absorption is caused by enhanced collisional damping at the upper hybrid region and only a small fraction being dissipated in the wave mode, though its field strength is large compared with that at the plasma boundary (E_W/E(a) ≈ 6). An estimation of the mean field strength within the coupling region shows, that such a low power as 1 mW/cm² absorbed in the hybrid region may produce fieldstrength exceeding the typical threshold values for non-linear excitation mechanism.