The warm-fluid equation derived from the drift kinetic equation is solved numerically to investigate the electrostatic low-frequency stability of an electron ribbon lieuin drifting in the crossed-fields of a planar magnetron. The temperature effect is manifested only for oblique propagation with respect to the drifting beam direction. The dispersion relation takes the form w=w(k⊥, K||)=w(k_||/k_⊥). where k_|| and k_⊥ are respectively the components of the surface wave vector k parallel and perpendicular to the magnetic field. The obliqueness of the propagation direction and the non-zero temperature give rise to a resonant instability, in addition to the diocotron instability, and thewavenumber corresponding to the maximum diocotron growth rate shifts as the temperature changes.
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