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Beam-plasma interaction is investigated in a model of plasma microwave oscillator: waveguide with spatially separated plasma and beam layers of finite thicknesses. Investigation is carried out in general form without specifying shape of the waveguide in cross section. Approach is based on perturbation theory over wave coupling. Spatial separation implies weak beam-plasma interaction that exhibits many specific features. Developing instability is caused by the growth of the negative energy beam wave. It is shown that upon weak coupling presence, dissipation leads to a new type of dissipative beam instability. Its maximal growth rate is inversely proportional to collision frequency in plasma. The growth rate of this instability is obtained for an arbitrary level of dissipation. Basic parameters of instability show that its properties cannot be neglected upon design of high-power, high-frequency plasma-filled generators/amplifiers based on relativistic e-beams.

The effect of dissipation on the instability caused by growing of the slow space charge wave of electron beam is studied. High level of dissipation leads to a new type of dissipative streaming instability. Its growth rate has a previously unknown, inverse proportional dependence on dissipation. The new dependence is a result of superposition of two factors, both of which lead to excitation of the beam wave with negative energy: increasing of beam current and dissipation. Basic parameters of the instability indicate that it can be an important additional factor upon design of high-power high-frequency devices based on relativistic e-beams.

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