Dissipative Instability Under Weak Beam-Plasma Coupling

Summary form only given. Plasma filling of microwave devices makes possible to pass through e-beams with current that is several times higher than limiting current in vacuum devices. In its turn this gives possibilities to increase power of output radiation and gradually change its frequency. Theory of these devices actually considers cylindrical waveguide with thin annular e-beam and coaxial thin annular plasma. In wide range of the system parameters beam-plasma instability is caused by growing of the beam slow wave with negative energy. The growth rate of this instability attains maximum in resonance of plasma wave with beam slow wave. Instability of this type can develop in considered systems in different cases. First of them is weak coupling of the beam and plasma fields when the beam actually is left on its own. Secondly this is the case of very high beam currents when it exceeds limiting vacuum current. For overlimiitng e-beams proper oscillations reveal themselves significantly for both weak and strong coupling and resulted instability is caused by growing of the beam slow wave. Present investigation considers interaction of spatially separated electron beam and plasma in waveguide in limit of weak beam-plasma coupling. An approach is used that is based on smallness of coupling coefficient. It is shown that strong dissipation in the system leads to a new type of dissipative beam instability with more critical (as compared to conventional), inverse proportional dependence on dissipation i.e. nu^-frac12rarrnu^-1.. This new type of dissipative instability is result of superposition of two factors that lead to exhibition of the beam wave with negative energy: decreasing of the beam-plasma coupling and dissipation. Growth rate of this instability is obtained for arbitrary level of dissipation.