Theory Of Microwave Generation By An Intense Relativistic Electron Beam In A Rippled Magnetic Field

Motivated by the recent results of Friedman and Herndon the authors have investigated theoretically the stability of a waveguide enclosed relativistic electron beam in a rippled magnetic field. The presence of the ripple can couple negative energy beam modes to positive energy waveguide modes to produce instability and thereby generate microwaves. In the experiment the thickness of the beam is small compared to all other relevant lengths. The nonlinear dynamics of an infinitessimally thin sheet beam is analyzed. The system is described by the orbit equations for the particles, Maxwell's equations, a set of nonlinear boundary conditions across the moving surface and a continuity equation for the moving surface. (Author).

The stability of waveguide modes in a waveguide along which is propagated an unneutralized, intense beam of relativistic electrons guided by an applied axial magnetic field is considered. It is found that the waveguide mode can interact unstably with the beam by resonating with the Doppler shifted cyclotron frequency of the beam. This instability appears to be the mechanism by which microwaves are produced in some recent experiments which are discussed. (Author).

Microwave generation experiments were carried out on a multimegavolt electron accelerator (3.3 MV, 80 kA). X-band microwave power as high as about 1 GW was measured. Results are compared with a non-linear theory of unstable resonant interaction between a TE waveguide mode of the electron drift tube and an electron cyclotron wave on the beam.

A Nobel Foundation Symposium on the subject: "Nonlinear Effects 1n Plasmas", was held at Aspenasgarden, Lerum, in the G6teborg area of Sweden from June 11-17, 1976. The Symposium was the 36th in the series of Nobel Foundation Symposia, which have been held mainly within the areas of physics, chemistry, medicine, literature and peace prizes. Some 30 leading experts from the United States, Soviet Union, Japan and Western Europe attended the Symposium. The purpose of the Symposium was to discuss various topics in the field of modern plasma physics. We had to select from this vast area of active research a suitable common theme with a great number of new and interesting contributions. We decided to devote our Sym posium in particular to nonlinear effects in plasmas and to emphasize some areas where important developments seemed to be taking place. In recent years basic theory and experiments in nonlinear plasma physics have been stimulated largely by the need for an energy source based on fusion of light nuclei. In many laboratories all over the world attempts are being made to come closer to the final goal by studying magnetically confined plasmas and systems of inertial con finement. Heating of plasmas to fusion temperatures remains a key problem. There are good reasons to believe that the nonlinear effects in plasmas will play an important role for fusion, a long-range program which is still largely in its basic research phase.