Optimum operation of gyrotwistrons

Relativistic gyrotwistrons, which can, in principle, achieve efficiencies in excess of 50%, are promising sources for driving particle accelerators. The optimum operating point for these devices represents a tradeoff between maximizing the available energy and minimizing the deleterious effects of velocity spread. The former increases with Doppler upshift and pitch ratio, while the effects of velocity spread are made worse by both of these quantities. We discuss the issues that go into deciding how to achieve maximum efficiency, with particular attention paid to the role of beam current