Efficiency improvement in relativistic Cerenkov devices due to dynamic cyclotron resonance

The efficiency enhancement in relativistic electron devices with linear electron beams guided by homogeneous magnetic fields due to combined Cerenkov-cyclotron interaction has been studied. The case considered is when electrons do not satisfy the cyclotron resonance condition initially, but start interaction with an electromagnetic field under the condition of pure Cerenkov synchronism. Then, the changes in electron energy due to the Cerenkov interaction lead some electrons to satisfy the cyclotron resonance condition. This effect, called the dynamic cyclotron resonance, may play a significant role in strongly nonlinear regimes of a primary interaction when an electron beam is separated into groups of accelerated and decelerated particles. For the case of the primary Cerenkov interaction, two groups of accelerated and decelerated electrons can exist which interact with the RF field under the same Cerenkov resonance condition, but due to dynamic shifts in electron cyclotron frequencies, can resonantly interact with different harmonics or the gyrofrequency. It is shown by numerical simulations that the efficiency can be improved 25-28% for typical Cerenkov interaction up to 50% by choosing an appropriate value of the focusing magnetic field.