Cyclotron Resonance at the Particle–Strong Wave Interaction

Abstract

In this chapter we will consider a charged particle interaction with a strong EM wave in the presence of a uniform magnetic field along the wave propagation direction when the resonant effect of the wave on the particle rotational motion in the static magnetic field is possible. In vacuum, as a result of the interaction of a charged particle with a monochromatic EM wave and uniform magnetic field the resonance created at the initial moment for the free-particle velocity automatically holds throughout the interaction process due to the equal Doppler shifts of the Larmor and wave frequencies in the field. This phenomenon is known as “Autoresonance”. This property of cyclotron resonance in vacuum makes possible the creation of a generator of coherent radioemission by an electron beam, namely a cyclotron resonance maser (CRM). From the point of view of quantum theory the relativistic nonequidistant Landau levels of the particle in the wave field become equidistant in the autoresonance due to the quantum recoil at the absorption/emission of photons by the particle. In addition, the dynamic Stark effect of the wave electric field on the transverse bound states of the particle does not violate the equidistance of Landau levels in the autoresonance. Then the inverse process, that is, multiphoton resonant excitation of Landau levels by strong EM wave and, consequently, the particle acceleration in vacuum due to cyclotron resonance, in principle, is possible. In a medium with arbitrary refractive properties (dielectric or plasma) because of the different Doppler shifts of the Larmor and wave frequencies in the interaction process the autoresonance is violated. However, the threshold (by the wave intensity) phenomenon of electron hysteresis in a medium due to the nonlinear cyclotron resonance in the field of strong monochromatic EM wave takes place. In contrast to autoresonance, the nonlinear cyclotron resonance in a medium proceeds with a large enough resonant width. This so-called phenomenon of electron hysteresis leads to a significant acceleration of particles, especially in the plasmalike media where the superstrong laser fields of relativistic intensities can be applied. The use of dielectriclike (gaseous) media makes it possible to realize cyclotron resonance in the optical domain (with laser radiation) due to an arbitrarily small Doppler shift of a wave frequency close to the Cherenkov cone, in contrast to the vacuum case where the cyclotron resonance for the existing maximal powerful static magnetic fields is possible only in the radio-frequency domain.