Realization of a relativistic mirror: Electromagnetic backscattering from the front of a magnetized relativistic electron beam

TLDR

Relativistic electron beams can act as mirrors for electromagnetic waves, enabling high‑frequency backscattering. The study demonstrates a relativistic mirror mechanism that could enable short‑pulse, ultrahigh‑power, tunable millimeter‑wave generators. An intense relativistic electron beam was injected into a cylindrical drift tube carrying a counter‑propagating 9.3 GHz, 170 kW electromagnetic wave. At an axial magnetic field of about 5 kG, the beam reflected the incident wave at ~40 GHz with power exceeding the incident power by several hundred kilowatts and nanosecond pulse duration, consistent with reflection from a refractive‑index discontinuity at the beam front near cyclotron resonance.

Abstract

An intense relativistic electron beam has been injected into a cylindrical drift tube containing a counterstreaming electromagnetic wave ${f}_{i}=9.3$ GHz and ${P}_{i}=170$ kW). Within a narrow range of axial magnetic field centered at 5 kG, a reflected wave at ${f}_{s}\ensuremath{\sim}40$ GHz was generated by the interaction of the beam with the incident wave. The reflected wave was observed to have a power of several hundred kilowatts (i.e., greater than the power of the incident wave) and a pulse duration on the order of nanoseconds. All observed experimental characteristics (viz. frequency shift, power amplification, pulse duration, and cyclotron resonance) were consistent with a model of reflection from the discontinuity in refractive index that is associated with an electron beam front near cyclotron resonance. This mechanism could be employed in a new class of short-pulse, ultrahigh power, tunable generators at millimeter and submillimeter wavelengths.