Experimental study of the microwave emission from electrons in air

TL;DR

This study experimentally confirms microwave emission from electrons in air via bremsstrahlung, measuring its angular distribution and yield, and extrapolates findings to ultra-high-energy cosmic ray conditions.

Contribution

It provides the first direct measurement of microwave emission from electron air interactions, clarifying emission mechanisms and scaling behavior.

Findings

Microwave emission is well described by bremsstrahlung model.

Emission is anisotropic, peaked in the forward direction.

Yield scales linearly with the number of electrons.

Abstract

We searched for the emission of microwave radiation in the Ku band generated by a 95 keV electron beam in air. We unequivocally detected the radiation, and measured its yield and angular dependence. Both the emitted power and its angular pattern are well described by a model, where microwave photons are generated via bremsstrahlung in the free-electron atomic-nucleus collisions, during the slowdown of the electrons. As a consequence, the radiation is not isotropic but peaked in the forward direction. The emission yield scales proportionally with the number of electrons. This contrasts a previous claim that the yield scales with the number squared, due to coherence. With a Monte Carlo simulation we extrapolate our results to the Ultra High Energy Cosmic Ray energy range.