Observation of inverse Čerenkov interaction between free electrons and laser light

Abstract

Momentum exchange was observed between laser light and an electron beam using the inverse \ifmmode \check{C}\else \v{C}\fi{}erenkov effect. This interaction was accomplished by introducing a gas with an index of refraction which reduced the phase velocity of the light wave to match the velocity of the electron. A 30-MW Nd: YAG 1.06-\ensuremath{\mu}m laser intersected 102-MeV electrons at an angle of 18 mrad in hydrogen gas. The beams overlapped in the interaction region for approximately ${10}^{5}$ optical wavelengths. The energy exchange by the inverse \ifmmode \check{C}\else \v{C}\fi{}erenkov effect was verified in two ways: First, a change was observed in the electron energy distribution in the presence of the laser, and second, this change was observed to be a function of the index of refraction, as determined by the pressure of the gas. A \ifmmode\pm\else\textpm\fi{}13% variation about the pressure for optimum energy exchange reduced the interaction by one-half. The results of the experiment agree with the predictions of a Monte Carlo computer simulation of the interaction. Methane gas was also investigated as a phase-matching medium. Possible applications include laser-driven particle accelerators and stimulated \ifmmode \check{C}\else \v{C}\fi{}erenkov devices, such as optical klystrons and traveling wave tubes.