A Nd : YAG laser has been used to modulate the energy of a free electron beam. With a single pass of the 2−MW laser light through the 100−MeV electrons we have measured a 37−keV increase in the width of the energy spectrum for the particles. The interaction occurred in helium at standard temperature and pressure over a distance corresponding to 105 optical wavelengths, and a phase synchronism condition was maintained by means of the inverse Cerenkov effect. That is, along the direction of motion of the electrons the phase velocity of the electromagnetic wave equaled the electron velocity. Therefore, the electrons remained in an electric field of constant phase, resulting in significant energy exchange.

Momentum modulation of a free electron beam with a laser

This paper presents an analysis of the bunching of an electron beam at optical wavelengths by means of the inverse Cerenkov effect. A laser light beam is used to modulate the velocity of the electrons, and this velocity modulation becomes charge‐density modulation after an appropriate drift distance. The effects of energy and angular spread of the electrons are included, as well as the impact of scatterings in the index‐matching medium. Bunching can occur with reasonable parameter values and the drift distance is on the order of several centimeters.

Analysis of bunching of an electron beam at optical wavelengths

An analysis and experimental verification of momentum modulation of relativistic electrons by laser light using the inverse Cerenkov effect is presented. As an alternative to the free-electron laser for achieving energy exchange between particles and photons, the inverse Cerenkov effect uses the index of refraction of a gaseous medium to retard the phase velocity of an electromagnetic wave, enabling the electrons to remain in a field of constant phase. The momentum modulation converts to charge-density modulation by allowing the electrons to drift, thus forming electron bunches separated by optical wavelengths. An analysis is presented for the maximum amount of energy exchange, the energy exchange distribution, and the optimum bunching distance. A computer simulation of the interaction process is also given. These results are compared with the observed momentum modulation of a 102 MeV electron beam by a 30 MW Nd:YAG 1.06 μm laser in both hydrogen and methane gases. Initial observation of coherent optical radiation from a 57 MeV electron beam using the same laser system is also presented. Laser-driven particle accelerators and optical klystrons are possible applications of this interaction.

Free-electron interactions with light using the inverse Cerenkov effect

Coherent emission of light in the uv or vuv range can be obtained from an undulator or optical klystron by harmonic generation using an external laser. We show that if the input is a multimode laser (instead of an ideal monochromatic coherent wave) the kinetics of growth of the rms beam modulation is completely different from the one described by the usual Webster formula, and we find a simple analytical expression for a Gaussian input (large number of modes). Numerical calculations show that with 3 modes the result is already close to the Gaussian case, and that the frequency bandwidth increases as the square root of the harmonic number.

Harmonic generation from a multimode laser in an optical klystron

Momentum modulation of relativistic electrons by laser light using the stimulated Cerenkov interaction is analyzed. This interaction is accomplished by introducing a gas whose index of refraction retards the phase velocity of the light wave. By intersecting the electron and laser beams at the Cerenkov angle, ϑc = cos−1(1/nβ), where n is the index of refraction of the gas, and β is the ratio of the electron velocity over the velocity of light in vacuum; the electrons remain in a field of constant phase resulting in a cumulative energy exchange. The laser‐induced change in particle momentum and the electron beam energy spectrum are analyzed as a function of the electron and laser‐beam characteristics. The results agree with a computer simulation of the interaction.

D.A. Varshalovich

Papers

Optical modulation of electron beam by inverseCˇerenkov effect

Quantum theory of optical frequency modulation of an electron beam.