M.Sc Thesis

M.Sc StudentVoin Miron
SubjectElectromagnetic Wake Amplification by Active Medium
DepartmentDepartment of Electrical and Computers Engineering
Supervisor PROF. Levi Schachter
Full Thesis textFull thesis text - English Version


Inelastic collisions of free electrons with atoms or molecules play a crucial role in a variety of phenomena.  Extensive theoretical and experimental studies were conducted on collisions of First Kind, when particles lose the energy to atoms.  Collisions of the Second Kind, occur when particles gain the energy from excited atoms or molecules, are as probable but attracted lesser attention.

This study considers a liner model for collective interaction of an electron beam with an active medium, through which the beam gains energy at the expense of the medium - a paradigm known as Particle Acceleration by Stimulated Emission of Radiation (PASER). Contrary to the first generation experiment wherein the bunch excites the medium and at the same time it is accelerated, here the trigger bunch excites an electromagnetic wake which has a discrete spectrum due to its confinement by a waveguide.

Specifically, the active medium consists of a high-pressure CO2 mixture, similar to that used in lasers, confined in a cylindrical waveguide. It is assumed that a single resonance of CO2 corresponding to 10.6 µm wavelength is active. A relativistic electron bunch - the trigger bunch, travels along the waveguide’s axis at velocity above Cherenkov’s. It generates an electromagnetic wake consisting of an infinite number of Bessel harmonics. The geometry of the waveguide may be selected such that one of the harmonics' frequencies is close to the active resonance frequency of the medium. In this case the resonant harmonic is amplified by the medium via stimulated emission of radiation resulting in exponential growth of the single mode of the wake as it propagates. It is shown that at a distance of several thousands of wavelengths behind the trigger the resonant mode dominates and the wake becomes effectively a monochromatic wave. The amplitude of the wake is limited by amount of energy stored in the medium and electric discharge of the gas mixture. It is estimated that fields on GV/m scale, may be reached exceeding the gradient in conventional (rf) accelerators by more than one order of magnitude. Beyond the implication on high energy physics this may revolutionize medical accelerators.

The analytic approach facilitates simple assessment of the effect of the various parameters on the accelerating gradient. Numerical simulations for a particular case of interest are presented.