Ph.D Thesis

Ph.D StudentToroker Zeev
SubjectLight Amplification in Ionized or Excited Medium
DepartmentDepartment of Electrical and Computers Engineering
Supervisors PROF. Levi Schachter
PROF. Nathaniel Fisch
Full Thesis textFull thesis text - English Version


Many important scientific applications are based upon the interaction of light with matter, such as the generation of high intensity laser pulses, the generation of high harmonics, particle acceleration, the development of X-ray lasers, inertial confinement fusion, and optical metrology. In this study two related topics are considered: resonant electro-magnetic field amplification either in ionized or in excited media.

In the first part, the backward Raman amplification (BRA) process to generate high intensity laser pulse is explored. The BRA is a resonant nonlinear interaction in which a long laser pulse (pump pulse) transfers, by the mediation of a plasma wave (Langmuir wave), most of its energy to a counter propagating short laser pulse (seed pulse). The nonlinear interaction process has been studied in extended regimes of pump pulse, seed pulse, and plasma density parameters by using fluid and kinetic models. For example, for homogeneous plasma, proper chirping of the seed pulse can reduce the required plasma length and pump fluence by a factor of two compared to a non-chirped seed. In addition, the Raman efficiency in the Langmuir wavebreaking regime was investigated. The major result is that moderate efficiency of BRA can occur for pump intensity up to a few times larger than the wavebreaking threshold. Here, the wavebreaking threshold is defined as the initial pump intensity that generates, through the BRA, longitudinal electron quiver velocity that is equal to the phase velocity of the plasma wave.

In the second part, Cerenkov wake amplification in a cylindrical waveguide filled with an active (excited) medium has been studied. The principal application of such radiation is for accelerating an electron beam to high kinetic energy of the order of GeV in a compact structure size. In this approach a short bunch of electrons, the trigger bunch, is injected into the medium and generates a Cerenkov wake which in turn is amplified, and after reaching saturation it accelerates a second trailing bunch. Since the structure resembles the two-beam accelerator, we call it active medium two-beam accelerator

 (AM TBA).

Based on a semiclassical model, high wake field amplitude at saturation is obtained for a large waveguide radius and high density of initially excited atoms. Short saturation length is reached for a trigger bunch traveling at a velocity slightly larger than the threshold velocity for Cerenkov radiation. In addition, we have studied the wake dynamics for a trigger bunch that is either modulated or composed of discrete distribution of electrons that are initially located randomly. Our numerical simulations indicate that for a number of electrons above a threshold value the phase-spread of the wake can be reduced to a fraction of radians.

Finally, the system efficiency and ability to accelerate a second train of micro bunches have been explored. Since the trailing bunch generates a wake that tends to reduce the amplified wake, we have shown by numerical simulations that it is possible to obtain either high efficiency but with relatively high energy spread or high accelerating gradient but with low efficiency.