OPTIMIZATION OF ENERGY GAIN IN THE BUBBLE REGIME OF LASER WAKEFIELD ACCELERATOR (LWFA)

Abstract

This work deals with optimization of the energy gain of the electrons inside the bubble in a LWFA. Acceleration of an electron to high energy in a short distance is now possible due to the development of laser Wakefield acceleration (LWFA). A compact-size high energy electron accelerator is possible with LWFA because the acceleration field in LWFA is 1000 times higher than that in the typical radio-frequency electron accelerator. The LWFA scheme also can generate an ultrashort electron bunch because the acceleration region in LWFA is very narrow. But the energy gain of electrons in a LWFA is limited by several factors, e.g. pump depletion, electron dephasing, and laser diffraction. To work around these limitation in LWFA requires increasing the laser plasma interaction length. So, in this work, a uniform density region followed by a ramp structure is employed as a plasma medium, through which laser propagates. Number of simulations have been conducted by varying the ramp length and keeping the laser parameters constant, and the effects were studied and simulation results were compared with each other. By varying the ramp length, variation in plasma channel density is achieved. As a result of variation in plasma density, the acceleration distance is directly affected, which in turns increases the energy gained by the electrons. The simulation results show that by employing upward density ramp, the energy gained by electrons have been increased from 100MeV to almost 300MeV. There is a significant increase in the bubble radius too, as the radius of the bubble increases the radial accelerating and focusing force have a dominant effect on the electron acceleration and the beam quality improves. Also with the increase in the density ramp length, the number of monoenergetic electrons getting trapped in the bubble increases. Another fact that is notice from this simulation is the bubble formation takes place in the uniform density region. And in the meantime the acceleration distance is also increased. As I am simulating the plasma with a 10 Tera-watt driver laser in non-linear regime (𝑎0 2 ≫ 1), laser pump depletion limitation will be more evident than other factors. And the simulation shows that with increasing ramp length, the pump depletion length also increases. That means, the electrons are accelerated for more plasma length compared to one with uniform plasma density profile. All simulations are done in 2D-PIC codes with the help of VORPAL codes developed and marketed by Tech-X Corporation. The Vorpal outputs are, in the HDF5 format, analyzed using a python solver.