Research on development of a density-tapered capillary gas cell for laser plasma acceleration

Abstract

Advanced particle accelerator concepts based on the high-power laser and the plasma interactions have been studied for the last three decades. They have been considered as suitable alternatives to the RF-based conventional accelerators. Femto-second (fs) and tera-watt (TW) scale lasers have become widely available and easily accessible even in small laboratories since 1980s. On the other hand, development of a proper plasma target is an important issue for more stable electron injections and higher energy electron beam generations. This thesis presents results of the numerical and experimental works aimed to develop the new gas/plasma target and its detailed diagnostics for the improved environment of the laser-plasma acceleration. The experimental study was undertaken using the 20 TW and 32 fs laser system in the Laser-Plasma Accelerator Laboratory (LPAL) at Gwangju Institute of Science and Technology (GIST). The laser system was established based on the chirped pulse amplification (CPA) technique with the Ti:sapphire oscillator as a fs seed generator. Three amplification stages including one regenerative amplifier and two multi-pass amplifiers were installed. The regenerative amplifier was optimized by suppressing the amplified spontaneous emission (ASE) value with the spectral-matching technique. This technique can provide an improved temporal contrast ratio for the amplified laser pulse. The two-staged multi-pass amplifier includes the 4-pass pre- and 4-pass main-amplifiers with 20- and 25-times amplification rate, respectively. The final pulse compression was done with the reflective grating pair down to 32 fs of the temporal pulse duration. This laser system also partially used for the experimental characterization of the newly designed gas and plasma capillary targets. The transverse interferometers were employed as the main tools of the gas and plasma diagnostics. In particular, a Nomarski interferometer with a nano-second (ns) probe pulse for the neutral gas distribution and a Mach-Zehnder interferometer with a fs probe pulse for the laser-induced plasma were used. In the laser wakefield acceleration, improving the electron injection rate and overcoming the energy gain limitations are the important issues. First, the density transition injection method has been studied and used for more controllable electron injection. I suggest a new scheme using the multi-gas injection for the stable density down-ramp generation in the relatively low density range and compare the detailed characteristics with the blade installed gas jet which has been used to generate a sharp density down-ramp. Second, the limited energy gain is mainly caused by the acceleration phase mis-matching (dephasing) between the laser-induced wakefield and the accelerated electron beam. The linearly increasing density distribution is suggested to suppress this dephasing issue. Therefore, a density tapered gas cell capillary has been designed based on the numerical simulation of three-dimensional computational fluid dynamics. The performance characteristics such as the fill-up time, density distribution, and so on, are also experimentally tested. The positively tapered plasma density was properly generated in the density range of 10^{18} ~ 10^{19} /cm^3 which is suitable for the laser wakefield accelerator. The density value was measured by the transverse Mach- Zehnder interferometer. Some additional preliminary works such as the first experimental results of the electron beam generation with the square cross-section capillary target are also presented. The particle-in-cell (PIC) simulation results are also given for the energy-tunable electron beam generation with the tapered density profile which is particularly aimed to the compact and tunable light source applications.