High repetition rate laser driven proton source and a new method of enhancing acceleration

Gao, Ying (2020): High repetition rate laser driven proton source and a new method of enhancing acceleration. Dissertation, LMU München: Fakultät für Physik

Abstract

In the past few decades and with the increasing availability of high-intensity laser systems, laser ion acceleration has evolved into a mature and promising source for experiments with energetic ions. In particular, the latest laser-driven proton energy has reached nearly 100 MeV. Most applications require a stable ion source with high repetition frequency. The methods and strategies for realizing such repetitive laser-ion sources vary dramatically, in particular with respect to the employed target technology. In view of the interest of our research group on solid thin targets, the first focus of this PhD work was on an automated target positioning system that is employed in various research topics. A pilot study with one thousand targets was conducted with the nano-Foil Target Positioning System at the ATLAS 300 at Laboratory for Extreme Photonics (LEX Photonics), which was able to deliver laser pulses with a pulse energy of up to 6J and a pulse duration 25fs. Through real-time monitoring of various parameters of the laser pulses and targets, we have evaluated the stability of the proton source at a repetition rate of 0.5 Hz. During this study, we artificially varied parameters that were controllable and studied their impact on the proton yield. While scientifically interesting, the results did not clearly reveal the basis that would allow for stabilizing the proton source. It is likely that spatial-temporal contrast fluctuations contribute, which cannot yet be monitored on shot-to-shot. The request for repetition rate poses challenges to optimization strategies that rely on targets more complicated than plain foils. Among currently favored methods, which are reviewed in this thesis, are mass-limited-targets (MLT). Their lateral size is comparable to the laser focus diameter and therefore the energized electrons remain confined to a microscopically small volume such that acceleration fields are increased, as is the ion energy. However, the rapid positioning of MLTs is experimentally challenging. In order to find alternatives, we tested the generation of transient micro-targets by manipulating an initially plain foil with a Laguerre-Gaussian (LG) pre-pulse. This pre-pulse was introduced in the frontend of the CLAPA 200 TW laser at Peking University and passed through a spiral phase plate (SPP) before sending it back with a 1.7 ns advance to the main laser pulse into the laser chain. In the far-field, i.e. in the focus on the target, the LG pre-pulse results in a donut-shaped intensity distribution and initiates a ring-shaped plasma that is left to expand. The main laser pulse focuses on the center of this ring. The experimental results revealed a doubling of proton energy under the right pre-pulse intensity conditions. The evolution of the ring-pre-plasma expansion is modeled and the interaction between the main pulse with the transiently micro-plasma is studied by particle-in-cell simulations. The simulation results can recover the experimental observation, in particular, the proton energy increase in the relevant parameter range. Our understanding is in line with expectations that energetic electrons remain concentrated around the central part of the quasi-isolated micro-target, even though the target is not fully isolated but surrounded by a low-density plasma by the time of laser-plasma interaction at peak intensity.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: ultra-short laser pulse, laser-ion acceleration, Laguerre Gaussian pulse，quasi-isolated target
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 2. Juli 2020
• 1. Bericht­erstatter:in: Schreiber, Jörg