Study Of Positron Production By Ultra-intense Short Pulse Laser

Since the prediction and subsequent discovery of the positron in the1930s, there have been extensive investigations of positrons in material defects and phase transitions owing to the annihilation property of the positron. In both fundamental and applied studies, positron source generation is a subject of great interest. A present, positrons generated by accelerators and β+decay from radioactive isotopes are commonly used in research. However, the positron beams from these positron sources generally have long pulse durations, thus low beam densities, which cannot be used for astrophysical experiments or other applications. In recent years, with the development of laser techniques, relativistic positron beams created by ultra-intense lasers have attracted considerable attention owing to their outstanding properties, such as high yield, high density, and short pulse.Considering the current research status of the laser-based positron source, we have conducted the following studies in this thesis:1) numerical simulation studies of the positron production by ultra-intense lasers;2) preliminary experimental investigation of the positron generation by an ultra-intense femtosecond laser;3) diagnostic technique studies of the copropagating X-rays generated in laser-positron production experiments.1) In the part of the laser-positron-generation simulation studies, two typical positron-generation schemes were investigated in detail with particle-in-cell (PIC) and Monte Carlo (MC) simulations.As for the direct positron generation by laser-solid interactions, a hot electron emission model was set up according to the parameters of currently available laser systems. The hot electron transport process was simulated by the MC code Geant4. The dependence of the positron yield on the target material, the target thickness, and the hot electron temperature was investigated in detail. The simulation results show that Au is the best target material, and the positron yield increases with the target thickness and the hot electron temperature. It is shown that the positron spectrum is quasi-Maxwellian distribution with the positron temperature increasing with the hot electron temperature when the sheath field on the target back is not considered. To evaluate the effect of the target back sheath field, we have established a electrostatic field model in the thick target case. The acceleration effect of the sheath field on the positron was simulated by introducing a electric field according to our sheath field model. The results indicate that quasi-monoenergetic positron beams can be generated owing to the sheath field.In the indirect positron-generation part, based on the parameters of a typical100TW ultra-short laser, the electron acceleration in an underdense plasma by the laser was simulated by the PIC code Vorpal, and the positron generation by the electron beam in a solid target was modeled by the MC code Geant4. The dependence of the positron yield on the plasma density, the plasma length, and the solid target thickness was investigated in detail. It was found that to generate high yield positrons, laser-gas interaction should work in the near-bubble regime with a plasma density slightly higher than that used for the production of quasi-monoenergetic electron beams, and the optimal plasma length is basically consistent with the laser pump depletion length. In addition, the dependence of the positron beam properties (temperature, pulse duration, and divergence angle) on the plasma length is given.With the same parameters of a laser system (e. g. the SILEX laser at China Academy of Engineering Physics), An extensive comparison of the properties of positron beams produced by the ultra-intense femtosecond laser in direct and indirect schemes has been performed with PIC and MC simulations. It is shown that the positron beam generated in the indirect scheme has a higher yield (1010), a higher temperature (28.8MeV), a shorter pulse duration (5ps), and a smaller divergence (8°) than in the direct case (109yield,4.4MeV temperature,40ps pulse duration, and60°divergence). In addition, it was found that the positron/gamma ratio in the indirect scheme is one order of magnitude higher than that in the direct one, which represents a higher signal/noise ratio in the positron detection.2) In the part of the experimental investigation of laser-positron production, we have performed a preliminary experiment exploration on the SILEX laser facility.Firstly, a magnetic spectrometer was developed for the positron measurement, and examined on a traditional electron accelerator to verify its spectrum-measurement reliability. It was found that the spectrometer recorded clear positron signals in the test experiment. The experiment performed on the SILEX laser system did not produce any obvious positron signals. But we observed an apparent electron signal. By using Geant4code, we developed a method to deduce the hot electron temperature and the sheath field potential on the back of the target. By matching the experiment result and the MC simulation result of the electron spectrum, we estimated an initial hot electron temperature of1.5MeV and a sheath field potential of3.5MV.The absence of a clear positron signal in the experiment may be attributed to:1. The laser intensity was too low, only～1018W/cm2, leading to a low hot electron temperature, thus a low positron yield;2. The laser energy of the femtosecond laser we used has low energy (-J), resulting a low positron yield;3. The magnetic spectrometer used in the experiment has a poor shielding consideration, which cannot effectively suppress the high flux gamma rays generated with positrons.3) In the part of the diagnostic technique studies of the copropagating X-rays, we have investigated the response characteristics of a single-photon-counting charge-coupled-device (CCD) and a multi-channel y spectrometer. Besides, we have simulated a Cherenkov radiator of PbF2used for the measurement of the time process of511keV annihilation photons using MC method, and obtained its response parameters.The X-rays generated in the positron-production process contains amounts of information. By measuring these X-rays, we can obtain some important physical quantities related with the positron production process, such as hot electron temperature, laser-to-hot-electron energy conversion efficiency, energy and beam-diameter of wakefield electrons, etc.By using standard radioactive sources, the single-photon-counting CCD (PI-LCX1300) with its spectrum measurement range of2-30keV was absolutely calibrated. Also, we studied the detection efficiency and the reason of the formation of the so-called split-pixel event using MC method. An algorithm used for the treatment of the split-pixel event was developed. Hight signal/noise ratio of the X-ray spectra was obtained, thus the detection effiency was derived. Results indicate that the manufacture-provided data is consistent with that obtained by the experimental calibration and the MC simulation, revealing that event losses caused by data treatment using our algorithm is negligible. The simulation of the split-pixel event shows that the energy deposition of the incident X-ray photon is highly concentrated, and the split-pixel event is mainly caused by the electron cloud shift in the CCD field region.The spectral measurement properties of the multi-channel γ spectrometer was investigated with MC simulations. The detection efficiency and the peak-to-compton ratio were obtained. According to the simulations, the detection range of this spectrometer is below500keV. In addition, we have performed the preliminary experiment test for this spectrometer to verify its performance.Finally, we have conducted a series of simulations of the PbF2Cherenkov radiator used for the measurement of the time process of511keV annihilation photons to obtain its production efficiency of optical photons and its time broadening effect. The simulation results show that the radiator only leads to very short time broadening (tens of ps), which can meet the need of single-shot positron-annihilation life spectroscopy.