Measurements of Hot Electron Spectra From High Irradiance Laser Plasmas

This thesis presents measurements of the production of hot electrons during laser interactions in the irradiance range 10^{16} -10^{20} W cm^{−2} . This intensity regime is accessible with modest ultra-short lasers at ∼ 10^{16} Wcm^{−2} where resonance absorption dominates the electron acceleration. At the higher irradiances (10^{18} -10^{20} W cm^{−2}), petawatt class lasers are required where J × B electron accelera- tion occurs. Fast electron generation using the VULCAN Petawatt laser (irradiances ∼ 10^{20} W cm^{−2}) and the LASERIX facility (irradiances ∼ 10^{16} W cm^{−2}) have been investigated. Using the Vulcan Petawatt laser, with pulse duration of 1 - 2 ps at intensities greater than 10^{20} W cm^{−2}, electron energies up to 120 MeV with temperatures 20 - 30 MeV have been observed. A pre pulse was used to create an expanding plasma in which the high irradiance pulse was incident and the scale length of the pre pulse produced plasma was measured using optical probing. It was found that the number of hot electrons produced at 10^{20} W cm^{−2} linearly increases with the measured pre pulse plasma scale length indicating that the electron acceleration is dependent on the pre-pulse plasma volume. 2D PIC simulations are in agreement with experimentally measured temperatures, while 1D PIC simulations only agree for shorter scale length plasmas (L≤ 7.5 μm). Plasma channels are formed with longer scale length plasmas, but these channels do not result in greater acceleration of electrons as they are not sufficiently nar- row. The role of hot electrons in heating solid targets is experimentally examined and it is found that radiation effects on target heating when a pre pulse is present dominate over hot electron heating

Plasma scale length effects on protons generated in ultra-intense laser–plasmas

The energy spectra of protons generated by ultra-intense (1020 W cm−2) laser interactions with a preformed plasma of scale length measured by shadowgraphy are presented. The effects of the preformed plasma on the proton beam temperature and the number of protons are evaluated. Two-dimensional EPOCH particle-in-cell code simulations of the proton spectra are found to be in agreement with measurements over a range of experimental parameter

Exotic dense-matter states pumped by a relativistic laser plasma in the radiation-dominated regime

In high-spectral resolution experiments with the petawatt Vulcan laser, strong x-ray radiation of KK hollow atoms (atoms without n = 1 electrons) from thin Al foils was observed at pulse intensities of 3 x 10(20) W/cm(2). The observations of spectra from these exotic states of matter are supported by detailed kinetics calculations, and are consistent with a picture in which an intense polychromatic x-ray field, formed from Thomson scattering and bremsstrahlung in the electrostatic fields at the target surface, drives the KK hollow atom production. We estimate that this x-ray field has an intensity of >5 x 10(18) W/cm(2) and is in the 3 keV range

Micron-scale mapping of megagauss magnetic fields using optical polarimetry to probe hot electron transport in petawatt-class laser-solid interactions

The transport of hot, relativistic electrons produced by the interaction of an intense petawatt laser pulse with a solid has garnered interest due to its potential application in the development of innovative x-ray sources and ion-acceleration schemes. We report on spatially and temporally resolved measurements of megagauss magnetic fields at the rear of a 50-μm thick plastic target, irradiated by a multi-picosecond petawatt laser pulse at an incident intensity of ~1020 W/cm2. The pump-probe polarimetric measurements with micron-scale spatial resolution reveal the dynamics of the magnetic fields generated by the hot electron distribution at the target rear. An annular magnetic field profile was observed ~5 ps after the interaction, indicating a relatively smooth hot electron distribution at the rear-side of the plastic target. This is contrary to previous time-integrated measurements, which infer that such targets will produce highly structured hot electron transport. We measured large-scale filamentation of the hot electron distribution at the target rear only at later time-scales of ~10 ps, resulting in a commensurate large-scale filamentation of the magnetic field profile. Three-dimensional hybrid simulations corroborate our experimental observations and demonstrate a beam-like hot electron transport at initial time-scales that may be attributed to the local resistivity profile at the target rear

Experimental investigation of photon attenuation behaviors for concretes including natural perlite mineral

Perlite mineral contains relatively high water and in general hydrated obsidian forms the perlite which is mainly an amorphous volcanic glass. Photon attenuation properties for different concrete types including natural perlite mineral and B4C have been experimentally investigated by using different radioactive point sources at 81, 276, 303, 356, 384, 662, 1173, 1275 and 1333 keV. SEM and EDAX analyses were carried out to control the crystal structure of the selected concrete types. In this work, HPGe detector based on gamma spectrometer was employed for all experiments. The results revealed that among the prepared concrete samples, the P6 concrete sample has the lowest HVL and MFP values and thus, having best ability to attenuate gamma rays in comparison to the other prepared concretes. Keywords: Radiation shielding, Attenuation coefficient, Gamma spectrometer, Perlit

Detailed analysis of hollow ions spectra from dense matter pumped by X-ray emission of relativistic laser plasma

X-ray emission from hollow ions offers new diagnostic opportunities for dense, strongly coupled plasma. We present extended modeling of the x-ray emission spectrum reported by Colgan et al. [Phys. Rev. Lett. 110, 125001 (2013)] based on two collisional-radiative codes: the hybrid-structure Spectroscopic Collisional-Radiative Atomic Model (SCRAM) and the mixed-unresolved transition arrays (MUTA) ATOMIC model. We show that both accuracy and completeness in the modeled energy level structure are critical for reliable diagnostics, investigate how emission changes with different treatments of ionization potential depression, and discuss two approaches to handling the extensive structure required for hollow-ion models with many multiply excited configurations