W. Bang

Optimum laser intensity for the production of energetic deuterium ions from laser-cluster interaction

We measured, using Petawatt-level pulses, the average ion energy and neutron yield in high-intensity laser interactions with molecular clusters as a function of laser intensity. The interaction volume over which fusion occurred (1–10 mm3) was larger than previous investigations, owing to the high laser power. Possible effects of prepulses were examined by implementing a pair of plasma mirrors. Our results show an optimum laser intensity for the production of energetic deuterium ions both with and without the use of the plasma mirrors. We measured deuterium plasmas with 14 keV average ion energies, which produced 7.2 × 106 and 1.6 × 107 neutrons in a single shot with and without plasma mirrors, respectively. The measured neutron yields qualitatively matched the expected yields calculated using a cylindrical plasma model.

Experimental study of fusion neutron and proton yields produced by petawatt-laser-irradiated D2-3He or CD4-3He clustering gases

We report on experiments in which the Texas Petawatt laser irradiated a mixture of deuterium or deuterated methane clusters and helium-3 gas, generating three types of nuclear fusion reactions: D(d,^{3}He)n, D(d,t)p, and ^{3}He(d,p)^{4}He. We measured the yields of fusion neutrons and protons from these reactions and found them to agree with yields based on a simple cylindrical plasma model using known cross sections and measured plasma parameters. Within our measurement errors, the fusion products were isotropically distributed. Plasma temperatures, important for the cross sections, were determined by two independent methods: (1) deuterium ion time of flight and (2) utilizing the ratio of neutron yield to proton yield from D(d,^{3}He)n and ^{3}He(d,p)^{4}He reactions, respectively. This experiment produced the highest ion temperature ever achieved with laser-irradiated deuterium clusters.

Thermal and log-normal distributions of plasma in laser driven Coulomb explosions of deuterium clusters

In this work, we explore the possibility that the motion of the deuterium ions emitted from Coulomb cluster explosions is highly disordered enough to resemble thermalization. We analyze the process of nuclear fusion reactions driven by laser–cluster interactions in experiments conducted at the Texas Petawatt laser facility using a mixture of D2+3He and CD4+3He cluster targets. When clusters explode by Coulomb repulsion, the emission of the energetic ions is “nearly” isotropic. In the framework of cluster Coulomb explosions, we analyze the energy distributions of the ions using a Maxwell–Boltzmann (MB) distribution, a shifted MB distribution (sMB), and the energy distribution derived from a log-normal (LN) size distribution of clusters. We show that the first two distributions reproduce well the experimentally measured ion energy distributions and the number of fusions from d–d and d-3He reactions. The LN distribution is a good representation of the ion kinetic energy distribution well up to high momenta where the noise becomes dominant, but overestimates both the neutron and the proton yields. If the parameters of the LN distributions are chosen to reproduce the fusion yields correctly, the experimentally measured high energy ion spectrum is not well represented. We conclude that the ion kinetic energy distribution is highly disordered and practically not distinguishable from a thermalized one.

Study of the yield of D-D, D-3He fusion reactions produced by the interaction of intense ultrafast laser pulses with molecular clusters

The interaction of intense ultrafast laser pulses with molecular clusters produces a Coulomb explosion of the clusters. In this process, the positive ions from the clusters might gain enough kinetic energy to drive nuclear reactions. An experiment to measure the yield of D-D and D-3He fusion reactions was performed at University of Texas Center for High Intensity Laser Science. Laser pulses of energy ranging from 100 to 180 J and duration 150fs were delivered by the Petawatt laser. The temperature of the energetic deuterium ions was measured using a Faraday cup, whereas the yields of the D-D reactions were measured by detecting the characteristic 2.45 MeV neutrons and 3.02 MeV protons. In order to allow the simultaneous measurement of 3He(D,p)4He and D-D reactions, different concentrations of D2 and 3He or CD4 and 3He were mixed in the gas jet target. The 2.45 MeV neutrons from the D(D,n)3He reaction were detecteded as well as the 14.7 MeV protons from the 3He(D,p)4He reaction. The preliminary results will be shown.

A laser application to nuclear astrophysics

In the last decade, the availability in high-intensity laser beams capable of producing plasmas with ion energies large enough to induce nuclear reactions has opened new research paths in nuclear physics. We studied the reactions 3He(d,p)4He and d(d,n)3He at temperatures of few keV in a plasma, generated by the interaction of intense ultrafast laser pulses with molecular deuterium or deuterated-methane clusters mixed with 3He atoms. The yield of 14.7 MeV protons from the 3He(d,p)4He reaction was used to extract the astrophysical S factor. Results of the experiment performed at the Center for High Energy Density Science at The University of Texas at Austin will be presented.

Double shock front formation in cylindrical radiative blast waves produced by laser irradiation of krypton gas

Radiative blast waves were created by irradiating a krypton cluster source from a supersonic jet with a high intensity femtosecond laser pulse. It was found that the radiation from the shock surface is absorbed in the optically thick upstream medium creating a radiative heat wave that travels supersonically ahead of the main shock. As the blast wave propagates into the heated medium, it slows and loses energy, and the radiative heat wave also slows down. When the radiative heat wave slows down to the transonic regime, a secondary shock in the ionization precursor is produced. This paper presents experimental data characterizing both the initial and secondary shocks and numerical simulations to analyze the double-shock dynamics.

The Texas petawatt laser and current experiments

The Texas Petawatt Laser is operational with experimental campaigns executed in both F/40 and F3 target chambers. Recent improvements have resulted in intensities of >2×1021 W/cm2 on target. Experimental highlights include, accelerated electron energies of >2 GeV, DD fusion ion temperatures >25 keV and isochorically heated solids to 10-50 eV.

Temperature measurements of cluster fusion plasmas using D −3 He or CD4 −3 He mixtures on the Texas Petawatt

We present a novel way of determining the plasma temperature in a laser-cluster fusion experiment on the Texas Petawatt laser, which uses the ratio of the 2.45 MeV neutron and 14.7 MeV proton yields.

All-reflective Ti:Sa power amplifier for Petawatt laser

An all-reflective Ti:Sa power amplifier capable of extracting up to 60 joules has been designed. The power amplifier consists of four passes through a 100mm Ti:Sa crystal really imaged by off-axis parabolas enclosed in a vacuum chamber.