Thomas E. Cowan

Energetic proton generation in ultra-intense laser–solid interactions

An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.

Electron, Photon, and Ion Beams from the Relativistic Interaction of Petawatt Laser Pulses with Solid Targets

In recent Petawatt laser experiments at Lawrence Livermore National Laboratory, several hundred joules of 1 μm laser light in 0.5–5.0-ps pulses with intensities up to 3×1020 W cm−2 were incident on solid targets and produced a strongly relativistic interaction. The energy content, spectra, and angular patterns of the photon, electron, and ion radiations have all been diagnosed in a number of ways, including several novel (to laser physics) nuclear activation techniques. About 40%–50% of the laser energy is converted to broadly beamed hot electrons. Their beam centroid direction varies from shot to shot, but the resulting bremsstrahlung beam has a consistent width. Extraordinarily luminous ion beams (primarily protons) almost precisely normal to the rear of various targets are seen—up to 3×1013 protons with kTion∼several MeV representing ∼6% of the laser energy. Ion energies up to at least 55 MeV are observed. The ions appear to originate from the rear target surfaces. The edge of the ion beam is very shar...

Nuclear fusion from explosions of femtosecond laser-heated deuterium clusters

As a form of matter intermediate between molecules and bulk solids, atomic clusters have been much studied. Light-induced processes in clusters can lead to photo-fragmentation, and Coulombic fission, producing atom and ion fragments with a few electronvolts (eV) of energy. However, recent studies of thephotoionization of atomic clusters with high intensity (>1016 W cm−2) femtosecond laser pulses have shown that these interactions can be far more energetic—excitation of large atomic clusters can produce a superheated microplasma that ejects ions with kinetic energies up to 1 MeV (ref. 10). This phenomenon suggests that through irradiation of deuterium clusters, it would be possible to create plasmas with sufficient average ion energy for substantial nuclear fusion. Here we report the observation of nuclear fusion from the explosions of deuterium clusters heated with a compact, high-repetition-rate table-top laser. We achieve an efficiency of about 105 fusion neutrons per joule of incident laser energy, which approaches the efficiency of large-scale laser-driven fusion experiments. Our results should facilitate a range of fusion experiments using small-scale lasers, and may ultimately lead to the development of a table-top neutron source, which could potentially find wide application in materials studies.

Hot electron production and heating by hot electrons in fast ignitor research

In an experimental study of the physics of fast ignition the characteristics of the hot electron source at laser intensities up to 10(to the 20th power) Wcm{sup -2} and the heating produced at depth by hot electrons have been measured. Efficient generation of hot electrons but less than the anticipated heating have been observed.

High energy proton acceleration in interaction of short laser pulse with dense plasma target

The generation of high energy protons from the interaction of a short laser pulse with a dense plasma, accompanied by a preformed low density plasma, has been studied by particle-in-cell simulations. The proton acceleration toward the laser direction in the preformed plasma is characterized by a time-dependent model and the peak proton energy is given. The effect of electron recirculation on the rear side sheath acceleration is discussed and it is found that the peak proton energy increases in inverse proportion to the target thickness. These results shed light on the peak proton energy dependence on laser intensity, laser pulse length, and target thickness. Finally the optimal parameters of the laser pulse for large ion peak energy and conversion efficiency are discussed.

High energy electrons, nuclear phenomena and heating in petawatt laser-solid experiments

The Petawatt laser at LLNL has opened a new regime of laser-matter interactions in which the quiver motion of plasma electrons is fully relativistic with energies extending well above the threshold for nuclear processes. In addition to -few MeV ponderomotive electrons produced in ultra-intense laser-solid interactions, we have found a high energy component of electrons extending to -100 MeV apparently from relativistic self-focusing and plasma acceleration in the underdense pre-formed plasma. The generation of hard bremsstrahlung, photo-nuclear reactions, and preliminary evidence for positron-electron pair production will be discussed.

Proton spectra from ultraintense laser-plasma interaction with thin foils: Experiments, theory, and simulation

A beam of high energy ions and protons is observed from targets irradiated with intensities up to 5×1019 W/cm2. Maximum proton energy is shown to strongly correlate with laser-irradiance on target. Energy spectra from a magnetic spectrometer show a plateau region near the maximum energy cutoff and modulations in the spectrum at approximately 65% of the cutoff energy. Presented two-dimensional particle-in-cell simulations suggest that modulations in the proton spectrum are caused by the presence of multiple heavy-ion species in the expanding plasma.

The use of an electron beam ion trap in the study of highly charged ions

The Electron Beam Ion Trap (EBIT) is a relatively new tool for the study of highly charged ions. Its development has led to a variety of new experimental opportunities; measurements have been performed with EBITs using techniques impossible with conventional ion sources or storage rings. In this paper, I will highlight the various experimental techniques we have developed and the results we have obtained using the EBIT and higher-energy Super-EBIT built at the Lawrence Livermore National Laboratory.

Fusion neutron and ion emission from deuterium and deuterated methane cluster plasmas

Experiments on the interaction of intense, ultrafast pulses with large van der Waals bonded clusters have shown that these clusters can explode with substantial kinetic energy and that the explosion of deuterium clusters can drive nuclear fusion reactions. Producing explosions in deuterated methane clusters with a 100 fs, 100 TW laser pulse, it is found that deuterium ions are accelerated to sufficiently high kinetic energy to drive deuterium nuclear fusion. From measurements of cluster size and ion energy via time of flight methods, it is found that these exploding deuterated methane clusters exhibit higher ion energies than explosions of comparably sized neat deuterium clusters, in accord with recent theoretical predictions. From measurements of the plume size and peak density, the relative contribution to the fusion yield from both beam target and intrafilament fusion is discussed.

Detailed study of nuclear fusion from femtosecond laser-driven explosions of deuterium clusters

Recent experiments on the interaction of intense, ultrafast pulses with large van der Waals bonded clusters have shown that these clusters can explode with sufficient kinetic energy to drive nuclear fusion. Irradiating deuterium clusters with a 35 fs laser pulse, it is found that the fusion neutron yield is strongly dependent on such factors as cluster size, laser focal geometry, and deuterium gas jet parameters. Neutron yield is shown to be limited by laser propagation effects as the pulse traverses the gas plume. From the experiments it is possible to get a detailed understanding of how the laser deposits its energy and heats the deuterium cluster plasma. The experiments are compared with simulations.