A. Machacek

Observation of a highly directional γ-ray beam from ultrashort, ultraintense laser pulse interactions with solids

Novel measurements of electromagnetic radiation above 10 MeV are presented for ultra intense laser pulse interactions with solids. A bright, highly directional source of γ rays was observed directly behind the target. The γ rays were produced by bremsstrahlung radiation from energetic electrons generated during the interaction. They were measured using the photoneutron reaction [63Cu(γ,n)62Cu] in copper. The resulting activity was measured by coincidence counting the positron annihilation γ rays which were produced from the decay of 62Cu. New measurements of the bremsstrahlung radiation at 1019 W cm−2 are also presented.

Production of radioactive nuclides by energetic protons generated from intense laser-plasma interactions

Nuclear activation has been observed in materials exposed to the ablated plasma generated from high intensity laser–solid interactions (at focused intensities up to 2×1019 W/cm2) and is produced by protons having energies up to 30 MeV. The energy spectrum of the protons is determined from these activation measurements and is found to be consistent with other ion diagnostics. The possible development of this technique for “table-top” production of radionuclides for medical applications is also discussed.

Energetic proton production from relativistic laser interaction with high density plasmas

Energetic protons up to 30 MeV have been measured from high intensity laser interactions (⩽5×1019 W/cm2) with solid density plasmas. Up to 1012 protons (> 2 MeV) were observed at the rear of thin aluminum foil targets and measurements of their angular deflection were made. Similar energies were measured from ions produced from the front of the foils. Nuclear activation and track detector measurements suggest that the protons measured at the rear originate from the front surface of the target and are bent by large magnetic fields that exist in the plasma interior, which are likely generated by a laser-produced beam of fast electrons.

Ultrahigh-intensity laser-produced plasmas as a compact heavy ion injection source

The possibility of using high-intensity laser-produced plasmas as a source of energetic ions for heavy ion accelerators is addressed. Experiments have shown that neon ions greater than 6 MeV can be produced from gas jet plasmas, and well-collimated proton beams greater than 20 MeV have been produced from high intensity laser solid interactions. The proton beams from the back of thin targets appear to be more collimated and reproducible than are high-energy ions generated in the ablated plasma at the front of the target and may be more suitable for ion injection applications. Lead ions have been produced at energies up to 430 MeV.

Ultrafast x-ray diffraction using a streak-camera detector in averaging mode

We demonstrate an apparatus for measuring time-dependent x-ray diffraction. X-ray pulses from a synchrotron are diffracted by a pair of Si(111) crystals and detected with an x-ray streak camera that has single-shot resolution of better than 1 ps. The streak camera is driven by a photoconductive switch, which is triggered by 100-fs laser pulses at a repetition rate of 1 kHz. The laser and the streak camera are synchronized with the synchrotron pulses. In the averaging mode, trigger jitter results in 2-ps temporal resolution. We measured the duration of 5-keV pulses from the Advanced Light Source synchrotron to be 70ps.

A versatile matrix-based solution for the two plasmon decay instability

A Floquet-based solution of the equations governing the two plasmon decay (TPD) instability which is valid over a range of plasma densities is presented. The capability to include a large number of modes allows the validity of conventional fluid-based analytic solutions at densities differing from quarter-critical to be assessed. The conventional assumptions are upheld in the absence of a magnetic field, however, it is shown that the extra modes have a significant effect once a static magnetic field is present in the plasma.

Modeling a sonoluminescing bubble as a plasma

Abstract We present detailed simulations of the optical spectra emitted from an Argon plasma whose conditions correspond to those thought to prevail within sonoluminescing bubbles. The model incorporates detailed atomic physics based on atomic data from the Opacity Project database, and includes bound–bound, bound–free, and free–free transitions. Line broadening is treated by use of the modified semi-empirical method. The spectral model is used as a postprocessor of hydrodynamic simulations. We find good agreement with experimental spectra and accurately reproduce experimental pulse widths. We also predict that whilst the majority of the optical emission corresponds to bound–free transitions, there remains the possibility of observing line emission in both the UV and IR regions of the spectrum.

A fluid-kinetic model for the two plasmon decay instability

A solution for the gain rates and plasmon frequencies of the two plasmon decay instability is obtained using a fluid-kinetic model. This work is of relevance to high temperature, short-pulse laser plasma interactions where appreciable instability is expected in regions of plasma where the phase velocity of the electrostatic waves may not be assumed to be much larger than the thermal velocity of the electrons. The model is evaluated by comparing its results for the stimulated Raman scattering process with a fully kinetic treatment.

Detailed simulations of sonoluminescence spectra

We present detailed simulations of the optical spectra emitted from an argon plasma whose conditions correspond to those thought to prevail within sonoluminescing bubbles. The model incorporates detailed atomic physics based on atomic data from the Opacity Project database, and includes bound-bound, bound-free and free-free transitions. Line broadening is treated using the modified semi-empirical method. The spectral model is used as a postprocessor of hydrodynamic simulations. While finding excellent agreement with the shape of experimental spectra, we calculate an intensity that is a factor of 100 greater than that in experiment. We also predict that whilst the majority of the optical emission corresponds to bound-free transitions, there remains the possibility of observing broad line emission in both the UV and IR regions of the spectrum.

Rapid generation of approximate optical spectra of dense cool plasmas

Abstract In recent experiments uniform plasmas have been generated at high densities and low temperatures, (typically electron densities of 1019 cm−3, and T ~ 2–5 eV). Additionally, such plasmas are also produced during free laser ablation — a topic of relevance to the deposition of thin solid films. Standard methods used to diagnose plasmas are difficult to apply at these conditions, as there is significant overlap of the broad spectral lines from different elements and ion stages. It is therefore of interest to attempt to calculate the entire spectrum in the appropriate wavelength regime. For most elements, the number of individual spectral line profiles that have been calculated as a function of density and temperature using the semi-classical method is very small, hindering such a synthesis of the full spectra. However, a technique for approximating line shapes simply and rapidly (the modified semi-empirical method) has previously been developed for individual lines. We utilise this method, coupled with an accurate database, to generate a large number of density dependent line profiles, and hence an approximation to the full spectrum. We evaluate the accuracy and utility of such an approach by comparison with the few extant semi-classical calculations. The method described facilitates the rapid generation of approximate spectra. It can also be used as a post processor to a hydrodynamic code to obtain both time dependent and time integrated spectra in the approximation that the laser-ablated plasma is both optically thin and in LTE.