Experimental demonstration of an electromagnetic pulse mitigation concept for a laser driven proton source.

The targets that are used to produce high-energy protons with ultra-high intensity lasers generate a strong electromagnetic pulse (EMP). To mitigate that undesired side effect, we developed and tested a concept called the "birdhouse." It consists in confining the EMP field in a finite volume and in dissipating the trapped electromagnetic energy with an electric resistor. A prototype was tested at a 10 TW 50 fs laser facility. The recorded average EMP mitigation ratio is about 20 for frequencies from 100 MHz to 6 GHz. The EMP mitigation ratio attains the level of 50 in the frequency range of 1-2 GHz where microwave emission is maximal. We measured the intensity of proton emission in two directions: along the laser propagation direction and along the edge of the proton beam. We observed that the "birdhouse" induces a two-fold increase of the intensity in the center of the proton beam and a two-fold reduction of the intensity on its edge. We did not observe any modification of the proton beam normalized spectrum.

Monoenergetic proton emission from nuclear reaction induced by high intensity laser-generated plasma.

A 10(16) W∕cm(2) Asterix laser pulse intensity, 1315 nm at the fundamental frequency, 300 ps pulse duration, was employed at PALS laboratory of Prague, to irradiate thick and thin primary CD(2) targets placed inside a high vacuum chamber. The laser irradiation produces non-equilibrium plasma with deutons and carbon ions emission with energy of up to about 4 MeV per charge state, as measured by time-of-flight (TOF) techniques by using ion collectors and silicon carbide detectors. Accelerated deutons may induce high D-D cross section for fusion processes generating 3 MeV protons and 2.5 MeV neutrons, as measured by TOF analyses. In order to increase the mono-energetic proton yield, secondary CD(2) targets can be employed to be irradiated by the plasma-accelerated deutons. Experiments demonstrated that high intensity laser pulses can be employed to promote nuclear reactions from which characteristic ion streams may be developed. Results open new scenario for applications of laser-generated plasma to the fields of ion sources and ion accelerators.

Shot-to-shot reproducibility in the emission of fast highly charged metal ions from a laser ion source.

The generation of fast highly charged metal ions with the use of the sub-nanosecond Prague Asterix Laser System, operated at a fundamental wavelength of 1315 nm, is reported. Particular attention is paid to shot-to-shot reproducibility in the ion emission. Au and Pd targets were exposed to intensities up to 5 × 10(16) W∕cm(2). Above the laser intensity threshold of ∼3 × 10(14) W∕cm(2) the plasma is generated in a form of irregular bursts. The maximum energy of protons constituting the leading edge of the fastest burst reaches a value up to 1 MeV. The fast ions in the following bursts have energy gradually decreasing with the increasing burst number, namely, from a value of about 0.5 MeV∕charge regardless of the atomic number and mass of the ionized species.

Laser produced streams of Ge ions accelerated and optimized in the electric fields for implantation into SiO2 substrates.

Ge crystals were prepared by means of laser-induced ion implantation technique. A Nd:YAG pulsed laser (repetition rate: 10 Hz; pulse duration: 3.5 ns; pulse energy: ∼0.5 J) was used both as an ion source and to carry out the ablation processes. The optimization of the laser-generated ion beam parameters in a broad energy and current density range has been obtained controlling the electrostatic field parameters. Numerical simulations of the focusing system, performed adopting an OPERA 3D code, and an investigation of the ion characteristics, using the ion time-of-flight method, have allowed to optimize the preparation parameters. The structural properties of the samples were investigated by means of x-ray photoelectron, micro-Raman spectroscopies, and scanning electron microscopy techniques. Experimental results show that, by appropriately varying the ion implantation parameters and by a post-preparation annealing treatment, it is possible to achieve the development of a micrometer-sized crystalline Ge phase and∕or an amorphous one.

Proton emission from thin hydrogenated targets irradiated by laser pulses at 10(16) W∕cm2.

The iodine laser at PALS Laboratory in Prague, operating at 1315 nm fundamental harmonics and at 300 ps FWHM pulse length, is employed to irradiate thin hydrogenated targets placed in vacuum at intensities on the order of 10(16) W∕cm(2). The laser-generated plasma is investigated in terms of proton and ion emission in the forward and backward directions. The time-of-flight technique, using ion collectors and semiconductor detectors, is used to measure the ion currents and the corresponding velocities and energies. Thomson parabola spectrometer is employed to separate the contribution of the ion emission from single laser shots. A particular attention is given to the proton production in terms of the maximum energy, emission yield, and angular distribution as a function of the laser energy, focal position, target thickness, and composition. Metallic and polymeric targets allow to generate protons with large energy range and different yield, depending on the laser, target composition, and target geometry properties.

Fast proton generation from ultrashort laser pulse interaction with double-layer foil targets.

The results of studies of fast-proton generation from foil targets irradiated by 1-ps laser pulse at 10(17) W/cm (2) are presented. It is shown that a considerable increase in proton energy and current is possible when a double-layer foil target containing a high- Z layer and a low- Z hydrogen-rich layer is used instead of a single-layer target. Proton energies and current increase with the Z of the high- Z layer and depend essentially on the target and the layer thicknesses. Above 10(9) forward-emitted protons of energy >100 keV have been recorded within a cone angle <3 degrees.