D. Doria

Ion acceleration in multispecies targets driven by intense laser radiation pressure

The acceleration of ions from ultrathin foils has been investigated by using 250 TW, subpicosecond laser pulses, focused to intensities of up to 3 × 10(20) W cm(-2). The ion spectra show the appearance of narrow-band features for protons and carbon ions peaked at higher energies (in the 5-10 MeV/nucleon range) and with significantly higher flux than previously reported. The spectral features and their scaling with laser and target parameters provide evidence of a multispecies scenario of radiation pressure acceleration in the light sail mode, as confirmed by analytical estimates and 2D particle-in-cell simulations. The scaling indicates that monoenergetic peaks with more than 100 MeV/nucleon are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.

Generation of neutral and high-density electron-positron pair plasmas in the laboratory

Electron–positron pair plasmas represent a unique state of matter, whereby there exists an intrinsic and complete symmetry between negatively charged (matter) and positively charged (antimatter) particles. These plasmas play a fundamental role in the dynamics of ultra-massive astrophysical objects and are believed to be associated with the emission of ultra-bright gamma-ray bursts. Despite extensive theoretical modelling, our knowledge of this state of matter is still speculative, owing to the extreme difficulty in recreating neutral matter–antimatter plasmas in the laboratory. Here we show that, by using a compact laser-driven setup, ion-free electron–positron plasmas with unique characteristics can be produced. Their charge neutrality (same amount of matter and antimatter), high-density and small divergence finally open up the possibility of studying electron–positron plasmas in controlled laboratory experiments.

A study of the parameters of particles ejected from a laser plasma

We report on the results concerning the characteristics and the behavior of expanding plasma generated by a Laser Ion Source ~LIS!. The LIS technique is an efficient means in producing of multi-charged ions utilizing pulsed laser beams. In order to extract Cu ions, in this experiment an XeCl excimer UV laser was employed, providing a power density on the target surface up to 5 3 10 8 W0cm 2 . Two typologies of diagnostic systems were developed in order to detect the plasma current and the ion energy. The time-of-flight ~TOF! measurements were performed exploiting either a Faraday cup or an Ion Energy Analyzer ~IEA!. This latter allowed getting quantitative information about the relative ion abundances, their kinetic energy and their charge state. To study the plasma characteristics we measured the total etched material per pulse at 70 mJ. It was 0.235 mg and the overall degree of ionization, 16%. The angular distribution of the ablated material was monitored by optical transmission analysis of the deposited film as a function of the angle with respect to the normal to the target surface. Applying a high voltage to an extraction gap a multi-charged ion beam was obtained; different peaks could be distinguished in the TOF spectrum, resulting from the separation of ions of hydrogen, adsorbed compounds in the target and copper.

Biological effectiveness on live cells of laser driven protons at dose rates exceeding 109 Gy/s

The ultrashort duration of laser-driven multi-MeV ion bursts offers the possibility of radiobiological studies at extremely high dose rates. Employing the TARANIS Terawatt laser at Queens University, the effect of proton irradiation at MeV-range energies on live cells has been investigated at dose rates exceeding 109 Gy/s as a single exposure. A clonogenic assay showed consistent lethal effects on V-79 live cells, which, even at these dose rates, appear to be in line with previously published results employing conventional sources. A Relative Biological Effectiveness (RBE) of 1.4±0.2 at 10% survival is estimated from a comparison with a 225 kVp X-ray source.

Characterization of a nonequilibrium XeCl laser-plasma by a movable Faraday cup

In this work the experimental results of a nonequilibrium laser-plasma induced by an ultraviolet 308 nm excimer laser are reported. All measurements were performed fixing the laser energy at 70 mJ. It was concentrated on a 0.0099 cm2 spot by a convergent focal lens of 15 cm focal length. The utilized target was a 99.99% pure Cu disk. An 8 cm in diameter movable Faraday cup was developed in order to detect the plasma flow pulse at different positions along a drift tube. Analyzing the time-of-flight pulse under different cup bias voltage, we were able to distinguish the electron pulse, the suprathermal ions, and the thermal evolution of the plasma. In addition, by applying a breakdown voltage as polarizing cup voltage, we characterized the duration of the neutral component. To determine the system particle production efficiency, the total etched material per pulse, 0.235 μg, and the fractional ionization were measured. The expelled particle flux distribution was measured by an optical transmission analysis ...

Charge losses in expanding plasma created by an XeCl laser

The emission of multiply charged Cuq+ ions from a plasma produced by 308 nm excimer laser is analyzed with respect to the distance from the irradiated target. The critical zone, outside which the charge states of ions of the expanding plasma are frozen, was determined to be approximately 20 cm from the target. This value was estimated using a charge-freezing criterion expressed by a distance dependence of the total charge carried by the ions Q∝L−2, which describes the dilution of plasma by its expansion into a vacuum without collisional recombination processes.

Time-Resolved Characterization of the Formation of a Collisionless Shock

We report on the temporally and spatially resolved detection of the precursory stages that lead to the formation of an unmagnetized, supercritical collisionless shock in a laser-driven laboratory experiment. The measured evolution of the electrostatic potential associated with the shock unveils the transition from a current free double layer into a symmetric shock structure, stabilized by ion reflection at the shock front. Supported by a matching particle-in-cell simulation and theoretical considerations, we suggest that this process is analogous to ion reflection at supercritical collisionless shocks in supernova remnants.

Time-of-flight profile of multiply-charged ion currents produced by a pulse laser

The ion emission from a Cu-plasma produced by a 308 nm excimer laser is analysed by time-of-flight spectroscopy and by deconvolution of measured ion currents. The ion current signals recorded by an ion collector outside the critical zone, where the charge-states of ions of the expanding plasma are frozen, have been deconvoluted by the use of the Kelly and Dreyfus equation. The meaningful recovered currents recorded for Cu+ and Cu2+ ions have been compared with the current signals reconstructed from the ion energy analyser spectra. A velocity distribution and the abundance of the above ions are presented. A time-resolved average charge-state of ions is also determined. The application of the law Q ∝ l−2, based on the dilution of the total charge, Q, carried out by ions at long distances, l, from the target, is shown to be fundamental for a characterization of the laser-produced plasma.

The TARANIS laser: A multi-Terawatt system for laser-plasma investigations

The multi-Terawatt laser system, terawatt apparatus for relativistic and nonlinear interdisciplinary science, has been recently installed in the Centre for Plasma Physics at the Queen’s University of Belfast. The system will support a wide ranging science program, which will include laser-driven particle acceleration, X-ray lasers, and high energy density physics experiments. Here we present an overview of the laser system as well as the results of preliminary investigations on ion acceleration and X-ray lasers, mainly carried out as performance tests for the new apparatus. We also discuss some possible experiments that exploit the flexibility of the system in delivering pump-probe capability.

Overcoming pulse mixing and signal tailing in laser ablation inductively coupled plasma mass spectrometry depth profiling

The laser ablation-induced plasma was used as a composition-controlled source for ion implantation in Si crystals. Then, laser ablation in combination with inductively coupled plasma mass spectrometry was used for the elemental depth profiling of the implanted samples. Monte Carlo simulations permitted us to conclude that a depth resolution of tens of nm would be necessary to define the shape of the implantation profiles, as is obtained using XPS and RBS, whereas a hundred nm depth resolution is sufficient to determine the total implanted dose. The detection power of LA-ICP-MS would routinely allow rapid analytical control on the trace level implanted dose. Nevertheless, this technique is limited in terms of depth profiling resolution due to pulse mixing and signal tailing induced during the aerosol transport. Raw signal processing procedures were developed for the minimization of shapeline dispersion, deconvolution of pulse mixing and more appropriate assessment of the implanted profiles. Shapeline dispersion could be corrected for by determining the signal waning constant and implementing this information for a non-affine alibi transformation of the LA-ICP-MS signal traces. Pulse mixing deconvolution was attained with an algorithm that considered accumulated signal intensity due to pulse-on-pulse stacking, i.e., the latest pulse on top of all antecedent individual pulses’ exponential tails proportionally.