O. Tresca

Refluxing of fast electrons in solid targets irradiated by intense, picosecond laser pulses

The propagation of fast electrons produced in the interaction of relativistically intense, picosecond laser pulses with solid targets is experimentally investigated using K-alpha emission as a diagnostic. The role of fast electron refluxing within the target, which occurs when the electrons are reflected by the sheath potentials formed at the front and rear surfaces, is elucidated. The targets consist of a Cu fluorescence layer of fixed thickness at the front surface backed with a propagation layer of CH, the thickness of which is varied to control the number of times the refluxing fast electron population transits the Cu fluorescence layer. Enhancements in the K-alpha yield and source size are measured as the thickness of the CH layer is decreased. Comparison with analytical and numerical modelling confirms that significant refluxing occurs and highlights the importance of considering this phenomenon when deriving information on fast electron transport from laser-solid interaction experiments involving relatively thin targets.

Surface transport of energetic electrons in intense picosecond laser-foil interactions

The angular distribution of energetic electrons emitted from thin foil targets irradiated by intense, picosecond laser pulses is measured as a function of laser incidence angle, intensity, and polarization. Although the escaping fast electron population is found to be predominantly transported along the target surface for incidence angles ≥65°, in agreement with earlier work at lower intensities, rear-surface proton acceleration measurements reveal that a significant electron current is also transported longitudinally within the target, irrespective of incident angle. These findings are of interest to many applications of laser-solid interactions, including advanced schemes for inertial fusion energy.

Relativistic plasma surfaces as an efficient second harmonic generator

We report on the characterization of the specular reflection of 50fs laser pulses in the intensity range 10 17 -10 21 Wcm 2 obliquely incident with p-polarization onto solid density plasmas. These measurements show that the absorbed energy fraction remains approximately constant and that second harmonic generation (SHG) achieves efficiencies of 22±8% for intensities approaching 10 21 Wcm 2 . A simple model based on the relativistic oscillating mirror concept reproduces the observed intensity scaling, indicating that this is 8 Author to whom any correspondence should be addressed.

Controlling the properties of ultraintense laser-proton sources using transverse refluxing of hot electrons in shaped mass-limited targets

Compared with conventional bulk metallic glasses, Ce-based and Zn-based bulk metallic glasses have received considerable attention because of their possible application as structural and functional materials. Kinetic fragility parameter m in amorphous material presents degree of deviations from the Arrhenius law above the glass transition temperature (T-g) of the material. Kinetic fragility parameter (m) and Kauzmann temperature (T-K) in (Ce0.72Cu0.28)(90-x) Al10Fex (x = 0, 5 or 10) and Zn38Mg12Ca32Yb18 bulk metallic glasses have been determined by differential scanning calorimetry (DSC). Results show that Zn38Mg12Ca32Yb18 presents a higher m than (Ce0.72Cu0.28)(90-x) Al10Fex (x = 0, 5 or 10). The activation energies E-g for glass transition are 1.51 eV (x = 0), 1.59 eV (x = 5) and 1.83 eV (x = 10) in (Ce0.72Cu0.28)(90-x) Al10Fex (x = 0, 5 or 10), and 3.59 eV in Zn38Mg12Ca32Yb18, respectively. The values of E-g increase with increasing the Fe content in (Ce0.72Cu0.28)(90-x) Al10Fex (x = 0, 5 or 10) bulk metallic glasses. Kinetic fragility parameter in of bulk metallic glasses increases with the glass transition temperature T-g of bulk metallic glasses, in agreement with previous investigations

Fast ion acceleration from thin foils irradiated by ultra-high intensity, ultra-high contrast laser pulses

Ion acceleration resulting from the interaction of ultra-high intensity (2u2009×u20091020 W/cm2) and ultra-high contrast (∼1010) laser pulses with 0.05–10 μm thick Al foils at normal (0°) and 35° laser incidence is investigated. When decreasing the target thickness from 10 μm down to 0.05 μm, the accelerated ions become less divergent and the ion flux increases, particularly at normal (0°) laser incidence on the target. A laser energy conversion into protons of ∼6.5% is estimated at 35° laser incidence. Experimental results are in reasonable agreement with theoretical estimates and can be a benchmark for further theoretical and computational work.

Scintillator-based ion beam profiler for diagnosing laser-accelerated ion beams

Next generation intense, short-pulse laser facilities require new high repetition rate diagnostics for the detection of ionizing radiation. We have designed a new scintillator-based ion beam profiler capable of measuring the ion beam transverse profile for a number of discrete energy ranges. The optical response and emission characteristics of four common plastic scintillators has been investigated for a range of proton energies and fluxes. The scintillator light output (for 1 MeV > Ep < 28 MeV) was found to have a non-linear scaling with proton energy but a linear response to incident flux. Initial measurements with a prototype diagnostic have been successful, although further calibration work is required to characterize the total system response and limitations under the high flux, short pulse duration conditions of a typical high intensity laser-plasma interaction.

Laser pulse propagation and enhanced energy coupling to fast electrons in dense plasma gradients

Laser energy absorption to fast electrons during the interaction of an ultra-intense (10(20) Wcm(-2)), picosecond laser pulse with a solid is investigated, experimentally and numerically, as a function of the plasma density scale length at the irradiated surface. It is shown that there is an optimum density gradient for efficient energy coupling to electrons and that this arises due to strong self-focusing and channeling driving energy absorption over an extended length in the preformed plasma. At longer density gradients the laser filaments, resulting in significantly lower overall energy coupling. As the scale length is further increased, a transition to a second laser energy absorption process is observed experimentally via multiple diagnostics. The results demonstrate that it is possible to significantly enhance laser energy absorption and coupling to fast electrons by dynamically controlling the plasma density gradient.

Enhanced proton beam collimation in the ultra-intense short pulse regime

The collimation of proton beams accelerated during ultra-intense laser irradiation of thin aluminum foils was measured experimentally whilst varying laser contrast. Increasing the laser contrast using a double plasma mirror system resulted in a marked decrease in proton beam divergence (20 ◦ to <10 ◦ ), and the enhanced collimation persisted over a wide range of target thicknesses (50 nm–6 µm), with an increased flux towards thinner targets. Supported by numerical simulation, the larger beam divergence at low contrast is attributed to the presence of a significant plasma scale length on the target front surface. This alters the fast electron generation and injection into the target, affecting the resultant sheath distribution and dynamics at the rear target surface. This result demonstrates that careful control of the laser contrast will be important for future laser-driven ion applications in which control of beam divergence is crucial.

Influence of laser irradiated spot size on energetic electron injection and proton acceleration in foil targets

The influence of irradiated spot size on laser energy coupling to electrons, and subsequently to protons, in the interaction of intense laser pulses with foil targets is investigated experimentally. Proton acceleration is characterized for laser intensities ranging from 2 x 10(18) - 6 x 10(20) W/cm(2), by (1) variation of the laser energy for a fixed irradiated spot size, and (2) by variation of the spot size for a fixed energy. At a given laser pulse intensity, the maximum proton energy is higher under defocus illumination compared to tight focus and the results are explained in terms of geometrical changes to the hot electron injection

Injection and transport properties of fast electrons in ultraintense laser-solid interactions

Fast electron injection and transport in solid foils irradiated by sub-picosecond-duration laser pulses with peak intensity equal to 4 x 10(20)W/cm(2) is investigated experimentally and via 3D simulations. The simulations are performed using a hybrid-particle-in-cell (PIC) code for a range of fast electron beam injection conditions, with and without inclusion of self-generated resistive magnetic fields. The resulting fast electron beam transport properties are used in rear-surface plasma expansion calculations to compare with measurements of proton acceleration, as a function of target thickness. An injection half-angle of similar to 50 degrees - 70 degrees is inferred, which is significantly larger than that derived from previous experiments under similar conditions