C. M. Brenner

High energy conversion efficiency in laser-proton acceleration by controlling laser-energy deposition onto thin foil targets

An all-optical approach to laser-proton acceleration enhancement is investigated using the simplest of target designs to demonstrate application-relevant levels of energy conversion efficiency between laser and protons. Controlled deposition of laser energy, in the form of a double-pulse temporal envelope, is investigated in combination with thin foil targets in which recirculation of laser-accelerated electrons can lead to optimal conditions for coupling laser drive energy into the proton beam. This approach is shown to deliver a substantial enhancement in the coupling of laser energy to 5–30 MeV protons, compared to single pulse irradiation, reaching a record high 15% conversion efficiency with a temporal separation of 1 ps between the two pulses and a 5 μm-thick Au foil. A 1D simulation code is used to support and explain the origin of the observation of an optimum pulse separation of ∼1 ps.

Carbon ion acceleration from thin foil targets irradiated by ultrahigh-contrast, ultraintense laser pulses

In this study, ion acceleration from thin planar target foils irradiated by ultrahigh-contrast (10 10 ), ultrashort (50fs) laser pulses focused to intensities of 7◊10 20 Wcm 2 is investigated experimentally. Target normal sheath acceleration (TNSA) is found to be the dominant ion acceleration mechanism when the target thickness is >50nm and laser pulses are linearly polarized. Under these conditions, irradiation at normal incidence is found to produce higherenergyions thanobliqueincidenceat35 withrespectto thetargetnormal. Simulations using one-dimensional (1D) boosted and 2D particle-in-cell codes support the result, showing increased energy coupling efficiency to fast electrons for normal incidence. The effects of target composition and thickness on the acceleration of carbon ions are reported and compared to calculations using analytical models of ion acceleration. 5 Author to whom any correspondence should be addressed.

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.

Dependence of laser accelerated protons on laser energy following the interaction of defocused, intense laser pulses with ultra-thin targets

The scaling of the flux and maximum energy of laser-driven sheath-accelerated protons has been investigated as a function of laser pulse energy in the range of 15-380 mJ at intensities of 10(16)-10(18) W/cm(2). The pulse duration and target thickness were fixed at 40 fs and 25 nm, respectively, while the laser focal spot size and drive energy were varied. Our results indicate that while the maximum proton energy is dependent on the laser energy and laser spot diameter, the proton flux is primarily related to the laser pulse energy under the conditions studied here. Our measurements show that increasing the laser energy by an order of magnitude results in a more than 500-fold increase in the observed proton flux. Whereas, an order of magnitude increase in the laser intensity generated by decreasing the laser focal spot size, at constant laser energy, gives rise to less than a tenfold increase in observed proton flux.

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 (2 × 1020 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.

Multi-pulse enhanced laser ion acceleration using plasma half cavity targets

We report on a plasma half cavity target design for laser driven ion acceleration that enhances the laser to proton energy conversion efficiency and has been found to modify the low energy region of the proton spectrum. The target design utilizes the high fraction of laser energy reflected from an ionized surface and refocuses it such that a double pulse interaction is attained. We report on numerical simulations and experimental results demonstrating that conversion efficiencies can be doubled, compared to planar foil interactions, when the secondary pulse is delivered within picoseconds of the primary pulse.