P. Gallegos

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.

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.

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.

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.

On the investigation of fast electron beam filamentation in laser-irradiated solid targets using multi-MeV proton emission

The transverse filamentation of beams of fast electrons transported in solid targets irradiated by ultraintense (5 × 1020 W cm−2), picosecond laser pulses is investigated experimentally. Filamentation is diagnosed by measuring the uniformity of a beam of multi-MeV protons accelerated by the sheath field formed by the arrival of the fast electrons at the rear of the target, and is investigated for metallic and insulator targets ranging in thickness from 50 to 1200 µm. By developing an analytical model, the effects of lateral expansion of electron beam filaments in the sheath during the proton acceleration process is shown to account for measured increases in proton beam nonuniformity with target thickness for the insulating targets.

Coherent x-ray generation in relativistic laser/gas jet interactions

We present experimental results, theory, and simulations demonstrating two novel sources of coherent X-ray radiation generated in the relativistic laser (>1018W/cm2) interaction with easily accessible, repetitive, and debris-free gas jet targets. The first source is based on a relativistic mirror reflecting a counter-propagating laser pulse. A strongly nonlinear breaking wake wave driven by an intense laser pulse can act as a semi-transparent relativistic flying mirror. Such a mirror directly converts counter-propagating laser light into a high-frequency (XUV or X-ray) ultrashort pulse due to the double Doppler effect. In the experimental demonstration with the 9 TW J-KAREN laser, the flying mirror generated in a He gas jet partially reflected a 1 TW pulse, providing up to ~1010 photons, 60 nJ (~1012 photons/sr) in the XUV range (12.8-22 nm). The second source is demonstrated with the laser power ranging from 9 to 170 TW in experiments with the J-KAREN and Astra Gemini lasers. The odd and even order harmonics generated by linearly as well as circularly polarized pulses are emitted forward out of the gas jet. The 120 TW laser pulses produce harmonics with ~3×1013photons/sr (~600 μJ/sr) in the 120±5 eV spectral range. The observed harmonics cannot be explained by previously known mechanisms (atomic harmonics, betatron radiation, nonlinear Thomson scattering, etc.). We introduce a novel mechanism of harmonic generation based on the relativistic laser-plasma phenomena (self-focusing, cavity evacuation, bow wave generation), mathematical catastrophe theory which explains the formation of structurally stable electron density singularities, spikes, and collective radiation of a compact charge driven by a relativistic laser.

Modified proton radiography arrangement for the detection of ultrafast field fronts

The experimental arrangement for the investigation of high-field laser-induced processes using a broadband proton probe beam has been modified to enable the detection of the ultrafast motion of field fronts. It is typical in such experiments for the target to be oriented perpendicularly with respect to the principal axis of the probe beam. It is demonstrated here, however, that the temporal imaging properties of the diagnostic arrangement are altered drastically by placing the axis (or plane) of the target at an oblique angle to the transverse plane of the probe beam. In particular, the detection of the motion of a laser-driven field front along a wire at a velocity of (0.95+/-0.05)c is described.

Ion source development and radiobiology applications within the LIBRA project

In view of their properties, laser-driven ion beams have the potential to be employed in innovative applications in the scientific, technological and medical areas. Among these, a particularly high-profile application is particle therapy for cancer treatment, which however requires significant improvements from current performances of laser-driven accelerators. The focus of current research in this field is on developing suitable strategies enabling laser-accelerated ions to match these requirements, while exploiting some of the unique features of a laser-driven process. LIBRA is a UK-wide consortium, aiming to address these issues, and develop laser-driven ion sources suitable for applicative purposes, with a particular focus on biomedical applications. We will report on the activities of the consortium aimed to optimizing the properties of the beams, by developing and employing advanced targetry and by exploring novel acceleration regimes enabling production of beams with reduced energy spread. Employing the TARANIS Terawatt laser at Queens University, we have initiated a campaign investigating the effects of proton irradiation of biological samples at extreme dose rates (> 109 Gy/s).

Acceleration of ions up to 20MeV/nucleon in the ultrashort, high-intensity regime

The measurements reported here provide scaling laws for the ion acceleration process in the regime of ultrashort (50 fs), ultrahigh contrast (1010) and ultrahigh intensity (> 1020W/cm 2), never investigated previously. The scaling of the accelerated ion energies was studied by varying a number of parameters such as target thickness (down to 10nm), target material (C and Al) and laser light polar- ization (circular and linear) at 35° and normal laser incidence. A twofold increase in proton energy and an order of magnitude enhancement in ion flux have been observed over the investigated thickness range at 35° angle of incidence. Further- more, at normal laser incidence, measured peak proton energies of about 20 MeV are observed almost independently of the target thickness over a wide range (50nm- 10 μm). 1.