S. Reed

Generation of 10 11 contrast 50 TW laser pulses

We demonstrate what we believe to be the highest-contrast (1011), multiterawatt, chirped-pulse amplification (CPA) Ti:sapphire laser by applying the modified cross-polarized-wave (XPW) generation method. This method produces a contrast improvement of 3 orders of magnitude using microjoule input energy. Microjoule energy can be achieved by direct amplification without the complications of a double CPA system. The 1011 contrast is sufficient for experiments on high-damage-threshold solid targets with focused intensities up to 1022 W/cm2.

Accelerating protons to therapeutic energies with ultraintense, ultraclean, and ultrashort laser pulses

Proton acceleration by high-intensity laser pulses from ultrathin foils for hadron therapy is discussed. With the improvement of the laser intensity contrast ratio to 10(-1) achieved on the Hercules laser at the University of Michigan, it became possible to attain laser-solid interactions at intensities up to 10(22) W/cm2 that allows an efficient regime of laser-driven ion acceleration from submicron foils. Particle-in-cell (PIC) computer simulations of proton acceleration in the directed Coulomb explosion regime from ultrathin double-layer (heavy ions/light ions) foils of different thicknesses were performed under the anticipated experimental conditions for the Hercules laser with pulse energies from 3 to 15 J, pulse duration of 30 fs at full width half maximum (FWHM), focused to a spot size of 0.8 microm (FWHM). In this regime heavy ions expand predominantly in the direction of laser pulse propagation enhancing the longitudinal charge separation electric field that accelerates light ions. The dependence of the maximum proton energy on the foil thickness has been found and the laser pulse characteristics have been matched with the thickness of the target to ensure the most efficient acceleration. Moreover, the proton spectrum demonstrates a peaked structure at high energies, which is required for radiation therapy. Two-dimensional PIC simulations show that a 150-500 TW laser pulse is able to accelerate protons up to 100-220 MeV energies.

Relativistic plasma shutter for ultraintense laser pulses

A relativistic plasma shutter technique is proposed and tested to remove the sub-100 ps pedestal of a high-intensity laser pulse. The shutter is an ultrathin foil placed before the target of interest. As the leading edge of the laser ionizes the shutter material it will expand into a relativistically underdense plasma allowing for the peak pulse to propagate through while rejecting the low intensity pedestal. An increase in the laser temporal contrast is demonstrated by measuring characteristic signatures in the accelerated proton spectra and directionality from the interaction of 30 TW pulses with ultrathin foils along with supporting hydrodynamic and particle-in-cell simulations.

Photonuclear Fission with Quasimonoenergetic Electron Beams from Laser Wakefields

Recent advancements in laser wakefield accelerators have resulted in the generation of low divergence, hundred MeV, quasimonoenergetic electron beams. The bremsstrahlung produced by these highly energetic electrons in heavy converters includes a large number of MeV γ rays that have been utilized to induce photofission in natural uranium. Analysis of the measured delayed γ emission demonstrates production of greater than 3×105 fission events per joule of laser energy, which is more than an order of magnitude greater than that previously achieved. Monte Carlo simulations model the generated bremsstrahlung spectrum and compare photofission yields as a function of target depth and incident electron energy.

Efficient initiation of photonuclear reactions using quasimonoenergetic electron beams from laser wakefield acceleration

Pulses of nearly monoenergetic relativistic electrons have been generated by laser wakefield acceleration and used to perform photonuclear activation of carbon, copper, and photofission in uranium. Using bremsstrahlung converter targets, the electron beams generated by this technique have been shown to be effective in producing high energy γ-rays (tens of MeV) that are necessary to efficiently induce photonuclear reactions. Quantitative γ-ray spectroscopy of the irradiated C, Cu, and U samples indicates that more than 105 reactions were produced per joule of laser energy. The activation yield measurements have been compared with Monte Carlo modeling of electromagnetic cascade and photonuclear processes occurring in the targets to infer the characteristics of the laser accelerated electron beams.

High-intensity laser-driven proton acceleration enhancement from hydrogen containing ultrathin targets

Laser driven proton acceleration experiments from micron and submicron thick targets using high intensity (2 × 1021 W/cm2), high contrast (10−15) laser pulses show an enhancement of maximum energy when hydrogen containing targets were used instead of non-hydrogen containing. In our experiments, using thin (<1μm) plastic foil targets resulted in maximum proton energies that were consistently 20%–100% higher than when equivalent thickness inorganic targets, including Si3N4 and Al, were used. Proton energies up to 20 MeV were measured with a flux of 107 protons/MeV/sr.

Energetic electron and ion generation from interactions of intense laser pulses with laser machined conical targets

The generation of energetic electron and proton beams was studied from the interaction of high intensity laser pulses with pre-drilled conical targets. These conical targets are laser machined onto flat targets using 7‐180 µJ pulses whose axis of propagation is identical to that of the main high intensity pulse. This method significantly relaxes requirements for alignment of conical targets in systematic experimental investigations and also reduces the cost of target fabrication. These experiments showed that conical targets increase the electron beam charge by up to 44 ± 18% compared with flat targets. We also found greater electron beam divergence for conical targets than for flat targets, which was due to escaping electrons from the surface of the cone wall into the surrounding solid target region. In addition, the experiments showed similar maximum proton energies for both targets since the larger electron beam divergence balances the increase in electron beam charge for conical targets. 2D particle in cell simulations were consistent with the experimental results. Simulations for conical target without preplasma showed higher energy gain for heavy ions due to ‘directed coulomb explosion’. This may be useful for medical applications or for ion beam fast ignition fusion.

Self-guided laser wakefield acceleration using ablated plasma targets

Laser wakefield acceleration (LWFA) was studied using ablated plasmas as the target medium. A low density laser-ablated plasma (carbon and fluorine) was produced by focusing a 100 mJ, 10 ns pulse from a Nd : YAG laser onto the surface of a plastic target to a peak intensity of 3 × 1010 W cm−2. A 30 fs interaction pulse from the HERCULES Ti : sapphire laser system with 30 TW laser power subsequently irradiated the plasma at a peak intensity of 3 × 1018 W cm−2. The plasma density profile was varied in situ by changing the time delay between the two laser pulses. It was observed that electron energies up to 120 MeV with monoenergetic features were observed. For larger delays, the electron beam charge increases while the transmittance of the interaction pulse decreases. This correlation suggests that pump depletion occurs due to wake excitation. The use of an ablated plasma target enables LWFA operation at much higher repetition rates due to the fast plasma dynamics and adds flexibility of plasma parameters such as temperature, charge state and ion composition.

Reply to Comment on "Generation of 10^1^1 contrast 50 TW laser pulses"

We demonstrate what we believe to be the highest-contrast (10(11)), multiterawatt, chirped-pulse amplification (CPA) Ti:sapphire laser by applying the modified cross-polarized-wave (XPW) generation method. This method produces a contrast improvement of 3 orders of magnitude using microjoule input energy. Microjoule energy can be achieved by direct amplification without the complications of a double CPA system. The 10(11) contrast is sufficient for experiments on high-damage-threshold solid targets with focused intensities up to 10(22) W/cm(2).

Proton acceleration from thin foils using ultraintense, high-contrast pulses

Proton production from ultrathin foils using 3times10²⁰ W/cm² and 10^-11 ASE contrast is explored. Maximum proton energy and laser transmittance for ultrathin foils are studied to achieve ion acceleration within the directed Coulomb explosion regime.