Mitsuru Yamagiwa

Laser ion-acceleration scaling laws seen in multiparametric particle-in-cell simulations.

The ion acceleration driven by a laser pulse at intensity I= 10(20)-10(22) W/cm(2) x (microm/lambda)(2) from a double layer target is investigated with multiparametric particle-in-cell simulations. For targets with a wide range of thickness l and density n(e), at a given intensity, the highest ion energy gain occurs at certain electron areal density of the target sigma = n(e)l, which is proportional to the square root of intensity. In the case of thin targets and optimal laser pulse duration, the ion maximum energy scales as the square root of the laser pulse power. When the radiation pressure of the laser field becomes dominant, the ion maximum energy becomes proportional to the laser pulse energy.

Temporal contrast enhancement of petawatt-class laser pulses

We demonstrate the temporal contrast enhancement in a petawatt-class Ti:sapphire chirped-pulse amplification (CPA) laser system. An extra saturable absorber, introduced downstream after a low-gain optical parametric chirped-pulse amplification (OPCPA) preamplifier, has improved the temporal contrast in the system to 1.4×10(12) on the subnanosecond time scale at 70 TW power level. We have achieved 28 J of uncompressed broadband output energy with this system, indicating the potential for reaching peak powers near 600 TW.

Radiotherapy using a laser proton accelerator

Laser acceleration promises innovation in particle beam therapy of cancer where an ultra-compact accelerator system for cancer beam therapy can become affordable to a broad range of patients. This is not feasible without the introduction of a technology that is radically different from the conventional accelerator-based approach. The laser acceleration method provides many enhanced capabilities for the radiation oncologist. It reduces the overall system size and weight by more than one order of magnitude. The characteristics of the particle beams (protons) make them suitable for a class of therapy that might not be possible with the conventional accelerator, such as the ease for changing pulse intensity, the focus spread, the pinpointedness, and the dose delivery in general. A compact, uncluttered system allows a PET device to be located in the vicinity of the patient in concert with the compact gantry. The radiation oncologist may be able to irradiate a localized tumor by scanning with a pencil-like particle beam while ascertaining the actual dosage in the patient with an improved in-beam PET verification of auto-radioactivation induced by the beam therapy. This should yield an unprecedented flexibility in the feedback radiotherapy by the radiation oncologist. Laser accelerated radiotherapy has a unique niche in a current world of high energy accelerator using synchrotron or cyclotron.

Tunable high-energy ion source via oblique laser pulse incident on a double-layer target.

The laser-driven acceleration of high quality proton beams from a double-layer target, comprised of a high-Z ion layer and a thin disk of hydrogen, is investigated with three-dimensional particle-in-cell simulations for an obliquely incident laser pulse. The proton beam energy reaches its maximum at a certain incidence angle, where it can be much greater than the energy at normal incidence. The proton beam propagates at some angle with respect to the target surface normal and with some tilt around the target surface, as determined by the proton energy and the incidence angle.

Electron bunch acceleration in the wake wave breaking regime

Results are presented from theoretical analysis and 2D PIC simulations of electron acceleration in a breaking wake plasma wave generated by a short intense laser pulse during its interaction with a finite-length underdense plasma layer. The high energy electron energy spectrum and transverse emittance are obtained. It is shown that, for laser pulse lengths above the plasma wake wavelength, the wakefield-accelerated electrons are further accelerated by the electromagnetic wave.

Spectral and dynamical features of the electron bunch accelerated by a short-pulse high intensity laser in an underdense plasma

The results of the theoretical consideration and two-dimensional particle in cell simulation of electron acceleration with a short-pulse intense laser propagating through a finite length underdense plasma layer are presented. The fast electron energy spectrum and emittance are analyzed for moderate to high intensity and for different plasma density. It is shown that for laser pulse lengths above the plasma wake wavelength the wakefield accelerated electrons are further accelerated by the electromagnetic wave.

MeV ion generation by an ultra-intense short-pulse laser : application to positron emitting radionuclide production

A new method is proposed for producing 18F, a positron emitter, via 18O(p,n)18F reactions with fast protons from the interaction of a relativistically-intense short-pulse laser with an underdense plasma layer. The fast ion concentration contributing to the nuclear reaction is estimated on the basis of a two-dimensional particle-in-cell simulation. The instantaneous production rate of 18F is found to be two orders of magnitude larger than by the standard method using a cyclotron.

Experimental verification of femtosecond laser ablation schemes by time-resolved soft x-ray reflective imaging

Pump and probe reflective imaging using a soft x-ray laser probe was applied to the observation of the early stage of femtosecond laser ablation process on platinum. In strongly excited area, drastic and fast reflectivity drop was observed. In moderately excited area, the decay of the reflectivity is slower than that in the strongly excited area, and the reflectivity reaches its minimum at t = 160 ps. In weakly excited area, laser-induced reflectivity change was not observed. In addition, the point where the reflectivity dip was observed at t = 10 ps and t = 40 ps, coincides with the position of the edge of reflectivity drop at t = 160 ps. These results give the critical information about the femtosecond laser ablation.

Simulation and experiments of the laser induced breakdown of air for femtosecond to nanosecond order pulses

3D + 1 dimensional simulations and experimental results for the laser induced breakdown of air are presented. The simulations include the laser propagation, multi-photon and impact ionization and heating of the electrons using accurate atomic and molecular data. For laser pulses of duration from 100 fs to 1 ns mechanisms for the breakdown of air based on the pulse duration and intensity ranging from optical field ionization to electron impact ionization are found. The laser energies at which the breakdown occurs are found to be in good agreement with experimental results.

Pressure formulae for liquid metals and plasmas based on the density-functional theory

First, pressure formulae for electrons under the external potential produced by fixed nuclei are derived both in surface integral and volume integral forms for an arbitrary volume chosen in the system; the surface integral form is based on a pressure tensor which is the sum of the kinetic and exchange-correlation parts in the density-functional theory, and the volume integral form represents the virial theorem with subtraction of the nuclear virial term. Secondly, on the basis of these formulae, the thermodynamical pressure of liquid metals and plasmas is represented in the forms of a surface integral and a volume integral including the nuclear contribution. From these results, we obtain a virial pressure formula for liquid metals, which is more accurate and simpler than the standard representation. On the basis of our formulation, some comments are made on pressure formulae derived previously and on a definition of pressure used widely.