Due to their particular properties, the beams of the multi-MeV protons generated during the interaction of ultraintense (I>10(19) W/cm(2)) short pulses with thin solid targets are most suited for use as a particle probe in laser-plasma experiments. The recently developed proton imaging technique employs the beams in a point-projection imaging scheme as a diagnostic tool for the detection of electric fields in laser-plasma interaction experiments. In recent investigations carried out at the Rutherford Appleton Laboratory (RAL, UK), a wide range of laser-plasma interaction conditions of relevance for inertial confinement fusion (ICF)/fast ignition has been explored. Among the results obtained will be discussed: the electric field distribution in laser-produced long-scale plasmas of ICF interest; the measurement of highly transient electric fields related to the generation and dynamics of hot electron currents following ultra-intense laser irradiation of targets; the observation in underdense plasmas, after the propagation of ultra-intense laser pulses, of structures identified as the remnants of solitons produced in the wake of the pulse. (C) 2002 American Institute of Physics

The acceleration of multi-MeV protons from the rear surface of thin solid foils irradiated by an intense (approximately 10(18) W/cm2) and short (approximately 1.5 ps) laser pulse has been investigated using transverse proton probing. The structure of the electric field driving the expansion of the proton beam has been resolved with high spatial and temporal resolution. The main features of the experimental observations, namely, an initial intense sheath field and a late time field peaking at the beam front, are consistent with the results from particle-in-cell and fluid simulations of thin plasma expansion into a vacuum.

A study of the properties of multi-MeV proton emission from thin foils following ultraintense laser irradiation has been carried out. It has been shown that the protons are emitted, in a quasilaminar fashion, from a region of transverse size of the order of 100-200 microm. The imaging properties of the proton source are equivalent to those of a much smaller source located several hundred microm in front of the foil. This finding has been obtained by analyzing proton radiographs of periodically structured test objects, and is corroborated by observations of proton emission from laser-heated thick targets.

The influence of the plasma density scale length on the production of MeV protons from thin foil targets irradiated at I lambda(2) = 5 x 10(19) W cm(-2) has been studied. With an unperturbed foil, protons with energy >20 MeV were formed in an exponential energy spectrum with a temperature of 2.5+/-0.3 MeV. When a plasma with a scale length of 100 microm was preformed on the back of the foil, the maximum proton energy was reduced to <5 MeV and the beam was essentially destroyed. The experimental results are consistent with an electrostatic accelerating mechanism that requires an ultrashort scale length at the back of the target.

Proton imaging is a recently proposed technique for diagnosis of dense plasmas, which favourably exploits the properties of protons produced by high-intensity laser-matter interaction. The technique allows the distribution of electric fields in plasmas and around laser-irradiated targets to be explored for the first time with high temporal and spatial resolution. This leads to the possibility of investigating as yet unexplored physical issues. In particular we will present measurements of transient electric fields in laser-plasmas and around laserirradiated targets under various interaction conditions. Complex electric field structures have been observed in long-scale laser-produced plasmas, while global target charge-up and growth of electromagnetic instabilities have been detected following ultraintense interactions with solid targets.

Basic issues for the design of moderate-gain fast ignition targets at total laser energy of 200–400kJ (with less than 100kJ for the igniting beams) are discussed by means of a simple integrated gain model. Gain curves are generated and their sensitivity to several parameters is analyzed. A family of scaled target is designed, based on 1D hydrodynamic simulations of the implosion stage and 2D model simulations of ignition and burn. It is found that ignition and propagating burn can be achieved by targets compressed by 100–150kJ, properly shaped laser pulses (with wavelength λc=0.35μm), and ignited by 80–100kJ pulses. This requires adiabat shaped implosions to limit Rayleigh-Taylor instability, at the same time keeping the fuel entropy at a very low level. In addition, the igniting beam should be coupled to the fuel with an efficiency of about 25%, and the hot-electron average penetration depth should be at most 1.2–1.5g∕cm2. According to the present understanding of ultraintense laser-matter interaction, this limits the wavelength of the ignition beam to λig≤0.5μm. With the same assumptions, energy gain G=100 can be achieved by targets driven by a 250kJ compression laser pulse and an 80–100kJ ignition pulse.

In the field of radiotherapy, Monte Carlo (MC) particle transport calculations are recognized for their superior accuracy in predicting dose and fluence distributions in patient geometries compared to analytical algorithms which are generally used for treatment planning due to their shorter execution times. In this work, a newly developed MC-based treatment planning (MCTP) tool for proton therapy is proposed to support treatment planning studies and research applications. It allows for single-field and simultaneous multiple-field optimization in realistic treatment scenarios and is based on the MC code FLUKA. Relative biological effectiveness (RBE)-weighted dose is optimized either with the common approach using a constant RBE of 1.1 or using a variable RBE according to radiobiological input tables. A validated reimplementation of the local effect model was used in this work to generate radiobiological input tables. Examples of treatment plans in water phantoms and in patient-CT geometries together with an experimental dosimetric validation of the plans are presented for clinical treatment parameters as used at the Italian National Center for Oncological Hadron Therapy. To conclude, a versatile MCTP tool for proton therapy was developed and validated for realistic patient treatment scenarios against dosimetric measurements and commercial analytical TP calculations. It is aimed to be used in future for research and to support treatment planning at state-of-the-art ion beam therapy facilities.

The interaction of a relativistic electron with a dense plasma is studied in the context of inertial fusion fast ignition. Expressions for the electron stopping power and deflection are given and implemented in a three-dimensional (3D) Monte Carlo code. Electron range and penetration depth are computed as functions of the electron energy and plasma parameters; approximate expressions are also proposed. Conditions for fast ignition are studied by including the 3D Monte Carlo code in a 2D hydrodynamic code. The required beam energy is determined as a function of mean electron energy for monoenergetic and exponential energy distributions and a uniform initial deuterium-tritium plasma with a density of 300 g cm −3 . A simple model is shown to agree with the code.