Interpretation of laser-produced ion beam diagnostics

Dipartimento di Energetica, Università di Roma “La Sapienza”, Roma, Italy In recent years many experiments have reported the production and the application as a diagnos-tic tool of laser-generated ion beams[1]. In particular, in the interaction of an ultraintense laser pulse with a thin solid target, protons and target ions can be accelerated to energies of the order of a few tens of MeV per nucleon. Laser-generated ion beams have typically a wide energy spectrum and contain up to 1011−13 ions per shot. The temporal duration of the ion beam source is comparable to that of the laser pulse used to generate it, and typically is about 1 ps long[2]. Moreover these ion beams are almost completely charge neutralised by an accompanying cloud of electrons stripped from the target. As a consequence the expansion of the beam is ballistic, making these ion sources suitable for diagnostic applications, such as the proton imaging tech-nique. rncuoc hqtokpirwnug ERC rwnug rtqvqp dgco TEHfgvgevqt cnwokpkwo hqknu rncuoc Figure 1: Top view of a typical experimental set-up for proton imaging. A particle-tracing code, PTRACE, has been origi-nally developed for helping scientists in the inter-pretation of experimental data, usually collected in the form of two-dimensional cross-sections of the ion-beam used as a probe. The code allows for trac-ing ions through user-prescribed electromagnetic structures, which can be described by analytical for-mulas or as field maps obtained from other simula-tion tools. The code also computes the deposition of the ion beam in nuclear track detectors or ra-diochromic film stack detectors for direct compari-son with actual data. The problem and its geometry Figure 1 presents a schematic diagram of typical experimental arrangement for the genera-tion of a proton beam using an ultraintense laser pulse. A chirped-pulse-amplified (CPA) pulse is focussed onto an thin foil of solid material (here an Aluminium foil) at an intensity in excess of 5 ·1018 W/cm2. On the back side of the irradiated foil the ions of the material are accelerate