A. Schiavi

Electric field detection in laser-plasma interaction experiments via the proton imaging technique

Due to their particular properties, the beams of the multi-MeV protons generated during the interaction of ultraintense (I>1019 W/cm2) 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 ...

Proton imaging: a diagnostic for inertial confinement fusion/fast ignitor studies

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 laser-irradiated 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.

Targets for direct-drive fast ignition at total laser energy of 200-400 kJ

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, t...

Inhibition of fast electron energy deposition due to preplasma filling of cone-attached targets

We present experimental and numerical results on the propagation and energy deposition of laser-generated fast electrons into conical targets. The first part reports on experimental measurements performed in various configurations in order to assess the predicted benefit of conical targets over standard planar ones. For the conditions investigated here, the fast electron-induced heating is found to be much weaker in cone-guided targets irradiated at a laser wavelength of 1.057μm, whereas frequency doubling of the laser pulse permits us to bridge the disparity between conical and planar targets. This result underscores the prejudicial role of the prepulse-generated plasma, whose confinement is enhanced in conical geometry. The second part is mostly devoted to the particle-in-cell modeling of the laser-cone interaction. In qualitative agreement with the experimental data, the calculations show that the presence of a large preplasma leads to a significant decrease in the fast electron density and energy flux...

Fast ignitor target studies for the HiPER project

Target studies for the proposed High Power Laser Energy Research (HiPER) facility [M. Dunne, Nature Phys. 2, 2 (2006)] are outlined and discussed. HiPER will deliver a 3ω (wavelength λ=0.35μm), multibeam, multi-ns pulse of about 250kJ and a 2ω or 3ω pulse of 70–100kJ in about 15ps. Its goal is the demonstration of laser driven inertial fusion via fast ignition. The baseline target concept is a direct-drive single shell capsule, ignited by hot electrons generated by a conically guided ultraintense laser beam. The paper first discusses ignition and compression requirements, and presents gain curves, based on an integrated model including ablative drive, compression, ignition and burn, and taking the coupling efficiency ηig of the igniting beam as a parameter. It turns out that ignition and moderate gain (up to 100) can be achieved, provided that adiabat shaping is used in the compression, and the efficiency ηig exceeds 20%. Using a standard ponderomotive scaling for the hot electron temperature, a 2ω or 3ω ...

Stopping and scattering of relativistic electron beams in dense plasmas and requirements for fast ignition

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.

Proton imaging detection of transient electromagnetic fields in laser-plasma interactions (invited)

Due to their particular properties (small source size, low divergence, short duration, large number density), the beams of multi-MeV protons generated during the interaction of ultraintense (I>1019 W/cm2) short pulses with thin solid targets are most suited for use as a particle probe in laser–plasma experiments. In particular, the proton beams are a valuable diagnostic tool for the detection of electromagnetic fields. The recently developed proton imaging technique employs the beams, in a point-projection imaging scheme, as an easily synchronizable diagnostic tool in laser–plasma interactions, fields, with high temporal and spatial resolution. The broad energy spectrum of the beams coupled with an appropriate choice of detector (multiple layers of dosimetric film) allows temporal multiframe capability. By allowing, for the first time, diagnostic access to electric-field distributions in dense plasmas, this novel diagnostic opens up to investigation a whole new range of unexplored phenomena. Results obtai...

Measurement of highly transient electrical charging following high-intensity laser–solid interaction

The multi-million-electron-volt proton beams accelerated during high-intensity laser–solid interactions have been used as a particle probe to investigate the electric charging of microscopic targets laser-irradiated at intensity ∼1019 W cm2. The charge-up, detected via the proton deflection with high temporal and spatial resolution, is due to the escape of energetic electrons generated during the interaction. The analysis of the data is supported by three-dimensional tracing of the proton trajectories.

Impulsive electric fields driven by high-intensity laser matter interactions

Theinteractionofhigh-intensitylaserpulseswithmatterreleasesinstantaneouslyultra-largecurrentsofhighlyenergetic electrons, leading to the generation of highly-transient, large-amplitude electric and magnetic fields. We report results of recent experiments in which such charge dynamics have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channeling in underdense plasmas have been studied in this way, as also the processes of Debye sheath formation andMeVionfrontexpansionattherearoflaser-irradiatedthinmetallicfoils.Laser-drivenimpulsivefieldsatthesurface of solid targets can be applied for energy-selective ion beam focusing.

Fluid and kinetic simulation of inertial confinement fusion plasmas

The main features of codes for inertial confinement fusion studies are outlined, and a few recent simulation results are presented. The two-dimensional Lagrangian fluid code DUED is used to study target evolution, including beam-driven compression, hydrodynamic stability, hot spot formation, ignition and burn. An electro-magnetic particle-in-cell (PIC) code is applied to the study of ultraintense laser–plasma interaction and generation of fast electron jets. A relativistic 3D collisionless fluid model addresses relativistic electron beam propagation in a dense plasma.