H. Ruhl

Laser light and hot electron micro focusing using a conical target

The laser light propagation inside the conical target had been studied by three-dimensional particle-in-cell simulations. It is found that the laser light is optically guided inside the conical target and focused at the tip of the cone. The intensity increases up to several tens of times in a several micron focal spot. It is the convergence of hot electrons to the head of the cone that is observed as a consequence of the surface electron flow guided by self-generated quasistatic magnetic fields and electrostatic sheath fields. As a result, the hot electron density at the tip is locally ten times greater than the case of using a normal flat foil.

Multispecies ion acceleration off laser-irradiated water droplets

In a recent experiment at Max-Born Institut, Berlin, strong modulations have been observed in energy spectra of MeV ions that were accelerated by ultrashort intense laser pulses, τL=35fs, I=1019W∕cm2, off water droplets. This experiment is studied analytically as well as by numerical particle-in-cell simulations in one and two dimensions and it is shown how classical isothermal fluid expansion models fail in the present case. The paper investigates alternative models which claim to describe the ion spectral modulations and a mechanism that generates proton spectra similar to the experimental ones due to a simultaneous acceleration of several ion components. Finally, the issue of directional anisotropy in the spectra of accelerated ions for the case of a spherical target is discussed.

Laser accelerated ions in ICF research prospects and experiments

The acceleration of ions by ultra-intense lasers has attracted great attention due to the unique properties and the unmatched intensities of the ion beams. In the early days the prospects for applications were already studied, and first experiments have identified some of the areas where laser accelerated ions can contribute to the ongoing inertial confinement fusion (ICF) research. In addition to the idea of laser driven proton fast ignition (PFI) and its use as a novel diagnostic tool for radiography the strong dependence on the electron transport in the target was found to be helpful in investigating the energy transport by electrons in fast ignitor scenarios. More recently an additional idea has been presented to use laser accelerated ion beams as the next generation ion sources, and taking advantage of the luminosity of the beams, to develop a test bed for heavy ion beam driven inertial confinement fusion physics. We review our recent experiments and simulations relevant to ICF research presenting a possible scenario for PFI as well as the prospects for next generation ion sources.

Super-intense quasi-neutral proton beams interacting with plasma: a numerical investigation

Due to the unique properties of the recently discovered sheath laser-ion source the investigation of super-intense, quasi-neutral ion beams interacting with plasma has become of substantial interest. Novel experiments in parameter regimes that bear relevance for future ion based fast ignition concepts can be envisioned. Simulations in three spatial dimensions of protons, charge and current neutralized by co-moving electrons, interacting with a low density plasma are presented. Quasi-neutral beam propagation and normalized proton beam momentum growth are discussed.

Anomalous inhibition of electron transport in laser–matter interaction at subrelativistic intensities

Electron transport in femtosecond laser-irradiated solid targets is investigated by means of one-dimensional particle-in-cell simulations that include a model of collisional ionization, binary collisions and field ionization, while treating ions as individual particles. In particular, heat and particle fluxes in conductor and insulator targets are compared at the onset of relativistic laser intensities, i.e., at I=1017 W/cm2. Simulations show that fast electrons generate a longitudinal electric field of the order 1011 V/m in the bulk material that acts to inhibit heat flux in insulators, while the electric fields observed in metals are weaker and electrons penetrate deeper into the target. The bulk heat transport is found to be similar in both materials and mainly Spitzer-like, with a noticeable contribution by fast electrons.

Ultra‐Low Emittance Proton Beams From A Laser‐Virtual Cathode Plasma Accelerator

The laminarity of high current multi‐MeV proton beams produced by irradiating thin metallic foils with ultra‐intense lasers has been measured. For proton energies >10 MeV, the transverse and longitudinal emittance are respectively 10 MeV. Magnetic stripping of the co‐moving electrons out of the beam after a few cm of debunching is not observed to induce emittance growth.