Ultra-intense Laser-Solid Interactions: Ion Acceleration and High Field Physics

Ultra-intense Laser-Solid Interactions: Ion Acceleration and High Field Physics

Prof. Paul McKenna (paul.mckenna@strath.ac.uk), Ross Gray and colleagues in the Department of Physics at the University of Strathclyde use ARCHIE-WeSt to simulate ultraintense laser-dense-plasma interactions in support of their ongoing experimental program to develop laser-driven sources of energetic ions and to investigate new regimes of high-field physics.

A very short (femtosecond to picosecond range), ultra-intense (>1020 W/cm2) laser pulse is used to irradiate a solid density thin foil target. The resulting interaction results in the production of multi-mega-ampere currents of relativistic electrons and very strong electric and magnetic fields.

The extremely high electric field gradients (TV/m) produced in these interactions results in rapid ionisation and acceleration of ions to multi-MeV/nucleon energies, over an accelerating distance of only a few microns! In comparison to conventional accelerator technologies, where acceleration takes place over many meters, this represents orders of magnitude reduction in the scale of the accelerator structure. It has been shown that laser-accelerated ion beams possess unique characteristics. The ion source is essentially ‘point-like’ in both space (micron scale) and time (picoseconds), and the resulting ion pulse is several orders of magnitude brighter than can be achieved with conventional technology. The simulations undertaken using ARCHIE-WeST focus on the development of new schemes for ion acceleration, made possible by developments in high power laser technology pushing the achievable laser intensity up to ~1022 Wcm-2. At these unprecedented intensities, a new scheme for ion acceleration using the radiation pressure of the laser pulse is predicted to produce a step change in the ion properties (energy, energy spread, brightness and directionally). This new pathway is important to push the properties of the laser-generated ion beams towards the parameters necessary for many applications, including hadron therapy for cancer treatment.

With the advent of multi-petawatt-class lasers on the horizon, unprecedented intensities of ~1023 Wcm-2 will soon be achievable, giving rise to a new regime of strong field QED-plasmas in which large fractions of the laser energy is absorbed into gamma-rays and subsequently electron-positron pair creation. The gamma-rays are produced by synchrotron-like radiation of electrons accelerated by the laser and the resulting radiation force can dominate the motion at ultra-high intensities. A program of simulations is underway using the ARCHIE-WeST HPC to investigate the onset of these effects.

Simulations aid in understanding laser-plasma interactions by enabling, a time-resolved measure of key interaction parameters (laser and plasma) which are inherently difficult to measure experimentally.  The comparison between experiment and simulation results leads to a more comprehensive understanding of these complex, dynamic interactions.

Figure: Propagation of high intensity laser pulse (>1020 W/cm2), from left to right, through a plasma with an increasing density gradient. The interaction of the laser pulse with the plasma, and vice versa, results in self-focusing of the laser pulse as well as an increase in the efficiency of laser coupling to high energy electrons. This in turn leads improvements in the production, both in number and energy, of ions.

For a list of the research areas in which ARCHIE-WeSt users are active please click here.