ELEMENTARY PARTICLE PHYSICS

High energy ion beams can be used for ion beam therapy of tumours. Compared with conventional radiation therapy with X-rays and gamma-rays, this approach has many advantages including high precision and low side-effects. Ion beams generated by synchrotron accelerators have been used in many medical institutions. However, a synchrotron accelerator has a large footprint (soccer field size) and is very expensive. With the rapid developments of the high power laser technology, laser-driven plasma-based ion acceleration has attracted much attention. To gain a deeper knowledge on laser-plasma ion acceleration including the spatial distribution of ion density, which are very helpful for upcoming experiments and future medical applications, researchers carried out 3D simulations on SuperMUC using the Plasma-Simulation-Code (PSC).

Ion wave breaking acceleration happens in laser-driven foam-like relativistically-transparent plasma targets. Ions can be self-trapped in the laser-driven charge-separation field via ion wave breaking and then accelerated to very high energy. The number of the accelerated ions can be controlled by adjusting external parameters such as laser intensity and target density. It allows designing controllable high-energy high-quality ion accelerators. To gain a deeper knowledge on laser-plasma ion acceleration including the spatial distribution of ion density, which are very helpful for upcoming experiments and future medical applications, researchers carried out 3D simulations on SuperMUC using the Plasma-Simulation-Code (PSC).

Introduction

With the development of material science, well-defined near-critical density plasma (NCDP) targets can be prepared in experiments. The NCDP attracts much attention due to the nonlinear particle dynamics and the strong coupling between the laser pulse and the plasma. X/gammaray emission from NCDP driven by ultra-intense laser pulses has been observed in Particle-in-Cell (PIC) simulations and attracted the interest of experimentalists. A group lead by Jörg Schreiber at Ludwig-Maximilians-Universität, Munich, has done a preliminary experiment, and another group lead by Manuel Hegelich at Texas University in Austin is interested in carrying out an experiment.

PIC simulations adjusting the results to the experimental parameters have to be run. Ion acceleration enhancement with NCDP has been observed experimentally by a team of researchers lead by Jörg Schreiber. PIC simulations have to be run in order to scan experimental parameters and compare with the experimental results. By analysing the simulation results of the laser interacting with NCDP, we were surprised to find that there exists a new ion acceleration regime never described before. We call it ion wave breaking acceleration (IWBA).

Wave breaking is one of the most interesting phenomena in plasma physics. Electron self-injected acceleration via wave breaking has lead many applications. Ions are traditionally treated as particles. We found that, when applying a fast rising laser-driven pulse, the background ions move collectively as a cold wave. When the ion wave is too strong, the wave breaks, then a small fraction of ions can be self-injected into a laser driven wake and accelerated efficiently. The final ion beam is collimated and monoenergetic. Such a beam has potential important applications, such as tumour treatment, material detection, and basic physics. Since the ion wave breaking dynamics is too complex to be solved analytically, PIC simulations are needed for understanding the physics.

Results and Methods

Simulation method

The Plasma-Simulation-Code (PSC) is a general purpose framework to solve the extended Maxwell-Vlasov-Boltzmann system of equations via the PIC approach [1]. Recent extensions comprise the self-field effects of radiation and electron-positron pair production in strong fields. The original FORTRAN version evolved to a modern modularized C simulation framework supporting bindings to FORTRAN as well as C/CUDA and features selectable field and particle pushers. The PIC approach is well-known for its good scaling capability via configuration space parallelization. A Hilbert-Peano space-filling curve is used for efficient, dynamic and adaptive load and memory balancing allowing for complex and dynamic geometries.

Result: X/gamma-ray emission from NCDP

In this sub-project, we investigated the X/gamma-ray emission when propagating an ultraintense laser pulse in a NCDP via 3D PIC simulations. Collimated radiation with peaked photon energy up to MeV is observed, as marked by a black arrow in Fig. 1. In order to resolve detailed electron dynamics, very high resolution is needed. For a full 3D simulation we used 7 109 simulation cells and 35 macro-particles per cell, this requires total memory of up to 20Tb.

Result: Ion acceleration enhancement with NCDP

In this sub-project we investigated ion acceleration when irradiating an ultra-intense laser pulse on a combined target, which is formed by a uniformed NCDP layer and an ultra-thin solid foil. A high-density collimated ion layer is observed in a high-resolution 2D simulation, as shown in Fig. 2. In order to resolve the ultra-thin solid foil, very high resolution is needed in simulations. Furthermore, in order to include the laser self-shaping effect in NCDP, a very large simulation box is needed. Therefore even for a full 2D simulation, total memory of up to 15Tb is required.

Result: IWBA

In this sub-project we investigated self-injected ion acceleration when propagating an ultraintense laser pulse in a relativistic self-transparent NCDP with full 3D PIC simulations. The ion wave growing and breaking are observed clearly in simulations, as shown in Fig. 3. The ion wave breaking process is extremely nonlinear and the background ions show complex kinetic behaviour. In order to resolve the details, large scale and high resolution are required in the simulations. About 0.1 million core-hours are needed for a full 3D simulation. Since this IWBA regime is barely discussed in literature, we have to establish the model from scratch. We have run thousands of 1D/2D simulations and dozens of full 3D simulations for different laser plasma parameters to understand the physics of IWBA. Some of the results have been published [2].