# Relativistic laser-plasma interaction, inertial fusion physics

Present members: Geissler, Florian Gruener, Hartmut Ruhl, Laser Plasma Theory Group, MPQ

### Activities of the Laser Plasma Theory Group:

### Relativistic laser plasma

Present-day lasers with powers in the range of terawatts to petawatts (10^{12} - 10^{15} W) reach focused intensities high enough to ionize target materials (gases, solids) instantaneously and to create dense ionized matter called plasma. Propagation of light in plasma critically depends on the free electron dynamics performing charge oscillations against the ion background at the plasma frequency ωp. At intensities in excess of 10^{18} W/cm^{2} the dynamics of a free electron in the laser field becomes relativistic. The laser plasma interaction then changes drastically, driving electrons mainly in the laser direction and producing large currents and magnetic fields. At much larger fields radiation reaction must be taken into consideration. Remaining in the limiting case of soft photons the creation of electron-positron pairs is inhibited. However, the dynamics of an electron in the radiation field changes drastically. At yet larger field strengths pair creation processes will set in.

We study those features theoretically and by simulation. In particular, we have access to particle-in-cell (PIC) codes, which run on either dedicated clusters or else on other super-computers available at the MPQ. Ultra high fields open up a number of fascinating new applications with table-top laser devices. Here we list a few examples:

### Laser particle accelerators and XFELs

The relativistic interaction of lasers with plasma is capable of forming stable nonlinear excitations in the plasma in which electrons can be accelerated to GeV energies with high efficiency. The acceleration of electrons in plasma has parameters similar to those obtained in large conventional electron accelerators. The acceleration lengths, however, are millimeters rather than kilometers. Presently we explore possibilities to drive free electron lasers (FELs) with electrons accelerated in plasma. The hope is that table-top devices capable of producing intense coherent soft X-rays can be designed.

### Attosecond pulses from relativistic mirrors

Solid surfaces irradiated by intense ultra short laser pulses can become relativistic mirrors capable of producing high harmonics of the laser frequency and attosecond pulse trains of energetic photons with high efficiency. The incident light ionizes the surface. In the presence of the laser light the dense electron cloud forms an oscillating mirror moving at almost the speed of light. The laser photons scattered back from the oscillating surface are shifted to higher frequencies. For incident laser pulses with phase control they may emerge as light bursts of a few attoseconds.

### Exploring the concept of Fast Ignition

The idea of Fast Ignition of pre-compressed nuclear fuel requires extreme energy density. Petawatt laser pulses are perceived as appropriate drivers. The associated research topic is the physics of transport of the igniter energy through very dense plasma by electrons.

### Radiation Reaction and QED vacuum

At ultra large intensities the quantum structure of the QED vacuum is supposed to show nonlinear, non-classical phenomena. One of the most prominent properties of the QED vacuum under optimized laser pulse conditions is the onset of cascades that consist of electrons, positrons and radiation. The required field strengths are sufficiently far away below the Schwinger limit.

### Contribution to IMPRS curriculum:

### Related PhD work at MPQ

The MPQ laser plasma theory group offers PhD work to students in any of the topics mentioned above, in particular to those interested in cutting-edge numerical work on non-standard hardware.

### Lectures offered:

### Computational Plasma Physics

#### Lecturer: Hartmut Ruhl (4+2 hours per week)

Content:

Plasma Physics comprises a wide range of topics. The lecture will give an introduction into the basic sets of equations and models used to describe plasma. Those are Maxwell equations and equations for radiation reaction, fluid equations, kinetic equations, quantized transport equations, and elementary QED processes in the realm of ultra high fields. The main part of the course is concerned with the development of numerical methods and their implementation on the computer. Exercises will improve the understanding of the presented material. Particular emphasis is given to laser plasma.