ELEMENTARY PARTICLE PHYSICS

Peeling foils with laser pulses

One of the exciting results of the group’s research was the so-called “peeler regime” of laser-plasma interaction [1]. Besides fundamental physics, the acceleration of ions is also of large interest for medical applications like tumor therapy, as protons enable a very precise energy deposition in a narrow region of tissue. Several schemes of ion acceleration have already been proposed, one of the most prominent being “Target Normal Sheath Acceleration” (TNSA). In TNSA, a laser pulse irradiates a solid foil (“target”), heating up the electrons and forming a sheath field. This sheath field then accelerates the ions. Different acceleration mechanisms aim at improving one or multiple aspects of TNSA, like the maximum ion energy obtained with respect to the energy of the incident laser pulse.

Xiaofei Shen, now assistant professor at Peking University, points out: “Our idea is rather simple. Instead of irradiating the flat side of the foil, we direct the laser such that it grazes along its surface. This excites a surface plasma wave, which extracts electrons along the surface and that is capable of accelerating ions.” The peeler scheme is depicted in Figure 1.

The acceleration mechanism delivers high-quality ion beams with a very narrow energy spread which is of interest for applications like tumor therapy, where a spread of 1 % is required for the controlled irradiation of tissue. Besides the ion beams, Shen was able to show that the peeler regime can produce an abundance of electrons and bright x-rays which can be useful in applications like ultrafast diagnostics and high energy-density physics.

“The next crucial step in research would be to show that the peeler can be realized in experiment”, says Shen.

Giving ions a spin

Another research effort went into the realization of spin-polarized ion beams. This study was conducted by Lars Reichwein, a postdoctoral researcher at HHU, in close collaboration with Prof. Markus Büscher of Forschungszentrum Jülich. The existence of spin, a fundamental property of matter, has significant implications for basic research but also for applications like nuclear fusion. Already in the 1980s it was estimated by Kulsrud and Goldhaber that spin-polarized fuels, i.e. particle ensembles with aligned spin, could lead to an increase of approximately 50 % in fusion yield. This could make polarized beams another promising step towards nuclear fusion as a sustainable source of energy.

Currently, the main difficulty in obtaining polarized ion beams from laser-plasma interaction lies within the fact that high polarization and high energy seem to be counteracting goals. Dr. Reichwein explains: “We start from an initially polarized target and use the laser pulse to accelerate some of the ions. The problem is that the strong fields of the laser necessary for acceleration also induce stronger depolarization. Therefore, we needed to look for new setups that improve on that.”

The setup they proposed is rather counterintuitive: instead of using a single laser pulse, two pulses are utilized [2].  The two pulses propagate side-by-side through the plasma, creating two channels void of particles (see Figure 2 for the setup). In the space between them, a central filament made of polarized ions is formed, which is accelerated by the fields inside the plasma.

This dual-pulse process leads to improved polarization compared to the conventional single-pulse mechanism: as the structure of the electromagnetic fields is significantly different, the polarization is increased to approximately 80 %, whereas before only 60 % could be obtained.

The results on improving the degree of polarization will be of interest for upcoming experimental campaigns: recently, the group of Markus Büscher presented the first proof-of-principle results of laser-accelerated, polarized Helium-3. More experiments showing the capabilities of laser-plasma interaction for spin-polarized beams are planned, with direct input from the theoretical side.

Grazing for quantum electrodynamics

A last sub-project of Pukhov’s group explored laser-plasma interaction for near-future laser parameters. With the growing availability of high-intensity lasers, probing effects formerly beyond experimental capabilities comes into reach. One of the topics that could be extensively investigated in the next decades and that is driving the development of high-intensity lasers is quantum electrodynamics (QED). One of the most prominent effects in this regime is pair creation, where a photon decays into an electron and its anti-particle, a positron.

Accordingly, the group of Prof. Pukhov commonly explores potential setups with near-future laser parameters that enable the study of QED effects. Laser-plasma based approaches are a promising pathway in order to reach regimes like the “non-perturbative regime of QED”, where currently not even a complete analytical theory exists.

Within the computational project, Marko Filipovic, a former postdoctoral researcher of Prof. Pukhov, proposed a setup where two intense, counterpropagating laser pulses irradiate a solid target at a small, grazing angle [3]. The laser pulses extract a large number of electrons from the solid surface which leads to the production of gamma photons via the so-called non-linear Compton scattering. When a photon decays in the presence of the counter-propagating laser pulse, an electron-positron pair can be created (cf. Figure 3). As a large number of these pairs are produced during the interaction, the researchers speak of “QED cascades” in this context.