The Study and Development of Pulsed High-field Magnets for Application in Laser-plasma Physics

The thesis at hand addresses design, characterization and experimental testing of pulsed high-field magnets for utilization in the field of laser-plasma physics. The central task was to establish a technology platform that allows to manipulate laser-driven ion sources in a way that the accelerated ions can be used in complex application studies, e.g. radiobiological cell or tumor irradiation.

Laser-driven ion acceleration in the regime of target normal sheath acceleration (TNSA) offers the unique opportunity to accelerate particles to kinetic energies of few 10MeV on the micrometer scale. The generated bunches are short, intense, show broad exponentially decaying energy spectra and high divergence. In order to efficiently use the generated particles, it is crucial to gain control over their divergence directly after their production. For most applications it additionally is favorable to reduce the energy spread of the beam. This work shows that the developed pulsed high-field magnets, so-called solenoids (cylindrical magnets), can efficiently capture, transport and focus laser-accelerated protons. The chromaticity of the magnetic lens thereby provides for energy selection.

Three prototype solenoids, adapted to fit different application scenarios, and associated current pulse drivers have been developed. The magnets generate fields of several 10 T. Pulse durations are of the order of one millisecond and thus the fields can be considered as quasi-static for laser-plasma interaction processes taking place on the ps- to ns-scale. Their high field strength in combination with abandoning magnetic cores make the solenoids compact and light-weight.

The presented experiments focus on a solenoid magnet designed for the capture of divergent laser-driven ion beams. They have been carried out at the 6MV tandetron accelerator and the laser acceleration source Draco of Helmholtz-Zentrum Dresden – Rossendorf as well as at the PHELIX laser of GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt. The results show that the developed technology platform breaks ground for a variety of practical applications of laser ion acceleration. It is shown that laser-driven ion beams can be efficiently injected into conventional accelerator structures to allow for phase space modulation. Furthermore, first practical studies on medical beam guidance systems are presented. Hence, the developed magnets allow to investigate feasibility and potential of the frequently proposed laser-based ion beam therapy of tumor diseases. The pulsed high-field magnets bring us one step closer to the realization of this ambitious endeavor, as they pave the way for compact and efficient beam guidance toward the patient but also, in the phase of translational research, allow to study the radiobiological properties of the novel particle source. In this context, worldwide first irradiation studies with laser-accelerated protons on volumetric tumors in the mouse model have been prepared and their feasibility studied, identifying already met radiobiological criteria and hurdles yet to overcome.