Application-oriented laser-plasma accelerators
Compact laser-plasma-based light and particle sources have matured significantly over the last decade regarding the required laser technology as well as an understanding of the underlying physical processes. Both developments have paved the way for establishing first accelerators for applications in medicine and science relying on the laser-plasma-based accelerator concept. The Junior Research Group Application-oriented laser-plasma accelerators will set up a laser-based ion accelerator at the HZDR-developed PENELOPE laser with the longer-term perspective of maturing the system into an applicable ion source. From a physical point of view, the steps towards such a source ion source require a thorough understanding of the ion acceleration and underlying electron dynamics at a pulse duration of 150 fs, the PENELOPE laser system’s pulse duration. On the technical side, the final goal of providing an ion source for applications will require an optimization and automation of the ion acceleration setup which is up to now not realized for laser-driven sources. That entails the development of optimized diagnostics for laser-driven ions.
- Online detectors for laser-driven ion beams
The development of laser-plasma-based accelerators for applications requires the provision of online diagnostic tools adapted to the properties of laser-accelerated ion pulses. These pulses feature exponentially decreasing energy spectra with cut-off energies of up to currently several 10 MeV for the class of application-relevant high-power, short pulse laser sources, and the particle emission occurs within an angle of ∼30°. Both, the energy spectrum as well as the ion pulse’s beam profile carry essential information about the acceleration process: The cut-off energy of the spectrum is used as a benchmark for the acceleration performance, and the spatial emission pattern can carry information about the target surface, plasma dynamics or spatio-temporal properties of the laser pulse. Whereas offline detectors capable of simultaneously resolving the spectral and spatial properties of laser-driven ion pulses are used successfully, the spectral and spatial online characterization of ion pulses at the ∼Hz repetition rate of application-relevant high-power laser sources is still a challenge. To overcome this shortcoming, we develop scintillator-based online area detectors which use the energy-range correlation of presently mainly protons in an absorber to yield spectral and spatial resolution.
An online, energy-resolving beam profile detector for laser-driven proton beam, J. Metzkes, K. Zeil, S.D. Kraft, L. Karsch, M. Sobiella, M. Rehwald, L. Obst, H.-P. Schlenvoigt, U. Schramm, Review of Scientific Instruments 87, 083310 (2016)
A scintillator-based online detector for the angularly resolved measurement of laser-accelerated proton spectra, J. Metzkes, L. Karsch, S. Kraft, J. Pawelke, C. Richter, M. Schuerer, M. Sobiella, N. Stiller, K. Zeil, and U. Schramm, Review of Scientific Instruments 83, 123301 (2012)
- Characterization of laser absorption and electron dynamics via bremsstrahlung detectors
Laser-plasma-based ion acceleration at solid targets is a two-step process, requiring mediation from an energetic electron population created in the laser-matter-interaction: When irradiating the surface of a solid, usually micrometer thick target with focused laser intensities of up to 1021 W/cm2, a plasma is generated and the plasma electrons gain up to MeV energies in the laser pulse field. These electrons – energetic enough to leave the target and hence charge it up – set up quasi-static electric sheath fields of up to TV/m magnitude at the target surfaces, serving the acceleration of ions to several 10 MeV kinetic energies. The ion source is partly the target bulk but mainly the naturally present hydro-carbon contaminant layer at the target surface. The challenge in understanding and optimizing the involved process of laser-driven ion acceleration lies in accessing the complex underlying plasma dynamics occurring on femto- to picosecond time- and micrometer and below spatial scales. One approach we follow is the experimental characterization of particles and radiation emitted from the interaction region to e.g. determine the efficiency of laser light conversion into electron kinetic energy in the primary laser-matter-interaction. We apply magnetic electron spectrometers, which are sensitive to the very fast electron population generated around the laser pulse peak. These are combined with detectors for hard x-ray bremsstrahlung – generated by the collisions of the electrons with target bulk ions – and hence resolve the electron distribution in the target bulk. Both detector types are applied in experimental studies at the DRACO facility during the setup time PENELOPE experimental site, where they will be used to study the pulse duration dependent ion acceleration performance.