Ion acceleration from relativistic laser nano-target interaction

Laser-ion acceleration has been of particular interest over the
last decade for fundamental as well as applied sciences. Remarkable
progress has been made in realizing laser-driven accelerators that
are cheap and very compact compared with conventional
rf-accelerators. Proton and ion beams have been produced with
particle energies of up to 50MeV and several MeV/u, respectively,
with outstanding properties in terms of transverse emittance and
current. These beams typically exhibit an exponentially decaying
energy distribution, but almost all advanced applications, such as
oncology, proton imaging or fast ignition, require
quasimonoenergetic beams with a low energy spread. The majority of
the experiments investigated ion acceleration in the target normal
sheath acceleration (TNSA) regime with comparably thick targets in
the μm range. In this thesis ion acceleration is investigated from
nm-scaled targets, which are partially produced at the University
of Munich with thickness as low as 3 nm. Experiments have been
carried out at LANL’s Trident high-power and high-contrast laser
(80,J, 500 fs, t=1054nm), where ion acceleration with these
nano-targets occurs during the relativistic transparency of the
target, in the so-called Breakout afterburner (BOA) regime. With a
novel high resolution and high dispersion Thomson parabola and ion
wide angle spectrometer, thickness dependencies of the ions angular
distribution, particle number, average and maximum energy have been
measured. Carbon C6+ energies reached 650MeV and 1GeV for unheated
and heated targets, respectively, and proton energies peaked at
75MeV and 120MeV for diamond and CH2 targets. Experimental data is
presented, where the conversion efficiency into carbon C6+
(protons) is investigated and found to have an up to 10fold (5fold)
increase over the TNSA regime. With circularly polarized laser
light, quasi-monoenergetic carbon ions have been generated from the
same nm-scaled foil targets at Trident with an energy spread of as
low as ±15% at a central energy of 35MeV. High resolution kinetic
simulations show that the acceleration is based on the generation
of ion solitons due to the circularly polarized laser. The
conversion efficiency into monoenergetic ions is increased by an
order of magnitude compared with previous results in the TNSA
regime. The advances in ion energies and the control over the
spectra mark an important basis for future research of laser-driven
ion acceleration and might enable laser-based implementation of
these applications in the future.