Laser-driven ion acceleration from carbon nano-targets with Ti:Sa laser systems

Over the past few decades, the generation of high energetic ion
beams by relativistic intense laser pulses has attracted great
attentions. Starting from the pioneering endeavors around 2000,
several groups have demonstrated muliti-MeV (up to 58 MeV for
proton by then) ion beams along with low transverse emittance and
ps-scale pulse duration emitted from solid targets. Owing to those
superior characteristics, laser driven ion beam is ideally suitable
for many applications. However, the laser driven ion beam typically
exhibits a large angular spread as well as a broad energy spectrum
which for many applications is disadvantageous. The utilization of
nano-targets as ion source provides a number of advantages over
micrometer thick foils. The presented PhD work was intended to
investigate laser driven ion acceleration from carbon nano-targets
and demonstrate the potential feasibility for biological studies.
Two novel nano-targets are employed: nm thin diamond-like-carbon
(DLC) foil and carbon nanotubes foam (CNF). Both are self-produced
in the technological laboratory at Ludwig-Maximilians-Universität
München. Well-collimated proton beams with extremely small
divergence (half angle) of 2 degrees are observed from DLC foils,
one order of magnitude lower as compared to micrometer thick
targets. Two-dimensional particle-in-cellsimulations indicate a
strong influence from the electron density distribution on the
divergence of protons. This interpretation is supported by an
analytical model. In the same studies, the highest maximum proton
energy was observed with a moderate laser intensity as low as
5*10^18W/cm^2. Parallel measurements of laser transmission and
reflection are used to determine laser absorption in the
nano-plasma, showing a strong correlation to the maximum proton
energy. This observation indicates significance of absorbed laser
energy rather than incident laser intensity and is supported by an
analytical model. The ion energy also depends on pulse duration, a
reduced optimum pulse duration is found as compared to micrometer
thick targets. This behavior is attributed to a reduction of
transverse electron spread due to the reduction of thickness from
micrometer to nanometer. These remarkable proton bunch
characteristics enabled irradiating living cells with a single shot
dose of up to 7 Gray in one nanosecond, utilizing the Advanced
Titanium: sapphire LASer (ATLAS)system at Max-Planck-Institut of
Quantum Optics (MPQ). The experiments represent the first
feasibility demonstration of a very compact laser driven nanosecond
proton source for radiobiological studies by using a table-top
laser system and advanced nano-targets. For the purpose of
providing better ion sources for practical application,
particularly in terms of energy increase, subsequent experiments
were performed with the Astra Gemini laser system in the UK. The
experiments demonstrate for the first time that ion acceleration
can be enhanced by exploiting relativistic nonlinearities enabled
by micrometer-thick CNF targets. When the CNF is attached to a
nm-thick DLC foil, a significant increase of maximum carbon energy
(up to threefold) is observed with circularly polarized laser
pulses. A preferable enhancement of the carbon energy is observed
with non-exponential spectral shape, indicating a strong
contribution of the radiation pressure to the overall acceleration.
In contrast, the linear polarization give rise to a more prominent
proton acceleration. Proton energies could be increased by a factor
of 2.4, inline with a stronger accelerating potential due to higher
electron temperatures. Three-dimensional (3D) particle-in-cell
(PIC) simulations reveal that the improved performance of the
double-layer targets (CNF+DLC) can be attributed to relativistic
self-focusing in near-critical density plasma. Interestingly, the
nature of relativistic non-linearities, that plays a major role in
laserwakefield-acceleration of electrons, can also apply to the
benefit of laser driven ion acceleration.