I will present data and supporting theoretical and numerical models from two experiments
involving high intensity lasers interacting with matter. These experiments do not represent
work at the very highest available laser intensities, rather they exploit more modest energy
systems, though still able to access relativistic intensities. This work was performed with
the intention of optimising the emission of secondary sources (electrons, protons and x-rays)
from these laser matter interactions and, in the process, furthering our understanding of the
mechanisms that produced them. This also provides a route to optimisation of experiments
on near-future high-intensity lasers where the increased repetition rate provides access to
large data sets, and thus enables use of techniques such as modification of the laser pulse
shape via a genetic algorithm.
The first experiment took place on the Astra Gemini laser at the Central Laser Facility.
The 4x10^17 Wcm2 ultra short pulse laser was used to irradiate argon clusters, of
radius varying from 6 nm to 11 nm. The x-ray emission of the laser irradiated clusters was
observed with a set of PIN diodes and a genetic algorithm was implemented that controlled
the high order spectral phase terms (up to fourth order) of the laser pulse using a Dazzler,
and optimised the x-ray emission as a result. The genetic algorithm improved the x-ray
emissions by a factor of ~3. The resultant laser pulse that produced the highest x-ray yield
was not as one might expect the shortest pulse possible, rather it exhibited a distinct linear
chirp and a low intensity foot on its leading edge. This was interpreted as the optimum
laser-energy coupling to the cluster electrons requiring some initial pre-ionisation before the
main pulse, along with a more subtle time varying frequency to match a transient resonance
in the electrons laser absorption.
The second experiment involved the Cerberus high-contrast OPCPA-Nd:Glass laser system
at Imperial College. An artificial pre-pusle of I ~10^14 Wcm2 was added to the main beam
of I~10^18 Wcm2 at various time delays, which subsequently irradiated 14 um aluminium foils. The x-rays, electrons and protons emitted by the target were observed by a variety
of diagnostics. An enhancement in the electron and proton energies by a factor of ~5 was
observed when the pre-pulse was timed to arrive 60 ps prior to the main pulse. It is theorised
that this is an effect of the laser self-focusing in the short scale length pre-plasma developed
at the front of the target, hence accelerating electrons with stronger fields, the enhanced electron
acceleration couples into proton acceleration through the mechanism known as target
normal sheath acceleration.