Can proton radiography be used to image imploding target in ICF experiments
2011
Generation of high intensity and well collimated multi energetic proton beams from laser-matter
interaction extend the possibility to use protons as a diagnostic to image imploding target in Inertial
Confinement Fusion experiments. An experiment was done at the Rutherford Appleton Laboratory
(Vulcan Laser Petawatt laser) to study fast electron propagation in cylindrically compressed targets,
a subject of interest for fast ignition. This was performed in the framework of the experimental road
map of HiPER (the European High Power laser Energy Research facility Project). In the experiment,
protons accelerated by a ps-laser pulse were used to radiograph a 220 m diameter cylinder (20 m wall,
filled with low density foam), imploded with 200 J of green laser light in 4 symmetrically incident
beams of pulse length 1 ns. Point projection proton backlighting was used to get the compression
history and the stagnation time. Detailed comparison with 2D numerical hydro simulations has
been done using a Monte Carlo code adapted to describe multiple scattering and plasma effects
and with those from hard X-ray radiography. These analysis shows that due to the very large mass
densities reached during implosion processes, protons traveling through the target undergo a very large
number of collisions which deviate protons from their original trajectory reducing proton radiography
resolution. Here we present a simple analytical model to study the proton radiography diagnostic
performance as a function of the main experimental parameters such as proton beam energy and target
areal density. This approach leads to define two different criteria for PR resolution (called "strong"
and "weak" condition) describing different experimental conditions. Finally numerical simulations
using both hydrodynamic and Monte Carlo codes are presented to validate analytical predictions.
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