Speaker
Antoine Yvon Compant La Fontaine
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.2030.pdf
Photon dose lowering by fast electron energy loss induced by return current as a
short-pulse high-intensity laser interacts on a metal solid target
A. Compant La Fontaine
CEA, DAM, DIF, F-91297 Arpajon, France
During the interaction of a short-pulse high-intensity laser with the preplasma produced by
the pulse’s pedestal in front of a high-Z metal solid target, high-energy electrons are
produced, which in turn create an X-ray source by interacting with the atoms of the converter
target. The current brought by the hot electron current is almost completely neutralized by a
return current driven by the background electrons of the conductive target, and the force
exerted on the hot electrons by the electric field induced by the Ohmic heating produced by
the background electrons reduces the energy of the hot electrons and thus lowers the X-ray
emission and photon dose. This effect is analyzed here by means of a simple 1-D temperature
model which contains the most significant terms of the relativistic Fokker-Planck equations
with electron multiple scattering, and the energy equations of ions, hot and cold electrons are
solved numerically. The energy loss of the hot electrons by Ohmic heating varies with
different parameters as the plasma scale length, the target thickness, and the laser
characteristics. For instance for a ps laser pulse with 10 microns spot size on a tantalum
target, the energy loss fraction by Ohmic heating is about 10 to 40%. Laser and plasma
parameters may be optimized to reduce this effect, for instance at small plasma scale length
or at small laser spot size. Conversely, the resistive heating is enhanced with a foam target or
at long plasma scale length and high laser spot size and intensity, at which the incident hot
electron bunch have a small mean emission angle given by the ponderomotive force. The
X-ray emission and dose produced by a laser interacting in a gas jet may thus be inhibited
under these circumstances. The resistive heating may also be maximized in order to reduce
the X-ray emission to lower the radiation level for instance in a safety radiological goal.
