Abstract
Plasma expansion following the interaction of an intense laser beam with the inner surface of gold hohlraums, emulating conditions relevant to indirect drive inertial confinement fusion (ICF), has been investigated by a radiographic technique which employs a beam of laser-accelerated protons. This probing technique has made it possible to measure the electric field distribution associated with the plasma front and its propagation throughout the interior of the hohlraum with a temporal and spatial resolution of the order of a few ps and μm, respectively. The data indicate that the expanding plasma slows down approaching the opposite walls, possibly due to the interaction with x-ray heated plasma from the non-irradiated walls. The electric field at the plasma front shows a bipolar structure, suggesting the presence of ion-acoustic soliton-like structures cotraveling with the front. Data obtained using enclosed hohlraums suggest the feasibility of this type of diagnosis in gas-filled hohlraums, as currently employed in ICF experiments.
Highlights
Plasma expansion following the interaction of an intense laser beam with the inner surface of gold hohlraums, emulating conditions relevant to indirect drive inertial confinement fusion (ICF), has been investigated by a radiographic technique which employs a beam of laser-accelerated protons
A short and intense laser pulse is focused onto a thin metallic foil in order to generate and accelerate an energetic and collimated proton beam with a temporal duration comparable to the laser pulse duration
Since the proton beam is created via target normal sheath acceleration (TNSA) [16], it exhibits a broad energy spectrum resulting in different times of flight for the different energy components; the multi-layer arrangement of the RadioChromic Films (RCF) stack allows for a temporal multiframe capability of the detector even in a single shot configuration
Summary
The proton probing technique (see figure 1) employs a proton beam, generated and accelerated during intense laser–matter interaction, as a charged particle probe for laser-plasma experiments. A short and intense laser pulse is focused onto a thin metallic foil (in the case of the present work a 20 μm thick Au foil) in order to generate and accelerate an energetic and collimated proton beam with a temporal duration comparable to the laser pulse duration Such a proton beam is directed to propagate through the region of interaction between a second intense laser pulse and the target, and undergoes deflections due to the transverse electric field in the region of interest. This assumption implies that the electric field gradient is much larger along the plasma front (y-axis) than along the other transverse axis (z-axis).
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