Time-resolved measurements of fast electron recirculation for relativistically intense femtosecond scale laser-plasma interactions

Abstract

This data was generated from a 2013 Gemini experiment (Central Laser Facility) investigating fast electron transport in solid targets. The data was obtained from an optical probe that was reflected off the rear surface of the targets and collected using a single lens imaging system, before being split between three cameras that were filtered by wavelength. A 10 mm apodised portion of a second Gemini laser was used as a rear surface optical probe, incident at an angle of 40 degrees relative to target normal in a p-polarised orientation.

The beam was chirped to a pulse length of between 0.3 and 5 ps by detuning the compressor. At the shortest pulse duration (300 fs) the temporal resolution was ~100 fs. By using a linear chirp, a linear relationship between wavelength and probe timing could be assumed, the shortest wavelengths (~780nm) probing earliest in time, the longest ~820nm) latest in time. In order to separate the time windows, three CCD cameras were using in the imaging line, with a different bandpass filter in front of each camera (see Green et al. J. Inst. 2014 http://dx.doi.org/10.1088/1748-0221/9/03/P03003 for more details). An f=40 cm achromatic lens was used to image the target rear surface, yielding a spatial resolution of 6um at 800 nm, and a magnification of x7.

The timing of the rear surface probe relative to the main pulse was also varied with the use of a timing slide. In order to make quantitative measurements of the rear surface reflectivity a reference image of each target was first obtained by just firing the probe beam of the Gemini laser. A second image was then taken with both the probe beam and main interaction pulse incident on the target, with the timing between the two varied on each shot. By normalising the on-shot reflectivity map with the reference reflectivity map obtained from the undisturbed foil, a clear 2D measurement of any change in target reflectivity was obtained.