Laser Plasma Section

High energy x-ray generation and fast electron transport in thin foils and wires

Fast electron transport studies

The interaction of an ultra-short (< 50 fs), ultra-high intensity (≥1018 Wcm-2) laser pulse with a solid target leads to the generation of fast electron having energy from few keV to MeVs, with a current density in the target as high as 1012 A cm-21. Figure 1 illustrates the generation and transport of fast electrons in laser solid interaction. The propagation of this high energy electron beam inside the bulk solid target leads to generation of inner shell x-ray line radiation as well as hard x-ray bremsstrahlung radiation. In addition, the propagation of this electron beam gives rise to self-generated huge magnetic fields, together with space-charge and inductive electric fields. These fields modify the transport properties of the fast electron beam inside the target. As a result, a significant fraction of the accelerated electrons can be "focused" and even "guided" over a long distance. There is a great interest in the research to understand the physical mechanisms involved in the generation of the relativistic electron beams and their propagation in the target in laser–plasma interaction due to its application in fast ignition approach to inertial confinement fusion (ICF), the acceleration of MeV ions generation and production of ultrafast duration x ray pulses.

Fast electron transport in high intensity laser matter interaction
Figure 1: Fast electron transport in high intensity laser matter interaction

We have carried out an initial study on the propagation of fast electrons through solid density matter. The experimental setup is shown in Fig. 2. Fast electrons were generated by focusing 45 fs, 10 TW Ti: Sapphire laser pulse to a maximum intensity of ~ 1.3 x1018 W-cm-2 on 7 μm thick Cu foil. X-ray source size variation (at the rear of the target) was studied by changing laser irradiation and target parameters to infer the electron transport process. The knife-edge technique was used for measuring x-ray source size from the rear surface as well as at front surface of the target. The source size obtained on the front side was 38 μm (~ 3. 8 times of the laser focal spot size of 10 μm) whereas a significantly larger source size 152 μm (~ 15.2 times the laser focal spot size) was measured at the target rear surface.

Schematic diagram of the experimental setup
Figure 2 : Schematic diagram of the experimental setup

A multi-layered target geometry was used for measurement of fast electron divergence. The target had a front propagation layer of Al of thickness in the range 7– 25 μm, with a 20 μm thick Cu tracer layer placed behind the front layer. The variation of the x-ray source size with the propagation layer thickness gives the half divergence cone angle. On irradiating the target at a laser intensity of ~1.3×1018 W-cm-2, the divergence of the fast electron was estimated to be 37°.

The variation of source size with Al layer thickness
Figure 3 : The variation of source size with Al layer thickness

Experimental measurements of the fast electron spectra were done from the rear side of foil targets using an indigenously developed electron spectrograph. The electron temperature was derived from the slope of the semi-log plot of the electron spectrum. An electron temperature of ~ 192 keV was estimated at a laser intensity of ~1.3×1018 W-cm-2.

Fast electron spectrum measured from the rear side of foil surface
Figure 4 : Figure 4 : Fast electron spectrum measured from the rear side of foil surface

BS Rao, V Arora, PA Naik, PD Gupta , Phys. Plasmas 19, 113118 (2012)
T. Mandal, V. Arora, M. Tayyab, S. Bagchi, R. Rathore, B. Ramakrishna, C. Mukharjee, J. A. Chakera, P. A. Naik, and P. D. Gupta. Applied Physics B: Lasers and Optics 119, 281 (March 2015).

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