Abstract
A key issue in realising the development of a number of applications of high-intensity lasers is the dynamics of the fast electrons produced and how to diagnose them. We report on measurements of fast electron transport in aluminium targets in the ultra-intense, short-pulse (<50 fs) regime using a high resolution temporally and spatially resolved optical probe. The measurements show a rapidly (≈0.5c) expanding region of Ohmic heating at the rear of the target, driven by lateral transport of the fast electron population inside the target. Simulations demonstrate that a broad angular distribution of fast electrons on the order of 60° is required, in conjunction with extensive recirculation of the electron population, in order to drive such lateral transport. These results provide fundamental new insight into fast electron dynamics driven by ultra-short laser pulses, which is an important regime for the development of laser-based radiation and particle sources.
Highlights
The development of many cutting-edge applications in the field of ultra-high intensity laser-plasma interactions is highly dependent on progressing our understanding of fast electron generation and transport
Based on our measurements and closely coupled simulations, we report that high energy electrons are accelerated into the target over a large half angle of ≈60°, with electron recirculation driving strong lateral transport of energy over areas significantly larger than the original laser focal spot and on timescales much longer than the pulse duration
Hybrid-PIC simulations of the interaction indicate that a large initial fast electron divergence angle coupled with electron recirculation are required to maintain the radial expansion and heating observed
Summary
The development of many cutting-edge applications in the field of ultra-high intensity laser-plasma interactions is highly dependent on progressing our understanding of fast electron generation and transport. A number of complementary techniques have been used to diagnose fast electron beam transport in dense targets in an attempt to characterise the electron beam temperature, flux and divergence Of these key fast electron beam properties, both diagnosing and controlling the beam divergen ce is perhaps the greatest challenge, with previous experimental measurements[11] indicating that the fast electron divergence half angle is on the order of 45° at intensities of ≈1020 Wcm−2. The degrees to which electron recirculation and the initial electron injection profile affect subsequent transport physics clearly impacts many areas of laser-plasma sources and requires the development of diagnostic approaches more suited to spatially and temporally resolving electron transport inside the target. Recent time-resolved measurements have provided spatially and temporally resolved data on a short-pulse interaction[32]
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