Description
Laser-solid interactions in the ultra-intense regime (>10$^{18}$ W cm$^{-2}$) rely primarily on the coupling of laser energy into the electron population of the target. Accelerated “fast” electrons drive all subsequent target behaviour, such as X-ray emission and proton acceleration. Targets with nanostructures on the front surface promise vast improvements in laser-electron coupling, with laser absorption of >90%, as opposed to <10% for a planar target [1]. Nanowires show promise in easy tailoring of the target for different purposes, such as X-ray radiography and the fast ignition regime of ICF [2].
Experimental measurements of the fast electron population allow diagnosis of laser electron coupling. Presented are results obtained from an experimental campaign from 2023 at ILIL, Pisa, during which the escaping fast electron populations from nanowire targets were diagnosed using Optical Transition Radiation (OTR) imaging [3] and emittance measurement using a pepper-pot diagnostic [4]. Emittance measurement indicates transverse position and divergence properties of the electron beam. OTR imaging also allows for diagnosis of any significant filamentation and bunching of escaped electrons. Planar targets are also diagnosed for suitable comparison.
Emittance measurements and OTR intensity measurements show no major change in performance of nanowire targets and indicate that the nanowire structures upon the target are destroyed by laser emission prior to the main pulse. Further study is ongoing in EPOCH [5] to supplement these observations.
[1] M. M. Murnane et al, Appl. Phys. Lett 62, no. 10, pp. 1068–1070, Mar. 1993
[2] G. Kulcsár et al., Phys. Rev. Lett. 84 no. 22, pp. 5149–5152, May 2000
[3] C. B. Schroeder et al., Phys. Rev. E 69 no. 1, p. 016501, Jan. 2004
[4] M. Zhang, FERMILAB-TM-1988, Oct. 1996
[5] T. D. Arber et al., Plasma Phys. Control. Fusion 57, no. 11, p. 113001, Sep. 2015