Transport of intense laser-produced electron beams in matter

Abstract

Fast electron transport in matter is a key issue for assessing the feasibility of fast ignition; however several important points are not clear yet. Therefore we realized an experiment with ultra-intense lasers ( 6 10 19 W cm −2 ) studying transport in metallic (Al) and insulating (CH) foil targets. The dynamics of fast electron propagation versus target thickness was investigated by optical selfemission from targets rear side. In Al targets we distinguished two-components in the fast electron population: moderately relativistic electrons and a highly collimated micro-bunched relativistic tail. A large ohmic heating at the rear of the thinner targets was observed due to the background return current. In CH, optical emission is mainly due to the Cherenkov effect and is much larger than in Al. We also observed that in insulators the fast-electron beam undergoes strong filamentation and the number of filaments increases with thickness. This behaviour was attributed to an ionization front instability.

Diagnosis of fast electron transport by coherent transition radiation

“…This result confirms CTR can be measured experimentally. However in most experiments, CCD is placed at the focus of convex lens [31,39], circular CTR is therefore diagnosed.…”

“…This makes CTR have high intensity in terahertz range [8]. Besides, in the studies of Liu et al and Batani et al, it is found that the intensity relationship between CTR and blackbody radiation is related to the thickness of the target [30,31]. In a thin target, the optical CTR has the same order of magnitude as the blackbody radiation, and may even be smaller than the blackbody radiation, but the CTR in terahertz range is stronger than that in blackbody.…”

Heat transport in solid target following relativistic laser–matter interaction

“…The interaction of relativistically intense (>10 18 W/cm 2 ), ultrashort (few tens of fs), high contrast ratio (10 10 ) laser pulses with solid targets is important for various potential applications, including isochoric heating of dense plasma [1][2][3], generation of high energy electrons [4][5][6], production of hard and soft x-rays [7][8][9], and fast ignition of laser fusion [10,11]. Ultrafast transport of energy out of the initially photo-excited volume governs the plasma temperature that is reached, the duration and source size of x-ray radiation, and the efficiency of energy deposition deep in the target.…”

“…The rapid movement of these hot electrons, however, creates strong electric and magnetic fields, which in turn complicate their transport pattern. Consequently, numerous recent experiments have focused on characterizing their transport in various directions: into the target along the surface-normal [4][5][6], into the target along the incident laser direction [12], along the specularly reflected laser direction [13], and along the target surface [11,14,15].…”

On the effect of rear-surface dielectric coatings on laser-driven proton acceleration

“…Such an effect, which was observed for plastic coatings thicker than 0.1 m, 31,30 has been attributed to the disruption of the fast electron current, which in many experiments has been observed to occur inside dielectric layers. 32 The laser-target coupling mechanisms giving rise to the fast electrons 33 and the transport dynamics of the fast electron current occurring inside the target 34,35 have thus emerged as a fundamental issue to be carefully addressed also in the study of laser-driven ion acceleration.…”