We report on shadowgraphic measurements showing the first space-and time-resolved snapshots of ultraintense laser pulse-generated fast electrons propagating through a solid target. A remarkable result is the formation of highly collimated jets (,20-mm) traveling at the velocity of light and extending up to 1 mm. This feature clearly indicates a magnetically assisted regime of electron transport, of critical interest for the fast ignitor scheme. Along with these jets, we detect a slower (ഠc͞2) and broader (up to 1 mm) ionization front consistent with collisional hot electron energy transport. 52.60. + h The fast ignitor scheme, which claims to relax some of the constraints hampering the standard approaches to inertial confinement fusion, has triggered a worldwide interest since its inception [1]. It hinges on the rapid additional heating of the core of a precompressed thermonuclear pellet due to the slowing down of a bunch of relativistic electrons generated by an ultraintense laser pulse. Now, the highly overcritical plasma surrounding the core should prevent any laser pulse from reaching it, whatever highintensity penetration mechanisms are at work (relativistic self-induced transparency [2] or ponderomotive hole boring [3]). An encouraging point is that particle-in-cell simulations predict a rather peaked hot electron distribution in the vicinity of the laser-solid interaction zone [4]. However, an efficient heating of the core requires the electron beam to remain collimated up to its final absorption zone, i.e., on a distance of several hundreds of microns. This can be achieved only through the pinching effect of the beam-driven magnetic field competing with multiple scattering. Therefore, fast electron transport from moderately to extremely dense regions appears as a key issue for the success of fast ignition, which must be thoroughly tackled both experimentally and theoretically.Over the past year, there has been a growing body of experimental evidence pointing to the existence of very collimated high intensity laser-produced electron jets traveling through solid targets. Tatarakis et al. have recently observed a narrow expanding plasma at the rear surface of thick plastic slabs irradiated by a 1 ps, 10 19 W͞cm 2 laser pulse [5]. By using a 2D Fokker-Planck hybrid code, they interpreted this localized rear heating as a magnetic field-enhanced electron energy deposition at the target/vacuum interface [6]. This effect has also been detected in other experiments [7]. Though very encouraging, these studies still provide an incomplete experimental picture of the phenomena arising in the bulk of the target.In the present paper, we report on optical shadowgraphic results showing what is, to our knowledge, the first comprehensive set of space-and time-resolved snapshots of fast electrons propagating through a solid target. In order to bypass the classical limitation of optical probing into an overcritical solid target, we use transparent glass slides. Our measurements pinpoint the existence of two types of fast...
We report on rear-side optical self-emission results from ultraintense laser pulse interactions with solid targets. A prompt emission associated with a narrow electron jet has been observed up to aluminum target thicknesses of 400 microm with a typical spreading half-angle of 17 degrees. The quantitative results on the emitted energy are consistent with models where the optical emission is due to transition radiation of electrons reaching the back surface of the target or due to a synchrotron-type radiation of electrons pulled back to the target. These models associated with transport simulation results give an indication of a temperature of a few hundred keV for the fast-electron population.
Fast electron generation and propagation were studied in the interaction of a green laser with solids. The experiment, carried out with the LULI TW laser (350 fs, 15 J), used K(alpha) emission from buried fluorescent layers to measure electron transport. Results for conductors (Al) and insulators (plastic) are compared with simulations: in plastic, inhibition in the propagation of fast electrons is observed, due to electric fields which become the dominant factor in electron transport.
We analyze recent experimental results on the increase of fast electron penetration in shock compressed plastic [Phys. Rev. Lett. 81, 1003 (1998)]. It is explained by a combination of stopping power and electric field effects, which appear to be important even at laser intensities as low as 10(16) W cm-2. An important conclusion is that fast electron induced heating must be taken into account, changing the properties of the material in which the fast electrons propagate. In insulators this leads to a rapid insulator to conductor phase transition.
We present the first results of fast electron deposition in a laser shock compressed plasma. The interaction of a 3 ps, 15 J laser pulse with solid polyethylene targets is used to produce fast electrons on one side of foil targets and a 2 ns duration laser pulse is used to drive a shock wave into the target from the opposite side. K a emission from chlorine fluor buried layers is used to measure the electron transport. The hot electron range in the shock compressed plastic is found to be approximately twice as large as the range in the solid density plastic. [S0031-9007(98)06642-3] PACS numbers: 52.35.Tc, 52.50.Lp, 62.50. + pThe fast ignitor scheme [1] gives a possible route to reducing the energy required to achieve breakeven and gain in laser driven inertial confinement fusion (ICF). This scheme requires that an intense, short ͑ϳ10 19 W cm 22 , 10 ps͒ laser pulse produces fast electrons which are then absorbed in a small region of dense compressed plasma in order to produce local heating and ignition [2,3]. Previous experiments have been conducted to measure the fast electron production and the deposition of their energy in solid density targets and reasonable agreement has been obtained with models [4-6].We report here experiments using the VULCAN laser to extend these measurements to the study of fast electron production and deposition in shock compressed plasmas using K a emission spectroscopy. The use of K a emission from buried layer fluors (fluorescent material) is now an established technique and has been widely used in the study of fast electrons from femtosecond laser plasma interactions [4,7].The experiment can be divided into three parts: (i) the study of shock wave dynamics and the determination of the parameters of the shock compressed material, (ii) the characterization of the fast electrons temperature, and (iii) the comparison of K a emission from shock compressed and solid density material.Fast electrons were produced on the "rear side" of plastic foil targets by focusing the VULCAN chirped pulse amplification (CPA) beam to a focal spot of 100 mm diameter using an f͞10 off axis parabola (OAP). The energies of the CPA beam used in these experiments was in the range 4 to 15 J and the pulse length was 3 ps. Maximum irradiances on target were approximately 6 3 10 16 W cm 22 . The CPA beam was incident at 30 ± on the target in order to maximize laser absorption following previous experiments [4]. The foil targets were compressed using two, 108 mm diameter frequency doubled long pulse beams (2 ns) of the VULCAN laser with a total energy of up to 160 J focused onto a spot of diameter 200 mm using random phase plates (RPP). The shock compression laser pulses were incident on the targets from the "front side," i.e., from the opposite side from the CPA beam. The targets in these experiments consisted of a PVC plastic fluor layer of 13.5 mm thickness, sandwiched between two thicknesses of polyethylene. The thickness of the polyethylene layer on the rear side of the target was varied from 10 to 150 mm. The...
The efficiency of ablation induced in poly(methyl methacrylate) (PMMA) by single soft X-ray pulses emitted from Z-pinch and laser-produced plasmas was determined. X-ray ablation of PMMA was found to be less efficient than that of teflon (PTFE). Nonthermal effects of the radiation on the polymer structure play a key role in the mechanisms of the ablation, i.e. the ablation can be explained by the formation of radiation-chemical scissions of the polymer chain followed by blowoff of low-molecular fragment fluid into the vacuum. The most promising application of this phenomenon seems to be micropatterning/micromachining.
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