A study on preheating effects in laser-driven shock waves is presented. Two different diagnostics were used: the color temperature measurement deduced by recording the target rear side emissivity in two spectral bands, and the rear surface reflectivity measurement by using a probe beam. In order to test the response of the two diagnostics to the preheating, three types of targets characterized by different radiative properties were used. The greater sensitivity of the second diagnostic compared with the first was demonstrated. A model which calculates the reflectivity using a one-dimensional hydrodynamic code data was developed. In this model, the wave propagation equations in the expanding plasma using an appropriate model for the electron–ion collision frequency applicable to the cold solid-hot plasma transition were solved. The comparison between the calculated and measured reflectivities allows us to estimate the preheating process.
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...
We present the results of the low-melting liquid metal droplets generation based on excited Rayleigh jet breakup. We discuss on the operation of the industrial and in-house designed and manufactured dispensing devices for the droplets generation. Droplet diameter can be varied in the range of 30-90 μm. The working frequency of the droplets, velocity, and the operating temperature were in the ranges of 20-150 kHz, 4-15 m/s, and up to 250 °C, respectively. The standard deviations for the droplet center of mass position both their diameter σ < 1 μm at the distance of 45 mm from the nozzle. Stable operation in the long-term (over 1.5 h) was demonstrated for a wide range of the droplet parameters: diameters, frequencies, and velocities. Physical factors affecting the stability of the generator operation have been identified. The technique for droplet synchronization, allowing using the droplet as a target for laser produced plasma, has been created; in particular, the generator has been successfully used in a high brightness extreme ultraviolet (EUV) light source. The operation with frequency up to 8 kHz was demonstrated as a result of the experimental simulation, which can provide an average brightness of the EUV source up to ∼1.2 kW/mm sr.
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