We report measurements of laser absorption for high-contrast ultrashort pulses on a variety of solid targets over an intensity range of 10'3 to 10's W/cm2. These data give an experimental determination of the target energy content and an indirect measure of dense plasma electrical conductivity. Our calculations accurately reproduce the behavior of aluminum targets, while the other materials show signs of additional absorption mechanisms. At high intensity all target materials reach a "universal plasma mirror" state and reflect about 90% of the incident light.

Simultaneous temporally and spatially resolved measurements of the phase change and reflectivity of S- and P-polarized femtosecond laser probes are obtained from hot expanded states produced by femtosecond laser heating of a solid aluminum target. The combined set of data provides an integral test of equation-of-state models in a regime up to 10 Mbar and densities of 0.01–1 times solid. The results suggest that target stoichiometry at the few Å level should be considered in the analysis of phase and reflectivity measurements in such experiments.

We have designed and built a Dense Plasma Focus (DPF) Z-pinch device using a kJ-level capacitor bank and a hollow anode, and fueled by a cylindrically symmetric gas puff. Using this device, we have measured peak deuteron beam energies of up to 400 keV at 0.8 kJ capacitor bank energy and pinch lengths of ∼6 mm, indicating accelerating fields greater than 50 MV/m. Neutron yields of on the order of 10(7) per shot were measured during deuterium operation. The cylindrical gas puff system permitted simultaneous operation of DPF with a radiofrequency quadrupole accelerator for beam-into-plasma experiments. This paper describes the machine design, the diagnostic systems, and our first results.

We report the first spatially and temporally localized measurements of 500 eV electron temperatures in solid density Al plasmas generated by a 3 3 10 17 W͞cm 2 , 170 fs laser. Expansion velocities of marker layers from various depths are sampled with mass-resolved ion time-of-flight spectroscopy. Hydrodynamic simulations relate the measured velocities to the sound speed, determining the temperature along the central axis of the plasma. Results are consistent with conductive heating of the first 1000 Å. [S0031-9007(96)00721-1] PACS numbers: 52.70.Nc, 52.40.Nk, 52.50.Jm, 52.65.Kj Recent advances in high intensity sub-ps laser technology have allowed new regimes of hot dense matter to be investigated [1][2][3][4][5][6]. Plasmas in the kilovolt temperature range at near solid densities are relevant to areas such as intense x-ray source development [6] and for inertial confinement fusion applications [7]. The macroscopic properties of these plasmas are determined by complex interactions such as laser absorption mechanisms at high intensities [4,5], and the generation and transport of radiation [8-10] and particle fluxes [11][12][13][14].Dense, high temperature plasmas are typically studied by x-ray spectroscopy, particle emission, and Doppler shift spectroscopy. Ion time-of-flight (TOF) measurements have been used previously to determine suprathermal electron temperatures of plasmas produced by lasers with pulse lengths from 1 ps [15] to 1 ns [16]. Ion energy distributions with MeV proton energies have recently been reported from 3 ps laser pulses [17]. Spatially and temporally averaged x-ray spectra of sub-ps laser produced plasmas have shown electron temperatures of a few hundred eV [9,10]. However, determining the peak temperature from x-ray spectra is problematic, since the fastest reported x-ray streak camera response is 0.89 ps [18], while simulations presented here indicate cooling times of a few hundred femtoseconds. Attenuation of cold Ka emission with depth in the target has been used to measure the suprathermal electron temperature [13,14] but does not measure the thermal electron temperature. Doppler shift measurements have been used to determine temperatures at lower intensities [19,20] but interpretation requires the inclusion of ponderomotive effects above 10 17 W͞cm 2 [21].For sub-ps laser heating, solid density plasmas are produced with scale lengths shorter than the laser wavelength.Hydrodynamic expansion of such plasmas should be relatively simple to measure and interpret theoretically. Ions from the surface expand during the laser pulse, and therefore are driven by an electron distribution which may contain a suprathermal component. For the plasma conditions of this Letter, expansion from depths greater than ϳ100 Å occurs after the laser pulse, and therefore is driven only by thermal electron pressure. Thus, expansion velocities from surface and embedded materials are indicative of suprathermal and thermal temperatures, respectively. The work presented in this Letter is novel in that embedded la...

Production of protons with energies of ϳ20 keV have been observed to originate from the interaction of a high intensity laser with a preformed underdense plasma. The energy and distribution of ions are explained by acceleration by the ponderomotive force resulting from filamentation. [S0031-9007(96) PACS numbers: 52.40.Nk, 52.35.Fp, 52.35.Mw, 52.50.Jm The propagation of high intensity laser pulses through plasmas has been studied for a number of years. Predictions of laser-target interactions for inertial confinement fusion (ICF) applications, for example, will change if the laser intensity is sufficiently high to form density channels via the ponderomotive force or exceed the threshold for the filamentation instability. Numerous theoretical works have studied filamentation at low intensities [1]. Presently nonlinear fluid simulations [2,3] are being used to simulate high intensities (.10 16 W͞cm 2 ) which can occur in ICF applications; it is important to experimentally verify their accuracy in this new regime. Experimental measurement of filamented density channels formed at high intensities is extraordinarily difficult via imaging techniques due to their small transverse dimensions (ϳ10 mm). On the other hand, as shown first in particle-in-cell simulations [4] and in the fluid simulations [2,3], intense laser pulses can produce a large transverse ponderomotive force which can accelerate ions to high velocities (y i ¿ c s , the sound speed) which can then be used to diagnose the laser intensity in the filament. Although the energy distributions of ions ejected from laser-plasma interactions have been studied extensively in the past [5], these ions have generally been accelerated by the ambipolar potential created by hot electrons which can be produced by processes such as resonance absorption [6].In this Letter, we describe the results of an experiment in which ion velocity distributions are observed which are produced when a focused laser beam interacts with preformed underdense plasmas. As we vary the incident laser intensity, the production of hot ions is correlated with the onset of filamentation. The separation of the plasma formation from the hot ion generation allows us to distinguish between the ion distributions from each process and to choose the maximum electron density reached by the interaction beam. We have observed proton energies of ϳ20 keV for incident intensities of 5 3 10 16 W͞cm 2 , a laser wavelength of 1.064 mm, and densities of 0.25n c , where n c is the critical density at which the incident laser frequency equals the electron plasma frequency. This result is confirmed by twodimensional fluid simulations, which show that the ion ejection is localized spatially to the filamentation region, and by particle-in-cell simulations which accurately predict the observed ion energies.A simple estimate shows that filamentation can easily produce ions with energies much greater than the initial thermal ion energy. We equate the kinetic energy of the ion to the potential energy set up by the ponde...