Electron Temperature Measurements of Solid Density Plasmas Produced by Intense Ultrashort Laser Pulses

Abstract

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...

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