E. Hill

Lineshape measurements of He-β spectra on the ORION laser facility

We have utilized a newly developed high-resolution X-ray spectrometer to measure the shapes of spectral lines produced from laser-irradiated targets on the Orion laser facility in the United Kingdom. We present measurements of the He-β spectra of chlorine and chromium from targets irradiated by either a long-pulse or a short-pulse laser beam. The experimental conditions provide a spread in plasma density ranging from about 1019 to about 1024 cm−3. We present spectral calculations that show that the relative intensities of the Li-like satellite lines can be used to infer the density in the lower range, especially if the lithiumlike satellite lines are well resolved. In addition, we use the Stark-broadened width of the He-β line to infer densities above about 1022 cm−3. In the case of a short-pulse irradiated chromium foil, we find that the He-like chromium is produced at a density of almost 8 g/cm3, i.e., solid density. In addition, we can infer the electron temperature from the observation of dielectronic...

Photoionized astrophysical plasmas in the laboratory

The time-dependent collisional-radiative code ALICE [E. G. Hill and S. J. Rose, High Energy Density Phys. 5, 302 (2009)] is used to model the spectrum from a laboratory photoionized silicon plasma [S. Fujioka et al., Nat. Phys. 5, 821 (2009)]. The results show a good agreement with the laboratory spectrum and lend support to the accompanying analytical discussion of photoionized laboratory spectra, their parametrization, and relevance to astrophysics.

Measurements of plasma spectra from hot dense elements and mixtures at conditions relevant to the solar radiative zone

X-ray emission spectroscopy has been used to study hot dense plasmas produced using high power laser irradiation of dot samples buried in low Z foils of plastic or diamond. By combining a high contrast short pulse (picosecond timescale) laser beam operating in second harmonic with long pulse (nanosecond timescale) laser beams in third harmonic, and with pulse shaping of the long pulse beams, a range of plasma temperatures from 400eV up to 2.5keV and electron densities from 5e22 up to 1e24/cc have been accessed. Examples are given of measurements of dense plasma effects such as ionization potential depression and line-broadening from the K-shell emission spectra of a range of low Z elements and mixtures and compared to model prediction. Detailed spectra from measurements of the L-shell emission from mid-Z elements are also presented for an example spectrum of germanium. These data are at conditions found in stellar interiors and in particular in the radiative zone of the sun. The plasma conditions are infe...

Ultrafast Imaging of Laser Driven Shock Waves using Betatron X-rays from a Laser Wakefield Accelerator

Betatron radiation from laser wakefield accelerators is an ultrashort pulsed source of hard, synchrotron-like x-ray radiation. It emanates from a centimetre scale plasma accelerator producing GeV level electron beams. In recent years betatron radiation has been developed as a unique source capable of producing high resolution x-ray images in compact geometries. However, until now, the short pulse nature of this radiation has not been exploited. This report details the first experiment to utilize betatron radiation to image a rapidly evolving phenomenon by using it to radiograph a laser driven shock wave in a silicon target. The spatial resolution of the image is comparable to what has been achieved in similar experiments at conventional synchrotron light sources. The intrinsic temporal resolution of betatron radiation is below 100 fs, indicating that significantly faster processes could be probed in future without compromising spatial resolution. Quantitative measurements of the shock velocity and material density were made from the radiographs recorded during shock compression and were consistent with the established shock response of silicon, as determined with traditional velocimetry approaches. This suggests that future compact betatron imaging beamlines could be useful in the imaging and diagnosis of high-energy-density physics experiments.

Thomson scattering in short pulse laser experiments

Thomson scattering is well used as a diagnostic in many areas of high energy density physics. In this paper, we quantitatively demonstrate the practicality of using Thomson scattering as a diagnostic of short-pulse laser-plasma experiments in the regime, where the plasmas probed are at solid density and have temperatures of many hundreds of eV using a backlighter produced with an optical laser. This method allows a diagnosis both spatially and temporally of the density and temperature distributions in high energy density laser-plasma interactions which is independent from, and would act as a useful complement to, the existing spectroscopic methods.