Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

The use of a single-photon-counting x-ray charge-coupled device (CCD) camera as an x-ray spectrometer is a well-established technique in ultrashort-pulse laser experiments. In single-photon-counting mode, the pixel value of each readout pixel is proportional to the energy deposited from the incident x-ray photon. For photons below 100 keV, a significant fraction of the events deposits all the energy in a single pixel. A histogram of the pixel readout values gives a good approximation of the x-ray spectrum. This technique requires almost no alignment, but it is very sensitive to signal-to-background issues, especially in a high-energy petawatt environment. Shielding the direct line of sight to the target was not sufficient to obtain a high-quality spectrum, for the experiments reported here the CCD camera had to be shielded from all sides with up to 10 cm of lead. © 2004 American Institute of Physics.

Abstract:

The full aperture backscatter system provides a measure of the spectral power, and integrated energy scattered by stimulated Brillouin (348-354 nm) and Raman (400-800 nm) scattering into the final focusing lens of the first four beams of the NIF laser. The system was designed to provide measurements at the highest expected fluences with: (1) spectral and temporal resolution, (2) beam aperture averaging, and (3) near-field imaging. This is accomplished with a strongly attenuating diffusive fiber coupler and streaked spectrometer and separate calibrated time integrated spectrometers, and imaging cameras. A new technique determines the wavelength dependent sensitivity of the complete system with a calibrated Xe lamp. Data from the calibration system are combined with scattering data from targets to produce the calibrated power and energy measurements that show significant corrections due to the broad band calibrations. (C) 2004 American Institute of Physics.

Abstract:

A high-energy, ultraviolet Thomson scattering probe beam has been implemented on the Omega laser facility at the University of Rochester. The new probe operates at a wavelength of 264 nm, with a maximum energy of 260 J in a pulse length of 1 ns. The probe is focused with an F/6.7 lens to a minimum focal spot of 40 mum within a pointing tolerance of <50 mum. Data obtained from this probe beam have provided new diagnostic information on plasmas relevant for inertial confinement fusion and atomic physics studies. (C) 2004 American Institute of Physics.

Abstract:

We have studied the focusing properties of two highly oriented pyrolitic graphite (HOPG) spectrometers, which differ in the degree of the mosaic spread: ZYA with a low mosaic spread (gamma=0.4degrees) and ZYH with a large mosaic spread (gamma=3.5degrees). In order to asses the crystal performance for a variety of different experiments, various Kalpha and Kbeta x-ray lines have been produced using a high-intensity (greater than or similar to10(17) W/cm(2)) short-pulse (similar to100 fs) laser beam focused onto Ti, V, Zn, and Cu foils. The measured spectral resolution of the HOPG crystals in both first and second order diffraction has been compared with theoretical predictions. Using known values for the peak reflectivity of HOPG crystals, we have also computed Kalpha x-ray conversion efficiencies of Ti, V, Zn, and Cu. These results are important to estimate the optimal conditions under which different types of HOPG monochromators can be used for the detection of weak x-ray signals as the one encountered in x-ray Thomson/Compton scattering experiments. (C) 2004 American Institute of Physics.

Abstract:

High-power lasers that fit into a university-scale laboratory can now reach focused intensities of more than 10 19 W cm -2 at high repetition rates. Such lasers are capable of producing beams of energetic electrons, protons and γ-rays. Relativistic electrons are generated through the breaking of large-amplitude relativistic plasma waves created in the wake of the laser pulse as it propagates through a plasma, or through a direct interaction between the laser field and the electrons in the plasma. However, the electron beams produced from previous laser-plasma experiments have a large energy spread, limiting their use for potential applications. Here we report high-resolution energy measurements of the electron beams produced from intense laser-plasma interactions, showing that-under particular plasma conditions-it is possible to generate beams of relativistic electrons with low divergence and a small energy spread (less than three per cent). The monoenergetic features were observed in the electron energy spectrum for plasma densities just above a threshold required for breaking of the plasma wave. These features were observed consistently in the electron spectrum, although the energy of the beam was observed to vary from shot to shot. If the issue of energy reproducibility can be addressed, it should be possible to generate ultrashort monoenergetic electron bunches of tunable energy, holding great promise for the future development of 'table-top' particle accelerators.