Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We report experimental results on laser-driven electron acceleration with low divergence. The electron beam was generated by focussing 750 mJ, 42 fs laser pulses into a gas-filled capillary discharge waveguide at electron densities in the range between 1018 and 1019cm-3. Quasi-monoenergetic electron bunches with energies as high as 500MeV have been detected, with features reaching up to 1 GeV, albeit with large shot-to-shot fluctuations. A more stable regime with higher bunch charge (20-45 pC) and less energy (200-300 MeV) could also be observed. The beam divergence and the pointing stability are around or below 1 mrad and 8 mrad, respectively. These findings are consistent with self-injection of electrons into a breaking plasma wave. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Abstract:

Dense titanium plasma has been heated to an electron temperature up to 1300 eV with a 100 TW, high intensity short-pulse laser. The experiments were conducted using Ti foils (5 μm thick) sandwiched between layers of either aluminum (1 or 2 μm thick) or plastic (2 μm thick) to prevent the effects of prepulse. Targets of two different sizes, i.e., 250 × 250 μm 2 and 1×1 mm2 were used. Spectral measurements of the Ti inner-shell emission, in the region between 4 and 5 keV, were taken from, the front-side (i.e., the laser illuminated side) of the target. The data show large shifts in the Kα emission from open-shell ions, suggesting bulk heating of the sample at near solid density, which was largest for reduced mass targets. Comparison with collisional radiative and 2D radiation hydrodynamics codes indicates a peak temperature of Te,peak= 1300 eV of solid titanium plasma in ∼0.2 μm thin layer. Higher bulk temperature (T e,bulk=100 eV) for aluminum tamped compared to CH tamped targets (Te,bulk=40 eV) was observed. A possible explanation for this difference is described whereby scattering due to the nuclear charge of the tamping material leads to modified electron transport behavior. © 2007 American Institute of Physics.

Abstract:

Selective enhancement (>103) of harmonics extending to the water window (∼4nm) generated in an argon gas filled straight bore capillary waveguide is demonstrated. This enhancement is in good agreement with modeling which indicates that multimode quasi-phase-matching is achieved by rapid axial intensity modulations caused by beating between the fundamental and higher-order capillary modes. Substantial pulse energies (>10nJ per pulse per harmonic order) at wavelengths beyond the carbon K edge (∼4.37nm, ∼284eV) up to ∼360eV are observed from argon ions for the first time. © 2007 The American Physical Society.

Abstract:

In this Letter we report on a near collective x-ray scattering experiment on shock-compressed targets. A highly coupled Al plasma was generated and probed by spectrally resolving an x-ray source forward scattered by the sample. A significant reduction in the intensity of the elastic scatter was observed, which we attribute to the formation of an incipient long-range order. This speculation is confirmed by x-ray scattering calculations accounting for both electron degeneracy and strong coupling effects. Measurements from rear side visible diagnostics are consistent with the plasma parameters inferred from x-ray scattering data. These results give the experimental evidence of the strongly coupled ionic dynamics in dense plasmas. © 2007 The American Physical Society.