Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Two critical issues related to the success of fast ignition inertial fusion have been vigorously investigated in a co-ordinated campaign in the European Union and the United States. These are the divergence of the fast electron beam generated in intense, PW laser-plasma interactions and the fast electron energy transport with the use of high intensity contrast ratio laser pulses. Proof is presented that resistivity gradient-induced magnetic fields can guide fast electrons over significant distances in (initially) cold metallic targets. Comparison of experiments undertaken in both France and the United States suggests that an important factor in obtaining efficient coupling into dense plasma is the irradiation with high intensity contrast ratio laser pulses, rather than the colour of the laser pulse itself. © 2010 IOP Publishing Ltd.

Abstract:

We investigate the influence of a tilted laser-pulse-intensity front on laser-wakefield acceleration. Such asymmetric light pulses may be exploited to obtain control over the electron-bunch-pointing direction and in our case allowed for reproducible electron-beam steering in an all-optical way within an 8 mrad opening window with respect to the initial laser axis. We also discovered evidence of collective electron-betatron oscillations due to off-axis electron injection into the wakefield induced by a pulse-front tilt. These findings are supported by 3D particle-in-cell simulations. © 2010 The American Physical Society.

Abstract:

The experimental study of the behavior of deuterium plasma with densities between 2× 1018 and 2× 1020 cm-3, subjected to a 6 TW, 30 ps, 3× 1018 W cm-2 laser pulse, is presented. Conclusive experimental proof that a single straight channel is generated when the laser pulse interacts with the lowest densities is provided. This channel shows no small-scale longitudinal density modulations, extends up to 2 mm in length and persists for up to 150 ps after the peak of the interaction. Bifurcation of the channel after 1 mm propagation distance is observed for the first time. For higher density interactions, above the relativistic self-focusing threshold, bubblelike structures are observed to form at late times. These observations have implications for both laser wakefield accelerators and fast ignition inertial fusion studies. © 2010 American Institute of Physics.

Abstract:

Latest developments in the field of laser-wakefield accelerators (LWFAs) have led to relatively stable electron beams in terms of peak energy, charge, pointing and divergence from mmsized accelerators. Simulations and LWFA theory indicate that these beams have low transverse emittances and ultrashort bunch durations on the order of ∼ 10 fs. These features make LWFAs perfectly suitable for driving high-brightness X-ray undulator sources and free-electron lasers (FELs) on a university-laboratory scale.With the detection of soft-X-ray radiation from an undulator source driven by laser-wakefield accelerated electrons, we succeeded in achieving a first milestone on this path. The source delivers remarkably stable photon beams which is mainly due to the stable electron beam and our miniature magnetic quadrupole lenses, which significantly reduce its divergence and angular shot-to-shot variation. An increase in electron energy allows for compact, tunable, hard-Xray undulator sources. Improvements of the electron beams in terms of charge and energy spread will put table-top FELs within reach. © 2010 American Institute of Physids.