51ÁÔÆæÈë¿Ú

Abstract:

Hydrodynamic optical-field-ionised (HOFI) plasma channels promise a route towards high repetition-rate, metre-scale stages for future laser plasma accelerators. These channels are formed by hydrodynamic expansion of a plasma column produced by optical field ionisation at the focus of a laser, typically from an axicon lens. Since the laser pulses used to generate the initial plasma column are of sub-picosecond duration, chromatic effects in the axicon lens could be important. In this paper we assess these effects using a numerical propagation code. The code is validated using analytical formulae and experimental data. For the parameter range investigated, dispersive effects are found to be of minor importance, reducing the peak on-axis intensity in the focal region by approximately 10%.

Abstract:

The phase retrieval problem occurs in a number of areas in physics and is the subject of continuing investigation. The one-dimensional case, e.g., the reconstruction of the temporal profile of a charged particle bunch, is particularly challenging and important for particle accelerators. Accurate knowledge of the longitudinal (time) profile of the bunch is important in the context of linear colliders, wakefield accelerators and for the next generation of light sources, including x-ray SASE FELs. Frequently applied methods, e.g., minimal phase retrieval or other iterative algorithms, are reliable if the Blaschke phase contribution is negligible. This, however, is neither known a priori nor can it be assumed to apply to an arbitrary bunch profile. We present a novel approach which gives reproducible, most-probable and stable reconstructions for bunch profiles (both artificial and experimental) that would otherwise remain unresolved by the existing techniques.

Abstract:

In multi-pulse laser wakefield acceleration (MP-LWFA) a plasma wave is driven by a train of low-energy laser pulses separated by the plasma period, an approach which offers a route to driving plasma accelerators with high efficiency and at high pulse repetition rates using emerging technologies such as fibre and thin-disk lasers. Whilst these laser technologies are in development, proof-of-principle tests of MP-LWFA require a pulse train to be generated from a single, high-energy ultrafast pulse. Here we demonstrate the generation of trains of up to 7 pulses with pulse separations in the range 150–170 fs from single 40 fs pulses produced by a Ti:sapphire laser.