Note: femtosecond laser micromachining of straight and linearly tapered capillary discharge waveguides

Gas-filled capillary discharge waveguides are important structures in laser-plasma interaction applications, such as the laser wakefield accelerator. We present the methodology for applying femtosecond laser micromachining in the production of capillary channels (typically 200–300 μm in diameter and 30–40 mm in length), including the formalism for capillaries with a linearly tapered diameter. The latter is demonstrated to possess a smooth variation in diameter along the length of the capillary (tunable with the micromachining trajectories). This would lead to a longitudinal plasma density gradient in the waveguide that may dramatically improve the laser-plasma interaction efficiency in applications

Plasma plume influence on laser pulse evolution in capillary discharge waveguides

Gas-filled capillary discharge waveguide is an useful medium for investigating high-power laser-plasma interactions over extended lengths because guiding can increase the interaction length to many Rayleigh lengths. The role of the gas plasma plume at the entrance of a CDW in increasing the laser intensity is under investigation. Experimentally have been performed different measurements of the plasma density profiles in the region adjacent to exit plane of capillary. Simulations of laser pulse evolution in this region, employing simulation codes and analytical functions, show that relativistic self-focusing may lead to an increase of the pulse intensity compared to the case without the plume. Measurements show that the on-axis plasma density over this region is close to that inside the waveguide (~1018 cm-3). Here the laser beam converges to the smallest focal spot and relativistic self-focusing leads to an increase in the laser intensity. If injection is guaranteed to occur early, the required length would only be of the order of 3-8 mm for our parameters, with important advantages of a lower discharge voltages for gas breakdown, and an easier alignment of laser beam © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

A high voltage pulsed power supply for capillary discharge waveguide applications

We present an all solid-state, high voltage pulsed power supply for inducing stable plasma formation (density ∼1018 cm−3) in gas-filled capillary discharge waveguides. The pulser (pulse duration of 1 μs) is based on transistor switching and wound transmission line transformer technology. For a capillary of length 40 mm and diameter 265 μm and gas backing pressure of 100 mbar, a fast voltage pulse risetime of 95 ns initiates breakdown at 13 kV along the capillary. A peak current of ∼280 A indicates near complete ionization, and the r.m.s. temporal jitter in the current pulse is only 4 ns. Temporally stable plasma formation is crucial for deploying capillary waveguides as plasma channels in laser-plasma interaction experiments, such as the laser wakefield accelerator

Linearly tapered discharge capillary waveguides as a medium for a laser plasma wakefield accelerator

Gas-filled capillary discharge waveguides are commonly used as media for plasma wakefield accelerators. We show that effective waveguides can be manufactured using a femtosecond laser micromachining technique to produce a linearly tapered plasma density, which enables the energy of the accelerator to be enhanced significantly. A laser guiding efficiency in excess of 82% at sub-relativistic intensities has been demonstrated in a 40 mm long capillary with a diameter tapering from 320 μm to 270 μm, which gives rise to an on-axis, time-averaged plasma density that varies from 1.0 × 1018 cm−3 to 1.6 × 1018 cm−3

Straight and linearly tapered capillaries produced by femtosecond laser micromachining

Gas-filled capillary discharge waveguides are a commonly employed medium in laser–plasma interaction applications, such as the laser wakefield accelerator, because they can simultaneously guide high-power laser pulses while acting as the medium for acceleration. In this paper, the production of both straight and linearly tapered capillaries using a femtosecond laser micromachining technique is presented. A tapered capillary is shown to possess a smooth variation in diameter (from 305 μm to 183 μm) along its entire 40 mm length, which would lead to a longitudinal plasma density gradient, thereby dramatically improving the laser–plasma interaction efficiency in applications. Efficient guiding with up to 82% energy transmission of the fundamental Gaussian mode of a low intensity, 50 fs duration laser pulse is shown for both types of capillary waveguide