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.