G.J.H. Brussaard

Direct measurement of synchronization between femtosecond laser pulses and a 3 GHz radio frequency electric field inside a resonant cavity

We demonstrate a method to measure synchronization between femtosecond laser pulses and the electric field inside a resonant 3 GHz radio frequency (RF) cavity. The method utilizes the Pockels effect in a crystal inside the RF cavity by measuring the retardation of the components of polarization as a function of RF phase. Resolution of the setup used is shown to be 29 ± 2 fs (root-mean-square, rms), with timing jitter between the laser pulses and the RF field inside the cavity of 96 ± 7 fs (rms). The method provides a tool to reduce jitter and improve time-resolution in ultrafast electron diffraction experiments.

Laser wakefield acceleration: the injection issue. Overview and latest results

External injection of electron bunches into laser-driven plasma waves so far has not resulted in ‘controlled’ acceleration, i.e. production of bunches with well-defined energy spread. Recent simulations, however, predict that narrow distributions can be achieved, provided the conditions for properly trapping the injected electrons are met. Under these conditions, injected bunch lengths of one to several plasma wavelengths are acceptable. This paper first describes current efforts to demonstrate this experimentally, using state-of-the-art radio frequency technology. The expected charge accelerated, however, is still low for most applications. In the second part, the paper addresses a number of novel concepts for significant enhancement of photo-injector brightness. Simulations predict that, once these concepts are realized, external injection into a wakefield accelerator will lead to accelerated bunch specs comparable to those of recent ‘laser-into-gasjet’ experiments, without the present irreproducibility of charge and final energy of the latter.

Experimental validation of a radio frequency photogun as external electron injector for a laser wakefield accelerator

A purpose-built RF-photogun as external electron injector for a laser wakefield accelerator has been thoroughly tested. Different properties of the RF-photogun have been measured such as energy, energy spread and transverse emittance. The focus of this study is the investigation of the smallest possible focus spot and focus stability at the entrance of the plasma channel. For an electron bunch with 10 pC charge and 3.7 MeV kinetic energy, the energy spread was 0.5% with a shot-to-shot stability of 0.05%. After focusing the bunch by a pulsed solenoid lens at 140 mm from the middle of the lens, the focal spot was 40 μm with a shot-to-shot stability of 5 μm. Higher charge leads to higher energy spread and to a larger spot size, due to space charge effects. All properties were found to be close to design values. Given the limited energy of 3.7 MeV, the properties are sufficient for this gun to serve as injector for one particular version of laser wakefield acceleration, i.e., injection ahead of the laser puls...