Generation of ion pulses in a fast pulsed EBIS supported by realistic ion trajectory simulations

Abstract

The time4ions group aims to conduct novel pump/probe experiments on particle systems, using ion pulses as the pump mechanism combined with a laser pulses as the probe mechanism. Such pump/probe experiments will be used to resolve the relaxation dynamics of collision cascades in materials induced by the ion pulse impact. Since such collision cascades occur on the sub-nanosecond timescale accurate measurements of the relaxation require ion pulses with below sub-nanosecond pulse widths. We generate ion pulses in two steps. In the first step, we utilize a femtosecond laser directed onto a cath- ode to produce pulsed photo-electrons by photo-emission in a electron beam ion source (EBIS). The EBIS is placed in a vacuum chamber filled with low pressure gaseous argon or nitrogen. In the second step, the pulsed photo-electrons interact with the gas, among other processes, via pulsed electron impact ionisation (EII). If EII occurs in a volume in the EBIS surrounded by a potential slope, we get pulsed ions with pulse widths de- pendent on the potentials set to the EBIS electrode assembly. The pulsed electrons and ions time of flight (TOF) is measured, and from their TOF distribution we evaluate the pulse width.Currently the main focus of the experiment lies in the process of ion pulse generation. The EBIS we use covers nearly the full solid angle around the cathode. The only solid angle through which the cathode is accessible is given by the exit opening of EBIS. A few complications arise in this setup. First we have to direct the femtosecond laser onto the cathode through the exit of the EBIS. This requires us to place a optical mirror close the beam axis of the EBIS, blocking a part of our ion/electron beam. We tested the EBIS in two distinct assembly configurations called the half-EBIS and full-EBIS operation. The advantage of the half-EBIS operation is a bigger opening solid angle for accessing the cathode, however we lose the ability to prevent EII outside the EBIS. The ions produced outside the EBIS have no temporal structure which distorts our pulse width during TOF measurements. In the full-EBIS operation we can prevent most electrons leaking out of the EBIS, however this comes at a cost of a reduced opening solid angle which forces us to place the optical mirror closer to the beam axis, cutting more of our particle beam.We have been able to produce electron pulses with both EBIS setups at pulse widths in the mid 100 ps magnitude range, with the electron pulse width showing a weak dependency on the cathode potential. In the half-EBIS operation we were not able to create reproducible ion pulses. In full-EBIS operation a clear ion pulse is not yet visible, however the TOF spectra shows temporal structure in direct relation to the femtosecond laser repetition rate. Furthermore measurements of pulsed photo-electron emission currents from the cathode are shown to be relatively small to the detriment of ion pulse generation.