The interaction of intense laser light with atomic matter was a widely explored field in the 2000s. However, the reaction products were a mixture of different charge states, and the ionization channels could not be distinguished from each other very well. In order to be able to separate the individual ionization events from each other, the HILITE Penning trap was built. It can provide ions of any specific element and charge state as a target. The setup is designed in a transportable fashion to be operated at any laser facility in order to cover a wide range of laser parameters – especially concerning laser intensity and laser wavelength.

Recent developments at the Helmholtz Institute Jena have boosted the performance of the experiment. Ion clouds with several 10,000 ions can be formed in the ion trap. To allow efficient laser experiments with a huge overlap of the laser pulses and the ion cloud, a high ion density is crucial. This is ensured by fast ion cooling to compress the ions to a small volume. The cooling of the ion cloud is done with the well understood and elaborated technique of resistive ion cooling. In recent publications, the team showed fast ion cooling for large ion clouds (see Figure 1a). The fast decay in the beginning is caused by resistive ion cooling where the ions lose 99.99% of their kinetic energy within roughly 50 ms [1]. This will allow for an experiment cycle time of less than 1 second which is comparable of the typical cycle time of a high-power laser. Furthermore, using the uncommon approach of a dual-hot-end resonator, it is possible to disentangle resistive ion cooling from the effect of ion dephasing. This increases the understanding of the processes inside the ion cloud and allows to maximise the harmonicity of the ion trap with a large ion ensemble [2]. After cooling, the FWHM of the ion distribution is found to be 600 µm as depicted in Figure 1b. Consequently, 95% of the stored ions have a distance less than 112 µm from the trap centre [1]. The corresponding ion density is about 10.000 mm^-3 which is sufficiently high to ensure a high number of interacting ions in a laser experiment.

Within the next months, the HILITE Penning trap setup will be prepared to be combined with the JETI200 laser, also located at the Helmholtz Institute Jena. In the experiments, ionisation cross sections will be measured for different ion species of carbon, nitrogen, oxygen, and neon. The used intensities will be in the relativistic regime. Due to the ionic target, the electric field of the weakest bound electron will be close to the laser’s electric field and the experimental results can be compared with theory which describes tunnel ionisation at relativistic electron energies. In addition, it is foreseen to detect photons in the x-ray regime which are produced in the sense of high-harmonic generation.

References:

[1] M. Kiffer, S. Ringleb, Th. Stöhlker and M. Vogel, Phys. Rev. A 109, 033102 (2024).
[2] S. Ringleb, M. Kiffer, Th. Stöhlker and M. Vogel, Eur. Phys. J. Plus (accepted for publication).