GSI Helmholtz Centre for Heavy Ion Research

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    Characterisation and cooling of captured ensembles of highly charged ions in a Penning trap

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    The electric field experienced by bound electrons in highly charged ions ranks among the strongest available to experimental studies. For moderate atomic numbers, the electric field in hydrogen-like ions is equivalent to current laser systems. Quantitative interaction experiments with ions require a single-species, well-controlled target. In particular, the target’s ion distribution and density must be characterised. This thesis addresses these requirements by producing and characterising a suitable ion target through dynamic capture in a Penning trap as part of the High-Intensity Laser-Ion Trap Experiment. Three key developments have been implemented. First, the ion optics were upgraded to control the initial conditions of the incoming ion bunches. Second, a phosphor screen detector was introduced to measure the radial distribution of the captured ion ensembles. Finally, a new Penning trap setup featuring a dedicated dual-electrode resonator was developed and integrated into the experiment. The presented experimental results demonstrate that the ion ensemble reaches maximum radial density when the incoming ions are aligned with the central axis. Under these conditions, the radial thermalisation process occurs within 50 μs; resulting in a Gaussian-shaped radial distribution characteristic of a weakly coupled ion ensemble in thermal equilibrium. In the axial direction, the applied resonator facilitates the resistive cooling process of the captured ions, reducing the centre-of-mass energy by more than 99 % within 30 to 100 ms. A model is developed to describe the time-resolved induced signal during resistive cooling. From this model, key parameters such as the ion number and axial equilibrium energy are extracted. The combination of these radial and axial results yields an estimated peak density of 50 000 per cubic millimetre. For an upcoming tunnel ionisation experiment, ionisation yields are estimated, with more than 100 ionisations expected per laser pulse

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