## 3 Description of the Ion Trap  
The 3D model of the trap assembly is shown in Figure 2a. The octupole trap consists of eight gold-coated molybdenum wires with a diameter of 100 \(\upmu\)m. They are symmetrically placed around the trap axis on a circle with radius of 3 mm. The wires are soldered onto a printed circuit board (PCB). A radio-frequency field is applied to the trap wires, along with an additional static voltage as an offset. The offset chosen corresponds to the voltages used in the Wiley-McLaren acceleration stage, which lowers the kinetic energy of the incoming ions such that the trapping potential can confine them. While the static voltage applied to the endcap electrodes provides axial confinement to the ions,  
Figure 2: Description of the ion trap. (**a**) A 3D model of the ion trap is shown with an illustration of the radio frequency (rf) trap wires, endcap electrodes, and shielding plates used in the setup. (**b**) A qualitative description of the electric potential as a function of the position along the trap’s main axis during three different stages. Firstly, the loading of ions from the source into the trap. Secondly, the axial confinement of ions. Finally, the unloading of ions from the trap to be further extracted onto the detector. the radio-frequency field applied to the wires confines the ions radially, such that the ions experience a position-dependent field proportional to \(r^{2n-2}\), where \(n\) is the pole order of the ion trap, with \(n=4\) for an octupole trap. The potential that the ions experience governs their spatial density distribution. Thus, while they are Gaussian-distributed in the axial direction due to the harmonic potential in this direction, they are described by a super-Gaussian distribution of order six in the radial direction [31]. The total potential exhibits a local maximum at the center of the trap along with off-center minima [32]. While the endcaps create a necessary confining potential in the axial direction, they also create a deconfining potential in the radial direction, which exhibits \(r^{2}\) dependency close to the center [33]. This is a feature common to all multipole traps with \(n>2\), and not only to octupole wire traps, thus creating multiple regions of local minima called \"pockets\". In addition to the rf wires and endcap electrodes, the trap consists of a set of vertical and axial shielding plates. The vertical shielding plates prevent the grounded MOT coils (not shown) from pulling down the potential in the trap center. The hole in the plates provide optical access for laser beams to create an MOT. The axial shielding plates provide the flexibility to shift the ion cloud during trapping or giving them additional acceleration during extraction, thus increasing the detection efficiency. The axial shieldings are important for controlling the slope of the potential during extraction, as seen in Figure 2b. The voltages applied to the endcap electrodes and shielding plates are optimized such that the trapped ion cloud is initially exposed to a linear acceleration region during extraction. This is necessary for obtaining a symmetric TOF spread. Additionally, the axial shieldings can also be used to modify the potential topology for the extraction process to enhance the mass resolution of the system. This is particularly useful for exploring systems with higher masses, where multiple ionic species are simultaneously trapped. A wire trap offers the advantage of providing optical access for laser beams for hybrid atom-ion experiments, and an octupole trap provides a large central field-free region where the influence of radio-frequency heating is reduced. However, one feature of wire traps that needs to be noted while designing a trap assembly is that all nearby electrodes and grounded plates will influence the potential, in the region enclosed by the wires, drastically. This effect is diminished for a trap using thicker rods, as the penetration of outer electrodes is greatly reduced.