DOCUMENTACIÓN

1.8 2 2.2 2.4 2.6 2.8 3 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 y /mm x/mm 0 0.5 1 1.5 2 2.5 E- fie ld/ kV mm -1 - 10 00 V + 10 00 V 0 V

Figure 2.12.:Detail of the simulation [105] of the electric field in a cross section perpendicular to the rf-axis in the middle of the differential pumping stage. Voltages of±1000 V on the rf-electrodes

were assumed. Maximum field strengths occur at sharp corners. Possible problems related to this were circumvented by using inlay electrodes with rounded and polished surface as can be seen in fig. 2.11 on the facing page. In this figure, a flat surface has been assumed instead. an electro-spray source is used to inject molecular ions into the experiment chamber (see chapter 6 on page 118), will be tolerable.

A trade-off between maximum disruptive strength and good vacuum isolation is realized in the clover leaf-shaped aperture with 0.5 mm clearance between the electrodes and the dps walls. A

simulation of the resulting electric field is shown in fig. 2.12. The experimental results presented in section 4.1.4 on page 72 showed that at rms rf-voltages as high as 1000 V arc-overs do not occur. The length of the dps (10 cm) was chosen so as to find a good compromise between large vacuum isolation and sufficiently short rf-guide.

2.5. Atom oven

Atomic ions are generated from atoms in a thermal beam by photoionization. The small resistively heatable tantalum tubes from which the atom vapor evaporates are referred to as atom ovens within this thesis. One of them can be seen in fig. 2.13 on the next page with its dimension given in the caption.

A current through the tantalum wires heats the attached tube and the metal inside up to the temperature at which the vapor pressure of the respective metal rises sufficiently above the residual gas pressure. The temperature is chosen for the flux of atoms in the thermal beam emitted from the tube to meet the experimental requirements on the photoionization rate. For the current experiments, an ion loading rate of approximately 1 s-1 was sufficient. The photoionization scheme of magnesium and the relevant atomic transitions are described in [97]. The optimum temperature

Figure 2.13.:Atom oven tantalum tube (1 mm outer diameter, 15 mm long) ready to be loaded with magnesium or barium. The back- side of the tube is closed by tight crimping. The front side is sealed with indium after barium chips were inserted under dry nitro- gen to prevent oxidation; see appendices B.2 and B.3 on page 143. During bake-out the sealing melts and allows evaporated barium atoms to exit. The tantalum heating wires looping around the tube are spot-welded and act also as support. Washers allow to mount the oven securely. In total nine of these tubes—a set of three tubes in each oven ensemble—are installed in tiamo. depends on the metal’s vapor pressure and additional experimental and geometric parameters. The higher the photoionization laser power, the lower is the atom flux needed to sustain a certain ionization rate. One the one hand, the rate depends on the overlap volume of thermal atomic beam and the laser beam. On the other hand, owing to the broadening of the transition frequency induced by the Doppler effect, the ionization rate is also function of the velocity distribution of the thermal beam depending on the angle between the beams and its temperature. It is hard to measure the oven temperature because its dimensions are tiny but the temperature distribution over the tube inhomogeneous though; cf. fig. B.6a on page 147. However, the oven does not glow in the visible spectral range but can be seen with night vision gear at regular operation conditions. The oven tubes were chosen to be tiny because in this way also the heat capacity is small and the oven stops emitting atoms shortly (a few seconds) after the heating current is interrupted. Appendix B.2 on page 143 provides more detailed information about the heating characteristics, appendix A.1.1.a on page 124 focuses on the integration of an automated loading procedure in the experimental control scheme.

When the atomic beam exits from the oven tube, its angular distribution is wide. The estimated opening angle of the emission cone surpasses 90°, cf. fig. B.6b on page 147. In order to prevent staining of the rf-electrodes with metal deposits, a restrictive aperture was placed in front of the oven. It is realized in a 200 µm wide slit-shaped opening at a distance of 21 mm as part of a protective housing (see fig. 2.14 on the next page) around the atom oven. The slit clips the atomic beam in one direction such that it is 1 mm wide and just fits through the 37.5 mm distant rf-guide. In the

other (axial) direction, no aperture is used in order to maximize the photoionization volume. For trapping experiments with two different atomic ion species, ovens for magnesium and barium were prepared and grouped into three ensembles which are composed of three single tubes each; see fig. 2.3 on page 26 and fig. 2.14 on the next page. One of these tubes contains a magnesium wire, the other two contain barium chips. Both fillings reflect the natural abundances of the respective metal’s isotopes. The magnesium oven is aligned perpendicular to the laser axis