Vacuum system setup

The scheme of the vacuum system for the future integration of the joint CEM detector in the single atom trap setup is illustrated in fig. 4.5. The vacuum system is segmented into three main parts (fig. 4.5(a)). The detector section (red) consists of a quartz glass cell (Suprasil) wherein the joint CEM system and the future single atom trap are situated. The source section (green) consists of the atom sources, an additional electron source and the electric feedthrough for the individual sources. In the pump section (blue), the pump systems, the electrical feedthroughs of the CEM cables, and the ion pressure gauge are set. For future single atom precision experiments, the entire UHV chamber is made of steel with a low magnetic permeability (316LN steel). All individual vacuum elements in the three sections are sized and connected by standard CF-40 flange tubing and consist mainly of commercially available components. For improved pumping of the individual vacuum pump systems, conical adapter flanges7 are used matching the different flange sizes and larger tube diameter of the 7_{Conical adaptor flanges yield improved pumping compared to their zero-length counterpart. Note however}

4.2. Joint CEM detector

corresponding pump entrance. To shield the CEM detector system from any parasitary stray light or emitted stray charges originating from the ambient vacuum system itself, all active vacuum components are mounted at least around one corner turn (fig. 4.5). Such active components are either the atom/electron sources, the ion gauge, or the ion pump system.

The entire vacuum chamber is pumped through the CF-40 six-way cross cube by a50 l·s−1

ion pump (Varian, VacIon Plus 55 StarCell). An attached all metal angle valve at the ion gauge chamber allows to connect a roughing pump system for initial pump down and bakeout of the entire vacuum chamber. To prevent the pumps from macroscopic particle intrusion during rough pumping at initial pump down, the ion getter pump, the turbo-molecular pump and also the ion gauge are mounted upside-down. The vacuum pressure of the entire chamber can be monitored down to levels below 10−10mbar with a high sensitivity ion gauge (Var- ian, UHV-24p). The ion pressure gauge is mounted in an attached, separate gauge chamber. The separate mounting prevents the operating gauge from measurement deviations induced by magnetic stray fields of the nearby ion getter pump. The spatial dimensions of the ion gauge chamber are designed to be sufficiently large to inhibit ambient wall degassing due to heating by the hot gauge filament. For the eventual adaption of a titanium sublimation pump system (Varian, TSP cartridge source integrated into a water-cooled TSP cryopanel), an addi- tional port at the six-way cube is blindflanged. Such a TSP system will particularly enhance the pumping speed8 for reactive, getterable gases like hydrogen and nitrogen. Moreover, at considerable low pressures it allows to switch off the entire ion getter pump system. By dis- mounting the external magnets of the ion getter pump, the TSP system will then solely pump the whole vacuum system. This will result in a significantly reduced magnetic perturbation of the entire single atom trap setup caused by the ion getter pump magnets.

The source section (fig. 4.5, green) holds three87_{Rb-atom sources (atomic vapor dispensers;} Alvatec and SAES getters) mounted upside-down in an upright, standard CF-40 tube attached to the three-way cross. An additionally mounted tungsten wire extracted from a commercial light bulb serves as electron emitting source for test purposes. During dispenser operation, the wire is held on a constant positive potential of +300 V which deflects/attracts emitted ions and electrons out of the thermal dispenser source. This effectively creates a charged particle trap, preventing stray charge bombardment of the overall CEM detection system and its associated cables. To further shield the corresponding cables from stray charges or induced cable noise from the atom sources, in the source section all CEM cables are additionally guided within a straight copper tube (Oxygen-free high thermal conductivity (OFHC) copper, inner diameter∼10 mm) for the whole length of the section. For neutral atom release, the atomic vapor dispensers are operated by ohmic heating.

Initially, the entire vacuum system is baked out at 240◦C for one week. A bakeout at such high temperatures will ensure that most bulk impurities out of the metal walls will be driven out of the material and subsequently be pumped. Prior to the initial bakeout, the glass cell with the joint CEM detector system is dismounted and the open CF-40 port at the source section of the main chamber is blindflanged. For pumping during bakeout, a roughing pump system and a 55 l·s−1 turbo-molecular pump (Leybold, Turbovac 50) are attached to the main chamber via the all metal angle valve (fig. 4.5). During the baking procedure, the atomic vapour dispensers are initialised, and then constantly heated with a low current

(I = 2.5−3.0 A). After cooling the apparatus down, the main chamber is opened again and

8_{Water-cooled TSP cryopanel pumping speed (at 20}◦

C): N2∼515 l·s−1, H2∼1200 l·s−1, and

the glass cell is reattached to the main chamber. In a second, consecutive bake-out stage, the whole system is then baked again for another one and a half weeks at110◦C. This comparably low bake-out temperature is owed to the low melting point of the glass-to-metal sealing of the glass cell made from indium (melting point: ∼157◦C). After the whole bakeout, a total pressure of the order of10−10mbaris achieved.