Atom cooling and trapping experiments must be carried out in a vacuum, other- wise the effects of the cooling or trapping technique would be negated by colli- sions with background gas. While it is possible to form a MOT with pressures as high as 10−7 mbar[35], it is desirable to maintain as good a vacuum as pos- sible. Not only will the better vacuum increase the size of the MOT, but it will decrease the likelihood of collisions affecting guided atoms in the dipole guides. To that end, the vacuum apparatus used in these experiments were designed to approach ultra high vacuum (UHV) operation3with a typical base pressure of our
vacuum systems being3×10−9mbar. In order to achieve UHV, there are stringent requirements for the preparation of the system and the choice of materials used within. Although a number of vacuum connection standards exist, the ConFlat standard, which consists of a gasket (usually copper) sandwiched between two ‘knife-edged’ flanges, yields the highest performance and was used exclusively in our UHV systems.
3.3.1
Materials
In UHV, outgassing becomes an issue. Not only can bulk outgassing prevent the system from reaching the desired vacuum level, small levels of local outgassing (i.e. outgassing from a small region such as a fingerprint) can interfere with the creation and guiding of the cold atoms. Consequently, materials were chosen and prepared so as to have as low an outgassing rate as possible. The vacuum com- patible materials used within this systems included304and316LNtype stainless steel, copper, MACOR (machinable glass ceramic), Kapton (a low outgassing polyimide film used for electrical insulation) and glass.
3Typically defined as pressures between10−9
and10−12mbar
3.3.2
Cleaning of vacuum components
Cleaning procedures for ultra high vacuum systems can be convoluted multi-step procedures that in extreme cases will include vapour washes and electropolishing. For our system however, a basic but effective cleaning practice was adequate:
1. The vacuum piece was cleaned in a solution of Liquinox (which is a residue free liquid detergent) and warm water. If the item was small enough this was done in an ultrasonic bath.
2. The piece was thoroughly rinsed under the cold water tap. Filtered or de- ionised water would have been preferable but tap water proved sufficient. The piece was dried using a clean room wipe.
3. If the piece was small enough, it was ultrasonically cleaned in a bath of isopropanol or acetone.
4. Before insertion to the vacuum system, the piece was wiped using a clean room wipe soaked in isopropanol or acetone. Inspection of the wipe in- dicated if any contamination remained on the piece. If the piece was still coarsely contaminated, the procedure was repeated in parts or in full. It is worth noting however, that the ‘odd’ contaminant within a vacuum system may not necessarily guarantee that the system won’t reach UHV. Indeed, on dis- mantling an inherited vacuum system, I discovered a quantity of blu tack within the chamber; this chamber had been operating in UHV for many years. How- ever, the best practice when working with UHV is to adopt a suitable cleaning procedure for the experiment’s needs and to strictly adhere to it.
3.3.3
Rubidium source
In all the atom guiding experiments detailed in this thesis, rubidium ‘alkali metal dispensers’ (SAES getters), colloquially known as getters, were used. These get- ters consisted of a12 mm×1.12 mm×1.35 mm(L×W×H) sealed metal container, containing a few milligrams of rubidium. When a current of between2and7.5 A
was passed through the getter, rubidium was released through a small slit in the getter. These getters provided a reliable and controllable source of rubidium in the atom guiding experiments.
3.3.4
Bake-out
In all of our vacuum systems, one or two ion getter pumps (Varian, models: Va- cIon Plus 40, Triode, 919-0201; and VacIon Plus 25, Triode, 911-5030), that were permanently attached and turned on, were used to maintain UHV in the system. These pumps are termed capture pumps, in that they capture, rather than remove, gases from the vacuum system. These pumps perform excellently in UHV, but as they can only operate in the pressures below∼ 10−2 mbar, they are unable to evacuate the system down from air. Consequently a preliminary ‘pumping down’ stage was needed.
This pumping down stage was performed using a turbomolecular pump, backed by a diaphragm pump (Varian, model: Turbo-Dry 70). This stage usually lasted for a few days and was always accompanied by a bake-out of the system. This bake-out involved heating the system, using heat tape, to a temperature of around
100◦C, primarily in order to speed up the evaporation of water adsorbed on the
inside surface of the system. Care was taken to raise and lower the temperature in a controlled manner, and to heat the system evenly. Once the pumping down stage was completed, the temperature was slowly lowered. At around 50◦C, the all-
metal valve was closed, isolating the vacuum chamber from the turbomolecular pump. The ion getter pump was then turned on. The ion getter pump’s controller provided a real-time reading of the current being drawn by the pump; this current was converted to a pressure. The pressure was carefully monitored at this stage, as an insufficient vacuum would result in the ion getter pump switching itself off to prevent damage. If the ion getter pump stayed on, and the pressure started to reduce, the turbomolecular pump was turned off. Care was taken at this stage to monitor the vacuum; if insufficient force have been used to close the all-metal valve, the loss of vacuum on turbomolecular pump side of the valve would result in a rapid loss of vacuum in the system. Provided the valve was correctly closed
the pressure of the system over the next24hourswould approach its eventual base pressure of around∼3×10−9mbar.