Representaciones de mujeres notables en las biografías de La Mujer

Investigations of atomic gases at sub-Kelvin temperatures can only be undertaken if the gas is thermally isolated from all sources of heating. This is accomplished by trapping the atoms in magnetic or optical fields within vacuum, in order to reduce

conductive heating. In order to minimise collisions between trapped and untrapped atoms and prolong the lifetime of the trapped cloud, ultra-high vacuum (UHV) conditions must be maintained.

To achieve these UHV conditions, we built a vacuum system based around a 10-way stainless steel cross, as shown in Figure 2.1, with the ConFlat connection standard. All ConFlat vacuum pieces terminate in a sharp knife-edge, and a seal is accomplished by placing a copper gasket between two of these terminations and tightening mechanically until the knives have cut into the soft copper of the gasket. This tightening must be performed evenly around the flange by alternately tighten- ing the screws, in order to avoid having the knife-edge enter the copper at one angle when the first screw is tightened only to be moved to another angle when another screw is tightened, which would reduce the quality of the seal. The six mutually- orthogonal flanges of the cross are 70mm in diameter, whilst the other four have a smaller diameter of around 2.5cm. The four smaller flanges and two of the larger ones are used for optical access for the MOT beams, while three more of the larger flanges also accommodate viewports to allow increased optical access.

Figure 2.1: The assembled vacuum system, showing all major components.

One of the remaining flanges is sealed by an electrical feedthrough (VacGen Part Number ZEFT34A). Four 125mm long, 5mm diameter stainless steel rods protrude into the vacuum chamber and the ends are tapped to provide screw terminals. The rods are insulated at the point where they are connected to the flange and exit the vacuum chamber. Inside the vacuum, pairs of these rods are connected by two alkali metal dispensers (SAES RB/NF/7/25 FT10+10 and LI/NF/7/25 FT10+10), in order that a current may be passed through them. The dispensers are steel

2.2. ULTRA-HIGH VACUUM SETUP 23

12×1.12×1.35 mm containers (Figure 2.2) of a few mg of alkali-metal chromate (Rb2CrO4 or Li2CrO4) and a reducing agent (ST101 - a mixture of Zr and Al).

These containers have a slit in the long side and emit atoms via a reduction reaction when the dispenser is heated, e.g. resistively. The reducing agent absorbs all other chemicals produced in the reaction, ensuring that the alkali metal alone is propelled out of the slit. The dispensers are installed 13cm from the trapping region, and exhibit a current threshold below which no atoms are emitted. Rubidium dispensers contain enough rubidium for us to run our experiment without changing dispenser for at least two years. When we have replaced them we have noted that the current threshold varies from one dispenser to the next: typical values range from 2.5A to 3A.

Figure 2.2: Schematic of a SAES alkali-metal dispenser.

The 10-way cross is connected via a 6-way cross to a 55l/s Varian Starcell ion pump and, during evacuation, a mechanical turbo pump (Varian Turbo - V70D).

The ion pump consists of two electrodes operated at high voltage. Free electrons travel between the electrodes and ionize gas particles in the vicinity, which are then attracted to the cathode where they remain. As this does not remove particles from the vacuum system, but merely captures them electrostatically, the ion pump has a long but finite lifetime. To avoid shortening the lifetime, the vacuum is initially achieved using the turbo pump which does actively remove particles from the system. The ion pump is then switched on to improve and maintain the UHV conditions.

To ensure a high-quality vacuum all components except the dispensers are firstly cleaned with methanol and de-ionised water in an ultrasonic bath, before being heated to a high temperature to release any contaminants from the surfaces of the components. (We note that components with glass-to-metal transitions, such as viewports, occasionally display some discolouring when cleaned with methanol or acetone. If this occurs, these viewports are not trusted to maintain UHV conditions anymore. In subsequent iterations we have used isopropanol to clean viewports.) High-temperature cleaning, also known as baking or bakeout, is achieved by wrap- ping the vacuum apparatus in resistive heating tapes and aluminium foil. Heating is performed by gradually increasing the current to the tapes by means of a variable transformer and monitored by attaching thermocouples to various parts of the appa- ratus. In order to avoid damaging any vacuum components, particularly glass, it is important to minimise both spatial and temporal gradients of temperature. Baking is performed with the turbo pump connected and switched on, while the ion pump is left off in order to preserve the lifetime of the pump.

While the temperature is high, the dispensers are activated and decontaminated (degassed) by passing a high current through them. We typically pass 5A through the dispensers, and watch the pressure rise on an ion gauge connected to the turbo pump. During degassing, the pressure rises quickly and then decays back toward the pre-degas pressure. Degassing is said to be complete when a much slower increase in the pressure, which is indicative of the emission of some of the dispenser’s atomic content, is observed. We heat the chamber to 250◦C for 14 days until the pressure, which rises upon heating, decays back below the pre-bakeout pressure. At this point the system is gradually cooled down to room temperature, before the ion pump is switched on to lower the pressure further. The turbo pump remains on until after the ion pump is switched on, as we have observed that the ion pump tends to emit some gas into the chamber when it is first switched on. Once this small pressure increase has disappeared, we close the valve and switch off the turbo-pump. After baking the whole vacuum at 250◦C for around a week, we obtain a base pressure of 10−10 mbar.