Ultra high vacuum, that is a pressure below 10−9mbar, is a crucial prerequisite for an ion trapping experiment. This minimizes adverse collisions with background gas particles that can lead to heating (decoherence) or to chemical reactions. The latter is particularly important, since photo-assisted chemical reactions are the dominant loss mechanism for trapped alkaline earth ions (Be+, Mg+, ...): During laser cooling, the ion will spend some time in the excited state. If it then collides with a hydrogen molecule from the background gas, there is a high probability that it will form a hydride (e.g. Mg+∗+H2 →MgH++H). In general, the resulting molecule will remain trapped, but, will not interact with the laser light anymore. Depending on the residual partial hydrogen pressure, a sample of trapped ions will therefore become increasingly “polluted” with molecular ions.
This section describes the vacuum system of both ion traps in detail, including hardware (pumps and components), cleaning and baking procedures and performance. Although the SI Unit of pressure is Pa (N/m2) we specify pressures in mbar (=100 Pa), which is customary in this field.
2.3.1. Preliminaries: design, materials, cleaning and baking
In order to reach the lowest pressures great care needs to be taken in choosing appropriate materials that are introduced into the vacuum system. Further, meticulous cleaning and finally careful baking are necessary.
The final pressure of the system will be given by the ratio of gas load to pumping speed. The gas load is caused by the vapor pressure of the materials used, absorbed water and hydrocarbons on the surfaces and diffusion through the walls (hydrogen!). Proper choice of the materials used will take care of the first and the last point. The second point requires minimizing the surface area (by design and smooth surfaces), cleaning and finally heating the apparatus while it is being pumped (“baking”). Without baking surface adsorbed water and hydrocarbons will make it hard to achieve a pressure below 10−8mbar. Careful design is further necessary to avoid so-called “virtual leaks”: This could be, for example, a simple thread. If it has no extra venting hole, a small volume is created inside the vacuum chamber that can hardly be pumped. Even after baking a tiny amount of water can be left that will prevent achieving low pressure. Welded parts must have their welded seam on the vacuum side for the same reason, even though this might be challenging in some situations.
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Suitable materials must therefore have a very low vapor pressure, a smooth surface and must be “bakeable”, that is withstand temperatures of 200◦C-400◦C. Frequent choices include, but are not limited to: A wide range of metals and alloys, such as stainless steel (304L), copper, gold, aluminum and tantalum, further certain ceramics and glass. Special UHV compatible plastics exist, e.g. polyimide (Vespelr, Kaptonr, both by DuPont), and even some adhesives have turned out to be suitable (e.g. Epotek 301, 353ND, H77, H35-175MPV). Materials that should be avoided include brass and zinc (careful: nuts, screws and washers!). Teflon has an excellently low vapor pressure but is suitable only at first sight because it cannot be baked. Soldering should be avoided, but, if necessary, special UHV solders are available. Wiring should only be done with solid wires, litz has a big surface and can be an annoying source of virtual leaks.
The following cleaning procedure gives good results while being moderately time consuming. The use of powder-free (Nitrile) gloves throughout the procedure is of course mandatory.
• Scrub with soap. Parts from the machine shop will be full of oil and turnings, the same applies for screws, nuts and bolts.
• Immerse in ultrasonic bath with strong degreaser.
• Rinse with distilled water. Demineralised water is suitable, too.
• Clean with methanol (Uvasolr). Either by wiping or in ultrasonic bath. Acetone is a better solvent, but sometimes leaves some residue behind.
• Rinse with distilled water.
• Blow dry with hot air. Do not wipe the parts dry, otherwise dust particles accumulate. The vacuum system should be assembled shortly after cleaning. The parts attract dust, even if covered. Ideally, the parts are still warm after blow drying, which reduces the amount of water introduced. Seals are fastened using a torque wrench. A little high-temperature compatible anti-seize (e.g. molybdenum disulfide) on the screws facilitates re-tightening and loosening.
Baking the apparatus means heating it to 200◦C-400◦C for about one week while pumping on it. The apparatus can either be heated by building an oven made from fire clay around it, or, by wrapping heating bands, stone wool and aluminum foil around it. Most vacuum systems will be made of stainless steel which is a very poor thermal conductor. For this reason the “oven” method has the clear advantage of more uniform heating. Due to size and other restrictions this may not be possible for every apparatus. If done carefully, the “wrap- ping” method works equally well, although great care needs to be taken to keep temperature gradients small. The weakest parts of a vacuum system are in general the vacuum windows (more precisely the solder of the glass-metal transition). Bad temperature monitoring and inhomogeneous heating (spatially and in time) can lead to a hot-spot near a window and subsequently to its failure. Other than that, the temperature should be as high as possible, again typically limited by the vacuum windows. This is because vapor pressures increase exponentially with temperature, so the amount of water and hydrocarbons removed will grow exponentially with temperature. Heating the apparatus up or cooling it down must be done carefully, that is the heating rate must be limited to the maximum specified rate of the most sensitive component. These are usually the vacuum windows and should be heated/ cooled
slower than 1 K/min. As an example, this is the baking procedure of the endcap trap using the “wrapping” method:
• Pump the system for about one week. Leak check. If everything is well cleaned and all seals are tight, a pressure around 10−8mbar with the turbo pump alone is achieved.
• “Bake” the electron gun and atomic oven by operating them with increasing currents until pressure settles.
• Gradually heat up the system. The pressure can rise to 10−5mbar or more. The ion pump including magnet is heated, too.
• Operate (hot) ion pump with increasing voltage. Repeatedly operate (“flash”) titanium sublimation pump (TSP) according to specification (47 A for 1 min in our case, every 3 mins).
• After 3-7 days the pressure will drop to 10−7mbar. Cool down.
• After cooling down, a pressure around 10−10mbar will be reached. Close all-metal angle valve.
• Flash TSP. Within a day the pressure will drop into the low 10−11mbar regime.
• About one-two weeks after cooling down, the pressure will have further dropped below 10−11mbar.
The baking procedure for the 6-rod trap is very similar.
2.3.2. Endcap trap Vacuum chamber
The vacuum chamber consists of the following main parts:
• A CF150 double cross serves as main experiment chamber. It is attached to the optical table using four ceramic feet, so it is thermally and electrically decoupled.
• Two custom made CF150 flanges are connected to the experiment chamber which hold the ion trap (see Fig. 2.9), allow laser access and provide a direct gas inlet over a precision leak valve.
• A custom made CF150 “re-entry window” allows to position imaging optics only 63 mm away from the trap center.
• A CF150 tube with three connectors (1×CF63, 2×CF40) sits on top of the experiment chamber. The ion pump and TSP are attached to the CF150 port. The turbo and backing pump are attached to an all metal angle valve which is connected to the CF63 port. The ion gauge is connected to a CF40 port.
The CF150 tube also houses two “baffles” which prevent ions from the ion pump to enter the experiment chamber, without reducing the conductivity significantly.
The ion trap is centered in the double cross. Four vacuum windows allow laser access either axially or enclosing an angle of 15◦ with respect to the axis. Ions trapped in segment C are located 333 mm from the axial laser access window and 262 mm from the “angled” port. Both distances are measured from the front (=air side) surface of the vacuum window.
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Pumps
The vacuum system utilizes four different types of pumps to achieve the lowest possible pressure as follows (see also table 2.3): A Varian SH100 scroll pump serves as backing pump with a pumping speed of 1.4 l/s and provides an oil-free vacuum with a base pressure of 6.6×10−2mbar. It should be noted that the scroll pump used is oil- but not “particle-free”. Abrasion of the teflon fitted bearings can cause dust to accumulate in the foreline which may be blown into the vacuum vessel upon venting. The teflon particles can be problematic for high voltage applications, as we experienced by dust-induced short cuts in a channeltron. This problem can be avoided by a careful venting procedure or by using newer, oil- and particle-free pumps7. The foreline vacuum is monitored with a Pirani type gauge. Ultra-high vacuum is then achieved with three different pumps: A 345 l/s turbo pump (DN100CF) evacuates the chamber to a base pressure of about 10−10mbar. The specified pumping speed is not reached in our system for two reasons: First, the base pressure of the backing pump is too high, for optimum performance the foreline pressure should be below 10−2mbar. Second, the pump is not attached to the vacuum vessel directly, but to an all-metal angle valve with reduced opening (DN63CF). It has a conductivity of C ∼100 l/s, so the net pumping speed is only (P
i1/Ci)−1 ∼78 l/s. The angle valve allows to separate the foreline vacuum system from the main experimental chamber which is pumped by an ion pump and a titanium sublimation pump (combined in a Varian VacIon Plus 300). Once the system has been pumped down and baked, the valve is closed and the main chamber is pumped by the ion pump and the TSP only. This has several advantages: Vibrations from the scroll and the turbo pump are eliminated, the hydrogen load is minimized (turbo pumps have bad compression ratios for light species) and in the event of a power shortage the chamber is not vented. The ion pump has a pumping speed of 240 l/s and is operated at the lowest voltage setting. The titanium sublimation pump is crucial to achieve pressures below 10−10mbar. At such low pressures the residual gas consists mainly of hydrogen which is pumped by the TSP extremely effectively: The nominal pumping speed is 1580 l/s. The pumping speed can be increased by water or cryo-cooling the TSP. The ion pump is operated permanently, the titanium layer in the TSP is renewed (“flashed”) only about once or twice a year.
Result
The entire system is depicted in Fig. 2.15. The ion gauge is specified down to 5×10−11mbar, and reads “under range”. As this thesis was written, the TSP had not been flashed for more than a year.
2.3.3. 6-rod trap
The vacuum system is similar to the one of the endcap trap. The major difference is that there is no TSP installed. Also, the pumping speed of the ion pump is significantly lower. For this reason the vacuum is not “pinched off” by an all-metal angle valve, but pumped permanently by the turbo pump as well.
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