Vacuum system design
We require atoms to be captured into a MOT out of a vapour at approximately 10-8 torr. However, because we need a longer trapping lifetime than a vacuum pressure at 10-8 torr affords us (Brian DeMarco via Dave McKay rule of thumb: 10s at 10-10, 100s at 10-11, 1000s. at 10-12 in a trap of just a few ER), we must transport the atoms into a higher vacuum chamber via a moving magnetic minimum. The two chambers are kept at different pressures because only a thin (.75" ID) tube connects them.
Table of Contents
- Scrub with a hot-water Alconox solution or in the ultrasonic cleaner.
- Do a thorough rinse with hot tap water (> 30 seconds).
- Scrub with acetone using lens paper or Kimwipes.
- Rinse with methanol. If sufficiently "dirty" scrub with methanol as well.
- Do a final rinse with DI water.
- It's preferable to clean the gaskets with lens tissue and methanol.
Closing Torques for All-Metal Valves
For closing the all-metal VG scienta (part # ZCR40R) valves the recommended closing torque is 7.7-12.1 Nm. They work by a standard knife-edge seal of a copper gasket and after bakeout that seal might need to be re-formed. The forming torque is 15.4nm. There is an index line to show where the valve should be closed for the appropriate torque. This line can be adjusted.
Main Chamber Valve
- March 4, 2009: Closed to the line marked on the valve, which corresponded to a system drop in pressure. No torque wrench was available, so the closing torque was not measured.
- March 18, 2009: Closed just past the line. Measured 13.1 Nm with Torque wrench.
- June 28, 2009: Closed just past the line. Measured 13.2 Nm with Torque wrench.
- November 25, 2009: Closed just past the line. Measured 13.3 Nm with Torque wrench.
MOT Chamber Valve
Maximum bake temperatures
|Part||Max Temp||Max Heating Rate||Comment|
|VAT Valve||200C (actuator), 300C (body)||10C/hour||Max gradient b/w body and actuator is 100C|
|Chamber window coatings||200C||1C/min||-|
|Sapphire window||400C||-||"as even and steady as possible - avoid thermal gradients at all costs"|
|Sapphire window coating||200C||-||-|
|Alvatec dispensers||150C||-||don't melt indium seal!|
|Duniway Ion pump||150C w magnets and running, 350C w/o||-||-|
|RGA||70C w control unit, 300C w/o||-||-|
|VG scienta valve||4,500C (open), 300C (closed)||-||both sides must be pumped when the valve is hot (and closed)|
|Oswald coil epoxy||?||-||-|
|Duniway Ti-sub pump||350C||-||-|
Accumulated Vacuum Lore and Wisdom
When the DB9 connector is plugged into the turbo control module (the metal piece on the side of the turbo where the power plugs in), the turbo is in remote control mode and the digital control unit (DCU) does nothing. When the power switch on the DCU is flipped to on the turbo automatically starts and when the power is switched off the turbo automatically spins down.
If the DB9 connector is removed the turbo is controlled by the DCU. Use the arrow keys to go through the parameters and push both arrows at the same time to change a parameter. Pressing both arrows gets a arrow on the screen at which point use the left and right arrows to change the parameter and then press both arrows again to lock in the change. Initially the DCU starts with a reduced parameter list, go to P:794 and change from 0 to 1 to get the full parameter list.
Turning on/off in Manual Mode
There are two turn on parameters, the pumping station on (which is also controlled by the on/off button on the front) and the turbo on. These are parameters 012 and 023 respectively. If the turbo parameter is set to on then the pump is controlled by the pumping station on/off parameter. However, the pumping station can be on with the turbo set to off. Note that the valve opens when the pumping station is off and closes when the pumping station is on.
- Make sure turbo (P:023) is set to off
- Turn on the pumping station (P:010). This closes the valve.
- Turn on the roughing pump
- Wait for the system to rough out.
- Turn on the turbo pump (P:023)
- Turn off the turbo pump (P:023) and wait for it to spin down.
- Turn off the roughing pump
- Turn off the pumping station. If the valve is set to open, it will close 6s after turning off the pumping station. Its probably best to close the manual valve after the automatic turbo valve and slowly bring the pump back to air.
The turbo has an automatic vent if connected.
- Parameter P:030 sets the vent mode
- 0: Auto
- 1: Off
- 2: On (Will vent on pumping station off or main power off)
- list item
- Parameter P:012 is vent enable (on/off)
Given that when the vent is enabled it will vent when the power goes off, it may be smart to DISABLE the vent when running the turbo for extended periods and only reconnect when a planned shut-off is to occur.