Vacuum Skill Lab

The end product that you will create in this lab is a beam splitter, which is a ‘half-silvered’ piece of glass that will transmit and reflect roughly equal amounts of light. Beam splitters are used to conduct interferometry experiments.

‘Silvering’ in this case does not involve actual silver, but rather the deposition of aluminium vapors on to a slide. Below are ‘cookbook’ instructions for both a cold start (starting with the diffusion pump off) and a ‘hot’ start (with the diffusion pump on) for the vacuum apparatus in Advanced Lab.

Cold start

1. Close valve 4, open valves 1, 2, 3, and 6.
2. Turn on the mechanical pump.
3. Wait till the pressure on thermocouple 1 (with the red LED display) reaches 50 mTorr.
4. Close valve 3.
5. Turn on the water to cool the diffusion pump.
6. Turn on the diffusion pump.
7. Wait till the pressure on thermocouple 1 reads 10 mTorr.
8. Turn on the Penning gauge (press the two red buttons - ‘Main power’ and ‘Penning Power’ on the vacuum gauge control box). There is a safety interlock in this system which prevents the Penning gauge from turning on before thermocouple 2 drops below 10 mTorr.
9. Wait till the pressure on the Penning gauge reads 0.2 mTorr.
10. Fill the cold trap with liquid nitrogen by pouring it through the funnel - keep pouring until you see little bubbles on the top of the cold trap.
11. Wait until the pressure drops to approximately 0.06 mTorr (read from the Penning gauge - you will have to read the number on the lower scale and multiply by $$10^{-5}$$).
12. Turn on the filament and turn the current up to 10 Amperes. You may have to adjust it later as well since the resistance will go up with the temperature of the filament.
13. When the slides seem adequately silvered (that is, you should still be able to faintly see the glowing filament through them), turn off the filament.
14. Turn off the Penning gauge.
15. Close valve 1.
16. Open valve 4 slowly to let the chamber equilibriate with the outside atmosphere.
17. Twist the bell jar and pull it off, and take your slides off of the housing.
18. You now have a beam splitter! Go test it out on a laser!

Hot start

In a hot start, the diffusion pump is already on, and you do not want to expose it to atmospheric pressure.

1. If the bell jar is at high vacuum and you need to put the slides in, close valve 1, and open valve 4 slowly until the bell jar is equilibriated with the outside atmosphere.
2. Place your slide on the housing, positioned above the filament and the aluminium hooks, and place the bell jar on top.
3. Open valve 3 slightly, until thermocouple 2 reads approximately 180-200 mTorr, then close it again.
4. Wait for the pressure in thermocouple 2 to drop to almost zero.
5. Repeat steps 3 and 4 until thermocouple 1 reads below 10 mTorr.
6. Open valve 1 and wait until you reached the desired pressure.