Vacuum Experiment

This experiment examines the operation and several properties of a typical "high" vacuum system, based on a diffusion pump. The ultimate pressure of the system will be measured and the pumpdown characteristics will be charted. The performance of various vacuum gauges will be examined. The residual gas analyzer will be used to study residual and intensionally added gases.  Finally evaporation techniques will be used to silver (actually, aluminize) a mirror.
 

Background:

1. How vacuum pumps work (specifically mechanical and diffusion pumps.)
2. How pressure gauges work (specifically thermocouple, ion, Pirani, and Penning gauges.)
3. The pumpdown procedure (listed below.)
4. Expected time dependence of the pressure in the bell jar (if there were no outgassing).

Apparatus:

1. Water-cooled diffusion pump vacuum system with:
1. Liquid N₂ trap
2. Evaporation system
3. Various gauges
4. Residual gas analyzer

Measurements:

1. Start up the system. Let it pump down for a couple of hours and measure the ultimate pressure of the system.
• Note, along the way, the pressure as a function of time. Start by timing from the opening of the roughing valve until the thermocouple gauge comes on scale, at 1 Torr. Below 1 Torr, compare the thermocouple and Pirani gauges with the McLeod gauge.  In the diffusion pump regime, turn on the Penning gauge first, to verify that the pressure is sufficiently low for the ion gauges. You will pump down more than once, so the first time is for familiarization.
2. Use the leak valve to establish a (nearly) constant chamber pressure. Compare the readings of all of the various types of gauges. What are the useful pressure ranges for each type of gauge? How consistent are the gauges? Are they more consistent than during pumpdown?
3. Using the gas cylinders, determine the sensitivity of the gauges to gas composition.
4. Use the evaporator to "silver" a mirror with aluminum. What factors should you keep in mind in order to avoid having the evaporated aluminum peel off? (Consult instructor for appropriate settings of heater voltage.)

Pump Down Procedure:

1. Insure that the gate valve and vent valve are closed and the bell jar is seated on baseplate.
2. Turn on the mechanical pump and start pumping down the chamber.
3. Turn on water for diffusion pump.
4. Close roughing valve and open foreline valve to start pumping out the diffusion pump.
• When foreline pressure is 30 millitorr, turn on the diffusion pump. (Make sure the water is on).
5. When chamber is roughed out (30 millitorr) and diffusion pump has warmed up:
1. Be sure the roughing valve is closed and foreline valve is open.
2. Open the gate valve to connect the diffusion pump to the bell jar.
3. Fill the LN₂ trap (requires about 6 liters of LN₂ from room temperature).

 Procedure for recording pumpdown curves:

Residual gas analysis procedure:

Back-to-Air Pressure Procedure:

1. Close the gate valve (to keep the diffusion pump under vacuum).
2. Turn off the ion gauges. (TC gauges do not have to be turned off.)
3. Be sure the roughing valve is closed.
4. Open vent valve.

Shut Down Procedure:

1. Close the gate valve.
2. Turn off the diffusion pump.
3. LEAVE THE WATER ON AND CONTINUE PUMPING THE FORELINE WITH THE  MECHANICAL PUMP  until the diffusion pump is cold. After this, you may close the foreline valve and turn the mechanical pump and water off. (It is suggested that you leave the water and mechanical pump on overnight).  If the bell jar is to be left under vacuum, vent the mechanical valve with the manual valve in back.

Evaporation Procedure:

1. Place about one aluminum bead (or 2 sq. inches of aluminum foil, rolled up) inside the filament coil. Place a clean glass plate on a holder over the filament.
2. Pump down to best vacuum possible, using liquid nitrogen in the trap.
3. ...

References:

1. S. Dushman, Scientific Foundations of Vacuum Technique. Wiley: 1962 (the standard guide).
2. A. Guthrie, Vacuum Technology. Wiley, 1963. (an excellent book except that there are errors in the conductance formulas in the appendices. Check against Dushman).
3. L.G. Carpenter, Vacuum Technology. American Elsevier, 1970. (a short, simple introduction).
4. A. Roth, Vacuum Technology, North Holland, 1976.
5. V. Atta, Vacuum Science and Engineering, McGraw-Hill, 1965.

6. J.F. O'Hanlon, A User's Guide to Vacuum Technology, Wiley, 1980.
7. G.L. Weissler and R.W. Carlson, Vacuum Physics and Technology, Methods of Experimental Physics, vol. 14, Academic, 1979.