Vacuum Experiment

This experiment examines the operation and several properties of a typical "high" vacuum system, based on a turbomolecular 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 intentionally added gases.  Finally evaporation techniques will be used to silver (actually, aluminize) a mirror.
 

One thing before starting:

Background:

1. How vacuum pumps work (specifically mechanical and turbomolecular 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. Turbomolecular pump system with:
1. Evaporation system
2. Various gauges
3. 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 pump turn on until the thermocouple gauge comes on scale, at 1 Torr. Below 1 Torr, compare the thermocouple and Pirani gauges. Once the pirani gauge reads zero, turn on the ion gauge. 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. Before doing this, check the following:
   1. Turn off the ion gauge at first and only turn it back on if the pressure is below 10^-4 Torr.
   2. Press the Low Speed button on the pump controller. This runs the turbo pump at a lower rotation rate to prevent damage to the pump blades.
   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. Before starting, make sure that the scroll pump is off and the turbo pump is not spinning (you will hear it if it is) . Vent the turbo pump using the small black lever on the back of the pump system. Carefully open the butterfly valve, it requires some torque and releases suddenly. Insure that all other vent valves are closed and the bell jar is seated on the baseplate.
2. Press the start/stop button on the turbopump controller. The scroll pump will start and begin to evacuate the chamber. Shortly thereafter, the turbo pump will be activated and the rotation rate will be displayed on the controller.

Procedure for recording pumpdown curves:

You can record the pressure in the chamber vs time using the Pasco interface. The ion gauge controller has two outputs, one for the ion gauge and one for the pirani gauge. Start recording data before turning on the pump to monitor the pressure. Once the pirani gauge reads zero pressure, turn on the ion gauge.

Residual gas analysis procedure:

Shut Down Procedure:

1. Make sure the ion gauge is off.
2. Close the butterfly valve.
3. Press the start/stop button on the turbo pump controller.
4. The turbo pump will begin to spin down very slowly. At this point it is safe to vent the chamber. Once the turbo pump spins down, vent the pump using the small black lever at the back of the pump system.

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.