Vacuum Experiment

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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:

You should look at one or more of the references listed below and be familiar with the following:

  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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  1. System Diagram


 

 


Measurements:

Interesting data

·        Pump down, atmospheric pressure to ~1mTorr

·        Venting from high vacuum to atmosphere

·        Effect of degassing of ion gauge

·        Effect of  addition of liquid nitrogen to the cold trap

·        Closing the high vacuum valve, see pressure change

It would be helpful to understand “vacuum” if you calculate

·        The number of molecules per cm3 at pressure of 10-7 Torr.

·        The force due to atmospheric pressure on a 2” diameter area, and how it would change going from 1 Torr to 10-6 Torr.

·        The mean free path of N2 at 10-6 Torr of pressure.

1.      Turn on the power of the mechanical pump. The valve setting should be

·        Foreline Valve                    : Open

·        High Vacuum Valve            : Open

·        Roughing Valve                  : Close

·        Vent Valve                         : Close

·        Pump Vent Valve               : Close.

  1. Observe and record

·        Sound of the pump

·        Pirani and Capacitive diaghragm MKS gauges.

When the system pressure is lower than ~20mTorr, open the cooling water valve just enough (discuss the flow rate with the instructor), turn on the power for the diffusion pump. Note the time. Heat up time is around 20 minutes. Observe and record

·        Pirani and Capacitive diaghragm MKS gauges

·        Cold Cathode gauge.

After the pressure goes below 10-4 Torr, turn on the filament of the ion gauge. Depending on the gauges available, you could monitor the pressure through Data Studio.

  1. Add liquid nitrogen to the cold trap after the pressure drops below ~10-5 Torr.
  2. Use the leak valve to establish a (nearly) constant chamber pressure. Compare the readings of all the various types of gauges. What are the useful pressure ranges for each type of gauges? How consistent are the gauges? Are they more consistent than during pump down?
  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.)

Valve positions

Looking at system diagram can you follow the reasoning behind the valve settings?

1.      Normal high vacuum running:

·        Foreline valuve                   : Open

·        High Vacuum Valve            : Open

·        Roughing Valve                  : Close

·        Vent Valve                         : Close

·        Pump Vent Valve               : Close

2.      Venting the chamber, bringing the bell jar to the atmospheric pressure, to open the system:

·        Foreline valve                     : Open

·        High vacuum valve              : Close first

·        Roughing valve                   : Close

·        Vent valve                          : Open, then close

·        Pump vent valve                 : Close

3.      Obtaining high vacuum in the chamber at atmospheric pressure:

·        Foreline valve                     : Close first

·        High vacuum valve              : Close

·        Roughing valve                   : Open afterwards

·        Vent valve                          : Close

·        Pump vent valve                 : Close

4.      Observe the pressure in the chamber (bell jar). If the pressure is lower than 20mTorr, then:

·        Foreline valve                     : Open afterwards

·        High vacuum valve              : Close

·        Roughing valve                   : Close first

·        Vent valve                          : Close

·        Pump vent valve                 : Close

5.      Observe the pressure above the diffusion pump. It should be around 10-6Torr. Then:

·        High vacuum valve              : Open.

6.      Observe the pressure in the bell jar.

 

Residual gas analysis

You should read the section in the manual on the operating principles of the RGA (Residual Gas Analyzer).

Familiarize yourself with the software to obtain the partial pressure vs. time and pressure vs. mass/charge graphics. Use of the extensive library feature. You should be able the mark the Mechanical Pump Oil (MPO) peaks.

Evaporation Procedure:

  1. Set the valve settings for vent.
  2. Open the bell jar.
  3. 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.
  4. Pump down to best vacuum possible, using liquid nitrogen in the trap.
  5. Evaporate aluminum.
  6. Measure the electrical resistance of the obtained films with four-point technique.
  7. Try to obtain thickness information through resistance measurements. Use traveling microscope for dimensional measurements.

References:

  1. Basic Vacuum Techiques, 2nd Ed. A, Chambers, R. K. Fitch & B. S. Halliday, Institute of Physics Publishing, 1998, UVa Ebook
  2. Total Pressure Measurements in Vacuum Technology, A. Berman, Academic Press, Inc., 1985, 396pp., UVa library Call number : QC165.B45 1985
  3. Handbook of Electron Tube and Vacuum Techniques, F. Rosebury, Addison-Wesley Pub. Co., 1965, UVa library Call number : TK7872.V3 R58
  4. Vacuum Sealing Techniques, A. Roth, Pergamon Press, 1966, UVa library Call number : TJ940.R6
  5. The Physical Basis of Ultrahigh Vacuum, P. A. Redhead, J. P. Hobson, E. V. Kornelsen, American Institute of Physics, 1993, 498pp., ISBN: 1-56396-122-9
  6. Modern Vacuum Practice, Nigel Harrison, Self-Published, 1989, 315pp., ISBN : 0-07-707099-2
  7. A User’s Guide to Vacuum Technology, 2nd Ed., J. O’Hanlon, John Wiley & Sons, 1989, 512pp., ISBN: 0-417-81242-0
  8. Experimental Innovation in Surface Science,  Njohn T. Yates Jr., AIP Press, 1998
  9. S. Dushman, Scientific Foundations of Vacuum Technique. Wiley: 1962 (the standard guide).
  10. A. Guthrie, Vacuum Technology. Wiley, 1963. (an excellent book except that there are errors in the conductance formulas in the appendices. Check against Dushman).
  11. L.G. Carpenter, Vacuum Technology. American Elsevier, 1970. (a short, simple introduction).
  12. A. Roth, Vacuum Technology, North Holland, 1976.
  13. V. Atta, Vacuum Science and Engineering, McGraw-Hill, 1965.
  14. J.F. O'Hanlon, A User's Guide to Vacuum Technology, Wiley, 1980.
  15. G.L. Weissler and R.W. Carlson, Vacuum Physics and Technology, Methods of Experimental Physics, vol. 14, Academic, 1979.


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