1. Vacuum Science and Technology in Accelerators
Ron Reid
Consultant
ASTeC Vacuum Science Group
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

2. The Production of Vacuum
Session 3
The Production of Vacuum
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

3. Aims To demonstrate the main types of vacuum pump used in accelerators
To understand the pumping mechanisms involved
To understand the advantages and limitations of each type of pump
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

4. Pumping for accelerators
Mechanical Pumps
Turbomolecular Pumps
Ion Pumps
Getter Pumps
Evaporable
Non evaporable
Cryopumps
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

5. Mechanical pumps
Mechanical pumps (displacement pumps) remove gas atoms from the vacuum system and expel them to atmosphere, either directly or indirectly
In effect, they are compressors and one can define a compression ratio, K, given by
K is a fixed value for any given pump for a particular gas species when measured under conditions of zero gas flow.
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

6. Mechanical pumps Here, we will only look at the turbomolecular pump.
Turbo pumps cannot pump from atmosphere and cannot eject to atmosphere, so they require roughing (forevacuum) pumps to reduce the pressure in the vacuum system before they can be started and backing pumps to handle the exhaust.
There are many types of roughing and backing pumps. Most accelerators now use clean (dry) pumps to avoid oil contamination in the system.
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

7. Turbomolecular pump principle
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

8. Turbomolecular pump principle
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

9. Turbomolecular pump principle
To maximise the compression ratio, blade tip velocities need to be comparable to molecular thermal velocities.
For a single blade, at zero flow
where α12 is the forward transmission probability
and α21 is the reverse transmission probability
It can be shown that
where Vb is the blade velocity
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

10. Turbomolecular pumps R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

11. Turbomolecular pumps
Turbo pumps come in a wide range of speeds – from a few l sec-1 to many thousands of l sec-1 and operate from 10-3 mbar to lower than 10-9 mbar
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

12. Turbomolecular pumps
Operation can be extended to higher pressure by adding a drag stage
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

13. Turbomolecular Pumps R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

14. Turbomolecular Pumps The choice of bearing type is important
Oil sealed
Greased
Greased ceramic ball
Magnetic
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

15. Ion Pumps Based on Penning Cell R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

16. Ion Pumps Based on Penning Cell R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

17. Ion Pumps Pumping in the basic diode Penning cell R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

18. Ion Pumps The Diode pump has poor pumping speed for noble gases
Remedies
Differential Ion; Noble Diode
“Heavy” cathode
Triode
Special Anode shape e.g. Starcell
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

19. Ion Pumps
Using a heavier cathode e.g. Tantalum increases reflected neutrals
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

20. Ion Pumps Triode Pumps use a different design R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

21. Ion Pumps Starcell configuration R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

22. Ion Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

23. Ion Pumps
Current (per cell) – and hence pumping speed – depends on voltage, magnetic field, pressure and history.
1.05 < n < 1.2
Pump life depends on quantity of gas pumped
> 20 years at 10-9 mbar
Prone to generate particulates
Leakage current unpredictable, so pressure indication below 10-8 mbar unreliable
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

24. Ion Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

25. Ion Pumps Diode Differential Diode Starcell Triode Voltage +7kV +2-5kV
Pumping Speed (Active gases)
Highest
Good
Lowest
Pumping Speed (Noble gases)
Higher
Starting Pressure
UHV
Low
Cost
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

26. Ion Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

27. Getter Pumps
When a gas molecule impinges on a clean metal film, the sticking probability can be quite high.
For an active gas with the film at room temperature, values can be between 0.1 and 0.8. These fall with coverage.
For noble gases and hydrocarbons sticking coefficients are very low (essentially zero)
Evaporated films, most commonly of titanium or barium, are efficient getters and act as vacuum pumps for active gases.
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

28. Getter Pumps
For vacuum use, the most common getter pump is the titanium sublimation pump
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

29. Getter Pumps
An important class of getter pumps are the Non Evaporable Getters (NEGs)
These are alloys of elements like Ti, Zr, V, Fe, Al which after heating in vacuo present an active surface where active gases may be gettered
Traditionally, the getters take the form of a sintered powder either pressed into the surface of a metal ribbon or formed into a pellet
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

30. Getter Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

31. Getter Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

32. Getter pumps
In recent times, thin films of getter material have been formed on the inside of vacuum vessels by magnetron sputtering
These have the advantage of
pumping gas from the vacuum chamber by gettering
and of stopping gases from diffusing out of the walls of the vessels
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

33. Getter Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

34. Getter Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

35. Cryogenic Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

36. Cryogenic Pumps There are two major classes of such pumps Liquid Pool
Liquid helium temperature (~4K)
Closed cycle
Refrigerator (~12K)
Supplemented by cryosorption
R J Reid
Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

37. Cryogenic Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures

38. Cryogenic Pumps R J Reid Vacuum Science and Technology in Accelerators
Cockcroft Institute Lectures