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Lecture 3 R J ReidVacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007 Vacuum Science and Technology in Accelerators Ron Reid.

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Presentation on theme: "Lecture 3 R J ReidVacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007 Vacuum Science and Technology in Accelerators Ron Reid."— Presentation transcript:

1 Lecture 3 R J ReidVacuum Science and Technology in Accelerators Cockcroft Institute Lectures Vacuum Science and Technology in Accelerators Ron Reid Consultant ASTeC Vacuum Science Group

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

3 Lecture 4 3 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures 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

4 Lecture 4 4 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Pumping for accelerators Mechanical Pumps Turbomolecular Pumps Ion Pumps Getter Pumps Evaporable Non evaporable Cryopumps

5 Lecture 4 5 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures 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.

6 Lecture 4 6 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures 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.

7 Lecture 4 7 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Turbomolecular pump principle

8 Lecture 4 8 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Turbomolecular pump principle

9 Lecture 4 9 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures 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 α 12 is the forward transmission probability and α 21 is the reverse transmission probability It can be shown that where V b is the blade velocity

10 Lecture 4 10 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Turbomolecular pumps

11 Lecture 4 11 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures 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 mbar to lower than mbar

12 Lecture 4 12 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Turbomolecular pumps Operation can be extended to higher pressure by adding a drag stage

13 Lecture 4 13 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Turbomolecular Pumps

14 Lecture 4 14 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures The choice of bearing type is important Oil sealed Greased Greased ceramic ball Magnetic Turbomolecular Pumps

15 Lecture 4 15 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps Based on Penning Cell

16 Lecture 4 16 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps Based on Penning Cell

17 Lecture 4 17 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps Pumping in the basic diode Penning cell

18 Lecture 4 18 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures 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

19 Lecture 4 19 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps Using a heavier cathode e.g. Tantalum increases reflected neutrals

20 Lecture 4 20 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps Triode Pumps use a different design

21 Lecture 4 21 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps Starcell configuration

22 Lecture 4 22 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps

23 Lecture 4 23 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps Current (per cell) – and hence pumping speed – depends on voltage, magnetic field, pressure and history < n < 1.2 Pump life depends on quantity of gas pumped > 20 years at mbar Prone to generate particulates Leakage current unpredictable, so pressure indication below mbar unreliable

24 Lecture 4 24 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps

25 Lecture 4 25 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps DiodeDifferential Diode StarcellTriode Voltage+7kV +2-5kV-5kV Pumping Speed (Active gases) HighestGood Lowest Pumping Speed (Noble gases) LowestGoodHigherHighest Starting PressureLowest GoodHighest UHVLow GoodHighest CostLowestHigherLowHighest

26 Lecture 4 26 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Ion Pumps

27 Lecture 4 27 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures 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.

28 Lecture 4 28 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Getter Pumps For vacuum use, the most common getter pump is the titanium sublimation pump

29 Lecture 4 29 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures 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

30 Lecture 4 30 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Getter Pumps

31 Lecture 4 31 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Getter Pumps

32 Lecture 4 32 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures 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

33 Lecture 4 33 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Getter Pumps

34 Lecture 4 34 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Getter Pumps

35 Lecture 4 35 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Cryogenic Pumps

36 Lecture 4 36 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Cryogenic Pumps There are two major classes of such pumps Liquid Pool Liquid helium temperature (~4K) Closed cycle Refrigerator (~12K) Supplemented by cryosorption

37 Lecture 4 37 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Cryogenic Pumps

38 Lecture 4 38 of 38 The Production of Vacuum R J Reid Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures Cryogenic Pumps


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