Vacuum II. G. Franchetti CAS - Bilbao. 30/5/2011 G. Franchetti 1

Transcription

1 Vacuum II G. Franchetti CAS Bilbao 30/5/2011 G. Franchetti 1

2 Index Creating Vacuum (continuation) Measuring Vacuum Partial Pressure Measurements 30/5/2011 G. Franchetti 2

3 Laminar flow Cold surface Diffusion Ejector pump Inlet Schematic of the pump operating pressure: mbar Outlet Boiling oil 30/5/2011 G. Franchetti 3

4 Laminar flow Cold surface Inlet P i Pump principle: The vacuum gas diffuses into the jet and gets kicked by the oil molecules imprinting a downward momentum P o Outlet The oil jets produces a skirt which separate the inlet from the outlet Boiling oil 30/5/2011 G. Franchetti 4

5 Inlet Outlet Diffusion 30/5/2011 G. Franchetti 5

6 Cold surface Problems Inlet Pi Back streaming BackMigration 30/5/2011 G. Franchetti 6 Po

7 Cold surface Cold surface Cures Inlet Baffle (reduces the pumping speed of 0.3) Pi Cold Cap 30/5/2011 G. Franchetti 7 Po

8 with The pumping speed S m is proportional to the area of the inlet port 100 mm diameter S m = ~ 250 l/s for N 2 LEYBOLD (LEYBODIFF) 30/5/2011 G. Franchetti 8

9 Capture Vacuum Pumps Principle Capture vacuum pumps are based on the process of capture of vacuum molecules by surfaces Getter Pumps (evaporable, nonevaporable) Sputter ion Pumps Cryo Pumps 30/5/2011 G. Franchetti 9

10 Getter Pumps Gas Surface Solid Bulk 30/5/2011 G. Franchetti 10

11 Definition of NEG Getters are materials capable of chemically adsorbing gas molecules. To do so their surface must be clean. For NonEvaporable Getters a clean surface is obtained by heating to a temperature high enough to dissolve the native oxide layer into the bulk. T = T a T = RT T = RT Native oxide layer > no pumping Heating in vacuum Oxide dissolution > activation Pumping NEGs pump most of the gas except rare gases and methane at room temperature P. Chiggiato 11

12 Sorption Speed and Sorption Capacity C.Benvenuti, CAS /5/2011 G. Franchetti 12

13 Choice of the coating technique for thin film: sputtering substrate to coat: vacuum chamber target material: NEG (cathode) driving force: electrostatic energy carrier: noble gas ions NEG composition Ti V Zr The trend in vacuum technology consists in moving the pump progressively closer to the vacuum chamber wall. The ultimate step of this process consists of transforming the vacuum chamber from a gas source into a pump. One way to do this is by exsitu coating the vacuum chamber with a NEG thin film that will be activated during the in situ bakeout of the vacuum system. 13

14 Dipole Coating Facility Quadrupole Coating Facility M.C. Bellachioma (GSI) 30/5/2011 G. Franchetti 14

15 Sputter ion pumps Anode E Cathode E P2 B Titanum P1 30/5/2011 G. Franchetti 15

16 Sputtering process trapped electrons The adsorbing material is sputtered around the pump 30/5/2011 G. Franchetti 16

17 A glimpse to the complexity ion bombardment with sputtering Adsorbed ions Trapped electron ionization process 30/5/2011 G. Franchetti 17

18 Example of Pumping speed J.M. Laffterty, Vacuum Science, J. Wiley & Son, /5/2011 G. Franchetti 18

19 Cryo Pumps Stick to the Wall!! v 1 m Cold Wall Dispersion forces between molecules and surface are stronger then forces between molecules 30/5/2011 G. Franchetti 19

20 Schematic of a cryo pump Vessel n w p w P w P c I w I c = pressure warm = pressure in the cold = flux Vessel Pump = flux Pump Vessel Pump n c p c cold molecules stick to the cold wall 30/5/2011 G. Franchetti 20

21 Now By using the state equation In the same way If no pumping although Relation between pressures Thermal Transpiration 30/5/2011 G. Franchetti 21

22 When the two pressures breaks the thermal transpiration condition a particle flow starts We define and We find P c depends on the capture process Cryocondensation: P w is the vapor pressure of the gas at T c 30/5/2011 G. Franchetti 22

23 Summary on Pumps N. Marquardt 30/5/2011 G. Franchetti 23

24 Gauges Liquid Manometers MacLeod Gauges Viscosity Gauges Thermal conductivity Gauges Hot Cathode Gauges AlpertBayard Gauges Penning Gauges 30/5/2011 G. Franchetti 24

25 Liquid Manometers P 2 Relation between the two pressure P 1 h issue: measure precisely h by eye / 0.1 mm but with mercury surface tension depress liquid surface High accuracy is reached by knowing the liquid density Mercury should be handled with care: serious health hazard 30/5/2011 G. Franchetti 25

26 McLeod Gauge First mode of use: P 2 The reservoir is raised till the mercury reaches the level of the second branch (which is closed) Δh Closed h 0 Quadratic response to P 2 Second mode of use: keep the distance Δh constant and measure the distance of the two capillaries linear response in Δh 30/5/2011 G. Franchetti 26

27 Viscosity Gauges φ ω Gas molecule It is based on the principle that gas molecules hitting the sphere surface take away rotational momentum R The angular velocity of the sphere decreases ρ = sphere density α = coefficient of thermal dilatation T = temperature v a = thermal velocity 30/5/2011 G. Franchetti 27

28 (K. Jousten, CAS 2007) F.J. Redgrave, S.P. Downes, Some comments on the stability of Spinning Rotor Gauges, Vacuum, Vol. 38, /5/2011 G. Franchetti 28

29 Thermal conductivity Gauges hot wire A hot wire is cooled by the energy transport operated by the vacuum gas Accommodation factor By measuring de/dt we measure P 30/5/2011 G. Franchetti 29

30 Energy Balance Energy loss by gas molecules W c /2 W R Energy loss by Radiation V W c /2 ε = emissivity Energy loss by heat conduction When the energy loss by gas molecule is dominant P can be predicted with contained systematic error 30/5/2011 G. Franchetti 30

31 Pirani Gauge Example of power dissipated by a Pirani Gauge vs Vacuum pressure The Gauge tube is kept at constant temperature and the current is measured So that. Through this value E 30/5/2011 G. Franchetti 31

32 Ionization Gauges Principle electrons V L accelerating gap 30/5/2011 G. Franchetti 32

33 E new electrons i = current proportional to the ionization rate i V positive ions 30/5/2011 G. Franchetti 33

34 Electrons ionization rate Sensitivity 30/5/2011 G. Franchetti 34

35 Hot Cathode Gauges Anode Electric field Grid 180V 30 V Hot Cathode 30/5/2011 G. Franchetti 35

36 Hot Cathode Gauges In this region the electrons can ionize vacuum particles Anode i Grid 180V 30 V Hot Cathode i A current between grid and anode is proportional to the vacuum pressure 30/5/2011 G. Franchetti 36

37 Limit of use Upper limit: Lower limit: it is roughly the limit of the linear response Xray limit Anode Xray new electron The new electron change the ionization current P m Xray Xray limit Grid P 30/5/2011 G. Franchetti 37

38 AlpertBayard Gauge Xray The Xray limit is easily suppressed by a factor ~ Hot Cathode Anode Xray Grid the probability that a new electron hits the anode is now very small 30/5/2011 G. Franchetti 38

39 Schematic of the original design (K. Jousten, CAS 2007) 30/5/2011 G. Franchetti 39

40 Penning Gauge (cold cathode gauges) E = electric field B = magnetic field anode cathode Electrons motion 30/5/2011 G. Franchetti 40

41 30/5/2011 G. Franchetti 41 One electron is trapped Under proper condition of (E,B) electrons get trapped in the Penning Gauge

42 One electron is trapped one vacuum neutral gas enters into the trap 30/5/2011 G. Franchetti 42

43 Ionization process Neutral vacuum atom Before Collision Collision Ionization Charged vacuum atom Electron at ionization speed Electron at ionization speed New electron 30/5/2011 G. Franchetti 43

44 Ionization Charged vacuum atom this ion has a too large mass and relatively slow velocity, therefore its motion is dominated by the electric field and not by the magnetic field Electron at ionization speed New electron 30/5/2011 G. Franchetti 44

45 Motion of stripped ion Ionization Charged vacuum atom New electron 30/5/2011 G. Franchetti 45

46 30/5/2011 G. Franchetti 46 Negative space charge formation More electrons are formed through the ionization of the vacuum gas and remains inside the trap

47 30/5/2011 G. Franchetti 47 When the discharge gets saturated each new ionization produce a current I

48 The time necessary for the discharge formation depends upon the level of the vacuum lower pressure longer time of formation Sensitivity of a SIP (the same as for the Penning Gauge). J.M. Lafferty, Vacuum Science,p /5/2011 G. Franchetti 48

49 Summary on Gauges N. Marquardt 30/5/2011 G. Franchetti 49

50 Partial Pressure Measurements These gauges allow the determination of the gas components Partial pressure gauges are composed Ion Source Mass Analyzer Ion current detection System data output 30/5/2011 G. Franchetti 50

51 Ion Sources Vacuum gas is ionized via electronimpact the rate of production of ions is proportional to each ion species Electronimpact ionization process e Before Impact v M After impact M m e inelastic scattering: kinetic energy transfer from the electron to the molecule e m e v e m e v 30/5/2011 G. Franchetti 51

52 The minimum energy to ionize M is called appearance potential At the appearance potential the production rate is low Electron energy (ev) 15 IP = 15 ev M e appearance potential A. von Engel, Ionized Gases, AVS Classics Ser., p. 63. AIP Press, /5/2011 G. Franchetti 52

53 Open source BayardAlpert gauge Ion source acceleration gap ion collector electrons source ionization region electrons ions E E V f V g V g < V c V c 30/5/2011 G. Franchetti 53

54 A schematic Mass analyzer F = ion transmission factor Ion Detection 30/5/2011 G. Franchetti 54

55 Ion Detection Faraday Cup Secondary electron Multiplier (SEM) G = Gain # electrons output # ion input Idea: an ion enters into the tube, and due to the potential is accelerated to the walls. At each collision new electrons are produced in an avalanche process d L V 0 = applied voltage V = initial energy of the electron K = δ V c where J.Adams, B.W. Manley IEEE Transactions on Nuclear Science, vol. 13, issue 3, p. 88 δ = secondary emission coefficient V c = collision energy 30/5/2011 G. Franchetti 55

56 Mass Analyzers Quadrupolar mass spectrometer y r 0 s Ion equation of motion x define: U, V constant 30/5/2011 G. Franchetti 56

57 By rescaling of the coordinates the equation of motion becomes Mathieu Equation The ion motion can be stable or unstable Stability of motion horizontal vertical q=0, a>0 unstable stable q=0, a<0 stable unstable The presence of the term q, changes the stability condition Development of stable motion Stable or unstable motion is referred to a channel which is infinitely long, but typically a length correspondent to 100 linear oscillation is considered enough Example: For N 2 at E k = 10 ev v s = 8301 m/s For a length of L = 100 mm 100 rf oscillation f = 8.3 MHz typically f ~ 2 MHz 30/5/2011 G. Franchetti 57

58 10 5 a Stability Chart of Mathieu equation 10 a 5 Stable in y q q 5 5 Stable in x /5/2011 G. Franchetti 58

59 a 10 a q a q 0 =0.706 a 0 =0.237 Stability region in both planes q 10 30/5/2011 G. Franchetti 59

60 a (0.706; 0.237) Given a certain species of mass M 1 there are two values U 1, V 1 so that q=q 0 and a=a 0 By varying V and keeping the ratio V/U constant, the tip of the stability is crossed and a current is measured at V=V 1 q i Stability region in both planes V 30/5/2011 G. Franchetti 60

61 Magnetic Sector Analyzer Example: A 90 0 magnetic sector mass spectrometer B is varied and when a current is detected then E z is the ions kinetic energy Resolving power W source = source slit width W collector = collector slit width J.M. Lafferty, Vacuum Science, p /5/2011 G. Franchetti 61

62 Omegatron 30/5/2011 G. Franchetti 62

63 The revolution time is independent from ion energy collector B If the frequency of the RF is 1/τ a resonant process takes place and particle spiral out E Resolving power 30/5/2011 G. Franchetti 63

64 Conclusion Creating and controlling vacuum will always be a relevant part of any accelerator new development. THANK YOU FOR YOUR ATTENTION These two lectures provides an introduction to the topic, which is very extensive: further reading material is reported in the following bibliography 30/5/2011 G. Franchetti 64

65 Kinetic theory and entropy,c.h. Collie, Longam Group, 1982 Thermal Physics, Charles Kittel, (John Wiley and Sons, 1969). Basic Vacuum Technology (2 nd Edn), A Chambers, R K Fitch, B S Halliday, IoP Publishing, 1998, ISBN Modern Vacuum Physics, A Chambers, Chapman & Hall/CRC, 2004, ISBN The Physical Basis of Ultrahigh Vacuum, P. A. Redhead, J. P. Hobson, E. V. Kornelsen, AIP, 1993, ISBN Foundation of Vacuum Science, J.M. Lafferty, Wiley & Sons, 1998 Vacuum in accelerators, CERN Accelerator School, CERN June 2007 Vacuum technology, CERN Accelerator School, CERN Acknowledgements: Maria Cristina Bellachioma 30/5/2011 G. Franchetti 65