Vacuum II G. Franchetti CAS - Bilbao 30/5/2011G. Franchetti1

Index 30/5/2011G. Franchetti2 Creating Vacuum (continuation) Measuring Vacuum Partial Pressure Measurements

Diffusion Ejector pump 30/5/2011G. Franchetti3 Laminar flow Inlet Outlet Cold surface Boiling oil Schematic of the pump operating pressure: – mbar

30/5/2011G. Franchetti4 Laminar flow Inlet Outlet Cold surface Boiling oil PiPi PoPo Pump principle: The vacuum gas diffuses into the jet and gets kicked by the oil molecules imprinting a downward momentum The oil jets produces a skirt which separate the inlet from the outlet

30/5/2011G. Franchetti5 Inlet Outlet Diffusion

Problems 30/5/2011G. Franchetti6 Laminar flow Outlet Cold surface Boiling oil Pi Po Inlet Back streaming Back-Migration

Cures 30/5/2011G. Franchetti7 Laminar flow Outlet Cold surface Boiling oil Pi Po Inlet Baffle (reduces the pumping speed of 0.3) Cold surface Cold Cap

30/5/2011G. Franchetti8 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)

Capture Vacuum Pumps 30/5/2011G. Franchetti9 Getter Pumps (evaporable, non-evaporable) Sputter ion Pumps Cryo Pumps Principle Capture vacuum pumps are based on the process of capture of vacuum molecules by surfaces

Getter Pumps 30/5/2011G. Franchetti10 Solid Bulk Surface Gas

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

30/5/2011G. Franchetti12 C.Benvenuti, CAS 2007 Sorption Speed and Sorption Capacity

13 energy carrier: noble gas ions substrate to coat: vacuum chamber target material: NEG (cathode) driving force: electrostatic Choice of the coating technique for thin film: sputtering NEG composition  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 “ex-situ” coating the vacuum chamber with a NEG thin film that will be activated during the “in situ” bakeout of the vacuum system.

Dipole Coating Facility Quadrupole Coating Facility 30/5/2011G. Franchetti14 M.C. Bellachioma (GSI)

Sputter ion pumps 30/5/2011G. Franchetti15 B E Anode Cathode E P1 P2 Titanum

Sputtering process 30/5/2011G. Franchetti16 + The adsorbing material is sputtered around the pump trapped electrons

30/5/2011G. Franchetti17 A glimpse to the complexity Adsorbed ions Trapped electron ionization process ion bombardment with sputtering

30/5/2011G. Franchetti18 Example of Pumping speed J.M. Laffterty, Vacuum Science, J. Wiley & Son, 1998

Cryo Pumps 30/5/2011G. Franchetti19 m v1v1 Stick to the Wall !! Dispersion forces between molecules and surface are stronger then forces between molecules Cold Wall

30/5/2011G. Franchetti20 cold Vessel nwnw pwpw ncnc pcpc Pump P w = pressure warm P c = pressure in the cold I w = flux Vessel  Pump I c = flux Pump  Vessel molecules stick to the cold wall Schematic of a cryo pump

30/5/2011G. Franchetti21 Now By using the state equation In the same way Ifno pumping although Thermal Transpiration Relation between pressures

30/5/2011G. Franchetti22 When the two pressures breaks the thermal transpiration condition a particle flow starts We find We define and P c depends on the capture process Cryocondensation:P w is the vapor pressure of the gas at T c

Summary on Pumps 30/5/2011G. Franchetti23 N. Marquardt

Gauges 30/5/2011G. Franchetti24 Liquid Manometers MacLeod Gauges Viscosity Gauges Thermal conductivity Gauges Hot Cathode Gauges Alpert-Bayard Gauges Penning Gauges

Liquid Manometers 30/5/2011G. Franchetti25 P2P2 P1P1 h Relation between the two pressure issue: measure precisely “h”by eye +/- 0.1 mm High accuracy is reached by knowing the liquid density but with mercury surface tension depress liquid surface Mercury should be handled with care: serious health hazard

McLeod Gauge 30/5/2011G. Franchetti26 Δh Closed h0h0 Quadratic response to P 2 First mode of use: The reservoir is raised till the mercury reaches the level of the second branch (which is closed) P2P2 Second mode of use: keep the distance Δh constant and measure the distance of the two capillaries  linear response in Δh

Viscosity Gauges 30/5/2011G. Franchetti27 Gas molecule ω φ It is based on the principle that gas molecules hitting the sphere surface take away rotational momentum The angular velocity of the sphere decreases R ρ = sphere density α = coefficient of thermal dilatation T = temperature v a = thermal velocity

30/5/2011G. Franchetti28 (K. Jousten, CAS 2007) F.J. Redgrave, S.P. Downes, “Some comments on the stability of Spinning Rotor Gauges”, Vacuum, Vol. 38,

Thermal conductivity Gauges 30/5/2011G. Franchetti29 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

Energy Balance 30/5/2011G. Franchetti30 V W c /2 WRWR  Energy loss by gas molecules  Energy loss by Radiation  Energy loss by heat conduction When the energy loss by gas molecule is dominant P can be predicted with contained systematic error ε = emissivity

30/5/2011G. Franchetti31 The Gauge tube is kept at constant temperature and the current is measured So that Through this value  E Pirani Gauge Example of power dissipated by a Pirani Gauge vs Vacuum pressure.

Ionization Gauges Principle 30/5/2011G. Franchetti32 electrons accelerating gap L V +

30/5/2011G. Franchetti33 V + IONIZATION positive ions new electrons i + = current proportional to the ionization rate E i-i-

30/5/2011G. Franchetti34 Electrons ionization rate Sensitivity

Hot Cathode Gauges 30/5/2011G. Franchetti35 Grid Anode Hot Cathode 30 V + 180V + Electric field

Hot Cathode Gauges 30/5/2011G. Franchetti36 Grid Anode Hot Cathode 30 V + 180V + In this region the electrons can ionize vacuum particles A current between grid and anode is proportional to the vacuum pressure i+

Limit of use 30/5/2011G. Franchetti37 Upper limit:it is roughly the limit of the linear response Lower limit:X-ray limit Grid Anode new electron X-ray The new electron change the ionization current P PmPm X-ray limit

Alpert-Bayard Gauge 30/5/2011G. Franchetti38 Grid Anode X-ray Hot Cathode the probability that a new electron hits the anode is now very small The X-ray limit is easily suppressed by a factor ~

30/5/2011G. Franchetti39 Schematic of the original design (K. Jousten, CAS 2007)

Penning Gauge (cold cathode gauges) 30/5/2011G. Franchetti40 B = magnetic field E = electric field Electrons motion anode cathode

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

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

Ionization process 30/5/2011G. Franchetti43 Neutral vacuum atom Electron at ionization speed Before CollisionCollision  Ionization Charged vacuum atom Electron at ionization speed New electron

30/5/2011G. Franchetti Ionization Charged vacuum atom Electron at ionization speed New electron 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

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

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

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

30/5/2011G. Franchetti48 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. 322

Summary on Gauges 30/5/2011G. Franchetti49 N. Marquardt

Partial Pressure Measurements 30/5/2011G. Franchetti50 These gauges allow the determination of the gas components Partial pressure gauges are composed Ion Source Mass Analyzer Ion current detection System data output

Ion Sources 30/5/2011G. Franchetti51 Vacuum gas is ionized via electron-impact  the rate of production of ions is proportional to each ion species Electron-impact ionization process e v inelastic scattering: kinetic energy transfer from the electron to the molecule meme M Before Impact After impact e v meme M+M+ e v meme

30/5/2011G. Franchetti52 The minimum energy to ionize M is called “appearance potential” Electron energy (eV) 15 IP = 15 eVM + + e - At the appearance potential the production rate is low appearance potential A. von Engel, Ionized Gases, AVS Classics Ser., p. 63. AIP Press, 1994

Ion source 30/5/2011G. Franchetti53 ionization region electrons source acceleration gap VfVf VgVg electrons ions ion collector E E VcVc Open source Bayard-Alpert gauge V g < V c

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

30/5/2011G. Franchetti55  Faraday Cup  Secondary electron Multiplier (SEM) 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 # electrons output # ion input G = Gain L d V 0 = applied voltage V = initial energy of the electron K = δ V c where δ = secondary emission coefficient V c = collision energy J.Adams, B.W. Manley IEEE Transactions on Nuclear Science, vol. 13, issue 3, p. 88

Mass Analyzers 30/5/2011G. Franchetti Ion equation of motion U, V constant r0r0 define:

30/5/2011G. Franchetti57 By rescaling of the coordinates the equation of motion becomes Mathieu Equation Stability of motion horizontalvertical q=0, a>0unstablestable q=0, a<0stableunstable The presence of the term q, changes the stability condition Development of stable motion The ion motion can be stable or unstable 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

30/5/2011G. Franchetti a q Stable in y a q Stable in x Stability Chart of Mathieu equation

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

30/5/2011G. Franchetti60 Stability region in both planes q 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 V i+i+

30/5/2011G. Franchetti61 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. 462

Omegatron 30/5/2011G. Franchetti62

30/5/2011G. Franchetti63 The revolution time is independent from ion energy If the frequency of the RF is 1/τ a resonant process takes place and particle spiral out collector B E Resolving power

Conclusion 30/5/2011G. Franchetti64 Creating and controlling vacuum will always be a relevant part of any accelerator new development. These two lectures provides an introduction to the topic, which is very extensive: further reading material is reported in the following bibliography THANK YOU FOR YOUR ATTENTION

30/5/2011G. Franchetti65 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