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

G. Franchetti 1

Vacuum II

G. FranchettiCAS - Bilbao

30/5/2011

G. Franchetti 2

Index

30/5/2011

Creating Vacuum (continuation)

Measuring Vacuum

Partial Pressure Measurements

G. Franchetti 3

Diffusion Ejector pump

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Lam

inar

flow

Inlet

Outlet

Cold

sur

face

Boiling oil

Schematic of the pump

operating pressure: 10-3 – 10-8 mbar

G. Franchetti 430/5/2011

Lam

inar

flow

Inlet

Outlet

Cold

sur

face

Boiling oil

Pi

Po

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


Inlet

Outlet

Diffusion

G. Franchetti 6

Problems

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Lam

inar

flow

Cold

sur

face

Pi

Po

Inlet

Back streaming

Back-Migration

G. Franchetti 7

Cures

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Lam

inar

flow

Cold

sur

face

Pi

Po

Inlet

Baffle

(reduces the pumping speed of 0.3)

Cold

sur

face

Cold Cap


with

The pumping speed Sm is proportional to the area of the inlet port

100 mm diameter Sm = ~ 250 l/s for N2

LEYBOLD (LEYBODIFF)

G. Franchetti 9

Capture Vacuum Pumps

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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

G. Franchetti 10

Getter Pumps

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Solid Bulk

Surface

Gas

11

Heating in vacuum Oxide dissolution -> activation

T = RT

T = Ta

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


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.

G. Franchetti 14

Dipole Coating Facility Quadrupole Coating Facility

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M.C. Bellachioma (GSI)

G. Franchetti 15

Sputter ion pumps

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B

-+ -

E

Anode

Cathode

E

P1

P2

Titanum

G. Franchetti 16

Sputtering process

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+

The adsorbing material is sputtered around the pumptrapped electrons


A glimpse to the complexity

Adsorbed ions

Trapped electron ionization process

ion bombardment with sputtering


Example of Pumping speed

J.M. Laffterty, Vacuum Science, J. Wiley & Son, 1998

G. Franchetti 19

Cryo Pumps

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m

v1

Stick to the Wall !!

Dispersion forces between molecules and surface are stronger then forces between molecules

Cold Wall


cold

Vessel

nw pw

nc pcPump

Pw = pressure warmPc = pressure in the coldIw = flux Vessel PumpIc = flux Pump Vessel

molecules stick to the cold wall

Schematic of a cryo pump


Now By using the state equation

In the same way

If no pumping although

Thermal Transpiration

Relation between pressures


When the two pressures breaks the thermal transpiration condition a particle flow starts

We find

We define and

Pc depends on the capture process

Cryocondensation: Pw is the vapor pressure of the gas at Tc

G. Franchetti 23

Summary on Pumps

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N. Marquardt

G. Franchetti 24

Gauges

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Liquid Manometers

MacLeod Gauges

Viscosity Gauges

Thermal conductivity Gauges

Hot Cathode Gauges

Alpert-Bayard Gauges

Penning Gauges

G. Franchetti 25

Liquid Manometers

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P2

P1 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

G. Franchetti 26

McLeod Gauge

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Δh

Closed

h0

Quadratic response to P2

First mode of use: The reservoir is raised till the mercury reaches the level of the second branch (which is closed)

P2

Second mode of use: keep the distance Δh constant and measure the distance of the two capillaries linear response in Δh

G. Franchetti 27

Viscosity Gauges

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Gasmolecule

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 densityα = coefficient of thermal dilatationT = temperatureva = thermal velocity


(K. Jousten, CAS 2007)

F.J. Redgrave, S.P. Downes, “Some comments on the stability of Spinning Rotor Gauges”, Vacuum, Vol. 38, 839-842

G. Franchetti 29

Thermal conductivity Gauges

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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

G. Franchetti 30

Energy Balance

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V

Wc/2

Wc/2

WR

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


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

.

G. Franchetti 32

Ionization Gauges Principle

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electrons

accelerating gap

L

V

+


V

+

IONIZATION

positiveions

new electrons i+ = current proportional

to the ionization rate

E

i-


Electrons ionization rate

Sensitivity

G. Franchetti 35

Hot Cathode Gauges

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Grid

Anode

Hot Cathode

30 V

+

180V

+Electric field

G. Franchetti 36

Hot Cathode Gauges

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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+

i+

G. Franchetti 37

Limit of use

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Upper limit: it is roughly the limit of the linear response

Lower limit: X-ray limit

Grid

Anodenew electronX-ray

X-ray

The new electron change the ionization current

P

Pm

X-raylimit

G. Franchetti 38

Alpert-Bayard Gauge

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Grid

Anode

X-ray

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 ~100-1000


Schematic of the original design (K. Jousten, CAS 2007)

G. Franchetti 40

Penning Gauge (cold cathode gauges)

30/5/2011

B = magnetic field

E = electric field

Electrons motion

anode

cathode


+

++ + + + + + +

++ + + + + + +

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

One electron is trapped

Under proper condition of (E,B) electrons get trapped in the Penning Gauge


+

++ + + + + + +

++ + + + + + +

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

One electron is trapped

one vacuumneutral gas enters into the trap

G. Franchetti 43

Ionization process

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Neutral vacuum atom

Electron at ionizationspeed

Before Collision Collision Ionization

Charged vacuum atom


New electron

+--

-


+

++ + + + + + +

++ + + + + + +

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Ionization

Charged vacuum atom


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


+

++ + + + + + +

++ + + + + + +

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Ionization Charged vacuum atom

New electron

+

--

Motion of stripped ion


++ + + + + + +

++ + + + + + +

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

++

++++++

++

++++++

--

--

-

-

-

--

Negative space charge formation

More electrons are formed through the ionization of the vacuum gas and remains inside the trap


++ + + + + + +

++ + + + + + +

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

++

++++++

++

++++++

--

--

-

-

-

--

--

-

-

--

-

-

-

--

-

When the discharge gets saturated each new ionization produce a current

I


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

G. Franchetti 49

Summary on Gauges

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N. Marquardt

G. Franchetti 50

Partial Pressure Measurements

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These gauges allow the determination of the gas components

Partial pressure gauges are composed

Ion Source

MassAnalyzer

Ion currentdetection System

dataoutput

G. Franchetti 51

Ion Sources

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Vacuum gas is ionized via electron-impact

the rate of production of ions is proportional to each ion species

Electron-impact ionization process

ev

inelastic scattering: kinetic energy transfer from the electron to the molecule

me

MBefore Impact After impact

e

vme

M+

e

vme


The minimum energy to ionize M is called “appearance potential”

Electronenergy(eV)

15IP = 15 eV M+ + 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

G. Franchetti 53

Ion source

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ionizationregionelectrons

source

acceleration gap

Vf

Vg

electronsions

ion collector

+

+

+

E

E

Vc

Open sourceBayard-Alpert gauge

Vg < Vc


+

+

+

+

+

+

+

+

A schematic

F = ion transmission factorMass analyzer

Ion Detection

G. Franchetti 55

Ion Detection

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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 inputG =

Gain

L

d V0 = applied voltage

V = initial energy of the electron

K = δ Vc

where

δ = secondary emission coefficientVc = collision energyJ.Adams, B.W. Manley IEEE Transactions on Nuclear Science, vol. 13, issue 3, 1966. p. 88

G. Franchetti 56

Mass Analyzers

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Quadrupolar mass spectrometer

x

y s

++

++ ++

++

Ion equation of motion

U, V constant

r0

define:


By rescaling of the coordinates the equation of motion becomes

Mathieu Equation

Stability of motionhorizontal 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

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 N2 at Ek = 10 eV vs = 8301 m/s

For a length of L = 100 mm 100 rf oscillation f = 8.3 MHz

typically f ~ 2 MHz


5 10 150

5

10

5

10

a

q

Stable in y

5 10 150

5

10

5

10

a

q

Stable in x

Stability Chart of Mathieu equation


5 10 150

5

10

5

10

a

q

a

Stability region in both planes

q

a

q0=0.706 a0=0.237


Stability region in both planes

q

a

(0.706; 0.237)

Given a certain species of mass M1 there are two values U1, V1 so that q=q0 and a=a0

By varying V and keeping the ratio V/U constant, the tip of the stability is crossed and a current is measured at V=V1

V

i+


Magnetic Sector Analyzer

Example: A 900 magnetic sector mass spectrometer

B is varied and when a current is detected then

Ez is the ions kinetic energy

Resolving power

Wsource = source slit widthWcollector = collector slit width J.M. Lafferty, Vacuum Science, p. 462

G. Franchetti 62

Omegatron

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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

G. Franchetti 64

Conclusion

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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


Kinetic theory and entropy ,C.H. Collie, Longam Group, 1982Thermal Physics, Charles Kittel, (John Wiley and Sons, 1969).Basic Vacuum Technology (2nd Edn), A Chambers, R K Fitch, B S Halliday, IoP

Publishing, 1998, ISBN 0-7503-0495-2Modern Vacuum Physics, A Chambers, Chapman & Hall/CRC, 2004, ISBN 0-8493-

2438-6The Physical Basis of Ultrahigh Vacuum, P. A. Redhead, J. P. Hobson, E. V.

Kornelsen, AIP, 1993, ISBN 1-56396-122-9Foundation of Vacuum Science, J.M. Lafferty, Wiley & Sons, 1998Vacuum in accelerators, CERN Accelerator School, CERN-2007-003 11 June 2007Vacuum technology, CERN Accelerator School, CERN 99-05 1999

Acknowledgements: Maria Cristina Bellachioma

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