F. Le PimpecF. Le Pimpec 11

Reaching and Reaching and Measuring XHVMeasuring XHV

((1010-12-12 Torr) Torr)

- Luminosity for accelerators

- Lifetime in storage rings

Reaching XHV is commercially easier than measuring it

A CERN modified Helmer gauge did measure 10-14 Torr

XHV is not official - Pressure 10-7 Torr are still called UHV

F. Le PimpecF. Le Pimpec 22

How DID they make light bulb How DID they make light bulb work ? work ?

Bulb Installed in 1901 at the Livermore’s (CA) Fire station. C filament - 4 W

♦ ♦ Since 1800 the electric arc Since 1800 the electric arc light was known. However, light was known. However, the life time of a filament the life time of a filament made of carbon was poor.made of carbon was poor.

♦ ♦ The problem was in The problem was in obtaining a high enough obtaining a high enough vacuum in the bulb and vacuum in the bulb and getting rid of the oxygen in getting rid of the oxygen in the residual gasthe residual gas

Use of the first Use of the first Evaporable Evaporable

GettersGetters

Phosphorus pumped lamp tend to have a red cast

F. Le PimpecF. Le Pimpec 33

Why Measure Total Pressure ?Why Measure Total Pressure ?Partial Pressure gives information on the contents of the vacuum

Total pressure can be computed from the partial P measurements

Operational in the same range (UHV)

The use of hot and cold gauge style device need calibration for every single species for accurate readings – chemistry sensitivity

RGA’s electronics are sensitive to the beam passage ! And are still not cheap compared to gauges !

Extractor

RGA

F. Le PimpecF. Le Pimpec 44

UHV - XHV Total Pressure UHV - XHV Total Pressure - Xray limitation due to the e- hitting the grid : Ions are desorbed from the Collector. Remedies : Modulation

- ESD from the gauges elements – Reducing emission current : Wrong The grid will pump then release molecules

Modify Extractor gauge with hidden collector (U. Magdeburg)

- Installing a hot gauge in a small tube – Transpiration effectDespite a higher pressure thegauge will read lower. Solution: nude gauges – but sensitivity to stray ions from surroundings

A

FP

F. Le PimpecF. Le Pimpec 55

Ti vs Other Getters in AcceleratorTi vs Other Getters in Accelerator

A) Pumping Speed calculated at RTB) Wide variations due to film roughnessC) For H2, competition between desorption

and diffusion inside the deposited layers

Ref. “Sorption of Nitrogen by Titanium Films,” Harra andHayward, Proc. Int. Symp. On Residual Gases in Electron Tubes, 1967

1cm2

Ref. “Le Normand CERN vacuum note”

Ba - Ca - Mg : High vapor pressure. Trouble if bake out is requestedZr - Nb - Ta : Evaporation temperature too high

Typical sublimation rate 0.1 to 0.5 g/hr

0.5 g/hr ; MTi =48g/mol0.5/(48*3600) = 2.9 10-6 mol.s-1

1ML~1015 atom.cm-2

2.9 10-6 mol.s-1. Avogadro = 1.74 1018 atom.s-1

1 ms is necessary to evaporate 1 ML3 minutes = 180E3 ms hence ML for 1cm2

F. Le PimpecF. Le Pimpec 66

Also True for Thin Films TiZr and Also True for Thin Films TiZr and TiZrVTiZrV

V. Baglin et al.

F. Le PimpecF. Le Pimpec 77

Ionodesorption by heavy energetic Ionodesorption by heavy energetic ions on technical surfacesions on technical surfaces

E. Mahner et al.

1.5 109 Pb53+ ions (per shot) under 89.2° grazing incidence and 4.2 MeV/u

F. Le PimpecF. Le Pimpec 88

Ion-desorption on Al surfacesIon-desorption on Al surfacesIon induced desorption

yield

A.G. Mathewson

M.H. Achard M.H. Achard-R. Calder-A.G. Mathewson

M.P. Lozano

1976 1976 1978 2001

15N2+ at

2 keV

15N2+ at

2 keV

K+ at2 keV

K+ at 1.4 keV Ar+ at 3 keV

Aluminiumas received

H2 4.5 – 10 2.3 3.6 - 10 18 4 - 7

CH4 0.55 – 0.95 0.2 0.3 - 0.9 1 0.5 – 0.8

CO 7 – 10 2.5 3 - 10.5 7 0.9 – 1.5

CO2 1.8 – 3.2 0.5 1 - 3.7 1.2 1 – 2.5

Aluminium after 24 hours baking

at 2000C (*)

H2 3.2 – 4 3.2 2.5

CH4 0.22 – 0.23 0.2 0.32

CO 2.8 – 2.9 2.2 1.5

CO2 0.75 - 1 0.18 0.35

(*) 300°C in the measurement of M.H. Achard

F. Le PimpecF. Le Pimpec 99

Electron CloudElectron Cloud

fast head-tail instabilityfast head-tail instability

NLC Fast Head tail - straight

M. Pivi

F. Le PimpecF. Le Pimpec 1010

TiN/Substrate & Electron TiN/Substrate & Electron CloudCloud

a) COII - N - 2Rb) COIIIL c) InjBELL1d) Inj BEIR1 e) E3L f) RFS2R

Roughness is an issue

F. Le PimpecF. Le Pimpec 1111

Getter & Electron CloudGetter & Electron Cloud

Hilleret et al.

TiZrV coating

2 h at 300C, CO injected at NEG T=60C

TiZrV coating

Low SEY : Choice for the NEG of the activation T and t .

Conditioning (photons e- ions)

Contamination by gas exposure, or by the vacuum residual gas, increases the SEY; even after conditioning.

Angles of incidence, of

the PE, change the shape of the curve at higher

energy

F. Le PimpecF. Le Pimpec 1212

TiN/SS

TiN/Al

Roughness is an issue

cos1 e

TiN/Substrate & Electron CloudTiN/Substrate & Electron Cloud

=0.5

NLC: 130 eV e-beam conditioning

≤ 10-9 Torr Vacuum Recontamination

F. Le PimpecF. Le Pimpec 1313Al disk with triangular shape

Alternative solution :Alternative solution :Playing with RoughnessPlaying with Roughness

Oxidized Cu disk

Very rough surfaces emits Very rough surfaces emits less SE, because SE can be less SE, because SE can be intercepted by surrounding intercepted by surrounding “walls”“walls”

F. Le PimpecF. Le Pimpec 1414

Al disk with triangular shape

Alternative solution :Alternative solution :Playing with RoughnessPlaying with Roughness

Oxidized Cu disk

SimulationSimulation

Reality

F. Le PimpecF. Le Pimpec 1515

Al disk with triangular shape

Alternative solution :Alternative solution :Playing with RoughnessPlaying with Roughness

SimulationSimulation

Reality

F. Le PimpecF. Le Pimpec 1616

Dynamic UHV – NEG Dynamic UHV – NEG solutionsolution

In accelerator Cu, Al or SS are the technical In accelerator Cu, Al or SS are the technical materials of choice, high conductivitymaterials of choice, high conductivity

– Cu and SS, can be baked at high temperature, Cu and SS, can be baked at high temperature, Al cannot (200°C) Al cannot (200°C) special design, or ways, to special design, or ways, to activate a NEG pumping solutionactivate a NEG pumping solution

– SS and NEG coating have a lower conductivity SS and NEG coating have a lower conductivity compared to Cu or Al, wakefield issues compared to Cu or Al, wakefield issues skin skin depth & vacuum chamber size determination depth & vacuum chamber size determination

– A leak during an activation might lead to A leak during an activation might lead to scrapping the chamber (2m of Be chamber, scrapping the chamber (2m of Be chamber, vertex detector, for LHC 10vertex detector, for LHC 1066 CHF) CHF)

– Cycles of venting/activation need to be Cycles of venting/activation need to be assessed for the lifetime of the machine assessed for the lifetime of the machine