Go/no-go test for the acceptance of

components for unbaked vacuum sectors

Paolo Chiggiato

(Thanks to Sophie Meunier for the preparation of the presentation)

4/6/2014 Document reference 2

Introduction

CERN policy: ‘All vacuum components must be tested before

installation in particle accelerators, even those already installed in

accelerators before modification’

Points to take into account to define the acceptance test:

• The tested elements could be only a part of a mechanical assembly.

• The outgassing rate of unbaked components change with pumping time.

• Hundreds of components could be regularly tested.

• Technicians prefer easy-to-apply go/no-go acceptance criteria.

4/6/2014 Document reference 3

Acceptance criteria for components of unbaked vacuum sectors

The acceptance test of components for unbaked vacuum sectors relies on:

1. The pumpdown curve.

AND

2. Relative comparison of RGA spectra.

Typical all-metal system used for

acceptance tests of unbaked

components

4/6/2014 Document reference 4

Acceptance criteria: pumpdown curve

The pressure reading is monitored with and without (background) the sample with

a strict procedure:

• The vacuum system is exposed to air overnight.

• The duration of the pumpdown is at least 10 h.

• The slope a of the P=P(t) function in log-log scale is calculated.

• The outgassing rate after 10 h of pumping is calculated and converted in

square meter of regular stainless steel.

The acceptance condition is:

• Virtual leaks, presence of organic materials or contamination are

detected

Q(10h): compatible with the application:

• If a ≤ −𝟎. 𝟖𝟓 , unexpected high roughness or open porosity are detected

a ≤ −𝟎. 𝟖𝟓

4/6/2014 Document reference 5

The estimated outgassing rate after 10 h of pumping of the four electrodes is:

Q (10h) = (9.1 10-7 - 6.6 10-8)*63 = 5.32 10-5 mbar.l/s.

The data is reported as equivalent m2 of stainless steel.

For the example: 18 m2 of stainless steel for the ≈ 0.5 m2 of electrode surface area

Electrodes for PS strip liner monitor BTM

Pre

ssure

[m

bar]

Pumping time[s]

a=-0.97

a=-1.02

background

Acceptance criteria: example of non-acceptance

4/6/2014 Document reference 6

4/6/2014 Document reference 7

Pumping time[h]

Pre

ssure

[m

bar]

background

a=-0.5

Acceptance criteria: example of non-acceptance

4/6/2014 Document reference 8

Acceptance criteria: RGA spectra

The aim of this measurement is to discriminate between acceptable and non

acceptable components on the basis of relative thresholds.

The procedure is repeated twice with exactly the same parameters and procedure,

with and without the component to be tested.

1. After 11 h of pumping, The RGA filament is switched-on.

2. Three hours after the ignition of the filament, mass scans are recorded (in

Faraday or electron-multiplier mode depending on the attained pressure).

4/6/2014 Document reference 9

Two criteria for the acceptance of the components based on RGA spectra:

1. All mass-peak between 18 amu and 44 amu shall be 100 times lower than the

18-amu mass-peak, except for 28 amu and 44 amu.

AND

2. All peaks for masses higher than 44 amu shall be 1000 times lower than the

18 amu mass-peak.

The purpose is to detect contamination due to ineffective (or lack of) cleaning,

post-cleaning manipulation, packaging, etc.

Acceptance criteria: RGA spectra

4/6/2014 Document reference 10

Passed the RGA test

4/6/2014 Document reference 11

Did not pass the RGA test

÷ 𝟏𝟎𝟎

÷ 𝟏𝟎𝟎𝟎

𝟏

4/6/2014 Document reference 12

Are the acceptance criteria too stringent ?

III. Systèmes Utilisés 12

Sector 1 Sector 2 Sector 3

Residual gas

analyser

All-metal sector valves

Pumping group for

Injection line

Pumping group for

training lab

A clean system, which comprises

several components found in vacuum

sectors, was exposed to N2, air, and

water vapor and then re-pumped.

• Venting of the test system with 4 different gas :

nitrogen, dry air (80% N2, 20% O2) , water vapor (25 mbar), ambient air

• Bake-out after water vapor and air venting

• Mass scan analysis after different pumping time : 5h, 7h et 24h.

9h00 16h00 9h00 12h00 14h30 16h00

Venting and re-pumping procedure

4/6/2014 Document reference 14

Pump-down measurement after venting: 𝑃 ∝ 1 𝑡 as expected

The pressure spikes are due to the ignition of the RGA filament.

Pump-down curves

CO and CO2 generation

on the RGA’s filament

4/6/2014 Document reference 15

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

0 10 20 30 40 50 60 70 80 90 100

Ion Current (A)

m/z

Mass spectrum test system

After bakeout

N2

Dry Air (N2+O2)

Water

Air

RGA used: Pfeiffer Prisma QMA200

(SEM 1200V )

RGA spectra

4/6/2014 Document reference 16

0.00E+00

1.00E-12

2.00E-12

3.00E-12

4.00E-12Io

n C

urr

en

t [A

]

Peak 55

0.00E+00

5.00E-13

1.00E-12

1.50E-12

2.00E-12

Ion

Cu

rren

t [A

]

Peak 57

• Hydrocarbon peaks are most likely generated by chemical reactions involving the

decomposition of water vapor onto the hot RGA’s filament.

• Such hydrocarbon peaks are several orders of magnitude lower than those regularly

measured during acceptance tests.

4/6/2014 Document reference 17

Conclusions

1. Acceptance tests for components of unbaked vacuum sectors are now

mandatory at CERN.

2. The acceptance criteria are based on:

a. The slope of the pumpdown curve and the outgassing rate after 10h of

pumping.

b. The relative intensities of the RGA mass spectrum.

3. Such criteria allow vacuum technicians to easily identify non-conformities.

They are now part of the normal practice at CERN.

4. Clean all-metal systems fulfil easily the requirements.

5. Part of the hydrocarbon peaks could be originated by the interaction of surface

carbon contamination and dissociated water on hot filaments.

6. The validity of the criteria will be soon checked by XPS and FTIR analysis.