Update of the vacuum system for the CLIC two-beam modules

C. GarionCERN/TE/VSC

LCWS, 23-26 September 2011

C. Garion, LCWS 2011 2/21

Outline

• Vacuum system in the CLIC module– requirements– Specificities

• Vacuum dynamics for a non-baked system

• Technological solutions for the vacuum system in the CLIC module

– Vacuum tests of two solutions– Comparison of the technologies

• Few words on dynamic vacuum

• Conclusion

C. Garion, LCWS 2011 3/21

Vacuum system in the CLIC module

Specificities1. Non-baked system vacuum is driven by water

2. Low conductance (beam pipe diameter ~ 10 mm) and large areas (~5000 cm2/AS)

Typical shape and dimensions of an accelerating structure disk

6

1. Field ionization studies resulted in a lowering of the vacuum threshold for fast ion beam instability to: pressure < 1 nTorr [G. Rumulo]

2. Non heating of the RF systems to guaranty the high precision assembly

3. Vibration free vacuum system for the quad stabilization

Requirements

Internal volume of an accelerating structureCourtesy of D. Gudkov

C. Garion, LCWS 2011 4/21

Vacuum dynamics for a non-baked system

Elements of theory

Non baked system: Main molecular specie is water sticking probability and sojourn time are not negligible (whereas for a baked system the time of flight is the most important parameter)

RT

Eexp0

10-13 s Temperature

Activation energy of desorption

Usually, vacuum technical surfaces exhibits a wide range of binding energy (distribution density).

Sticking probability depends also the sticking factor and also on the coverage.

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

0 100 200 300 400 500 600

tem

ps [

s]

T[K]

CO/CO2

organics

water

H2 chem

Courtesy of M. Taborelli

C. Garion, LCWS 2011 5/21

Vacuum dynamics for a non-baked system

Elements of theory

For the design of a vacuum system the outgassing rate is usually used. For an non baked system, a simplified evolution law is used:

lim1 qh

qqh

From an engineering point of view:

D. Edwards Jr. Journal of Vacuum Science and Tech.,14(1977)606

q [mbar.l/s/cm2] ~ 2.10-9 / t[h] (valid for all metals)

C. Garion, LCWS 2011 6/21

Technological solutions for the vacuum pumping system in the CLIC module

Courtesy of A. Samoshkin

External pumps on a central tank

C. Garion, LCWS 2011 7/21

Technological solutions for the vacuum pumping system in the CLIC module

Courtesy of D. Gudkov

Small NEG pump connected to main RF components

Compact pumps

Vacuum tank

Tank Support

C. Garion, LCWS 2011 8/21

Vacuum tests and simulations of the vacuum system in the CLIC module

Tkqt

Tcp

2

2

x

Pca

t

PS

Heat transfer equation:

Gas flow equation (1D):

Dedicated accelerating structure

Vacuum model

C. Garion, LCWS 2011 9/21

Vacuum tests and simulations of the vacuum system in the CLIC module

Solution 1: Test set-up

Cap with Indium sealing

Penning Gauge

Vacuum manifolds (25*28, 30*30 mm2)

Penning Gauge

Pirani Gauge

Insulation valve Turbomolecular pump

C. Garion, LCWS 2011 10/21

Good agreement between the measurements and the simulation Non significant influence of manifold size7.10-9 mbar is reached after 100 hours of

pumping

Vacuum tests and simulations of the vacuum system in the CLIC module

Solution 1: Test results

Evolution of pressure

Pressure field after 100h of pumping

Pressure profile along beam axis after 100h of pumping

C. Garion, LCWS 2011 11/21

Nextorr pump from SAES

NEG cartridge

Ion pump

Vacuum manifold

Penning GaugeInsulation valve

Solution 2: Test set-up

Vacuum tests and simulations of the vacuum system in the CLIC module

Compact NEG and ion pump

Turbomolecular pump

C. Garion, LCWS 2011 12/21

Solution 2: Test results

Vacuum tests and simulations of the vacuum system in the CLIC module

Good agreement between the measurements and the simulation3.10-9 mbar is reached after 100 hours of pumping

Evolution of pressure

Pressure field after 100h of pumping

C. Garion, LCWS 2011 13/21

Extension of the vacuum model to the CLIC module

Static pressure field

Vacuum model of the module (Not up to date version ) for the solution 1

Pressure profile for the main beam

Pressure profile for the drive beam

C. Garion, LCWS 2011 14/21

1. Full vacuum model of the module with parameters as close as possible to present baseline

2. Installation of a Residual Gas Analyzer:• influence of methane and noble gases

• Optimization of ion pumps : number and location

3. Influence of the SiC damping loads:• increase the gas load

• reduce the vacuum conductance

4. Influence of compact loads

5. Influence of venting duration, venting gas

Technological solutions for the vacuum pumping system in the CLIC module

Next steps

C. Garion, LCWS 2011 15/21

Technological solutions for the vacuum pumping system in the CLIC module

Comparison of the two solutions

Criteria Central tank Small NEG cartridges

Vacuum performances - +

Vacuum forces - +

Integration in the module - +

Procurement/assembly - +

Control - +

Cost (hardware) + -

C. Garion, LCWS 2011 16/21

Dynamic vacuum in the accelerating structures

Different vacuum inside the PETS and the accelerating structures can be considered:• Static: pressure after pump down without RF power and beams • Dynamic: during breakdown• Dynamic: during RF pulses without breakdown

S. Calatroni et al.

C. Garion, LCWS 2011 17/21

Assumptions: – 1012 H2 molecules released during a breakdown [Calatroni et al.] – Gas is at room temperature (conservative)

Requirement: Pressure<10-9 mbar 20ms after breakdownMonte Carlo simulation or thermal analogy

Dynamic vacuum in the accelerating structures

Longitudinal distribution as a function of time

Vacuum degradation remains localized close to the breakdown and seems not to be an issue.

Maximum pressure during time

C. Garion, LCWS 2011 18/21

Vacuum in the accelerating structures with RF

Principle: Field emission leading to electron stimulated desorption and/or to local heating

Estimation of dynamic pressure:1. Dark current simulations from SLAC2. ESD data on unbaked copper at high e- energy from CERN

[Pasquino, Calatroni]3. Introduce these into MC models and get gas distribution, with

reasonable assumptions on molecule’s speed

Direct measurement: feasibility study on going [K. Osterberg]

C. Garion, LCWS 2011 19/21

Vacuum in the accelerating structures with RF

1E11 1E12 1E13 1E14 1E15 1E161E-4

1E-3

0.01

0.1

1

De

s Y

ield

(#

mo

l/e-)

Dose (e-/cm2)

H2 CH4 H2O N2/CO C2H6 CO2

ESD @ 10 kV

1E15 1E161E-4

1E-3

0.01

0.1

De

s Y

ield

(m

ol/e

-)

Dose (e-/cm2)

H2 CH4 H2O N2/CO C2H6 CO2

PCV082C @ 10 kV

Influence of Dose on ESD

Conditioning after ~1014 e-/cm2

Decrease of desorption yield with 1/dose0.5

Study on the influence of the heat treatment on going

C. Garion, LCWS 2011 20/21

Vacuum in the accelerating structures with RF

Influence of e- energy on ESD

Increase up to 8 keV, then constant (or even small decrease) Study on the influence of the heat treatment on going

0 2 4 6 8 10 12 14

1E-3

0.01

0.1

De

s Y

ield

(m

ol/e

-)

Energy (keV)

H2 CH4 H2O N2/CO C2H6 CO2

19_PCV082C

5 10 15 20

1E-3

0.01

0.1

De

s Y

ield

(m

ol/e

-)

Energy (keV)

H2 CH4 H2O N2/CO C2H6 CO2

19_PCV082C, Dose: 2.7E15

54_SSH104C

C. Garion, LCWS 2011 21/21

Conclusions

Test set-up and vacuum models have been developed to study the static vacuum in the CLIC two beam module.

A good agreement between the measurements and the estimations is achieved.

Two technological solutions have been (and are being) tested. The solution based on NEG cartridge combined with a small ion pump is promising.

Dynamic vacuum study is on going.