Update of the vacuum system for the CLIC two-beam modules€¦ · C. Garion, LCWS 2011 3/21 Vacuum...
Transcript of Update of the vacuum system for the CLIC two-beam modules€¦ · C. Garion, LCWS 2011 3/21 Vacuum...
Update of the vacuum system for
the CLIC two-beam modules
C. Garion CERN/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
Specificities
1. 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 structure
Courtesy 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 q
h
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 size
7.10-9 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 Gauge Insulation 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 simulation
3.10-9 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 breakdown
Monte 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 SLAC
2. 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 1E16
1E-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 1E16
1E-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.