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Several Technical Measures to Improve Ultra-High and Extreme-High Vacuum*
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Changkun Dong1, Parixit Meharotra2 and Ganapati Rao Myneni3
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Physics Department, Old Dominion University, Norfolk, VA 23529
2 University of Virginia, Charlottesville, VA 22904
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3 SRF Institute, Jefferson Lab, Newport News, VA 23606
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UHV and XHV pressures are generally achieved by reducing the outgassing rate (hydrogen) of the
stainless steel (ss) chamber walls and by providing large pumping speeds. Titanium nitride coatings
were applied to ss walls to reduce the permeation rate of hydrogen but are not cost effective and so
cannot be widely used. Copper and aluminum thin film coatings have been tried but the coating
process itself introduces severe outgassing problem even though it has the potential of reducing the
hydrogen permeation from the ss walls. Aluminum and Copper alloy (particularly beryllium copper,
which is toxic) chambers have also been built since they have the lowest outgassing rate but have
several disadvantages including the cost.
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1
In this work, we investigated the use of inexpensive coatings of silica and titanium oxide on
UHV/XHV chambers/components to reduce the adsorption/diffusion of water (which cracks into
oxygen and hydrogen in the material and acts as the internal source of hydrogen) through the
chamber walls thereby minimizing hydrogen outgassing. Further, we have also implemented backing
of the turbo pump with an ion pump for reducing the vacuum chamber pumpdown times into
UHV/XHV pressure range. The results of these investigations will be presented in this paper.
*This work supported by U.S DOE contract DE-AC05-84ER401540
Several Technical Measures to Improve Ultra-High and Extreme-High Vacuum
2
3
Experiments
• Improve vacuum with various coatings on stainless steel chamber
- Simple Ti compound, double sides thin/thick silica and outside silica coating
- Comparison test with AFM silver coated BeCu chamber
• Vacuum performance with ion pump backed turbo system
0
RGA
10
20
30
60
70
80
90
Stainless steel
Outgassing rate,
Torr l/s cm2
1.0 x 10-12
1.00E-07
EG
1.00E-10
Experiments from drag pump assisted turbo system
EG
Ti compound
coating
~ 7.75 x 10-13
Qw-v : outgassing and permeation
baking at 180 C
Silica and AMF chambers showed similar performance
- Ultimate pressure of the silica coated chamber is slightly lower
Turbo system operated successfully with ion pump backing
- Ultimate pressure ~ 2x10-10 Torr obtained in a short period (4 hours)
time , min
1.00E-06
SRS RGA
0
500
1000
1500
2000
2500
3000
3500
4000
drag pump off
drag pump on
1.00E-07
7000
8000
no coating
thin double coating
thick double coating
outside coating
Ti coating
1.E-08
1.E-09
baking chamber at ~180 C
1.E-10
pumping comparison of S.S. chamberws with different treaments
Silica thin
coating
~ 3.25 x 10-13
Silica thick
coating
~ 3.0 x 10-13
Silica outside
coating
~ 2.5 x 10-13
Before baking ->
Pumping
Pumping
Gas molecular
Gas molecular
Pumping
Gas molecular
4500
5000
Baking ->
H2 from wall to vacuum,
QV-W : adsorption and permeation
H2 from vacuum to wall,
Ql
Pumping
Pumping
Gas molecular
Gas molecular
baking at ~ 180 C
1.00E-10
System performance is better without the drag pump backup
-- Pressure is ~ 45% lower without drag pump
Pumping
Gas molecular
After baking ->
: leakage H2.
After system baking:
Pumping
Pumping
Gas molecular
No coating
No coating: Qw-v > Qv-w Both sides coating: Qw-v ≈ Qv-w
Gas molecular
Out coating: Qw-v < Qv-w
PH2 ‌ no coating > PH2 ‌ both coating > PH2 ‌ out coating
•Outside coated chamber presented best vacuum
• Silica coating better than AMF chamber
• Vacuum improved by a simple Ti compound coating
• Turbo system operated successfully with ion pump backing
Acknowledgment: Varian Vacuum Technologies, Pfeiffer Vacuum
Pumping
Gas molecular
Bothside coating Outer coating
1.00E-09
M D 4T
Diaphragm
pump
6000
RGA measurement,
then turn-off drag pump
Summary
1.00E-08
TM U071P
Turbo
TC600
Drag Pump
5000
QH2, ext = Qw-v – Qv-w + Ql
Varian
Starcell
Ion Pump
Tested
Chamber
EG
4000
1.E-07
+ P’1H2 / KH2
1.00E-08
1.00E-09
Start-up Varian
PTS300
Scroll Pump
3000
Outgassing rates with various chamber treatments
Material
PH2 = (QH2 / SH2)ext + (QH2 / SH2)int
AMF chamber
silico chamber
Varian V70
Turbo
2000
For partial pressure, like H2,
100
1.00E-06
Tested
Chamber
1000
Proposed pumping models for surface treatments
Pumping of silica coating and AMF chambers
time , h ou r
40
50
time, min
0
1.E-06
pressure, Torr
1
Changkun Dong , Parixit Mehrotra , Ganapati R. Myneni
In this work, we investigated the
1 Physics Department, Old Dominion University, Norfolk, VA 23529
use of inexpensive coatings on
2 University of Virginia, Charlottesville, VA 22904
UHV/XHV Components with
3 SRF Institute, Jefferson Lab, Newport News, VA 23606
silica and titanium oxide films
to reduce the adsorption/diffusion
of water which acts as the internal source of hydrogen through the chamber walls.
Pumping performance improved
Further, we implemented backing of the turbo pump with an ion pump for reducing
after surface treatments
the vacuum chamber pump down times.
- Ultimate vacuum more than 65%
*Ganapati Myneni - CEBAF Invention Disclosure: Low Cost Surface Diffusion
lower with surface silica coatings
Barrier for UHV/XHV Components and Chambers
- Outside coated chamber presented lowest
pressure: vacuum improved over 70%