Outgassing%20of%20Stainless%20Steel%20Vacuum%20Chambers%20and%20The%20Vacuum%20Pumping%20Speed%20and%20Capacity%20Evaluation%20of%20a%20Titanium%20Sublimation%20Pump - PowerPoint PPT Presentation

Title: Outgassing%20of%20Stainless%20Steel%20Vacuum%20Chambers%20and%20The%20Vacuum%20Pumping%20Speed%20and%20Capacity%20Evaluation%20of%20a%20Titanium%20Sublimation%20Pump


1
Outgassing of Stainless Steel Vacuum Chambers
andThe Vacuum Pumping Speed and Capacity
Evaluation of a Titanium Sublimation Pump

• Christian Hammill, Wayne State University
• Dr. Yulin Li Dr. Xianghong Liu, Cornell
  University

2
My Project at a glance
3
Contents

• Outgassing of stainless steel chambers
• Introduction and purpose
• What is the goal of this project?
• Methodology
• What methods are used to determine ?
• What do these methods involve?
• What is the difference in these methods?
• Results
• Conclusions

• Vacuum characteristics of the TiSP
• Introduction and purpose
• What is a TiSP?
• What do we want to determine from the TiSP?
• Methodology
• How do we determine Q(t) and STi(t)?
• Results
• Conclusions

4
SST Outgassing Introduction

• XHV must be achieved in parts of ERL (i.e.
  photo-chamber)
• Stainless Steel outgassing hinders this
• When put under vacuum
• 400C bakeout reduces outgassing
• Does this last forever?!
• My mission do stainless steel chambers that have
  been stored properly in N2 for 8 months keep low
  outgassing property?

5
SST Outgassing Methodology I

• Vacuum system comprised of two parts
• Testing Chamber
• Enclosed by oven
• Heating gun/ air blower
• Water cooling coils/ heating tape
• SRG/ sample chamber

• Sensor Chamber
• Valve to Testing Chamber
• RGA
• CCG
• Ion Pump Turbo Pump

6
SST Outgassing Methodology II

• Two methods of determining
• Rate of Rise method
• Where V is the volume of the chamber (28L) and As
  is inner surface area of the sample chamber (7500
  cm2)
• Throughput method
• Where is Sip the pumping speed of the ion pump
  and As is inner surface area of the sample
  chamber
• ?P (pressure increase due to outgassing) is
  explained later

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SST Outgassing Methodology III

• Both methods follow a similar procedure
• Set up and leak check
• All vacuum flanges are connected and properly
  tightened
• Then, entire vacuum system is pumped down and
  checked for gas leaks
• Bakeout
• The adsorbed water molecules (from air exposure)
  are eliminated via temperatures gt120C
• Measurement
• is determined using either ROR method or
  throughput
• Repeat
• Baking temperature is increased each time (150C
  to 200C to 250C)

8
The Rate of Rise Method

• In RoR measurements, the sample chamber is closed
  off from any pumping
• This allows gas to accumulate in the closed
  system
• With a constant outgassing rate, , a linear
  pressure rise is expected
• The rate of rise in the pressure, , is
  measured to calculate such that

9
The Spinning Rotor Gauge

• SRG consists of a magnetized rotor ball in a
  gimble tube, and a removable head that contains
  sets of coils
• The Rotor is magnetically levitated and spinning
  
• Molecules in the chamber collide with the rotor,
  ever slightly slowing down the rotor, the
  molecular drag
• The SRG measures the pressure by measuring the
  slowing down rate ofthe rotor due to the
  molecular drag

10
Throughput method

• Used as crosscheck for RoR method
• The pressure at the CCG is recorded while the
  sample chamber is still closed off from the
  sensor chamber gt baseline pressure
• The chamber is then opened and the pressure is
  left to settle then recorded
• The difference between the settled pressure and
  the baseline pressure is known as the change in
  pressure, ?P
• Then, using the pumping speed of the ion pump,
  Sip (9L/s), we calculate

11
A Typical RoR Result
Pressure measured by SRG clearly show a linear
rise in time. The fitted slope dP/dt
2x10-12 Torr/s.
Very stable temperature control ?T 0.17 C
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A Typical Throughput Result
Sip is determined via a pump down of sample
chamber
Fitting (blue) curve, P(t)
Gas composition at point A
In this case, ?P1.5x10-11 Torr, Sip 9 L/s
therefore, 1.9x10-14 TorrLs-1cm-2
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Outgassing Results Summary
Current project data
Tbake(C) (10-15 TorrLs-1cm-2) (10-15 TorrLs-1cm-2)
Tbake(C) Rate of rise Throughput
120 2.6 30
120 6.0 19
120 7.5 n/a
150 25 n/a
150 18 n/a
150 15 n/a
200 6.7 15
200 6.7 28
250 9.3 35
250 7.5 38
Data from 8 months ago
TBake (C) (10-15 TorrLs1cm-2)
150 19.0
200 16.0
250 14.0
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SST Outgassing Conclusions

• The outgassing rates measured in this study are
  comparable with the results from 8 months ago.
• This indicates that the extremely low outgassing
  property can be maintained.
• The comparison between the rate of rise method
  and the throughput results is very good, when one
  considers at least two factors
• The measurements were done at very different
  pressure ranges (10-6 torr for rate of rise, and
  10-10 torr for the throughput).
• They both use very different gauges (SRG vs.
  CCG).
• Therefore, in certain places in the ERL (i.e. the
  photo-cathode) one can vent out the vacuum,
  properly store the stainless steel components in
  N2, and not damage the outgassing properties of
  these components

15
TiSP Performance Introduction

• After measuring the properties of the electron
  beam, it must be safely terminated at the beam
  dump in the Cornell Proto-type Photo-cathode
  Injector
• Very large gas loads of H2 gas are generated at
  the beam dump when this happens
• A large TiSP will be used together with two large
  ion pumps to control the H2 gas load
• Will this do the trick?
• My mission evaluate the pumping performance
  (particularly the pumping speed and the pumping
  capacity) of the TiSP to determine if it is
  suitable for this application

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TiSP Performance methodology I

• Experimental Setup
• The test system was leak checked and baked at
  170C
• Ultra-high purity hydrogen is used to measure the
  TiSP pumping performance
• The flow-rate of H2 was experimentally determined
  before any Ti-sublimation,
• After Ti-sublimation, H2 flows through the TiSP
  chamber, and the pressures are monitored by two
  cold cathode gauges

17
TiSP performance methodology II

• Ti Sublimation
• There are 3 Ti cartidges on the end of the
  chamber
• Each filament is heated via voltage power source
• This heating causes the Ti to sublimate in the
  chamber
• This process is known as flashing
• The Ti is flashed in the chamber for a certain
  period of time at a certain power

Ti cartrige with 3 filaments
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Determining STi(t)

• After flashing, we open the H2 valve again and
  let the gas flow
• Because the Ti layer inside the chamber is so
  reactive, it will capture much of the gas as it
  enters the chamber
• This causes the pressure to decrease at the CCG
• At the CCG there are now 2 pumping speeds
• The effective speed of the turbo, , and
  the pumping speed of the chamber, STi(t).
• Knowing this, we derive the formula for the
  pumping speed of the chamber over time
• ? this
  follows the Equation, SP

19
Determination of Q(t)

• We want to determine the total amount of H2 gas
  pumped away by the TiSP chamber, Q(t).
• As the Ti film pumps more and more H2 is pumped
  away it becomes saturated with H2
• As it becomes more saturated, its pumping speed
  decreases
• The accumulated Q(t) is directly related to
  STi(t) such that

20
TiSP Performance Results I

• STi(t) was measured at 2 flashing settings
• (A) 3 minutes at 170W
• (B) 5 minutes at 195W
• Higher Ti-flashing power longer duration gt
  much thicker Ti layer
• The STi(t)s are plotted against the Q(t)s
• Limited Ti flashing and H2 saturation cycles were
  done
• Thick, H2 rich, Ti films are known to be flaky

21
TiSP Performance Results II

• Q(t) is arbitrary and depends on its application.
  
• Here, we chose Q(t) to be the point where STi(t)
  drops to 100L/s
• (i.e. 10 of its initial pumping speed).
• With this definition, one sees from that
• QA 4 TorrL
• QB 25 TorrL
• With maximum pumping speeds of
• STi, A(t) 900L/s STi, B(t) 1200L/s
• As a side note, it took 80 hours for B to
  saturate from the beginning to 100L/s

Hydrogen Pumping by Ti
Step 1 Dissociative adsorption
H2 ? 2Hads
Step 2 Bulk diffusion
Hads ? HBulk
Step 1 depends on surface reactivity of Ti
film Step 2 depends on Ti film thickness and
solubility
22
TiSP Performance Conclusions

• It can be seen that the max STi 1200 L/s
  that it is capable of pumping out 25 TorrL of
  H2 gas.
• The estimated gas load at the beam dump (A5
  section of the Cornell Proto-type Photo-cathode
  Injector) can be as high as 1.8x10-3 TorrL/s.
• Considering that two other ion pumps will be
  sharing half of the load, the TiSP will have to
  handle a gas load of 9.0x10-4 TorrL/s.
• With a little math, we can see that the TiSP can
  go 8 hours before needing to go through a
  flashing cycle again.
• This is considered to be sufficient for the
  injector operations
• Therefore the TiSP is suitable for the ERL
  prototype project

23
Acknowledgements

• Thanks are due to
• My mentors Dr. Yulin Li and Dr. Xianghong Liu
  who have assisted me in the writing of this
  paper, the assembling of the experiments, and the
  overall guidance though this project.
• The lab technical staff Tim Giles, Tobey Moore,
  and Brent Johnson.
• Dr. Rich Galik and Dr. Claude Pruneau who are the
  assemblers of this REU project and made it able
  for me to attend the summer here.
• The National Science Foundation who every year
  make it possible for students much like myself to
  have experiences like this one.