Vacuum System

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Vacuum System

• Presented by Dong Haiyi
• Accelerator Center,IHEP,China
• April 27, 2006

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Contents

• Characteristics of BEPCII Vacuum System
• Vacuum Chamber
• TiN Coating of Vacuum Chambers
• RF Shielding Bellows
• Photon Absorbers
• Installation and Leak detection of Vacuum System
• In-vacuum wiggler
• Summary

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1.Characteristics of BEPCII Vacuum System

• To meet the need of beam lifetimes more than 10
  hours, the dynamic pressure is lower than
  5?10-9Torr
• To reduce the interaction of photoelectrons and
  secondary electrons with positron beam, the
  chambers of positron ring will be made into
  antechambers and coated by Titanium nitride on
  its inner surface
• Copper absorbers and masks are used to prevent
  synchrotron radiation from falling on vacuum
  chambers,bellows and welding

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Distribution of the vacuum devices in a quadrant
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2.Vacuum Chamber

• Aluminum alloy 5083-H321 is chosen as the arc
  vacuum chamber material.
• In the straight sections, the vacuum chambers
  are fabricated from 316LN stainless steel,
  Al-alloy(Al-5083-H321) and Al-alloy(Al-6061).
• Standard stainless-steel conflat flanges with
  copper gaskets are used due to its reliability.

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Half of Arc Chamber
The vacuum chamber consists of the upper and
lower pieces. The two pieces made by an oil-less
machining process to improve the contour
precision and reduce the outgassing rate, are
welded together.
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Leak Detecting and Baking Test for Antechambers
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Pumping Down for Antechamber

(Thermal Outgassing Rate 3.3?10-13TorrI/scm2,
Ultimate Pressure 5?10-11Torr)
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Vacuum Testing for Straight Vacuum Chamber (316LN
stainless steel)
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Vacuum Testing for Straight Vacuum Chamber
(Al-6061)
Pumping Slot
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2.Vacuum Chamber (cont.)

• All the arc chambers(80 units) have been tested,
  which ultimate pressure is less than 5?10-10Torr.
• 10/40 straight chambers(s.s.), 19/25 straight
  chambers(Al-6061) and 16/52 straight
  chamber(Al-5083-H321) have been pumped down and
  reached the design pressure.

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3.TiN Coating of Vacuum Chambers

• The inner surface of the positron ring vacuum
  chambers will be coated with ?100 nm of Titanium
  Nitride (TiN) in order to minimize the secondary
  electron yield (SEY) and thus avoid the so-called
  electron clouds instability caused by electron
  multipacting.
• TiN coating of the positron vacuum chambers has
  been done by using DC sputtering, and all the
  arc chambers(40 units) of positron ring have been
  coated TiN, TiN-coating of the straight chambers
  is being prepared.

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The Vacuum Chamber Bakeout Before TiN Coating
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Discharge Characteristics of DC sputtering
Cathode Potential 12501300 V Process
Pressure 1.51.710-1 mbar Gas Composition 70
Ar 30 N2 Current Density 1.01.110-4 A/cm2
Time to Discharge 6 8 hours
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BEPCII TiN-coated Samples
Color is due to three factors  Thickness,
stoichiometry, and structure.  Films deposited
at low energy tend to have porous columnar
structure, which traps light and water.  These
appear dark gray or black. This is not
necessarily a problem, except that the film may
be somewhat gassy.  Gold color does not guarantee
low yield.
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SEY measured in KEK
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SEY of BEPCII TiN-coated samples measured in SLAC
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SEY
Electron Energy (eV)
SEY of BEPCII TiN-coated samples measured in
Shanghai Fudan University
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Analysis of TiN Coating Property
The measurement results of BEPCII TiN-coated
samples Ti/N ratio 1.10-1.19 SEY lt1.95 TiN
film thickness 100-200nm Coating adhesion good
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4. RF Shielding Bellows

• All the RF bellows including 9 kinds of types
  have been sent to IHEP.
• There has been a leak detection for every RF
  bellows, only one of 180 RF bellows had a leak.
• Heating tests for RF bellow prototypes have been
  done by using microwave power.
• Expansion and contraction of RF bellows,
  hydraulic pressure test of cooling channels have
  also been done for a part of RF bellows.
• Four RF bellows have been connected to vacuum
  chambers in situ.

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Double-Fingers RF Bellows
(VALQUA LTD.)

• Fingers Shield-Finger
• Spring-Finger
• Spring-Finger Inconel alloy
• Contact-Finger CuBe (C1720)
• Max. Contraction 18 mm
• Max. Expansion 6mm
• Offset 2 mm
• Bend 50 mrad
• Tilt 0

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PLS type RF Bellows
(AnZ Corp.)

• 1.Structurally strong and flexible (offset 5
  mm)
• 2. Functionally stable and reliable 3. Smooth
  inner surface 4. Low broad band
  impedance 5. Long operation

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Setup of Heating Test for RF shielding Bellows
We carried out the heating test using a 199.5 MHz
microwave which are used for the BEPC RF cavities
. The TEM mode of the coaxial transmission line
is very suitable for the test because the wall
current has only an axial component, in
principle, just like a real wall current.
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Temperature rise of contact fingers as a function
of input powers for two kinds of RF shielding
Bellows
BEPCII
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RF Bellows
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5. Photon Absorber

• The absorbers are made of oxygen free copper and
  have a direct cooling water system.
• According to the shapes of the cross section, the
  absorbers had been divided into 10 serials which
  include 49 kinds of types.
• The hydraulic pressure difference and flow
  quantity is measured for one of every kind of
  type absorber.
• There are the hydraulic pressure test and leak
  detection for every absorber.
• 132/175 absorbers have been produced , most of
  which have been installed inside the vacuum
  chambers.

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Photon Absorbers
As there will be the dedicated SR mode and the
colliding mode in double rings, the absorbers
will be designed with two faces to accept the
synchrotron radiation produced by the electron
and the positron, respectively. In addition, in
order to reduce the synchrotron radiation density
of impinging on the face of the absorbers, the
face is sloped at 22.5grazing incidence angle.
The rectangular absorber is located near BPM to
increase the cutoff frequency.
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Max. temperature192?,Max. temperature on cooled
wall111?, Max. thermal stress 85Mpa, Max.
distortion 0.69mm.
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Milling V-shape Groove of Absorber
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Preparation of Absorber Brazing in the hydrogen
furnace
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Testing of the hydraulic pressure difference and
flow quantity
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6. Installation and Leak detection of Vacuum
System

• 70/80 arc vacuum chambers have been installed
  with ion pumps, NEG pumps, TSP,BPM and photon
  absorbers.
• All the components on the arc chamber have been
  detected for leak and preserved in a vacuum
  condition.
• The rectangular conflat flanges with copper
  gasket have been connected in situ.

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TSP
The pumping system of Half Cell
NEGP
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Rectangular Flanges Connection
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7. In-vacuum wiggler

• One of the main difficulties for fabricating
  in-vacuum wiggler is how to achieve the
  ultra-high vacuum in the situation of the large
  outgassing rates produced by the permanent-magnet
  blocks.
• The static pressure of 4?10-10Torr has been
  achieved by coating TiN on the permanent-magnet
  blocks and the reasonable pumpdown techniques.

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The Pumping Configuration of In-vacuum Wiggler
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In-vacuum Wiggler Bakeout
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Pumpdown Process of In-vacuum Wiggler

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Overall Vacuum Quality Control

• Each components that will be installed in vacuum
  system are cleaned strictly and detected for
  leak, which leak rate is less than
  2?10-10TorrL/s.
• The special vacuum devices such as injection
  kickers, RF superconducting cavities and beam
  monitors, have been tested due to vacuum
  requirements, and a pressure of less than
  5?10-10Torr has been achieved.
• A detailed installation procedure of vacuum
  devices has been established, especially be
  careful of anything that did not belong to the
  vacuum system were left inside the vacuum
  chambers.

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8. Summary

• All the arc vacuum chambers have been fabricated
  and tested, and TiN coating of the arc chambers
  had also been completed. SEY of all the
  TiN-coating samples is less than 2.0. The mass
  production of straight vacuum chambers is still
  critical path.
• All kinds of vacuum pumps, gauges, valves and RF
  bellows have been sent to IHEP and tested.
• Most of photon absorbers have been produced and
  tested, the progress of fabrication of absorbers
  may match with that of installation of vacuum
  chambers.
• Installation of vacuum system is smooth, 70/80
  arc vacuum chambers have been installed with
  pumps,BPM and photon absorbers. 4 RF bellows and
  6 rectangular flanges have been assemblied in
  situ.