Vacuum System

Presented by Dong HaiyiAccelerator Center,IHEP,China

April 27, 2006

Contents1. Characteristics of BEPCII Vacuum System2. Vacuum Chamber3. TiN Coating of Vacuum Chambers4. RF Shielding Bellows5. Photon Absorbers6. Installation and Leak detection of Vacuum Syste

m7. In-vacuum wiggler8. Summary

1.Characteristics of BEPCII Vacuum System

To meet the need of beam lifetimes more than 10 hours, the dynamic pressure is lower than 510-9Torr;

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;

The main parameters of vacuum system for BEPCII storage rings

BEPCII Parameter BEPC

Electron ring Positron ring Energy [GeV] 2.8 1.89 Current [A] 0.132 0.91 Circumference of ring [m] 240.4 237.5 Dipole magnet radius [m] 10.35 10.35(O), 9.15(I) Bending magnet field [T] 0.9028 0.7293 SR critical energy [keV] 4.7 1.64 Total SR power [kW] 69.38 112.31 SR linear density [kW/m] 1.07 1.73(O), 1.95(I) Desorption coefficient 210-6 210-6 Total SR gas load [TorrL/s] 1.7910-5 8.3210-5 Linear SR gas load [TorrL/sm] 2.7510-7 1.4510-6 Pumping speed of DIP[L/sm] 110 Dynamic pressure [Torr] 310-9 510-9 Beam stay clear[mmmm][HV] 10543 Aperture of arc chamber [mmmm][HV]

12058 10852 10852

Distribution of the vacuum devices in a quadrant

2.Vacuum Chamber Aluminum alloy 5083-H321 is chosen as the

arc vacuum chamber material. In the straight sections, the vacuum chamber

s 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.

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.

Half of Arc Chamber

Leak Detecting and Baking Test for Antechambers

Pumping Down for

Antechamber(Thermal Outgassing Rate: 3.310-13Torr·I/s·cm2, Ultimate Pressure : 510-11Torr)

Vacuum Testing for Straight Vacuum Chamber (316LN stainless steel)

Vacuum Testing for Straight Vacuum Chamber (Al-6061)

Pumping Slot

2.Vacuum Chamber (cont.)

All the arc chambers(80 units) have been tested, which ultimate pressure is less than 510-10Torr.

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.

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.

The Vacuum Chamber Bakeout Before TiN Coating

Discharge Characteristics of DC sputtering

Cathode Potential:

1250 ～ 1300 V

Process Pressure:

1.5 ～ 1.7×10-1 mbar

Gas Composition:

70% Ar — 30% N2

Current Density:

1.0 ～ 1.1×10-4 A/cm2

Time to Discharge:

6 ～ 8 hours

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.

BEPCII TiN-coated Samples

SEY measured in KEK

SEY of BEPCII TiN-coated samples measured in SLAC

Electron Energy (eV)

SEY

SEY of BEPCII TiN-coated samples measured in Shanghai Fudan University

Max. Secondary Electron Yield of TiN-Coated Chambers

Al 2.7

0

0.5

1

1.5

2

2.5

3

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

Serial Number of vacuum Chambers

SEY

Analysis of TiN Coating Property

The measurement results of BEPCII TiN-coated samples

Ti/N ratio: 1.10-1.19

SEY: <1.95

TiN film thickness: 100-200nm

Coating adhesion: good

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.

Fingers: Shield-Finger + Spring-Finger

Spring-Finger : Inconel alloyContact-Finger: CuBe (C1720)Max. Contraction : 18 mm Max. Expansion : 6mmOffset : ±2 mmBend: ±50 mradTilt: 0

Double-Fingers RF Bellows

(VALQUA LTD.)

“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

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.

Temperature rise of contact fingers as a function of input powers for two kinds of RF shielding Bellows

Peak wall current

RMS wall current

Peak wall current density

RMS wall current density

Peak electric field at wall

58.1 A 6.1 A 355.8 A/m 37.4 A/m 1.34105 V/m

BEPCII

RF Bellows

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.

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 22.5°grazing incidence angle. The rectangular absorber is located near BPM to increase the cutoff frequency.

Max. temperature:192℃ ， Max. temperature on cooled wall:111℃ ，

Max. thermal stress: 85Mpa, Max. distortion: 0.69mm.

Milling V-shape Groove of Absorber

Preparation of Absorber Brazing in the hydrogen furnace

Testing of the hydraulic pressure difference and flow quantity

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.

TSP

NEGP

The pumping system of Half Cell

Rectangular Flanges Connection

7. In-vacuum wiggler One of the main difficulties for fabricating in-vacu

um 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 410-10Torr has been achieved by coating TiN on the permanent-magnet blocks and the reasonable pumpdown techniques.

The Pumping Configuration of In-vacuum Wiggler

In-vacuum Wiggler Bakeout

Pumpdown Process of In-vacuum Wiggler

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 210-10Torr·L/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 510-

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.

8. Summary All the arc vacuum chambers have been fabricated and tested, and Ti

N 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.