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Vacuum System
Sources & accelerators Contents > Accelerators > Vacuum System
Storage Ring Ultra Vacuum System

The vacuum system was designed to ensure an average pressure of 10-9 mbar with a beam of 500 mA. The main characteristics are the following ones:

 

    The conventional pumping system is composed of 180 triode ion pumps and 100 Titanium sublimation pumps. The total pumping speed on the ring is 55000 l/s.

  • In addition, NEG coating of the vacuum vessels is used to reduce the initial conditioning time of the vaccum system thanks to its low desorption rate:
    - The coating is deposited on all the straight vacuum vessels of the storage ring (56% of the circumference):
    quadrupole/sextupole type vacuum vessels and Insertion devices
    - The vacuum chamber material is extruded aluminium.
    - The NEG material is a Ti-Zr-V alloy deposited by magnetron sputtering
    - The activation is made by heating the chamber at 180°C for 24 hours (during bake out of the vacum system)

  • Materials:
    -All the straight parts like vaccum vessels in the multipole magnet and in the insertion devices are made from extruded aluminium tube (Aluminium grade is 6060 T6)
    -All the special vacuum vessels like for bending magnets or bellows and BPM are made of stainless steel 316L or 316 LN.

 
 
 
View of one cell installed in the storage ring 

Design of the quadrupole type vacuum vessels

Pumping ports made in packing aluminium are brazed on the main tube. The Conflat flanges are made with an aluminium-stainless steel bi-metallic material. The aluminium part is brazed on the tube and the stainless steel part bears the gasket that will ensure tightness. 

 
 
   
Detailed view of water connections First of series of the quadrupole type vacuum vessels


The Bending magnet vacuum vessel

Because of the lattice compactness, the vacuum chamber is stretching from the dipole to the quadrupole and to the sextupole coming downstream.
The bending magnet part is made by machining stainless steel sheets in two half shells brazed in the medium plane with electron beam.

The multipole magnet part is made by folding plates then brazing the different parts with electron beam.

 
 

The final assembling of the chamber was made with a specific bench allowing to locate each element regards to the reference located at the support foot level. The assembling of two adjacent girders (the dipole lays on the edge of each girder) allowed sucessful validation of all the vacuum chamber magnet interfaces.

 
Assembling of dipole vacuum chamber

Vacuum vessels supports are designed to ensure:

- Stability of vacuum system during operation of the ring
- Realignment of the vacuum system during realignment operation of the girders in order to avoid any contact between
magnets and vacuum vessels
- Vacuum vessels movements during bake out.

   
     
   
Different types of supports

Bake out System

Vacuum system must be baked at 180°C for 24 hours for the activation of the NEG.
As the clearance between magnets and vacuum vessels must been kept a system of thin Kapton foils (thickness <0.4mm) with printed circuits have been developed. It supplies 0.6 W/cm².

The connections and thermocouples are far from the magnet poles. During the bake out, water circulates in the magnet coils.

 
 
Heaters on dipole and quadrupole vacuum vessels

Power dissipation
The total power radiated in the bending magnets is 472 kW (14.4 kW in each bending magnet). Only few percents are delivered to the beamlines. The remaining power has to be stopped inside the vacuum vessels. A set of absorbers has been designed to prevent critical area from direct irradiation which could cause failure of the vacuum system.
 
 
 Section of the dipole vacuum vessel showing the crotch, the second absorber and the longitudinal absorber

 
   
3D view of the crotch absorber Power distribution on the crotch absorber

 
Temperature distribution on the crotch absorber Stress distribution on the crotch absorber
 
 
First of series of the crotch abosrber


Impedance of the vacuum vessel
 Image current, circulating in the walls of the vacuum vessels, generates electromagnetic fields which could affect the electron beam stability. In order to minimise this effect, the impedance of the vacuum vessel has been minimised by making smooth transition from one cross-section to another.
Two examples are given below:
- BPM - Bellows module
- Gap between Conflat flanges.
 
RF shield of the bellows

This design has been used successfully at SLS and at another recent synchrotron light sources facilities.

 
BPM-bellows module


Flange design with RF shield

Impedance calculation shows that the first design (0.2 mm<gap<0.4 mm) leads to a low instability threshold. With RF shield at the level of the two horizontal faces of the cross section, the maximum size of the remaining cavity is
0.4mm x 1mm and the resulting impedance is acceptable.

 


Resistive wall impedance
  • Calculations have been performed to evaluate the effect of the NEG on the resistive wall effect:
    - Critical parameters are:

    • Thickness of the coating

    • Roughness of the coating

  • Nevertheless, the thickness must be kept large enough to reach the expected behaviour regarding the photon stimulated desorption.
    - Non uniform thickness distribution

 
Thickness distribution


Roughness of the coating
  • The final roughness depends on the initial roughness of the substrate: Ra=0.3µm RMS
  • The roughness of the coating depends on the conditions of the film growth. The substrate roughness has been kept after coating.
 
NEG Coating on the first test chamber (SAES Getter)
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