Vacuum Systems for Synchrotron Light Sources
- PDF / 4,855,385 Bytes
- 7 Pages / 604.8 x 806.4 pts Page_size
- 99 Downloads / 236 Views
and applied research in such fields as solid state physics, biology, chemistry, surface science, and technology. The électron storage ring must provide an ultrahigh vacuum environment for the électron beam to minimize électron residual gas collision which would
shorten the beam Jifetime. This article will discuss the design of électron storage ring vacuum Systems and materials, and how the choice of materials can affect the machine design. A typical électron storage ring is shown in Figure 1. It consists of an injector (linac and booster), transport System, storage rings, and expérimental photon beam Unes. Thèse machines vary in size from a few meters in circumference for a compact light source used for x-ray lithography, to a few hundred meters in circumference for high energy physics. Vacuum System and Chamber Design The vacuum System for an électron storage ring is an all-metal ultrahigh vacuum System. The operating pressure is in the low 10"9 torr range with stored électron beam, and 10 10 torr without beam. Certain unique vacuum problems must be faced in électron storage ring
X-RAY STORAGE NNG
UOKT SOURCE
BROOKHAVEN
AVENUE
Figure 3. Electron storage ring at the National Synchrotron Light Source, Brookhaven National Labomtory.
MRS BULLETIN/JULY1990
35
Vacuum Systems for Synchrotron Light
design: photon-stimulated gas desorption, power dissipation in the chamber walls, impédance changes due to changes in the chamber cross section, conductance limitations, accurate placement of the chamber, and ail of those sundry problems associated with a large bakeable all-metal UHV System. Some of thèse characteristics are illustrated schematically in Figure 2. Two excellent papers that address many of thèse issues hâve been written by N . Mistiy 1 (system design) and H. Wiedeman 2 (impédances and instabilities). Gas Desorption The ultimate pressure (P) in a static vacuum System of constant p u m p i n g speed (S) and constant gas load (Q) is determined by the ratio Q/S; i.e., P = Q/S, where P is in torr, Q in torr liter/s, and S in liter/s. If there are no leaks, the gas load Q is the product of the surface area times a thermal gas desorption coefficient (q) which differs for each material and varies with time and température. In an électron storage ring, photons that strike the chamber walls desorb large numbers of gas molécules (PSD) which must be added to the thermal gas desorption above. In fact, the PSD is generally of such a magnitude that it completely overshadows the thermal gas desorption and détermines the system pressure. Andritschky et al. 3 and Grobner et al. 4 are authors of just two of a number of papers which discuss this phenomenon with quantitative results. Figure 3 is a curve showing the PSD yield in terms of molecules/photon versus accumulated photons/meter, that is photon dose per area. It lists values for aluminum, stainless steel, and copper. Materials of Construction The most commonly used materials for light source vacuum chambers are aluminum, stainless steel, or a combination of the two. W
Data Loading...
