LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

L C L S C O N C E P T U A L D E S I G N R E P O R T
Two effects, namely the geometric wall wakefield and the resistive wall wakefield, can negatively
influence beam performance. The 6 mm OD × 5 mm ID stainless steel tubing needs to have a
small surface roughness. Various roughness-reducing procedures have been investigated. The
semiconductor industry has an ongoing need for ultra pure gas transport systems, and much R&D
effort was invested in recent years to satisfy these demands. Present state-of-the-art technology
identified fully austenitic stainless steel Type 316-L with very stringent chemical composition
limits as a successful starting material. It is extruded into seamless tubing with special high
quality dies, to great straightness, and under very clean conditions. The tubing is then cleaned and
electropolished in lengths up to 6 m. Tubing with a surface roughness of Ra ~125 nm (rms) is
readily available off the shelf and for modest cost. Flat surfaces are readily electropolished to Ra
~50 nm, and such a value might also be achievable inside a tube with more R&D effort. Another
quantity in surface roughness evaluation is the total peak-to-valley depth of roughness, Rt or Rmax,
which varies from 4 to 12 times the Ra value.
An R&D program is underway at SLAC/SSRL to use such high quality industrial tubing as
starting material and then improve the surface quality to Ra < 100 nm (rms) by further
electropolishing. The best results to date from small samples (12 to 15 mm long × 5 to 6 mm
wide) cut out at random locations from 0.5 m long tubing sections showed Ra values from a best
of 10 nm to the 50 nm (rms) range. The azimuthal values were always somewhat lower than the
longitudinal ones. The measurements were made with an atomic force microscope and distances
traversed with the stylus were ~55 µm azimuthally and ~70 µm longitudinally. It appears that the
surface quality of the commercial products can be significantly improved to achieve values of
surface roughness that will not negatively impact the performance of the LCLS undulator.
Electro-polishing 3.4 m long tubing sections will, however, present additional challenges.
Since stainless steel has a high electrical resistivity, the resistive wall wake is increased. To
reduce this effect, a thin layer of copper (> skin depth) is deposited onto the highly electro-
polished surface using a thin nickel substrate for better adhesion. This process is then followed by
further electro-polishing the copper surface. Best values achieved to date are Ra ~120 nm (rms),
but further refinements in the process are possible and should make the surface inside the
undulator vacuum chamber smooth enough that it does not materially degrade the electron beam.
8.8.2 Vacuum System Requirements and Description
The vacuum system for the undulator must have the following attributes to provide a good
environment for the electron and photon beams:
• The vacuum system components must provide low impedance to avoid producing beam
instabilities and minimize higher-order mode heating.
• Adequate pumping must be provided to maintain a low beamline pressure (