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
to an ambient temperature Ta ~300 K ≡ 27°C, one finds Ts = (
1072°C.
4 ′′ r / + Ta
) 1/4 = 1345K =
q εσ
Even if one factored in a small reduction due to the contribution of natural convection, steady
state temperatures of this magnitude are too high for the stainless steel chamber and for the
adjacent permanent magnet material. The MPS system will detect such an errant beam condition
and rate-limit the beam
In addition to stainless steel, three other potential magnet vacuum chamber materials were
analyzed. Aluminum was found to comfortably withstand the consequences of single rf-bunch
hits; but for continuous exposure to a 1.8 kW beam, the steady state temperature exceeds the
melting point. OFE copper was found to be able to take individual bunch hits but was marginal
for repeated exposure at the same location; i.e., single bunch thermal stresses were modestly
above the endurance limit. Continuous beam exposure resulted in steady state temperatures near
the melting point, and this material is not suitable for a vacuum chamber. Early detection of errant
beam would remove this handicap for both copper and aluminum. A proposed ceramic (Al2O3)
vacuum chamber concept has also been analyzed. Single pulse temperature and stress rises were
modest, and the steady state temperature was well within the capability of this material. When
compared to stainless steel, neither of these materials was cost effective; and the ceramic chamber
presented additional engineering challenges.
In summary, the proposed stainless steel vacuum chamber can comfortably accept missteered
beam pulses inside the undulator and will not suffer any damage for σ ≥ 32 µm and Pav = 1.8 kW.
However, a continuous beam exposure must be detected and beam delivery terminated before
significant temperature increases in the chamber and adjacent magnetic material occur.
8.8.1.6 Adjustable Collimators to Protect Undulator and Vacuum Chamber
The analysis of various collimator concepts resulted in selection of a jaw design with multiple
materials. For many reasons, such as fabrication, water-cooling, compactness, etc., it is still
highly desirable to use copper as the primary power absorber material. For fully annealed OFE
copper to withstand the exposure to a very large number of pulses, the cyclic thermal stresses
should not exceed 3.45 to 4.15 × 10 7 N/m 2 (5000 to 6000 psi). Working backwards, the effective
transverse beam size for an assumed Gaussian distributed beam should be σ ≳ 50 µm at the beam
entrance face of a copper collimator jaw where
( ) −
Π e = 1. But the transverse beam size at that
location is only σ ~ 38 µm. To guarantee long term survival of the copper, the transverse beam
size must be increased. Using a spoiler of a lower Z material with appropriate mechanical
properties is a simple and passive method of achieving this goal. In the past a titanium alloy (Ti-
6Al-4V) has been successfully used for this purpose. Modeling with the Monte Carlo code EGS
resulted in a minimum spoiler thickness required of ~0.3 Xo. This will protect the front part of the
copper near the interface with the titanium. However, the region of the highest power density and
therefore temperature rise for the beam energies of interest and for copper is at a depth of ~3 Xo.
Since there is significant shower multiplicity to that depth without a commensurate transverse
spread of the beam, the minimum size of the beam needs to increase to σ ~160 µm at the front
U N D U L A T O R ♦ 8-45