New Device Developments
ELETTRA is developing an electromagnetic elliptical wiggler in collaboration
with BESSY and MAXlab, capable of switching the radiation helicity at
up to 100 Hz. The novel configuration is a double electromagnet with an open-sided
structure. Further elliptical undulators and also a mini-gap in-vacuum undulator
are being considered in conjunction with new beamline proposals that are presently
being evaluated. The ESRF are working on two new prototypes, a 20 Hz pulsed
helical device and a fast pulsed undulator which will provide radiation only
from a single bunch when the machine is operated in "hybrid" mode.
The ESRF also reported on recent tests of an in-vacuum undulator built by SPRING-8.
The device was operated down to 5 mm gap, but was limited by problems caused
by heating of the steel sheet covering the magnets.
Effect of Small ID Gaps
The "first-order" effect of reducing the insertion device gap, and
hence the beam-stay-clear, is a reduction in lifetime due to the elastic gas
scattering term. In this respect a low vertical beta function is clearly an
advantage; at the ESRF for example, the new low vertical beta means that a minimum
beam stay clear of 5 mm can now be accepted instead of 7 mm for the same 20%
reduction in beam lifetime. For smaller gaps, instabilities and r.f. heating
become important and so the resistivity of the chamber becomes an issue. At
the SRS a new wiggler chamber with grooves for the poles is made from Titanium.
The cost of the material is comparable to stainless steel, it is easily welded
and maintains its shape. At the ESRF the resistive wall impedance has increased
steadily due to the installation of ID vessels; further increase will be avoided
however by the use of stainless steel vessels with a copper plating. At ELETTRA
no resistive wall effects have so far been seen, a possible explanation being
that the natural tune spread is sufficient to Landau damp the instability. The
design and use of small gap devices must also take into account shims and tolerances
which become a significant fraction of the gap.
Effect of Helical Devices on Beam Dynamics
As a general topic, the effects of elliptical devices on non-linear beam dynamics
seems not to have received a lot of attention. Studies performed by ELETTRA
and BESSY on the elliptical wiggler shows significant non-linear effects, and
work is in progress to find a pole configuration to increase the dynamic aperture.
To aid in the interpretation of dynamic aperture calculations in general, it
was felt that more comparisons between prediction and measurement would be useful.
Optimum Lattice Functions
Various approaches to the definition of optimum beta functions were discussed.
For brightness optimization for example, it was pointed out if there is a large
mismatch between the photon and electron beam emittance then the resulting effective
emittance is less sensitive to the beta value. It was also pointed out that
a proper analysis would need to take into account the energy spread. The benefits
of a large beta and positive alpha in the ID straights in order to produce a
small beam size at the sample was illustrated for the case of the ESRF. Such
a solution cannot be applied to all straights however since some beamlines already
include horizontal focusing. There are implications also for the number of independent
quadrupoles in the insertion straight. For SOLEIL the beam sizes and divergences
at 25 m from the source were calculated for various photon energies, undulator
lengths and couplings as a function of the beta function. It was concluded that
a beta function in excess of 10 m is optimum, the dependence being more pronounced
in the horizontal plane.
Radiation Damage
No examples of radiation damage of insertion devices during normal operation
were reported. As a precaution, all devices are opened at injection in ELETTRA.
At the ESRF, only the IDs in the first two high beta straights after injection
were opened. prepared by C.J. Bocchetta and R.P.
Walker