4th Annual Workshop on European Synchrotron Light Sources

Trieste, November, 18th/19th 1996

Session 7: Insertion Devices

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