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Der Supraleitende Asymmetrische Wiggler - SAW

To satisfy the present need for the generation of intense circular polarized synchrotron radiation in the x-ray part of the spectrum a 5.3 Tesla superconducting asymmetric multipole wiggler with a semi-cold vacuum chamber was designed at DELTA and manufactured by the German company ACCEL. Its delivery was in January '98. A special coil arrangement consisting of NbTi-wires allows two operation modes:
  • symmetrical with a sine-like field of 10 periods with a peak field of 2.79 Tesla.
  • asymmetrical with 5 periods and a peak value of 5.3 Tesla.

The critical energy for the circular polarized photons is about 4.1 keV for the symmetrical mode and 7.9 keV for the asymmetrical state.

The order for the wiggler was placed in February 1995. First field investigations took place in October 1997. Both required peak values were aimed. These results were obtained by the pulsed wire method, which was used succesfully at DELTA during the field measuring of the FEL undulator. The first field integral was determined with a short rectangular pulse. The corresponding magnetic field was proportional to the derived signal. To get an exact measurement of the field integral at the end of the wiggler, a periodical rectangular pulse (t=1.3 ms for the asym. case and t=0.65 ms for the sym. mode) was used. A step-pulse was needed to get the second field integral. The first and second field integral were trimmed to zero accurately to avoid an effect on the electron trajectory at the end of the wiggler and to cancel out a (horizontal) shift of the ideal closed orbit.

The wiggler reached all values in the scope of the design parameters. Its behaviour in the case of a quench, that means the responding of the quench protection system, was tested and fulfilled the technical specifications. The exhaust steam rate amounted to 201 l/h gaseous helium, corresponding 194 mW. Liquid helium has to be refilled at least after three weeks under running conditions. The evaporated helium gas will be confined and liquified again with a refrigerator at the department of physics at the University of Dortmund. In order to suppress the heat intrusion coming from current carried by the coils, especially when the wiggler is excited, the coils are cooled with evaporated helium gas which is forced to circulate by a pump.

Before installation at the end of this year, some quadrupoles have to be converted. Two quadrupole families will be installed in the lower symmetric point of DELTA and together with seven additional quadrupole power supplies one has the flexibility of new optics being concerned with the wiggler. The wiggler has a strong influence on the optics, mainly focussing properties on the beam and due to its small vacuum chamber (10 mm full gap), there are vertical aperture limits (i.e. a small vertical beta-function) to fulfill. A first proposal for an optic for the state of a built in wiggler without a magnetic field was done within the scope of a diploma thesis. The focussing and the behaviour of the wiggler on electron dynamics have already been examined. The dynamic aperture was calculated and is sufficient (not decreasing the lifetime). Before installation, it is planned to test the designed optics in order to ensure safe machine operation. Therefore, scrapers in the west side of the ring are used to measure the cross- section of the beam. Furthermore, at the place of the wiggler the beam line will be contracted vertically to simulate the narrow part of the wiggler gap during injection. Until spring 1999 the photon beam line will be built up.

The following table summarizes the most characteristic features of the SAW:


sym. Mode

asym. Mode

max. peak field

2.69 T

5.3 T


14.4 cm

28.8 cm

number of periods






critical photonenergy

4.1 keV

7.9  keV

opening angle of the SR

± 13 mrad

± 25 mrad

first field integral

-0.4 Tmm

1.6 Tmm

second field integral

-700 Tmm²

1310 Tmm²