Superconducting HERA dipole magnet.

ALPS II (as well as ALPS I) will be operating with long superconducting dipole magnets of the HERA accelerator. These 8.8-meter magnets are designed to be operated at the temperature of about 4K, which requires a powerful cryogenic supply to be able to operate 2*100 m magnet string.

The achievable power buildup of an optical cavity inside a string of dipoles depends on the available aperture of the vacuum pipe in the dipoles. The diameter of the circulating photon beam near the focusing mirror will become larger with increasing length of the optical resonator. Therefore the clipping losses in the resonator will increase with length, limiting the number of dipoles per string for a given aperture.

The inner diameter of the vacuum pipe in the magnets is 55 mm. However, due to the curvature of the dipoles built for HERA, the free horizontal aperture is reduced to ~ 35mm. In order to increase a horizontal aperture of the curved magnets a simple method was developed to straighten the cold mass (iron yoke) of the HERA dipoles yielding an aperture of 50mm, which allowing for a 2*10 dipole setup for ALPS II. The straightening of the yoke and thus the beam pipe is achieved by a brute force deformation from the outer vacuum vessel at the 3 planes of support of the dipole.

The figure shows the position of the center of the beam pipe before and after the application of the deforming forces. As the deformation of the yoke is elastic, the deforming force has to be maintained during operation at cryogenic temperatures. Therefore pressure props were designed, which keep the thermal flux from the vacuum vessel at room temperature to the yoke at liquid Helium temperature within acceptable limits. The props at the ends of the dipole must allow for the length change of the yoke during cool down and warmup and yet maintain the deforming pressure.

The HERA dipole, which was used for the ALPS I experiment has been straightened on the test bench and was operated continuously for 30 hours at the design current of 5700 A for ALPS II corresponding to 5.3 Tesla.

For further information about the ALPS optical system please contact:

Dieter Trines
Contact person for: Magnets
Institute: DESY
Tel.: +49-40-8998-3149
email: Dieter.Trines(at)