ALPS

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Magnets

Any Light Particle Search

Magnets

Strong superconducting dipoles are available for the setup of ALPS II from the 6.3 km long lepton-proton-collider ring HERA. Some of these dipoles will be used to set up two long straight strings of magnets for the ALPS II experiment in a straight section of the HERA tunnel. The geometry of the tunnel allows the installation of two straight strings with 12 HERA dipoles, each.

A superconductiong HERA dipole magnet, for the ALPS II experiment, about to be installed into the HERA Tunnel

Schematic view of the HERA dipole. 1 Helium vessel containing cold mass, 2 Suspension, 3 Radiation shield, 4 Vacuum vessel, 5 Helium pipes.

A laser beam will be injected into the magnetic field of a string of superconducting dipole magnets to produce axion like particles. After passing a light tight wall, the ALPs can reconvert into photons in a second string of HERA s.c. dipoles. The sensitivity of the experiment will be increased by two mode-matched optical cavities before and behind the wall.

However, the aperture and the performance of the optical resonators would be limited by the curvature of the magnets from HERA, optimized for the storage of protons, beyond a certain length. As the sensitivity of the search scales with the length of the magnetic field, the aperture for the optical resonators was increased by straightening the curved magnet yoke.

Schematics of straightening. Left: Before applying the deforming force, Right: The deformation forces the pipe to develop two 'camel humps,' exaggerated in the figure for better illustration. This deformation yields the largest achievable horizontal aperture.

A simple method was developed to straighten the cold mass (iron yoke) yielding an aperture of about 50mm (the inner diameter of the beam pipe is 55.3 mm), allowing for a 2*12 dipole setup for ALPS II matching the length allowed by the tunnel geometry. The straightening of the yoke and thus the beam pipe was achieved by a deformation of the yoke at the 3 planes of support of the dipole.

The figure below 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 (two near the ends of a dipole -indicated by arrows in the figure- and one in the middle) 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 near 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.

All dipoles were operated on a test bench up to their quench current twice. In addition all dipoles were operated continuously at the nominal operating current of the ALPS II experiment for about 8 hours. All dipoles have quench currents well above the nominal operating current of 5963 A.

Finally, we succeeded to obtain dipoles with sufficiently large horizontal apertures and sufficiently large quench currents for two strings of 12 dipoles each, plus two spares.