The straightening procedure
The straightening of the magnet and the vacuum pipe, to increase the horizontal aperture of the beam tube for the optical resonators, was performed on the test bench. It was achieved by fixing the position of the ‘cold mass’ at the outer suspensions of the magnet and pushing (and thus deforming) the cold mass in the middle (see Fig. 8). The cold mass was fixed in position by inserting and tightening Titanium pressure props (see Figs. 9 and 16) between the cold mass and the vacuum vesselFootnote 6. By a transportation fixture (see Fig. 10) on the opposite side of the cryostat, the cold mass was prevented from moving, while tightening a pressure propFootnote 7.
The deformation in the middle was achieved by a steel screw (‘pressure screw’) (see Fig. 11) exerting a force of about 40 kN, inserted between the vacuum tank and the Helium vessel at the lower flange of the vacuum vessel.
When the aimed for deformation was achieved, another Titanium pressure prop was inserted (see Fig. 12) from the upper flange above the pressure screw. After tightening the pressure prop, the pressure screw and the transportation fixtures were removed. With all three pressure props installed and tightened, the deformation of the yoke is maintained by the tension of the yoke, counteracted by the vacuum tank via the pressure props. Figure 13 shows the cryostat with three Titanium pressure props inserted before the removal of the transportation fixtures.
The ends of the beam tube cannot be straightened independently, as no force can be applied beyond the outer suspension planes. The ends just rotate around the fix points, given by the outer pressure props, due to the bending of the beam tube in the middle (see Fig. 8). The middle of the cold mass was bent in a way, to force the beam tube to develop two ‘camel humps’ (see Fig. 8 and also Fig. 15). This deformation yields the largest achievable horizontal apertureFootnote 8. In the figure the deformation is exaggerated for better illustration.
The pressure prop in the middle (see “The pressure props” section) of the cryostat constitutes a fix point for the thermal shrinkage/expansion during cool down/warm up of the cold mass.
Finite element calculations, determining the additional stress on the Helium vessel by the straightening procedure, were performed [25] and presented to the agency for pressure vessel safety (TUEV). The calculations showed that all stresses are well below the limits set by pressure vessel regulations.
There is a detailed report on the work required for the straightening [26], partially presented on the PATRAS workshop 2018 [27].
The survey of the vacuum pipe
The position of the center of the beam pipe before, during, and after the deformation was measured by the DESY survey group with a laser tracker and a so-called mouse with a reflector attached. The mouse was pulled by a string through the vacuum pipe along the length of the magnet, while the laser tracker continuously measured the position of the reflector (see Fig. 14) through an open flange in the middle of the end box of the test bench.
A typical result of straightening a dipole vacuum tube is shown in Fig. 15, in comparison with the original curved shape of the vacuum pipe. The success of the deformation was judged by the horizontal aperture achieved.
The result of the survey of the beam pipe, i.e. the position of the beam pipe center line after straightening, was transferred to marks, welded to the outside of the vacuum vessel. When setting up the straight magnet strings for ALPS II in the HERA tunnel, these survey marks will allow alignment of the dipoles to yield the largest possible overall horizontal aperture within the strings.
The curvature of the beam pipe before the straightening slightly varied among the dipoles. The maximum deviation from the straight line connecting the ends of the beam tube (17.9 mm in the example shown in Fig. 15) varied by ±2.5 mm between the extremes of 16 and 21 mm. The average was 18.4 mm. There is a correlation between this maximum deviation and the achieved aperture after the straightening procedure. In general, the larger the achieved final aperture, the smaller is the deviation from a straight line connecting the ends of the original beam tube.
The beam tubes also showed deviations from the horizontal plane, i.e. in the vertical direction by ±1 to 2 mm, both before and after straightening. These deviations reduce the vertical aperture from 55.3 mm (diameter of the beam tube) to 51.3 mm at most. This value equals the largest horizontal aperture obtained by the straightening procedure (see “Results” section) and does therefore not deteriorate the performance of the optical resonators.
During one cryogenic test, the mouse with reflector was installed into the vacuum pipe in the middle of a dipole, which was on the test bench at the time, to monitor the position of the vacuum pipe after cool down of the dipole. The position of the reflector in the cold vacuum pipe could be measured with the laser tracker through a quartz window on a flange in the end box of the test bench.
The measurement showed that the horizontal aperture increase, achieved by the deformation of the cold mass, is reduced by the cool down of the magnet by about 0.5 millimeter, caused by the thermal shrinkage of the pressure props and the cold mass. The impact on the performance of the two 120 m long optical resonators for ALPS II is tolerable (see “Results” section).
To check whether the straightening of the dipole would suffer from transportation between DESY and the HERA hall East for installation into the tunnel, a straightened dipole (BR 221) was put on a transport trailer and driven from the DESY site on public roads to the HERA hall East and back. Then the center line of the beam tube was measured again. It was identical to the one measured before the transportation.
The pressure props
As the deformation of the yoke is elastic, the deforming force by the pressure props must be maintained, also at cryogenic temperatures. However, the pressure props constitute a thermal short between the Helium vessel at 4K and the vacuum vessel at room temperature. To keep the additional heat flow to the 4K level of the cryogenics supply as low as possible, the props were made from a thin walled (1.5 mm) Titanium tube (see Figs. 9, 10, 12, and 16), a material with low thermal conductivity, comparably low thermal expansion, and large mechanical strength. In addition, the contact area at both temperature levels is small for the props at the outer suspensions, as the ends of the props form sections of a sphere. The thermal flux from room temperature to the yoke at liquid Helium temperature was estimated to about 1 Watt per prop from the thermal conductivity of the thin walled Titanium tube (see “Results” section).
The props near the ends of the dipole must allow for the length change of the yoke with respect to the vacuum vessel (≈ 30 mm total) during cool down and warm up and yet maintain the deforming force. Shaping the ends of the props as sections of a sphere, allows for a tilt of the props during cool down or warm up (see Fig. 16), without changing the distance between the vacuum vessel and the cold massFootnote 9 except for the thermal shrinkage of the props and the cold mass (see above).
The deforming forces are always perpendicular to the surfaces of the support cups, (see Figs. 16 and 9) resting on the Helium and the vacuum vessel, thus reducing the momentum on the prop and the danger of it tipping over. The support cups of the outer pressure props at the vacuum vessel stay fixed, while the cups on the Helium vessel move with the shrinkage/expansion of the Helium vessel.
The Titanium tube of the pressure prop in the middle was screwed into a stainless steel strip (5 mm thick and 50 mm wide), which matches the inner surface of the vacuum vessel (see Fig. 12). The steel strip remained in the cryostat after straightening the yoke. The middle pressure prop has a flat contact area to the Helium vessel and does not tilt during thermal cycles. It thus constitutes a fix point for the thermal shrinkage/expansion during cool down/warm up of the cold mass.
To place the pressure props at the right position and angle within the cryostat, special mounting tools were developed (see Fig. 17 and also Fig. 12). After positioning the prop, the assembly tool was removed. It should be noted, that all props are finally held in position inside the cryostat only by the spring pressure between the vacuum vessel and the Helium vessel.
The proper choice of materials and the concept were validated by a test of an outer pressure prop in vacuum at liquid Nitrogen temperature. A force of 40 kN was exerted on the prop in a test device while moving one side back and forth by ±15 mm several hundred times. No problem with the prop was encountered.
The new suspensions
The insertion of pressure props at any of the three positions, foreseen in the midplane of the magnet, was impossible with the original suspensions in place. The radiation shield box, connected to the suspension, was blocking the insertion of a prop (see Figs. 18 and 10). Therefore the original suspensions of the cold mass with the radiation shield box had to be removed from the cryostat for the insertion of a pressure prop.
In the beginnings of the straightening studies, which were performed on a defective dipole used as an exhibit, all three suspensions were removed at the same time. The cold mass was still safely suspended by the remaining three suspensions. However, the yoke bent vertically by several millimeters, causing concerns over an undesirable reduction of aperture.
Therefore, in the final procedure, applied to all straightened dipoles, only one suspension was removed at a time, according to the following sequence: First, one of the outer suspensions was removed, then the corresponding pressure prop was installed. After a modification of the suspension (see below) it was reinstalled, now carrying again the weight of the cold mass. Then followed the same steps for the other outer suspension. Finally, the suspension in the middle was removed and modified. After the straightening of the magnet, the middle suspension was reinstalled in the same way as the other suspensions. The vertical deformation was small for this procedure (a few tenth of a millimeter) and could easily be restored by the new suspensions (see below).
Obviously, an installed pressure prop prevented the re-installation of the original suspension. Therefore a slit was machined into the radiation shield box, to allow for the re-installation of the suspension, when the pressure prop was in place. Also the G10 loops, supporting the cold mass, were replaced by an open structure, realized by Titanium strips (10 mm wide and 1 mm thick) to pass by the inserted pressure prop (see Fig. 18). The thermal flow to 4K due to the modified suspension was increased by 0.3 Watt per suspension i.e. 0.9 Watt per dipole (see “Results” section).
The free length of the support screws (see Fig. 18 right) at the top of the suspensions were measured before their removal from the cryostat. Before removing the shield box from an original suspension, the positions of the ears – for the fastening screws to the radiation shield – were marked on a special tool, while the upper part of the G10 loop had tight contact with the transfer pivot of the tool (see Fig. 19 left). After machining, the shield box was aligned in the device to the marks, before fastening it to the new suspension, positioned by the transfer pivot of the tool. These measures ensured, that the ears of the radiation shield box were at the same position on the new suspension as on the original one with respect to the support screw.
After the new suspension was inserted into the cryostat, the shield box was screwed to the radiation shield. An especially tailored package of super insulation foils, sliding over the pressure prop, was attached to the shield box. Then the length of the support screw on top of the suspension was adjusted to its original – before measured – value, ensuring that the new suspension – and thus the radiation shield – was at the same position as the original one. Finally a support bracket was fastened by nuts to the suspension. The bracket, now carrying the cold mass (see Fig. 19 right), was pushed up by nuts with only little force against the support pivot of the cold mass. The nuts were secured by a counter nut.
Finally the lower end of the suspension and the support pivot were shielded against thermal radiation by an Aluminium cap, covered with super insulation foil and attached to the radiation shield by two screws.