Superconducting dipole magnet


Peter

Project leader: 
Peter Senger

p.senger(at)gsi.de

M1

The magnet with the feeder box on its adjustable support structure. 

The CBM superconducting dipole magnet provides a vertical magnetic field with a magnetic field integral of 1 Tm which is needed to obtain a momentum resolution of Δp/p=1 % for track reconstruction at FAIR beam energies. The magnet gap has a height of 147 cm and a width of 330 cm in order to accommodate the STS detector system with a polar angle acceptance of ±25° and a horizontal acceptance of ±30°. The total weight of the magnet is about 160 t. The yoke is equipped with field clamps in order to reduce the stray field to an acceptable level in the area of the RICH-detector.

M2

Vertical cut through the magnet.

Coil structure and cooling 

The magnet consists of a pair of superconducting circular coils and a warm iron yoke with framed shape. Each coil is suspended in a stainless steel vacuum vessel. 

The coil conductor is of the “wire-in-channel” type, i.e. a NbTi strand is embedded in a rectangular copper conductor. The copper to superconductor ratio is 5:1. Each coil requires a conductor with a length of almost 20 km. As only conductors with a maxi-mum length of 6 km can be delivered, each coil winding pack consists of 4 supercon-ducting sections connected with soldered joints. The total number of turns is 4100, the operating current at maximum field is 311 A, the stored energy of the magnet is 5.36 MJ.  

Mag1

Each coil will be indirectly cooled with LHe in a helical tube surrounding the coil. The LHe tube is connected via copper strips to copper plates above and below the coil winding pack. The copper plates are pressed towards the coil winding pack by stain-less steel plates to assure good thermal contact.

Coil support system 

The mechanical support of the coils has to fix the coils and to compensate the Lorentz force equivalent to 300 t, and, at the same time, to prevent heat exchange between the cold mass of the coil (LHe temperature) and the coil cryostat (room temperature). This support is realized by 6 tension rods and 12 bushings. The tension rods fix the coils, while the bushings compensate the pressure of the Lorentz force. The bushings consist of two stainless steel tubes and three G11 (fiberglass laminate) tubes, which are arranged concentrically and inserted into each other. The inner bushing is fixed, whereas the outer bushing is sliding and arranged on bronze sliding plates. This plate is directly connected to the cryostat and the sliding support plate is connected with the outer G11 bushing. The bronze sliding plates allow shrinkage of the coils during cool down and under nominal current. They guide the sliding in radial direction because during cool down the coil will shrink in radial direction. Therefore, the G11 bulk with the support plate can slide on the bronze sliding system in radial direction.

Cryogenic system 

A liquid helium thermosiphon system is used for the cooling of the coils. It consists of a vessel with liquid Helium at a temperature of 4.6 K located inside the feeder box above the two coils. The working principle of a thermosiphon is based on the natural circulation of helium in the system. The flow is created by the density difference be-tween the heated fluid with a gaseous component in the return line and the cold fluid in the supply line. The circulation can be additionally driven by a heater in the return line.

M6 M7

The cryogenic distribution is provided by the branch box, which is connected to the feeder box by a transfer line.

Quench protection and detection 

A passive quench protection system in the form of cold rectifier diodes is foreseen for the CBM magnet coils. Each coil is sub-divided into 4 sections, and each section is protected by 4 pairs of back-to back diodes. This redundant combination of diodes takes into account two failure scenarios of diodes for both current directions: The damaged diode is short-circuited and allows current to flow through, and the other case, the diode no longer allows current to flow. In the case of a quench the power supply’s output is deactivated via its interlock function and the stored energy is dissi-pated within the coils and the diode racks. The quench detection is based on a bal-anced voltage principle with two coils being balanced against one another. Voltage taps located at the inlet and outlet of the coils at the joint between the two coils serve as quench sensing. The quench protection is realized with a system of cold diodes. The diodes are connected to copper blocks inside the so-called feeder box, and are cooled to LHe temperature. The feeder box contains the LHe bath, the current leads, the bus bars, and the cold diodes on copper blocks.

Magn2

Support structure and alignment system 

The magnet support consists of four solid feet made of 30 mm steel sheets. Trape-zoidal threaded spindles are centered in the foot construction, which allows the mag-net to move in the vertical direction. At the upper end of the spindles there are ball sockets carrying an adjustment table, which allows the shift of the magnet in horizon-tal direction on sliding bronze plates. The alignment parameters are as follows: 

  • Alignment range: ± 40 mm 

  • Position accuracy: ± 0.5 mm 

  • Orientation accuracy (roll): ± 0.5 mrad 

  • Rotation about a vertical axis: ± 4° 

  • Horizontal earthquake tolerance: 12% of gravitational acceleration

BIL

The design and construction of the magnet is performed by Bilfinger Nuclear and Energy Transition (BNET, Würzburg) in collaboration with GSI.