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SuShi septum is an acronym for Superconducting Shield septum. The project is a collaborative effort between CERN and Wigner Research Centre for Physics (Budapest, Hungary) to evaluate the feasibility of using a superconducting shield to create a high field extraction septum magnet for high-energy accelerators.

A series of videos displaying the winding and assembly of the magnet

The research group is grateful for the support of
- the SM18 and the Magnetic Measurements teams (CERN)
- CERN TE-ABT group
- FCC Study group
and receives funding from
- the European Commission under the FP7 Research Infrastructures project EUCARD-2, grant agreement no. 312453
- the European Union’s Horizon 2020 research and innovation programme (ARIES) under grant agreement No 730871
- the FCC Study group
- the Hungarian National Research, Development and Innovation Office under grants #K124945 and 2019-2.1.6-NEMZ_KI-2019-00008
- the Ministry of Innovation and Technology of Hungary from National Research, Development and Innovation Fund under Contract TKP-17-1/PALY2020

     

Particle colliders store two beams of charged particles rotating in opposite directions. The two beams cross each other at well defined positions (the interaction points) along the ring, where they are focused to a very small size in order to increase the probability of collisions. At other locations in the ring the two beams are separated. The operation of these rings is typically cyclic.

In the first step the injector accelerator (which might be either a linear accelerator, or another accelerator ring) fills the ring (in both directions)

The two beams are then accelerated to the nominal energy of the ring.

In the collision phase the energy of the particles is kept constant and the two beams are crossed at the interaction points. The physics experiments register the particles produced in the collisions. This phase is about 10 hours in case of the LHC.

When the intensity of the circulating beams, and hence the collision rate has decreased below a certain limit, the beams are dumped from the ring, and the cycle starts from the first step again.

The energy stored in the circulating beams can be enormous. For the LHC it is equivalent to the kinetic energy of a TGV fast train running at a speed of 150 km/h (see other examples). For the Future Circular Collider the stored energy (8.4 GJ) corresponds to the kinetic energy of 24 TGV fast trains. Since these beams have a typical size below a millimeter, their energy is extremely destructive even when their intensity is too low for the physics experiments, and can easily destroy the machine itself if control is lost. Therefore the beams must also be dumped in case of the indication of any failure in the system.

The beam extraction system is illustrated in Fig. 1 below. When the ring is filled, a particle free abort gap is created in the fill pattern. A fast pulsed kicker magnet system gives an initial, typically small kick to the beam. Even though these magnets reach their nominal field very quickly (typically on the μs scale), during their risetime they would sweep the beam downstream, therefore their trigger is synchronized with the passage of the abort gap through the kickers.

Figure 1: Schematic illustration of the extraction scheme from a high-energy accelerator

After some distance the small angle of the kicked beam leads to a spatial separation from the circulating trajectory. The beam then runs into a special septum magnet, which creates a high field at the location of the extracted beam, and zero field at the location of the circulating beam. The high field of this device deviates the extracted beam further, which then enters the dump beamline.

The beam power of the LHC is so large that it would destroy any material if it impacts at a single point. This is even more true for the FCC: its beam would penetrate through about 300 m of copper, evaporating the material on its way. The beam energy therefore must be distributed over a large surface on the beam dump. This is realized by a system of magnets in the dump line, the dilution kickers, which sweep the beam along a spiral pattern on the surface of the beam dump block.

When a conductor material is exposed to changing external magnetic fields, eddy currents are induced on its surface in such a way that their magnetic fields try to compensate the change of the external field inside the material. This is Lenz's law. For example if a conductor material is initially in zero magnetic field, and it is then exposed to a quickly rising external field, the eddy currents will try to keep the field level at zero inside the material. Due to the finite resistivity of usual materials these currents quickly decay, and the magnetic field will finally penetrate the material with some delay.

Superconductors are characterized by zero resistance. This means that the eddy currents will have very long lifetimes (days or weeks). If a superconductor was cooled down to the superconducting state in zero external field, a ramped-up field will be excluded from the material even if the ramp is slow, and the field is then kept at a high level for a long time. Since the eddy currents try to oppose any change of the magnetic field, the superconductor must be cooled to the superconducting state in zero external field, otherwise it would trap the initial field.

This phenomenon is not to be confused with Meissner's effect, in which a superconductor material expels the magnetic field upon its phase transition to the superconducting state even if cooled down in a non-zero external field. This effect only works up to a few mT field level, which is insufficient for our purpose.

Figure 1: Schematic illustration of the principle of the CCT-SuShi septum. Blue arrows indicate the magnetic field in the midplane. Color indicates the magnitude of the shielding currents on the surface of the shield.

As explained in the "Beam extraction" section, a septum magnet must create a high magnetic field at the location of the extracted beam, and zero field at the location of the circulating beam. This can be realized by installing a superconducting shield around the circulating beam, and an external excitation magnet which produces a high magnetic field, as illustrated in Fig. 1.

In this concept the homogeneous magnetic field near the shield is created by a canted cosine theta-like superconducting magnet. This magnet configuration is an old idea which is rediscovered recently and the subject of many active R&D projects.

3D animation (click here if you are equipped with a virtual reality set)
Keybindings in third person view: Left drag - move; Right drag - rotate; (Shift)Scroll - Zoom;
Keybindings in first person view: WASD,Space,Shift - Move; Left drag - look around; Scroll - Change movement speed;
3D CAD model
3D simulation of the coil and magnetic field in the presence of the passive superconducting shield
3D geometry of the exotic winding
Field pattern (lines) and current distribution (color) in the 2D cross section
Test winding
Test winding: improved layer jump insert
Test winding: improved layer jump insert and glass-fiber tape
Participation in the winding of the 2.2 m LHC corrector magnet at CERN
Yoke lamination on the wire EDM machine (Camilleon Ltd)
Current lead guide in G10
Joint box in glass-fiber reinforced peek
Layer jump insert in glass-fiber reinforced peek
Lock ring in G10
Machining of the formers
The formers
Close-up of the grooves in the formers
The support tube
Split saw-tooth clamps to support the shield inside the aperture
✉barna.daniel@wigner.hu
⇋Wigner Research Centre for Physics, H-1121 Budapest, Konkoly-Thege Miklós ú 29-33, Hungary
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