CERN Accelerating science

Towards an FCC-hh Detector Magnet System

by Erwin Bielert, on behalf of the FCC-hh detector magnet working group

The Future Circular Collider (FCC) study is well on its way, and will culminate in the publication of the Conceptual Design Report (CDR) early 2019. The most ambitious proposed future machine is a hadron-hadron accelerator (FCC-hh) of 100 km in circumference foreseeing proton-proton collisions at a center of mass energy of 100 TeV. This collision energy is about 7 times higher than the presently largest and most powerful machine, the LHC, can deliver. This factor 7 has a huge impact on the design of the accelerator, as new technologies are required, as well as the exploration of a new realm in dimensions. Besides the collider, the increased energy also has a significant impact on the design of the experiments for measuring tracks and momenta of the particles created during the collisions. This impact is not only on the design of the various detectors for tracking and calorimetry and their readout, but also on the magnet system as the key ingredient for bending charged particles, thereby identifying their charge and momentum. The superconducting detector magnets have to provide a higher magnetic field over a larger tracking distance and this thus implies much larger detector dimensions, meaning a new challenge for the magnet designers! 

Recent developments 

Last year, Tom Taylor presented a nice historic overview in the CERN Courier about  superconducting magnets and their technological development. In addition Akira Yamamoto  reviewed the main steps in the development of superconducting detector magnets at the 2017  European Conference on Applied Superconductivity in Geneva, see the EP-newsletter. Both refer  to a proposed “twin solenoid” magnet system for the FCC-hh general-purpose experiment. More  details of this design by Matthias Mentink can be found in a 2014 EP-newsletter. However, the  crystallization process of the baseline detector went on, as presented in several publications.  Most recently, the FCC-hh detector magnet working group provided an entertaining overview of the evolution of the design towards a smaller, simpler and especially more cost-effective  solution. The result is the new CDR-ready baseline detector and its magnet system shown in  Figure 1, comprising a 20 meter long central solenoid with 10 m free bore, delivering 4 T in the  center at the interaction point. It is accompanied by two smaller solenoids at either end covering  the forward directions and delivering 3.2 T on beam axis in a 5 m free bore across 4 m.  

Figure 1. FCC-hh detector overview (left) and superconducting magnet system cold masses (right).  

Baseline and options 

Since the main solenoid is the largest single part of the detector system, installation scenarios  have to be developed properly. The diameter of the main shaft to the experimental cavern needs  to have a minimum size to allow for the lowering of the solenoid with axis vertical where after it  is turned 90 degree to its operational position. Also the cranes need to be able to handle at least  2.5 kt, to safely deal with the heavy magnet system components and calorimeter units as well.  Installation steps discussed in detail during the FCC week 2017 in Berlin have not been  significantly changed since. 

Besides delivering 4 tesla magnetic field in the very large volume, the magnet system has to operate reliably and inherently safely. Like for any other superconducting system, not only a proper magnetic performance must be demonstrated but also electrically, thermally and mechanically the design has to be sound. The stored energy over the cold mass is 12 kJ/kg, which is comparable  to the CMS solenoid today. However, the 12.5 GJ stored energy of the much larger FCC-hh system  is unprecedented for a detector.

Large scale magnets like this one make use of active as well as  passive quench protection meaning that part of the energy is extracted from the system and the  remaining energy is absorbed by the cold mass itself. Therefore, the cold mass mainly consists of aluminium alloy rather than superconducting cable. To spread the energy as homogenous as possible, quench heaters are foreseen in both the main solenoid and in the forward solenoids.  When the system is working properly, 73% of the energy is extracted and the peak temperature  in the main coil will be at 60 K level, while the forward solenoids can reach 90 K. Failing quench heaters will result in much higher, but still safe temperatures of about 130-140 K. 

Optional for the baseline is replacing the forward solenoids by forward dipole coils. For particles created during the collisions in IP, moving in a direction with a very small angle relative to the beam, the magnetic field is almost parallel to the trajectory and therefore the Lorentz force acting on these particles very small. The result is almost straight trajectories for which it is very difficult to measure the particle’s momentum. This is also the main reason why solenoids are usually used in collider experiments: for most of the angles, the field is reasonably perpendicular to the  trajectories of the particles created during the collisions.

For high pseudo rapidity particles flying  at low angle in the forward direction, more parallel to the particle beam, a forward dipole magnet may be an interesting option despite their obvious technical disadvantages. One drawback is that a forward dipole magnet breaks the rotationally symmetric system, and causes a large torque additional to the already huge forces, and it has negative impact on the particle beam as well that would require corrections. The system of cold masses with this option on is shown in Figure 2, alongside the impact of a magnetic dipole field on the integrated magnetic field along the trajectories for several pseudo-rapidities (eta). 

Figure 2. Superconducting cold mass of main solenoid accompanied by two forward dipole coils  (left) and the impact forward dipole coils would have on the integrated magnetic field as a  function of distance along the trajectory for several pseudo-rapidities. 

 

A novel design for which targeted R&D is required is also part of the overall investigation for FCC detector magnets. Extremely thin in terms of material build and radiation transparent coils would allow for advanced experiments. Since magnetic field is not required in the volume of the calorimeters,  but in the inner trackers only, ideally the solenoid would be placed in between inner tracker and calorimeters. The argument against is that energy measurements get disturbed by the presence of too much material used to build the magnet cold mass and cryostat. So the challenge is to develop a next generation of ultra-thin solenoids with less than one unit of radiation length. Interesting steps forward have been taken, new ideas are being tested and results can be expected in the years to come. 

Conclusion 

To conclude, it has been shown design-wise that it is technically possible to construct a record  size detector solenoid, 20 m long, 10 m free bore, with a stored energy of 13 GJ. There are still  many challenges for the years to come.

Fundamental physics research, in particular particle physics, has always pushed the performance of superconductors, either for accelerator magnets  or for detector magnets. There has always been, and this project shows that there continues to be,  a demand for better performing superconductors, the understanding of their behaviour under  ever more extreme conditions of electromagnetic forces, high radiation levels and thermal cycling from room to cryogenic temperature.