The LHC is a 27 km superconducting collider situated under the French-Swiss border just outside Geneva. The LHC first injected beam in September 2008. A serious incident caused significant damage to the machine and the recovery took until the end of 2009. The LHC then operated through to the beginning of 2013 before starting a two year shutdown which finished in April 2015. Since then the LHC has been operating, somewhat haltingly, close to its design energy.

An introduction to the basics of the LHC and its standard operation model is presented. An overview of the ups and downs of the last few years and the present re-start is given with attention to the challenges inherent in the operation of a large scale superconducting facility. Some of these involve mastery of large-scale cryogenics, quench protection, vacuum, controls and technical infrastructure systems. However, a number of more subtle challenges are related to features of superconducting magnets and their operation at high field. Further interesting considerations come with the introduction of high energy, high intensity proton beams into this superconducting environment with the attendant limits on particle loss throughout the nominal LHC operational regime.

CERN is planning an upgrade to the LHC – the High Luminosity LHC (HL-LHC) - for deployment in around 2023. This is involves pushing superconducting magnet technology to new limits. The plans and key features of the upgrade are briefly outlined

After nearly half a century of dominance by NbTi, other superconducting materials are finally making their way into use for accelerator magnets. Quadrupoles using Nb3Sn will be installed as part of the luminosity upgrade for the LHC. Recently, the High Temperature Superconductors, Bi-2212 and ReBCO have been added to the list of candidate materials. But, despite the tantalizing potential for achieving dipole fields more than twice that of NbTi, there are many challenges that still need to be overcome. Interest in high field dipoles has been given a boost by new proposals to build a high-energy proton-proton collider to follow the LHC, and programs around the world are taking on the task to answer the need. After a brief review of current progress, the talk will describe the key issues facing future development and present a roadmap for moving high field accelerator magnet technology forward.


Work supported by the US Department of Energy, Office of High Energy Physics

In 2014 CERN launched the conceptual design for a new 100 TeV circular proton-proton collider with collision energy 7 times higher than in the present LHC. A new 100 km long tunnel is needed as well as caverns for four experiments. The conceptual design report is due in 2018, where after next steps will be considered, eventually leading to a working collider by medio 2045. The factor 7 increase in collision energy enforces a scaling up of the detector magnets in size and magnetic field for maintaining particle detection resolution. The options are being explored and the study covers the scaling up needed. The first system uses in the style of the present ATLAS detector, toroids for muon tracking and a solenoid for the inner detector. New is that two dipole magnets are incorporated for covering the low angle forward directions. In a similar way the second option features a scaled-up CMS type solenoid also in combination with two dipole magnets. An important issue in the solenoid design is the level of shielding the magnetic field. A light iron yoke design is envisaged in order to limit the iron content, or very promising, is using an actively shielded solenoid also called the twin solenoid design and leading to a relatively light system. The magnets are of unprecedented size featuring outer dimensions of 20-30 m in diameter and overall length of 40 to 50 m. The magnetic field in the large toroid and solenoid will go up to some 2.5 and 5 to 6 tesla, respectively, still feasible with NbTi technology. The magnetic stored energies are very high and in the range of 40-60 GJ. Various options of these record breaking magnets will be presented.

A 100 kA current-capacity superconductor is required for the helical coils of FFHR-d1, the LHD-type helical fusion reactor. We propose the High-Temperature Superconducting (HTS) STARS (Stacked Tapes Assembled in Rigid Structure) conductor for this use. The STARS conductor is mechanically strong due to its structure, comprised of simply-stacked ReBCO tapes, a copper stabilizer and a rigid stainless-steel jacket. The helical coils of large-diameter and complex-shape of FFHR-d1 can be constructed by connecting segmented STARS conductors exploiting the advantage of HTS, such as high cryogenic stability and low refrigeration power.
For the development of the STARS conductor suitable for the helical fusion reactor, we have fabricated and tested 30-kA-class and 100-kA-class prototype samples, which have 20 and 54 GdBCO tapes (10 mm width, ~600 A critical current at 77 K, self-field), respectively. The critical current of the samples were measured at various temperatures and magnetic fields: e.g., 45 kA at 20 K, 6.1 T for the 30 kA-class sample and 67.4 kA at 38 K, 4.3 T for the 100 kA-class sample. The experimental results showed that the conductors reached the critical current determined by the self-consistent distribution of magnetic field and current density of entire tapes, without having a premature quench even at a fast ramp-up rate of 1 kA/s, although the structure of the STARS conductor is prone to cause inhomogeneous current distribution among tapes.
A numerical analysis of the current-carrying characteristics of the samples is performed based on the coupled thermal-electromagnetic analysis using the finite element method. In this analysis, the quench current of the conductors, which are determined by the balance between heat generation of the conductor and cooling by the heat conduction, is also investigated at various temperatures, magnetic fields and ramp-up rates of the transport current.

While ITER is being built in Cadarache, France, a European “roadmap to fusion electricity by 2050” has been recently approved by the European Commission. It foresees the design of the DEMO reactor as the step following ITER, aimed at the production of electricity from fusion energy. The DEMO superconducting (SC) magnets will pose some new challenges compared to ITER, related to the increased size and to the higher field and current. The DEMO toroidal field (TF) magnets, composed by winding pack (WP) without radial plates, encapsulated in a steel casing, should be designed to guarantee a temperature margin DT_mar (with respect to current sharing) larger than the minimum value of 1.5 K during the plasma pulses. These activities are being carried out under the EUROfusion umbrella in the Work Package MAG (WPMAG).
Here we present a comprehensive thermal-hydraulic model of a DEMO TF coil, developed using the 4C code, which has been validated and applied in the recent years to the SC magnet systems of several operating tokamaks, (EAST, KSTAR) and designs (ITER, JT60-SA).
The new WP design proposed by ENEA is considered, where rectangular cable-in-conduit conductors (CICC) with a novel design, based on the introduction of two, instead of the customarily single, low-impedance channels for the supercritical helium coolant, are wound in layers to obtain a graded (Nb3Sn + NbTi) WP. The casing around the WP is modeled in detail, including its cooling channels, together with a simplified cryogenic circuit for the WP and for the casing: 4C is currently the only tool inside WPMAG able to cope with such a level of complexity.
We then apply the DEMO TF model to the analysis of the thermal-hydraulic transient during a plasma pulse, during which a non-negligible nuclear heat load is deposited on the magnet, and the resulting minimum DT_mar is evaluated in the entire WP.

The work has been partially supported by EuroFusion – WPMAG

The design of European DEMO, i.e. the future fusion tokamak planned after ITER, is being developed under the coordination of Eurofusion consortium. This work presents the recent design optimization of the toroidal field (TF) winding pack and its corresponding react-and-wind conductor, and a new design study of the central solenoid (CS).
The optimization of the TF coil was driven by the results of the mechanical analysis that revealed an unacceptable stress accumulation in some locations of the previous proposal of the TF winding pack.The design study of the CS coil was done with the aim to maximize the magnetic flux for the given spatial constraints, and consequently also the plasma burn duration. The results of the design study, namely the optimized inner radius, current density, structural material distribution and the field map, define the initial information that will be needed in future for designing the DEMO CS winding pack and conductor. Opposite to former, similar studies, no upper limit is set for the peak field of the CS, implicitly allowing the use of HTS conductors whenever the current density of Nb₃Sn at the operating field is too low.