The critical current in the vortex phase of a type-II superconductor such as NbSe2 displays a striking anomaly in the vicinity of the superconductor-to-normal-metal transition. Instead of going to zero smoothly, it rebounds to a sharp and pronounced maximum, just before vanishing at the transition. This counter-intuitive phenomenon, known as the peak effect, has remained an unsolved problem for 40 years. Here we use a scanning a.c. Hall microscope to visualize the real-space distribution of the critical current in NbSe2. We show that in the peak-effect regime two distinct vortex-matter phases with intrinsically different pinning strengths coexist on a macroscopic scale. The composition of the two-phase mixture and the transformation of one phase into another are responsible for the history effects and anomalous voltage response commonly seen when external parameters such as temperature, magnetic field or transport current are varied. We argue that the observed phase coexistence is, in fact, the hallmark of a disorder-driven non-thermal phase transition.

Future high-energy accelerators will need very high magnetic fields in the range of 20 T. The EuCARD-2 work-package-10 is a collaborative push to take HTS materials into an accelerator quality demonstrator magnet. The demonstrator will produce 5 T standalone and between 17 T and 20 T, when inserted into the 100 mm aperture of Fresca-2 high field out-sert magnet. The HTS magnet will demonstrate the field strength and field quality that can be achieved. An effective quench detection and protection system will have to be developed to operate with the HTS superconducting materials. This paper presents a ReBCO magnet design using multi strand Roebel cable that develops a stand-alone field of 5 T in a 40 mm clear aperture and discusses the challenges associated with good field quality using this type of material. A selection of magnet designs is presented as result of a first phase of development.Presented at: ASC 2014, 10-15 August, Charlotte, USA Geneva, Switzerland February 2015 Abstract -Future high-energy accelerators will need very high magnetic fields in the range of 20 T. The EuCARD-2 workpackage-10 is a collaborative push to take HTS materials into an accelerator quality demonstrator magnet. The demonstrator will produce 5 T standalone and between 17 T and 20 T, when inserted into the 100 mm aperture of Fresca-2 high field out-sert magnet.The HTS magnet will demonstrate the field strength and field quality that can be achieved. An effective quench detection and protection system will have to be developed to operate with the HTS superconducting materials. This paper presents a ReBCO magnet design using multi strand Roebel cable that develops a stand-alone field of 5 T in a 40 mm clear aperture and discusses the challenges associated with good field quality using this type of material. A selection of magnet designs is presented as result of a first phase of development. IndexTerms-Accelerator magnet, EuCARD-2, Superconducting Magnets, HTS magnet design, quench protection, YBCO Roebel cable, ReBCO.

In weakly pinning type-II superconductors the vortex lattice can flow coherently under the action of a Lorentz force. This type of collective vortex motion in a random potential is manifested by channeling of the vortex flow along preferred paths which are stable in time. We present direct experimental observations of such vortex flow "channels" obtained with magnetic decoration experiments on NbSe 2 single crystals.

We summarize critical current measurements and parameterizations of the data of 112 round wires and extracted strands that were reacted with the first 17 coils for the high-gradient quadrupole (HQ) magnets for the US LHC Accelerator Research Program (LARP). We standardize the strand parameterizations and coil 'short sample' calculations, and demonstrate that the entire critical current database can be captured in two scaling parameters per coil. These parameters summarize the short sample performance for each coil for either HQ magnet tests, or mirror tests of individual coils. We also demonstrate that for RRP R conductors, generic strain scaling parameters can be derived for at least four substantially different wire configurations, and standardize self-field corrections for LARP. The parameterized conductor performance is used to judge the performance of the HQ magnets and mirror tests. We find that although the HQ magnets reach around 86% of their short sample limitations, they are limited by factors other than the critical current of the conductor. Individual coils in mirror tests reach up to 98% of the expected performance, and do appear limited by the critical current of the conductor. Detailed analysis of short sample performance through accurate parameterizations simplifies the accessibility of short sample data, and enables accurate judgment of magnet performance as well as conductor and cable quality.

Among the components to be upgraded in LHC interaction regions for the HiLumi-LHC projects are the inner triplet (or low-β) quadrupole magnets, denoted as Q1, Q2a, Q2b, and Q3. The new quadrupole magnets, called MQXF, are based on Nb3Sn superconducting magnet technology and operate at a gradient of 132.6 T/m with a conductor peak field of 11.4 T. The Q1 and Q3 are composed by magnets (called MQXFA) fabricated by the US Accelerator Upgrade Project (AUP) with a magnetic length of 4.2 m. The Q2a and Q2b consists of magnets (called MQXFB) fabricated by CERN with a magnetic length of 7.15 m. After a series of short models, constructed in close collaboration by the US and CERN, the development program is now entering in the prototyping phase, with CERN on one side and BNL, FNAL, and LBNL on the other side assembling and testing their first long magnets. We provide in this paper a description of the status of the MQXF program, with a summary of the short model test results, including quench performance, and mechanics, and an update on the fabrication, assembly and test of the long prototypes.