A Practical Fit for the Critical Surface of NbTi

Known expressions for the critical temperature, critical field and Pinning force in NbTi are combined into a self-consistent fit formula that provides the critical current density as a function of temperature and field. The main advantage of such a fit is the extended validity range. Data available in literature and measurements on LHC strands are used to demonstrate the accuracy of the fit. The Jc data-sets used to cover a range of field from 0 T to 9 T and temperature from 1.9 K to 9 K. The standard deviation of the fits presented is of the order of 5 % or better. This accuracy is generally sufficient for design purposes, extrapolation and scaling of measured results. Better accuracy, e.g. for short sample limit prediction, can be achieved restricting the domain of validity

Magnet Quench 101

This paper gives a broad summary of the physical phenomena associated with the quench of a superconducting magnet.Comment: 9 pages, Contribution to WAMSDO 2013: Workshop on Accelerator Magnet, Superconductor, Design and Optimization; 15 - 16 Jan 2013, CERN, Geneva, Switzerlan

Flower, a Model for the Analysis of Hydraulic Networks and Processes

We have developed in the past years a model that describes hydraulic networks that are typical of the cryogenic interconnection of superconducting magnets. The original model, called Flower, was used mostly to provide consistent boundary conditions for the operation of a magnet. The main limitations were associated with the number and nature of modelling elements available, and to the maximum size of the model that could be solved. Here we present an improvement of the model largely relaxing the above limitations by the addition of new modelling elements, such as parallel flow heat exchangers, and by a significant improvement in the numerics of the solver, using sparse matrix storage and solution techniques. We finally show a typical application to the case of a magnet quench in the LHC string

Field Errors Decay and "Snap-Back" in LHC Model Dipoles

The magnetic field in accelerator magnets decays when the current is kept constant during the particles injection phase, and returns quickly (snaps back) to the original values as soon as ramping is restarted. Here we show results of measurements of the decay of the field errors in 10 m long LHC model dipole magnets. In accordance with previous findings, precycles and stops at intermediate current levels influence the decay. We discuss a possible mechanism causing the decay and snap-back, based on the internal field change in the cable

Field Measurements

The measurement of the magnetic field is often the final verification of the complex design and fabrication process of a magnetic system. In several cases, when seeking high accuracy, the measurement technique and its realization can result in a considerable effort. This note describes most used measurement techniques, such as nuclear magnetic resonance, fluxmeters and Hall generators, and their typical range of application. In addition some of less commonly used techniques, such as magneto-optical, SQUIDs, or particle beams methods, are listed

Experimental Evidence of Boundary Induced Coupling Currents in LHC Prototypes

The field quality of 10 m long LHC dipole models has been measured with short rotating coils to explore its dependence on time and position. Multipoles exhibit a longitudinal periodic variation, with period equal to the twist pitch length. This periodicity is shown here to have at least two components with very different time constants. The amplitude of the component with the shorter time constant, in the range of 100 to 300 s, depends on position and time. Larger amplitudes are measured at early times after a ramp and close to regions with incomplete cable transposition with respect to the non-uniform external field change. As the multipoles periodicity is due to current imbalance in the cables, we attribute the short time scale variations to the presence of space and time decaying boundary induced coupling currents (BICC's) in the cable. An estimate of their value is give