Last year a record central field of 11 T at first excitation at 4.4 K has been achieved with the experimental LHC model dipole magnet MSUT by utilising a high J/sub c/ powder-in-tube Nb/sub 3/Sn conductor. This is the first real breakthrough towards fields well above 10 T at 4 K. The clear influence of magnetisation and coupling currents on the field quality, the quench behaviour and the temperature development in the coils has been measured and is discussed. For application in high-field accelerator magnets (10-15 T dipoles, 300-400 T/m quadrupoles) these experimental results clearly reveal the potential, the present limitations and the necessary improvements of Nb/sub 3/Sn technology with respect to strand, cable and coil design and manufacturing. A brief review of developments in this field is presented. The focus is on accelerator dipole magnets but the key issues for quadrupole magnets are quite similar.

/pdf/application-of-nb-sub-3-sn-superconductors-in-high-field-4y6qhxzs9e.pdf

Application of Nb/sub 3/Sn superconductors in high-field accelerator magnets

We describe here a new harmonic coil system for the field measurement of the superconducting, twin aperture LHC dipoles and the associated corrector magnets. Besides field measurements the system can be used as an antenna to localize the quench origin. The main component is a 16 m long rotating shaft, made up of 13 ceramic segments, each carrying two tangential coils plus a central radial coil, all working in parallel. The segments are connected with flexible Ti-alloy bellows, allowing the piecewise straight shaft to follow the curvature of the dipole while maintaining high torsional rigidity. At each interconnection the structure is supported by rollers and ball bearings, necessary for the axial movement for installation and for the rotation of the coil during measurement. Two such shafts are simultaneously driven by a twin-rotating unit, thus measuring both apertures of a dipole at the same time. This arrangement allows very short measurement times (typically 10 s) and is essential to perform cold magnetic measurements of all dipoles. The coil surface and direction are calibrated using a reference dipole. In this paper we describe the twin rotating coil system and its calibration facility, and we give the typical resolution and accuracy achieved with the first commissioned unit.

/pdf/twin-rotating-coils-for-cold-magnetic-measurements-of-15-m-3vbebndj3r.pdf

Twin rotating coils for cold magnetic measurements of 15 m long LHC 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.

/pdf/field-errors-decay-and-snap-back-in-lhc-model-dipoles-5c5tnxaroh.pdf

Field errors decay and "snap-back" in LHC model dipoles

This paper presents a study on the ramping losses of a high temperature superconductor (HTS) magnet consisting of multiple no-insulation (NI) (RE)Ba2Cu3Ox coils. The (RE)Ba2Cu3Ox (REBCO) conductor is the second generation HTS thin tape, where RE stands for rare-earth. During a ramping operation of the NI HTS magnet, losses are generated both across turn-to-turn resistances and inside superconducting layers. The former comes with radial current, which is called “turn-to-turn loss;” the latter one is induced by flux creep and jump, called “magnetization loss.” The modeling and experimental studies on the ramping losses have been reported on single NI pancake coils in the previous part. In a HTS magnet consisting of multiple NI coils, the electromagnetic coupling between coils has a considerable influence on the distribution of ramping losses. Here, the experimentally validated model is used to investigate a HTS magnet consisting of 14 single pancake REBCO coils. The results show that both the turn-to-turn l...

/pdf/non-uniform-ramping-losses-and-thermal-optimization-with-md98qkl06p.pdf

Non-uniform ramping losses and thermal optimization with turn-to-turn resistivity grading in a (RE)Ba2Cu3Ox magnet consisting of multiple no-insulation pancake coils

We describe the systems for AC loss and magnetic field measurements developed for the LHC superconducting magnets. AC loss measurements are performed using an electric method, while field measurements are performed using either fixed pick-ups or rotating coils. We present results obtained on 1-m long model dipoles, and compare the results of the different methods in terms of average interstrand resistance.

/pdf/measurement-of-ac-loss-and-magnetic-field-during-ramps-in-13zhuhjett.pdf

Measurement of AC loss and magnetic field during ramps in the LHC model dipoles

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 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 given.

/pdf/experimental-evidence-of-boundary-induced-coupling-currents-23ecv0d6dk.pdf

Experimental evidence of boundary induced coupling currents in LHC prototypes

Extensive power tests of the LHC dipole magnets required the development of new techniques to study the quench and training behaviour. Magnetic measurements of short and long model dipoles have allowed to understand and quantify the time dependent behaviour of the field quality during the current flat top needed during beam injection. The experience gained is employed for the design of the measuring tools presently under construction for the series measurements of the LHC dipole magnets. The economically important issue of how many magnets have to be measured in the superconducting state is addressed in view of the field quality required for the performance of the LHC.

/pdf/towards-series-measurements-of-the-lhc-superconducting-dtqhmec5ln.pdf

Towards series measurements of the LHC superconducting dipole magnets

We have measured the magnetic field at room temperature and at 1.8 K on more than twenty, 1-m long, single aperture LHC superconducting dipole models. The magnets feature either a 5-block coil geometry or the baseline 6-block geometry foreseen for the LHC. Comparison of warm and cold measurements show that the coil geometry is essentially unchanged during cooldown. We have therefore used mechanical measurements taken on the coil and collars during assembly to estimate the azimuthal coil length. Based on these measurements we show here that the sensitivity of allowed harmonics on coil size is in good agreement with the prediction obtained from the numerical model used for designing the LHC magnets.

/pdf/coil-size-and-geometric-field-quality-in-short-model-dipoles-3s508pc4ds.pdf

Coil size and geometric field quality in short model dipoles for LHC

Individually powered twin-aperture superconducting quadrupoles with a coil bore of 70 mm will be installed in certain areas of the LHC insertions where increased geometrical acceptance is required. A 1-m twin-aperture model of the quadrupole, featuring a four-layer coil with a nominal gradient of 160 T/m at 4.5 K, has been built and tested. In this report we present the quench performance of the magnet, and discuss the results of the quench protection studies and magnetic field measurements.

Performance of the 1-m model of the 70 mm bore twin-aperture superconducting quadrupole for the LHC insertions

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