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

One of the main issues for the operation of the LHC accelerator at CERN is the field errors generated by persistent and coupling currents in the main dipoles at injection conditions, i.e., 0.54 T dipole field. For this reason we are conducting systematic magnetic field measurements to quantify the above effects and compare them to the expected values from measurement on strands and cables. We discuss the results in terms of DC effects from persistent current magnetization, AC effects with short time constant from strand and cable coupling currents, and long-term decay during constant current excitation. Average and spread of the measured field errors over the population of magnets tested are as expected or smaller. Field decay at injection, and subsequent snap-back, show for the moment the largest variation from magnet to magnet, with weak correlation to parameters that can be controlled during production. For this reason these effects are likely to result in the largest spread of field errors over the whole dipole production.

/pdf/persistent-and-coupling-current-effects-in-the-lhc-3ub8w8szt4.pdf

Persistent and coupling current effects in the LHC superconducting dipoles

The time decay of field harmonics during current plateaus is a known drawback of superconducting accelerator magnets. The present understanding of this phenomenon refers to a combination of flux creep and of the interaction between the redistribution of currents in Rutherford cables and the filament magnetization. Current cycles of decreasing amplitude, called here degaussing, were found to reduce significantly the decay observed in accelerator magnets. This paper reports on the measured reduction of decay obtained in short dipole models for the LHC and on one experiment with a single LHC strand.

/pdf/degaussing-and-decay-reduction-in-the-short-superconducting-4pmb44msfv.pdf

Degaussing and decay reduction in the short superconducting dipole models for the LHC

In superconducting particle accelerators a fast change of the magnetic field occurs during the first few seconds after the start of an energy ramp. Standard magnetic measurements using a coil rotating at 1 Hz do not have the time resolution required to completely resolve this phase, usually called snapback. For this reason we have developed a new and fast system dedicated to sextupole measurements. The basic component consists of three Hall plates mounted on a ring. In an ideal case this arrangement compensates the main dipole field and produces a signal proportional to the sextupole only. Mechanical tolerances and differences in the sensitivity of the Hall plates are compensated by instrumentation amplifiers and an in situ fine adjustment of the probe orientation. Using this hybrid compensation technique we have measured sextupole variations in an LHC dipole prototype during snapback at a rate of 5 Hz. In this paper we present details on the device and the results of our measurements.

/pdf/a-fast-sextupole-probe-for-snapback-measurement-in-the-lhc-3tefs08nd2.pdf

A fast sextupole probe for snapback measurement in the LHC dipoles

AC loss in cored, stabrite-coated, superconducting cables in response to external compaction and variation of core thickness and width

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

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