Search

We present the structure and features of a quench code for the analysis of normal operation and safety-relevant transients. The code gives a self-consistent simulation of 3-D quench development in the winding pack of a force-flow cooled coil, including the evolution of the operating current and magnetic field and the change of the helium state at the coil inlet and outlet manifolds due to the transient response of the cryogenic plant. >

An optimisation procedure for the design of CICC's is applied to a parametric scan on the effect of the superconductor critical properties (Jc, Tc) operating conditions (B, strain) and other design parameters (RRR, strand diameter, limit on the strand Cu:non-Cu ratio). The parameter with the strongest influence on the optimal design performance is Tc, while surprisingly large variations in Jc can be accommodated at minor loss in performance. >

A preliminary safety analysis of the superconducting magnet system of a ‘next generation’ tokamak is performed. Such tokamaks must satisfy nuclear class safety standards to permit their operation. Designs are not sufficiently advanced to permit the full safety case to be made. However scoping calculations of the form presented here enable potential problems to be identified or eliminated. With a number of constraints on the design it is shown that ‘passive’ safety could be achieved in such a magnet system i.e. the magnets can not cause failure in adjacent nuclear components.

A method for the calculation of the hysteresis loss in a superconducting slab for large, bipolar changes of the applied magnetic field is presented. The method is purely analytical and uses the Kim model for the critical current density J/sub c/. Arbitrary change in the external magnetic field can be handled, before or after full penetration. Magnetization, dissipated power and energy are computed as a function of time, independent of the presence of closed loops in the magnetic field change. Scaling relations are given to extend the results to the hysteresis loss in a superconducting cylinder in a changing transverse field. An example of an application shows excellent agreement between measured and computed dissipated energy. >

Experiments of quench propagation and stability margin have been performed on a NbTi hollow conductor test module allowing heat conduction only between adjacent layers, while the turns in a layer were thermally insulated. Some of the experimental data on heat propagation and stability margin are reported, and the results of two selected runs are compared with the prediction of a 2-D version of the SARUMAN code. It is shown that the simulation can reproduce the general features of the quench propagation experiments, in one case reaching a quantitative agreement. >

Stability margin, ac response (losses and current distribution) and protection issues (hot-spot temperature and maximum quench pressure) are the three primary aspects considered in the design of a superconducting magnet for application in an experimental fusion reactor. This paper reviews these three aspects in particular for cable-in-conduit conductors (CICCs). Their combination, together with the requirements on the performance of the magnet, determines a set of constraints to be satisfied at all operating points. This can be achieved only by means of compromises in the design, e.g. selecting the material fractions in the cable as a function of the specified margin and of the achievable current density. Here are presented some of the basic guidelines to be used for an effective design of a CICC, based in particular on simplified stability and hot-spot models.

The eddy currents induced in the casing of the superconducting TF coils of ITER during the inductive plasma start-up are a source of heat which must be removed in order to guarantee the stable operation of the superconductor. Furthermore, the ohmic power is deposited at cryogenic temperatures and may represent a significant fraction of the total heat load on the cryogenic system. The necessity of minimizing this heat deposition by means of insulating breaks and geometry changes, still satisfying fabricability requirements, is of obvious importance. Here the heat deposition is computed using a 3-D eddy currents code for different configurations of the coil casing. Some solutions are proposed for the reduction of the local and global heat loads.

The authors present a procedure for the analysis of the operating margin of a pulsed magnet, based on the sequential calculation of magnetic field, AC losses, temperature, temperature margin, and energy margin. The procedure is suited to the analysis of supercritical helium, force-flow cooled magnets. An example of application to the ITER (International Thermonuclear Experimental Reactor) central solenoid and the diverter coils is given, showing the capabilities achieved by an integrated analysis code and the level of detail in the information gained. >

In large superconducting magnets built using force-flow cooled conductors, such as those being designed for next generation fusion machines, the quench propagation is a three-dimensional phenomenon. In this paper we develop a method for the analysis of quenches in 3-D which is extremely versatile and comprehensive. The method is based on the parallel solution of a set of 1-D problems represented by the helium flow, heat conduction and quench propagation along the conductor length. Transverse heat exchange among conductors is then explicitly inserted in the model, thus achieving the desired 3-D capability. In the model developed for the 1-D analysis we have foreseen the possibility of taking into account the thermal gradients in the cable cross-section, and the changes of magnetic field and operating current which are typical of a quench transient.

The AC losses requirement for the superconducting coils of the Next European Torus (NET) project are discussed on the basis of the pulsed field scenario and plasma disruptions. The AC losses test results on the prototype 40 kA cable-in-conduit superconductors developed for NET are presented. The performance achieved is compared with the target. Some recommendations are made for work to be done. >

The Large Hadron Collider (LHC) at CERN is equipped with a feed-forward control system known as the field description for the LHC (FiDeL) which is designed to predict the magnetic field and its multipoles, hence reducing the burden on beam based feedback. FiDeL consists of a physical and empirical parametric field model based on magnetic measurements at warm and in cryogenic conditions. It is particularly critical during beam injection when the field decays and at the beginning of acceleration when the field snaps back. It is known that the decay amplitude is largely affected by the powering history of the magnet, particularly by the precycle flattop current and duration and the preinjection preparation duration. Recently, we have collected data that quantify the dependence of the decay amplitude on the precycle ramp rate. This paper presents the results of the measurements performed to investigate this effect, and the method included in FiDeL to model the precycle dependence.With this complete picture of dynamic changes, we finally discuss the effect on the data taken at nominally constant field, along the magnet loadline. We show that a correction for dynamic changes is required for adequate magnet characterization., peer-reviewed

In this paper we report the main test results obtained on the Poloidal Field Conductor Insert coil (PFI) for the International Thermonuclear Experimental Reactor (ITER), built jointly by the EU and RF ITER parties, recently installed and tested in the CS Model Coil facility, at JAEA-Naka. During the test we (a) verified the DC and AC operating margin of the NbTi Cable-in-Conduit Conductor in conditions representative of the operation of the ITER PF coils, (b) measured the intermediate conductor joint resistance, margin and loss, and (c) measured the AC loss of the conductor and its changes once subjected to a significant number of Lorentz force cycles. We compare the results obtained to expectations from strand and cable characterization, which were studied extensively earlier. We finally discuss the implications for the ITER PF system.

A new platform for magnetic measurements at the European Organization for Nuclear Research (CERN) is presented. The key concepts and the architecture of its main components, a multi-purpose digitizer (fast digital integrator - FDI) and a flexible software framework (flexible framework for magnetic measurements - FFMM), are detailed. The experimental results of the metrological characterization of FDI exhibit a significant advance with respect to the state of the art. Furthermore, the FFMM implementation is shown to provide a reusable and scalable measurement software. Finally, the results of the on-field validation of the platform for field measurements on superconducting magnets in the CERN test facility are discussed. © 2008 Elsevier Ltd. All rights reserved.

Next generation fusion reactors such as the Next European Tokamak (NET) and the International Thermonuclear Experimental Reactor (ITER) have design targets for the conductor performance in their superconducting coils. These coils require high fields and must tolerate both AC and DC conditions. There are a range of superconductor configurations (monolithic, cable in conduit) and materials (Nb 3 Sn, NbTi) that are candidates for the conductors. This report quantifies the performance of these candidates in the expected environment of the NET/ITER coils, and shows that Nb 3 Sn cable in conduit type conductors can be expected to satisfy the present design targets of NET/ITER.

Stability is a key point in the design of large conductors for application in fusion machines such as NET or ITER. In the electromagnetically noisy environment characterizing the surroundings of a plasma, the superconductor should be stable against all expected heat deposition, including a.c. losses driven by plasma disruptions. Since all sources of thermal perturbations cannot be adequately identified, predicted or calculated, the design must allow for suitable margins. To ensure this, stability must be considered as a design criterion which is checked by means of analytical and numerical methods and later calibrated against experimental measurements of transient heat transfer and stability behaviour. In this the procedure developed at NET is described for the design and analysis of the stability margin of conductors for the toroidal and poloidal field coils. These methods are applied to the generation of stability curves and maps which are used to set operating limits for the 40 kA NET conductor. The design criteria and analytical methods can be appropriately modified for other conductors and coil applications.

The major part of the program involves the industrial fabrication of long lengths of full-size conductors and the manufacturing of test coils that will be thoroughly tested in a single test facility (TOSKA, at KfK Karlsruhe, Germany). A total of 3.5 km of 40-kA conductor is required to form circular model coils with an inner bore of 2 m and a peak field of over 13.5 T. Short (5-m) lengths of full-size 40-kA conductors have been manufactured. The present results on the industrial fabrication of 12-T, 40-kA cable-in-conduit conductors for the Next European Torus (NET) indicate that the machine design criteria can be satisfied and that the conductor is suitable for large-scale industrial fabrication.

The kernel design of the flexible framework for magnetic measurements (FFMM), under development at the European Organization for Nuclear Research (CERN), is described. The design aims at satisfying the critical scenario of short-term requirement variability for the small batches of magnets after the commissioning for the large hadron collider (LHC). The experimental test results of software flexibility and scalability are illustrated for a case study of superconducting magnets in warm and cold on-field conditions.

The performance of the iterative procedure used for the explicit characteristic—Galerkin method in the solution of advection-dominated problems is investigated here in the implicit case. The procedure is conditionally stable, and here its stability limit is compared to that of time stepping. It is shown that in a limited range of values of the implicitness parameter θ it is possible to generate explicit methods by use of the iterative solution procedure. This is true, in particular, at θ = ⅓; with this choice the algorithm generated is more accurate, has a wider stability margin and lower cost (for linear problems) than the usual choice θ = 0. Because of these properties, this algorithm might be used for further improvement of the performance of fluid-dynamics codes.

A statistical behavioral modeling approach for assessing dynamic metrological performance during the concept design of accurate digitizers is proposed. A surface-response approach based on statistical experiment design is exploited in order to avoid unrealistic hypothesis of linearity, optimize simulation effort, explore systematically operating conditions, and verify identification and validation uncertainty. An actual case study on the dynamic metrological characterization of a Fast Digital Integrator for highperformance magnetic measurements at European Organization for Nuclear Research (CERN) is presented.

The normal and superconducting magnets for the LHC ring have been carefully examined to insure that each of about 1900 assemblies is suitable for the operation in the accelerator. Hardware experts and accelerator physicists have contributed to this work that consisted in magnet acceptance, and sorting according to geometry, field quality and quench level. This paper gives a description of the magnet approval mechanism that has been running since four years, reporting in a concise summary the main results achieved.

Premature training quenches are caused by transient energy released within the LHC dipole magnet coils while it is energized. Voltage signals recorded across the magnet coils and on the so-called quench antenna carry information about these disturbances. The transitory events correlated to transient energy released are extracted making use of continuous wavelet transform. Several analyses are performed to understand their relevance to the so called training phenomenon. The statistical distribution of the signals amplitude, the number of events occurring at a given current level, the average frequency content of the events are the main parameters on which the analysis have been focalized. Comparisons among different regions of the magnet, among different quenches in the same magnet and among magnets made by different builders are reported. Conclusions about the efficiency of the raw data treatment and the relevance of the parameters developed with respect to the magnet global behavior are finally given.

A superconducting particle accelerator like the LHC (Large Hadron Collider) at CERN, can only be controlled well if the effects of the magnetic field multipoles on the beam are compensated. The demands on a control system solely based on beam feedback may be too high for the requirements to be reached at the specified bandwidth and accuracy. Therefore, we designed a suitable field description for the LHC (FIDEL) as part of the machine control baseline to act as a feed-forward magnetic field prediction system. FIDEL consists of a physical and empirical parametric field model based on magnetic measurements at warm and in cryogenic conditions. The performance of FIDEL is particularly critical at injection when the field decays, and in the initial part of the acceleration when the field snaps back. These dynamic components are both current and time dependent and are not reproducible from cycle to cycle since they also depend on the magnet powering history. In this paper a qualitative and quantitative description of the dynamic field behavior substantiated by a set of scaling laws is presented., peer-reviewed

A significant increase of critical current in high magnetic field, up to 31 T, was recorded in long tapes manufactured by employing a double-disorder route. In a double-disordered high-temperature superconductor (HTS), a superimposing of intrinsic and extrinsic disorder takes place in a way that (i) the intrinsic disorder is caused by local stoichiometry deviations that lead to defects of crystallinity that serve as pining centers in the YBa2Cu3O x−δ matrix and (ii) the extrinsic disorder is introduced via embedded atoms or particles of foreign material (e.g. barium zirconate), which create a set of lattice defects. We analyzed possible technological reasons for this current gain. The properties of these tapes over a wider field-temperature range as well as field anisotropy were also studied. Record values of critical current as high as 309 A at 31 T, 500 A at 18 Tm and 1200 A at 5 T were found in 4 mm wide tape at 4.2 K and B perpendicular to tape surface. HTS layers were processed in medium-scale equipment that allows a maximum batch length of 250 m while 22 m long batches were provided for investigation. Abnormally high ratios (up to 10) of critical current density measured at 4.2 K, 19 T to critical current density measured at 77 K, self-field were observed in tapes with the highest in-field critical current. Anisotropy of the critical current as well as angular dependences of n and α values were investigated. The temperature dependence of critical current is presented for temperatures between 4.2 and 40 K. Prospects for the suppression of the dog-bone effect by Cu plating and upscale of processing chain to >500 m piece length are discussed.

A self-calibrating digital instrument for flux measurements on magnets for accelerators used in basic research on subnuclear particles is proposed. The instrument acquires voltage arising from rotating coils transducers with a theoretical resolution of 10 ppt and a maximum sampling frequency of 800 kS/s. Then, samples are integrated on-line and suitably processed in order to improve time resolution and flux accuracy. This allows the limits of state-of-the-art digital fluximeters, related mainly to new-generation rotating coils, with trigger rate of 20 kHz and coils speed of 10 rps, to be overcome. The instrument has been prototyped at Magnetic Measurement and Testing (MTM) Group of European Laboratory for Nuclear Research (CERN), under a framework of cooperation with the University ofSannio. Details on hardware and firmware conception, as well as on experimental results of the instrument principle validation, and of the preliminary metrological characterization of the prototype, are provided. © 2006 IEEE.

High energies physics is nowadays challenging the unprecedented opportunity of studying and investigating the ultimate constituents of matter and their interaction. Thanks to its superior energy level, Large Hadron Collider (LHC), currently under construction at CERN (Geneva), will allow a deep insight into elementary particles world. To create and study elementary particles, it is, in fact, necessary to assure that particles collides with one another with sufficient energy. Particles are kept on stable orbits by means of powerful superconducting dipole magnets, the magnetic field of which is capable of suitably bending particles trajectories; proper measurements of magnetic field turn out to be an advisable and strategic task. Standard procedure, based on the application of traditional DFT algorithm to flux samples acquired through a rotating coils system, has proven to be unreliable when magnets are supplied with non-stationary current ramping up to the value corresponding to the nominal operative magnetic field. The authors present hereinafter a digital signal processing approach to overcome standard procedure limitations. The approach is based on interpolation techniques and allows the evolution versus time of harmonic coefficients of the magnetic field to be accurately reconstructed. Several tests have been conducted on simulated flux samples to assess the performance of the proposed approach. Obtained results are presented and compared to those granted by standard procedure.

CERN is currently assembling the LHC (Large Hadron Collider) that will accelerate and bring in collision 7 TeV protons for high energy physics. Such a superconducting magnet-based accelerator can be controlled only when the field errors of production and installation of all magnetic elements are known to the required accuracy. The ideal way to compensate the field errors obviously is to have direct diagnostics on the beam. For the LHC, however, a system solely based on beam feedback may be too demanding. The present baseline for the LHC control system hence requires an accurate forecast of the magnetic field and the multipole field errors to reduce the burden on the beam-based feedback. The field model is the core of this magnetic prediction system, that we call the field description for the LHC (FIDEL). The model will provide the forecast of the magnetic field at a given time, magnet operating current, magnet ramp rate, magnet temperature, and magnet powering history. The model is based on the identification and physical decomposition of the effects that contribute to the total field in the magnet aperture of the LHC dipoles. Each effect is quantified using data obtained from series measurements, and modeled theoretically or empirically depending on the complexity of the physical phenomena involved. This paper presents the developments of the new finely tuned magnetic field model and, using the data accumulated through series tests to date, evaluates its accuracy and predictive capabilities over a sector of the machine., peer-reviewed

Beam loss and emittance dilution during ramping from injection to collision energy is observed in the Tevatron, now in its collider run-II stage. It is well known that the sextupole (b2) components in the superconducting dipole magnets decay during the injection plateau and snap back rapidly at the start of the ramp. These so called dynamic effects, which were originally discovered in the Tevatron, are compensated with the chromaticity correctors, distributed around the ring. Imperfect control of the chromaticity during injection and snapback can contribute to the beam loss. Therefore a thorough investigation of the b2 compensation in the Tevatron was launched, including beam chromaticity measurements and offline magnetic measurements on Tevatron dipoles. This paper reports the status of this investigation. A companion paper describes in detail the results of the magnet measurements. This work was partly conducted as a collaboration between FNAL and CERN.

After the commissioning and the first powering of the main circuits in autumn 2001 in its shorter version, the facility was completed to a full cell of LHC in the regular part of an arc and commissioned in July 2002. During this second run, which accumulated more than 4000 hours below 2 K, a very dense experimental program was carried-out to validate the final versions of the technical systems and design choices such as the bus-bar cables running along the magnet cold masses inside the cryostats. The program included the investigation of thermo-hydraulics of quenches, quench propagation, power converter controls and tracking between power converters. The cryogenic process dynamics were studied in length; predictive control techniques were tested and their performance assessed. During a short shutdown starting in December 2002, the facility was stripped of all instrumentation contributing to increased heat loads and heat load measurements will be performed in a last run during the first half of 2003. The paper describes the facility and details the results obtained during the experimental program.

The preseries production of the LHC main superconducting dipoles is presently being tested at CERN. The foremost features of these magnets are: twin structure, six block two layer coils wound from 15.1 mm wide graded NbTi cables, 56 mm aperture, polyimide insulation and stainless steel collars. This paper reviews the main test results of magnets tested to day in both normal and superfluid helium. The results of the training performance, magnet protection, electrical integrity and the field quality are presented in terms of the specifications and expected performance of these magnets in the future accelerator.

We have performed experiments and simulations of the current distribution process in a CICC with the aim to understand better the coupled thermal, hydraulic and electric process that leads to a stable or unstable transient cable behaviour. The cable, wound from 128 Nb$_{3}$Sn and pure copper strands, has been tested in the SULTAN facility. A resistive heater, glued on the jacket of the conductor, has been used to start the transient, and the response has been monitored with arrays of Hall plates. In this paper we report the results of simulations, especially the computed Hall signals, and compare them to the experimental data. Based on the experimental results and their interpretation we postulate that large temperature gradients must develop in the helium stream in the cable cross sections during the transient heat pulse.

The validation of numerical codes for the calculation of current distribution and AC loss in superconducting cables versus experimental results is essential, but could be affected by approximations in the electromagnetic model or incertitude in the evaluation of the model parameters. A preliminary validation of the codes by means of a comparison with analytical results can therefore be very useful, in order to distinguish among different error sources. We provide here a benchmark analytical solution for current distribution that applies to the case of a cable described using a distributed parameters electrical circuit model. The analytical solution of current distribution is valid for cables made of a generic number of strands, subjected to well defined symmetry and uniformity conditions in the electrical parameters. The closed form solution for the general case is rather complex to implement, and in this paper we give the analytical solutions for different simplified situations. In particular we examine the influence of different boundary conditions, the effect of a localised resistance in the middle of the cable such as in the case of quench and the effects of localized time dependent magnetic fluxes acting on the cable.

In recent years the problem of current distribution in multistrand superconducting cables has received increasing attention for large scale superconductivity applications due to its effect on the stability of fusion magnets and the field quality of accelerator magnets. A modelling approach based on distributed parameters has revealed to be very effective in dealing with long cables made of some tens or hundreds of strands. In this paper we present a fully analytical solution equation for a distributed parameters model in cables made of an arbitrary number of strands, whose validity is subjected to symmetry conditions generally satisfied in practical cables. We give in particular analytical formulae of practical use for the estimation of the maximum strand currents, time constants and redistribution lengths as a function of the cable properties and the external voltage source.

The CONDOPT (CONDuctor OPTimization) experiment has been recently completed in SULTAN. The current sharing behavior of Nb/sub 3/Sn samples was assessed as a function of the number of cyclic loads experienced during current sweeps in a 10 T background field. We present here results of a computer analysis performed with the code THEA/sup /spl trade// (for consistent Thermal, Hydraulic and Electric Analysis) in support of the interpretation of the experimental results. We focus in particular on the critical current and current sharing temperature runs, providing details on the features and effects of current distribution among cable sub-stages.

In many applications of superconductivity the magnetization of a saturated cylindrical superconducting filament is perturbed by an external field change. Especially in superconducting accelerator magnets local field changes have an impact on the magnetization in the coil. We analyze the behavior of the current distribution and magnetization of a single filament for field changes with different amplitudes and at arbitrary angles with respect to the original background field. Formulas are developed and the behavior of the magnetization is demonstrated.

The design of a cryogenic sensing element for superconducting DC current transformers working up to 100 kA at 4.2 K is proposed. Limitations of μ-metal magnetic characteristics at cryogenic conditions are overcome by a soft Ni 81-Mo 5-Fe magnetic alloy (referred to as cryogenic permalloy). An optimized geometrical design allows cores saturation due to mechanical constraints to be avoided. The effectiveness of the design is demonstrated through electromagnetic simulations exploiting measurement results from the characterization of cryogenic permalloy samples. Moreover, a preliminary lab-scale cryogenic DCCT prototype for a nominal current of 20 kA is implemented and characterized. A typical precision of the transducer below ±0.05% is achieved along with nonlinearity better than ±0.05%.

A crucial issue for a superconducting coil in order to be safely used in the magnetic system of a fusion reactor is stability against all foreseen disturbances. To simulate the fusion machine conditions, including off-normal events, e.g. plasma disruptions, the energy deposition has to be spread over a "long length" cable in conduit conductor (CICC) and a background magnetic field is needed. We have therefore designed and built an experiment consisting of an instrumented NbTi test module inserted in a pair of co-axial pulsed copper coils. A 0.6 m diameter superconducting coil provides a background magnetic field up to 3 T. Calibration of the energy inductively coupled between the pulsed coils and the module has been obtained measuring the system temperature increase just after the pulse by means of thermometers positioned along the conductor. Stability vs. operating current I/sub op/ has been examined for different helium temperatures and different background magnetic fields. The finite element code Gandalf for the stability and quenching transients analysis in forced flow cooled superconducting coils has been run to check the matching with the experimental results. (3 refs).

The magnetic field in superconducting accelerator magnets has a fine structure with longitudinal periodicity. This periodic pattern, with period identical to the cable twist pitch, is originated by uneven current distribution within the cable. Here, the authors present results of measurements of the periodic pattern performed in an LHC dipole model. They report in particular the results obtained powering the magnet with simple current steps and typical operation cycles as will be used during accelerator operation. The main result of the analysis is the time variation of the amplitude of the periodic pattern, from which they infer the evolution of the current distribution in the cable. They discuss the dependence of the pattern amplitude on ramp and pre-cycle parameters.

The sections in this article are 1 Superconductor Stability 2 Forced-Flow–Cooled Conductors 3 Stability Margin of Cable-In-Conduit Conductors in Supercritical Helium 4 Stability Margin of Cable-In-Conduit Conductors in Superfluid Helium 5 Calculation of the Stability Margin in He-I 6 Calculation of Stability margin in He-Ii 7 Conclusions and Research Directions 8 Appendix: Transient Heat Transfer Keywords: current-sharing temperature; cable-in-conduit conductors; internally cooled superconductors; stability; cryostability; recovery; stability margin; disturbance spectrum; limiting current; lower limiting current; multiple stability; training; ramp-rate limitation; helium; supercritical helium (He-I); superfluid helium (He-II); forced-flow cooling; superfluid heat conduction; heat transfer; transient; forced flow of helium; superfluid helium; boundary layer; kapitza resistance

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.

Thermal-hydraulic modeling of cable-in conduit conductors (CICC) with a two-channel bundle/hole (B/H) topology contains several uncertainties in the BM coupling model. Here we study numerically with the 2-fluid MITHRANDIR code some effects of the major coupling parameters, i.e., degree of perforation of the B/H interface and heat transfer coefficient through it, on quench propagation in a two-channel CICC. A semi-quantitative discussion of the results is presented.

Several short (1 m) and long (10 m) dipole models have been tested within the scope of the on-going R&D programme for LHC at CERN. We report here the results of measurements of field quality in these dipoles, focussing on the contribution of cable magnetization. We show that the results obtained over a significant (> 10) number of magnets at 1.8 and 4.2 K are in reasonable agreement with calculati ons of the dependence of allowed harmonics on field. The calculations are based on the Bean model of filament magnetization and assume an approximate Jc(B) dependence, calibrated against low field mea surements of strand magnetization. The field quality measurements at low field also correlate satisfactorily to measurements of cable critical current at high field and geometric filament diameter.

Mithrandir [1] – a 1-D code for the analysis of thermal-hydraulic transients in cable-in-conduit super-conductors (CICC) with cooling channel – has been extended to helium II by including Gorter-Mellink heat transport. The code treats the general case of different thermodynamic properties and flow velocities for the helium in the cable bundle region and that in the cooling channel, which are coupled to conductor and jacket at different temperatures. A detailed validation of the code against a thermal-hydraulic experiment in helium II is presented, showing good agreement, and code convergence is demonstrated numerically. We present results of simulations of the QUench Initiation and Propagation Study (QUIPS) experiment, concentrating mostly on the initial phase of the quench. We show that the simulation is at least qualitatively in agreement with experimental data.

In the Quench Experiment on Long Length (QUELL), in the SULTAN facility, quench propagation was studied in a well instrumented sample wound using a Nb3Sn cable-in-conduit conductor (CICC) with central cooling channel, cooled by supercritical helium I. A few selected runs have been analyzed here by means of two different computer models — Mithrandir and Gandalf, using the same set of common input parameters, helium and material properties, heat transfer and fluid dynamic correlations. The predictions of the two codes are compared in detail with each other and against experimental data. Both codes are 1-D tools for the description of thermal-hydraulic transients in CICCs with cooling channel; Mithrandir differs from Gandalf mainly because it does not assume, as the latter does, the same thermodynamic state for the helium in the cable bundle region and the helium in the cooling channel. It turns out that although the 1-fluid model (Gandalf) gives a reasonable agreement with the experiment, 2-fluid (Mithrandir) modeling is more accurate both qualitatively and quantitatively.

Tune, chromaticity and orbit of the LHC beams have to be precisely controlled by synchronising the magnetic field of quadrupole, sextupole and corrector magnets. This is a challenging task for an accelerator using superconducting magnets, whose field and field errors will have large dynamic effects. The accelerator physics requirements are tight due to the limited dynamic aperture and the large energy stored in the beams. The power converters need to be programmed in order to generate the magnetic functions with defined tolerances. During the injection process and the energy ramp the magnetic performance cannot be predicted with sufficient accuracy, and therefore real-time feedback systems based on magnetic measurements and beam observations are proposed. Beam measurements are used to determine a correction factor for some of the power converters. From magnetic measurements the excitation of small magnets to compensate the sextupolar (b/sub 3/) and decapolar (b/sub 5/) field components in the dipole magnets will be derived. To meet these requirements a deterministic control system is envisaged.

In QUELL (QUench Experiment on Long Length) a cable-in-conduit-conductor with central cooling hole has been tested under fusion reactor relevant conditions. A first comparison is presented here between the results of a recently developed 2-fluid code-Mithrandir-and those of the reference 1-fluid code-Gandalf-for the case of the QUELL conductor. Mithrandir allows for different thermodynamic properties of the helium in the hole and that in the bundle, thereby providing a more accurate description of the physics involved when a central cooling hole is present.

Heat treatments and phase formation of Nb3Sn internal tin superconductors are more complicated than bronze route conductors due to the need to convert low melting/low decomposition temperature Sn-rich phases to higher temperature Cu-rich Cu–Sn phases. Conventionally, the Cu–Sn phase development in internal tin wires and hence heat treatment optimization and microstructure control are typically interpreted as a matter of outward Sn diffusion from the Sn core towards the Nb filaments, and Cu diffusion in the opposite direction is simply assumed. In this paper, we present a perspective of Cu diffusion, based on our investigation of phase development. We shall show that the conventional Sn diffusion perspective cannot explain some of our observations, in particular the subelement core phase development. We shall also show that the distribution of Kirkendall pores is opposite to that of the coarse Nb3Sn grains, thus establishing a direct relationship between copper diffusion and coarse Nb3Sn grain formation and distribution. We shall compare wires of different local Cu:Nb area ratio (LAR) and show how Cu diffusion appears to control the Cu–Sn phase formation across the subelement and the final Nb3Sn microstructure (and hence influences the critical current density). Drawing from what we learnt from our observation, we managed to modify a standard heat treatment and obtained up to over 20% improvement in critical current density in some of the wire designs we investigated.

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.