Loss and dynamic magnetic field measurements in LHC dipoles

Knowledge of AC loss and dynamic magnetic field distortion in the main LHC dipoles is both important for the assessment of the accelerator performance and providing insight into the properties of assembled magnets. We measured the loss due to the current cycling in a few 1-meter long model dipoles, 15-meter long dipole prototypes and pre-series magnets. As expected the loss depends linearly on the rate of the current change. From the slope of this dependence, the contact resistance between the strands of the opposite layers of the cable, R/sub c/, was evaluated for the inner winding of the dipole. We discuss the method to estimate the R/sub c/ value in the outer winding. The R/sub c/ value has been also derived independently from measurements of the magnetic field. For this, the ramp rate dependent component of the main field as well as of the harmonics has been measured. The main magnetic field measurements were performed using both stationary coils and Hall probes. Rotating coils were used to perform the harmonic measurements.     

Magnetic alignment measurements at room and cryogenic temperature of LHC cryodipoles and associated correctors at CERN

Considerable effort is spent at CERN on magnetic alignment measurements of main lattice LHC dipoles, including field direction, curved axis shape and position of in-built correctors, essential to verify the geometry of the assembly and to guarantee correct installation with respect to the reference beam orbit. The current baseline includes measurements of a statistically relevant percentage of cold masses and cryostated magnets before, during and after cryogenic tests. For this, we use a range of scanning probes based either on harmonic coils or fixed coils in AC mode, with laser and telescope trackers to measure position with respect to cryostat fiducials. The dipole is usually powered in "quadrupole mode" to create a convenient magnetic reference. In this paper, we first recall objectives, equipment and methods. Then, we report the status of the test activities, showing results obtained on the first pre-series dipoles, including cross-checks of various measurement systems and correlation between measurements at room and cryogenic temperatures.     

Accurate calculation of fringe fields in the LHC main dipoles

The ROXIE program developed at CERN for the design and optimization of the superconducting LHC magnets has been recently extended in a collaboration with the University of Stuttgart, Germany, with a field computation method based on the coupling between the boundary element (BEM) and the finite element (FEM) technique. This avoids the meshing of the coils and the air regions, and avoids the artificial far field boundary conditions. The method is therefore specially suited for the accurate calculation of fields in the superconducting magnets in which the field is dominated by the coil. We will present the fringe field calculations in both 2d and 3d geometries to evaluate the effect of connections and the cryostat on the field quality and the flux density to which auxiliary bus-bars are exposed     

Results of 3-dimensional structural FE-modeling of the coilend-regions of the LHC main dipoles

The transition region between the straight part and the ends of the coils of the LHC model and prototype dipole magnets are often identified as the origin of training quenches. In order to study how the discontinuities in the material properties of these regions affect coil pre-stress and possibly gain more insight in the quench behavior, a program was set up at CERN to analyze by 3D-FEM these particular regions. The ACCEL team, who performed a similar analysis for the main quadrupoles of the Superconducting Supercollider SSC, is entrusted with this program. In this paper we report on the results of 3D-modeling and analysis of the coil return end region, including the complete coil mass, of a 1-m single bore model magnet. This magnet represents all relevant features of the “two-in-one” LHC main dipole design concerning the winding configuration, the collar pack, the yoke, and the outer shell representing the He-vessel. The transition region between coil ends and straight section is modeled by slicing the magnet down to individual collar laminations per elementary level. The two-layer winding pack is represented with all individual conductor blocks, wedges, end spacers, and the interlayer spacer. Results will be presented for load cases with pre-stress after assembly at room temperature, after cool-down, and under operation at maximum current. Critical stress locations were identified in the transition into the pole free section of the magnet and in the bent part. Shimming of the coils, as well as impact from material choices and suitable alternatives are discussed     

Shape analysis of the LHC pre-series dipoles

This paper presents the results of a comprehensive analysis, from the geometric point of view, of the pre-series LHC dipoles. The progressive change of the imposed magnet shape has been monitored from the first assembly stage until after the cold test. Data concerning the error on sagitta, extremity positions and sextupolar corrector positions are provided for the pre-series magnets. Implications of aligning out-of-tolerance dipoles by the extremities are also discussed.     

Correlations between field quality and geometry of components in the collared coils of the LHC main dipoles

The Large Hadron Collider (LHC) (1995), a proton-proton superconducting accelerator, will consist of about 8400 superconducting magnet units, all operating in superfluid helium at a temperature of 1.9 K. The design of the superconducting main dipole magnets for the LHC is guided by the requirement of an extremely high field quality in the magnet aperture which is mainly defined by the layout of the superconducting coil and the position of the conductors. In order to avoid conductor movements within the magnet cross-section, the superconducting coils are held in place by surrounding stainless steel collars. In this paper, we review the dependence of field harmonics in the LHC main dipoles on dimensions of the hardware components of the collared coils. An analysis of the dimensional measurements of these components which are used in the collared coils produced so far is given. Sensitivity tables which are worked out through a coupled magneto-static model give the variation of the multipoles on collars, copper wedge dimensions and cable geometry. A Monte Carlo method is used to simulate the effects of possible errors on the multipoles.     

Magnetic field measurements of the LHC-IR MQXA magnets at room temperature

KEK has completed 13 of the 18 MQXA quadrupole magnets supplied for the LHC interaction region. The field measurements at room temperature were performed before and after the cold test. The multipole components by the warm measurements showed a good correlation with those by the cold measurements. It is concluded that field quality of the magnets can be evaluated by the warm measurements.     

Thermionic-field emission through silicon Schottky barriers at room temperature

Current transport via thermionic-field emission through Ni-Si diodes having implanted surface layers has been studied for surface fields between 10⁷ and 10⁸ V m^?1. The well defined potential profile in these structures enables a close comparison to be made between the calculated and measured thermionic-field emission currents and it is shown that the change of reverse current with field can be accurately predicted using the appropriate electron effective mass in the direction of current flow. For typical barrier heights, the field dependence of the electron current through a triangular barrier with image force correction is very nearly exponential with an effective tunnelling distance between 25 and 30 Å at 300K.     

Quench performance and field quality of the LHC preseries superconducting dipoles

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. The paper reviews the main test results of magnets tested to day in both normal and superfluid helium. The results of 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.     

Superconducting magnets for the LHC main lattice

The main lattice of the Large Hadron Collider (LHC) employs about 1600 main magnets and more than 4000 corrector magnets. All superconducting and working in pressurized superfluid helium bath, these impressive line of magnets fills more than 20 km of the underground tunnel. With almost 70 main dipoles already delivered and 10 main quadrupoles almost completed, we passed the 5% of the production and now all manufacturers have fully entered into series production. In this paper the most critical issues encountered in the ramping up in such a real large scale fabrication is addressed; uniformity of the coil size and of prestress, special welding technique, tolerances on curvature (dipoles) or straightness (quadrupoles) and of the cold mass extremities, harmonic content and, most important, the integrated field uniformity among magnets. The actual limits and the solution for improvements are discussed. Finally a realistic schedule based on actual achievements is presented.     

MOSFET's negative transconductance at room temperature

Negative transconductance is reported for the first time at T=300 K for NMOS transistors fabricated with different technologies and oxide thickness in the 3-20 nm range. The effects of drain bias, channel length, oxide thickness as well as substrate doping and bias on the phenomenon are investigated. The results are interpreted in terms of surface-roughness limited mobility, and parameters for mobility modeling at high effective fields are extracted     

Amorphous magnetic semiconductors with Curie temperatures above room temperature

Recently, amorphous magnetic semiconductors as a new family of magnetic semiconductors have been developed by oxidizing ferromagnetic amorphous metals/alloys. Intriguingly, tuning the relative atomic ratios of Co and Fe in a Co-Fe-Ta-B-O system leads to the formation of an intrinsic magnetic semiconductor. Starting from high Curie-temperature amorphous ferromagnets, these amorphous magnetic semiconductors show Curie temperatures well above room temperature. Among them, one typical example is a p-type Co_28.6Fe_12.4Ta_4.3B_8.7O₄₆ magnetic semiconductor, which has an optical bandgap of ~2.4 eV, room-temperature saturation magnetization of ~433 emu/cm³, and the Curie temperature above 600 K. The amorphous Co_28.6Fe_12.4Ta_4.3B_8.7O₄₆ magnetic semiconductor can be integrated with n-type Si to form p–n heterojunctions with a threshold voltage of ~1.6 V, validating its p-type semiconducting character. Furthermore, the demonstration of electric field control of its room-temperature ferromagnetism reflects the interplay between the electricity and ferromagnetism in this material. It is suggested that the carrier density, ferromagnetism and conduction type of an intrinsic magnetic semiconductor are controllable by means of an electric field effect. These findings may pave a new way to realize magnetic semiconductor-based spintronic devices that work at room temperature.     

Selection of the cross-section design for the LHC main dipole

With the aim of selecting the most suitable design for the series production of the LHC main dipoles, several possible configurations were analysed with respect to admissible component tolerances and structural stability, field level, field quality, number and weight of parts. Two alternatives designs, featuring common collars made out of aluminium alloy and austenitic steel, respectively, were finally compared in detail, Although both designs are almost equivalent at nominal conditions, the austenitic steel collar structure turned out to be far less sensitive to components dimensional variations. This paper reports the main results of the above evaluations, which lead to the choice of austenitic steel collars for the LHC main dipoles     

Deformations and displacements of the LHC superconducting dipoles induced by standard and nonstandard operational modes

A full-scale and fully-instrumented working model of the LHC lattice cell has been tested at CERN between March and December 2002. Aside of the current, pressure and temperature sensors controlled by an industrial supervision system, a novel device has been set to monitor magnet positions with respect to the surrounding cryostat. The series of operating modes to test cryogenics, current leads and quench recovery electronics offered the chance to investigate potentially harmful deformations of the superconducting structure. In this paper, we present a survey of displacements and deformations experienced by the LHC cell magnets during thermal cycles, current ramps and resistive transitions. Although the system complexity prevented from complete modeling, a preliminary phenomena explanation is given.     

Charge accumulation of quantum dots at room temperature

The accumulation of charge in InGaAs quantum dots has been measured at room temperature by the photoelectrochemical capacitance-voltage (CV) technique for the first time. A carrier per quantum dot ratio greater than four has been observed. The use of atomic force microscopy and low temperature and room temperature photoluminescence (PL) confirm the existence of quantum dots. Also, a possible excited state is indicated by room temperature PL in a sample with small quantum dots.     

Edge photoluminescence of single-crystal silicon at room temperature

The edge photoluminescence of single-crystal silicon (c-Si) with a peak at ～1.09 eV at room temperature is observed for structures that consist of nanocrystalline silicon (nc-Si) and c-Si. The structures are obtained by pulsed-laser deposition of an nc-Si film onto a c-Si substrate. The photoluminescence signal increases as both the density of surface states at the nc-Si/c-Si boundary and the scattering of the edge emission from c-Si in the nc-Si film decreases.     

Diffusion of iodine into CdTe at room temperature

Studies on the diffusion of iodine into CdTe at a temperature of 20°C using four widely differing types of diffusion sources are compared and discussed. The concentration profiles were measured using either a radiotracer sectioning (RTS) technique or secondary ion mass spectrometry (SIMS). The profiles were composed of four parts to which a computer package consisting of the sum of four complementary error functions (erfc) gave accurate fits, providing four empirical values of the diffusivity. The diffusivities for the fastest component in all four cases were in agreement (～2 × 10^?14 cm² s^?1) and were consistent with previously published data. These results indicate that when iodine is diffused from the vapour it is not a suitable long-termstable dopant in devices where sharp junctions are required.     

Statistical studies of the robustness of the LHC main dipolemechanical structure

This paper describes two methods used to study the effect of the tolerances of the components on the structure of the LHC main dipole. The first method, called semi-statistical, is useful for the determination of the acceptable variance of the dimensions of magnet components. The second one, fully statistical, allows the study of the combined effect of many parameters. The use of these two methods allowed to evaluate with good confidence the robustness of two different dipole cross-section designs, featuring austenitic and aluminium alloy collars, respectively