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.     

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.     

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     

Status of the production of the LHC superconducting corrector magnets

The Large Hadron Collider (LHC) will be equipped with a large number (6400) of superconducting corrector magnets. These magnets are powerful, with typical peak fields of 3-4 T on the coils, but at the same time compact and of low cost. There are many types: sextupoles, octupoles and decapoles to correct the main dipole field, dipoles, quadrupoles, sextupoles and octupoles to condition the proton beams and several nested correctors from dipole to dodecapole in the inner triplets. The sizes vary from 6 kg, 110 mm long, nested decapole-octupole spool pieces to 1800 kg, 1.4 m long, trim quadrupoles. The fabrication of the 11 different types of magnets is assured by 10 contracts placed at 6 firms, two of which are in India. A number of magnets are now in series production, others in their pre-series production. The paper describes the present state of the fabrication and the testing of these magnets.     

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     

Production follow-up of the LHC main dipoles through magnetic measurements at room temperature

In this paper, we review the tools used for controlling the production of the LHC main dipoles through warm magnetic measurements. For the collared coil measurements, control limits are based on the statistics relative to the pre-series production. For the cold mass, the difference between collared coil and cold mass is considered, allowing a very stringent test. In both cases, measurements are split in straight part average, variations and coil ends contributions. Two different alarm levels exist in case the measured field is out of limits. The analysis can be carried out at the manufacturer and allows detection of anomalies in the measured magnetic field. These can be either due to wrong measurements or caused by assembly defects. Techniques used to work out information on the magnet assembly from the field harmonics are outlined. We summarize the experience gathered on about 180 collared coils and 120 cold masses, pointing out the bad cases and investigating the reliability of the measurements.     

Effective length and input impedance of the NIST standard dipoles

Computed values for the effective length and input impedance for a set of standard dipoles used at the National Institute of Standards and Technology are presented. These dipoles are used as calculable reference antennas in the calibration of dipole-like electromagnetic interference antennas in an open-area test site. Estimates for the uncertainty in the computed values of the effective length and antenna impedance are also given     

Construction and qualification of the pre-series MQM superconducting quadrupoles for the LHC insertions

The LHC insertions will be equipped with individually powered MQM superconducting quadrupoles, produced in three versions with magnetic lengths of 2.4 m, 3.4 m, and 4.8 m. The quadrupoles feature a 56 mm aperture coil, designed on the basis of an 8.8 mm wide Rutherford-type NbTi cable for a nominal gradient of 200 T/m at 1.9 K and 5390 A. A total of 96 quadrupoles are in production in Tesla Engineering, UK. In this report we describe the construction of the pre-series MQM quadrupoles and present the results of the qualification tests.     

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.     

Fabrication of the pre-series wide aperture superconducting quadrupoles for the LHC insertions

Individually powered superconducting quadrupoles with a coil bore of 70 mm will be installed in the LHC insertions, in areas where increased geometrical acceptance and improved field quality are required. The quadrupoles feature a four-layer coil, designed on the basis of two graded 8.3 mm wide Rutherford-type NbTi cables. The magnets have a magnetic length of 3.4 m and a nominal gradient of 160 T/m at 4.5 K and 3610 A. A total of 26 quadrupoles are in production at ACCEL Instruments (Germany). In this report, we present the experience in fabrication of the pre-series magnets and the results of the initial qualification tests.     

Development of superconducting tuning quadrupole corrector (MQT) prototypes for the LHC

The main quadrupoles of the Large Hadron Collider (LHC) are connected in families of focusing and defocusing magnets. In order to make tuning corrections in the machine a number of quadrupole corrector magnets (designated MQT) are necessary. These 56 mm diameter aperture magnets have to be compact, with a maximum length of 395 mm and a coil radial thickness of 5 to 7.5 mm, while generating a minimum field gradient of 110 T/m. Two design options have been explored, both using the "counter-winding" system developed at CERN for the fabrication of low cost corrector coils. The first design, with the poles composed of two double-pancake coils, each counter-wound using a single wire, superposed to create 4-layer coils, was developed and built by ACCEL Instruments GmbH. A second design where single coils were counter-wound using a 3-wire ribbon to obtain 6-layer coils was developed at CERN. This paper describes the two designs and reports on the performance of the prototypes during testing.     

Random errors induced by the superconducting windings in the LHCdipoles

The problem of estimating the random errors in the LHC dipole is considered. The main contributions to random errors are due to random displacements of the coil position with respect to nominal design and to the variation of the magnetization of the superconducting cable. Coil displacement can be induced either by mechanical tolerances or by the manufacturing process. Analytical and numerical scaling laws that provide the dependence of the random errors due to random displacements on the multipolar order are worked out. Both simplified and more realistic models of the coil structure are analysed. The obtained scaling laws are used to extract from experimental field shape data the amplitude of the coil displacements in the magnet prototypes. Finally, random errors due to interstrand resistance variation during the ramp are estimated     

Principles developed for the construction of the high performance, low-cost superconducting LHC corrector magnets

The Large Hadron Collider (LHC) needs more than 6000 superconducting corrector magnets. These must be sufficiently powerful, have enough margin, be compact and of low cost. The development of the 11 types of magnets was spread over several years and included the magnetic and mechanical design as well as prototype building and testing. It gradually led to the systematic application of a number of interesting construction principles that allow to realize the above mentioned goals. The paper describes the techniques developed and presently used in practically all the LHC corrector magnets ranging from dipoles to dodecapoles.     

S-N transition of HIGH-Tc superconducting ring caused by induced current

The experimental investigation results of superconducting-normal state transitions in a high-T_c superconducting ring with induced current are presented. A transformer device is used, where the superconducting ring forms a secondary winding of a transformer. Using the analysis method based on an equivalent circuit of the transformer, the voltage and current in the ring vs time are obtained from oscilloscope traces of the voltage and current in the primary winding of the device. The dynamic voltage-current characteristic of the ring is found to have a hysteresis. This hysteresis is explained by the inertia of thermal processes and propagation of a resistive zone in the ring.     

Analysis of the operational characteristics of a heater-triggertype high-Tc superconducting power supply

This paper deals with the design and fabrication of a heater-trigger type high-T_c superconducting (HTSC) power supply, and its characteristics has been analyzed through experiments. A heater-trigger HTSC power supply consists of two heaters, an electromagnet, and a YBCO superconducting bulk. In this experiment, a 0.6-T magnet and a 2.3-A dc heater current were used and 190 s and 380 s were used for the pumping period. In order to measure the pumping current with respect to the magnet flux changes, hall sensors were installed at the surface of the YBCO bulk and inside of an iron core. The experimental observations have been compared with the theoretical predictions. In this experiment, the pumping current has reached about 12 A. In computer simulation, the maximum pumping current of the system has been predicted to be about 13 A     

Design and fabrication of a 1 m model of the 70 mm bore twinaperture superconducting quadrupole for the LHC insertions

For reasons of geometrical acceptance, 70 mm bore twin aperture quadrupoles are required in the LHC insertions. For an operating gradient of 160 T/m at 4.5 K, a design based on a four layer coil wound from two graded 8.2 mm NbTi conductors has been developed. Three 1 m single aperture quadrupoles of this design have been built and successfully tested. Thereafter, the magnets have been disassembled and the coils re-collared using self-supporting collars. In this paper, we describe the design features of the twin aperture quadrupole, and report on the initial collaring tests and procedures for collaring and final assembly of the 1 m magnet     

Effects of the operational modes on the temperature dependence of the Gunn diode admittance

the analytical expressions for the large-signal admittances of Gunn diodes including the temperature dependence have been derived theoretically for the quenched- and delayed-domain modes by using a piece-wise linear approximation for the velocity-field characteristic. As the main physical parameters affecting the temperature dependence of the Gunn diode admittance, the following have been considered; the domain transit time, the domain formation time, the domain extinction time, the low-field mobility and the dielectric constant. The analysis shows that the diode conductance decreases with increasing microwave voltage and temperature for both the quenched- and delayed-domain modes, whereas the dependence of the diode susceptance on the microwave voltage and temperature is different between the two modes. This difference is explained by the temperature dependence of the domain transit time, the domain formation time, and the low-field mobility. The variation of oscillation frequency due to the change of the diode admittance has also been discussed.     

Analysis of the operational characteristics of a high-Tc superconducting power supply with the Bi-2223 pancake load

This paper presents the design and fabrication of a high-Tc superconducting (HTSC) power supply with the Bi-2223 pancake load, as well as its characteristics as determined through experiments. HTSC power supply consists of two heaters, an electromagnet, a Bi-2223 solenoid, and a Bi-2223 pancake load. In this experiment, a 331-mH electromagnet and 0.8-A dc heater current were used, and 8.5 and 17 s were used for pumping periods, respectively. Mechanism of the superconducting switch is used for heater-trigger. In order to measure the pumping-current with respect to the magnet flux changes, a hall sensor was installed at the center of the Bi-2223 pancake load. The experimental observations have been compared with the theoretical predictions. In this experiment, the pumping-current has reached about 0.6 A with 0.01-V electromagnet voltage.     

Identification of steady-state operational modes of the seriesresonant DC-DC converter based on loosely coupled transformers inbelow-resonance operation

This paper identifies all the steady-state operational modes of a series-loaded resonant DC-DC power converter (SRC) in below-resonance operation, taking into account the coupling quality of the isolating transformer. A novel operational plane is defined, and ten operational modes are identified in this plane. Borders between neighboring domains of these operational modes are analytically defined. Results are given in normalized form and experimentally verified     

Flexible organic transistors on standard printing paper and memory properties induced by floated gate electrode

Integrating electronic devices with unconventional substrate has been a popular research direction. Among these substrates, cellulose fiber paper has advantages in low-cost, recyclable and bio-degradable. We demonstrated directing printing of all contact electrodes on standard untreated Fuji Xerox printer paper without using planarization layer. The screen-printed gate electrodes based on silver nanoparticles can smooth out the paper substrate surface by two orders of magnitude and allow us to use parylene and DNTT as the dielectric and active layer directly. The transistors show average mobility of 0.297 cm² V⁻¹ s⁻¹ and on/off ratio larger than 10⁵. The low leakage current allows us to demonstrate memory properties by employing the floated gate method. The devices show excellent memory retention time for more than 10,000 s. The unique flexibility and combustibility of the organic transistors on paper substrate manifest their applications as next generation of green electronics.