Harmonic measurements in superconducting magnets for a high energy accelerator

For the multipole field measurement in superconducting magnets for an accelerator application, a practical method was successfully introduced. The method uses a slow rotating coil assembly with an integator and a shaft encoder connected to a computer system. The accuracy and the quickness of the measurement was sufficient for the mass production test of the Fermilab Energy Doubler magnets.     

Analysis of voltage signals from superconducting accelerator magnets

We present two techniques used in the analysis of voltage tap data collected during recent tests of superconducting magnets developed by the Superconducting Magnet Program at Lawrence Berkeley National Laboratory. The first technique was used on a quadrupole to provide information about quench origins that could not be obtained using the time-of-flight method. The second technique illustrates the use of data from transient flux imbalances occurring during magnet ramping to diagnose changes in the current-temperature margin of a superconducting cable. In both cases, the results of this analysis contributed to make improvements on subsequent magnets.     

Acoustic emission during quench training of superconducting accelerator magnets

Acoustic emission (AE) sensing is a viable tool for superconducting magnet diagnostics. Using in-house developed cryogenic amplified piezoelectric sensors, we conducted AE studies during quench training of the US LARP’s high-field quadrupole HQ02 and the LBNL’s high-field dipole HD3. For both magnets, AE bursts were observed, with spike amplitude and frequency increasing toward the quench current during current up-ramps. In the HQ02, the AE onset upon current ramping is distinct and exhibits a clear memory of the previously-reached quench current (Kaiser effect). On the other hand, in the HD3 magnet the AE amplitude begins to increase well before the previously-reached quench current (felicity effect), suggesting an ongoing progressive mechanical motion in the coils. A clear difference in the AE signature exists between the untrained and trained mechanical states in HD3. Time intervals between the AE signals detected at the opposite ends of HD3 coils were processed using a combination of narrow-band pass filtering; threshold crossing and correlation algorithms, and the spatial distributions of AE sources and the mechanical energy release were calculated. Both distributions appear to be consistent with the quench location distribution. Energy statistics of the AE spikes exhibits a power-law scaling typical for the self-organized critical state.     

Superconducting magnets for very high fields: status in Japan

Three projects on superconducting magnets for very high fields are now in progress in Japan. The Japan Atomic Energy Research Institute constructed TMC-1 having an inner diameter of 600 mm which could produce 11.1 T by using a multifilamentary Nb₃Sn conductor. The National Research Institute for Metals has developed a multifilamentary Nb₃Sn wire with Ti doping and fabricated a high-stability superconducting magnet which will produce a magnetic field of 15 T in an 180 mm bore. Combined with this magnet, a new superconducting magnet system, which may break the field record in the superconducting state, is under construction. The High Field Laboratory for Superconducting Materials at Tohoku University constructed a hybrid magnet, in which a Nb₃Sn superconducting magnet was combined with a polyhelix type water-cooled magnet and produced a total magnetic field of 29.3 T in a 32 mm bore, where 10 T was produced by the superconducting magnet with an inner bore of 430 mm. The present status of these projects is briefly described.     

Development of stable high field Nb-Ti superconducting magnets

This Paper gives an analysis of the disturbance spectrum within an adiabatic superconducting magnet, and discusses the technical requirements for constructing a stable high field Nb-Ti magnet. Using Nb-Ti wires made in China we wound six superconducting magnets smeared with wax-Al₂O₃ or wax-Gd₂O₃ powder, except for magnet SM-3. All the magnets produced central fields higher than 8.6 T and one of them produced a central field up to 9.24 T at 4.2 K. The quenching current on the load lines was >95% as much as the critical current of short samples, and no training effect was found, except for magnet SM-3. These magnets could be energized to maximum central field at a high speed, typically 4 T min ¹, and operated stably up to 8.6 T after quenching 10 times and eight cycles between 4.2 K and room temperature. This indicates the effectiveness of the techniques developed in this Paper.     

Development of Nb3Sn superconducting wires for high field magnets at Kobe Steel and JASTEC

Research and development activities and some recent results related to Nb₃Sn superconducting wires in Kobe Steel, Ltd. (KSL) and Japan Superconductor Technology Inc. (JASTEC) are introduced. First, an outline of the activities is described briefly from a historical point of view. Following that, improvements in the characteristics (i.e., critical current density (J_c), n-value and mechanical properties) of bronze-processed Nb₃Sn wires are reviewed. Finally, the status of development for the Ta–Sn powder-in-tube (TS-PIT) process is briefly discussed.     

Ionisation chamber containing boron as a neutron detector in medical accelerator fields

A combination of the recombination principle of H(10) measurements with the use of the ionisation chambers containing boron has been presented, in order to increase the relative sensitivity of the chamber to neutrons by a factor close to the radiation quality factor of photoneutrons. Three types of the chambers were investigated. Two of them were filled with BF(3) and the third one contained electrodes covered with B(4)C. All the chambers were placed in paraffin moderators. The response of the chambers was investigated, depending on gas pressure and polarising voltage. The results showed that it was possible to obtain nearly the same response of the chamber to H(10) for photons and neutrons in a restricted energy range; however, further investigations are needed to make an optimum design.     

A novel high temperature superconducting magnetic flux pump for MRI magnets

This paper presents a kind of minitype magnetic flux pump made of high temperature superconductor. This kind of novel high temperature superconducting (HTS) flux pump has not any mechanical revolving parts or thermal switches. The excitation current of copper coils in magnetic pole system is controlled by a singlechip. The structure design and operational principle have been described. The operating performance of the new model magnetic flux pump has been preliminarily tested. The experiments show that the maximum pumping current is approximately 200 A for Bi2223 flux pump and 80 A for MgB₂ flux pump operating at 20 K. By comparison, it is discovered that the operating temperature range is wider, the ripple is smaller and the pumping frequency is higher in Bi2223 flux pump than those in MgB₂ flux pump. These results indicate that the newly developed Bi2223 magnetic flux pump may efficiently compensate the magnetic field decay in HTS magnet and make the magnet operate in persistent current mode, this point is significant to the magnetic resonance imaging (MRI) magnets. This new flux pump is under construction presently. It is expected that the Bi2223 flux pump would be applied to the superconducting MRI magnets by further optimizing structure and improving working process.     

Enhanced performance of a bolometric detector at mK temperatures in high magnetic fields

Results on the operation of a bolometer detector in high magnetic fields and at very low temperatures are presented. We tested a diamond absorber on a Ge thermistor in magnetic fields up to 8 Tesla and temperatures down to 10 mK using an⁵⁵Fe X-ray and¹⁰⁹Cd electron source. Heavily doped Ge shows a giant magnetoresistance effect. This was used to tune the thermistor impedance to an optimal value.     

Magnetic field stability of superconducting magnets

A simple qualitative model is proposed to explain the fast initial change of the magnetic field of superconducting magnets which operate in the persistent mode. Using this model the additional component of the transport current which is induced by the decaying screening currents can be represented. The analytical results are in good qualitive agreement with the experimental curves. By a suitable cyclic approach to the frozen-in current it is possible to attain very good stability from the start.     

Measurement of the energy loss in superconducting magnets

A device is described which measures the energy losses in superconducting magnets. It is excited by alternating current, and is based on the use of a Hall probe as a wattmeter. The signal from the probe, proportional to the power of the losses, integrated over a cycle of current change, corresponds to the losses in the magnet. A feature of the device consists in the use of a special measuring magnet and a system for balancing the reactive voltage, so that the losses can be measured in high-power superconducting magnets.     

Development,manufacture, and experience with superconducting beam transport magnets at CERN

This paper describes five superconducting magnets built or under construction at CERN during the past five years. These magnets have all been designed as secondary beam trans-transport magnets for the CERN 28 GeV C^?1 proton accelerator. The first two magnets, dc quadrupole magnets, has been successfully built and operated, the third, a dipole magnet, is awaiting tests, and the remaining two magnets are being built at present.     

Design and test of superconducting beamline dipole magnets

Two prototype superconducting beamline dipoles have been designed, fabricated, and tested. Each has an oval-shaped bore 0.095 m × 0.146 m and has an active length of about 1 m. The windings were impregnated with epoxy under vacuum and assembled on a heavy walled bore tube which provided structural support in combination with a stainless steel wire structural banding. The total amount of structural banding utilized was the major difference between magnets. The design for the magnets is described together with test results. Performance was essentially identical with each magnet requiring 14 quenches to surpass the air core central field design point of 2.5 Wb/m². Voltage distributions within the magnets during the quenching process were measured and controlled through utilization of a system of protective resistors. Measured results with and without the protective circuitry are given as well as calculated transients based on a simplified model.     

Induced radioactivity in rare-earth permanent magnets at 2.5 GeV electron accelerator

Rare-earth permanent magnets such as Nd₂Fe₁₄B and Sm₂Co₁₇ are the main components of the insertion devices of synchrotron radiation facilities and are used at other particle accelerators. Due to radiation safety concern the induced radioactivity in permanent magnets at a 2.5 GeV electron accelerator were studied with Monte Carlo simulations and experiments. The saturated activity of each radioactive isotope was estimated with FLUKA code (2005 version) and compared with measurement results. Three models of NEOMAX Nd₂Fe₁₄B magnets and one trivial Sm₂Co₁₇ magnet were chosen as the test magnets. The remnant dose rate at 1 m from the magnet and its cooling time dependence are calculated with the induced activities from the view of radiation protection. These are investigated under unique radiation environments due to different target conditions. The transverse distributions of induced radioactivities of several representative isotopes, which were measured at different target conditions, agree well with the electromagnetic shower characteristics at different target conditions as well as the spatial distributions of photon and neutron fluences.     

Critical current of superconducting magnets wound from an anisotropic superconducting conductor

A superconducting magnet wound from an anisotropic superconductor, critical current calculation procedure is shown. For axisymmetric solenoids with rectangular cross-section and uniform current density the coefficient K_φ(α,β) = B_φ^max/B_φ was calulated and is presented in graphical form in the plane for the angles φ = 30°, 60°, 90°. Using the coefficient K_φ(α,β) it is possible to determine the critical current for a given angle ψ. In the case of the I_c-Bβ_ψ characterisctics of the superconductor expressed analytically, the term to determine I_cφ was derived. The critical current of the solenoid is determined by the lowest value of the critical currents I_ψ. Some results of our experimental investigations, showing typical examples of a critical current anisotropy are presented.     

Behaviour of power leads for superconducting magnets

In this paper we have attempted to gather together much of the information required by the designer of dc superconducting magnets. The new aspects of this work are: more information concerning performance of leads operating away from optimum conditions; comparisons of ideal cooling and a more realistic model for gas cooling; properties of gas-cooled leads on loss of coolant.     

Stabilization of large superconducting magnets: Experimental models

The four experiments reported here address the problem of stabilization in large superconducting magnets. The first two experiments concern frictional heating caused by conductor motion; the last two experiments measure minimum quench propagation currents in simulated cooling conditions.     

Thermal stability of superconducting magnets: dynamic criteria

The stability of a specific forced convection cooled conductor is analysed with two dynamic models of different complexity. We compare the dynamic analysis with static criteria. First we introduce a dynamic model of the conductor where the coolant is modelled as a constant temperature bath. A constant heat transfer coefficient is assumed. We found a critical energy that is not a function of the disturbed length for small sizes, and is proportional to the disturbed length for larger sizes.To introduce transient effects in the helium flow we solved a two dimensional flow and thermal model of the coolant. We present some characteristics of the transient behaviour. Critical energy appears to increase from 4 mJ to 15 mJ when transient effects are considered.     

Sensitivity of a superconducting nanowire detector for single ions at low energy

We report on the characterization of a superconducting nanowire detector for ions at low kinetic energies. We measure the absolute single-particle detection efficiency η and trace its increase with energy up to η = 100%. We discuss the influence of noble gas adsorbates on the cryogenic surface and analyze their relevance for the detection of slow massive particles. We apply a recent model for the hot-spot formation to the incidence of atomic ions at energies between 0.2 and 1 keV. We suggest how the differences observed for photons and atoms or molecules can be related to the surface condition of the detector and we propose that the restoration of proper surface conditions may open a new avenue for SSPD-based optical spectroscopy on molecules and nanoparticles.