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.     

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.     

A simple model for the prediction of quench point in cable-in-conduit conductors

A simple 0D analytical model for the prediction of the take-off point observed in critical-current or current-sharing tests in cable-in-conduit conductors (CICC) is presented. This may be referred to also as the quench point and defined as the point where the stationary thermal equilibrium between the conductor and helium is lost. Assuming a power-law parameterization for the current–voltage characteristic of the superconductor, specified by the index n and voltage criterion E_c, the quench point can be unambiguously defined as the contact point between the heat-generation curve and the cooling curve. The model offers a straightforward approach to estimating the quench behavior of CICC and provides a clear view of the relationships between conductor parameters and its quench properties. The results of the model are close to the experimental results thus validating it as a useful design criterion.     

Protection of large superconducting magnets: maximum permissible undetected quench voltage

For a quench detection scheme based on voltage measurement, a formula is derived for the maximum level of quench voltage that can be left undetected. This voltage sets a minimum voltage that must be detected by any voltage detection circuit. For ‘stable’ superconducting magnets wound with copper substrate composite conductor, this voltage is tens of millivolts, independent of current density. A formula is derived for the maximum permissible circuit delay time for the actuation of a dump circuit.     

Simulation of quench for the cable-in-conduit-conductor in HT-7U superconducting Tokamak magnets using porous medium model

A cable-in-conduit-conductor (CICC) consists of superconducting cable, copper, supercritical helium and conduit. To keep the operating temperature of superconducting cable lower than its current sharing temperature, the supercritical helium is forced flow through the CICC. The supercritical helium through the cable bundle has the complex directional changes due to the interaction between the supercritical helium and strands. The structure of CICC is characterized with the porous medium. The quench characteristics of CICC are analyzed by the model which the temperature difference between the strands and helium is assumed to be very small due to the heating induced flow to generate high heat transfer coefficient of supercritical helium. A moving mesh method is developed for the numerical solution of the problem with the steep drop for temperature and density of supercritical helium in the short front region of the normal zone. The computational mesh is obtained by equidistribution of a monitor function tailored for the functional variation of the arguments for density, temperature and velocity of supercritical helium. Existence and uniqueness of the discretised equations using a moving mesh are also established. The coupled equation for porous medium is solved using the finite element method with the artificial viscosity term. The validation of the code is tested by comparing it with the other codes with good accuracy. The converged properties of numerical solution due to quench in CICC are studied. We present preliminary estimates of the maximum conductor temperature rise and helium pressure during a quench in the inner layer of toroidal field (TF) magnet for HT-7U. The quench scenarios with different dump time constants of 6.25, 12, and 21.1 s are considered. The goal of such work is to guide the protection scheme and a detailed prediction of the quench evolution of magnet.     

Photoneutron target development for the RPI linear accelerator

Two new photoneutron targets have been developed for neutron time-of-flight experiments, the axial water-moderated target (AWMT) and the bare bounce target (BBT). These targets operate without any lead shielding nearby and both have superior neutron resolution compared to the older bounce target. The BBT has been selected over the AWMT for general time-of-flight measurements because it exhibited lower neutron background in the keV energy region.     

Towards a better understanding of the physics of the two-volume model of accelerator magnet quench thermohydraulics

The LHC currently under construction at CERN will make intensive use of high-field, twin aperture superconducting magnets operating in static baths of pressurized helium II at 1.9 K and at about 100 kPa. As long as magnet construction guarantees pressure homogeneity after a resistive transition by allowing sufficient radial venting, the quench thermohydraulics can be depicted by a mathematical model based on the assumed helium split between two hypothetical volumes of confined and bulk helium. The first phase of the process is dominated by fast adiabatic compression of bulk helium driven by the expanding confined helium while in the second phase the whole amount of the helium undergoes isochoric heating. We make the first conjecture that the confined helium is not solely composed from the helium filling the cable porosities, but also from a thin helium layer surrounding the coil. The second conjecture is that the onset of the isochoric heating phase is a result of fast heat transfer via the magnet collars to the bulk helium, still resting in a superfluid state, quantitatively overtaking in importance the adiabatic compression route as a means of energy transfer. Our hypothesis of the transition mechanism from adiabatic compression of the bulk helium to isochoric heating has been confirmed numerically and has been experimentally justified with String 2 measurements.     

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.     

Assessment of concepts for Fatigue Crack initiation and propagation life prediction

A great variety of fatigue life prediction concepts has evolved during the last decades. Nevertheless, there is a lack of satisfying fundamental theories for the evaluation of fatigue damage under variable amplitude loading; progress has been made, however, in the development of numerical prediction concepts which are backed up by experimental work and are, therefore, used as relative concepts. In the paper, some basic requirements concerning the assessment of fatigue life prediction methods are discussed. A survey on different concepts is given and capabilities and limitations are elucidated by typical results selected from the literature.     

Note on the modified Aitken accelerator for use in the residual force method

Some considerations regarding the application of the modified Aitken accelerator to solution of problems of elasto-plasticity by the residual force method are discussed. An efficient technique for continuing iterations after an acceleration is given.     

纳米晶复合永磁材料研究进展

就近年来纳米晶双相复合永磁材料的发展状况,从该类合金的矫顽力理论和提高磁性能的方法2个方面为重点做简要的评述.     

Probabilistic prediction models for crack initiation and progression of spray sealed pavements

Cracking is one of the primary distress modes in spray (chip)-sealed pavement surface performance and its prediction is a major concern for pavement engineers. In order to identify, manage and asses effectively and efficiently cracked pavement at a network level, a probabilistic modelling approach is utilised to develop cracking initiation and progression models. This study aims to predict the probability of pavement cracks occurring using a binary logistic model and cracks progression over time using an ordinal logistic regression model. These models have been developed to take into account the effect of variations among observations, among sections and among highways. Readily available historical time series data (from 2004 to 2011) from 40 highway segments have been collected and prepared for modelling. These time series include surface cracking as a performance parameter and traffic loading, expansion potential of subgrade soil, climate condition, condition of drainage system and pavement strength as predictor parameters. Cracking data include all types of cracking: transverse, longitudinal and crocodile cracking and is reported as a percent of the affected area. The study estimates the probability of crack initiation at a certain time and predicts the probability of a pavement maintaining its current level of cracking. It is found that with the 50% estimated probability, about 82% of the observations are correctly predicted by the crack initiation model and 65% of the observations are correctly predicted by the crack progression model. The study has concluded that the effect of time is stronger than the other variables on crack initiation and progression. Also, the effect of traffic loading is stronger than the effect of initial pavement strength in crack initiation phase. However, the effect of pavement strength at any time is stronger than the effect of traffic loading in crack progression phase. The predicted probabilities have been successfully validated using another data-set from the same network and the results indicate that the developed probability models are well estimating the crack conditions and have the ability to predict future conditions accurately.     

Analytical modeling of bone-cement interface and failure prediction

Artificial joint replacement is becoming increasingly important in orthopedics. Several hundreds of thousands of operations, especially of the hip joint, have already been performed. The design of such joints depends largely on how accurately they can be modeled analytically such that their load transmission characteristics can be determined. One of the most critical areas of investigation is the interface existing between bone and implant in the orthopedic prostheses applications. This investigation examines the effect of varying modulus between bone and cement (PMMA) on the failure of the joint. An energy criterion is used rather than the stress criteria normally applied in the open literature.

The results of the present study show that the modulus variation within the interface can have a significant influence on the stress and energy fields in the region near a material interface. It is found that if the interface modulus has a gradual variation, the predicted stress fields resemble those with a single material interface model which has a higher (stiffer) average modulus. The interface modulus variation and average modulus are shown to have a significant effect on the predicted location and onset of failure which is of primary interest. The present modeling scheme is intended to demonstrate some of the effects which might locally be found near the bone and cement interface in a prosthetic joint.     

Measuring,monitoring, and testing equipment for development and manufacture of permanent magnets

The paper presents a list of measuring and testing equipment used in the development and manufacture of permanent magnets based on modern hard-magnetic materials.Translated from Izmeritel'naya Tekhnika No. 7, pp. 43–44, July, 1993.     

Quench development in magnets made with multifilamentary NbTi cable

An experimental study of the normal zone propagation and total resistance as a function of time is described. The normal zone propagation velocity in a single strand was measured as a function of both current and magnetic field with particular interest in the neighborhood of the short sample limit. The study proceeded from measurements in single multifilamentary strands to measurements in 23 strand cables under different cooling environments to finally measurements in actual ramping dipole magnets made with this cable. Interpretation of the results led to the determination of safety limits for the Energy Doubler/Saver magnets and to safeguards implemented by an energy dumping circuit which effectively protects these magnets from self-destruction.     

Life prediction for rolling contact fatigue crack initiation of kiln wheels

This paper investigates the rolling contact fatigue life of kiln wheels with respect to the axis line deflection related with the applied supporting loads on wheels. Fatigue crack initiation criterions for elastic shakedown, plastic shakedown, and ratcheting material responses are applied to assess wheels responses with two sets of axial line deflection, one is measured in field and the other is optimal adjustment for the measured axial line deflection. The finite element simulations are performed by using the Bilinear material mode for nonlinear and kinematic hardening within ANSYS 11.0. By comparing life prediction from different criterions, it is showned that the low-cycle fatigue is the predominated failure. Results from different axial line deflection indicate that the optimum adjustment can greatly enhance the whole life of the supporting structure, that is useful for kiln adjustment and maintenance.