Numerical Analysis of the Screening Current-Induced Magnetic Field in the HTS Insert Dipole Magnet Feather-M2.1-2

Screening currents are field-induced dynamic phenomena which occur in superconducting materials, leading to persistent magnetization. Such currents are of importance in ReBCO tapes, where the large size of the superconducting filaments gives rise to strong magnetization phenomena. In consequence, superconducting accelerator magnets based on ReBCO tapes might experience a relevant degradation of the magnetic field quality in the magnet aperture, eventually leading to particle beam instabilities. Thus, persistent magnetization phenomena need to be accurately evaluated. In this paper, the 2D finite element model of the Feather-M2.1-2 magnet is presented. The model is used to analyze the influence of the screening current-induced magnetic field on the field quality in the magnet aperture. The model relies on a coupled field formulation for eddy current problems in time-domain. The formulation is introduced and verified against theoretical references. Then, the numerical model of the Feather-M2.1-2 magnet is detailed, highlighting the key assumptions and simplifications. The numerical results are discussed and validated with available magnetic measurements. A satisfactory agreement is found, showing the capability of the numerical tool in providing accurate analysis of the dynamic behavior of the Feather-M2.1-2 magnet.Comment: 14 pages, 18 figure

High-temperature superconducting screens for magnetic field-error cancellation in accelerator magnets

Accelerators magnets must have minimal magnetic field imperfections to reduce particle-beam instabilities. In the case of coils made of high-temperature superconducting (HTS) tapes, the magnetization due to persistent currents adds an undesired field contribution, potentially degrading the magnetic field quality. In this paper we study the use of superconducting screens based on HTS tapes for reducing the magnetic field imperfections in accelerator magnets. The screens exploit the magnetization by persistent currents to cancel out the magnetic field error. The screens are aligned with the main field component, such that only the undesired field components are compensated. The screens are self-regulating, and do not require any externally applied source of energy. Measurements in liquid nitrogen at 77 K show for dipole-field configurations a significant reduction of the magnetic field error up to a factor of four. The residual error is explained via numerical simulations accounting for the geometric defects in the HTS screens, achieving satisfactory agreement with experimental results. Simulations show that if screens are increased in width and thickness, and operated at 4.5 K, field errors may be eliminated almost entirely for the typical excitation cycles of accelerator magnets

High Temperature Superconductor Accelerator Magnets

For future particle accelerators bending dipoles are considered with magnetic fields exceeding $20T$. This can only be achieved using high temperature superconductors (HTS). These exhibit different properties from classical low temperature superconductors and still require significant research and development before they can be applied in a practical accelerator magnet. In order to study HTS in detail, a five tesla demonstrator magnet named Feather-M2 is designed and constructed. The magnet is based on ReBCO coated conductor, which is assembled into a $10kA$ class Roebel cable. A new and optimized Aligned Block layout is used, which takes advantage of the anisotropy of the conductor. This is achieved by providing local alignment of the Roebel cable in the coil windings with the magnetic field lines. A new Network Model capable of analyzing transient electro-magnetic and thermal phenomena in coated conductor cables and coils is developed. This model is necessary to solve critical issues in coated conductor accelerator magnets, such as thermal stability, quench propagation and field quality. The electrical part of the Network Model is validated using a series of benchmark simulation tests and measurements. The model is then used to calculate the dynamic field quality in coil structures. It is concluded that dynamic compensation of the field quality is needed. This can be achieved by a series of persistent current shim coils that are inserted inside the aperture, which is a novel concept introduced in this thesis. The thermal part of the Network Model is validated by comparing it to an earlier experimentally validated one-dimensional single tape model. Using this a quench is modeled in a section of Roebel cable and later in the full Feather-M2 coil. It is shown that the initialization of the quench consists of three phases: Thermal Drift, Pre-Quench and Quench. The Drift phase can be detected using temperature sensors and the Pre-Quench phase using an array of pick-up coils. These quench detection techniques, provided sufficient margin is maintained, could allow for safe operation of HTS coils up to very high current densities. An iterative layout optimization algorithm is implemented and used to study layouts at the magnetic field limit of Nb$_3$Sn. It is shown that grading of the Nb$_3$Sn is essential for reaching magnetic fields beyond $14T$ with $20\%$ margin and that operation at $1.9K$ and improving the critical current density of the Nb$_3$Sn conductor are highly recommended. Additionally, preliminary results are shown for layouts using HTS in the high magnetic field region of the coil in $20T$ magnets

Parameterization of the critical surface of REBCO conductors from Bruker

Parameterization of the critical surface of REBCO conductors will help determine the performance of such high temperature superconducting tapes

Mechanical design of a nested 4-layer Canted Cosine Theta (CCT) dipole

Nested CCT dipoles could be used in particle accelerators and proton therapy machines to bend a beam of charged particles in any direction. In the present study, a mechanical design of a 4 layer nested CCT dipole is evaluated with Finite Element Method (FEM). A full parametric 3D model of a 2.5 Tm 4-layer CCT dipole has been developed using the APDL scripting in the ANSYS software. The, so called, bottom up approach with direct generation of nodes and elements has been utilized, optimizing for the speed of the model generation as the time consuming meshing was bypassed. The properties of the Nb-Ti strands with the surrounding CTD-101K epoxy were obtained with a dedicated homogenization model. Resulting orthotropic properties were fully accounted for in the simulation. The shear stresses in the bonding composite layer: Kapton+S2-glass+CTD-101K were computed. With the results above the limit of 10 MPa, the necessity to provide additional rigidity against the torque was confirmed – castellated design. The shear stresses were mostly caused by thermal effects, and only ~25 % was caused by the Lorentz forces. The influence of the boundary conditions was analysed, leading to their optimal choice limiting the deformation due to Lorentz forces to 91 μm

Guiding secondary school students during task selection

Secondary school students often learn new cognitive skills by practicing with tasks that vary in difficulty, amount of support and/or content. Occasionally, they have to select these tasks themselves. Studies on task-selection guidance investigated either procedural guidance (specific rules for selecting tasks) or strategic guidance (general rules and explanations for task selection), but never directly compared them. Experiment 1 aimed to replicate these studies by comparing procedural guidance and strategic guidance to a no-guidance condition, in an electronic learning environment in which participants practiced eight self-selected tasks. Results showed no differences in selected tasks during practice and domain-specific skill acquisition between the experimental groups. A possible explanation for this is an ineffective combination of feedback and feed forward (i.e. the task-selection advice). The second experiment compared inferential guidance (which combines procedural feedback with strategic feed forward), to a no-guidance condition. Results showed that participants selected more difficult, less-supported tasks after receiving inferential guidance than after no guidance. Differences in domain-specific skill acquisition were not significant, but higher conformity to inferential guidance did significantly predict higher domain-specific skill acquisition. Hence, we conclude that inferential guidance can positively affect task selections and domain-specific skill acquisition, but only when conformity is high

Quench Behavior of the HL-LHC Twin Aperture Orbit Correctors

A study was performed to understand the quench behavior and ensure adequate quench protection of the canted cosine theta (CCT) twin aperture orbit corrector magnet, a superconducting magnet under development as part of the high-luminosity upgrade of the Large Hadron Collider (HL-LHC). The cosine theta geometry features canted superconducting coils, which together produce a magnetic dipole field. The NbTi/Cu strands are placed in slots inside formers that maintain the shape of the coils. The presence of these formers affects the quench behavior of the magnet by preventing direct thermal contact between adjacent groups of strands. At the same time, a discharge of the stored energy over an external resistor results in significant eddy current heating inside the formers, which quickly brings the entire superconducting magnet to a normal state. A calculation model was developed that describes the electrical and thermal behavior of this type of magnet, and the results of this model are compared to experimental observations on a 0.5 m CCT model coil. It is found that the calculation results and experimental observations are generally consistent, although the simplified manner in which the eddy current heating is described in the model leads to a modest overestimation of the hotspot temperature. The calculation results indicate that a proposed quench protection configuration, featuring a discharge over a 0.7 $\Omega$ energy extractor and a 0.05 $\Omega$ crowbar, is sufficient to protect both the 0.5 m CCT model magnet and the 2.2 m CCT prototype magnet, resulting in hotspot temperatures of 63 and 193 K, and peak voltages to ground of 300 and 310 V, respectively

Quench Protection of Very Large, 50-GJ-Class, and High-Temperature-Superconductor-Based Detector Magnets

An investigation is performed on the quench behavior of a conceptual 50-GJ 8-T high-temperature-superconductor-based solenoid. In this design, a 50-kA conductor-on-round-core cable-in-conduit conductor utilizing ReBCO technology is envisioned, operating at 40 K. Various properties such as resistivity, thermal conductivity, and heat capacity are very different at this temperature, which affects the quench behavior. It is found that the envisioned conductor is very stable with a minimum quench energy of about 2 kJ. However, the quench propagation velocity is typically about 20 mm/s, so that creating a wide-spread normal zone throughout the coil is very challenging. Moreover, an extraction voltage exceeding 20 kV would be required to ensure a hot-spot temperature below 100 K once a thermal runaway occurs. A novel concept dubbed “rapid quench transformation” is proposed whereby the superconducting conductor is co-wound with a normal conductor to achieve a high degree of inductive coupling. This geometry allows for a significant electric noise reduction, thus enabling low-threshold quench detection. The secondary circuit is connected in series with a stack of diodes, not allowing current transfer during regular operation, but very fast current transfer once a quench is detected. With this approach, the hot-spot temperature can be kept within 20 K of the cold mass temperature at all times, the hot-spot temperature is well below 100 K, and just under 80% of the stored energy can be extracted during a quench