Andrew V. Gavrilin

Analysis of the normal transition event of the LHD helical coils

Normal transitions and a subsequent quench were experienced with the pool-cooled helical coils of the Large Helical Device (LHD) during its excitation test. Although the initiated normal zone once started to recover, a disruptive transverse propagation followed and triggered an emergency discharging program. The cryogenic stability of the composite-type superconductor has been studied by sample experiments as well as by numerical calculations. Due to the rather long magnetic diffusion time constant in the pure Al stabilizer, transient stability of the conductor seems to play an important role for driving finite propagation of a normal zone. The cause of the final quench is also discussed from the viewpoint of cooling deterioration due to a possible accumulation of He bubbles.

Analysis of the Quench Protection System of a Series Connected Hybrid Magnet

An active quench protection system is planned to safely guard the superconducting coils of a 36 T hybrid magnet during a quench. The quench circuit includes a 0.1 Omega external discharge resistor, blocking diodes, and high-current circuit breakers for isolation of the power supplies during a dump. The protection system will rely on redundant hardware encoded circuits for detection of quench voltages. A quench modeling program has been created that incorporates GANDALF as the thermal and hydraulic module. Quench simulations have been performed to model temperatures, pressures, and mass flow rates of the helium and conductor. The response of the system under a protected quench has been analysed and it is shown that the coils would be adequately protected with a quench detection voltage of 1 V. Energy and helium expulsion rates during a quench are computed to determine the capacity and configuration of the cryogenic refrigeration system during off-normal events.

Stability Characteristics of the Aluminum Stabilized Superconductor for the LHD Helical Coils

Stability tests have been carried out on the aluminum stabilized composite-type superconductors developed and used for the pool-cooled helical coils of the Large Helical Device (LHD). The longitudinal voltage of a normal zone shows a short-time rise before reaching a final value, which seems to be explained by the rather slow diffusion process of transport current into the pure aluminum stabilizer and the copper housing. The propagation velocity has a finite value even below the recovery current, and it differs depending on the direction of transport current. A numerical analysis dealing with transient thermal and electromagnetic relaxation processes in the aluminum stabilizer well explains the experimentally observed results.

Computer simulation and experimental study of quench in superconducting epoxy-impregnated multi-layer coil

The quench behaviour of a single superconducting epoxy-impregnated coil is analyzed in full by means of a computer code developed, in comparison with detailed experiment in which time-dependent temperature of composite wire (cable) at different points of the winding, current decay, electrical voltages across turns, normal zone front velocities in all directions are measured. The 3D transient heat conduction equation with a source term reduces to the set of 1D transient non-linear differential-integral-difference heat balance equations governing dynamically interdependent processes of heat propagation along helix-shaped wire within the winding layers and of heat transfer (in radial and axial directions) through inter-turn insulation of finite thickness. The heat balance equations coupled with the circuit equation for the transport current are solved numerically by the finite difference method. The effect of the helical-discrete structure of the winding formed by helix-shaped metallic wire, the turns and layers of which are separated by weakly conducting insulating material, on the winding temperature profile and the normal zone front velocity is demonstrated.

Computer Simulation of Normal Zone Propagation in the LHD Helical Coils

An approach to high-accuracy computer simulation of normal zone propagation in the helical coils of the Large Helical Device is suggested. Main peculiarities of complicated electromagnetic and thermal processes in the helical coil cable are taken properly into account by means of a simple model. A comparison was made between the simulation results and experimental data.

Optimized HTS current leads

The problem of optimizing HTS current leads by varying their cross-section along the length is investigated both experimentally and numerically at 500 A current level. Bi-2223-based HTS multi-filament composite tapes were used with two types of matrices: pure Ag and (Ag+1 at.% Au) alloy. The warm ends of the HTS parts of the current leads were cooled with liquid nitrogen. Very low evaporation rates in the case of Au-doped matrix and rather long time constants to reach thermal equilibrium were observed.

Design of Cryogenic System for SCH Magnets

Two series-connected hybrid (SCH) magnets are under development at the National High Magnetic Field Laboratory. The first SCH is for the Helmholtz Centre Berlin (HZB) in Germany. The HZB SCH will be a horizontal bore, 30 T magnet and will be used for neutron scattering experiments. The second SCH is for the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, FL. The NHMFL SCH will be a vertical bore, 36 T magnet. Both SCH Magnets combine a set of resistive Florida-Bitter coils with a superconducting outsert coil constructed of cable-in-conduit conductor (CICC). The two SCH magnets are designed for various operating scenarios including those with multiple ramp cycles at various rates. Both of the superconducting magnets are forced flow cooled with supercritical helium at 4.5 K. A standard refrigerator with a capacity of about 150-200 W at 4.5 K will be used to supply the cooling power and the forced mass flow rate. The cryogenic system of the SCH magnet consists of a helium refrigerator, a valve box with subcooler, a magnet cryostat and cryolines. In this paper, the design of the cryogenic system is described.

Analysis on the cryogenic stability and mechanical properties of the LHD helical coils

Transient normal-transitions have been observed in the superconducting helical coils of LHD. Propagation of a normal-zone is analyzed with a numerical simulation code that deals with the magnetic diffusion process in a pure aluminum stabilizer. During excitation tests, a number of spike signals are observed in the balance voltage of the helical coils, which seem to be caused by mechanical disturbances. The spike signals are analyzed by applying pulse height analysis and the mechanical properties of the coil windings are investigated.

Modeling of electromagnetic and thermal diffusion in a large pure aluminum stabilized superconductor under quench

Low temperature composite superconductors stabilized with extra large cross-section pure aluminum are currently in use for the Large Helical Device in Japan, modern big detectors such as ATLAS at CERN, and other large magnets. In these types of magnet systems, the rated average current density is not high and the peak field in a region of interest is about 2-4 T. Aluminum stabilized superconductors result in high stability margins and relatively long quench times. Appropriate quench analyses, both for longitudinal and transverse propagation, have to take into account a rather slow diffusion of current from the superconductor into the thick aluminum stabilizer. An exact approach to modeling of the current diffusion would be based on directly solving the Maxwells equations in parallel with thermal diffusion and conduction relations. However, from a practical point of view, such an approach should be extremely time consuming due to obvious restrictions of computation capacity. At the same time, there exist certain ways that simplify mathematical models for the thermal and electromagnetic diffusion processes for the purpose of rapidly calculating the propagation velocity and effective simulating of quench behavior. These models explained here were tested and applied to quench simulation in the above-mentioned magnet systems.

Transient Stability Analysis of the Superconducting Outsert in the NHMFL Series Connected Hybrid Magnet System

Transient stability analysis of the superconducting outsert of a 36 T series-connected hybrid (SCH) magnet being developed at the NHMFL is performed. The latest design version of the SCH outsert primary coil will consists of three radial sections, designated as a low-field, a midfield and a high-field ones, each wound with a different superconducting cable-in-conduit conductor (CICC) using a cable of multi-filamentary strands inside a stainless steel jacket that confines slowly flowing supercritical helium (under 3 atm pressure at 4.5 K). There will be also an outer shield coil wound with a CICC that uses NbTi/Cu strands. The outsert cooling system concept delivers helium to all of the winding layers (each layer has an inlet and outlet), aiming to sustain a wide range of duty cycles required by diverse science experiments, conducted with the magnet. The transient stability is discussed in terms of temperature margin, limiting current, and energy margin as well and analyzed for several operation scenarios/duty cycles and situations resulting in different patterns of deposition and evacuation of heat due to AC losses in the outsert sections.