Quench Time Research Articles

Magnetohydrodynamic simulations of massive gas injection into Alcator C-Mod and DIII-D plasmas

Disruption mitigation experiments using massive gas injection (MGI) on Alcator C-Mod [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] and DIII-D [Luxon and Davis, Fusion Technol. 8, 441 (1985)] have shown that magnetohydrodynamics (MHD) plays an important role. The three-dimensional MHD code NIMROD [Sovinec et al., J. Comput. Phys. 195, 355 (2004)] has been extended to include atomic physics taken from the KPRAD code to perform simulations of MGI. Considerable benchmarking of the code has been done against Alcator C-Mod for neon and helium gas jet experiments. The code successfully captures the qualitative sequence of events observed in MGI experiments up to the end of the thermal quench. Neon jet simulations also show quantitative agreement with the experimental thermal quench onset time. For helium gas jets, we show that a small percent boron density can significantly alter the results even in the presence of a helium jet with three orders of magnitude higher density. The thermal quench onset time is considerably overpredicted unless boron radiation is included. A DIII-D helium jet simulation shows a faster rise time for total radiated power than the experiment, but comparable amplitude. Similar to the important role of boron in C-Mod, carbon radiation is a significant factor in DIII-D helium jet simulations and experiments.

Current redistribution around the superconducting-to-normal transition in superconducting Nb-Ti Rutherford cables

Sufficient thermal-electromagnetic stability against external heat sources is an essential design criterion for superconducting Rutherford cables, especially if operated close to the critical current. Due to the complex phenomena contributing to stability such as helium cooling, inter-strand current and heat transfer, its level is difficult to quantify. In order to improve our understanding, many stability tests were performed on different cable samples, each incorporating several point-like heaters. The current redistribution around the heat front is measured after inducing a local normal zone in one strand of the cable. By using voltage taps, expansion of the normal zone is monitored in the initially quenched strand as well as in adjacent strands. An array of Hall probes positioned at the cable edge is used to scan the self-field generated by the cable by which it becomes possible to estimate the inter-strand current transfer. In this paper it is demonstrated that two different stability regimes can be distinguished depending on the local conditions for local normal zone recovery through heat and current transfer to adjacent strands. It is shown that in the first regime every normal zone will lead to a quench, while in the second regime a normal zone in one strand can recover. Combining the predictions developed using a novel version of the numerical network model CUDI and new measurement results, it is possible to derive characteristic quench decision times as well to calculate and predict the influence of a change in cable parameters.

Quench studies on a layer-woundBi2Sr2CaCu2Ox/AgX coil at 4.2 K

To evaluate the controlled quench behavior of high temperature superconducting(HTS) coils, particularly when using HTS coils in a hybrid configuration as aninsert in a low temperature superconducting magnet, a layer-wound solenoid usingBi2Sr2CaCu2O wire was instrumented with several strip heaters to generate quenches in the axial andazimuthal directions. An array of distributed voltage taps and thermocoupleswere used to monitor the quench signals. Minimum quench energies (MQE) andquench propagation velocities (NZPVs) were determined. Results show that quenchenergies were moderate. NZPVs were slow but quench reaction times were ofthe same order as reaction times obtained at low quench energy densities inNb3Sn coils.

Study of Electromagnetic Repulsion Switch to High Speed Reclosing and Recover Time Characteristics of Superconductor

Using a high-temperature superconductor, we constructed and tested a model superconducting fault current limiter (SFCL). The superconductor and vacuum interrupter as the commutation switch were connected in parallel using a bypass coil. When the fault current flows in this equipment, the superconductor is quenched and the current is then transferred to the parallel coil due to the voltage drop in the superconductor. This large current in the parallel coil actuates the magnetic repulsion mechanism of the vacuum interrupter and the current in the superconductor is broken. Using this equipment, the current flow time in the superconductor can be easily minimized. On the other hand, the fault current is also easily limited by large reactance of the parallel coil. This system has many merits. So, we introduced to electromagnetic repulsion switch.There is duty of high speed re-closing after interrupting fault current in the electrical power system. So the SFCL should be recovered to superconducting state before high speed re-closing. But, superconductor generated heat at the time of quench. It takes time to recover superconducting state. Therefore it is a matter of recovery time. In this paper, we studied recovery time of superconductor. Also, we proposed electromagnetic repulsion switch with reclosing system.

Kibble-Zurek mechanism in a quenched ferromagnetic Bose-Einstein condensate

It is shown that spin vortices are created through the Kibble-Zurek mechanism in the quantum phase transition of a spin-1 ferromagnetic Bose-Einstein condensate when the applied magnetic field is quenched to below a critical value. It is also shown that the spin correlation functions have finite correlation lengths, and that the magnetization at widely separated positions grows in random directions, resulting in spontaneous creation of spin vortices. We numerically confirm the scaling laws that the winding number of spin vortices is proportional to the square root of the length of a closed path and, for a slow quench, is proportional to ${\ensuremath{\tau}}_{\text{Q}}^{\ensuremath{-}1∕6}$ with ${\ensuremath{\tau}}_{\text{Q}}$ being the quench time. The relevance of spin conservation to the Kibble-Zurek mechanism is discussed.

전압증가에 따른 자속구속형 초전도 한류기의 전류제한 및 회복특성 분석

자속구속형 초전도 한류기는 1차 권선과 초전도 소자가 직렬로 연결된 2차 권선을 갖는 변압기로 구성되어 있다.<BR> 이 초전도 한류기는 결선방향에 따라 감ㆍ가극 결선으로 나누어진다. 1차 권선과 2차 권선의 인덕턴스 비에 따라 자속구속형 초전도 한류기는 사고전류의 제한특성을 제어할 수 있다. 본 논문에서는 인가전압에 따른 자속구속형 초전도 한류기의 전압ㆍ전류 특성을 분석하였다. 감ㆍ가극 결선된 자속구속형 초전도 한류기의 인가전압을 증가시켰을 때, 초기에 제한되는 전류 및 초전도 소자의 편치 시간은 증가하였다. 초전도 소자의 회복시간은 인가전압이 증가함에 따라 증가하였다. 그러므로 자속구속형 초전도 한류기의 회복특성은 초전도 소자의 소비전력이 증가하기 때문에 초전도 소자에서 소비되는 에너지에 크게 의존한다는 것을 확인할 수 있었다.

Integrated view of disruption dynamics on internal electromagnetic and plasma structures in the small tokamak HYBTOK-II

An integrated view of disruption dynamics on internal plasma and associated electromagnetic structures with a high time resolution is presented in a systematic way. The main results are as follows: (i) observation of electron temperature oscillation due to rotation of magnetic islands of m(poloidal)/n(toroidal) = 3/2 mode numbers before the current quench, (ii) finding of rapid pump-out (∼20 µs) of plasma current and particles from the central core region to the edge just at the current quench start time and then returning to a peaked profile during the following slow decay phase and (iii) a new proposal for the evaluation of current quench time and the electron temperature dependence of the current quench time, the so-called τ/S scaling.

Recovery characteristics of flux-lock type superconducting fault current limiter

The flux-lock type superconducting fault current limiter (SFCL) has attractive characteristics that the current limiting level can be adjusted by a winding direction and the inductance ratio between two coils. We changed the winding direction and the number of coils to compare the resistive type SFCL with the flux-lock type SFCL. The initial limiting current (Iini) and quench characteristic were dependent on the winding direction and the inductance ratio of two coils. As a winding number was increased from 21 to 42, Iini and quench characteristic were proportionally increased. In additive polarity winding, Iini was 10.2A and the quench time (Tq) was 0.53ms, which was faster than that of a subtractive polarity winding.The consumed energy and recovery characteristics in a superconducting element showed the same tendency. Recovery characteristics in the flux-lock type SFCL were dependent on the consumed energy of a superconducting element. The recovery time was related to a heat energy and it was represented as the consuming time of the heat energy. As the heat energy was shown in H=0.24I2Rt, the recovery time was shortened in the following order: a subtractive polarity winding, a resistive type and an additive polarity winding. It was known that the recovery time was proportional to a consumed energy of a superconducting element.

Coated Conductor Based FCL With Controllable Time Response

Performance of fault current limiter demonstrators based on either a single coated conductor (CC) tape or CC assemblies tested in current limitation modus is reported. A primary goal of this work was to achieve a quick reaction of the fault current limiter (FCL) device, and to study possibilities to provide a reliable and controllable time performance of limiting devices. An improved homogeneity of coated conductors was achieved employing a new alternating beam assisted deposition technique. This technique was used for processing of biaxially textured buffer layers with enhanced quality. Typically, the tapes yielded a critical current of 200-300 A per cm width at 77 K and self field. In demonstrators based on single CC tape, a 10 mus transient time of quench was observed. A stack of coated conductors jointed in parallel was employed as an active FCL element. Two alternative cases are studied when (i) CC tapes exhibit an equal and homogeneously distributed critical current and (ii) the critical currents and contact resistances are different in different tapes. As a result, it was experimentally shown that the quench duration can be varied from 10 mus (in case (i)) to 2 ms (in case (ii)). Such artificially smoothed quenches with pre-determined time dependence should play an important role in AC and DC networks where current surges have to be reliably suppressed. We found that the quenching behavior of the entire electrical circuits is consistent with a finite element model that takes into account a non-linear type of resistivity of CC tape. Short recovery time of 0.5 ms was achieved in CC based fault current limiters.

Operational characteristics of hybrid-type SFCL with closed and open cores

We investigated the operational characteristics of the hybrid-type superconducting fault current limiter (SFCL) with the closed and the open cores, which induced the variation of the magnetic flux between the primary and the secondary windings. The experimental set-up of the hybrid-type SFCL with the closed and the open cores were prepared and the experimental analyses for the current limiting characteristics were performed. The peak value of the fault current in the hybrid-type SFCL with the open core was higher than that of the closed core at the first cycle after fault occurrence. However, in the case of the hybrid-type SFCL with the open core, the limiting current level after fault occurrence was decreased less than that of the hybrid-type SFCL with the closed core, because the magnetic leakage reluctance of the open core was higher than that of the closed core. The quench time (Tq) and the arrival time (Ta) for the peak voltage (VSC) in the hybrid-type SFCL with the closed core were faster than that of the hybrid-type SFCL with the open core due to the increase of the mutual flux. We verified that the consumption power in the hybrid-type SFCL with the open core was larger owing to the increase of leakage flux by the reduction of mutual inductance between primary and secondary windings.

Entropy of entanglement and correlations induced by a quench: Dynamics of a quantum phase transition in the quantum Ising model

Quantum Ising model in one dimension is an exactly solvable example of a quantum phase transition. We investigate its behavior during a quench caused by a gradual turning off of the transverse bias field. The system is then driven at a fixed rate characterized by the quench time ${\ensuremath{\tau}}_{Q}$ across the critical point from a paramagnetic to ferromagnetic phase. In agreement with Kibble-Zurek mechanism (which recognizes that evolution is approximately adiabatic far away, but becomes approximately impulse sufficiently near the critical point), quantum state of the system after the transition exhibits a characteristic correlation length $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\xi}}$ proportional to the square root of the quench time ${\ensuremath{\tau}}_{Q}$: $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\xi}}=\sqrt{{\ensuremath{\tau}}_{Q}}$. The inverse of this correlation length is known to determine average density of defects (e.g., kinks) after the transition. In this paper, we show that this same $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\xi}}$ controls the entropy of entanglement, e.g., entropy of a block of $L$ spins that are entangled with the rest of the system after the transition from the paramagnetic ground state induced by the quench. For large $L$, this entropy saturates at $\frac{1}{6}\phantom{\rule{0.2em}{0ex}}{\mathrm{log}}_{2}\phantom{\rule{0.2em}{0ex}}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\xi}}$, as might have been expected from the Kibble-Zurek mechanism. Close to the critical point, the entropy saturates when the block size $L\ensuremath{\approx}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\xi}}$, but---in the subsequent evolution in the ferromagnetic phase---a somewhat larger length scale $l=\sqrt{{\ensuremath{\tau}}_{Q}}\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}{\ensuremath{\tau}}_{Q}$ develops as a result of a dephasing process that can be regarded as a quantum analog of phase ordering, and the entropy saturates when $L\ensuremath{\approx}l$. We also study the spin-spin correlation using both analytic methods and real time simulations with the Vidal algorithm. We find that at an instant when quench is crossing the critical point, ferromagnetic correlations decay exponentially with the dynamical correlation length $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\xi}}$, but (as for entropy of entanglement) in the following evolution length scale $l$ gradually develops. The correlation function becomes oscillatory at distances less than this scale. However, both the wavelength and the correlation length of these oscillations are still determined by $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\xi}}$. We also derive probability distribution for the number of kinks in a finite spin chain after the transition.

Disruption scenarios, their mitigation and operation window in ITER

The impacts of plasma disruptions on ITER have been investigated in detail to confirm the robustness of the design of the machine to the potential consequential loads. The loads include both electro-magnetic (EM) and heat loads on the in-vessel components and the vacuum vessel. Several representative disruption scenarios are specified based on newly derived physics guidelines for the shortest current quench time as well as the maximum product of halo current fraction and toroidal peaking factor arising from disruptions in ITER. Disruption simulations with the DINA code and EM load analyses with a 3D finite element method code are performed for these scenarios. Some margins are confirmed in the EM load on in-vessel components due to induced eddy and halo currents for these representative scenarios. However, the margins are not very large. The heat load on various parts of the first wall due to the vertical movement and the thermal quench (TQ) is calculated with a 2D heat conduction code based on the database of heat deposition during disruptions and simulation results with the DINA code. For vertical displacement event, it is found that the beryllium (Be) wall does not melt during the vertical movement, prior to the TQ. Significant melting is anticipated for the upper Be wall and the tungsten divertor baffle due to TQ after the vertical movement. However, its impact could be substantially mitigated by implementing a reliable detection system of the vertical movement and a mitigation system, e.g. massive noble gas injection. Some melting of the upper Be wall is anticipated at major disruptions. At least several tens of unmitigated disruptions must be considered even if an advanced prediction/mitigation system is implemented. With these unmitigated disruptions, the loss of the Be layer is expected to be within ≈30–100 µm/event out of a 10 mm thick Be first wall.

Disruption mitigation on Alcator C-Mod using high-pressure gas injection: Experiments and modeling toward ITER

High-pressure gas-jet injection disruption mitigation is investigated on Alcator C-Mod. The experimental results are encouraging for ITER. Gas-jets delivers reliable and substantial disruption mitigation of thermal loads and halo currents at the ITER field and pressure. The gas injection is compatible with a metallic wall. The gas-jets provide sufficient radiative energy dissipation on a timescale even faster than required for ITER. Runaway electrons are suppressed, however a better understanding of this suppression is still required for ITER. Present 0-D calculations of the gas delivery and global plasma response match important experimental data such as overall quench time and the current quench L/R time. Frictional dissipation may be important for gas delivery down the long tubes that will probably be required for ITER. However deep gas and/or impurity penetration is not required for mitigation, relieving technical requirements for the gas pressure in ITER. Initial studies with the non-ideal MHD simulation NIMROD show that radiation-induced MHD plays a critical role in thermal quench particle and energy dynamics. The application of a newly available version of NIMROD, with more complete atomic physics, shows promise in matching the experimental trends and could be an invaluable tool for increasing our confidence in designing efficient disruption mitigation for ITER.

Observation of q-profile dependence in noble gas injection radiative shutdown times in DIII-D

Massive injection of argon was used to terminate DIII-D discharges with different radial profiles of the safety factor q. An increasing delay before the onset of the central temperature collapse was observed as the overall (core and edge) q of the target plasma was increased. Additionally, an increase in the length of the current quench time was observed, consistent with reduced magnetohydrodynamic (MHD) mode amplitudes and reduced MHD mixing. These results are supported by measurements of heat and impurity mixing during the thermal collapse which observe that thermal transport and impurity mixing are not smooth functions of time but are accelerated when low-order (n=1) MHD modes are destabilized. These results demonstrate that low-order MHD modes play a central role in gas jet shutdowns and density-limit disruptions in tokamaks.

Gas jet disruption mitigation studies on Alcator C-Mod

Damaging effects of disruptions are a major concern for Alcator C-Mod, ITER and future tokamak reactors. High-pressure noble gas jet injection is a mitigation technique which potentially satisfies the operational requirements of fast response time and reliability, while still being benign to subsequent discharges. Disruption mitigation experiments using an optimized gas jet injection system are being carried out on Alcator C-Mod to study the physics of gas jet penetration into high pressure plasmas, as well as the ability of the gas jet impurities to convert plasma energy into radiation on timescales consistent with C-Mod's fast quench times, and to reduce halo currents given C-Mod's high-current density. The dependence of impurity penetration and effectiveness on noble gas species (He, Ne, Ar, Kr) is also being studied. It is found that the high-pressure neutral gas jet does not penetrate deeply into the C-Mod plasma, and yet prompt core thermal quenches are observed on all gas jet shots. 3D MHD modelling of the disruption physics with NIMROD shows that edge cooling of the plasma triggers fast growing tearing modes which rapidly produce a stochastic region in the core of the plasma and loss of thermal energy. This may explain the apparent effectiveness of the gas jet in C-Mod despite its limited penetration. The higher-Z gases (Ne, Ar, Kr) also proved effective at reducing halo currents and decreasing thermal deposition to the divertor surfaces. In addition, noble gas jet injection proved to be benign for plasma operation with C-Mod's metal (Mo) wall, actually improving the reliability of the startup in the following discharge.

Zurek-Kibble Mechanism for the Spontaneous Vortex Formation inNb−Al/Alox/NbJosephson Tunnel Junctions: New Theory and Experiment

New scaling behavior has been both predicted and observed in the spontaneous production of fluxons in quenched Nb-Al/Al(ox)/Nb annular Josephson tunnel junctions (JTJs) as a function of the quench time, tau(Q). The probability f(1) to trap a single defect during the normal-metal-superconductor phase transition clearly follows an allometric dependence on tau(Q) with a scaling exponent sigma = 0.5, as predicted from the Zurek-Kibble mechanism for realistic JTJs formed by strongly coupled superconductors. This definitive experiment replaces one reported by us earlier, in which an idealized model was used that predicted sigma = 0.25, commensurate with the then much poorer data. Our experiment remains the only condensed matter experiment to date to have measured a scaling exponent with any reliability.

Timescale and magnitude of plasma thermal energy loss before and during disruptions in JET

In this paper we analyse and discuss the thermal energy loss dynamics before and during JET disruptions that occurred between 2002 and 2004 in discharges which reached >4.5 MJ of thermal energy. We observe the slow thermal energy transients with diamagnetic loops and the fast ones with electron cyclotron emission and soft x-ray diagnostics. For most disruption types in JET, the plasma thermal energy at the time of the thermal quench is substantially less than that of the full performance plasma, typically in the range of 10–50% depending on plasma conditions and disruption type. The exceptions to this observation are disruptions in plasmas with a strong internal transport barrier (ITB) and in discharges terminating in a pure vertical displacement event, in which the plasma conserves a very high energy content up to the thermal quench. These disruption types are very sudden, leaving little scope for the combined action of soft plasma landing strategies and intrinsic performance degradation, both requiring >500 ms to be effective, to decrease the available thermal energy. The characteristic time for the loss of energy from the main plasma towards the PFCs in the thermal quench of JET disruptions is in the range 0.05–3.0 ms. The shortest timescales are typical of disruptions caused by excessive pressure peaking in ITB discharges. The available thermal energy fraction and thermal quench duration observed in JET can be processed (with due caution) into estimates for the projected PFC lifetime of the ITER target.

Semi-Empirical Correlation for Quench Time of Inductively Coupled Fault Current Limiter

Experimental results on the quench time of an inductively coupled fault current limiter are presented. The quench time is defined as the period between the initiation of the fault and the time when the device begins to limit the current. The experimental results show that the fault current limiter begins to limit the current well before the superconductor is fully quenched. A semi-empirical model, based on a lumped-parameter circuit analysis, is developed to predict the quench time. The model assumes that the average power dissipated in the fault current limiter is used to heat up the superconductor. The derived equation indicates that the quench time depends on the properties of the superconductor, the design parameters of the fault current limiter, and the system parameters. There is very little dissipation in the superconductor when the system voltage is less than the critical voltage of the superconductor. Therefore, the overvoltage (system voltage minus the critical voltage), instead of the system voltage, should be used in calculating the quench time. The quench time calculated from the semi-empirical correlation agrees well with experimental data.

Considerable Stability Increase in Superconducting Windings Doped With Extremely High Specific Heat Substances

The influence of some rare-earth based intermetallic compounds with extremely high specific heat capacities on the transient stability of NbTi superconducting windings was experimentally investigated. The experiments were carried out with three NbTi wire wound small round coils doped with CeCu/sub 6/, HoCu/sub 2/, and (for comparison) with boron nitride usually used as an epoxy resin filler. The intermetallic compounds, CeCu/sub 6/ and HoCu/sub 2/, were introduced as filling powders into an epoxy resin based composite in a wet winding process. Doping of about 5.7 vol.% of such compounds increases minimum quench energies several times even for short (of order of 1 ms) electromagnetic disturbances. The first results are also reported of the training history comparison in three NbTi Rutherford cable wound oval-shaped windings doped with half amount (about 2.9 vol.%) of the same compounds. It is shown that even such doping is very helpful in increasing of the first quench current. The prospects and most promising applications are briefly discussed.

Effect of Quench and Interpass Time on Dynamic and Static Softening during Hot Rolling

The categories of interpass time encountered in hot rolling are characterized. These are shown to fall into three broad categories: short (in rod rolling); intermediate (in strip processing); and long (in plate schedules). The effects of interpass time on recrystallization and carbonitride precipitation are described, and particularly on the interaction between these two mechanisms. The interactions, in turn, determine the final microstructure produced as well as the mechanical properties.