Magnetic Field Ripple Research Articles

EFFECT OF TOROIDAL MAGNETIC FIELD RIPPLE VALUE ON THE l=1, m=1 YAMATOR MAGNETIC SURFACES

To form a ripple toroidal magnetic field, the calculated l=1, m=1 Yamator model is equipped with N=16 toroidal magnetic field coils of ac radius. In order to study the effect of toroidal magnetic field ripple value δ on the magnetic surface parameters the calculation was performed for different values of radius ac. In the paper the range of ac radius values is identified within the scope of which the magnetic surface parameters are close to that of the l=1, m=1 Yamator with ideal axially symmetric toroidal magnetic field (δ=0).

Ripple modifications to alpha transport in tokamaks

Magnetic field ripple is inherent in tokamaks since the toroidal magnetic field is generated by a finite number of toroidal field coils. The field ripple results in departures from axisymmetry that cause radial transport losses of particles and heat. These ripple losses are a serious concern for alphas near their birth speed $v_{0}$ since alpha heating of the background plasma is required to make fusion reactors into economical power plants. Ripple in tokamaks gives rise to at least two alpha transport regimes of concern. As the slowing down time $\unicode[STIX]{x1D70F}_{s}$ is much larger than the time for an alpha just born to make a toroidal transit, a regime referred to as the $1/\unicode[STIX]{x1D708}\propto \unicode[STIX]{x1D70F}_{s}$ regime can be encountered, with $\unicode[STIX]{x1D708}$ the appropriate alpha collision frequency. In this regime the radial transport losses increase as $v_{0}\unicode[STIX]{x1D70F}_{s}/R$, with $R$ the major radius of the tokamak. The deleterious effect of ripple transport is mitigated by electric and magnetic drifts within the flux surface. When drift tangent to the flux surface becomes significant another ripple regime, referred to as the $\sqrt{\unicode[STIX]{x1D708}}$ regime, is encountered where a collisional boundary layer due to the drift plays a key role. We evaluate the alpha transport in both regimes, taking account of the alphas having a slowing down rather than a Maxwellian distribution function and their being collisionally scattered by a collision operator appropriate for alphas. Alpha ripple transport is found to be in the $\sqrt{\unicode[STIX]{x1D708}}$ regime where it will be a serious issue for typical tokamak reactors as it will be well above the axisymmetric neoclassical level and can be large enough to deplete the alpha slowing down distribution function unless toroidal rotation is strong.

Control of peaks of terahertz radiation and tuning of its frequency and intensity

We propose to employ dark hollow laser beams (DHBs) for obtaining multifocal terahertz (THz) radiations which are quite useful in medical applications. The number of peaks in the THz field can be suppressed if we apply external magnetic field and use intense lasers so that the relativistic effects are prominent. It means we can achieve strong bifocal THz radiation with this scheme. However, multifocal THz radiation can be reconstituted by increasing the beating frequency of the lasers. The separation of the peaks can be controlled by wisely choosing the orders of the lasers, and also by varying the strength of the magnetic field. The magnetic field is found to enhance the THz field, and also it tunes the frequency of the THz field. The DHBs of higher and equal orders are found to be most significant for efficient THz radiation generation with respect to the role of the magnetic field and density ripples.

Toroidal magnetic field ripple and ion orbit loss effects on edge toroidal rotation in J-TEXT Tokamak plasmas

The edge toroidal rotation on J-TEXT is in the counter-current direction and sensitive to electron density and safety factor. During a density ramp-up phase, the initial toroidal rotation in the edge region (e.g. r/a = 0.7–1), having one speed with little radial dependence, evolves to develop a radial profile when the edge density is sufficiently large. The effects of ripple-induced thermal loss on plasma rotation are predicted by neoclassical theory for those discharges, and comparisons with the measurements are performed. The predictions agree well with the measurements over a range of plasma conditions, confirming that using the ripple effect to interpret the phenomenon is appropriate in certain parameter regimes, while in others the neoclassical effects are not sufficient to predict the experimental observations.

Effects of Resonant Helical Field on Toroidal Field Ripple in IR-T1 Tokamak

The toroidal magnetic field which is created by toroidal coils has the ripple in torus space. This magnetic field ripple has an importance in plasma equilibrium and stability studies in tokamak. In this paper, we present the investigation of the interaction between the toroidal magnetic field ripple and resonant helical field (RHF). We have estimated the amplitude of toroidal field ripples without and with RHF (with different q = m/n) ( m = 2, m = 3, m = 4, m = 5, m = 2 & 3, n = 1) using “Comsol Multiphysics” software. The simulations show that RHF has effects on the toroidal ripples.

Magnetic design and method of a superconducting magnet for muon [formula omitted]/EDM precise measurements in a cylindrical volume with homogeneous magnetic field

A magnetic field design method of magneto-motive force (coil block (CB) and iron yoke) placements for g−2/EDM measurements has been developed and a candidate placements were designed under superconducting limitations of current density 125 A/mm2 and maximum magnetic field on CBs less than 5.5 T. Placements of CBs and an iron yoke with poles were determined by tuning SVD (singular value decomposition) eigenmode strengths. The SVD was applied on a response matrix from magneto-motive forces to the magnetic fields in the muon storage region and two-dimensional (2D) placements of magneto-motive forces were designed by tuning the magnetic field eigenmode strengths obtained by the magnetic field. The tuning was performed iteratively. Magnetic field ripples in the azimuthal direction were minimized for the design.The candidate magnetic design had five CBs and an iron yoke with center iron poles. The magnet satisfied specifications of homogeneity (0.2 ppm peak-to-peak in 2D placements (the cylindrical coordinate of the radial position R and axial position Z) and less than 1.0 ppm ripples in the ring muon storage volume (0.318 m <R<0.348 m and -0.05 <Z< 0.05 m) with 3.0 T strength and a slightly negative BR (magnetic field radial component) at Z> 0.0 m) for the spiral muon injection from the iron yoke at top.

Manufacturing and assembly of IWS support rib and lower bracket for ITER vacuum vessel

ITER Vacuum Vessel (VV) is made of double walls connected by ribs structure and flexible housings. Space between these walls is filled up with In Wall Shielding (IWS) blocks to (1) shield neutrons streaming out of plasma and (2) reduce toroidal magnetic field ripple. These blocks will be connected to the VV through a supporting structure of Support Rib (SR) and Lower Bracket (LB) assembly. SR and LB are two independent components manufactured from SS 316L(N)-IG, Total 1584 support ribs and 3168 lower bracket of different sizes and shapes will be manufactured for the IWS. Two lower brackets will be welded with one support rib to make an assembly. The welding between SR and LB is a full penetration welding. Total 1584 assemblies of different sizes and shapes will be manufactured. Sufficient experience gained from manufacturing and testing of mock ups, final manufacturing of IWS support rib and lower bracket has been started at the site of IWS manufacturer M/s. Avasarala Technologies Limited (ATL). This paper will describe, optimization of water jet cutting speed on IWS material, selection criteria for K type weld joint, unique features of fixture of assembly, manufacturing of Mock ups, and welding processes with NDTs.

A path to stable low-torque plasma operation in ITER with test blanket modules

New experiments in the low-torque ITER Q = 10 scenario on DIII-D demonstrate that n = 1 magnetic fields from a single row of ex-vessel control coils enable operation at ITER performance metrics in the presence of applied non-axisymmetric magnetic fields from a test blanket module (TBM) mock-up coil. With n = 1 compensation, operation below the ITER-equivalent injected torque is successful at three times the ITER equivalent toroidal magnetic field ripple for a pair of TBMs in one equatorial port, whereas the uncompensated TBM field leads to rotation collapse, loss of H-mode and plasma current disruption. In companion experiments at high plasma beta, where the n = 1 plasma response is enhanced, uncorrected TBM fields degrade energy confinement and the plasma angular momentum while increasing fast ion losses; however, disruptions are not routinely encountered owing to increased levels of injected neutral beam torque. In this regime, n = 1 field compensation leads to recovery of a dominant fraction of the TBM-induced plasma pressure and rotation degradation, and an 80% reduction in the heat load to the first wall. These results show that the n = 1 plasma response plays a dominant role in determining plasma stability, and that n = 1 field compensation alone not only recovers most of the impact on plasma performance of the TBM, but also protects the first wall from potentially damaging heat flux. Despite these benefits, plasma rotation braking from the TBM fields cannot be fully recovered using standard error field control. Given the uncertainty in extrapolation of these results to the ITER configuration, it is prudent to design the TBMs with as low a ferromagnetic mass as possible without jeopardizing the TBM mission.

RETRACTED: A first principles study on resonant perturbation field effects on toroidal field ripples in tokamaks

Abstract The toroidal magnetic field which is created by toroidal coils has ripples in torus space. This magnetic field ripple is important for plasma equilibrium and stability studies in tokamaks. In this paper, we have presented the investigation of interaction between the toroidal magnetic field ripple and Resonant Perturbation Field (RPF) using the COMSOL Multiphysics software in the tokamaks. We have simulated the amplitude of toroidal field ripples without and with RPF (with different q = m/n) (m = 2, m = 3, m = 4, m = 5, m = 2 & 3; n = 1). The simulations show that RPF lead to modulation of toroidal field ripples.

Application of a non-steady-state orbit-following Monte-Carlo code to neutron modeling in the MAST spherical tokamak

As part of the verification and validation of a newly developed non-steady-state orbit-following Monte-Carlo code, application studies of time dependent neutron rates have been made for a specific shot in the Mega Amp Spherical Tokamak (MAST) using 3D fields representing vacuum resonant magnetic perturbations (RMPs) and toroidal field (TF) ripples. The time evolution of density, temperature and rotation rate in the application of the code to MAST are taken directly from experiment. The calculation results approximately agree with the experimental data. It is also found that a full orbit-following scheme is essential to reproduce the neutron rates in MAST.

Ambipolarity in a tokamak with magnetic field ripple

In view of the recognized importance of electrostatic fields regarding turbulent transport, the radial electric field in a tokamak with magnetic field ripple is reconsidered. Terms in the ambipolarity condition involving the radial derivative of the field are derived from an extended drift-kinetic equation, including effects of second order in the gyroradius. Such terms are of interest in part because of their known importance in rotational relaxation equations for the axisymmetric case. The electric field is found to satisfy a nonlinear differential equation that is universal in a certain sense, and that implies spatial relaxation of the potential to its conventionally predicted value.

Multi-scale full-orbit analysis on phase-space behavior of runaway electrons in tokamak fields with synchrotron radiation

In this paper, the secular full-orbit simulations of runaway electrons with synchrotron radiation in tokamak fields are carried out using a relativistic volume-preserving algorithm. Detailed phase-space behaviors of runaway electrons are investigated in different dynamical timescales spanning 11 orders. In the small timescale, i.e., the characteristic timescale imposed by Lorentz force, the severely deformed helical trajectory of energetic runaway electron is witnessed. A qualitative analysis of the neoclassical scattering, a kind of collisionless pitch-angle scattering phenomena, is provided when considering the coupling between the rotation of momentum vector and the background magnetic field. In large timescale up to 1 s, it is found that the initial condition of runaway electrons in phase space globally influences the pitch-angle scattering, the momentum evolution, and the loss-gain ratio of runaway energy evidently. However, the initial value has little impact on the synchrotron energy limit. It is also discovered that the parameters of tokamak device, such as the toroidal magnetic field, the loop voltage, the safety factor profile, and the major radius, can modify the synchrotron energy limit and the strength of neoclassical scattering. The maximum runaway energy is also proved to be lower than the synchrotron limit when the magnetic field ripple is considered.

Ripple Field AC Losses in 10-MW Wind Turbine Generators With a MgB2Superconducting Field Winding

Superconducting (SC) synchronous generators are proposed as a promising candidate for 10-20-MW direct-drive wind turbines because they can have low weights and small sizes. A common way of designing an SC machine is to use SC wires with high current-carrying capability in the dc field winding and the ac armature winding is made with copper conductors. In such generators, the dc field winding is exposed to ac magnetic field ripples due to space harmonics from the armature. In generator design phases, the ac loss caused by these ripple fields needs to be evaluated to avoid local overheating and an excessive cooling budget. To determine the applicability of different design solutions in terms of ac losses, this paper estimates the ac loss level of 10-MW wind generator designs employing a MgB 2 SC field winding. The effects on ac losses are compared between nonmagnetic and ferromagnetic teeth with different numbers of slots per pole per phase. The necessity of an electromagnetic shield is then discussed based on the obtained loss levels. The results show that the total ac loss is so small that ferromagnetic teeth can be applied in the generator design without using an electromagnetic shield.

Neoclassical parallel flow calculation in the presence of external parallel momentum sources in Heliotron J

A moment approach to calculate neoclassical transport in non-axisymmetric torus plasmas composed of multiple ion species is extended to include the external parallel momentum sources due to unbalanced tangential neutral beam injections (NBIs). The momentum sources that are included in the parallel momentum balance are calculated from the collision operators of background particles with fast ions. This method is applied for the clarification of the physical mechanism of the neoclassical parallel ion flows and the multi-ion species effect on them in Heliotron J NBI plasmas. It is found that parallel ion flow can be determined by the balance between the parallel viscosity and the external momentum source in the region where the external source is much larger than the thermodynamic force driven source in the collisional plasmas. This is because the friction between C6+ and D+ prevents a large difference between C6+ and D+ flow velocities in such plasmas. The C6+ flow velocities, which are measured by the charge exchange recombination spectroscopy system, are numerically evaluated with this method. It is shown that the experimentally measured C6+ impurity flow velocities do not contradict clearly with the neoclassical estimations, and the dependence of parallel flow velocities on the magnetic field ripples is consistent in both results.

Current progress of Chinese HCCB TBM program

ITER is an unique opportunity to test tritium breeding blanket mock-ups in integrated Tokamak operating conditions. Helium-cooled ceramic breeder (HCCB) test blanket module will be the primary option of the Chinese ITER TBM program. China plans to test its own test blanket modules (TBM) during ITER different operation phase (H-H,D-D, D-T). A preliminary design of HCCB TBM have been completed in 2013. In order to reduce the effects of magnetic field ripple, the HCCB TBM design has been updated with reduced RAFM mass recently. Related R&D on key components fabrication, welding technology, materials development and mock-up test of TBM modules and components have being implemented. In this paper, the current status on design and related R&D, as well as the development of auxiliary system (TES, CPS,HCS, etc.,) of Chinese HCCB TBS program were introduced.

RETRACTED ARTICLE: Tokamak Coils Materials and Toroidal Field Ripples Calculation Using the Comsol Multiphysics

The ITER superconducting magnet systems consists of four main sub-systems: toroidal field (TF) coils, central solenoid coils, poloidal field coils, and correction coils. Like many other ITER systems, the magnet components are supplied in-kind by six domestic agencies. The technical specifications, manufacturing processes and procedures required to fabricate these components are particularly challenging. The management structure and organization to realize this procurement within the tight ITER construction schedule is very complex. On the other hand, toroidal magnetic field ripple in tokamak is an important issue in plasma equilibrium and stability studies. Toroidal magnetic field is created by toroidal torus with finite number of coils, therefore the field has a ripple in torus space. In this paper, we have reviewed the ITER magnetic coils materials, and also we have estimated the amplitude of TF ripples and its dependence to numbers of coils using the “Comsol Multiphysics” software. The calculations which performed for three: 8, 16 and 32 toroidal coils, indicates that increasing the number of toroidal coils lead to reduction of magnetic field ripple and lead to more stable plasma, but diagnostic access to plasma is reduces.

Modeling of non-axisymmetric magnetic perturbations in tokamaks

A numerical model to evaluate the effects of the non-axisymmetric magnetic perturbations on magnetic topology and magnetic field ripple in tokamaks is presented in this paper. It is illustrated by using an example magnetic field perturbation induced by a coil system on the EAST tokamak. The influence of the choice of the coordinates on the spectrum is presented. The amplitude of resonant components of the spectrum are found to be independent of the coordinates system, while that of the non-resonant components are not. A better way to describe the edge topology by using the Chirikov parameter profile is proposed and checked by the numerical Poincaré plot results. The contribution of the magnetic perturbation on local toroidal field ripple can be significant. One approximate method to model the helical ripple on the perturbed flux surface induced by a given non-axisymmetric magnetic field perturbation is presented. All of the spectrum analysis is applicable in case the plasma response is taken into account in the input of perturbed magnetic field.

Evolution of the Tore Supra Lower Hybrid Current Drive System for WEST

The WEST-project (W-tungsten Environment in Steady-state Tokamak) involves equipping Tore Supra with a full tungsten divertor, capable of withstanding heat load of 10MW/m2 in steady-state conditions, in discharges sustained by Lower Hybrid Current Drive (LHCD). The LHCD generator, recently upgraded to deliver 9.2MW/1000s, is equipped with sixteen TH2103C klystrons powering two launchers. The WEST transformation involves reducing the plasma volume, thus moving the launchers ∼10cm closer to the tokamak centre. The toroidal curvature of the launchers no longer fits the plasma curvature due to the strong magnetic field ripple effect, leading to a degradation of the LH wave coupling, especially with the Full Active Multijunction Launcher (FAM). The toroidal curvature radius of the FAM launcher mouth will therefore be reshaped from 1700mm to 2300mm. The machining process is described in this article. In order to improve the coupling of the LH wave, the local gas injection has been modified to help to meet the requirement of 7MW/1000s of LH power coupled to the plasma in the WEST scenarios.Finally, the curvature radius of the waveguide septa are rounded to minimize the excitation of suprathermal electrons near the plasma edge, which can induce high power loads on the plasma facing components.

Design of a Wedge-Shaped Toroidal Field Winding for KTX Device

The KTX device is a reversed field pinch (RFP) device currently under construction. Its maximum plasma current is designed as 1 MA with a discharge time longer than 100 ms. Its major radius is 1.4 m and its minor radius is 0.55 m. One of the most important problems in the magnet system design is how to reduce the TF magnetic field ripple and error field. A new wedge-shaped TF coil is put forward for the KTX device and its electromagnetic properties are compared with those of rectangular-shaped TF coils. The error field Bn/Bt of wedge-shaped TF coils with 6.4 degrees is about 6% as compared with 8% in the case of a rectangular-shaped TF coil. Besides, the wedge-shaped TF coils have a lower magnetic field ripple at the edge of the plasma region, which is smaller than 7.5% at R=1.83 m and 2% at R=1.07 m. This means that the tokamak operation mode may be feasible for this device when the plasma area becomes smaller, because the maximum ripple in the plasma area of the tokamak model is always required to be smaller than 0.4%. Detailed analysis of the results shows that the structure of the wedged-shape TF coil is reliable. It can serve as a reference for TF coil design of small aspect ratio RFPs or similar torus devices.

Coupling currents and hysteresis losses in MgB2 superconductors

MgB2 multi-filamentary superconductors are widely considered for use in dc applications. To expand the possible application range to ac apparatuses, the development of a low ac loss wire is needed. This development involves several steps, and a solid understanding of the loss mechanisms is important to optimize that process as well as for evaluating dc wires exposed to current or magnetic field ripple. In this study we discuss the coupling currents and their influence on hysteresis loss as well as on coupling current loss. We give a phenomenological explanation of the origin and behaviour of the coupling currents and describe the loss patterns for hysteresis loss and coupling current loss separately. Finally, we interpret measured ac losses in an MgB2 wire cut into different lengths representing different equivalent twist pitches. Under certain circumstances short sample lengths are shown to give inaccurate measurement results. On the other hand, short sample lengths of non-twisted wires can be used to estimate the twist pitch necessary to electromagnetically decouple the superconducting filaments. For the studied MgB2 wire, with a pure titanium matrix, the losses were mainly hysteretic, although the level of these losses was lowered when reducing the equivalent twist pitch, and thereby the coupling currents. At 35 K the reduction in loss started at an equivalent twist pitch of approximately 36 mm, where the filaments were assumed practically fully coupled. The decoupling then continued all down to the shortest equivalent twist pitch of 9 mm.