Helical System Research Articles

Characteristics of local helical coil system for dynamic ergodic divertor

AbstractHeat and particle flux control is one of the most important and difficult problems in steady‐state fusion devices. The Dynamic Ergodic Divertor (DED) is an advanced concept in which a rotational perturbed magnetic field is applied to make the tokamak edge magnetic structure stochastic so that it may decrease the heat and particle flux onto the local target in the conventional divertor. This rotational perturbed field is created by two sets of local helical coils (LHC) with a phase difference of 90° in the supplied ac currents. In DED experiments, it is important to study the perturbed magnetic field structure generated by these coil currents. In this paper, a new coil configuration is considered, which has an additional series of return windings so as to improve the mode quality of the rotating helical field (RHF). We obtain the mode structure of RHF using a new Fourier analysis. From this result, we confirm that the parasitic mode of RHF and the coil self‐inductance are decreased by the new coil configuration. © 2002 Scripta Technica, Electr Eng Jpn, 139(1): 16–23, 2002; DOI 10.1002/eej.1142

Design of Quasi-Axisymmetric Stellarator CHS-qa.

A new compact helical system CHS-qa has been designed based on the advanced stellarator concept of the quasi-axisymmetry in the magnetic field configuration. This new concept of quasi-axisymmetry makes it possible to realize the two-dimensional vacuum magnetic field structure (a tokamak-like configuration) without using plasma current. In this system, the neoclassical transport is greatly reduced compared with conventional helical systems because the helical ripples are very small. Good MHD stability for the interchange and ballooning modes are obtained. The possibility of finding the confinement improvement is also discussed from the viewpoint of plasma rotation and maximum-J criterion.

Vibratory Behavior in Multi Stage Helical Gear System. Theoretical Study on the Coupled Vibration of the Intermediate Axis.

Many of industrial gear systems are the multi-stage and with two gear pairs on the intermediate axis. In such multi-axes systems, two meshes on the intermediate axis cause coupled vibration. In this paper, the behavior of coupled vibration of such helical gear system composed of three axes will be theoretically examined. The exciting force caused by axial movement of the distributed load on the tooth flank will be considered as well as the exciting force caused by tooth flank error and by the change of tooth spring stiffness. As a result, it will be clarified that the intermediate axis has two vibratory modes; one is the coupled vibration between tooth normal displacements at both of two gear pairs. The other is the coupled vibration between rotational displacement in the plane of action at the first gear pair and the tooth normal displacement at the second gear pair.

Transition of Toroidal Alfven Eigenmode to Global Alfven Eigenmode in CHS Heliotron/Torsatron Plasmas Heated by Neutral Beam Injection.

A transition of a core localized type toroidal Alfven eigenmode with n = 1 toroidal mode number to two n = 1 global Alfven eigenmodes was observed in NBI-heated plasmas in the Compact Helical System (CHS) heliotron torsatron. This transition phenomenon is interpreted based on the temporal evolution of the rotational transform near the plasma center caused by the increase in the beam-driven current.

Particle transport diagnostics on CHS and LHDwith tracer-encapsulated solid pellet injection

A method using a tracer-encapsulated solid pellet (TESPEL) has been developed to diagnose particle transport. TESPEL consists of polystyrene as the outer part and another material as the inner core. At first, it was confirmed at the compact helical system (CHS) that the tracer of lithium was deposited locally in the plasma. Moreover, the behaviour of the tracer particles was observed by charge exchange recombination spectroscopy. TESPEL was also injected into the large helical device (LHD). In this case, the Kα line of titanium was observed without the necessity of a neutral beam, and it was shown that this method is promising.

Dose and Pitch Relationship for a Particular Multislice CT Scanner

With single-slice helical CT, an increased pitch can decrease the radiation dose to the patient if all other parameters are constant. The purpose of this study was to determine whether the same relationship holds for a particular multislice helical CT system (Somatom Plus 4 VZ multislice helical CT scanner, version A11A) in our department. The measured radiation dose to the phantom was identical for all pitch selections on the multislice helical CT system we tested. This unexpected result was because of an automatic proportionate increase in the tube current when the pitch selection was increased. Radiologists and physicists should exercise caution when extrapolating dose reduction strategies from single-slice to multislice helical CT systems, and they must acquire a detailed understanding of the multislice helical CT scanner of their chosen manufacturer.

The Preparation of Helical Cyclophanes Containing Five-membered Rings

Cycloadditions between the indenone derivatives prepared in situ from the bromides 12a and 12b and 4-vinyl[2.2]paracyclophane (11) yielded the Diels−Alder adducts 13a and 13b in acceptable yields. When these were heated in the presence of DDQ in toluene, the fluorenophanes 14a and 14b were produced in good yields. The diol 17 obtained from the tetralonophane 15 on treatment with POCl3/pyridine was dehydrated to the diene 18, which could be trapped by p-benzoquinone (19) to provide the cycloadducts 20 and 21; the former was oxidized to the latter by DDQ treatment. Although the even more highly annelated helical phane system 22 could be prepared from 17 and 12a, its aromatization to the ketone 24 or the hydrocarbon 25 failed. All new compounds were characterized by their spectroscopic data, in particular by extensive NMR investigations. A single-crystal X-ray structure analysis for the parent hydrocarbon phenanthreno[2.2]paracyclophane (5) is reported.

Inward turbulent particle transport in a helical plasma heated by neutral beam injection

In neutral-beam-injection (NBI) heated plasmas of the Compact Helical System (CHS) heliotron/torsatron [K. Matsuoka et al., Plasma Physics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 441], inward turbulent particle flux is observed at a particular radial location near the plasma edge. The observed reversal of the particle flux is connected to the change in the cross-phase between electron density and poloidal electric field fluctuations. This phenomenon is observed in the parameter regime of low collisionality at the electron temperature Te>15 eV, electron density ne<3×1018 m−3, which approximately corresponds to the effective collision frequency normalized by the transit frequency of a circulating particle ν*≲1. The large inward particle flux (≳1×1020 m−2 s−1) correlates with the positive shear of the radial electric field Er′ (≳1×106 V m−2), but does not have clear correlation with the radial electric field Er and the curvature Er″.

Experimental investigation of the ripple induced losses of perpendicularly injected beam ions in the low aspect ratio helical system CHS

Confinement of perpendicularly injected beam ions has been investigated experimentallyboth in the standard configuration and in the drift optimized configuration of theCHS heliotron/torsatron by means of the neutron technique. The experimental resultsindicate that the confinement of trapped beam ions is poor in both configurations. Theobserved loss rate of trapped beam ions was very high and this high rate is not explainedby losses due to charge exchange or collisional scattering from the timescale perspective.Full gyro-motion following orbit calculations showed that in the peripheralregion of CHS plasmas, trapped beam ions are promptly lost because they immediatelyintersect the vacuum chamber wall due to their large Larmor radii. It also suggested thatin the core domain the collisionless transition behaviour is the key to understanding the rapid lossof beam ions. It was shown that the magnetic moment of perpendicularly injected beamions is not well conserved in CHS. The rapid losses of perpendicularly injected beamions are probably due to non-adiabatic effects resulting in the transition behaviour of energeticions.

Helical harmonics for static fields.

Helical harmonic solutions of Laplace's equation are derived using a right-handed nonorthogonal helical coordinate system and a modified separation of variables technique. Unlike the Cartesian or cylindrical coordinate systems, in the helical system, two different sets of solutions are admitted, one right handed and one left handed. Consequently, the helical harmonics are used to solve the boundary value problem of two nested helices with different radii set to different potential values.

Helical localized wave solutions of the scalar wave equation

A right-handed helical nonorthogonal coordinate system is used to determine helical localized wave solutions of the homogeneous scalar wave equation. Introducing the characteristic variables in the helical system, i.e., u = zeta - ct and v = zeta + ct, where zeta is the coordinate along the helical axis, we can use the bidirectional traveling plane wave representation and obtain sets of elementary bidirectional helical solutions to the wave equation. Not only are these sets bidirectional, i.e., based on a product of plane waves, but they may also be broken up into right-handed and left-handed solutions. The elementary helical solutions may in turn be used to create general superpositions, both Fourier and bidirectional, from which new solutions to the wave equation may be synthesized. These new solutions, based on the helical bidirectional superposition, are members of the class of localized waves. Examples of these new solutions are a helical fundamental Gaussian focus wave mode, a helical Bessel-Gauss pulse, and a helical acoustic directed energy pulse train. Some of these solutions have the interesting feature that their shape and localization properties depend not only on the wave number governing propagation along the longitudinal axis but also on the normalized helical pitch.

Gyrokinetic study of the electron temperature gradient instability in plasmas with slightly hollow density profiles

The electron temperature gradient driven instabilities in plasmas with slightly hollow density profiles are studied. The gyrokinetic integral eigenvalue equation valid for a sheared slab configuration is employed. The Debye shielding effect on the modes is investigated. The effects of a sheared E×B flow on the modes are considered. Six modes with even or odd parities are found to be simultaneously unstable. The mixing length estimate for the transport is calculated. The flow shear suppression on the modes, as well as on the transport, is demonstrated. The correlations of the results with helical system Heliotron/Torsotron and tokamak experiments are discussed.

Direct numerical simulation of turbulent flows in curved and helically coiled pipes

Fully developed, statistically steady turbulent flow in straight, curved and helically coiled pipes is studied by means of direct numerical simulation to show the influence of curvature and torsion on turbulent flow. The incompressible Navier–Stokes equations, expressed in an orthogonal helical coordinate system, are integrated numerically by using second order central schemes in space and time. Besides the mean flow quantities and the rms values, vector plots and the stream function are shown to give an impression of the induced mean secondary flow.

Observation of toroidal flow antiparallel to the drift direction in the hot electron mode plasmas in the compact helical system.

A toroidal flow antiparallel to the <E(r) x B(straight theta)> drift direction is observed in the hot electron mode plasmas when a large positive electric field and a sharp electron temperature gradient are sustained inside the internal transport barrier in the Compact Helical System. This toroidal flow reaches up to 5x10(4) m/s at the plasma center, and it is large enough to reverse the toroidal flow driven by a tangentially injected neutral beam. These observations clearly show the plasma favors flow in the minimum nablaB direction at the transport barrier.

Susceptibility, Magnetization Process and ESR Studies on the Helical Spin System RbCuCl3

The static and dynamic magnetic properties of an $S=1/2$ stacked triangular antiferromagnet RbCuCl$_{3}$ with a helical spin structure due to lattice distortion were investigated by susceptibility, high-field magnetization process and ESR measurements. The susceptibilities were analyzed by high-temperature expansion approximation in terms of ferromagnetic intrachain coupling and the antiferromagnetic interchain coupling. The magnetization saturates at $H_{\rm s}\approx 66.8$ T at 1.5 K, the value of which is twice that of CsCuCl$_3$. A small magnetization jump indicative of a phase transition was observed at $H_{\rm c}=21.2$ T $\approx (1/3)H_{\rm s}$ for $H{\perp}c$. ESR modes observed for $H\parallel c$ are well described by the calculation based on the helical spin structure. From the present measurements, the ferromagnetic intrachain and two kinds of antiferromagnetic interchain exchange interactions, and the planar anisotropy energy were determined as $J_0/k_{\rm B}=25.7$ K, $J_1/k_{\rm B}=-10.6$ K, $J_1'/k_{\rm B}=-17.4$ K, and ${\Delta}J_0/k_{\rm B}=-0.45$ K, respectively.

Extraction of Runaway Electrons from Helical Magnetic Confinement System

Possibility of helical device as a high current electron source by utilizing a new method of runaway electrons extraction has been investigated. Magnetic force lines of the helical system with extraction coils and electron orbits in the given magnetic configuration have been calculated to design an experimental device for demonstrate electrons extracting from the helical device. Numerical results show it can be made.

Raman Optical Activity of a Purelyσ-Bonded Helical Chromophore: (−)-(M)-σ-[4]Helicene

The recent synthesis of enantiomerically pure (−)-(M)-σ-[4]helicene has provided an archetype helical model system for vibrational optical activity, comparable to what π-helicenes represent in the field of electronic optical activity. We present the first measurements and the first calculations of the Raman optical activity (ROA) of this interesting molecule. Observed and calculated ROA is large throughout the vibrational spectrum, in agreement with expectations, and spectacular effects, with Δ values close to 0.5%, occur in the 900-cm−1 region. Agreement between the experimental spectrum and the theoretical one, calculated with density-functional theory for the vibrational part and Hartree-Fock linear response theory for the molecular electronic tensors, is excellent, clearly the best that has been achieved to date in the field. This allows us to place confidence in the results of an analysis of Raman and ROA scattering generation in the molecule, obtained by a newly developed graphical procedure for extracting this kind of information from ab initio calculations. One finds that relative contributions made by carbon and hydrogen atoms can be comparable in size, but can also vary considerably, even between closely lying vibrations, and that, for most vibrations, the generation of ROA difference intensity is distributed rather differently than that of Raman intensity over the shape of the molecule. The sign of the ROA is, for the set of vibrations in the 900-cm−1 region, which we analyze in detail, determined by coupling terms between the two halves of the molecule, while Raman intensity is primarily generated within the two fragments, with coupling terms between them only adding to or substracting from it.

Vibration Analysis of a Multi-Stage Helical Gear System Including a Gearbox.

A new method for calculating the three-dimensional vibration behavior of a multi-stage helical gear system including a gearbox has been developed. This method utilizes three-dimensional multi -stage gear vibration analysis and FEM analysis. The calculated three-dimensional vibration of gears is used in a FEM model, which gives the vibration responses of the gearbox. The calculated and measured eigen frequencies and vibration responses of the gearbox agree well. For obtaining precise vibration responses of the gearbox, therefore, it is shown to be effective to consider the modeling of gear shaft, gear meshing, bearing and leg of gearbox in FEM analysis.

Vibration Analysis of a Multi-Stage Gear System Including Drive Mechanism Elements. Proposal of Three-Dimentional Vibration Model and Eigenvalue Analysis of Helical Gear System.

We have developed a new method for building a vibration model of an actual gear-drive system. Each gear has six degrees of freedom: three translational and three rotational motions. Stiffness matrixes representing each gear shaft and tooth meshing were made and overlapped into one stiffness matrix for the whole system. This enabled easy modelling of complex gear-drive systems such as multi-stage gears and branched gears. The results are in good agreement with experimental results with respect to the major resonance frequencies and vibration modes.

Photon-counting CCD detector as a tool of x-ray imaging

A new x-ray imaging technique to measure magnetic axis and two-dimensional soft x-ray energy spectra for a long-pulse discharge has been developed by utilizing a soft x-ray photon-counting charge-coupled-device (CCD) camera. This system consists of pinholes, Be filters, and a 1024×1024 frame-transfer back-illumination CCD detector (the imaging area has 1024×512 pixels). By choosing appropriate combinations of pinholes and Be filters, the x-ray flux is adjusted to the level suited for photon-counting mode and imaging mode, respectively. The Shafranov shift is derived from a two-dimensional soft x-ray intensity map measured in the imaging mode in the compact helical system (CHS) and large helical device. Two-dimensional profiles of electron temperature and two-dimensional profiles of high-Z impurity Kα radiation intensity are derived from two-dimensional energy spectra of x rays measured in photon-counting mode for the CHS hot-electron-mode plasma.