Estimation of 14 MeV neutron rate from triton burn-up in future W7-X deuterium plasma campaigns

Fast ion confinement is of major importance for the ignition of a burning fusion plasma. In future deuterium plasma campaigns of the Wendelstein 7-X stellarator, W7-X, the amount of triton burn-up is one possible measure for fast ion confinement. A well-established technique to observe triton burn-up is the 14 MeV neutron rate. In this paper, it is estimated whether an existing scintillating fibre neutron detector is also suited to measure triton burn-up in W7-X with sufficient accuracy. An estimation is presented, which can be applied to any tokamak or stellarator design and is one-dimensional in the minor radius. The inputs are profiles of density, temperature, and differential volume element as well as the triton slowing-down time. The estimation calculates the thermal deuteron fusion rate and the associated deuteron-triton fusion rate; thus, the triton burn-up generated 14 MeV neutron rate. It neither takes triton diffusion nor explicit losses into account. This thermally generated fusion rate is compared to the neutral beam injection heating induced beam-plasma fusion rate.     

Wendelstein 7‐X: An Alternative Route to a Fusion Reactor

With the ITER tokamak magnetic confinement fusion is making the decisive step to realize a burning fusion plasma and prepare the basis for a demonstration power plant (DEMO). Despite the advantages of an intrinsically steady state magnetic field and better stability properties, the development of stellarators lags behind by about 1 1/2 device generations, because of the difficulties to realize the desired magnetic field configuration. The goal of the optimized stellarator Wendelstein 7‐X is to overcome the principal deficiencies of the stellarator concept and demonstrate its reactor capability, combining sufficiently good thermal and fast ion confinement with reactor relevant ß and collisionality under steady state conditions. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental confirmation of stable, small-debye-length, pure-electron-plasma equilibria in a stellarator

The creation of the first small-Debye length, low temperature pure electron plasmas in a stellarator is reported. A confinement time of 20 ms has been measured. The long confinement time implies the existence of macroscopically stable equilibria and that the single particle orbits are well confined despite the lack of quasisymmetry in the device, the Columbia non-neutral torus. This confirms the beneficial confinement effects of strong electric fields and the resulting rapid E x B rotation of the electrons. The particle confinement time is presently limited by the presence of bulk insulating materials in the plasma, rather than any intrinsic plasma transport processes. A nearly flat temperature profile is seen in the inner part of the plasma.     

New approach to transport improvement in a helical magnetic axis system by introduction of effective toroidal curvature

A new approach to transport improvement in a helical magnetic axis stellarator is proposed. First of all, the proposal is presented for the L = 1 system. The effective toroidal curvature term epsilon(T), defined as the sum of the usual toroidal curvature and one of the nearest satellite harmonics of the helical field, determines confinement conditions of localized trapped particles. There exists a certain correlation between the smallness of epsilon(T) and the omnigeneity. This approach would give rise to the possibility of a stellarator design study in a wider parameter domain than quasisymmetry approaches.     

Edge Turbulence During Limiter Biasing Experiments in the TJ-II Stellarator

The influence of limiter biasing on plasma confinement, turbulence and plasma flows has been investigated in the TJ-II stellarator. Experimental results show that it is possible to modify global particle confinement and edge plasma parameters with both positive and negative biasing. Significant and minor modifications in the structure of plasma fluctuations have been observed during the transition to improved confinement regimes induced by limiter biasing. These results show evidence of electric field induced improved confinement via multiple mechanisms. The investigation of the relaxation of plasma potential and electric fields shows evidence of two different characteristic decay times.     

New advanced operational regime on the W7-AS stellarator

A promising new plasma operational regime on the Wendelstein stellarator W7-AS has been discovered. It is extant above a threshold density and characterized by flat density profiles, high energy and low impurity confinement times, and edge-localized radiation. Impurity accumulation is avoided. Quasistationary discharges with line-averaged densities n(e) to 4 x 10(20) m(-3), radiation levels to 90%, and partial plasma detachment at the divertor target plates can be simultaneously realized. Energy confinement is up to twice that of a standard scaling. At B(t) = 0.9 T, an average beta value of 3.1% is achieved. The high n(e) values allow demonstration of electron Bernstein wave heating using linear mode conversion.     

Energy-confinement scaling for high-beta plasmas in the W7-AS stellarator

High-beta energy-confinement data are subjected to comparisons of scaling invariant, first-principles physical models. The models differ in the inclusion of basic equations indicating the nature of transport. The result for high-beta data of the W7-AS stellarator is that global transport is described best with a collisional high-beta model, which is different from previous outcomes for low-beta data. Model predictive calculations indicate the validation of energy-confinement prediction with respect to plasma beta and collisionality nu*. The finding of different transport behaviors in distinct beta regimes is important for the development of fusion energy based on magnetic confinement and for the assessment of different confinement concepts.     

Theoretical Optimization of Stellarators

Net current free toroidal ("stellarator") confinement is studied with a combination of several methods: a complete set of analytical vacuum fields for finding favorable vacuum field configurations; three-dimensional MHD codes for finite-?, equilibrium computations; the expansion of a general toroidal equilibrium around its magnetic axis as guideline for the computational search in configurational space and for finite-?, MHD stability; Monte Carlo simulations for particle containment; continuous modular coil systems generating the configurations considered. Results are: vacuum field configurations with sizeable Q = 0, 1, 2, 3 helical fields, substantial twist number (? 1/2), significant reduction of the parallel current density, and vacuum magnetic well exist for a toroidal aspect ratio of 15-20 and can be generated by modular coils whose excursions from meridional planes are small compared to the toroidal period length. In these configurations, the finite-? toroidal shift is strongly reduced, so that a larger ? value (factor 2-4) than in the equivalent Q = 2 stellarator can be achieved. Stability calculations do not exclude the possibility of stable equilibria of this kind with (?) ? 0.05-0.1; transport calculations without electrical field show improvement-as compared to the Q = 2 stellarator-in the collisional and plateau regimes.     

ESTELL: A Quasi‐Toroidally Symmetric Stellarator

This work presents the physics design for a simple quasi‐axially symmetric stellarator. A plasma configuration described by a modest number of Fourier coefficients was found to establish this symmetry with good accuracy. The low rotational transform results in a relatively simple coil set exhibiting low curvatures and comfortable clearance between adjacent coils. As another consequence, the maximum achievable plasma pressure will be limited to about 0.5%. An experiment along the lines proposed would allow an exploration of the confinement properties of a quasi‐axially symmetric configuration. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

仿星器研究的进展

本文综合介绍了仿星器(由外部线圈产生旋转交换的任何稳态环形磁约束装置)进展,包括经典仿星器研究、扭曲器和模块式仿星器、仿星器位形优化及仿星器作为聚变堆的优点,最后对我国在这方面的研究对策提出一些看法。     

Effect of unstable MHD modes on the confinement of a stellarator plasma

The results of an experimental study of the influence of unstable MHD modes on plasma confinement in an L-2M stellarator are presented. The spectral and statistical characteristics of turbulent plasma simultaneously at both the edge and center of the plasma are investigated. It is shown that at constant power of electron-cyclotron heating of the plasma the energy content of the plasma depends strongly on the strength of the externally applied vertical magnetic field used to adjust the position of the plasma column. Appreciable degradation of plasma confinement is observed for values of the vertical field such that ideal MHD modes become unstable in the greater part of the plasma column. This in turn is due to the formation of a magnetic configuration with a magnetic “hump.” At the same time, in the edge plasma the instability of resistive-balloon modes grows, and turbulent particle transport increases. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 6, 407–412 (25 March 1999)     

Modular Coils: A Promising Toroidal Coil System

The concept of modular coils originated from a need to find reactor-relevant stellarator windings. Considerations of 1) basic principles of modular coils, 2) types of coils, 3) types of configurations (general, helically symmetric, helically asymmetric, with magnetic well, with magnetic hill), 4) types of rotational transform profile, and 5) structure and origin of ripples are given. These results show that modular coils can offer a wide range of vacuum magnetic field configurations, some of which cannot be obtained with the classical stellarator or torsatron coil configuration.     

High-beta equilibria of drift-optimized compact stellarators

Compact stellarator configurations have been obtained with good neoclassical confinement that are stable to both pressure- and current-driven modes for high values of beta. These configurations are drift-optimized tokamak-stellarator hybrids with a high-shear tokamak-like rotational transform profile and /B/ that is approximately poloidally symmetric. The bootstrap current is consistent with the required equilibrium current and, while larger than that in existing stellarators, is typically only a small fraction (1/3-1/5) of that in an equivalent tokamak. These configurations have strong magnetic wells and consequently high interchange stability beta limits up to beta=23%. Because of the reduced bootstrap current, these configurations are stable to low-n ideal MHD kink modes with no wall stabilization for values of beta ( approximately 7%-11%) significantly larger than in an equivalent advanced tokamak.     

Experimental and Theoretical Studies on Filling a Stel a Rator with Laser-Produced Plasma

Experiments have been performed on filling a stellarator with a noncurrent-carrying laser-produced plasma. Simultaneous plasma production by means of pulsed laser beams at four separate positions on the toroidal magnetic axis has been found to drastically enhance the trapping efficiency of produced plasma by stellarator field, to as high as 50 percent in contrast with about 10 percent in the case of plasma production at one position. This figure of 50 percent could be further improved to nearly 75 percent by spatially isotropic plasma productions which could not be realized in the present experiments owing to technical restrictions on the stellarator employed. The enhancement of trapping efficiency may be attributed to the reduction of toroidal plasma drift due to rotational transform coming into effect earlier in the multiposition production case. Some approximate theoretical analyses and considerations on toroidal drift motion of laser-produced plasma stream within stellarator field have also been presented, and the theoretical prediction on these analyses appears to be consistent with experimental results obtained.     

Role of rational surfaces on fluctuations and transport in the plasma edge of the TJ-II stellarator

It has been shown that transport barriers in toroidal magnetically confined plasmas tend to be linked to regions of unique magnetic topology such as the location of a minimum in the safety factor, rational surfaces or the boundary between closed and open flux surfaces. In the absence ofE×B sheared flows, fluctuations are expected to show maximum amplitude near rational surfaces, and plasma confinement might tend to deteriorate. On the other hand, if the generation ofE×B sheared flows were linked to low order rational surfaces, these would be beneficial for confinement. Experimental evidence ofE×B sheared flows linked to rational surfaces has been obtained in the plasma edge region of the TJ-II stellarator. Presented at the Workshop on the Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June, 2000.     

Spontaneous edge <Emphasis Type="Italic">E</Emphasis> × <Emphasis Type="Italic">B</Emphasis> sheared flow development studies in the TJ-II stellarator

The influence of plasma density and edge gradients in the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. It has been experimentally observed that the generation of spontaneous perpendicular sheared flow (i.e. the naturally occurring shear layer) requires a minimum plasma density or gradient. It has been found that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increase. The resulting shearing rate is close to the one required to trigger a transition to improved confinement regimes with reduction of edge turbulence, showing that spontaneous sheared flows and fluctuations keep themselves near marginal stability. Presented at the Workshop “Electric Fields, Structures and Relaxation in Edge Plasmas”, Tarragona, Spain, July 3–4, 2005.     

Application of the Haar Wavelet to the Analysis of Plasma and Atmospheric Fluctuations

The parameters of turbulence measured by means of a Doppler reflectometer at the plasma periphery in an L-2M stellarator and in atmospheric vortices (typhoons and tornadoes) are investigated using the wavelet methods with involvement of the Haar function. The periods of time taken for the transition (a bound of parameters) to occur in the L-2M stellarator plasma and in atmospheric processes are estimated. It is shown that high-and low-frequency oscillations of certain parameters, in particular, pressure, that occur in atmospheric vortices decay or increase at different moments of time, whereas the density fluctuation amplitudes that occur in plasma at different frequencies vary in a synchronous manner.     

A Spectrally Resolved Motional Stark Effect Diagnostic for the TJ‐II Stellarator

A spectrally resolved motional stark effect (MSE) diagnostic has been implemented for the TJ‐II stellarator to quantify the magnitude and pitch of components of the magnetic field created in this magnetic confinement device. The system includes a compact diagnostic neutral beam injector (DNBI) that provides a short pulse of accelerated neutral hydrogen atoms with an e^–1 beam radius of 2.1 cm to stimulate the Doppler‐displaced Balmer Hαs emissions, which are the basis for this diagnostic. Measurement of the wavelength separation of the Stark splitting of the Hα spectrum, as well as of the relative line intensities of its components, allow the local magnitude and direction of the internal magnetic field components to be measured at 10 positions across the plasma. The use of a DNBI extends such measurements to the electron cyclotron resonance (ECR) heated phases of plasmas while also overcoming the need for the complicated inversion techniques that are required when such measurements are performed with a heating neutral beam injector (NBI). Moreover, the use of the shot‐to‐shot technique with reproducible discharges further simplifies fits to the MSE spectra as nearby impurity spectral emission lines can be eliminated or significantly reduced. After outlining the principles of this technique and the diagnostic set‐up, magnetic field measurements made during ECR or NBI heating phases are reported for a range of magnetic configurations and are compared with vacuum magnetic field estimates in order to evaluate the capabilities and limitations of this diagnostic for the TJ‐II. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental investigation of the magnetic configuration dependence of turbulent transport

The dependence of turbulent transport on magnetic field properties is measured in detail on a plasma in a stellarator configuration. Pronounced poloidal asymmetries of fluctuation amplitudes and turbulent transport are observed. The transport maximum is located in regions where normal curvature of the magnetic field is negative and simultaneously the geodesic curvature has positive values. A major role of the local magnetic shear cannot be confirmed. The results can have important implications for the optimization of stellarators and the power influx into the scrape-off layer.     

Equilibrium-based iterative tomography technique for soft X-ray instellarators

A method to deduce local values of plasma emissivity from chord measurements has been developed and tested using data from a 30 detector soft X-ray array in the Wendelstein 7A stellarator. This technique, based on the calculated distribution of toroidal magnetic flux as the initial guess for the plasma emissivity, uses an iterative scheme to algebraically determine the distribution of the detector signals, and solves some drawbacks of the currently available tomography techniques. This method is especially suited for stellarator devices in which plasma cross sections are markedly noncircular and change as a function of the toroidal angle