Validating neutral-beam current drive simulations in the TJ-II stellarator
Sadig Mulas, Álvaro Cappa, José Martínez-Fernández, Daniel López-Bruna, José Luis Velasco, Teresa Estrada, Jesús Manuel Gómez-Manchón, Macarena Liniers, Ignacio… 展开作者
Sadig Mulas, Álvaro Cappa, José Martínez-Fernández, Daniel López-Bruna, José Luis Velasco, Teresa Estrada, Jesús Manuel Gómez-Manchón, Macarena Liniers, Ignacio Pastor, Francisco Medina, Enrique Ascasíbar, TJ-II Team
In this paper, we analyze the results of neutral-beam current drive (NBCD) experiments, performed in the TJ-II stellarator, with the aim of validating the theoretical predictions. Both parallel and anti-parallel injection with respect to the magnetic field were explored using co (NBI1) and counter (NBI2) beams at different injected beam power and plasma densities. The fast-ion current driven by both beams was simulated with the Monte Carlo code ASCOT and the electron response to the fast-ion current was calculated analytically using a model valid for an arbitrary magnetic configuration and a low collisionality plasma. The model reproduces with rather good agreement the toroidal current measured in NBI2 plasmas while the current driven by NBI1 is less than half the predicted one. Possible reasons for this discrepancy are discussed.
The global gyrokinetic toroidal code (GTC) has been recently upgraded to do simulations in non-axisymmetric equilibrium configuration, such as stellarators. Linear simulation of ion temperature gradient (ITG) driven instabilities has been done in Wendelstein7-X (W7-X) and Large Helical Device (LHD) stellarators using GTC. Several results are discussed to study characteristics of ITG in stellarators, including toroidal grids convergence, nmodes number convergence, poloidal and parallel spectrums, and electrostatic potential mode structure on flux surface.
The first fast ion experiments in Wendelstein 7-X were performed in 2018. They are one of the first steps in demonstrating the optimised fast ion confinement of the stellarator. The fast ions were produced with a neutral beam injection (NBI) system and detected with infrared cameras (IR), a fast ion loss detector (FILD), fast ion charge exchange spectroscopy (FIDA), and post-mortem analysis of plasma facing components. The fast ion distribution function in the plasma and at the wall is being modelled with the ASCOT suite of codes. They calculate the ionisation of the injected neutrals and the …
Optimised stellarators and other magnetic-confinement devices having the property that the average magnetic curvature is favourable for all particle orbits are called maximum-$J$ devices, and have recently been shown to be immune to trapped-particle instabilities driven by the density gradient. Gyrokinetic simulations reveal, however, that another instability can arise, which is also associated with particle trapping but causes less transport than typical trapped-electron modes. The nature of this instability is clarified here. It is shown to be similar to the "ubiquitous mode" in tokamaks, an…
Sawtooth-like core electron temperature crashes have been observed in W7-X experiments with electron cyclotron current drive. We present nonlinear single-fluid magnetohydrodynamic simulations of this phenomenon using the newly developed stellarator modeling capability of the M3D-$C^1$ code. The near-axis current drive gives rise to two $ι=1$ resonances in the equilibrium rotational transform profile so that two consecutive $(1,1)$ internal kink modes are seen in the simulations. A small-amplitude crash at the inner resonance occurs first, which may correspond to the sawtooth precursors observe…
The control of impurity accumulation is one of the main challenges for future stellarator fusion reactors. The standard argument to explain this accumulation relies on the, in principle, large inward pinch in the neoclassical impurity flux caused by the typically negative radial electric field in stellarators. This simplified interpretation was proven to be flawed by Helander et al. [Phys. Rev. Lett. 118, 155002 (2017)], who showed that in a relevant regime (low-collisionality main ions and collisional impurities) the radial electric field does not drive impurity transport. In that reference, …
In this work, the role which three-dimensional shaping plays in the generation of rotational transform in toroidal magnetically confinement devices is explored. The susceptance matrix as defined by Strand & Houlberg (2001) is presented and compared to simulations of three dimensional MHD equilibria. The dependence of the edge rotational transform on axisymmetric shaping is briefly explored. It is then shown how simple three dimensional shaping can drive rotational transform in a simply shaped stellarator. It is found that modes with high poloidal mode number drive edge transform with little ef…
In stellarators, zonal flow activity depends sensitively on geometry of the three dimensional magnetic field, via an interplay of mechanisms that is not fully understood. In this work, we investigate this by studying three magnetic configurations of the Wendelstein 7-X stellarator. We find that variation in linear zonal flow damping is accompanied by variation in nonlinear drive, and identify key geometric features that control these effects. Understanding the resulting balance is important for the development of reduced models of turbulent transport.
The accumulation of impurities in the core of magnetically confined plasmas, resulting from standard collisional transport mechanisms, is a known threat to their performance as fusion energy sources. Whilst the axisymmetric tokamak systems have been shown to benefit from the effect of temperature screening, that is an outward flux of impurities driven by the temperature gradient, impurity accumulation in stellarators was thought to be inevitable, driven robustly by the inward pointing electric field characteristic of hot fusion plasmas. We have shown in Helander et. al. (2017b) that such scree…
The first neutral beam injection (NBI) experiments in Wendelstein 7-X (W7-X) stellarator were conducted in the summer of 2018. The NBI system is used to heat the magnetically confined plasma by neutralising an accelerated hydrogen ion beam and directing it into the plasma, where the resulting energetic ions release their energy to heat the plasma. The modelling of the NBI fast ion experiments has commenced, including estimation of the shine-through and the orbit-losses. The stellarator has a wide-angle infra red (IR) imaging system to monitor the machine plasma facing component surface tempera…
Results of a three-dimensional, flux-driven, electrostatic, global, two-fluid turbulence simulation for a 5-field period stellarator with an island divertor are presented. The numerical simulation is carried out with the GBS code, recently extended to simulate plasma turbulence in non-axisymmetric magnetic equilibria. The vacuum magnetic field used in the simulation is generated with the theory of Dommaschk potentials, and describes a configuration with a central region of nested flux surfaces, surrounded by a chain of magnetic islands, similarly to the diverted configurations of W7-X. The hea…
A potential threat to the performance of magnetically confined fusion plasmas is the problem of impurity accumulation, which causes the concentration of highly charged impurity ions to rise uncontrollably in the center of the plasma and spoil the energy confinement by excessive radiation. It has long been thought that the collisional transport of impurities in stellarators always leads to such accumulation (if the electric field points inwards, which is usually the case), whereas tokamaks, being axisymmetric, can benefit from "temperature screening", i.e., an outward flux of impurities driven …
Good magnetic surfaces, as opposed to magnetic islands and chaotic field lines, are generally desirable for stellarators. In previous work, M. Landreman et al. [Phys. of Plasmas 28, 092505 (2021)] showed that equilibria computed by the Stepped-Pressure Equilibrium Code (SPEC) [S. P. Hudson et al., Phys. Plasmas 19, 112502 (2012)] could be optimized for good magnetic surfaces in vacuum. In this paper, we build upon their work to show the first finite-$β$, fixed- and free-boundary optimization of SPEC equilibria for good magnetic surfaces. The objective function is constructed with the Greene's …
Fully-ionised carbon impurity flow is studied in ion-root, neutral beam heated plasmas by means of Charge Exchange Recombination Spectroscopy (CXRS) in the TJ-II stellarator. Perpendicular flows are found to be in reasonable agreement with neoclassical calculations of the radial electric field. The parallel flow of the impurity is obtained at two locations of the same flux surface after subtraction of the calculated Pfirsch-Schlüter parallel velocity. For the medium density plasmas studied, $\bar{n}_{\rm e}\in(1.2-2.4)\times 10^{19}$ m$^{-3}$, the measured impurity flow is found to be inconsis…
Main ions influence impurity dynamics through a variety of mechanisms; in particular, via impurity-ion collisions. To lowest order in an expansion in the main ion mass over the impurity mass, the impurity-ion collision operator only depends on the component of the main ion distribution that is odd in the parallel velocity. These lowest order terms give the parallel friction of the impurities with the main ions, which is typically assumed to be the main cause of collisional impurity transport. Next-order terms in the mass ratio expansion of the impurity-ion collision operator, proportional to t…
Initial evaluation of miniature ultra-high-field commercial stellarator reactors with breeding external to resistive coils
Laboratorio Nacional de Fusión, CIEMAT（国家聚变实验室，CIEMAT）；Departamento de Física, Universidad Carlos III de Madrid（马德里卡洛斯三世大学物理系）；Oak Ridge National Laboratory (ORNL)（橡树岭国家实验室）；University of Texas, Austin（德克萨斯大学奥斯汀分校）
We investigate the linear theory of the ion-temperature-gradient (ITG) mode, with the goal of developing a general understanding that may be applied to stellarators. We highlight the Wendelstein 7X (W7-X) device. Simple fluid and kinetic models that follow closely from existing literature are reviewed and two new first-principle models are presented and compared with results from direct numerical simulation. One model investigates the effect of regions of strong localized shear, which are generic to stellarator equilibria. These "shear spikes" are found to have a potentially significant stabil…
Stellarator News, an international journal of the stellarator community, is Published by Fusion Energy Division, Oak Ridge National Laboratory, James A. Rome, Editor In the March 1995 issue . . . **** Exerpts from the U.S. Congress Office of Technology Assment report on TPX and Alternate Concepts. **** Edge transport and turbulence studies on U-3M The turbulent-driven particle flow is shown to be comparable with the equilibrium flow at the boundary of the configuration under Alfven-heating conditions in the U-3M torsatron. **** Topological aspects of island divertor studies on W7-AS The struct…
Tight tolerances have been a leading driver of cost in recent stellarator experiments, so improved definition and control of tolerances can have significant impact on progress in the field. Here we relate tolerances to the shape gradient representation that has been useful for shape optimization in industry, used for example to determine which regions of a car or aerofoil most affect drag, and we demonstrate how the shape gradient can be computed for physics properties of toroidal plasmas. The shape gradient gives the local differential contribution to some scalar figure of merit (shape functi…
We demonstrate a fast adjoint-based method to optimize tokamak and stellarator equilibria against a pressure-driven instability known as the infinite-$n$ ideal ballooning mode. We present three finite-$β$ (the ratio of thermal to magnetic pressure) equilibria: one tokamak equilibrium and two stellarator equilibria that are unstable against the ballooning mode. Using the self-adjoint property of ideal MHD, we construct a technique to rapidly calculate the change in the growth rate, a measure of ideal ballooning instability. Using the~\texttt{SIMSOPT} framework, we then implement our fast adjoin…
Global gyrokinetic analysis of Wendelstein 7-X discharge: unveiling the importance of trapped-electron-mode and electron-temperature-gradient turbulence
本文首次对类 W7-X 仿星器放电进行含动理学电子、平衡径向电场及电磁碰撞效应的非线性径向全局回旋动理学模拟，揭示芯部湍流电子热通量主要由 TEM 和 ETG 驱动，并强调有限电子温度梯度的关键作用。
X-ray emission spectroscopy is a well-established technique used to study continuum lowering in dense plasmas. It relies on accurate atomic physics models to robustly reproduce high-resolution emission spectra, and depends on our ability to identify spectroscopic signatures such as emission lines or ionization edges of individual charge states within the plasma. Here we describe a method that forgoes these requirements, enabling the validation of different continuum lowering models based solely on the total intensity of plasma emission in systems driven by narrow-bandwidth x-ray pulses across …
Asymptotic scaling theory of electrostatic turbulent transport in magnetised fusion plasmas
Princeton Plasma Physics Laboratory（普林斯顿等离子体物理实验室）；University of Maryland（马里兰大学）；University of Oxford（牛津大学）；Princeton University（普林斯顿大学）；Institute for Research in Electronics and Applied Physics（电子与应用物理研究所）；United Kingdom Atomic Energy Authority（英国原子能管理局）；University of New Hampshire（新罕布什尔大学）；University of Otago（奥塔哥大学）
Local gyrokinetic simulations use a field-aligned domain that twists due to the magnetic shear of the background magnetic equilibrium. However, if the magnetic shear is strong and/or the domain is long, the twist can become so extreme that it fails to properly resolve the turbulence. In this work, we derive and implement the "non-twisting flux tube," a local simulation domain that remains rectangular at all parallel locations. Convergence and runtime tests indicate that it can calculate the heat flux more efficiently than the conventional flux tube. For one test case, it was 30 times less comp…
The first experimental campaigns have proven that, due to the optimization of the magnetic configuration with respect to neoclassical transport, the contribution of turbulence is essential to understand and predict the total particle and energy transport in Wendelstein 7-X (W7-X). This has spurred much work on gyrokinetic modelling for the interpretation of the available experimental results and for the preparation of the next campaigns. At the same time, new stellarator gyrokinetic codes have just been or are being developed. It is therefore desirable to have a sufficiently complete, document…
This work focuses on the relationship between L-H (or L-I) transitions and MHD activity in the low magnetic shear TJ-II stellarator. It is shown that the presence of a low order rational surface in the plasma edge (gradient) region lowers the threshold density for H-mode access. MHD activity is systematically suppressed near the confinement transition. We apply a causality detection technique (based on the Transfer Entropy) to study the relation between magnetic oscillations and locally measured plasma rotation velocity (related to Zonal Flows). For this purpose, we study a large number of dis…
