Island Divertor Configuration Research Articles

Characteristics of the SOL ion-to-electron temperature ratio on the J-TEXT tokamak with different plasma configurations

Accurate measurement of the average plasma parameters in the edge region, including the temperature and density of electrons and ions, is critical for understanding the characteristics of the scrape-off layer (SOL) and divertor plasma transport in magnetically confined fusion research. On the J-TEXT tokamak, a multi-channel retarding field analyzer (RFA) probe has been developed to study average plasma parameters in the edge region under various poloidal divertor and island divertor configurations. The edge radial profile of the ion-to-electron temperature ratio, τ i/e, has been determined, which gradually decreases as the SOL ion self-collisionality, , increases. This is broadly consistent with what has been observed previously from various tokamak experiments. However, the comparison of experimental results under different configurations shows that in the poloidal divertor configuration, even under the same , τ i/e in the SOL region becomes smaller as the distance from the X-point to the target plate increases. In the island divertor configuration, τ i/e near the O-point is higher than that near the X-point at the same , and both are higher than those in the limiter configuration. These results suggest that the magnetic configuration plays a critical role in the energy distributions between electrons and ions at the plasma boundary.

Analysis of hydrogen fueling, recycling, and confinement at Wendelstein 7-X via a single-reservoir particle balance

A single-reservoir particle balance for the main plasma species hydrogen has been established for W7-X. This has enabled the quantitative characterization of the particle sources in the standard island divertor configuration for the first time. Findings from attached scenarios with two different island sizes with a boronized wall and turbo molecular pumping are presented. Fueling efficiencies, particle flows and source locations were measured and used to infer the total particle confinement time τ p. Perturbative gas injection experiments served to measure the effective particle confinement time . Combining both confinement times provides access to the global recycling coefficient . Hydrogen particle inventories have been addressed and the knowledge of particle sources and sinks reveals the core fueling distribution and provides insight into the capability of the magnetic islands to control exhaust features. Measurements of hydrogen fueling efficiencies were sensitive to the precise fueling location and measured between 12% and 31% with the recycling fueling at the strike line modeled at only 6%, due to much higher densities. 15% of the total 5.2 × 1022 a/s recycling flow ionizes far away from the recycling surfaces in the main chamber. It was shown that 60% of recycled particles ionize above the horizontal and 18% above the vertical divertor target, while the remainder of the recycling flow ionizes above the baffle (7%). Combining these source terms with their individual fueling efficiencies resolves the core fueling distribution. Due to the higher fueling efficiency in the main chamber, up to 51% of the total 5.1 × 1021 s−1 core fueling particles are entering the confined plasma from the main chamber. τ p values in the range of 260 ms were extracted for these discharges. Together with τ p, the global recycling coefficient was resolved for every measurement and a typical value close to unity was obtained. An increase of the island size, resulted in no change of τ p, but doubled , indicating the feasibility of the control coils as an actuator to control exhaust features without affecting core confinement properties.

Conceptual Design of the Power Supply for Magnetic Island Divertor Configuration on J-TEXT

The ac power supply system of the magnetic island divertor coil is widely applied to reduce the peak thermal load of the boundary target plate. It was found that this system should have the current frequency in the range of 0-100 Hz and the amplitude of 5 kA at 100 Hz. In this article, a new main circuit is proposed for the variable-frequency power supply system. The proposed circuit consists of a six-pulse, thyristor rectifier and a parallel, H-bridge inverter. The control strategy applies the carrier phase-shifted, sinusoidal pulsewidth modulation (CPS-SPWM) to form an almost sinusoidal current and calculate the LC filter parameters of the bus and inverter. Based on the requirements and load parameters of the power supply, it is found that the voltage and current double-closed loop PI control accelerates the system dynamic response and improves the nonlinear, load adaptability of the system. Moreover, the phase-shifting transformer with high-overload capability is applied in the proposed power supply to reduce the volume of the transformer and in advance charging. On the other hand, the water-cooling mode is adopted and the structure of water-cooled plate is designed. Moreover, optimal device and layout selection is performed to achieve reasonable modularization. Finally, the proposed method is applied to complete the main components of the space layout of the power supply. In the studied cases, the power density of 0.42 W/L is obtained.

Measurement of the edge ion temperature in W7-X with island divertor by a retarding field analyzer probe

Knowledge of the edge ion temperature (Ti) is important to understand the edge physics in fusion plasmas. The Ti edge profile has been successfully measured at W7-X in an island divertor configuration using a retarding field analyzer probe on a reciprocating manipulator. The experimental observations show that in the standard configuration, an ion temperature shoulder at the plasma boundary has been observed near the location where a sudden change of the magnetic field connection length appears in the scrape-off layer (SOL) region. The value of the Ti shoulder was observed to be dependent on the edge island structure modified by the control coils, SOL kinetic power, and electron density. A significant reduction and flattening in the edge ion temperature profile has been also observed during the nitrogen seeding from the upper horizontal divertor port. In addition, the edge ion-to-electron temperature ratio has been calculated, which gradually increases with the major radius and decreases with the normalized ion collisionality. An extended two-point model including upstream ion temperature is introduced to understand the experimental measurement.

First demonstration of radiative power exhaust with impurity seeding in the island divertor at Wendelstein 7-X

Radiative power exhaust by impurity seeding was demonstrated for the first time in island divertor configurations at the stellarator Wendelstein 7-X. Feasibility of stable plasma operation was shown during seeding with both neon (Ne) and nitrogen (N2). High radiative power losses (80%) were found to reduce the divertor heat loads globally by 2/3 with both seeding gases injected at a single toroidal location into one of five magnetic islands. Heat flux detachment was achieved for the price of a loss of (%) in the stored energy. Ne seeding allows for sustained enhancement of edge radiation with a very slow decay of line emission of several tens of seconds after the end of the injection indicating a high recycling of this noble gas at the carbon main plasma facing components. In N2 seeded discharges it is shown that a response of line emission and plasma parameters is in correlation to the puff duration which indicates a higher level of absorption of this seeding gas in the wall. Continuous N2 seeding results in global cooling of the scrape-off layer (SOL) and decay of radiation over several seconds after the injection. Damping of counter-streaming SOL flows, and divertor particle fluxes induced by Ne and N2 seeding have been measured and provide evidence for a reduction of the convective part of the divertor heat fluxes. Losses in density in response to seeding can be compensated by feedback controlled divertor fueling. The controlled reduction of heat fluxes within this complex 3D edge island geometry is a very promising finding concerning detachment control in a future all-metal divertor.

Spectroscopic studies of fuel recycling and impurity behaviors in the divertor region of Wendelstein 7-X

The first divertor operation phase (OP1.2a) was carried out on Wendelstein 7-X in the second half of 2017. Fuel recycling and impurity behaviors in the divertor region were investigated by employing a newly built ultraviolet–visible–near infrared overview spectroscopy system. The characteristic spectral lines of the working gases (hydrogen and helium), intrinsic impurities (carbon, oxygen and iron), and seeded impurities (neon and nitrogen) were identified and analyzed. The divertor electron temperature and density were measured using He I (667.8, 706.5, and 728.1 nm) line intensity ratios. The Hα (656.3 nm), He I (587.6 nm), C II (514.5 nm), and O I (777.2 nm) emissions were investigated over a wide range of operating conditions. The results showed that fuel and impurity emissions in the divertor region exhibit a strong dependence on magnetic topology and plasma conditions. The levels of Hα, He I, C II, and O I emissions are all reduced moving from the standard configuration to the high mirror configuration, and even further reduced for the high iota configuration, which is associated with decreasing connection length in these island divertor configurations. The H/He influx ratio shows that the plasma is a mixture of helium and hydrogen. The neutral and impurity influxes from the divertor target tend to increase with increasing divertor electron temperature.

Conceptual design of the island divertor coils on the J-TEXT tokamak

A set of new resonant magnetic perturbation (RMP) coils is designed for the J-TEXT tokamak to form the island divertor (ID) configuration [F. Karger, Phys Lett A, 61 (1977) 385]. This design is aiming at investigating the effect of ID on the peak heat load reduction in a tokamak. The RMP field generated by these new coils can produce magnetic island in the plasma boundary. The field line of this magnetic island is opened by divertor targets, so as to form the island divertor configuration. Therefore, the edge island should be a vacuum island, which can be described by an analytical mode with assuming linear superposition of the plasma equilibrium field and the magnetic perturbation in a vacuum approximation. Numerical calculations have been carried out to calculate several parameters for the island divertor configuration such as the perturbed magnetic field generated by the coils, its spectrum, the field line tracing and the Poincaré plot. Conceptual design is based on the m/n = 4/1 magnetic island, which can be generated by either 16 out-vessel saddle coils or 8 in-vessel S-type modular coils. Here, m and n are the poloidal and toroidal mode numbers respectively. The ID configuration can be then established by the boundary 4/1 island intersecting with proper divertor targets.

Investigation of 3D effects on heat fluxes in performance-optimized island divertor configurations at Wendelstein 7-X

The 3D effects on divertor heat loads have been investigated for performance-optimized island divertor configurations at Wendelstein 7-X with 3D modeling and IR camera measurements. A new high mirror configuration optimized for more stable island divertor operation due to reduced bootstrap currents and a more even heat load distribution between the main divertor targets has been investigated for the first time numerically and experimentally.Transport calculations with EMC3-EIRENE show a strong dependence of the heat flux distributions on the configurations and the details of the 3D island geometry. IR camera measurements confirm the predictions concerning the global heat load distributions for the standard configuration but show more even distributions between the main wetted divertor plates in the high mirror configurations. The local heat load profiles show offsets in their position of up to 5 cm to each other and averaged peak heat fluxes varying between 0.5 MWm−2 and 2.2 MWm−2 depending on the divertor module considered. These heat flux asymmetries complicate the local matching of profiles between experiment and 3D modeling.The 3D equilibrium of a high mirror high-performance scenario predicted by the HINT code has been investigated with EMC3-EIRENE for the first time to anticipate plasma response in higher performance scenarios. The island divertor is preserved for β=3% despite enhanced stochasticity. However, the islands are increased in size while the path lengths for parallel transport are reduced causing a substantial change in the divertor heat flux patterns.

Bootstrap current control studies in the Wendelstein 7-X stellarator using the free-plasma-boundary version of the SIESTA MHD equilibrium code

The recently developed free-plasma-boundary version of the SIESTA MHD equilibrium code (Hirshman et al 2011 Phys. Plasmas 18 062504; Peraza-Rodriguez et al 2017 Phys. Plasmas 24 082516) is used for the first time to study scenarios with considerable bootstrap currents for the Wendelstein 7-X (W7-X) stellarator. Bootstrap currents in the range of tens of kAs can lead to the formation of unwanted magnetic island chains or stochastic regions within the plasma and alter the boundary rotational transform due to the small shear in W7-X. The latter issue is of relevance since the island divertor operation of W7-X relies on a proper positioning of magnetic island chains at the plasma edge to control the particle and energy exhaust towards the divertor plates. Two scenarios are examined with the new free-plasma-boundary capabilities of SIESTA: a freely evolving bootstrap current one that illustrates the difficulties arising from the dislocation of the boundary islands, and a second one in which off-axis electron cyclotron current drive (ECCD) is applied to compensate the effects of the bootstrap current and keep the island divertor configuration intact. SIESTA finds that off-axis ECCD is indeed able to keep the location and phase of the edge magnetic island chain unchanged, but it may also lead to an undesired stochastization of parts of the confined plasma if the EC deposition radial profile becomes too narrow.

Hydrogen emissions from peripheral plasmas in local island divertor and helical divertor configurations

Superdense core plasmas with a highly peaked electron density profile have been obtained in reduced recycling discharges in the Large Helical Device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)]. The polarization separation spectroscopy technique has been applied to characterize Hα emissions in the peripheral region of superdense core plasmas. Fitting positions of the high intensity Hα line are located just outside of the last closed flux surface in the local island divertor configuration. In the helical divertor configuration, high intensity emissions are observed around the inner and outer X points and along the divertor legs. In both configurations the hydrogen atoms have an inward velocity along the lines of sight.

Divertor plasma and neutral particles behavior under the local island divertor configuration in the Large Helical Device

In the Large Helical Device (LHD), the local island divertor (LID) experiment has been conducted to investigate how to improve the core confinement with edge plasma control. An advantage of the LID configuration over the intrinsic helical divertor is the high pumping efficiency with the closed divertor module (neutralizer and pumping duct) and high speed pump-system. The particle evacuation efficiency, evacuated particle number/fueled particle number, has been estimated in experiment. This efficiency depends on the relative position of the closed divertor module (divertor head) to m/n=1/1 magnetic island’s outer separatrix corresponding to the divertor leg, and the efficiency is revealed to be 0.6–1. A new operational regime with high core density (>4×1020m−3) and a strongly peaked density profile has been discovered in the LID discharges. It is considered that a large effective pumping efficiency is necessary to enter this operational regime.

3D Divertor Transport Study of the Local Island Divertor Configuration in the Large Helical Device

The divertor/SOL transport of the Local Island Divertor (LID) configuration in the Large Helical Device (LHD) is studied. Since the LID configuration has an inherent three dimensional magnetic structure, the 3D edge transport code, EMC3-EIRENE, is applied in order to elucidate the 3D divertor transport physics. The main observations reported here are: 1. The effective temperature cooling due to the integrated perpendicular energy loss along the long connection length. 2. The significant plasma flow acceleration caused by the upstream source that is attributed to the geometrical significance of the LID configuration. 3. The effect of the flow acceleration on the impurity screening considering a parallel transport along the SOL. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Quest for a Divertor Effect in TJ-II

Neutral beam injection (NBI) heating in the TJ-II stellarator faces the particular challenge of unwanted particle sources, which for relatively low injected powers drive the plasma to collapse at relatively low values of line density. This effect is aggravated by an enhancement of particle confinement that occurs as density increases. At present, candidate magnetic topologies that make use of intrinsic islands at the plasma edge are being investigated experimentally. This concept uses magnetic configurations having a rotational transform with a rational value at the edge, i.e., an island divertor (ID). Recently, impurity injection experiments have been performed for ID and limiter configurations to investigate the divertor effect in TJ-II. Indeed, the enhanced screening of injected impurities, as well as the low intrinsic plasma contamination found in these ID plasmas, points to such magnetic configurations as being good candidates for the NBI experimental program in TJ-II.

Neoclassical effect on strike point patterns in local island divertor configuration of LHD

Monte Carlo simulation based on test particle representation is carried out in order to investigate the neoclassical effect on a strike point pattern of ions on the local island divertor (LID) head in the Large Helical Device configuration. According to collisionality, the particle orbits mainly contributing the particle flux from the core region to the LID head vary; in the present paper we call it the neoclassical effect on the edge transport phenomena. The pattern on the LID head is numerically observed by tracing the guiding centre orbits of the test particles under the effects of Coulomb collision and anomalous diffusion. We find that the strike point patterns on the LID head are varied according to collisionality. For the case of λmfp/Lc > 1, the pattern is characterized by the passing particle orbits along the field lines of the island separatrix, where λmfp is the mean free path and Lc ≈ 250 m is the connection length given as a length along a field line connecting the core region to the LID head. On the other hand, for the case of λmfp/Lc ≪ 1, the particles intensively strike the edge of the LID head, and the pattern is given by the trapped particle orbits. The trapped particle orbits mainly contribute the particle flux from the core region through the inside of the island, and finally the particles arrive at the edge of the LID head.

Three-dimensional Monte Carlo simulations of W7-X plasma transport: density control and particle balance in steady-state operations

This paper presents self-consistent three-dimensional (3D) plasma transport simulations in the boundary of stellarator W7-X obtained with the Monte Carlo code EMC3-EIRENE for three typical island divertor configurations. The chosen 3D grid consists of relatively simple nested finite toroidal surfaces defined on a toroidal field period and covering the whole edge topology, which includes closed surfaces, islands and ergodic regions. Local grid refinements account for the required high resolution in the divertor region. The distribution of plasma density and temperature in the divertor region, as well as the power deposition profiles on the divertor plates, are shown to strongly depend on the island geometry, i.e. on the position and size of the dominant island chain. Configurations with strike-point positions closer to the gap of the divertor chamber generally favour the neutral compression in the divertor chamber and hence the pumping efficiency. The ratio of pumping to recycling fluxes is found to be roughly independent of the separatrix density and is thus a figure of merit for the quality of the configuration and of the divertor system in terms of density control. Lower limits for the achievable separatrix density, which determine the particle exhaust capabilities in stationary conditions, are compared for the three W7-X configurations.

3D Edge Modeling and Island Divertor Physics

AbstractThe paper presents an overview on the state of the art of 3D fluid transport modeling in the boundaries of 3D toroidal confinement devices and on applications to island divertor physics. Typically, such edge configurations are characterized by the coexistence of closed magnetic surfaces, islands and open stochastic regions, e.g. in helical devices like W7‐AS, W7‐X, LHD and in tokamaks like TEXTOR‐DED. Two main approach branches falling within the current numeric catalogue of the 3D modeling are the finite volume andMonte Carlo methods. They differ essentially in the elementary treatment of the local transport. While in a finite volume method interpolation of the fluid fluxes through the interfaces by appropriate choice of a shape function is essential for the discretization process, the full fluid dynamics are, in a Monte Carlo approach, simulated by means of a local stochastic process, with the fluxes passing through cell boundary surfaces being a net result of the random process. In this paper, we present the numerics and strategies proposed in different models. Concerning the practical applications to a realistic 3D experiment, W7‐AS provides not only a practical fully 3D island divertor configuration but also sufficient experimental data for code validation. We present the main simulation results from the 3D edge Monte Carlo code EMC3/EIRENE and discuss the island divertor physics with respect to tokamak divertors. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carbon concentration in the Wendelstein 7-AS stellarator measured with the high-energy Li beam

The carbon concentration in the plasma of the stellarator experiment Wendelstein 7-AS is measured in the framework of the first island divertor campaign to examine divertor impurity control. Dedicated plasma density and electron cyclotron resonance heating power scans are performed and the influence of both the magnetic configuration and boronization of the vessel are investigated. A new diagnostic system is employed to measure C6+ densities via charge-exchange collisions with the neutral high-energy Li beam in the plasma edge and the core region. Carbon concentrations with respect to electron density are derived by using the density evaluation of the Li(2p) light profile. The results obtained during a period of approximately 2500 shots demonstrate that boronization reduces the carbon core concentration by a factor of three to a base level of 0.5–1%, which is not affected by further boronizations. The carbon concentration in island divertor configurations is found to be far below those obtained in a limiter configuration, which is attributed to a more than a factor two better impurity screening. The analysis of the carbon influx by spectroscopic measurements of C1+ radiation suggests that the carbon core concentration in discharges of different densities and heating power is adequately described using the electron density at the last closed flux surface normalized to the line-averaged electron density as a scaling parameter. This simple global approach underlines the important role of the balance of impurity sources and impurity dilution. The effect of changes in impurity confinement on the measured carbon concentrations is also discussed.

Electric fields and currents in an island divertor configuration

One candidate for an optimized particle and energy exhaust concept in stellarators is the island divertor. Due to the special topology of such a design, its physics characteristics can be quite different from standard tokamak divertors or Limited Stellarator Plasmas. We have extended the capabilities of the two-dimensional B2-solps5.0 multifluid Scrape-Off layer plasma simulation code to handle diverted island geometries, including all relevant physics, in particular drifts and currents. We focus here on the structure of the electric potential and electric field in this island configuration. We show the establishment of potential and current flow patterns favoring density accumulation inside the island. This electric potential distribution is non-trivial and quite different from the φ=3 kT ansatz used in previous studies.

Island divertor in a helical-axis heliotron device (Heliotron J)

A new configuration with ι/2 π ∼ 0.5 is proposed as an island divertor configuration in Heliotron J, a small size l=1/ M=4 helical-axis heliotron device. In this case, both the m=8/ n=4 edge islands and the last closed flux surface can keep simultaneously the moderate size. As a first step of an island divertor design with this configuration, edge field characteristics are numerically investigated assuming a target plate at r target=0.33 m (5 cm inside from the chamber wall). It is shown that the divertor fields cross the target at four discrete regions per a field period. The connection length from the edge to the target plate is in the range of 50–400 m. The total value of the `wetted area' for the whole torus is about 600 cm 2. The incidence angle of the diverted field lines to the target plate is less than 10°.

Island Divertors for Helical Devices

Basic features of the island divertor concepts for low-shear advanced stellarators like W7-AS and W7-X (intrinsic island divertors) and for high-shear heliotrons like CHS and LHD (local island divertors, LID) and first results for both concepts are shortly reviewed. The diverting fields of island divertors are either intrinsic (low-shear case) or externally imposed (high- shear case). The associated field perturbations are very small compared to tokamak poloidal field divertors, which explains the high flexibility of island divertor configurations, but sufficiently large to generate divertor-viable islands. Although the physics of island divertors is expected to be similar to that of tokamak poloidal field divertors, leading geometrical parameters significantly differ from those of comparable-size tokamaks. Furthermore, strong three-dimensional effects arise from toroidally discontinuous target plates. For both the intrinsic and local island divertor configurations, the island structure and the plasma divers on have been verified experimentally. For W7-AS, stable high recycling conditions could be demonstrated in a stellarator for a simple island divertor geometry. In CHS with the LID field switched on, a strong increase of the pumping efficiency was measured, resulting in a 50% reduction of the core density for the same gas-puff rate as in the non-LID case. 3D numerical transport studies predict, for low densities, significant momentum losses associated with the island divertor geometry. For high densities, low island temperatures lead to detachment of the ionisation front from the targets and to a strong toroidal modulation of the plasma parameter due to the discontinuous targets.