Magnetic Flux Expansion Research Articles

Snowflake divertor configuration studies in National Spherical Torus Experiment

Experimental results from NSTX indicate that the snowflake divertor (D. Ryutov, Phys. Plasmas 14, 064502 (2007)) may be a viable solution for outstanding tokamak plasma-material interface issues. Steady-state handling of divertor heat flux and divertor plate erosion remains to be critical issues for ITER and future concept devices based on conventional and spherical tokamak geometry with high power density divertors. Experiments conducted in 4–6 MW NBI-heated H-mode plasmas in NSTX demonstrated that the snowflake divertor is compatible with high-confinement core plasma operation, while being very effective in steady-state divertor heat flux mitigation and impurity reduction. A steady-state snowflake divertor was obtained in recent NSTX experiments for up to 600 ms using three divertor magnetic coils. The high magnetic flux expansion region of the scrape-off layer (SOL) spanning up to 50% of the SOL width λq was partially detached in the snowflake divertor. In the detached zone, the heat flux profile flattened and decreased to 0.5–1 MW/m2 (from 4–7 MW/m2 in the standard divertor) indicative of radiative heating. An up to 50% increase in divertor, Prad in the snowflake divertor was accompanied by broadening of the intrinsic C III and C IV radiation zones, and a nearly order of magnitude increase in divertor high-n Balmer line emission indicative of volumetric recombination onset. Magnetic reconstructions showed that the x-point connection length, divertor plasma-wetted area and divertor volume, all critical parameters for geometric reduction of deposited heat flux, and increased volumetric divertor losses were significantly increased in the snowflake divertor, as expected from theory.

Parallel Heat Flux from Low to High Parallel Temperature along a Magnetic Field Line

In a long mean-free-path plasma where temperature anisotropy can be sustained, the parallel heat flux has two components with one associated with the parallel thermal energy and the other with the perpendicular thermal energy. In a kinetic simulation with magnetic flux expansion toward an absorbing boundary, the parallel heat flux of the parallel thermal energy is found to flow from a low to high parallel temperature region. This unusual behavior is understood with the help of an analytical calculation of the drift-kinetic model using the same upstream source in the simulation.

Dependence of divertor heat flux widths on heating power, flux expansion, and plasma current in the NSTX

We report the dependence of the lower divertor surface heat flux profiles, measured from infrared thermography and mapped magnetically to the mid-plane on loss power into the scrape-off layer (PLOSS), plasma current (Ip), and magnetic flux expansion (fexp), as well as initial results with lithium wall conditioning in NSTX. Here we extend previous studies [R. Maingi et al., J. Nucl. Mater. 363–365 (2007) 196–200] to higher triangularity ∼0.7 and higher Ip⩽1.2MA. First we note that the mid-plane heat flux width mapped to the mid-plane, λqmid, is largely independent of PLOSS for PLOSS⩾4MW. λqmid is also found to be relatively independent of fexp; peak heat flux is strongly reduced as fexp is increased, as expected. Finally, λqmid is shown to strongly contract with increasing Ip such that λqmid∝Ip-1.6 with a peak divertor heat flux of qdiv, peak∼15MW/m2 when Ip=1.2MA and PLOSS∼6MW. These relationships are then used to predict the divertor heat flux for the planned NSTX-Upgrade, with heating power between 10 and 15MW, Bt=1.0T and Ip=2.0MA for 5s.

Divertor heat flux mitigation in high-performance H-mode discharges in the National Spherical Torus Experiment

Experiments conducted in high-performance 1.0 and 1.2 MA 6 MW NBI-heated H-mode discharges with a high magnetic flux expansion radiative divertor in NSTX demonstrate that significant divertor peak heat flux reduction and access to detachment may be facilitated naturally in a highly shaped spherical torus (ST) configuration. Improved plasma performance with high βt = 15–25%, a high bootstrap current fraction fBS = 45–50%, longer plasma pulses and an H-mode regime with smaller ELMs has been achieved in the strongly shaped lower single null configuration with elongation κ = 2.2–2.4 and triangularity δ = 0.7–0.8. Divertor peak heat fluxes were reduced from 6–12 to 0.5–2 MW m−2 in ELMy H-mode discharges using the inherently high magnetic flux expansion fm = 15–25 and the partial detachment of the outer strike point at several D2 injection rates. A good core confinement and pedestal characteristics were maintained, while the core carbon concentration and the associated Zeff were reduced. The partially detached divertor regime was characterized by an increase in divertor radiated power, a reduction in ion flux to the plate and a large neutral compression ratio. Spectroscopic measurements indicated the formation of a high-density, low-temperature region adjacent to the outer strike point, where substantial increases in the volume recombination rate and C II, C III emission rates were measured.

Divertor heat flux mitigation in the National Spherical Torus Experiment

Steady-state handling of divertor heat flux is a critical issue for both ITER and spherical torus-based devices with compact high power density divertors. Significant reduction of heat flux to the divertor plate has been achieved simultaneously with favorable core and pedestal confinement and stability properties in a highly shaped lower single null configuration in the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 2000] using high magnetic flux expansion at the divertor strike point and the radiative divertor technique. A partial detachment of the outer strike point was achieved with divertor deuterium injection leading to peak flux reduction from 4–6MWm−2to0.5–2MWm−2 in small-ELM 0.8–1.0MA, 4–6MW neutral beam injection-heated H-mode discharges. A self-consistent picture of the outer strike point partial detachment was evident from divertor heat flux profiles and recombination, particle flux and neutral pressure measurements. Analytic scrape-off layer parallel transport models were used for interpretation of NSTX detachment experiments. The modeling showed that the observed peak heat flux reduction and detachment are possible with high radiated power and momentum loss fractions, achievable with divertor gas injection, and nearly impossible to achieve with main electron density, divertor neutral density or recombination increases alone.

Observations of simultaneous coronal loop shrinkage and expansion during the decay phase of a solar flare

We report what we believe are the first direct and unambiguous observations of simultaneous coronal magnetic flux loop shrinkage and expansion during the decay phase of a solar flare. The retracting and expanding loops were observed nearly face-on (i.e., with the loop major axis approximately orthogonal to the line of sight) in emission in imaging data from the Yohkoh Soft X-ray Telescope (SXT). The retracting loop is observed to shrink with a speed of 118 ± 66 km s -1 . The faint outward moving loop-like feature occurred ~200´´ above the shrinking loop during the time of the shrinking loop. We estimate the speed of the outward moving loop was ~129 ± 74 km s -1 . We interpret the shrinking loop and simultaneous outward moving loop as direct evidence for reconnected magnetic field lines during a flare.

Boundary plasma and divertor phenomena in MAST

The boundary and divertor plasma of a large spherical tokamak (ST), with significant auxilliary heating, is explored in detail for the first time. The extreme geometry of the ST is found to play a key role, giving rise to strong asymmetries in the transport of heat and particle fluxes (including those during ELMs) towards the outer divertor targets. Preliminary modelling reveals the very significant contribution of magnetic flux expansion to the particle flux balance of the ST scrape-off layer. Divertor detachment at the inboard targets is observed in L-mode at moderate core densities for plasmas with up to 1 MW of auxilliary heating.

Latitudinal velocity structures up to the solar poles estimated from interplanetary scintillation tomography analysis

The Ulysses spacecraft observed high‐speed wind at high latitudes up to 80° and found that the high‐speed solar wind increased in velocity gradually with latitude and that the velocity had asymmetry between Northern and Southern Hemispheres. We have investigated the velocity increase up to the polar regions for the Carrington rotations of 1908–1915 in the year 1996. For this purpose we have made tomographic analyses of the latitudinal structure of the solar wind speed using interplanetary scintillation data obtained at heliocentric distances of 0.1–0.9 AU and latitudes up to 90°. The tomographic analysis method was modified from its previous version [Kojima et al., 1998] so that it could obtain more reliable solutions with better sensitivity in the polar region than the previous method. The results from the observations in 1996 showed that the velocity increased with latitude and had the N–S asymmetry as observed by Ulysses. These features persisted during the period analyzed. Since the asymmetry was found in rather short period observations of several Carrington rotations and at distances within 0.9 AU, it is caused neither by temporal evolution of the solar wind structures nor by interactions in the solar wind in interplanetary space. These global latitudinal velocity structures agree qualitatively with the magnetic flux expansion factor.

Synoptic Maps of Solar Wind: Comparison of IPS Data and the NOAA/SEC Version of the Wang and Sheeley Model

Since the 1970s, the Solar-Terrestrial Environment Laboratory, Japan, has been publishing synoptic maps of solar wind velocity prepared using the technique of interplanetary scintillation. These maps, known as V-maps, are useful to study the global distribution of solar wind in the heliosphere. As the Earth-orbiting satellites are unable to probe regions outside the ecliptic, it is important to exploit the scope of interplanetary scintillation to study the solar wind properties at these regions and their relation with coronal features. It has been shown by Wang and Sheeley that there exists an inverse correlation between rate of magnetic flux expansion and the solar wind velocity. The NOAA/Space Environment Center daily updated version of the Wang and Sheeley model has been used to produce synoptic maps of solar wind velocity and magnetic field polarity for individual Carrington rotations. The predictions of the model at 1 AU have been found to be in good agreement with the observed values of the same. The present work is a comparison of the synoptic maps on the source surface using the interplanetary scintillation measurements from Japan and the NOAA/SEC version of the Wang and Sheeley model. The two results agree near the equatorial regions and the slow solar wind locations are consistent most of the times. However, at higher latitudes within ±60°, the wind velocities differ considerably. In the Wang and Sheeley model the highest speed obtained is ∼ 600 km s−1 whereas in the IPS results velocities as high as 800 km s−1 have been detected. The paper discusses the possible causes for this discrepancy and suggestion to improve the agreement between the two results.

Enhanced ZENK ( zif-268) expression after passive avoidance task does not correlate with memory formation in neonatal chicks striata

WEST is a full metallic tokamak with an extensive set of diagnostics for heat load measurements. In this paper, heat loads on the lower divertor of WEST are investigated using two independent methods. A first method relies on the thermal inversion of temperature measurements from arrays of thermal sensors embedded a few millimeters below the surface, while the second consists in the inversion of black body surface temperatures measured by infra-red (IR) thermography. The challenge of IR based temperature measurements in the full metal environment of WEST is addressed through a simplified model, allowing to correct for global reflections and low surface emissivities of tungsten surfaces. A large database (>100 L-mode discharges) is investigated. It is found that the energy absorbed by an outer divertor tile during a plasma discharge is closely estimated by the two diagnostics, over a large set of experimental conditions. A similar match is also found for the peak heat flux value on the outer target. The toroidal modulation of target heat loads by magnetic ripple is found to be consistent with the geometrical projection of a parallel heat flux component. Additionally, the heat flux channel width at the target is found to scale linearly with the magnetic flux expansion as expected. These observations give confidence in the robustness of the data from both diagnostics, and confirm the simple geometrical rules at use in the description of heat flux deposition on divertor targets. However, it is shown that the heat flux channel width estimated from infra-red thermography is about three times lower than the width estimated from embedded measurements, which is still under investigation.

Acute and chronic effects of ethanol on the activation of HPA axis

WEST is a full metallic tokamak with an extensive set of diagnostics for heat load measurements. In this paper, heat loads on the lower divertor of WEST are investigated using two independent methods. A first method relies on the thermal inversion of temperature measurements from arrays of thermal sensors embedded a few millimeters below the surface, while the second consists in the inversion of black body surface temperatures measured by infra-red (IR) thermography. The challenge of IR based temperature measurements in the full metal environment of WEST is addressed through a simplified model, allowing to correct for global reflections and low surface emissivities of tungsten surfaces. A large database (>100 L-mode discharges) is investigated. It is found that the energy absorbed by an outer divertor tile during a plasma discharge is closely estimated by the two diagnostics, over a large set of experimental conditions. A similar match is also found for the peak heat flux value on the outer target. The toroidal modulation of target heat loads by magnetic ripple is found to be consistent with the geometrical projection of a parallel heat flux component. Additionally, the heat flux channel width at the target is found to scale linearly with the magnetic flux expansion as expected. These observations give confidence in the robustness of the data from both diagnostics, and confirm the simple geometrical rules at use in the description of heat flux deposition on divertor targets. However, it is shown that the heat flux channel width estimated from infra-red thermography is about three times lower than the width estimated from embedded measurements, which is still under investigation.

Volumetric heating in coronal streamers

The addition of a volumetric heat source to a coronal streamer model produces distinct, important changes in the model. Originally, such heating was added to meet the observational requirement for a thin current sheet above streamers. Here we report additional consequences of a volumetric heat source, together with the effects of redistribution of heat through thermal conduction. Specifically, we address the question of whether a heat source will allow a truly steady state to be achieved in the presence of thermal conduction, something known to be impossible for an adiabatic gas. The heat source causes a slow, continuing expansion and stripping of magnetic flux from the top of a streamer until the streamer essentially evaporates and the field is fully open to the interplanetary medium after an interval which depends on the magnitude of the source but typically varies from weeks to months. We find that thermal conduction does not quench the evaporation. We also find that the heat source, in the absence of other processes, must depend on the magnetic field geometry to simulate both the thinness of current sheets above streamers and the low density/high flow speed in coronal holes. Finally, we find that the expansion is not necessarily continuous.

Engineering design of the TITAN-II divertor

The design of the toroidal-field divertor for the TITAN-II reversed-field-pinch reactor is described. Strong radiation from the core and edge plasma spreads the heat load over the first wall and divertor areas of the high-power-density reactor, and careful shaping of the divertor target plate restricts the maximum heat flux to ~7.5 MW/m 2 . A further feature leading to manageable heat fluxes is the “open” configuration for the divertor, in which the target plate is located close to the null point to take advantage of the expansion of magnetic flux. The divertor target is constructed from a single material, a tungsten-rhenium alloy, for both the armor and coolant tubes, to avoid stress concentrations which arise at the interface between different materials. This alloy was chosen for its high ductility and good thermal and mechanical properties, which are retained at high temperatures, and for its excellent resistance to erosion by sputtering in the anticipated divertor plasma conditions. Detailed finite-element modeling of the divertor target indicates a peak equivalent thermal stress in the armor of ~500 MPa. Fabrication of the divertor plate is based on brazing the bank of coolant tubes to the armor, all components being manufactured using powder metallurgy techniques. The coolant for the divertor plate ia an aqueous LiNO 3 solution, with a lithium atomic percentage of 6.4%, as used throughout the fusion power core. Consideration of the thermal and physical properties of this solution allows coolant conditions to be chosen such that an adequate safety margin for critical heat flux is provided.

Patterns of active region magnetic field development

We discuss some characteristics of the appearance and development of magnetic fields within active regions as well as the large-scale ordering of activity into complexes of activity. It is not possible to separate a study of the evolution of active regions from a study of the model of the activity cycle. Many of the results obtained in the last few years concerning the development of active regions and large-scale activity have not been easily explained within any of the solar activity models. A chronological scheme of the development of a ‘typical’ C- or D-type active region is presented. We point out that the appearance of magnetic flux at the solar surface seems always to be a relatively rapid event, occurring during the course of a day or so. If the region does not receive more magnetic flux to make it a large region or if there is not a resurgence of activity later in its lifetime, the rest of the development is a gradual expansion and mixing of magnetic flux with the surrounding background field pattern.