Soft x-ray tomography using L1 regularization for MHD modes with limited sight lines in JT-60SA.

It is the first time that fast negative-ion loss signals are observed by Fast-ion Loss Detector (FILD) and analyzed in EAST neutral beam injection experiment. Velocity-space distributions of lost fast negative and positive ions reconstructed by the FILDSIM code are basically consistent. Orbit analysis backtracked by the ASCOT code shows that the fast negative ions are generated at the low-field side (LFS) scrape-off layer, whereas the fast positive ions are generated near the last closed flux surface (LCFS) at both the high- and LFSs. The existence of fast negative-ion loss indirectly suggests a very high neutral particle density near both the main limiter and antenna limiter. The differing birth locations of fast positive and negative ions result in distinct loss behaviors during ELM H-mode and n = 3 rotating RMP experiments. The asymmetry of neutral particle density in tokamak suggest that previous studies may have underestimated the fraction of charge exchange (CX) losses due to the assumption of a uniform neutral particle density outside the LCFS. Therefore, simulations evaluating the CX loss fraction in the future should account for a three-dimensional distribution of neutral particle density, including local gas puffing and the neutral particle density around various limiters.

Effects of a radial magnetic perturbation (MP) on electron cyclotron (EC) driven fast electrons are investigated in the Low Aspect ratio Torus Experiment (LATE) device. When a weak MP is applied to plasmas maintained solely by EC heating and current drive, plasma current and electron density increase, whereas energetic trapped electrons located outside the last closed flux surface (LCFS) on the low field side (LFS) decrease. The effect of MPs on energetic trapped electrons has an important role in improving EC-driven plasma discharges.

In the Large Helical Device (LHD), direct oblique launching of the fundamental extraordinary (X-) mode from the high magnetic field side (HFS) is available without installation of any additional launching equipment on the inner side wall of the torus. In the experiment, power absorption was observed in two separated regions by the X-mode launching. The central electron density was about 8% of the cutoff density. The result of numerical analysis with the ray-tracing calculation suggests that most power of the launched X-mode is damped out as the X-mode in the fundamental electron cyclotron resonance (ECR) layer before it reaches the upper hybrid resonance (UHR) layer where the electron Bernstein wave (EBW) occurs which is excited via the slow X (SX-) B mode conversion process. Only about 0.2% of the launched power is mode converted to the EBW and is then absorbed at a maximum. One of the two separated power absorption regions observed in the experiment agrees well with the power absorption region of the X-mode suggested by the ray tracing. The other one agrees well with that of the O-mode despite the setting of the X-mode launching. Supposedly mixed waves of the X- and the O-mode might be launched in the experiment. We assumed that the incident transverse electromagnetic waves in vacuum couple with the electromagnetic modes in the plasma at the last closed flux surface (LCFS). However, the coupling point was supposedly located outside the LCFS.For the few rays that can reach the UHR layer we have recognized that the parallel component of the refractive index N∥ becomes close to zero and power absorption as the X-mode weakens when the rays pass through the fundamental ECR layer. A numerical investigation assuming a higher central electron density, that is 23% of the cutoff density, suggests a scenario of effective EBW excitation. The ray that passes through the centre of the focused Gaussian beam launched from the HFS so that N∥ becomes close to zero near the ECR layer, Z can reach the UHR layer without being damped out and excite EBW. About 71% of power of the launched X-mode is mode converted to the EBW and absorbed in the Doppler-shifted ECR layer.Observation of the parametric decay waves suggests that the decay wave was excited in the ‘exterior’ UHR layer which is located outside the LCFS before the launched X-mode reaches the HFS. The combination of tunnelling, reflection and mode-coupling processes, the so-called ‘Budden's problem’, is suggested to occur in the evanescent region between the ‘exterior’ right handed cyclotron cutoff and the UHR layer outside the LCFS. The tunnelling rate should be considered for estimating the power that can penetrate inside the LCFS from the HFS, in particular in the higher density regime where the excitation of the EBW is expected.

This paper presents a novel ‘prediction-correction method’ for treating molecule-assisted recombination (MAR) in the three-dimensional (3D) boundary plasma transport code—EMC3-Eirene. In this approach, certain MAR products are first isolated and removed from the particle trajectories in Eirene and recorded as predictions, which are then implicitly corrected/reprocessed in EMC3 and subsequently fed back into Eirene as an ‘external’ source to compensate for the removed particles. Compared to the conventional scheme used in boundary plasma modelling, this new method exhibits enhanced numerical stability. It should be broadly applicable to other 2D fluid-kinetic edge plasma transport calculations. Using a typical detached plasma from W7-X as an example, we apply the latest version of the EMC3-Eirene code to perform, for the first time, a self-consistent analysis of the role of volume recombination processes, including MAR and electron–ion recombination (EIR), in a 3D divertor, namely the island divertor. Intrinsic carbon is assumed to be the only impurity species, and the radiation fraction f rad is used as a control parameter. The simulation demonstrates that both EIR and MAR increase with f rad , with the total volume recombination rate reaching approximately 30% of the total neutral source at f rad = 0.9. The MAR contribution is typically around a factor of three of that of the EIR when the radiation is located in the edge magnetic islands outside the last closed flux surface. There are no noteworthy effects of volume recombination on detachment performance regarding the power load on the target, the neutral pressure in the divertor chamber, or the distribution of impurity radiation. Nevertheless, volume recombination significantly changes the relative population of atoms and molecules in front of the targets, which may be generally important for boundary plasma spectroscopy in fusion devices.

We report on the development of the X-ray CCD for the soft X-ray imager (SXI) onboard ASTRO-H. SXI CCDs are P-channel, back-illuminated type manufactured by Hamamatsu Photonics K. K. Experiments with prototype CCD for the SXI shows the device has a depletion layer as thick as 200μm, high efficiency for hard X-rays. By irradiating soft X-rays to the prototype CCD for the SXI. At the same time, we found a significant low energy tail in the soft X-ray response of the SXI prototype CCD. We thus made several small size CCD chips with different treatment in processing the surface layers. CCDs with one of the surface layers treatment show a low energy tail of which intensity is one order of magnitude smaller than that of the original SXI prototype CCD for 0.5keV X-ray incidence. The same treatment will be applied to the flight model CCDs of the SXI. We also performed experiments to inject charge with the SXI prototype CCD, which is needed to mitigate the radiation damage in the orbit. We investigated the operation conditions of the charge injection. Using the potential equilibration method, charges are injected in each column homogeneously, though the amount of the charge must be larger than 20ke^-.

Solar wind charge exchange produces emissions in the soft X-ray energy range which can enable the study of near-Earth space regions such as the magnetopause, the magnetosheath and the polar cusps by remote sensing techniques. The Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) and Lunar Environment heliospheric X-ray Imager (LEXI) missions aim to obtain soft X-ray images of near-Earth space thanks to their Soft X-ray Imager (SXI) instruments. While earlier modeling works have already simulated soft X-ray images as might be obtained by SMILE SXI during its mission, the numerical models used so far are all based on the magnetohydrodynamics description of the space plasma. To investigate the possible signatures of ion-kinetic-scale processes in soft X-ray images, we use for the first time a global hybrid-Vlasov simulation of the geospace from the Vlasiator model. The simulation is driven by fast and tenuous solar wind conditions and purely southward interplanetary magnetic field. We first produce global X-ray images of the dayside near-Earth space by placing a virtual imaging satellite at two different locations, providing meridional and equatorial views. We then analyze regional features present in the images and show that they correspond to signatures in soft X-ray emissions of mirror-mode wave structures in the magnetosheath and flux transfer events (FTEs) at the magnetopause. Our results suggest that, although the time scales associated with the motion of those transient phenomena will likely be significantly smaller than the integration time of the SMILE and LEXI imagers, mirror-mode structures and FTEs can cumulatively produce detectable signatures in the soft X-ray images. For instance, a local increase by 30% in the proton density at the dayside magnetopause resulting from the transit of multiple FTEs leads to a 12% enhancement in the line-of-sight- and time-integrated soft X-ray emissivity originating from this region. Likewise, a proton density increase by 14% in the magnetosheath associated with mirror-mode structures can result in an enhancement in the soft X-ray signal by 4%. These are likely conservative estimates, given that the solar wind conditions used in the Vlasiator run can be expected to generate weaker soft X-ray emissions than the more common denser solar wind. These results will contribute to the preparatory work for the SMILE and LEXI missions by providing the community with quantitative estimates of the effects of small-scale, transient phenomena occurring on the dayside.

A system for studying the spatiotemporal dynamics of fluctuations in the boundary of the W7-X plasma using the "Gas-Puff Imaging" (GPI) technique has been designed, constructed, installed, and operated. This GPI system addresses a number of challenges specific to long-pulse superconducting devices, such as W7-X, including the long distance between the plasma and the vacuum vessel wall, the long distance between the plasma and diagnostic ports, the range of last closed flux surface (LCFS) locations for different magnetic configurations in W7-X, and management of heat loads on the system's plasma-facing components. The system features a pair of "converging-diverging" nozzles for partially collimating the gas puffed locally ≈135mm radially outboard of the plasma boundary, a pop-up turning mirror for viewing the gas puff emission from the side (which also acts as a shutter for the re-entrant vacuum window), and a high-throughput optical system that collects visible emission resulting from the interaction between the puffed gas and the plasma and directs it along a water-cooled re-entrant tube directly onto the 8 × 16 pixel detector array of the fast camera. The DEGAS 2 neutral code was used to simulate the Hα (656nm) and HeI (587nm) line emission expected from well-characterized gas-puffs of H2 and He and excited within typical edge plasma profiles in W7-X, thereby predicting line brightnesses used to reduce the risks associated with system sensitivity and placement of the field of view. Operation of GPI on W7-X shows excellent signal-to-noise ratios (>100 at 2 Mframes/s) over the field of view for minimally perturbing gas puffs. The GPI system provides detailed measurements of the two-dimensional (radial and poloidal) dynamics of plasma fluctuations in the W7-X edge and scrape-off layer and in and around the magnetic islands outside the LCFS that make up the island divertor configuration employed on W7-X.

Measurements of electrostatic fluctuations on the edge of the Advanced Toroidal Facility (ATF) torsatron are used to study the role of the edge turbulence in the particle transport in this current-free magnetic configuration. Spatial profiles of the plasma electron density n{sub e}, temperature T{sub e}, and fluctuations in density ({tilde n}{sub e}) and in the plasma floating potential ({tilde {phi}}{sub f}) are measured at the edge in electron cyclotron heated plasmas using a Langmuir probe array. At the last closed flux surface (LCFS), r/a {approx} 1, T{sub e} {approx} 20--40 eV, and n{sub e} {approx} 10{sup 12} cm{sup {minus}3} for a line-averaged electron density {bar n}{sub e} = (3--6) {times} 10{sup 12} cm{sup {minus}3}. The relative fluctuation levels decrease as the probe is moved into the core plasma. For T{sub e} > 10 eV, {tilde n}{sub e}/n{sub e} {approx} 5%, and e{tilde {phi}}{sub f}/T{sub e} {approx} 2{tilde n}{sub e}/n{sub e} at r/a = 0.95. The measured fluctuations spectra are broadband (40--300 kHz) with {bar k}{rho}{sub s} {approx} 0.05--0.1, where {bar k} is the average wave number of the fluctuations and {rho}{sub s} is the ion Larmor radius at the sound speed. Near the LCFS, the density fluctuations can be approximated by {tilde n}{sub e}/n{sub e} {approx} 0.4/{bar k}L{sub n}, where L{sub n} is the gradient scale length of n{sub e}. The propagation direction of the fluctuations reverses to the electron diamagnetic direction at r/a < 1. The phase velocity of the fluctuations and the electron drift velocity are comparable. The particles flux estimated from the fluctuations is consistent with fluxes obtained from the particle balance using the H{sub {alpha}} spectroscopic measurements. Many of the features seen in the ATF edge fluctuations resemble those of ohmically heated plasmas in the Texas Experimental Tokamak.

Reactor similarity for plasma–material interactions in scaled-down tokamaks as the basis for the Vulcan conceptual design

The Soft X-ray Imager (SXI) is a compact X-ray telescope with a wide $15.5^{\circ } \times 26.5^{\circ }$ 15.5 ∘ × 26.5 ∘ field-of-view (FOV) and is part of the scientific payload of the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission. SMILE is a joint science mission between the European Space Agency (ESA) and the Chinese Academy of Sciences (CAS) and is due for launch in 2026. SXI is capable of viewing large portions of Earth’s magnetosphere from the vantage point of the SMILE orbit. SXI is designed to detect the soft X-rays from the Solar Wind Charge eXchange (SWCX) process produced when heavy ions of solar wind origin interact with neutral particles in Earth’s exosphere. SWCX provides a mechanism for boundary detection within the magnetosphere, such as the position of Earth’s magnetopause, because the solar wind heavy ions essentially do not enter regions of closed magnetic field lines. The sensitivity of the SXI is such that it can potentially track movements of the magnetopause on timescales of a few minutes and the orbit of SMILE will enable such movements to be tracked for segments lasting many hours. The SXI has a large grasp (FOV times effective area) and a CCD-based detector system with an energy resolution comparable to contemporary X-ray astrophysics missions. SXI will, therefore, acquire high precision spectra capable of discriminating the composition of many of the solar wind heavy ions within the regions of interest. SXI is led by the University of Leicester (UL) in the United Kingdom (UK) with collaborating organisations on hardware, software and science support within the UK, Europe, China and the United States.

Effect of N2, Ne and Ar seeding on Alcator C-Mod H-mode confinement

Since high density operation is advantageous for building an efficient fusion reactor, understanding the density limit in tokamaks has been seen as one of the most important issues. This paper reports a series of measurements around the last-closed flux surface (LCFS) in L-mode plasmas by using a thermal helium beam diagnostic. Fluctuation analysis has been employed to characterize the poloidal flow and the turbulence structure. A reversal of the poloidal flow in the scrape-off layer and concomitant cooling of the outer divertor plasma are observed as the density is raised. While, in the confined region, the change in the density barely affects the poloidal flow, a higher density shifts the fluctuation power spectral densities toward lower frequencies and wave numbers. The eddy tilting of this region is consistent with what is expected from the magnetic shear effect. A radially coherent low frequency mode appears in the case of the highest density investigated in this study (n¯e/ne,GW = 0.51), and higher frequencies near the LCFS are modulated by this mode.

Development of an ICRH antenna system at W7-X for plasma heating and wall conditioning

Soft x-ray (SXR) cameras in a tokamak are limited spatially by ports of the vacuum vessel, and SXR tomography (SXT) technology is developed for reconstructing a two-dimensional SXR profile. However, traditional SXT is time-consuming and has difficulty achieving abundant and quick reconstructions for a tokamak. Based on experimental SXR data and Fourier–Bessel SXT codes at the EAST tokamak, three typical neural networks are built and trained. All the trained neural networks complete reconstruction within several milliseconds on a personal computer and succeed in constraining the SXR profile to match most of the data. In particular, the best-performing fully convolutional neural network provides SXR reconstruction images on the 2D evolution of a sawtooth, and shows its generalization. In the future, it is possible to provide an outstanding deep learning substitute to give abundant and quick SXT images instead of traditional SXT, after training for a few days.

X-ray Astronomy Recovery Mission (XARM) scheduled to be launched in early 2020’s carries two soft X-ray telescopes. One is Resolve consisting of a soft X-ray mirror and a micro calorimeter array, and the other is Soft X-ray Imaging Telescope (Xtend), a combination of an X-ray mirror assembly (XMA) and an X-ray CCD camera (SXI). Xtend covers a field of view (FOV) of 38′ × 38′ , much larger than that of Resolve (3′ × 3 ′ ) with moderate energy resolution in the energy band from 0.4 keV to 13 keV, which is similar to that of Resolve (from 0.3 keV to 12 keV). Simultaneous observations of both telescopes provide complimentary data of X-ray sources in their FOV. In particular, monitoring X-ray sources outside the Resolve FOV but inside the Xtend FOV is important to enhance the reliability of super high resolution spectra obtained with Resolve. Xtend is also expected to be one of the best instruments for low surface brightness X-ray emissions with its low non X-ray background level, which is comparable to that of Suzaku XIS. The design of Xtend is almost identical to those of Soft X-ray Telescope (SXT) and Soft X-ray Imager (SXI) both on board the Hitomi satellite. However, several mandatory updates are included. Updates for the CCD chips are verified with experiment using test CCD chips before finalizing the design of the flight model CCD. Fabrication of the foils for XMA has started, and flight model production of the SXI is almost ready.