Fast Losses Research Articles

TAE-induced fast ion losses and transport at the Globus-M/M2 spherical tokamaks

A multi-diagnostic study of the fast ion losses and redistribution during toroidal Alfvén eigenmodes at the Globus-M and Globus-M2 spherical tokamaks was performed. Mode amplitude and frequency evolution are discussed. Local energy-resolved spatial transport was examined using an active neutral particle analyzer. Losses and transport dependences on the mode amplitude were obtained. The change in the ion transport with the increase in plasma current and toroidal magnetic field is reported. The experimental data are compared with the modeling results.

Investigation of energetic ion losses induced by long-lived saturated internal mode with energetic particle diagnostics in the HL-2A tokamak

Several sets of energetic particle diagnostics, including a set of neutron flux monitoring systems, a solid-state neutral particle analyzer and a fast ion loss probe (FILP), have been used to investigate the energetic ion losses induced by the long-lived saturated internal mode (LLM) in the HL-2A tokamak. Clear experimental evidence for different levels of energetic ion losses induced by LLM, sawtooth and minor disruption has been observed. A numerical calculation for the evolution of neutron emissions was carried out with the FBURN code, and it shows that the neutron emission drop rate linearly increases with the LLM amplitude and no threshold perturbation amplitude exists, illustrating that the loss mechanism for LLM induced energetic ion loss is dominantly convective. In addition, measurement results of the FILP demonstrate that LLM tends to expel energetic ions with relatively low energy (keV) and high pitch angle (), and can suppress the prompt loss of energetic ions with high energy and low pitch angle to a certain degree. Furthermore, the physical process for LLM induced energetic ion loss can be explained by orbit calculations, which show that LLM induced lost energetic ions will transport from center to peripheral region first, and then get lost out of plasma. The experimental observations are successfully reproduced by calculations using the ORBIT code combined with both the NUBEAM code and the MARS-K code. The paper clearly describes the whole physical process of LLM induced energetic ion loss for the first time in the HL-2A tokamak.

Optimization of the operational domain for ICRH scenarios in WEST from statistical analysis

In the WEST tokamak, the Ion Cyclotron Resonance Heating (ICRH) system plays a substantial role in increasing the plasma temperature. However, its efficiency can be lowered by two main phenomena: ripple-induced fast ion losses and technical limits on the antenna current. In this work, a new technique using IR thermography was employed to measure the flux of particles coming from the trapping of fast ions in the toroidal magnetic field ripple. These measurements provided the opportunity to fit parametric scaling laws in order to predict the ion flux intensity and the total power loss for experiments with a plasma current of kA and a major radius of the cyclotron resonance layer of m depending on the heating power and the line-averaged electron density. Furthermore, another semi-empirical parametric scaling was developed to evaluate the coupling resistance depending on controllable parameters such as the line averaged electron density and the radial outer gap between the separatrix and the ICRH antenna. These laws were used to define an operational domain from the database of previous experiments made during campaign C4. The settled operational domain suggests that high power ( MW) and high electron density ( m−3) discharges are suitable for optimized steady-state high confinement scenarios in WEST using ICRH.

ORBIT simulations of fast ion power loads on the wall of the Divertor Tokamak Test

Neutral beam injection in tokamaks produces a population of fast ions, which interact with 3D magnetic fields in a variety of ways, often resulting in energetic particle losses in very short times to the wall. Careful design of neutral beams and active control of error fields helps to keep these losses to a minimum. Nevertheless, past experience in tokamaks in the 1980s and detailed simulations for future machines, such as International Thermonuclear Experimental Reactor, suggest that very localized fast ion losses (‘hot spots’) can be present, even if the overall losses are low. In this paper, we discuss this issue in the Divertor Tokamak Test (DTT) project, and we show that in the standard single-null full-power scenario of the DTT, fast ions produce two hot spots, corresponding approximately to the beam injection and exit toroidal angles: the former being mainly due to prompt losses/passing particles, while the latter is due to trapped ions. However, the maximum power load in these spots is of the order kW m−2, below the tolerance of plasma-facing components of the machine.

Spin wave dispersion of ultra-low damping hematite ( α−Fe2O3 ) at GHz frequencies

Low magnetic damping and high group velocity of spin waves (SWs) or magnons are two crucial parameters for functional magnonic devices. Magnonics research on signal processing and wave-based computation at GHz frequencies focused on the artificial ferrimagnetic garnet ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (YIG) so far. We report on spin wave spectroscopy studies performed on the natural mineral hematite ($\ensuremath{\alpha}{\text{-Fe}}_{2}{\text{O}}_{3}$), which is a canted antiferromagnet. By means of broadband GHz spectroscopy and inelastic light scattering, we determine a damping coefficient of $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and magnon group velocities of a few 10 km/s, respectively, at room temperature. Covering a large regime of wave vectors up to $k\ensuremath{\approx}24\phantom{\rule{0.16em}{0ex}}\mathrm{rad}/\textmu{}\mathrm{m}$, we find the exchange stiffness length to be relatively short and only about 1 \AA{}. In a small magnetic field of 30 mT, the decay length of SWs is estimated to be 1.1 cm similar to the best YIG. Still, inelastic light scattering provides surprisingly broad and partly asymmetric resonance peaks. Their characteristic shape is induced by the large group velocities, low damping, and distribution of incident angles inside the laser beam. Our results promote hematite as an alternative and sustainable basis for magnonic devices with fast speeds and low losses based on a stable natural mineral.

Layer thickness characterization of Faraday cup fast ion loss detectors.

Faraday cup fast ion loss detectors have attractive properties for fusion applications, as they can measure wide ranges of energy, are intrinsically neutron-hardened, and can be packaged in very small form factors. The latter allows them to be installed as arrays, offering opportunities to decouple fast ion loss location and magnitude in fully three-dimensional magnetic fields. In this work, we characterize the layer thicknesses of detector prototypes using spectral reflectance measurements, confocal laser scanning microscopy, as well as raster electron microscopy with a focused ion beam. We find that the measured layer thicknesses agree well enough with the specification to allow for accurate measurements. The as-manufactured heights are on the high side, increasing reliability. The data presented here further sets the basis for future optimizations in manufacturing.

First measurements of a scintillator-based fast-ion loss detector in reversed Ip/Bt at the ASDEX Upgrade tokamak

Fast-α losses in future fusion power plants are expected to follow the co- and counter-current directions due to the isotropy of the fusion reaction. At the ASDEX Upgrade tokamak, the co-current direction (achieved with positive plasma current and negative toroidal magnetic field) has been extensively studied thanks to an array of five fast-ion loss detectors (FILD). A new fast-ion loss detector compatible with reversed Ip/Bt (negative plasma current and positive magnetic toroidal field, defining positive as counter-clockwise when the machine is viewed from the top), namely RFILD, has been designed and commissioned to study the behaviour of counter-current fast-ion losses and to expand our knowledge on QH-mode, I-mode, and the L-H transition. The detector is a modified version of the midplane manipulator fast-ion loss detector that features a new probe head orientation and collimator geometry, optimised using the FILDSIM code. A fast CMOS camera is used as a high spatial resolution acquisition system, while an array of photomultipliers (up to 1 MHz) serves as a fast channel.First measurements have been taken in I-mode, QH-mode, L-mode and H-mode with core densities ranging between 4·1019 and 9·1019 m−3, IP between -0.8 and -0.6 MA and Bt of 2.5 T approximately. NBI fast-ion losses were observed for the first time in reversed Ip/Bt, producing high pitch (60 – 80°) losses. ICRH losses in H-minority heating configuration have also been observed in I-mode experiments. These losses correspond to the outer leg of banana particle trajectories. An upgraded double collimator detector design is also presented, that will increase the measurable particle range in future experimental campaigns (co- and counter-current particles simultaneously). Thus, we expand our understanding on the mechanisms behind fast-ion losses at the ASDEX Upgrade tokamak in the reversed Ip/Bt configuration.

3D simulation of orbit loss and heat load on limiters of ICRF-NBI synergy induced fast ions on EAST

Fast ions synergy induced by ion-cyclotron range of frequency (ICRF) and neutral beam injection (NBI) are of interest not only because of their advantage of heating the plasma and drive currents, but also because of their disadvantage of damaging plasma surface components and driving MHD instabilities. In this paper, we calculate the fast ion loss and the deposition distribution of the lost particles on the limiters in EAST under the synergistic effect of the ripple field and collisions with the full-orbit-following simulation program ISSDE for the first time. The previous models to study the NBI fast ion loss by the action of ICRF are relatively simple and consider fewer influencing factors. Most studies on fast ion loss have used toroidal uniform boundaries. In this work, we consider the distribution of ICRF-NBI synergy induced fast ions with different minority H concentrations. After setting the limiter boundary, we consider the prompt fast ion loss caused by the equilibrium field and the fast ion loss caused by the ripple field and collision. Under the action of minority-ion ion-cyclotron resonant heating, the NBI fast ion distribution function has spread in the high-energy part, especially for the minority H concentration of 1%, and the fast ions show each anisotropic distribution near the resonance band on the poloidal dimension. The synergistic loss caused by the ripple field and collision will first be greater than the loss caused by either factor, and then reach a final loss fraction of 3.8%. The heat load power density of the lost fast ions on different limiters is not uniform, as well as on each limiter, which is related to the distance from the limiter to the plasma, the relative position between the limiters and the parallel direction of most fast ions. Once the study of ICRF-NBI synergy induced fast ion loss caused by the action of ripple and collision has been done, we can do optimization in a targeted manner. Such as adding ferromagnetic inserts to reduce the ripple loss and optimizing the limiters’ position to reduce or control the generation of impurities.

Effect of ideal internal MHD instabilities on NBI fast ion redistribution in ITER 15 MA scenario

Transport of fast ions is a crucial issue during the operation of ITER. Redistribution of neutral beam injection (NBI) fast ions by the ideal internal magnetohydrodynamic (MHD) instabilities in ITER is studied utilizing the guiding-center code ORBIT (White R B and Chance M S 1984 Phys. Fluids 27 2455). Effects of the perturbation amplitude of the internal kink, the perturbation frequency of the fishbone instability, and the toroidal mode number of the internal kink are investigated, respectively, in this work. The internal kink mode can cause NBI fast ions transporting in real space from regions of to where labels the normalized plasma radial coordinate. The transport of fast ions is greater as the perturbation amplitude increases. The maximum relative change of the number of fast ions approaches 5% when the perturbation amplitude rises to 500 G. A strong transport is generated between the regions of and in the presence of the fishbone instability. Higher frequency results in greater transport, and the number of fast ions in is reduced by 30% at the fishbone frequency of 100 kHz. Perturbations with higher will lead to the excursion of fast ion transport regions outward along the radial direction. The loss of fast ions, however, is not affected by the internal MHD perturbation. Strong transport from to does not influence the plasma heating power of ITER, since the NBI fast ions are still located in the plasma core. On the other hand, the influence of fast ion transport from to needs further study.

Design of Probe to Investigate Energetic Electrons in Lower Hybrid Wave Plasmas in the TST-2 Spherical Tokamak

The plasmas whose current is driven by the lower-hybrid wave (LHW) alone have been investigated on the TST-2 spherical tokamak. We are developing a probe to obtain the information about energetic electrons generated by the LHW. We investigated a detector, motivated by the lost fast-ion probe (LIP) or fast ion loss detector (FILD), which can identify the energy and pitch angle of captured ions. However, the pitch angle of electrons of interest is expected to be so small that it was found that a conventional configuration with the normal vector of an entrance-orifice surface almost perpendicular to the magnetic field line was not applicable. Thus, we accessed the feasibility of the configuration with two tiny orifices by which orbits, or the energy and pitch angle,of detected electrons can be well specified. We carried out numerical calculations by using calculation conditions and geometries which are manufacturable. The results suggest that it is difficult to identify the energy of detected electrons, but that it is possible to identify their pitch angle well. Based on the results, we designed the diagnostic system with 3 channels which will detect energetic electrons with the pitch angle of 3 - 5, 5 - 9, and 9.5 - 12.5 degrees, respectively.

The physics basis to integrate an MHD stable, high-power hybrid scenario to a cool divertor for steady-state reactor operation

Coupling a high-performance core to a low heat flux divertor is a crucial step for ITER and a Fusion Pilot Plant or DEMO. Experiments in DIII-D recently expanded the steady-state hybrid scenario to high density and divertor impurity injection to study the feasibility of a radiating mantle solution. This work presents the physics basis for trade-offs between density, current drive and stability to tearing modes (TMs) at high β. EC power is crucial to tailor the plasma profiles into a passively stable state, and to eject impurities from the core. Off-axis EC depositions decrease the heating efficiency, but calculated electron heat transport coefficients show that this effect is partially mitigated by improved confinement inside the EC deposition. Additionally, the reduction in pressure is recovered by increasing the density. This favourable scaling of confinement with density was observed in high power plasmas for years, and this work provides a comprehensive explanation. ELITE predictions indicate that a path in peeling-ballooning stability opens up for certain conditions of density, power, q 95 and shaping, allowing the edge pressure to continue increasing without encountering a limit. In the core, calculated anomalous fast-ion diffusion coefficients are consistent with density fluctuation measurements in the toroidicity-induced Alfvén eigenmode range, showing that smaller fast-ion losses contribute to the enhanced confinement at high density. The edge integration study shows that divertor heat loads can be reduced with Ne and Ar injection, but this eventually triggers a cascade of n = 1, 2, 3 core TMs. We can now show that impurity radiation in the core is small and it is not the cause for the drop in confinement at high Ar and Ne injection rates. The overlap between the core TMs is consistent with the loss of pressure as estimated by the Belt model for the coupled rational surfaces. Optimization of these trade-offs has achieved plasmas with sustained H 98y2 = 1.7, f GW = 0.7 and ∼85% mantle radiation. The scenario and its variations at higher density and on- vs off-axis EC heating has been studied as a candidate for an integrated solution for several reactor designs, such as ITER, ARC, and the ARIES-ACT1 case, showing promising results in terms of fusion power and gain.

Impact of β n and spectrum of n = 1 applied fields on fast ion losses in DIII-D

The effect of n = 1 magnetic perturbations (MPs) on prompt fast ion losses in DIII-D has been investigated using the light ion beam probe technique. The effect of normalized beta, β n , and the plasma response to the MPs is studied for several MP spectra. Magnetics data show a strong dependence of plasma response to β n that depends on the phase difference in perturbation coils, . For most phases, the response increases with β n , however, the response is suppressed for phases between 180∘ and 240∘. Experimental data from scintillator based fast ion loss detectors shows that relative fluctuation of lost ions (20%–30% of the steady signal) does not diminish across the L- to H-mode transition for , despite the 34% decrease in radial response field and 50% decrease in poloidal response field. Simulations of the DIII-D discharges using M3D-C1 and ASCOT5 find that prompt losses from neutral beam injection in each investigated case hit the first wall at several concentrated locations: the vessel floor, midplane diagnostic ports, vessel ceiling, and inner wall. Simulated losses from co-injection neutral beams are born outside the last closed flux surface, corresponding to radial orbit displacements of 3–6 cm due to the MPs.

Optimizing numerical simulation of beam ion loss due to toroidal field ripple on EXL-50U spherical torus

Realization of high performance plasma of EXL-50U is very sensitive to NBI (neutral beam injection) heating, and it is expected that the fast ions of NBI are confined well and their energy is transferred to the background plasma by collision moderating. In this paper, the loss of fast ion ripple is simulated based on the equilibrium configuration, fast ion distribution and device waviness data given by the integrated simulation. It is found that the loss fraction of fast ion ripple is about 37%, and the local hot spot is about 0.6 MW/m<sup>2</sup>, which is unacceptable for the experimental operation of the device. The optimization method includes moving the plasma position and adding FI (ferritic steel plug-in) to reduce the ripple degree, increasing the <i>I</i><sub>p</sub> (plasma current) and optimizing the NBI injection angle. The results show that the ripple distribution must be controlled and the <i>I</i><sub>p</sub> must be increased to more than 600 kA, so that the fast ion loss can be reduced to 3%–4% and the local heat spot can be reduced by an order of magnitude. In this paper, the evaluation methods of fast ion ripple loss in device design are summarized, including the fast ion distribution in phase space, the overlap degree of ripple loss area, and the particle tracking on the time scale of total factor slowing down. The engineering and physical ways to reduce ripple loss are also summarized to provide simulation support for integrated simulation iterative optimization and plant operation.

Enhancing the photoelectrochemical activity of monoclinic BiVO4 by discretizing oxygen vacancies: insights from DFT calculations.

Monoclinic bismuth vanadate (BiVO4) has emerged as an excellent optically active photoanode material due to its unique physical and chemical properties. Experiments reported that the low concentration of oxygen vacancies enhances the photoelectrochemical (PEC) activity of BiVO4, but the high concentration of oxygen vacancies decreases the charge carrier lifetime. Using time-domain density functional theory and molecular dynamics, we have demonstrated that the distribution of oxygen vacancies has strong effects on the static electronic structure and nonadiabatic (NA) coupling of the BiVO4 photoanode. The localized oxygen vacancies create charge recombination centers within the band gap and enhance the NA coupling between the VBM and the CBM, resulting in fast charge and energy losses. By contrast, the discrete oxygen vacancies can eliminate the charge recombination centers and decrease the NA coupling between the VBM and the CBM, enhancing the PEC activity of monoclinic BiVO4. Our study suggests that the PEC performance of a photoanode can be improved by changing the distribution of oxygen vacancies.

Investigation of the synergistic effects of ICRF + NBI heating in EAST plasma discharges

Following an upgrade of the neutral beam injection (NBI) system, obvious synergy between combined NBI and ion cyclotron resonance frequency (ICRF) heating was observed in recent experiments conducted at EAST. To investigate the effects of beam-ions accelerated by radiofrequency (RF) wave, analyses are performed by using TRANSP code based on the experimental results. The calculated results argue that only a small fraction of the ICRF power is absorbed by the beam ions in the ICRF + NBI synergistic heating of the (H)D plasma. To enhance the beam–RF interactions in synergistic heating and achieve high plasma performance of EAST, different experimental conditions, including multiple injection powers and diverse beam injection options, were explored. Beam injected fast ions are passing particles, trapped particles observed when synergy heating between ICRF and NBI. In particular, by varying the injection direction of the beam ions it was observed that more tangential beam yields better synergy in comparison with more perpendicular beam. The neutron emission rate of D–D fusion in tokamaks is improved and less fast ion loss is produced with tangential NBI + ICRF heating. Also, the effect of the ratio P ICRF/P NBI on ICRF + NBI combined heating is demonstrated, the higher power boosts the fusion enhancement. The study of the performed synergistic heating provides an important reference for the subsequent combined NBI + ICRF heating experiments on EAST.

Interplay between beam-driven chirping modes and plasma confinement transitions in spherical tokamak ST40

Experiments on the high field spherical tokamak ST40 have led to the recent observation of interplay between beam-driven modes of sweeping frequency (chirping modes) and transitions to the enhanced global confinement regime (H-mode) and back to the low confinement regime (L-mode). The H-modes of plasma confinement are identified from decreased intensity of Dα signal and from clear distinctions in the edge gradients of the visible plasma boundary (observed as a sharp plasma edge in camera images). The beam-driven chirping modes, identified as ideal magnetohydrodynamics beta-induced Alfvén acoustic eigenmodes modes, are observed in Mirnov coil signals, interferometry, and soft x-ray diagnostics. A moderate amplitude ‘primer’ chirping mode usually precedes an H–L transition. This is followed by a ‘dominant’ chirping mode with higher amplitude during the L-mode. The L–H transition back to the improved confinement occurs on a longer time scale of tens of ms, consistent with the slowing down time scale of fast beam ions. A dramatic decrease in toroidal plasma rotation is systematically observed associated with chirping modes sweeping down to zero frequency. Resonance maps built for the beam-driven chirping modes with the ASCOT (accelerated simulation of charged particle orbits in torodoial devices) code show that the resonant beam ions have orbits near the trapped-passing boundary. The ASCOT modelling assesses how losses of the resonant fast ions caused by the chirping modes with high enough amplitude modify the torque, potentially affecting the plasma rotation.

Identification of fast ion wall loads in Wendelstein 7-X from thermographic measurements

Fast ion wall loads can result in excessively high heat fluxes to the plasma-facing components (PFCs). To allow for the development of mitigation strategies, and thereby protect the PFCs, the fast ion losses have to be predicted by faithful models. To ensure that fast ion models are an accurate representation of the real world, they need to be verified. The neutral-beam experiments performed in Wendelstein 7-X (W7-X) allow to investigate and verify models of the fast ion losses in the stellarator configuration. Infrared thermographic measurements were used to obtain the heat flux to both the baffle plates and the divertor. We found evidence of fast ion wall loads on the baffle plates, with loads between 100 kW m−2 and 1 MW m−2. The loads are attributed to fast ions which escape the main plasma via magnetic ripples. The fast ion wall loads on the baffle plates show up-down and toroidal asymmetry. The experimental results were compared to numerical simulations performed by the BEAMS3D and ASCOT codes. Qualitative agreement in up-down asymmetry is found, but the magnitude and toroidal asymmetry are not yet well predicted by the simulations. The asymmetries of the strike lines on the divertor suggest that fast ions also play a role here. Specifically, a second strike line emerged consistently in the high-iota configuration on the horizontal divertor. The shape and magnitude of the strike lines changed considerably during the neutral-beam injection (NBI) operation phase. Although no damage to steel components of W7-X was found, fast ion loads to the baffle plates could possibly limit the NBI operation in the upcoming campaigns of W7-X.

Energetic particle loss mechanisms in reactor-scale equilibria close to quasisymmetry

Collisionless physics primarily determines the transport of fusion-born alpha particles in 3D equilibria. Several transport mechanisms have been implicated in stellarator configurations, including stochastic diffusion due to class transitions, ripple trapping, and banana drift-convective orbits. Given the guiding center dynamics in a set of six quasihelical and quasiaxisymmetric equilibria, we perform a classification of trapping states and transport mechanisms. In addition to banana drift convection and ripple transport, we observe substantial non-conservation of the parallel adiabatic invariant which can cause losses through diffusive banana tip motion. Furthermore, many lost trajectories undergo transitions between trapping classes on longer time scales, either with periodic or irregular behavior. We discuss possible optimization strategies for each of the relevant transport mechanisms. We perform a comparison between fast ion losses and metrics for the prevalence of mechanisms such as banana-drift convection (Velasco et al 2021 Nucl. Fusion 61 116059), transitioning orbits, and wide orbit widths. Quasihelical configurations are found to have natural protection against ripple-trapping and diffusive banana tip motion leading to a reduction in prompt losses.

Impact of Neutral Beam and Plasma Shaping on the Current Drive Efficiency and Fast Ion Losses in a Compact Tokamak FNS-ST

Fusion neutron source (FNS)-spherical tokamak (ST) will be a FNS based on ST ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R =0.5$ </tex-math></inline-formula> m, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$a =0.3$ </tex-math></inline-formula> m, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k =2.75$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$B =1.5$ </tex-math></inline-formula> T) with a steady-state neutron yield ~1018 n/s. The noninductive current drive and neutron generation is to be maintained by neutral beam injection (NBI). Beam in toroids (BTOR) code (Python) is used to reproduce NBI and plasma shaping with high degree of detail and to perform parametric studies of neutral beam (NB) driven effects. The beam model light neutral beam (LNB) is inherited from the original beam transmission (BTR) code, used since 2005 for NBI design and optimizations. The BTOR methodology combines NB statistical description with high-performance analytical models of particle tracing, which allows one to get results several orders faster, if compared to the conventional (Monte–Carlo) approach, and still with a good agreement. The results obtained for the FNS-ST device prove that the entire beam efficiency highly depends on the beam–plasma mutual size and shaping, including the beam cross section and aiming, plasma aspect ratio, elongation, and triangularity. The results are especially important for a steady-state current drive and fusion control in low aspect ratio tokamaks; the BTOR method is also applicable to a conventional tokamak design.

Conceptual Design of a Scintillator-Based Fast-Ion Loss Detector for the Wendelstein 7-X Stellarator

A conceptual design of a scintillator-based fast-ion loss detector (FILD) has been developed for the Wendelstein 7-X stellarator (W7-X). Simulations using the Monte Carlo codes ASCOT5 and BEAMS3D have been performed to calculate the expected flux of neutral beam injection (NBI)-generated fast hydrogen ions onto the conceptual detector probe head. These fast-ion loss fluxes have been calculated for several magnetic field configurations as well as probe insertion positions. At the maximum insertion position, both co- and counter-going losses with high incident pitch angles are observed; however, at retracted positions, only co-going fast ions reach the probe head. The FILDSIM code has been used to optimize the geometry of the detector entrance and collimating elements to achieve a wide velocity space coverage as well as a high velocity–space resolution. A synthetic FILD signal is calculated for the expected loss distribution via forward modeling using the instrument response function. The synthetic signal is found to largely retain the velocity space features of the loss distribution.