Fusion Plasma Devices Research Articles

Examining transport and integrated modeling predictive capabilities for negative-triangularity scenarios

Abstract This paper investigates the predictive capabilities of TGYRO and TGLF models in assessing the
performance of negative triangularity (NT) plasmas compared to positive triangularity (PT) plasmas in fusion devices. TGYRO predicts kinetic profiles, while TGLF analyzes turbulent transport. The study reveals that TGYRO reasonably predicts NT profiles similar to PT, although it overpredicts the high-power scenarios where there is increased experimental MHD activity. TGLF analysis finds reduced linear growth rates in NT and altered flux spectra relative to PT. Additionally, the TGLF SAT0 saturation model is observed to predict high-k transport and a reduction of particle transport with the electron temperature gradient. These findings are further corroborated by core-pedestal modeling using the STEP (Stability Transport Equilibrium Pedestal) workflow, showing stronger confinement improvements in NT, particularly at higher power densities for the SAT0 saturation model. The study underscores the importance of accurately capturing turbulence saturation mechanisms for NT in order to project its performance accurately in fusion reactors.

Plasma dynamics and collisional magnetic presheath structure in unmagnetized divertor plasma in fusion devices

A model of strongly ionized plasma dynamics in a slightly inclined magnetic field with ions, unmagnetized with respect to ion–ion collisions, typical for detached regimes, is presented. It is demonstrated that far from the material surface, the parallel dynamics remains almost the same as in the magnetized plasma. In the wall vicinity, parallel plasma motion is transformed into perpendicular classical diffusion originating from electron–ion collisions, forming a diffusive layer. At a distance of few ion gyroradii from the material surface, a collisional magnetic presheath is formed, where ions move toward the surface with the velocity of the order of the sound speed, in spite of the fact that an ion exhibits many collisions before reaching the surface. For typical inclination angles of magnetic field of the order of few degrees, the electrostatic potential in the diffusive layer and in the presheath deviates considerably from the Boltzmann potential for electrons, which is typical for collisionless magnetic presheath. Moreover, the normal electric field in some regions is directed away from the surface, which is important for impurities to interact with the surface.

Electron beam profile measurement using enhanced dual-techniques in high heat flux test facility at Institute for Plasma Research.

During operation of an ITER-like plasma fusion device, Plasma Facing Components (PFCs) of a divertor are subjected to steady-state and transient heat loads of up to 20 MW/m2. At High Heat Flux Test Facility (HHFTF) at Institute for Plasma Research, PFCs are subjected to such heat flux scenarios using 200 kW electron beam (EB). A heat flux distribution on PFCs is significantly influenced by the EB profile and scanning technique. A novel dual-technique approach is used to measure EB cross-sectional profiles in HHFTF. A single setup is designed, fabricated, and installed in HHFTF consisting of (1) the knife-edge technique involving graphite and tungsten rectangular tiles bonded on a copper block adjacent to each other to form a sharp edge and (2) the wire scanner technique wherein tungsten wires are placed at equal distances. Current collected by the setup is measured as EB is rastered across the wire and tile boundaries. Current measured as a function of EB position is used to calculate the EB profile using a customized LabVIEW software application. Experiments are conducted by varying EB power from 28 to 200 kW and correspondingly the full width at half maximum of the EB profile grows from 5.30 to 16.04mm, as observed by both these techniques. The COMSOL Multiphysics simulation of the wire scanner technique is performed to verify the experimental results. X-ray and infra-red imaging diagnostics of HHFTF are also used to measure the EB profile in a unique and first-of-its-kind way, and they are found to agree well with the dual-technique measurements.

Neutron shielding calculation for DEMO-Prad/SXR measurement system

In plasma fusion devices, the selection of shielding materials is one of the challenges for plasma diagnostic and control systems under high radiation levels during long operation times. This study presents the effect of shielding materials on neutron flux for a radiated power and soft x-ray core intensity measurement system for a DEMOnstration power plant. A calculation model of neutron shielding was used to investigate the neutron shielding performance of borides and carbides for a large distance from plasma to the diagnostic system. The neutron fluxes were characterized for three points close to the measurement system location. The related neutronic calculations were performed with an ADVANTG hybrid code to obtain neutron flux distribution and attenuation rate depending on the thickness of shielding materials. The results indicate that B4C, W2B5, and WB4 are the most effective options to serve as shielding material due to the effect of boron on neutron shielding effectiveness.

Electron density measurement by the three boundary channels of HCOOH laser interferometer on the HL-3 tokamak

Far-infrared (FIR) interferometer is widely used to measure the electron density in the magnetically confined fusion plasma devices. A new FIR laser interferometer with a total of 13 channels (8 horizontal channels and 5 oblique channels) is under development on the HL-3 tokamak by using the formic-acid laser (HCOOH, f = 694 GHz). In order to investigate the boundary electron density activity during the divertor discharge, three horizontal interferometry channels located at Z = −97, −76, 76.5 cm have been successfully developed on HL-3 in 2023, and put into operation in recent experimental campaign, with a time resolution of < 1.0 μs and line-integrated electron density resolution of ~ 7.0 × 1016 m−2. This paper mainly focuses on the optical design of the three-channel interferometry system, as well as optical elements and recent experimental result on HL-3.

Quasioptical modeling of the electron cyclotron emission diagnostic

We report the first applications of the quasioptical (QO) code PARADE (PAraxial RAy DEscription) for modeling the electron cyclotron emission (ECE) diagnostic. Geometrical optics (GO) has been exploited to predict the ECE intensity received by an antenna, but is not sufficient since GO drops the information lying on the transverse cross section of the antenna sensitivity. Hence, we studied a QO model of ECE measurement based on the PARADE code. By introducing a weighting operator Wˆ, induced by the envelope profile of a wave beam virtually injected from an ECE receiving antenna, an integral equation, formally identical to the conventional GO model, is newly derived from the wave-action conservation law. A QO ECE prediction module −E3 is also newly developed, is applied to the JT-60SA tokamak, and shows that radiation temperature is corrected from conventional GO prediction. The −E3 module and the PARADE code are applicable to any fusion plasma devices. By using this ECE prediction package, more realistic calculations of radiation temperature are expected by taking the actual structure of an ECE receiving antenna and plasma parameters as well.

Fractal Escape Basins for Magnetic Field Lines in Fusion Plasma Devices

Fractal Escape Basins for Magnetic Field Lines in Fusion Plasma Devices

Simulation and data processing techniques to design optimized PPR systems on plasma fusion devices

Plasma position reflectometry (PPR) will be used for measuring the plasma position and shape in DEMO. To ensure the reliability, PPR systems are optimized for the expected range of plasmas foreseen during the operation. The study and optimization of PPR systems is a highly demanding computational task and the current approach to define the simulation setup and process the simulation results is time-consuming and does not allow the study of a large number of configurations in useful time.In this article we automate the simulation process required for studying and optimizing PPR systems with the REFMUL family of full-wave FDTD codes. A general overview of the fundamental concepts of PPR systems is presented in the context of reflectometry simulations. Different solutions are proposed to produce realistic reflectometer models from CAD files, realistic plasma models, minimize the complexity of the input definition, manage the simulations in HPCs and analyze the simulation results of PPR systems. A new algorithm based in the In-phase and Quadrature (I/Q) detection scheme is developed to extract the phase derivative and the detected signal amplitude from the synthetic signals of a generic simulation. These techniques allow a brute-force approach to reflectometry problems, being essential for the study of reflectometry systems. The algorithms are validated using the 2017 DEMO baseline scenario to test more than 100 different reflectometers at different poloidal positions in the same toroidal section. The application of our methodology can be extended to different reflectometry techniques and other fields.

Integrating batched sparse iterative solvers for the collision operator in fusion plasma simulations on GPUs

Batched linear solvers, which solve many small related but independent problems, are increasingly important for highly parallel processors such as graphics processing units (GPUs). GPUs need a substantial amount of work to keep them operating efficiently and it is not an option to solve smaller problems one-by-one. Because of the small size of each problem, the task of implementing a parallel partitioning scheme and mapping the problem to hardware is not trivial. In recent history, significant attention has been given to batched dense linear algebra. However, there is also an interest in utilizing sparse iterative solvers in a batched form.An example use case is found in a gyrokinetic Particle-In-Cell (PIC) code used for modeling magnetically confined fusion plasma devices. The collision operator has been identified as a bottleneck, and a proxy app has been created for facilitating optimizations and porting to GPUs. The current collision kernel linear solver does not run on the GPU—a major bottleneck. As these matrices are sparse and well-conditioned, batched iterative sparse solvers are an attractive option.A batched sparse iterative solver capability has recently been developed in the Ginkgo library. In this paper, we describe how Ginkgo's batched solver technology can integrate into the XGC collision kernel and accelerate the simulation process. Comparisons for the solve times on NVIDIA V100 and A100 GPUs and AMD MI100 GPUs with one dual-socket Intel Xeon Skylake CPU node with 40 cores are presented for matrices from the collision kernel of XGC. Further, the speedups observed for the overall collision kernel are presented in comparison to different modern CPUs on multiple supercomputer systems. The results suggest that Ginkgo's batched sparse iterative solvers are well suited for efficient utilization of the GPU for this problem, and the performance portability of Ginkgo in conjunction with Kokkos (used within XGC as the heterogeneous programming model) allows seamless execution on exascale-oriented heterogeneous architectures.

Helium plasma immersion ion implantation studies of tungsten and tungsten heavy alloys for fusion plasma facing components

Plasma fusion devices will require plasma-facing components (PFCs) which can withstand the extreme environment at the edge of a hot fusion plasma. Despite the excellent properties of tungsten(W) as a hard refractory metal, adverse effects such as embrittlement, melting, and morphological evolution have been observed in W when it is bombarded by a high-fluence of low-energy ions including helium. This study investigates the effect of helium ion bombardment on pure tungsten and a tungsten heavy alloy (W-HA)(NAECOMET 1000). Pure tungsten and NAECOMET 1000 samples were implanted with 3 keV helium ions with fluences ranging from to , using Plasma Immersion Ion Implantation (PIII). After PIII treatment, samples were analysed using scanning electron microscopy (SEM) and atomic force microscopy (AFM), which revealed differences in surface morphology and topography. Although the melting and cracking of the Ni/Cu binder phase in the NAECOMET 1000 samples was seen under all implantation conditions, the Ni/Cu presence did somewhat slow the formation of W fuzz in comparison to pure tungsten. X-ray diffraction (XRD) studies showed peak shifts increasing with helium ion fluence for both the pure W and NAECOMET 1000 samples, as well as increase in mean crystallite size, confirming the distortion of the lattice. XPS compositional analysis showed a strong oxidation (>97) near the metal surface, after helium PIII treatment, for both pure W and NAECOMET 1000 W-HA samples. Some conclusions about the potential suitability of W-HA materials for fusion plasma PFCs are drawn.

Conceptual design of a GEM (gas electron multiplier) based gas Cherenkov detector for measurement of 17MeV gamma rays from T(D, γ)5He in magnetic confinement fusion plasmas.

The only method for assessing the fusion power throughput of a deuterium-tritium (DT) reactor presently relies on determining the absolute number of 14MeV neutrons produced in the DT plasma. An independent method, developed and investigated during the recent DT campaign at the Joint European Torus, is based on the absolute counting of 17MeV gamma rays produced by the competing T(D, γ)5He reaction that features a very weak branching ratio (about 3-6 × 10-6) when compared to the main T(D,n)4He reaction. The state-of-the-art spectrometer used for gamma-ray measurements in magnetic confinement fusion plasmas is LaBr3(Ce) scintillator detectors, although they require significant neutron shielding to extract a relatively weak gamma-ray signal from a much more abundant neutron field. A better approach relies on a gamma-ray detector that is intrinsically insensitive to neutrons. We have advanced the design of a gamma-ray counter based on the Cherenkov effect for gamma-rays whose energy exceeds 11MeV, optimized to work in the neutron-rich environment of a steady-state, magnetically confined fusion plasma device. The gamma-rays interact with an aluminum window and extract electrons that move into the radiator emitting photons via the Cherenkov effect. Since the Cherenkov light consists of few photons (25 on average) in the far UV band (100-200nm), a pre-amplifier is required to transport the photons to the neutron-shielded location, which may be a few meters away, where the readout elements of the detector, either a silicon or standard photomultiplier tube, are placed. The present work focuses on the development of a scintillating GEM (Gas Electron Multiplier) based pre-amplifier that acts as a Cherenkov photon pre-amplifier and wavelength shifter. This paper presents the result of a set of Garfield++ simulations developed to find the optimal GEM working parameters. A photon gain of 100 is obtained by biasing a single GEM foil to 1kV.

Receiver circuit improvement of dual frequency-comb ka-band Doppler backscattering system in the large helical device (LHD).

Doppler-backscattering (DBS) has been used in several fusion plasma devices because it can measure the perpendicular velocity of electron density perturbation v⊥, the radial electric field Er, and the perpendicular wavenumber spectrum S(k⊥) with high wavenumber and spatial resolution. In particular, recently constructed frequency comb DBS systems enable observation of turbulent phenomena at multiple observation points in the radial direction. A dual-comb microwave DBS system has been developed for the large helical device plasma measurement. Since it is desirable to control the gain of each frequency-comb separately, a frequency-comb DBS system was developed with a function to adjust the gain of the scattered signal intensity of each channel separately. A correction processing method was also developed to correct the amplitude ratio and the phase difference between the in-phase and quadrature-phase signals of the scattered signals. As a result, the error in Doppler-shift estimation required to observe vertical velocity and the radial electric field was reduced, which enables more precise measurements.

Design and development of LN2 cooled cryopump for application in high heat flux test facility

• A liquid nitrogen cooled sorption cryopump is developed for the High Heat Flux Test Facility at IPR. • Design details, fabrication details, operational concept and thermo-structural analysis results are described. • The pump geometrical concept is novel because it provides ease of assembly, integration and trouble free operation in horizontal and vertical direction mounting. • The pump has 250 mm opening and offers pumping speed of ∼5106.6 l/s for water vapour and ∼1197 l/s for nitrogen using liquid nitrogen as coolant. • The pump integration and operation with High Heat Flux Test Facility is discussed. High Heat Flux Test Facility (HHFTF) at Institute for Plasma Research is established for testing thermal performance of Plasma Facing Components (PFC) and Plasma Facing Materials (PFM) that are expected to withstand steady-state and transient heat flux of the order of 10 MW/m 2 and 20 MW/m 2 , respectively, expected in plasma fusion devices. During high heat flux testing of PFCs and PFMs, water vapour and other trapped gases are released. For the pumping need, a cryocooler based commercial cryopump and a turbo-pump is used. In addition to that, an indigenously developed liquid nitrogen cooled cryopump is mounted to the High Heat Flux (HHF) test facility. The developed cryopump has 250CF inlet flange and mounted vertically downward to an angle port (50°) to the D-shaped vacuum chamber of the HHF facility. The pump has integrated liquid nitrogen bath to provide stable cooling to the cryopanels and radiation shield. Cryopanels are made of copper and coated with micro-porous activated charcoal. The pump's geometrical concept is novel because it provides ease of assembly, integration and trouble free operation in horizontal and vertical direction mounting. The pump offers pumping speed of ∼5106.6 l/s for water vapour and ∼1197 l/s for nitrogen using liquid nitrogen as coolant. The pump geometry, thermal and structural analysis, fabrication and experimental details are discussed in this paper. After integrating the cryopump to the HHF test facility, performance studies were carried out and initial test results are also discussed.

The Plasma Characterization of Argon and Helium Gases in Inertial Electrostatic Confinement Fusion Plasma Device

The Plasma Characterization of Argon and Helium Gases in Inertial Electrostatic Confinement Fusion Plasma Device

Arc behaviour on different materials in ASDEX Upgrade

• The erosion of plasma facing components in a tokamak environment was systematically studied using different materials. • The key parameter for the erosion is melting temperature of the material. • For magnetic steel much stronger erosion is found compared to stainless steel. Arcs, a source of dust particles and a localized erosion mechanism of the plasma-facing components, are found in all major fusion plasma devices. Measurements of arcs require diagnostics with high temporal and local resolution, which are not available at arc dominated locations in ASDEX Upgrade (AUG). To understand the erosion by arcing and to allow extrapolation for future fusion devices different materials are used to scan the material properties. In AUG, inserts were installed at the inner baffle region to measure the erosion by arcing. The use of polished inserts allows an accurate determination of the arc traces by depth maps obtained by laser profilometery. It turned out that the melting temperature of the materials is the main parameter for erosion. For tungsten mounted at the inner baffle, a region which is deposition dominated, an erosion rate by arcing of 1.2·10 13 at cm −2 s −1 is measured. For Beryllium, 9.5·10 13 at cm −2 s −1 is extrapolated from its thermal properties. As martensitic–ferritic low-activation steel is under discussion for the use in DEMO, magnetic steels were also investigated. Comparing stainless steel with magnetic steel, much deeper and wider craters are found in the latter one: they reach a depth of −80 μm. The erosion of magnetic steel by arcs is 40 times higher compared to stainless steel, which has almost the same physical properties.

A novel approach for the improvement of the sensitivity of the flow visualization system on supersonic molecular beam of tokamak fuelling

Supersonic molecular beam injection is a robust alternative to control the particles in fusion plasma devices. It is widely used in many experimental studies on the plasma physics. Direct visualization of beam makes it possible for precise optimization of the beam characteristics. However, a normal visualization system does not fully meet the requirements of the measurement of the beam distribution in the low gas density vacuum environment. This paper reports a newly designed multi-pass visualization system without changing the schlieren mirrors. This multi-pass system is designed to make the light passing the testing area several times, where the deflection angle of the light would simultaneously increase. Thus, the sensitivity of the schlieren image is enhanced. The testing result proves the improvement of the sensitivity, which makes it possible to optimized the beam structure in low gas density working conditions.

Investigations into growth of whistlers with energy of energetic electrons

Runaway electrons have great significance in fusion plasmas as they are considered to be responsible for the excitation of instabilities and for damage to the first wall components. The threshold for the generation of the runaway electron population in tokamaks strongly depends on the toroidal magnetic field and bulk electron temperature. The presence of these runaway electrons with a critical density and magnetic field value are correlated with the magnetosonic whistler activity. The Rosenbluth condition applies to the destabilization of upper hybrid modes, that remains mostly stable in large-scale tokamaks like Joint European Tokamakand International Thermonuclear Experimental reactor. On the other hand, whistlers with a frequency in the lower hybrid range are more likely to be destabilized in a lower magnetic field and temperature, along with the whistlers’ excitation threshold. The process of destabilization of whistlers by energetic electrons thus has a finite overlap between fusion and basic plasma devices, and the outcome from the latter can be scalable to fusion machines. We report that in the linear large volume plasma device, the destabilization of quasi-longitudinal whistlers, driven by the reflected particles from the mirror, takes place in the bulk plasma region i.e. away from the mirror in the energetic belt region, and follows the frequency ordering of . Our explanation involves an alternate limit of quasi-linear analysis, commonly employed for recovering the instability threshold of runaways scattering off whistlers in fusion plasmas, and the growth rate due to the reflected particle is proportional to the inverse square root of electron temperature, where the reflected particle fraction is (Sharma and Vlahos 1984 Astrophys. J. 280 405). The measurements required to obtain the energy scaling of runaway electrons with whistler instability are critical for large devices but remain scarce. We have emphasized the analytic correlation between the two regimes in these investigations.

Scale Symmetry of Stochastic Surface Clustering under Plasma Influence in Fusion Devices

Titanium, tungsten, carbon, lithium, and beryllium surface structure were analyzed after plasma irradiation in fusion devices. Exceptional extreme high-temperature plasma load in fusion devices leads to specific surface clustering. It is strictly different from any other conditions of material’s clustering. The hierarchical granularity with cauliflower-like shape and surface self-similarity have been observed. Height’s distribution is deviated from the Gaussian function. The relief roughness differs qualitatively from the ordinary Brownian surface and from clustering under other conditions. In fusion devices, the specific conditions regulate material surface clustering faced to plasma. Ions and clusters melt on the surface and move under the effect of stochastic electromagnetic field driven by the near-wall turbulent plasma. In such a process, long-term correlations lead to the growth of surface with a self-similar structure. The multiscale synergistic effects influence the self-similarity–fractal growth from nanometers to millimeters. Experimental results illustrate universality of stochastic clustering of materials irradiated with plasma in fusion devices.

Electron impact single ionization differential cross sections of W (6s), W (5d), W (5p) and W (4f)

Electron-induced triple differential cross sections (TDCSs) have been obtained for the few outer sub-shells of tungsten atoms, namely W (6s), W (5d), W (5p) and W (4f). Tungsten and related materials have been recommended as the prime candidates for wall materials in fusion plasma devices. The electron-induced cross-section data are strongly required for the diagnostics and modeling of plasma in such devices. We observe that the trends of TDCSs obtained for the ionization of W (5d) sub-shells are very different from the TDCSs of W (6s), W (5p) and W (4f) in terms of binary and recoil peak locations as well as in the change of magnitude of TDCS with an increasing value of momentum transfer which is due to the open d-shell. The angle and energy-resolved cross-section data of tungsten atoms may be of vital importance in order to understand the spectroscopy of W atoms as wall materials as well as an impurity.

Investigation of Hydrogen Glow Discharge Cleaning Side Effects on Tungsten

Hydrogen glow discharge cleaning (H-GDC) is a routine conditioning procedure for the modern tokamaks and the future fusion machines, including ITER. Due to the low energy of hydrogen ions in glow discharge plasmas, the probability of any considerable damage to the plasma-facing components was mainly ignored. In this work, Tungsten, as the primary candidate for the plasma-facing materials in tokamaks, is considered for studies regarding effects of the H-GDC procedure on the material during a routine vacuum vessel conditioning in Damavand tokamak. After performing routine H-GDC using pure hydrogen, the formation of loosely attached nanostructured bundles (NSBs) on the surface of the tungsten samples was observed. The presence of the NSBs, which can be a source of dust, would be significant due to their possible effects on the functionality of the future fusion plasma devices. The NSBs are observed on the tungsten samples with the surface temperature of <370 K after 2.5–4 hours of hydrogen ion bombardment with incident energy of ≈120 eV and fluence of ≈(2–3.5) × 1022 m–2. The surface modifications of specimens exposed to H‑GDC were examined using a material probe experiment and several surface analysis techniques such as SEM, EDX, XRD, and ERDA. Therefore, the formation of loose nanostructures on the wall of plasma confinement vessels due to H-GDC draws attention to probable damaging effects of this phenomenon upon functionality and outcomes of tokamaks.