George Tynan

Abstract The transition between the ELMy H-mode and the EDA H-mode is studied on Alcator C-Mod using an experimental database and recently-developed predictive pedestal models. High-resolution Thomson scattering measurements are used to inspect both the separatrix and pedestal operational space using two independent fit functions, each tailored to analyze either the pedestal or the separatrix and near-SOL profiles. The pedestal density, n e ped , and the separatrix density, n e sep are compared to main chamber neutral measurements. n e ped is sensitive to neutral sources only in the ELMy H-mode regime and not in the EDA H-mode regime. Density fluctuation spectra reveal that quasi-coherent structures become stronger at higher densities and more coherent in the EDA relative to the inter-ELM phases of ELMy H-modes, before weakening again at the highest values of n e ped . The Saarelma-Connor pedestal density prediction model is validated for ELMy H-modes up to n e ped = 2.0×10 20 m -3 . An additional transport channel driven by resistive ballooning modes (RBM), D RBM , scaling directly with α t , a collisional turbulence control parameter, and inversely with k RBM 2 q cyl , the product of the square of the characteristic RBM turbulence wavenumber and the cylindrical safety factor, is shown to improve the prediction for EDA H-modes, finding good model agreement up to n e ped = 3.0×10 20 m -3 . For comparison, EPED scans in n e ped are then performed at three values of n e sep /n e ped . Increasing this ratio moves the peeling-ballooning branch transition to lower n e ped , increasing n ped in the peeling branch and decreasing it in the ballooning branch. Agreement is found for large ELM H-modes. SPARC pedestal density predictions for an ELMy and an EDA/QCE-like H-mode are performed using the extension to the standalone Saarelma-Connor model and are found consistent with earlier assumptions used in EPED modeling. Inclusion of D RBM significantly weakens the density gradient near the separatrix, lowering n e ped by approximately 20%.

To successfully show that fusion is an attractive energy source, the ARC $^{\scriptstyle \mathrm{TM}}$ fusion power plant will need to operate with a robust, integrated power and particle exhaust solution. To maximise ARC’s fusion power output while avoiding excessive erosion of the plasma-facing components, we will need to radiatively dissipate most of the power crossing the last-closed flux surface, injecting radiating impurities such as argon or neon to access divertor detachment. Divertor detachment will need to be integrated with a high-performance core plasma, and with efficient impurity pumping to prevent the accumulation of helium ash in the core. To access and control detachment in high-performance plasmas, we have designed ARC with up–down-symmetric divertors, with secondary X-points in long, tightly baffled outer legs. Using a core-edge modelling workflow, we predict that with this divertor design, ARC will access detachment with modest argon seeding in the divertor ( ${c_{Ar,div}}\sim {0.9\,\%}$ ), which should have minimal impact on the core ( ${\Delta Z_{\textit{eff},\textit{core}}}\lt {0.5}$ ) for reasonable argon enrichment ( ${c_{Ar,div}/c_{Ar,\textit{core}}}={6}$ ). Due to the high predicted divertor neutral pressure ( ${p_{\textit{div}}}\sim {20\,\mathrm{Pa}}$ ), we predict that ARC will sufficiently pump helium to limit ash accumulation in the core ( ${c_{\textit{He},\textit{core}}}\lt {2\,\%}$ ) for a helium enrichment of ${c_{\textit{He},\textit{div}}/c_{\textit{He},\textit{core}}}={0.4}$ . ARC’s divertor design is expected to increase the stability of a detachment front in the outer divertor leg, helping to prevent divertor reattachment during smaller heat-flux transients such as scrape-off-layer filaments associated with the quasi-continuous exhaust regime. However, this buffering will not be sufficient to prevent divertor reattachment during large type-I edge-localised modes (ELMs), and as such these will need to be avoided on ARC. Experiments on SPARC will be used to select an integrated scenario which avoids or mitigates type-I-ELMs while maintaining access to detachment, good core fusion performance and sufficient impurity exhaust. SPARC experiments will also be used to finalise ARC’s divertor design, by studying the impact of magnetic and first-wall geometry on detachment stability, impurity enrichment and neutral baffling under conditions similar to those expected for ARC. In conclusion, our analysis finds that ARC will have a viable power and particle exhaust solution which is compatible with high-power operations, and this solution will be validated in experiments on SPARC.

Nuclear reactors are often modeled as inflexible baseload generators with fixed downtimes and restrictive ramping limits. In practice, however, operational flexibility for a reactor is coupled with its fuel-cycle. A critical physics constraint arises from xenon poisoning, in which the buildup of the neutron absorbing xenon following a power ramp-down suppresses core reactivity, thereby limiting power maneuvers and reactor restarts. Existing power system models represent these effects using static constraints, neglecting the crucial dependence of xenon-induced inflexibility on the instantaneous state of a reactor core. This study offers a physics-informed unit commitment framework that embeds fuel-cycle dynamics into dispatch modeling. Our approach tracks the reactivity margin and enforces core-state-dependent constraints on allowable minimum generation and restart downtimes, while endogenously scheduling refueling outages. To demonstrate the formalism, we apply the framework to a representative nuclear fleet operating alongside high shares of renewable energy and storage. The results show that operational mode significantly affects system outcomes; flexible nuclear operation slows reactivity degradation, extends fuel-cycles by up to 10%, reduces renewable curtailment, lowers production costs, and delays the onset of refueling outages. These findings highlight the importance of fuel-cycle-aware flexibility modeling and provide a computationally tractable approach for assessing the role of flexible nuclear power in decarbonized grids.

The foundations of irradiation damage theory were laid in the 1950s and 60s within the framework of chemical reaction kinetics. While helpful to analyze qualitative aspects of irradiation damage, the theory contained gaps that delayed its implementation and applicability as a predictive tool. The advent of computer simulations with atomistic resolution in the 80s and 90s revealed a series of mechanisms that have proved essential to understand key aspects of irradiation damage in crystalline solids. However, we still lack a comprehensive model that can connect atomic-level defect physics with experimental measurements of quantitative features of the irradiated microstructure. In this work, we present a mesoscale model that draws from our improved understanding of irradiation damage processes collected over the last few decades, bridging knowledge gained from our most sophisticated atomistic simulations with defect kinetics taking place over time scales many orders of magnitude larger than atomic interaction times. Importantly, the model contains no adjustable parameters and combines several essential pieces of irradiation damage physics: each playing an irreplaceable role in the context of the full model but of limited utility if considered in isolation. Crucially, we carry out a set of experiments carefully designed to isolate the key irradiation damage variables and facilitate validation. Using tungsten as a model material, we find exceptionally good agreement between our numerical predictions and experimental measurements of defect densities and defect cluster sizes.

The foundations of irradiation damage theory were laid in the 1950s and 60s within the framework of chemical reaction kinetics. While helpful to analyze qualitative aspects of irradiation damage, the theory contained gaps that delayed its implementation and applicability as a predictive tool. The advent of computer simulations with atomistic resolution in the 80s and 90s revealed a series of mechanisms that have proved essential to understand key aspects of irradiation damage in crystalline solids. However, we still lack a comprehensive model that can connect atomic-level defect physics with experimental measurements of quantitative features of the irradiated microstructure. In this work, we present a mesoscale model that draws from our improved understanding of irradiation damage processes collected over the last few decades, bridging knowledge gained from our most sophisticated atomistic simulations with defect kinetics taking place over time scales many orders of magnitude larger than atomic interaction times. Importantly, the model contains no adjustable parameters and combines several essential pieces of irradiation damage physics: each playing an irreplaceable role in the context of the full model but of limited utility if considered in isolation. Crucially, we carry out a set of experiments carefully designed to isolate the key irradiation damage variables and facilitate validation. Using tungsten as a model material, we find exceptionally good agreement between our numerical predictions and experimental measurements of defect densities and defect cluster sizes.

Abstract The density limit is investigated in the DIII-D negative triangularity plasmas which lack a standard H-mode edge. We find the limit may not be a singular disruptive boundary but a multifaceted density saturation phenomenon governed by distinct core and edge transport mechanisms. Sustained, non-disruptive operation is achieved at densities up to 1.8 times the Greenwald limit ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">G</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) until the termination of auxiliary heating. Systematic power scans show distinct power scalings for the core ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mi>e</mml:mi> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>SOL</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.27</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) and edge ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mi>e</mml:mi> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>SOL</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.42</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) density limits. The edge density saturation is triggered by the onset of a non-disruptive, high-field side radiation front and the associated cooling, which clamps the edge density below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">G</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . In contrast, the core density continues to rise until it saturates, a state characterized by enhanced core turbulence. Core transport evolves from a diffusive to an intermittent, avalanche-like state, as indicated by heavy-tailed probability density functions (kurtosis ≈ 6), increased Hurst exponents, and a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> </mml:math> -type power spectrum. These findings suggest that the density limit in the low-confinement regime is determined by a combination of edge radiative cooling and core turbulent transport. This distinction provides separate targets for control strategies aimed at extending the operational space of future fusion devices.

A two-photon absorption laser induced fluorescence (TALIF) diagnostic system is designed for measuring the density of deuterium atoms in the scrape-off laser (SOL) region of the Huanliu-3 (HL-3) tokamak. The system employs laser pulses at 10 Hz with an energy of 10 mJ per pulse (pulse width <8 ns) at 205 nm. To enhance the fluorescence intensity, a semi-confocal lens was designed to achieve a relatively large solid angle for fluorescence collection. A calibration experiment was conducted, establishing the relationship between fluorescence intensity and atomic deuterium (D) density using krypton (Kr) atoms as a reference. Experimental verification of the whole TALIF system has been successfully performed in a plasma environment. The feasibility of the TALIF for the measurements of the neutral particles is discussed, and an experimental result in the HL-3 tokamak edge plasma region is shown in this paper.

Abstract Proto-MPEX (Materials Plasma Exposure eXperiment), a linear plasma device (LPD), using a high power radio frequency (RF) (⩾100 kW, 13.56 MHz) helicon plasma source, has suffered RF sheath-induced window erosion. The sputtered impurities from the window surface transport toward the downstream target affecting plasma-material interaction studies. The rectified sheath voltage formed was high enough to cause window erosion by light ions due to its low-Z components (e.g. Si 3 N 4 and AlN). Hence, we are proposing impurity mitigation strategies, which involve the application of a Faraday screen to lower the rectified sheath voltage and the application of high-Z refractory coatings on the existing window plasma-facing surface. In this work, we report the erosion of two different coatings, i.e., tantalum oxide (Ta 2 O 5 ) and hafnium oxide (HfO 2 ) on a silicon nitride (Si 3 N 4 ) window material under conditions of low-energy deuterium (D) ion impact, well below the Ta 2 O 5 sputtering energy threshold (i.e. 250 eV). The test samples were RF-biased and exposed to a high D-ion fluence (∼10 26 m −2 ) in Plasma Interaction with Surface Component Experimental Station (PISCES-A) LPD. The experimentally measured sputtering yields of the high-Z refractory coatings below the sputtering energy threshold of Ta 2 O 5 indicate an increased erosion due to the plasma impurities, especially due to oxygen. Improving the vacuum level could reduce the oxygen impurities and increase the lifespan of the coated helicon window for plasma operation. Post-surface analysis indicates no preferential erosion of oxygen or enrichment of the high-Z surface component. This is likely due to an effective energy transfer from the impurity ion for sputtering of the high-Z component in the coating. With the reduced rectified sheath voltage at the window surface and improved vacuum conditions, the proposed high-Z refractory coatings offer a promising solution for reducing window erosion in future MPEX plasma operations at ORNL.

Nuclear reactors are often modeled as inflexible baseload generators with fixed downtimes and restrictive ramping constraints. In practice, however, a reactor's operational flexibility is closely tied to its fuel cycle and associated reactivity margin. A key physical constraint for power maneuverability is xenon poisoning, caused from the transient buildup of neutron-absorbing xenon following a power reduction. This transient can delay or prevent subsequent power ramp-up due to suppressed core reactivity. Additionally, if a reactor is shutdown during periods of low reactivity, restart times can vary significantly, leading to prolonged downtimes. This work introduces a physics-informed modeling framework that embeds fuel cycle dynamics within a unit commitment (UC) formulation. The framework tracks reactivity margin, dynamically enforces xenon induced constraints, and endogenously schedules refueling outages based on core conditions. By capturing intracycle reactivity evolution, the model enables operation dependent nuclear dispatch that reflects both techno-economic requirements and irreducible nuclear physics limits. Application to a representative reactor fleet shows that flexible operation can slow reactivity degradation and extend fuel cycles. Results further demonstrate that different operational modes substantially affect VRE utilization, curtailment, and nuclear fleet capacity factors. These findings highlight the importance of fuel cycle aware flexibility modeling for accurate reactor scheduling and integration of nuclear power into energy system models.

Abstract The tungsten (W) surface of the ITER divertor might provide suitable surface temperatures and helium (He) influxes for the formation of microscopic fuzz structures. The change of surface morphology can influence the angular distribution of the sputtered W, and thus might alter the migration of W on the divertor. In this work, the influence of angular distribution of sputtered W atoms on the migration of W in the ITER divertor region has been investigated with ERO2.0 simulations. Using unidirectional angular distributions, it is demonstrated that, while the migration of W ions is restricted by the field lines and thus not sensitive to the angular distribution, the migration of neutral W atoms is determined by the direction of their initial velocity due to the line-of-sight redeposition mechanism. Neutral atom deposition at a location far from the erosion location is suppressed by ionization of atoms. Angular distributions of W sputtered from a flat surface and a fuzzy surface are obtained with TRI3DYN simulations assuming W sputtering by neon (Ne) ions, the energy and angular distributions of which are obtained from a Monte–Carlo simulation of their motions in the magnetic presheath. The flat surface favors sputtering in the forward direction of the incident Ne ions, while the fuzzy surface enhances sputtering in the backward direction. Despite the clear differences in their angular distributions, the deposition profiles from the two surface types are not significantly different because the suppression of neutral atom deposition caused by ionization dominates the deposition profile, resulting in the neutral atom deposition peaking close to the strike points regardless of the angular distributions. The results imply that the angular distribution changes caused by the formation of fuzz on the divertor region might not significantly affect the W migration in the ITER tokamak.

Bulk targets of W 38 Ta 36 Cr 15 V 11 complex concentrated alloy (CCA) were produced by spark plasma sintering at the Vecchio materials research lab at the University of California–San Diego. Targets fabricated were reasonably dense, BCC phase, compositionally close in match to that of El-Atwani et al. (2019), and homogeneous, as measured by electron microscopy, energy dispersive x-ray microanalysis (EDX), EDX mapping, and x-ray diffraction. The targets were exposed to deuterium plasma of associated 100 eV ion fluences of 2 × 1 0 26 m −2 at 523 K, 773 K and 1023 K in the Pisces-RF high-flux linear plasma device. Plasma exposure is found to produce surface compositional enrichment in W and Ta, as determined by Auger electron spectroscopy and EDX, that is caused by a depletion of surface Cr and V, the former being observed with optical emission spectroscopy in the target plasma. Accompanying the compositional change, transient grating spectroscopy analysis, taken pre- and post-plasma exposure on the 1023 K exposed target, revealed ∼ 30 % reduction in surface thermal diffusivity from 9 . 4 × 1 0 − 6 m 2 s −1 to 6 . 7 × 1 0 − 6 m 2 s −1 . Lastly, D retention in the CCA targets was found to be characterized by a singular thermal release peak at ∼ 900 K, and retention varied from 2 × 1 0 23 m −2 to 3 × 1 0 20 m −2 for exposure in the temperature range 523–1023 K, as measured by thermal desorption spectrometry. This level of D retention is found to be high relative to pure tungsten.

Abstract The separatrix operational space (SepOS) model (Eich and Manz 2021 Nucl. Fusion 61 086017) is shown to predict the L–H transition, the L-mode density limit, and the ideal magnetohydrodynamic ballooning limit in terms of separatrix parameters for a wide range of Alcator C-Mod plasmas. The model is tested using Thomson scattering measurements across a wide range of operating conditions on C-Mod, spanning <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mover> <mml:mi>n</mml:mi> <mml:mo accent="true">―</mml:mo> </mml:mover> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> </mml:mrow> </mml:math> 0.3–5.5 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>20</mml:mn> </mml:mrow> </mml:msup> <mml:mstyle scriptlevel="0"/> </mml:mrow> </mml:math> m −3 , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">t</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2.5</mml:mn> </mml:mrow> </mml:math> –8.0 T, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> = 0.1–1.2 T. An empirical regression for the electron pressure gradient scale length, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>λ</mml:mi> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , against a turbulence control parameter, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">t</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , and the poloidal fluid gyroradius, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> <mml:mo>,</mml:mo> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , is constructed for H-modes and found to require positive exponents for both regression parameters, indicating turbulence widening of near-scrape-off layer widths at high <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">t</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and an inverse scaling with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , consistent with results on ASDEX Upgrade. The SepOS model is also tested in the unfavorable drift direction and found to apply well to all three boundaries, including the L–H transition as long as a correction to the Reynolds energy transfer term, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi>RS</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> is applied. I-modes typically exist in the unfavorable drift direction for values of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">t</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>≲</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> </mml:math> . Finally, an experiment studying the transition between the Type-I ELMy and EDA H-mode is analyzed using the same framework. It is found that a recently identified boundary at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>

This work details the construction and first in operando transient grating spectroscopy measurements conducted on PISCES-RF during plasma operation. A preliminary study on a tungsten sample with varied plasma species (Ar and D 2 ), ion flux (4 × 10 21 to 6 × 10 22 ion/m 2 /s), ion energy (5 to 80 eV), and surface temperature (20 to 200 °C) is presented. Prior to plasma exposure, the thermal diffusivity and surface acoustic wave (SAW) speed, (6.9±0.2) × 10 −5 m 2 /s and (2.66±0.02) km/s, were measured in situ with a thermal grating period of 12.5 μ m . During Ar plasma exposure, these two quantities vary according to the sample surface temperature. At low flux and varied ion energy, D 2 plasma exposure also results in thermal diffusivity and SAW speed varying with surface temperature alone. At high flux, D 2 plasma exposure results in no convincing change to the thermal diffusivity, but the SAW speed is reduced. After returning to 20 °C, post plasma exposures, the thermal diffusivity and SAW speed almost recover to the initial value prior to exposure. • First in operando measurement of thermal diffusivity and surface acoustic wave frequency with concurrent high flux plasma exposure. • Thermal diffusivity and surface acoustic wave frequency found to be only temperature dependent in low flux plasma (Ar or D 2 ). • Surface acoustic wave frequency reduced more than temperature dependence alone in high flux D 2 plasma.

Numerical diffuse scattering cross-section calculations are used to establish a rigorous basis for determining the concentration and size distribution of dislocation loops in irradiated single crystals from integral X-ray diffuse scattering (XRDS) measurements. Differential XRDS intensities for prismatic {111} type dislocation loops are numerically calculated as a function of loop radius R and wavevectors q relative to Bragg reflections in tungsten. The results show the well known 1/ q 2 Huang scattering form at small q that transitions to a ∼1/ q 4 dependence associated with the Stokes–Wilson approximation for q ≳ 1/ R . More importantly, they show further that the 1/ q 4 falloff is not the asymptotic large- q form of the diffuse scattering for small loops ( R &lt; 200 Å) as has often been assumed. Rather, for loop sizes as small as R ≃ 5 Å with strong curvature, the calculations show definitively that the scattering transitions to a robust 1/ q 5 falloff at larger q that arises due to the local strains near the dislocation core defining the circumference of the dislocation loops. The presence of this 1/ q 5 asymptotic form for both small and large loops is experimentally confirmed using an integral XRDS measurement around the 110 reflection on self-ion-irradiated tungsten combined with numerically calculated integral XRDS cross-sections. Accordingly, the historical two-region theoretical treatment of the cross-sections for integral XRDS is extended to a three-region model that has direct sensitivity to the (first-moment) dislocation line lengths of dislocation loops. These developments enable the use of both numerical and analytically modeled cross-sections to make accurate integral XRDS determinations of dislocation loop sizes and concentrations using modest-intensity laboratory X-ray sources.

Abstract The dynamic analysis of fusion power plant (FPP) fuel cycles highlights the challenge of achieving tritium self-sufficiency in future FPPs. While state-of-the-art fuel cycle models offer valuable insights into the necessary design parameters for attaining tritium self-sufficiency, none of these models currently consider the impact of tritium trapping within fuel cycle components. However, detailed analysis of individual components reveals that substantial amounts of tritium can be trapped within the first wall, divertors, and breeding blanket systems, suggesting that tritium trapping may significantly influence the FPP ability to achieve self-sufficiency. The compounded effects of additional tritium traps generated by irradiation effects and component replacements further exacerbate this challenge. The novelty of this work is the integration of an explicit, physics-based model for tritium trapping, evolution of damage-induced traps, and component replacements into a dynamic, system-level model of a fuel cycle. The results show an increase of a factor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>3</mml:mn> </mml:msup> <mml:mo>−</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>4</mml:mn> </mml:msup> </mml:mrow> </mml:math> of tritium inventory in the first wall and vacuum vessel of an ARC-class FPP when accounting for the aforementioned phenomena. This, coupled with the replacement of components subject to significant tritium trapping, slows down fuel cycle dynamics, resulting in an extended tritium doubling time (50% increase), higher start-up inventory (30% increase), and higher required tritium breeding ratio (2%–5%) compared to a scenario without tritium trapping.

Hydrogen-isotope (HI) fuel retention in the walls of magnetic fusion devices must be minimized to achieve tritium self-sufficiency and for safety considerations. Retention in codeposited layers (i.e., layers formed from eroded material while simultaneously exposed to HIs) is projected to be a dominant source. This work investigates retention in tungsten (W)–deuterium (D) codeposited films as a function of D 2 partial pressure. Tungsten films were deposited onto quartz substrates via magnetron sputtering in an Ar-D 2 mixed background with the D 2 partial pressure, p D 2 , varied from 0.13 to 1.6 Pa; the substrate temperature, T s , varied from 310 K to 623 K; and the W deposition rate, r d , varied from ∼ 1.0 to 4.2 × 1 0 19 m −2 s −1 . After deposition, D release was measured using thermal desorption spectroscopy (TDS). For films deposited at T s ∼ 310 K, D/W increased from 0.13 to 0.44 Pa, was constant to 0.89 Pa, and decreased at 1.6 Pa. However, the TDS spectra for films at all p D 2 showed a similar distribution of D in traps, with a single release peak at 400–700 K. For films with p D 2 = 0.89 Pa, D/W decreased exponentially with increasing T s , in agreement with the T s scaling in the literature, while the release peak in the TDS spectra shifted to higher desorption temperatures. For films deposited near room temperature, the measured D/W ratio decreased by 3 × as r d increased, in agreement with the r d − 0 . 8 scaling in the literature, while the tail of the D release increased from 700 to 1200 K for the higher r d . Hence, retention mitigation strategies for ITER (e.g., baking of the vacuum vessel at 493 K) may not be sufficient, especially for films formed with high r d . • W-D films were prepared at various conditions during the co-deposition process. • At ∼ 300 K, D retention depends non-monotonically on D 2 pressure from 0.13 to 1.6 Pa. • D release remains unchanged with D 2 pressure. • D retention decreases as r d −0.8 , while the release tail increases from 700 to 1200 K. • D retention decreases exponentially with substrate temperature from 310 to 623 K.

D 2 molecular band emission observed at a wavelength range of λ ∼ 557 − 643 nm is utilized to measure electron density, n e , in D plasmas of the PISCES-A and PISCES-RF linear plasma devices. The D 2 band is divided at λ = 593 nm to make an intensity ratio, D 2L ( ∼ 557 – 593 nm )/D 2R ( ∼ 593 – 643 nm ), where D 2L consists predominantly of the g 3 Σ g + 3 d σ , h 3 Σ g + 3 s σ , i 3 Π g 3 d π , j 3 Δ g 3 d δ → c 3 Π u 2 p π transitions, while D 2R mainly includes the d 3 Π u 3 p π → a 3 Σ g + 2 s σ Fulcher band emission. It is experimentally found that D 2L /D 2R depends strongly on n e with little T e dependence in ranges of n e ∼ ( 0 . 031 − 6 . 1 ) × 1 0 18 m −3 and T e ∼ 2 . 3 − 13 . 9 eV . This observed trend is consistent with collisional-radiative model calculations using Yacora on the Web. • An intensity ratio of D 2 molecular bands, D 2L ( ∼ 557 – 593 nm )/D 2R ( ∼ 593 – 643 nm ), is defined. • D 2L /D 2R is sensitive to electron density. • D 2L /D 2R depends little on electron temperature.

Abstract Long-wavelength density fluctuations ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>k</mml:mi> <mml:mrow> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mi>i</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> &lt;1) are studied using beam emission spectroscopy (BES) at the edge of DIII-D L-mode plasmas ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>ρ</mml:mi> </mml:mrow> </mml:math> = 0.88–1.1) in scenarios with electron cyclotron heating (ECH) power ramp ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>P</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>ECH</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> up to 1.5 MW), neutral beam injection (NBI) power ramp ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>P</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>NBI</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> up to 2.5 MW), and injected torque scan (−1 &lt; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>inj</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> &lt;0.6 Nm). We find that broadband turbulent density fluctuations ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> </mml:math> ∼ 20–120 kHz) have a non-Gaussian distribution. The skewness of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>δ</mml:mi> <mml:mi>n</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>n</mml:mi> </mml:mrow> </mml:math> changes sign from negative at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>ρ</mml:mi> </mml:mrow> </mml:math> &lt;0.95–0.97 to positive at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>ρ</mml:mi> </mml:mrow> </mml:math> &gt; 0.97, indicating the prevalence of density ‘voids’ at inner radii and density ‘blobs’ at outer radii and outside of the separatrix. The turbulence intensity flux <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>⟨</mml:mo> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mrow> <mml:mover> <mml:mi>v</mml:mi> <mml:mo stretchy="true">~</mml:mo> </mml:mover> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mtext>r</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mrow> <mml:mover> <mml:mi>n</mml:mi> <mml:mo stretchy="true">~</mml:mo> </mml:mover> </mml:mrow> </mml:mrow> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> </mml:mrow> <mml:mo>⟩</mml:mo> </mml:mrow> </mml:mrow> </mml:math> is calculated to characterize turbulence spreading at the plasma edge. During ECH/NBI power ramps and at counter- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mtext>p</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> injected torque, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>⟨</mml:mo> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mrow> <mml:mover> <mml:mi>v</mml:mi> <mml:mo stretchy="true">~</mml:mo> </mml:mover> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mtext>r</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mrow> <mml:mover> <mml:mi>n</mml:mi> <mml:

Abstract Linear plasma devices (LPD) utilizing a helicon plasma source, a high density light ion source, can generate impurities due to progressive erosion of the radio frequency (RF) transmission window caused by rectified sheath voltage. These source-born impurities can entrain and be transported by the plasma toward a target, affecting plasma-material interaction studies. Earlier work on material testing in Prototype-Materials Plasma Exposure eXperiment at ORNL revealed significant source impurity deposition on downstream targets. However, using a similar RF source, no target impurity deposition is observed in Plasma Interaction Surface Component Experimental Station (PISCES)-RF despite evidence of RF window erosion in the source region, thereby motivating the present work. Experimentally, using various magnetic field configurations upstream of the PISCES-RF plasma source and seeding titanium (Ti) impurities at various axial locations, impurity transport and deposition along the machine axis were investigated. It was found that Ti deposition was localized to the side of the plasma source where the Ti impurity was seeded. In contrast, aluminum (Al) deposition, originating from the sputtering of the helicon window, occurred predominantly upstream of the plasma source, suggesting an asymmetry in the axial transport of eroded RF window material. These observations suggest a stagnation of the parallel plasma flow immediately downstream of the plasma source, with impurity ions remaining unmagnetized near the source upstream. Al deposition in magnetic field-free regions in PISCES-RF indicates that sputtered Al impurities likely remained neutral due to their large ionization mean-free path under PISCES-RF conditions. Plasma modeling and simulation supported this, indicating that Al-neutrals transport toward the helicon source upstream for low electron density cases. It was found that the Larmor radius of the Al ions was greater than the plasma radius towards the source upstream and remained weakly magnetized in PISCES-RF, meaning that plasma source-born impurities are not efficiently entrained in the plasma flow. These findings provide critical insights into impurity transport in helicon plasma-based LPDs.

Pulsed laser deposition and subsequent thermal treatment were used to create compact tungsten layers with various grain sizes ( 1 μ m , 100 nm, and 10 nm). The layers were self-damaged at room temperature up to 0.23 dpa using 20.3 MeV W ions and annealed in vacuum at 473 K or 673 K prior to exposure to deuterium plasma to populate the surviving displacement defects. Thermal desorption spectroscopy was used to measure deuterium retention and study the desorption profile. Rate-equation modeling of the desorption spectra was performed to study the behavior of individual traps at both annealing temperatures. After annealing the 1 μ m and 100 nm samples at 473 K, the deuterium retention was found to decrease by 24% and 9%, respectively, whereas the total deuterium retention in the 10 nm sample did not change. Annealing at 673 K reduced total deuterium retention by additional 27%, 14%, and 34% in the case of the samples with a grain size of 1 μ m , 100 nm, and 10 nm, respectively.

Abstract We reply to the comment by Li et al (submitted to Nucl. Fusion with this response) on our recent paper Nishijima et al (2023 Nucl. Fusion 63 126003). In this response, we address the existence of an incident ion energy, E i , threshold for the deuterium (D) supersaturated surface layer (DSSL) formation with a newly conducted D plasma exposure experiment at E i ∼ 20 eV. It is also further demonstrated, based on new experiments where the ion flux, fluence, and sample temperature are scanned, that non-kinetic (ballistic) processes play a role in the DSSL formation and growth. In addition, the effect of impurities in our plasma is discussed also with a new analysis of the surface composition made after a D plasma exposure.

The thermal transport and elastic properties of materials are often measured using the laser-induced transient grating spectroscopy (TGS) technique. The analysis of the TGS signal usually involves fitting well-known expressions, derived assuming the limiting cases of opaque or transparent materials, to the measured data. In this paper, the system of thermoelastic equations is analytically solved for an isotropic homogeneous material assuming finite laser penetration depth, which is an important consideration when the penetration depth is on the order of the acoustic wavelength. The need to use such a solution is discussed and compared to the expression for opaque material. The solution is benchmarked against TGS signal measured on single-crystal silicon with {100} surface orientation and is found to significantly improve the accuracy of the calculated thermal diffusivity as compared to using the expression for opaque material.

• Corrigendum: the upper limit on the condensable impurity fraction in Pisces-RF target plasma is corrected to 10 ppb, not 0.1 pp as stated in the manuscript.

Tungsten targets produced by the additive manufacturing (AM) method of electron-beam powder-bed fusion, or 3–D metal printing, are exposed to high flux D plasma in the Pisces-RF linear plasma device with the plasma-exposed surface normal to the AM build direction. D retention was measured by thermal desorption mass spectrometry following exposure to D plasma with an associated ∼50eV D+ ion flux. D+ fluence, and operational temperature, in the ranges 5×1024–5×1026 m−2 and 400–1000 K, are explored. D retention values for the AM W are compared to identically plasma exposed’conventional’ sintered W and it is found that total D retention is similar. However, the D thermal release is notably different. Desorption from the AM W shows reduced D retention in traps typical of sintered W, and moderately increased trapping in defect types of higher trap release energy. The dependence of D retention on fluence is also different for the AM W, revealing an uptake slower than expected from Fickian diffusion, while that for sintered W is consistent and in agreement with previous poly-crystaline W results from Pisces-B. Hydrogen transport modelling of the fluence dependence suggests that interconnected pathways for D release back to the surface during plasma-exposure can account for the slower D uptake in the AM W.

The influence of a fuzzy surface on the physical sputtering of Mo in He plasmas has been studied with hyperspectral imaging (HSI) measurements and simulations that couple the TRI3DYN code with an impurity transport code. The 2D profiles of the Mo I line emission intensity from HSI images reveal that the sputtering yield, Y, is reduced to ∼40% of the smooth-surface value due to the presence of a fuzz layer, while the angular distribution of the sputtered Mo atoms might not change significantly. The simulations reproduce the Y reduction successfully, but indicate that fuzz causes an increase in the small-angle distribution of sputtered atoms. However, the increase is too small to produce an observable change in the Mo I emission profiles. A simple analytical model that assumes a single collision mean free path for a fuzz layer and considers only the primary sputtering events qualitatively reproduces the Y reduction and the small-angle distribution enhancement, explaining the geometrical effect of fuzz on physical sputtering.

Abstract Fuzz formation on a heated tungsten surface in the presence of a helium-containing plasma and tungsten deposition source was investigated. Tungsten samples were exposed at 1123 K to pure helium plasma with ion incident energy of 76 eV, W/He ion flux ratio of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mn>0.4</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> , and varied helium ion fluence from 0.18 to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>3.4</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>26</mml:mn> </mml:mrow> </mml:msup> </mml:math> m −2 . Fuzz thickness was measured by cross-sectional scanning electron microscopy to increase from 0.22 to 15 µ m with increasing helium ion fluence. No indication of saturation in fuzz thickness at high fluence was observed, in contrast to fuzz produced on a tungsten surface without tungsten deposition. Additional tungsten samples were exposed at 1123 K to pure helium plasma with ion incident energy of 76 eV, helium ion fluence of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mn>3.4</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>26</mml:mn> </mml:mrow> </mml:msup> </mml:math> m −2 , and varied W/He ion flux ratio from 0.26 to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>3.0</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> . Fuzz thickness increased from 7.5 to 120 µ m with increasing W/He ion ratio. A final sample exposed at 1123 K to a mixed helium-deuterium plasma with ion incident energy of 76 eV, helium ion fluence of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.18</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>26</mml:mn> </mml:mrow> </mml:msup> </mml:math> m −2 , and W/He ion flux ratio of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.2</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> developed nearly identical fuzz structures to that developed in a pure He plasma. As a function of deposited tungsten fluence, all results were found to trace out a single layer-growth curve given by a power law relation, indicating that fuzz thickness is independent of the W/He ion flux ratio in the range investigated and independent of any deuterium present in the plasma. As a result, for tungsten plasma facing walls in magnetic fusion devices at 1000–2000 K with 10 −4 W/He ion flux ratio, fuzz with thicknesses greater than hundreds of microns may form in as little as 10 4 s (in the absence of ELM-induced erosion or annealing), and may more significantly affect its thermophysical properties than fuzz generated without a tungsten deposition source.

Abstract Properties of deuterium (D) supersaturated surface layers (DSSLs) formed in tungsten (W), such as thickness, internal microstructures, and D retention, are experimentally investigated as a function of the incident ion energy, E i . W samples were exposed to D plasmas in the PISCES-A linear plasma device in a range of E i ∼ 45–175 eV, while other plasma exposure parameters were fixed: sample temperature, T s , ∼423 K, ion flux, Γ i , ∼1.2 × 10 21 m −2 s −1 , and fluence, Φ i , ∼3.0 × 10 24 m −2 . High-resolution, cross-sectional, transmission electron microscopy observations confirm that (1) a DSSL forms even at the lowest E i ∼ 45 eV, (2) the DSSL thickness, Δ t DSSL , is found to decrease with decreasing E i from ∼11–12 nm at E i ∼ 175 eV to ∼5–6 nm at ∼45 eV, and to agree with approximately the maximum implantation depth calculated using SDTrimSP, and (3) high-density D nanobubbles with a diameter of ∼1 nm or less exist inside the DSSL, which is deemed to validate a theory-predicted vacancy stabilization process due to trapping of a solute D atom(s). Utilizing a D areal density of ∼4.2 × 10 19 m −2 in the first 14 nm from the surface at E i ∼ 75 eV from nuclear reaction analysis and the measured E i dependence of Δ t DSSL , our previous laser-induced breakdown spectroscopy data is updated: both dynamic and static D retention increase with decreasing E i , and the D/W atomic fraction during plasma exposure reaches ∼0.3 at E i ∼ 45 eV. A possible DSSL formation mechanism is proposed.

The helicon-plasma based-linear plasma-materials-interaction device, Pisces-RF is introduced. Steady-state plasma parameters in the target region are electron density up to ∼1019 m−3, electron temperature up to ∼10 eV, and ion flux up to ∼1023 m−2s−1 in D2 and He, with an associated maximum peak target heat flux of ∼1 MWm−2 and a few MWm−2 when accelerating ions with an electrical bias. Attached and detached target plasma regimes with both gaseous species can be achieved, as characterized by high and low Te (<1eV) plasmas with high fractions of singly ionized atomic ions. The downstream target plasma is inferred to have a low concentration of condensing or sticking impurity ions (< 0.1 ppb), as confirmed by accumulation experiments on electrically-floating targets in deposition-dominated low Te plasma conditions. The validation of Pisces-RF target PMI on W is confirmed by the observation of D retention values and W fuzz-layer thicknesses, obtained in D2 and He plasmas, that are in good agreement with prior literature. Lastly, the integration of Pisces-RF with a tandem ion-accelerator, to study the synergistic effects of simultaneous displacement damage and plasma irradiation, which we refer to as burning-plasma-material-interaction (BPMI), is discussed.

It has been demonstrated that the electron density, ne, and temperature, Te, are successfully evaluated from He I line intensity ratios coupled with machine learning (ML). In this paper, the ML-aided line intensity ratio technique is applied to deuterium (D) plasmas with 0.031 &amp;lt; ne (1018 m−3) &amp;lt; 0.67 and 2.3 &amp;lt; Te (eV) &amp;lt; 5.1 in the PISCES-A linear plasma device. Two line intensity ratios, Dα/Dγ and Dα/Dβ, are used to develop a predictive model for ne and Te separately. Reasonable agreement of both ne and Te with those from single Langmuir probe measurements is obtained at ne &amp;gt; 0.1 × 1018 m−3. Addition of the D2/Dα intensity ratio, where the D2 band emission intensity is integrated in a wavelength range of λ ∼ 557.4–643.0 nm, is found to improve the prediction of, in particular, ne, and Te. It is also confirmed that the technique works for D plasmas with 0.067 &amp;lt; ne (1018 m−3) &amp;lt; 6.1 and 0.8 &amp;lt; Te (eV) &amp;lt; 15 in another linear plasma device, PISCES-RF. The two training datasets from PISCES-A and PISCES-RF are combined, and unified predictive models for ne and Te give reasonable agreement with probe measurements in both devices.

Ballistic helium transport in tungsten during fuzz growth was investigated to provide insight into the helium-induced nanostructuring process which adversely affects the near-surface properties of tungsten plasma-facing components. An analytical model of helium ion transport in the vacuum region within the fuzz layer was developed, whereby He ions are assumed to move in straight-line trajectories with mean free paths determined from fuzz/nanotendril dimensions. Reflection of ions from tendril sides was considered, with He ions allowed to implant into the bulk at the base of the fuzz layer if they maintain ≥5eV of energy necessary to overcome the He–W surface barrier potential, resulting in an increase in the effective He mean free path. Using the model results for helium implantation in the bulk, a helium fluence-fuzz thickness relation was achieved, which matches well with experimental data in the literature, and implies that the fuzz growth rate is consistent with ballistic implantation into the bulk through the growing porous fuzz layer.

The annealing of ∼1 μm thick tungsten–deuterium co-deposit layers produced at 320 ± 25 K is found to induce changes in the desorptive release behavior and crystal structure. The changes in co-deposits were revealed by annealing in the temperature range 273–1273 K, examining the crystal structure with glancing angle X-ray diffraction, re-populating traps remaining following the anneal, by D plasma exposure at 373 K, then re-examining the co-deposits with glancing angle X-ray diffraction and scanning electron microscopy, before performing thermal desorption mass spectrometry to ascertain the effect of annealing on trap concentrations. The results indicate that low temperature W-D co-deposit layers are prone to the development of a significant fraction of deuterium retention sites, with a predominant thermal release that begins at ∼400 K and ceases approaching ∼1000 K. Increased annealing temperature is found to progressively depopulate and remove low temperature deuterium retention sites, as suggested by an inability to re-populate those sites with the deuterium plasma. Scanning electron micro-graphs show marked differences at the anneal temperature extremes, with surface conversion from a dendritic appearance at 320 K to a nodular appearance at 1273 K. X-ray diffraction reveals two W phases initially present as 320 K, namely the α-W BCC and β-W A15 crystal phases, the latter of which rapidly anneals out above 490 K leaving only increased Bragg reflection from the α-W phase at the higher annealing temperature.

Electric field profile structure—especially its shear—is a natural order parameter for the edge plasma, and characterizes confinement regimes ranging from the H-mode (Wagner et al. 1982 Phys. Rev. Lett. 49 , 1408–1412 ( doi:10.1103/PhysRevLett.49.1408 )) to the density limit (DL) (Greenwald et al. 1988 Nucl. Fusion 28 , 2199–2207 ( doi:10.1088/0029-5515/28/12/009 )). The theoretical developments and lessons learned during 40 years of H-mode studies (Connor &amp; Wilson 1999 Plasma Phys. Control. Fusion 42 , R1–R74 ( doi:10.1088/0741-3335/42/1/201 ); Wagner 2007 Plasma Phys. Control. Fusion 49 , B1–B33 ( doi:10.1088/0741-3335/49/12b/s01 )) are applied to the shear layer collapse paradigm (Hong et al. 2017 Nucl. Fusion 58 , 016041 ( doi:10.1088/1741-4326/aa9626 )) for the onset of DL phenomena. Results from recent experiments on edge shear layers and DL phenomenology are summarized and discussed in the light of L <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo stretchy="false">→</mml:mo> </mml:math> H transition physics. The theory of shear layer collapse is then developed. We demonstrate that shear layer physics captures both the well known current (Greenwald) scaling of the DL (Greenwald 2002 Plasma Phys. Control. Fusion 44 , R27–R53 ( doi:10.1088/0741-3335/44/8/201 ); Greenwald et al. 2014 Phys. Plasmas 21 , 110501 ( doi:10.1063/1.4901920 )), as well as the emerging power scaling (Zanca, Sattin, JET Contributors 2019 Nucl. Fusion 59 , 126011 ( doi:10.1088/1741-4326/ab3b31 )). The derivation of the power scaling theory exploits an existing model, originally developed for the L <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo stretchy="false">→</mml:mo> </mml:math> H transition (Diamond, Liang, Carreras, Terry 1994 Phys. Rev. Lett. 72 , 2565–2568 ( doi:10.1103/PhysRevLett.72.2565 ); Kim &amp; Diamond 2003 Phys. Rev. Lett. 90 , 185006 ( doi:10.1103/PhysRevLett.90.185006 )). We describe the enhanced particle transport events that occur following shear layer collapse. Open problems and future directions are discussed. This article is part of a discussion meeting issue ‘H-mode transition and pedestal studies in fusion plasmas’.