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DOE OSTI2020

Unipolar arc plasmas on nanostructured tungsten surfaces under perpendicular magnetic field

Abstract Unipolar arcing is an important phenomenon in terms of wall erosion and the subsequent plasma contamination in nuclear fusion devices. In this study, unipolar arc plasmas are experimentally explored under the presence of the external magnetic field perpendicular to a tungsten (W) sample on which a ‘fuzz’ nanostructure surface is formed. It is found that the fuzz layer thickness, d fuzz , plays a dominant role in determining the motion of arc spots, the arc ignition probability, and arc plasma parameters. High-speed imaging reveals that the motion of arc spots becomes more collective, as d fuzz increases. Interestingly, arc spots make a circle while moving outward. With increasing d fuzz , the amount of eroded W atoms increases, as indicated from an observed increase in the total visible emission intensity, while the arc ignition probability is found to drop. The physical mechanisms are discussed, in detail, based on spectroscopic measurements of the electron temperature, obtained from Boltzmann plots of both W I and W II, and the electron density, determined from Stark broadening of W I lines.

Physics↗

DOE OSTI2025

Chemical Vapor Deposition-Grown Hexagonal Boron Nitride and Graphene for Tandem Dielectric Capacitive Polymer Devices

To address the low energy density of polymeric capacitors, this work explores how tandem combinations of antagonistic properties can enhance energy storage. We introduce multilayer architecture integrating chemical vapor deposition (CVD)-grown ≈2 nm hexagonal boron nitride (hBN) and graphene with ferroelectric polyvinylidene fluoride (PVDF) and linear polyetherimide (PEI) layers to suppress leakage paths, reduce loss, and promote Maxwell–Wagner–Sillars polarization. Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and plasma focused ion beam (PFIB) analyses confirm robust material integration. In conclusion, for the tandem device, E BD ≈600 V/μm, while individual layers show lower values (PVDF|Graphene (Gr)|PVDF ≈ 50 V/μm and PEI|hBN|PEI ≈ 275 V/μm), yielding an overall ≈6000% enhancement and demonstrating the effectiveness of the 2D multilayer design.

Chemical vapor deposition (CVD)↗

DOE OSTI2022

Tungsten doped diamond shells for record neutron yield inertial confinement fusion experiments at the National Ignition Facility

We report on fabrication and characterization of layered, tungsten doped, spherical about 2 mm diameter microcrystalline diamond ablator shells for inertial confinement fusion (ICF) experiments at the National Ignition Facility. As previously reported, diamond ICF ablator shells can be fabricated by chemical vapor deposition (CVD) on solid spherical silicon mandrels using an ellipsoidal microwave plasma reactor. In the present work, we further developed these ablator shells by embedding a W -doped diamond layer sandwiched between two undoped diamond regions. W incorporation in diamond was achieved by adding tungsten hexacarbonyl to the CH 4 /H 2 CVD feed gas. We observe that the W doping concentration decreases with increasing deposition rate which, in turn, is controlled by adjusting the total gas pressure. Cross sectional microstructural analysis reveals sharp interfaces between doped and undoped regions of the diamond shell and uniform W distribution with concentrations up to about 0.3 at.%. At higher W concentrations (>0.3 at.%) formation of tungsten carbide precipitates is observed. Using a 3-shock 1.6 MJ laser pulse, the targets described in this work produced the first laser driven implosion to break the 1 × 10 16 neutron yield barrier, followed by experiments (described in future publications) with similar targets and slightly more laser energy producing yields as high as 4 × 10 17 .

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Zonal shear layer collapse and the power scaling of the density limit: old L-H wine in new bottles

Edge shear layer collapse causes edge cooling and aggravates radiative effects. This paper details on the microscopic dynamics of the emergence of power (Q) scaling of density limit (DL) from the shear layer collapse transport bifurcation scenario. The analysis is based on a novel 4-field model, which evolves turbulence energy, zonal flow energy, temperature gradient and density, including the neoclassical screening of zonal flow response. Bifurcation analysis yields power scaling of critical density for shear layer collapse as $n_{crit}\sim Q^{1/3}$. The favorable Q scaling of the DL emerges from the fact that the shear layer strength increases with Q, thus preventing shear layer collapse. This in turn reduces particle transport and improves particle confinement. RMP induced ambient stochastic fields degrade the shear layer by inducing decoherence in the Reynolds stress. As a result the particle transport increases and particle confinement degrades. This leads to the emergence of unfavorable stochastic field intensity ($b_{st}^{2}$) scaling of the critical density as $n_{crit}\sim(1+b_{st}^{2})^{-5/3}$. All fields, including zonal flow shear, exhibit hysteresis when the power (Q) is ramped cyclically across the bifurcation point. The hysteresis is due to dynamical delay in bifurcation on account of critical slowing down. Furthermore, the dynamical hysteresis here is fundamentally different from the hysteresis associated with the existence of bi-stable states.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

PISCES-RF: a liquid-cooled high-power steady-state helicon plasma device

Radio-frequency (RF) driven helicon plasma sources can produce relatively high-density plasmas (n > 1019 m -3 ) at relatively moderate powers (<2 kW) in argon. However, to produce similar high-density plasmas for fusion relevant gases such as hydrogen (H), deuterium (D) and helium (He), much higher RF powers are needed. For very high RF powers, thermal issues of the RF-transparent dielectric window, used in the RF source design, limit the plasma operation timescales. To mitigate this constraint, we have designed, built and tested a novel helicon plasma source assembly with a fully liquid-cooled RF-transparent window which allows steady state operations at high power (up to 20 kW) and successfully produces high-density plasma with both argon and H. Deionized (DI) water, flowing between two concentric dielectric RF windows, is used as the coolant. We show that a full azimuthal blanket of DI water does not prevent high-density plasma production. From calorimetry on the DI water, we measure the net heat removed by the coolant at steady state conditions. Using infra-red imaging, we calculate the constant plasma heat deposition and measure the final steady state temperature distribution patterns on the inner surface of the ceramic layer. The heat deposition pattern follows the helical shape of the antenna. We also show the consistency between the heat absorbed by the DI water, as measured by calorimetry, and the total heat due to the combined effect of the plasma heating and the absorbed RF. These results are being used to answer critical engineering questions for the 200 kW RF device materials plasma exposure experiment being designed at the Oak Ridge National Laboratory as a next generation plasma material interaction device.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2023

Atomic layer etching (ALE) of III-nitrides

Atomic layer etching (ALE) was performed on (Al, In, Ga)N thin films using a cyclic process of alternating Cl 2 gas absorption and Ar + ion bombardment in an inductively coupled plasma etcher system. The etch damage was characterized by comparing photoluminescence of blue single quantum well light-emitting diodes before and after the etch as well as bulk resistivities of etched p-doped layers. It was found that etched surfaces were smooth and highly conformal, retaining the step-terrace features of the as-grown surface, thus realizing ALE. Longer exposures to the dry etching increased the bulk resistivity of etched surfaces layers slightly, with a damaged depth of ∼55 nm. With further optimization and damage recovery, ALE is a promising candidate for controlled etching with atomic accuracy. Finally, it was found that Al 0.1 Ga 0.9 N acts as an etch barrier for the ALE etch, making it a suitable etch to reveal buried V-defects in III-nitride light emitting diodes.

Atomic-layer etching↗

DOE OSTI2024

Optimizing Aluminum Bonding: Exploring Surface Roughness and Contact Angle Effects through Plasma and Acid Etching Contrasts

The use of adhesives to replace traditional techniques like welding and riveting has been on the rise in various industries as increasingly complex geometries and situations for bonding is required. It’s well known in the scientific community that an adhesive bond is only as efficient as the surface preparation used beforehand. Plasma treatments have been shown to gently clean the surface by only removing the top-most layer from the adherend and “charging” the surface instead of berating it. Whereas chemical treatments “etch” the surface in hopes of increasing surface area by increasing surface roughness. This method can be difficult to control quantitatively and can generate waste leading to serious environmental concerns. This study aims to address the encountered differences between plasma treatments and acid etching on aluminum substrates for adhesive bonding. Treating the surface of adherends plays a crucial role in promoting a good bond between adhesive and adherend. In this study, the effects of plasma and acid treatments are categorized using surface roughness measurements as well as contact angle which have been shown to correlate to mechanical strength. When evaluating different surface treatments, it is notable to mention that not only surface roughness and contact angle play a role in determining which method is most effective but also the consequences of the treatment itself. It was found that although chemical etching was the most effective in increasing roughness and decreasing contact angle, it generates a large amount of waste and is not sustainable compared to plasma work which uses a renewable process. The results from this work provide a deeper understanding of the relationship between surface treatments and surface roughness/contact angle and how this relates to a stronger adhesive bond.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Optimizing Aluminum Bonding: Exploring Surface Roughness and Contact Angle Effects through Plasma and Acid Etching Contrasts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

The role of RF-induced E×B flows in the mitigation of scrape-off-layer convective transport during ion cyclotron resonance heating

Abstract While multiple experiments have reported a decrease in intermittent fluctuations in the far Scrape-Off-Layer (SOL) during ion cyclotron resonance heating (Antar et al 2010 Phys. Rev. Lett. 105 165001, Li et al 2022 Nucl. Eng. Technol. 54 207–19, Antar et al 2012 Nucl. Fusion 52 103005), the physical mechanism behind this observation has not been fully established yet. In this work, we demonstrate, for the first time, a direct correlation between the amplitude of RF-induced E×B flows and turbulence suppression in the far SOL. Using the Gas Puff Imaging (GPI) diagnostic on Alcator C-Mod, we show again that Ion Cyclotron Range of Frequencies can significantly alter the flow in the SOL and introduce a shear layer in regions magnetically connected close to the antenna (Cziegler et al 2012 Plasma Phys. Control. Fusion 54 105019). With the 4-strap field-aligned antenna operated in dipole phasing, the ratio of the power coupled by the central two straps to the power coupled by the outer two straps was varied. The resulting RF-induced radial electric field magnitude thus varied, and we show that the impact on the far SOL turbulence correlates with the modified E×B velocity. We then apply a newly-developed blob tracking algorithm (Han et al 2022 Sci. Rep. 12 18142) to higher-resolution GPI videos in order to directly observe the process of blob shearing by RF-induced E×B flows. We show that the radially sheared poloidal flows act as a transport barrier by stretching, stopping, and destroying filaments, which is consistent with the observed difference in turbulence statistics.

Diab, R. (ORCID:0000000342065868)↗

DOE OSTI2024

Permeation Properties of Disordered Metal-Organic Framework Membranes Made by Vapor Phase Ligand Treatment

Zeolitic imidazolate frameworks (ZIFs) are nanoporous molecular sieves offering exciting opportunities for membrane- and adsorption-based gas separations. The Tsapatsis group developed an all-vapor-phase ligand induced permselectivation (LIPS) method for the fabrication of ZIF nanocomposite membranes. The LIPS method consists of a combination of atomic layer deposition of a dense oxide inside the mesopores of a porous support followed by transformation of the dense oxide deposit to nanoporous ZIF by exposure to sublimated vapors of an imidazolate ligand. It enables the formation of thin nanocomposite films consisting of ZIF deposited inside pores with diameters smaller than 10 nm. Vapor-phase conversion of the impermeable ZnO to ZIF increases flux of certain gases more than others. Vapor phase linker/ligand treatment (VPLT) can further modify the properties of LIPS and other membranes. The combination of LIPS with VPLT provides an all-vapor methodology for making and modifying ZIF membranes with the ability to control the ZIF deposit composition and the level of confinement in a variety of mesoporous supports. The propylene/propane separation performance of LIPS/VPLT ZIF membranes confined (entirely or partially) inside mesopores is remarkably superior to that of most conventional ZIF membranes consisting of unconfined (deposited on the external surface of porous supports) polycrystalline films. The accomplishments of this award are: (i) understanding of the LIPS process using mathematical models of atomic layer deposition (ALD), (ii) development of surface modification methods (vapor, e-beam, plasma, and X-ray treatments) to tune permeation properties, (iii) microstructural control of ZIF-8 films using ALD (support modification, direct ALD, and by use of in situ permeability monitoring), (iv) molecular simulations, and (v) thin film characterization and adsorption measurements using spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Field Emission Properties of Cu-Filled Vertically Aligned Carbon Nanotubes Grown Directly on Thin Cu Foils

Copper-filled vertically aligned carbon nanotubes (Cu@VACNTs) were grown directly on Cu foil substrates of 0.1 mm thicknesses at different temperatures via plasma-enhanced chemical vapor deposition (PECVD). By circumventing the need for additional catalyst layers or intensive substrate treatments, our in-situ technique offers a simplified and potentially scalable route for fabricating Cu@VACNTs with enhanced electrical and thermal properties on thin Cu foils. Comprehensive analysis using field emission scanning microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) mappings, and X-ray diffraction (XRD) revealed uniform Cu filling within the VACNTs across a range of synthesis temperatures (650 °C, 700 °C, and 760 °C). Field emission (FE) measurements of the sample synthesized at 700 °C (S700) showed low turn-on and threshold fields of 2.33 V/μm and 3.29 V/μm, respectively. The findings demonstrate the viability of thin Cu substrates in creating dense and highly conductive Cu-filled VACNT arrays for advanced electronic and nanoelectronics applications.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Optimization of simulated high-field side lower hybrid current drive coupling using machine learning predictions of scrape-off layer density

Lower hybrid current drive (LHCD) is a potential source of non-inductive off-axis current drive (CD) for tokamaks. Although LHCD has been successfully deployed on a number of tokamaks, it is highly sensitive to the scrape-off layer (SOL) conditions local to the LHCD launcher. Large gaps between the launcher and plasma core, SOL turbulence, or edge density perturbations due to edge-localized modes can hamper CD or cause large reflected power. These coupling issues in part motivated the installation of an LHCD launcher on the high-field side (HFS) of DIII-D. On the HFS, the SOL is less turbulent and more controllable compared to the low-field side. This quiescence may result in more predictable edge conditions and thus a more predictable CD. Here, in this work, HFS SOL reflectometry measurements are predicted from global plasma parameters using machine learning models. The SOL predictions coupled with the full-wave simulation of the LHCD launcher allow for the prediction of reflected power, directivity, and arcing risk before the discharge. Launcher performance is then optimized using multi-objective Bayesian optimization, finding the shot parameters that result in an optimal SOL density that maximizes CD while minimizing the risk of arcing. The predictions and optimizations of LHCD performance are then accelerated using a surrogate model of the full-wave LHCD simulation.

Bayesian optimization↗

DOE OSTI2024

Radiatively cooled magnetic reconnection experiments driven by pulsed power

We present evidence for strong radiative cooling in a pulsed-power-driven magnetic reconnection experiment. Two aluminum exploding wire arrays, driven by a 20 MA peak current, 300 ns rise time pulse from the Z machine (Sandia National Laboratories), generate strongly driven plasma flows (MA≈7) with anti-parallel magnetic fields, which form a reconnection layer (SL≈120) at the mid-plane. The net cooling rate far exceeds the Alfvénic transit rate (τcool−1/τA−1≫1), leading to strong cooling of the reconnection layer. We determine the advected magnetic field and flow velocity using inductive probes positioned in the inflow to the layer, and inflow ion density and temperature from analysis of visible emission spectroscopy. A sharp decrease in x-ray emission from the reconnection layer, measured using filtered diodes and time-gated x-ray imaging, provides evidence for strong cooling of the reconnection layer after its initial formation. X-ray images also show localized hotspots, regions of strong x-ray emission, with velocities comparable to the expected outflow velocity from the reconnection layer. These hotspots are consistent with plasmoids observed in 3D radiative resistive magnetohydrodynamic simulations of the experiment. X-ray spectroscopy further indicates that the hotspots have a temperature (170 eV) much higher than the bulk layer (≤75 eV) and inflow temperatures (about 2 eV) and that these hotspots generate the majority of the high-energy (>1 keV) emission.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2020

Etching of Si 3 N 4 induced by electron beam plasma from hollow cathode plasma in a downstream reactive environment

An etching system based on the interaction of electrons extracted from a direct current (DC) hollow-cathode (HC) Ar plasma and injected towards a Si3N4 covered silicon substrate located in the downstream reactive environment created by an additional remote CF 4 /O 2 plasma source was developed and evaluated. By controlling the properties of the injected beam electrons, this approach allows to deliver energy to a surface functionalized by exposure to reactive species, and initiate surface etching. The energy of the primary beam electrons is controlled by the acceleration voltage relative to the HC discharge. Ar atoms flow from the high-pressure HC discharge into the low pressure downstream reactive environment in the process chamber. For an acceleration voltage greater than the ionization potential of Ar and/or process gas species, the energetic primary beam electrons produce a secondary plasma in the process chamber, and can also cause additional dissociation. We have characterized the properties of the secondary plasma and also surface etching of Si 3 N 4 as a function of process parameters, including acceleration voltage (0-80 V), discharge current of the HC discharge (1-2 A), pressure (3.5-20 mTorr), source to substrate distance (1.5 to 5 cm), and feed gas composition (20% and 80% O2 in CF 4 /O 2 ). The electron energy probability function (EEPF) measured with a Langmuir probe about 2.5 cm below the extraction ring suggests several major groups of electrons for this situation, including high energy primary beam electrons with an energy that varies as the acceleration voltage is changed and low energy electrons produced by beam electron-induced ionization of the Ar gas in the process chamber. When a remote CF 4 /O 2 plasma is additionally coupled to the process chamber, Si 3 N 4 surfaces can be functionalized, and by varying the energy of the beam electrons, Si 3 N 4 etching can be induced by electron-neutral synergy effect with plasma-surface interaction. For conditions without beam electron injection, the remote plasma etching rate (ER) of Si 3 N 4 depends strongly on the O 2 concentration in the CF 4 /O 2 processing gas mixture and can be suppressed for O 2 -rich process conditions by the formation of a SiONF passivation layer on the Si 3 N 4 surface. The combination of the HC electron beam source with the remote plasma source (HCEB-RP) makes it possible to induce Si 3 N 4 etching for O 2 -rich remote plasma conditions where remote plasma by itself produces negligible Si 3 N 4 etching. The electron enhanced etching of Si 3 N 4 depends strongly on O 2 /CF 4 mixing ratio reflecting changing arrival rates of O and F species at the surface. Optical emission spectroscopy was used to estimate the ratio of gas phase F and O densities and found to be controlled by the gas mixing ratio and independent of HC EB operating conditions. At this time the detailed sequence of events operative in the etching mechanism is unclear. While the increase of the electron energy is ultimately responsible for initiating surface etching, at present we cannot rule out a role of ions from the simultaneously produced secondary plasma in plasma surface interaction mechanisms.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2024

The effect of plasma shaping on high density H-mode SOL profiles and fluctuations in TCV

Abstract The impact of plasma shaping on the properties of high density H-mode scrape-off layer (SOL) profiles and transport at the outer midplane has been investigated on Tokamakà configuration variable. The experimental dataset has been acquired by evolving the upper triangularity while keeping the other parameters constant. The scan comprises δ u p values between 0.0 and 0.6, excluding negative triangularity scenarios. Within this study, a transition from type-I edge localised modes to the quasi-continuous exhaust regime takes place from low to high δ u p . The modification of the upstream SOL profiles has been assessed, in terms of separatrix quantities, within the α t turbulence control parameter theoretical framework (Eich et al 2020 Nucl. Fusion 60 056016). The target parallel heat load and the upstream near-SOL density profiles have been shown to broaden significantly for increasing α t . Correspondingly, in the far SOL a density shoulder formation is observed when moving from low to high δ u p . These behaviours have been correlated with an enhancement of the SOL fluctuation level, as registered by wall-mounted Langmuir probes as well as the thermal helium beam diagnostic. Specifically, both the background and the filamentary-induced fluctuating parts of the first wall ion saturation current signal are larger at higher δ u p , with filaments being ejected more frequently into the SOL. Comparison of two pulses at the extremes of the δ u p scan range, but with otherwise same input parameters, shows that the midplane neutral pressure does not change much during the H-mode phase of the discharge. This indicates that indirect effects of the change in geometry, linked to first wall recycling sources, should not play a significant role. The total core radiation increases at high δ u p , on account of a stronger plasma–wall interaction and resulting larger carbon impurity intake from the first wall. This is likely associated to the enhanced first wall fluctuations, as well as a smaller outer gap and the close-to-double-null magnetic topology at high shaping.

Stagni, A. (ORCID:0000000180841544)↗

DOE OSTI2020

Development of a tomographic reconstruction method for axisymmetric D α emission profiles in the ITER plasma boundary

A tomographic method for reconstructing the axially symmetric D α emission profile in the ITER scrape-off layer and divertor from D α intensity measurements in the fields of view (FoV) of Vis/IR TV and Divertor Impurity Monitor diagnostics is under development. The method takes into account the strong background signal due to stray light from divertor produced by reflections from the metal wall of the vacuum chamber. The method allows to filter out the reflections in D α signals measured in the main chamber. Here, the accuracy of this method is tested within the framework of a synthetic diagnostic, which uses the Raysect and Cherab numerical codes for ray-tracing simulations. The D α emission profiles, simulated with the SOLPS and OEDGE codes, are used as synthetic experimental data. Uncertainty in light reflection properties of the first wall (FW) is the major factor affecting the accuracy of the D α emission reconstruction. The dependence on the uncertainty in the FW surface roughness and on the ratio of diffusive to specular reflection of the error in recovering useful D α signals (the direct light without reflections) in the FoV of the ITER main chamber H-alpha and Visible Spectroscopy Diagnostic is obtained.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Analysis of synthetic electron cyclotron emission from the high field side of HL-2M tokamak plasmas

A synthetic electron cyclotron emission (ECE) diagnostic is used to interpret ECE signals from preset plasma equilibrium profiles, including magnetic field, electron density, and electron temperature. According to the simulation results, the electron temperature (T e ) profile covering the harmonic overlap region can be obtained by receiving ECE signals at the high field side (HFS) of the HL-2M plasma. The third harmonic ECE at the low field side (LFS) cannot pass through the second harmonic resonance layer at the HFS unless the optical thickness (τ) of the second harmonic becomes gray (τ ≤ 2). In addition, the impact of the relativistic frequency down-shift has been evaluated and corrected. The measurable range of the HFS ECE has been calculated by scanning different parameters (electron density, temperature, and magnetic field). Higher plasma parameters allow a wider radial range of electron temperature measurements. The minimum inner measurable position can reach R = 120 cm (r/a = -0.89) when the product of core temperature (T e0 ) and density (n e0 ) is greater than 35 × 10 19 keV m -3 , which is extended by more than 30 cm inward compared with that of the LFS measurement. The HFS ECE will greatly improve the diagnostic ability of ECE systems on the HL-2M tokamak.

47 OTHER INSTRUMENTATION↗

DOE OSTI2021

Physics-Based Optical Neuromorphic Classification

Typical approaches to classify scenes from light convert the light field to electrons to perform the computation in the digital electronic domain. This conversion and downstream computational analysis require significant power and time. Diffractive neural networks have recently emerged as unique systems to classify optical fields at lower energy and high speeds. Previous work has shown that a single layer of diffractive metamaterial can achieve high performance on classification tasks. In analogy with electronic neural networks, it is anticipated that multilayer diffractive systems would provide better performance, but the fundamental reasons for the potential improvement have not been established. In this work, we present extensive computational simulations of two - layer diffractive neural networks and show that they can achieve high performance with fewer diffractive features than single layer systems.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗