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DOE OSTI2025

Erosion of high-Z refractory coatings for helicon plasma source window

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.

RF bias↗

DOE OSTI2011

Final Report for "SciDAC Center for Simulation of Wave-Plasma Interactions (CSWPI)"

This project has provided for the participation of the PI (Dr. David Smithe) and other Tech-X Corporation personnel in the larger CSWPI (Center for the Study of Wave Plasma Interactions) SciDAC project. In addition to collaborative efforts in several areas, the PI's thrust research area has focused on the tokamak edge, in the vicinity of the antenna structure, and on the specific physics of interest to the edge, including RF sheath formation. This interest is being driven by a desire to understand measured anomalous power losses that are associated with the edge region. Realistic true 3D geometry simulation capability of the operating antenna, immersed in the edge plasma, has been developed. Geometry accuracy is at the same degree as CAD drawings themselves. In addition, the plasma model includes an RF sheath potential calculation for cells where the plasma is in contact with the metallic structure of the antenna. This capability, in a time-domain simulation, is presently unique. This model can be used to immediately estimate local sheaths (and possibly sputtering damage caused by them), and will be used in the future to model long-range sheaths. Modeling on this scale, in 3D, clearly requires very large scale parallel processing and advanced computing. Ultimately, the desire is to provide quantitative prediction of parasitic edge loss, and edge plasma behavior, analogous to the quantitative core models that have long been a staple of the RF modeling regime.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2017

Construction Guide to Next-Generation High-Performance Walls in Climate Zones 3-5 - Part 2: 2x4 Walls

Part 2 of this Construction Guide to High-Performance Walls in Climate Zones 3-5 provides straightforward and cost-effective strategies to construct durable, energy-efficient walls. It addresses walls constructed with 2x4 wood frame studs, wood structural panel (WSP) sheathing as wall bracing and added backing for foam sheathing, a layer of rigid foam sheathing insulation up to 1.5 inches thick over the WSP, and a cladding system installed over the foam sheathing in low-rise residential buildings up to three stories high. Walls with 2x6 framing are addressed in Part 1 of the Guide.

2x4 wood frame studs↗

DOE OSTI2017

Construction Guide to Next-Generation High-Performance Walls in Climate Zones 3-5 - Part 2: 2x4 Walls

2x4 wood frame studs↗

DOE OSTI2016

Electrostatic analyzer measurements of ionospheric thermal ion populations

Here, we define the observational parameter regime necessary for observing low-altitude ionospheric origins of high-latitude ion upflow/outflow. We present measurement challenges and identify a new analysis technique which mitigates these impediments. To probe the initiation of auroral ion upflow, it is necessary to examine the thermal ion population at 200-350 km, where typical thermal energies are tenths of eV. Interpretation of the thermal ion distribution function measurement requires removal of payload sheath and ram effects. We use a 3-D Maxwellian model to quantify how observed ionospheric parameters such as density, temperature, and flows affect in situ measurements of the thermal ion distribution function. We define the viable acceptance window of a typical top-hat electrostatic analyzer in this regime and show that the instrument's energy resolution prohibits it from directly observing the shape of the particle spectra. To extract detailed information about measured particle population, we define two intermediate parameters from the measured distribution function, then use a Maxwellian model to replicate possible measured parameters for comparison to the data. Liouville's theorem and the thin-sheath approximation allow us to couple the measured and modeled intermediate parameters such that measurements inside the sheath provide information about plasma outside the sheath. We apply this technique to sounding rocket data to show that careful windowing of the data and Maxwellian models allows for extraction of the best choice of geophysical parameters. More widespread use of this analysis technique will help our community expand its observational database of the seed regions of ionospheric outflows.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2019

A Very Unlucky Intraoperative Event During a Chimney Technique for Juxtarenal Aneurysm Exclusion

A two-vessel Ch-EVAR procedure was scheduled within the instructions for use of the devices. After deploying, it appeared that the long sheath inserted through an arm access was discovered to be trapped behind the aortic stentgraft barbs. Trying to remove the sheath, the graft was displaced upward and the procedure was converted for a three-vessel Ch-EVAR incorporating the superior mesenteric artery. Simultaneous ballooning of the target vessels and the stentgraft permitted to exert a strong pullback traction on the blocked sheath, allowing its removal. A segment of the sheath distal end was left trapped in the aortic stentgraft. At 12 months follow-up, the patient is free of any complications and there are no occlusions of the abdominal visceral branches or embolic events in peripheral arteries.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2011

The mechanism of DNA ejection in the Bacillus anthracis spore-binding phage 8a revealed by cryo-electron tomography

The structure of the Bacillus anthracis spore-binding phage 8a was determined by cryo-electron tomography. The phage capsid forms a T = 16 icosahedron attached to a contractile tail via a head-tail connector protein. The tail consists of a six-start helical sheath surrounding a central tail tube, and a structurally novel baseplate at the distal end of the tail that recognizes and attaches to host cells. The parameters of the icosahedral capsid lattice and the helical tail sheath suggest protein folds for the capsid and tail-sheath proteins, respectively, and indicate evolutionary relationships to other dsDNA viruses. Analysis of 2518 intact phage particles show four distinct conformations that likely correspond to four sequential states of the DNA ejection process during infection. Comparison of the four observed conformations suggests a mechanism for DNA ejection, including the molecular basis underlying coordination of tail sheath contraction and genome release from the capsid.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2017

Electromagnetic simulations of JET ICRF ITER-like antenna with TOPICA and SSWICH asymptotic codes

Multi-megawatt Ion Cyclotron Range of Frequencies (ICRF) heating is routinely used in the JET tokamak. To increase the ICRF heating power available from the A2 antennas, the ICRF ITER-Like Antenna (ILA) was reinstalled for the 2015 JET ITER-like wall experimental campaign. The application of high levels of ICRF power typically results in increased plasma wall interaction which leads to the observation of enhanced influx of metallic impurities in the plasma edge. It is assumed that the impurity production is mainly driven by the parallel component of the Radio-Frequency (RF) antenna electric near-field, E// (parallel to the confinement magnetic field of the tokamak), that is rectified in a thin boundary layer (RF sheath). Torino Polytechnic Ion Cyclotron Antenna (TOPICA) code was used to compute E // field maps in front of the ILA and between its poloidal limiters in the presence of plasma using measured density profiles and various antenna feedings. E // field maps calculated between the poloidal limiters were used to estimate the poloidal distribution of RF-sheath Direct Current (DC) potential in this private region of the ILA and make relative comparison of various antenna electrical settings. For this purpose we used the asymptotic version of the Self-consistent Sheaths and Waves for Ion Cyclotron Heating Slow-Wave (SSWICH-SW) code. These estimations can help to study the formation of RF sheaths around the antenna and even at distant locations (~3m away).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2014

Effect of electron temperature anisotropy on near-wall conductivity in Hall thrusters

The electron velocity distribution in Hall thrusters is anisotropic, which not only makes the sheath oscillate in time, but also causes the sheath to oscillate in space under the condition of low electron temperatures. The spatial oscillation sheath has a significant effect on near-wall transport current. In this Letter, the method of particle-in-cell (2D + 3 V) was adopted to simulate the effect of anisotropic electron temperatures on near-wall conductivity in a Hall thruster. Results show that the electron-wall collision frequency is within the same order in magnitude for both anisotropic and isotropic electron temperatures. The near-wall transport current produced by collisions between the electrons and the walls is much smaller than experimental measurements. However, under the condition of anisotropic electron temperatures, the non-collision transport current produced by slow electrons which reflected by the spatial oscillation sheath is much larger and closes to measurements.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2014

Response of the plasma to the size of an anode electrode biased near the plasma potential

As the size of a positively biased electrode increases, the nature of the interface formed between the electrode and the host plasma undergoes a transition from an electron-rich structure (electron sheath) to an intermediate structure containing both ion and electron rich regions (double layer) and ultimately forms an electron-depleted structure (ion sheath). In this study, measurements are performed to further test how the size of an electron-collecting electrode impacts the plasma discharge the electrode is immersed in. This is accomplished using a segmented disk electrode in which individual segments are individually biased to change the effective surface area of the anode. Measurements of bulk plasma parameters such as the collected current density, plasma potential, electron density, electron temperature and optical emission are made as both the size and the bias placed on the electrode are varied. Abrupt transitions in the plasma parameters resulting from changing the electrode surface area are identified in both argon and helium discharges and are compared to the interface transitions predicted by global current balance [S. D. Baalrud, N. Hershkowitz, and B. Longmier, Phys. Plasmas 14, 042109 (2007)]. While the size-dependent transitions in argon agree, the size-dependent transitions observed in helium systematically occur at lower electrode sizes than those nominally derived from prediction. The discrepancy in helium is anticipated to be caused by the finite size of the interface that increases the effective area offered to the plasma for electron loss to the electrode.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2014

Three dimensional simulations of plasma filaments in the scrape off layer: A comparison with models of reduced dimensionality

This paper presents simulations of isolated 3D filaments in a slab geometry obtained using a newly developed 3D reduced fluid code, written using the BOUT++ framework. First, systematic scans were performed to investigate how the dynamics of a filament are affected by its amplitude, perpendicular size, and parallel extent. The perpendicular size of the filament was found to have a strong influence on its motions, as it determined the relative importance of parallel currents to polarization and viscous currents, whilst drift-wave instabilities were observed if the initial amplitude of the blob was increased sufficiently. Next, the 3D simulations were compared to 2D simulations using different parallel closures; namely, the sheath dissipation closure, which neglects parallel gradients, and the vorticity advection closure, which neglects the influence of parallel currents. The vorticity advection closure was found to not replicate the 3D perpendicular dynamics and overestimated the initial radial acceleration of all the filaments studied. In contrast, a more satisfactory comparison with the sheath dissipation closure was obtained, even in the presence of significant parallel gradients, where the closure is no longer valid. Specifically, it captured the contrasting dynamics of filaments with different perpendicular sizes that were observed in the 3D simulations which the vorticity advection closure failed to replicate. However, neither closure successfully replicated the Boltzmann spinning effects nor the associated poloidal drift of the blob that was observed in the 3D simulations. Although the sheath dissipation closure was concluded to be more successful in replicating the 3D dynamics, it is emphasized that the vorticity advection closure may still be relevant for situations where the parallel current is inhibited from closing through the sheath due to effects such as strong magnetic shear around X points or increased resistivity near the targets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2015

Effects of wall electrodes on Hall effect thruster plasma

This paper investigates the physical mechanisms that cause beneficial and detrimental performance effect observed to date in Hall effect thrusters with wall electrodes. It is determined that the wall electrode sheath can reduce ion losses to the wall if positioned near the anode (outside the dense region of the plasma) such that an ion-repelling sheath is able to form. The ability of the wall electrode to form an ion-repelling sheath is inversely proportional to the current drawn—if the wall electrode becomes the dominant sink for the thruster discharge current, increases in wall electrode bias result in increased local plasma potential rather than an ion-repelling sheath. A single-fluid electron flow model gives results that mimic the observed potential structures and the current-sharing fractions between the anode and wall electrodes, showing that potential gradients in the presheath and bulk plasma come at the expense of current draw to the wall electrodes. Secondary electron emission from the wall electrodes (or lack thereof) is inferred to have a larger effect if the electrodes are positioned near the exit plane than if positioned near the anode, due to the difference in energy deposition from the plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2015

The limits of the Bohm criterion in collisional plasmas

The sheath formation within a low-pressure collisional plasma is analysed by means of a two-fluid model. The Bohm criterion takes into account the effects of the electric field and the inertia of the ions. Numerical results yield that these effects contribute to the space charge formation, only, if the collisionality is lower than a relatively small threshold. It follows that a lower and an upper limit of the drift speed of the ions exist where the effects treated by Bohm can form a sheath. This interval becomes narrower as the collisionality increases and vanishes at the mentioned threshold. Above the threshold, the sheath is mainly created by collisions and the ionisation. Under these conditions, the sheath formation cannot be described by means of Bohm like criteria. In a few references, a so-called upper limit of the Bohm criterion is stated for collisional plasmas where the momentum equation of the ions is taken into account, only. However, the present paper shows that this limit results in an unrealistically steep increase of the space charge density towards the wall, and, therefore, it yields no useful limit of the Bohm velocity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2015

Non-invasive probe diagnostic method for electron temperature and ion current density in atmospheric pressure plasma jet source

The electrical probe diagnostics are very hard to be applied to atmospheric plasmas due to severe perturbation by the electrical probes. To overcome this, the probe for measuring electron temperature and ion current density is indirectly contacted with an atmospheric jet source. The plasma parameters are obtained by using floating harmonic analysis. The probe is mounted on the quartz tube that surrounds plasma. When a sinusoidal voltage is applied to a probe contacting on a quartz tube, the electrons near the sheath at dielectric tube are collected and the probe current has harmonic components due to probe sheath nonlinearity. From the relation of the harmonic currents and amplitude of the sheath voltage, the electron temperature near the wall can be obtained with collisional sheath model. The electron temperatures and ion current densities measured at the discharge region are in the ranges of 2.7–3.4 eV and 1.7–5.2 mA/cm{sup 2} at various flow rates and input powers.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2017

A nonlinear electromagnetics model of an asymmetrically-driven, low pressure capacitive discharge

It is well-known that standing waves having radially center-high voltage profiles exist in high frequency driven capacitive discharges. Capacitive sheaths can also nonlinearly excite driving frequency harmonics near the series resonance that can be spatially near-resonant, and therefore enhance the on-axis power deposition. The powered-electrode/plasma/grounded-electrode sandwich structure of an asymmetrically excited cylindrical discharge forms a three electrode system in which both z-symmetric and z-antisymmetric radially propagating wave modes can exist. We develop a nonlinear electromagnetics model for this system with radially- and time-varying sheath widths, incorporating both symmetric and antisymmetric modes, and the plasma skin effect. Waves generated in the electrostatic wave limit are also treated. The discharge is modeled as a uniform density bulk plasma with either homogeneous or Child law sheaths at the electrodes, incorporating their nonlinear voltage versus charge relations. The model includes a finite power source resistance and a self-consistent calculation of the dc bias voltages. The resulting set of nonlinear partial differential equations is solved numerically to determine the symmetric and antisymmetric mode amplitudes and the nonlinearly-excited radially-varying harmonics. Two examples driven at lower frequency (30 MHz) or at higher frequency (60 MHz) are examined. The first case displays the excited dominant ninth harmonic near the series resonance frequency with the fundamental frequency having a weak standing wave. The higher frequency case shows a more obvious standing wave effect, enhanced by the nonlinear harmonics. Finally, the symmetric and antisymmetric modes are of similar amplitudes over the driving electrode, summing to give a larger bottom sheath.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

Effect of insulator surface conditioning on the pinch dynamics and x-ray production of a Ne-filled dense plasma focus

The dense plasma focus (DPF) can be an intense source of x rays, wherein the insulator sleeve strongly dictates the electrical breakdown, which subsequently affects the formation of a plasma sheath and a collapse phase. Experiments on a 25 kJ DPF (operated at 4.4 kJ) are carried out to demonstrate the influence of insulator surface morphology on the pinch structure, dynamics, and x-ray yield using a Ne fill. Two borosilicate insulators are directly compared, one with a smooth finish and the other machined with four circumferential grooves traversing the perimeter of the exterior insulator surface. Comparisons are made through same-shot imaging diagnostics of the evolving plasma sheath during breakdown, rundown, and at the pinch in addition to the time-resolved measurements of emitted x rays via filtered photodiodes. The presence of structures on the insulator sleeve reduces x-ray production across all fill pressures by a factor of 2.8 ± 2.4 on average and reduces the highest x ray producing shots by a factor of 5. ± 1.8. Observations of sheath asymmetry and inhomogeneity at lift-off are observed and correlated with subsequent observations of off-axis radial collapse. Taken together, this suggests that local variations in the insulator surface decrease the spatial uniformity of the sheath, leading to an azimuthally asymmetric focus, reduced electron densities, and, ultimately, degraded x-ray production.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

On the Ohmic-dominant heating mode of capacitively coupled plasma inverted by boundary electron emission

Electron emission from the boundary is ubiquitous in a capacitively coupled plasma (CCP) and precipitates nonnegligible influence on the discharge properties. Here, we present Particle-in-Cell/Monte Carlo Collision simulation of an Ohmic-dominant heating mode of the capacitively coupled plasma, where the stochastic heating vanishes and only Ohmic heating sustains the discharge due to sheath inversion by boundary electron emission. The inverted CCP features negative sheath potential without Bohm presheath, hence excluding plasma heating due to sheath edge oscillation. The particle and energy transport of the proposed heating mode is analyzed. The influence of boundary electron emission flux, source voltage, and neutral pressure on the transition between classic and Ohmic-dominant CCP heating modes is shown with designated simulation scans. Further, a modified inverse sheath–plasma coupling due to excessive ionization is discovered. In the end, key indicators of the proposed heating mode in plasma diagnostics are provided for future experimental verifications.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2015

Through the ring of fire: γ-ray variability in blazars by a moving plasmoid passing a local source of seed photons

Blazars exhibit flares across the electromagnetic spectrum. Many γ-ray flares are highly correlated with flares detected at optical wavelengths; however, a small subset appears to occur in isolation, with little or no variability detected at longer wavelengths. These “orphan” γ-ray flares challenge current models of blazar variability, most of which are unable to reproduce this type of behavior. We present numerical calculations of the time-variable emission of a blazar based on a proposal by Marscher et al. to explain such events. In this model, a plasmoid (“blob”) propagates relativistically along the spine of a blazar jet and passes through a synchrotron-emitting ring of electrons representing a shocked portion of the jet sheath. This ring supplies a source of seed photons that are inverse-Compton scattered by the electrons in the moving blob. The model includes the effects of radiative cooling, a spatially varying magnetic field, and acceleration of the blob's bulk velocity. Synthetic light curves produced by our model are compared to the observed light curves from an orphan flare that was coincident with the passage of a superluminal knot through the inner jet of the blazar PKS 1510–089. In addition, we present Very Long Baseline Array polarimetric observations that point to the existence of a jet sheath in PKS 1510–089, thus providing further observational support for the plausibility of our model. An estimate of the bolometric luminosity of the sheath within PKS 1510–089 is made, yielding L{sub sh}∼3×10{sup 45} erg s{sup −1}. This indicates that the sheath within PKS 1510–089 is potentially a very important source of seed photons.

79 ASTRONOMY AND ASTROPHYSICS↗