Shallow Angles Of Incidence Research Articles

Compact Radiative Divertor Experiments at ASDEX Upgrade and Their Consequences for a Reactor.

We present a novel concept to tackle the power exhaust challenge of a magnetically confined fusion plasma. It relies on the prior establishment of an X-point radiator that dissipates a large fraction of the exhaust power before it reaches the divertor targets. Despite the spatial proximity of the magnetic X point to the confinement region, this singularity is far away from the hot fusion plasma in magnetic coordinates and therefore allows the coexistence of a cold and dense plasma with a high potential to radiate. In the compact radiative divertor (CRD) the target plates are placed close to this magnetic X point. We here report on high performance experiments in the ASDEX Upgrade tokamak that indicate the feasibility of this concept. Despite the shallow (projected) field line incidence angles of the order of θ_{⊥}=0.2°, no hot spots were observed on the target surface monitored by an IR camera, even at a maximum heating power of P_{heat}=15 MW. And even with the X point located exactly on the target surface and without density or impurity feedback control, the discharge remains stable, the confinement good (H_{98,y2}=1), hot spots absent, and the divertor in a detached state. In addition to its technical simplicity, the CRD scales beneficially to reactor-scale plasmas that would benefit from an increased volume of the confined plasma, more space for breeding blankets, smaller poloidal field coil currents, and-potentially-an increased vertical stability.

Atypical wound trajectory after a tangential pistol shot

Three intermediate-range shots from a Browning, model 1955, 7.65 mm caliber, pistol were fired from the driver’s seat of a car at a woman in the passenger seat. She sustained three wounds: An, ultimately fatal, penetrating head wound, a graze wound across her forehead, and a tangential, perforating, wound, with bullet entry over the medial sternum and exit through the right flank. Neither postmortem CT nor forensic autopsy discovered bony thoracic injuries or perforations of the thoracoabdominal cavities. There was pulmonary contusion in the medial lobe of the right lung and hemorrhage in the adipose tissue around the right kidney. The tangential bullet had left an almost 40-cm-long wound channel through a pronounced layer of subcutaneous fat. Based on 3D reconstructed CT-data determinations, a straight bullet trajectory between entry and exit wounds would have traversed the abdominothoracic cavities, right lung, and liver. The actual trajectory, however, described a prominent curve, without signs of deflection by bone. Postulated explanations for this unusual bullet track are that the woman was twisting her body in a dynamic scene when the bullet struck; further, due to its shallow angle of incidence on the skin, the bullet was deflected to an intracutaneous path. Additionally, soft tissue resistance may have caused the bullet to yaw. Caution should, thus, be exercised when reconstructing bullet trajectories solely from entry and exist wounds, also for bullet wounds through basically homogenous soft tissues.

In situ gas cell for the analysis of adsorption behaviour on surfaces using X-ray spectroscopy

A gas cell for in-situ measurements of Volatile Organic Compounds (VOCs) and their adsorption behaviour on different surfaces by means of X-ray Fluorescence (XRF) and X-ray Absorption Fine-Structure (XAFS) spectroscopy has been developed. The cell is especially designed to allow for the efficient excitation and detection of low-Z elements such as carbon, oxygen or nitrogen as main components of VOCs. Two measurement modes are available. In the surface mode, adsorption on a surface can be studied using XAFS by fluorescence detection under shallow angles of incidence. The transmission mode enables the simultaneous investigation of gaseous samples via XAFS in transmittance and fluorescence detection modes. Proof-of-principle experiments were performed at the PTB plane grating monochromator beamline for soft X-ray radiation at the synchrotron radiation facility BESSY II. The flexible design and high versatility of the cell are demonstrated with the investigation of ethanol (EtOH) as one of the most abundant VOCs. The comparison of Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectra under transmission and fluorescence detection in the gas phase with measurements of adsorbed molecules on a Si-wafer surface both at the C and O-K absorption edges proves the applicability of the cell for the monitoring of adsorption processes.

Small dams: determining the minimum waterbody surface area that can be successfully detected using Sentinel-1 SAR imagery

Water is a scarce resource in South Africa, and approximately 62% of the water used in South Africa is for irrigation. This water is stored in many small dams scattered across the country. If not managed correctly, they could have a negative effect on catchment areas and on the availability of water. As such, there is a need for a new monitoring and management system to be developed. This study determined the minimum surface area that would be required for a waterbody to be detected on Sentinel-1 Synthetic Aperture Radar imagery. A Random Forest classifier was used to detect waterbodies on a Sentinel-1 image calculated from a time series of imagery taken over a period of three months. Steep incidence angles outperformed shallow incidence angles, with the classification having an overall accuracy of 80%. Detection rates were almost 90% for waterbodies of one hectare and greater, with no false positives, and a 10% false negative rate. These findings provide the foundation for developing a detection and monitoring system, which would allow for the better management of water resources in South Africa.

Rice Crop Monitoring Using Sentinel-1 SAR Data: A Case Study in Saku, Japan

Global warming affects rice crop production, causing deterioration of rice grain quality. This study used C-band microwave images taken by the Sentinel-1 satellites to monitor rice crop growth with the aim to understand microwave backscatter behavior, focusing on decreases in panicle water contents with ripening, which affect C-band backscatter. Time-series changes illustrated a similar tendency across all four analysis years, showing that VV/VH ratio at an incidence angle of 45–46° stopped decreasing to be stable over the reproductive and ripening periods due to reductions in the panicle water content that allowed for greater microwave penetration into the canopy, thereby increasing panicle-related backscatter. Furthermore, multivariate regression analysis combined with field observations showed that VV and VH with the shallow incidence angles were significantly negatively correlated with panicle water content, which well demonstrated backscatter increases with plant senescence. Furthermore, it was observed that backscatter behaviors were highly consistent with changes in crop phenology and surface condition. Accordingly, Sentinel-1 images with shallow incidence angles and high revisit observation capabilities offer a strong potential for estimating panicle water content. Therefore, it seems reasonable to conclude that C-band SAR data is capable of retrieving grain filling conditions to estimate proper harvesting time.

Investigating the Effect of Lake Ice Properties on Multifrequency Backscatter Using the Snow Microwave Radiative Transfer Model

Recent investigations using polarimetric decomposition and numerical models have helped to improve understanding of how radar signals interact with lake ice. However, further research is needed on how radar signals are impacted by varying lake ice properties. Radiative transfer models provide one method of improving this understanding. These are the first published experiments using the Snow Microwave Radiative Transfer (SMRT) model to investigate the response of different imaging SAR frequencies (L, C, and X-band) at HH and VV polarizations using various incidence angles (20°, 30°, and 40°) to changes in ice thickness, porosity, bubble radius, and ice-water interface roughness. This is also the first use of SMRT in combination with a thermodynamic lake ice model. Experiments were for a lake with tubular bubbles and one without tubular bubbles under difference scenarios. Analysis of the backscatter response to different properties indicate that increasing ice thickness and layer porosity have little impact on backscatter from lake ice. X-band backscatter shows increased response to surface ice layer bubble radius; however, this was limited for other frequencies except at shallower incidence angles (40°). All three frequencies display the largest response to increasing RMS height at the ice-water interface, which supports surface scattering at the ice-water interface as being the dominant scattering mechanism. These results demonstrate that SMRT is a valuable tool for understanding the response of SAR data to changes in freshwater lake ice properties and could be used in the development of inversion models.

Spectral Element Modeling of Acoustic to Seismic Coupling Over Topography

AbstractAcoustic waves in the atmosphere are commonly recorded on seismometers as they couple into the ground. These signals, here called ground coupled airwaves, are not commonly considered in numerical modeling of infrasound propagation, which often assumes a rigid unmeshed boundary. Starting from an analytically‐tractable spherical wave model, we analyze how the coupling of an acoustic wave into a planar elastic halfspace changes over a wide range of scenarios. We use energy admittance to quantify acoustic to seismic coupling over both a planar elastic halfspace and meshed topography. Our spectral element and analytic calculations have different maxima as a function of incidence angle, with very high admittance values for near‐vertical incidence (maximum ≈78%). Energy admittance calculations at shallow incidence angles are much smaller (less than 1%). In simulations over the complex topography of Sakurajima Volcano, we attribute the variable spatial pattern of energy admittance to changes in earth parameters between each model. The observed pressure difference over the simulated 15 km region appears to be . While this value is relatively small, the cumulative addition over 100s of km and multiple acoustic bounce points may be significant. Acoustic to seismic coupling along the propagation path may bias long distance yield estimates, particularly when infrasound propagates over regions with steep topography or particularly slow seismic velocities, such as alluvial planes.

Angular dependence measurements of Magnum-PSI plasmas using MAST-U angled-tip Langmuir probes

Abstract Measurements made using flush-mounted Langmuir probes in tokamaks are difficult to interpret when operating at grazing angles of magnetic field incidence due to the effects of sheath expansion on the probe collection area. The Super-X divertor on the upgraded Mega Amp Spherical Tokamak (MAST-U) can have very shallow angles of magnetic field incidence to plasma facing components (1–10 ° ), making the use of conventional flush-mounted probes problematic. A novel probe tip geometry, based on the angled-tip design used successfully on JET and DIII-D, has therefore been used in MAST-U to mitigate sheath expansion effects by increasing the projected probe extent. To verify whether the new design of probe tip allows temperature ( T e ) and density ( n e ) measurements to be performed accurately at low angles of incidence, a 4-probe array based on this design was used on Magnum-PSI. Parameter scans were made on a range of hydrogen plasmas in conditions comparable to those expected in MAST-U. Plasma parameters were extracted from the measured IV characteristics - using a novel fitting approach which minimised a goodness-of-fit parameter ( T e δ T e ) - and compared to Magnum-PSI Thomson scattering measurements. The measured plasma parameters show that the MAST-U angled-tip design seems to successfully mitigate the effects of sheath expansion at low angles of magnetic field incidence. The standard MAST-U Langmuir probe also shows an apparent upper operational limit of θ = 8 ° . This covers the vast majority of expected plasma configurations in MAST-U when running both conventional and Super-X configurations.

Study of detachment in future ASDEX Upgrade alternative divertor configurations by means of EMC3-EIRENE

ASDEX Upgrade is preparing a hardware modification of its upper divertor in order to study alternative divertor configurations, like the X-divertor (XD) and the snowflake divertor (SF), that are discussed as a possible solution for the power exhaust problem. Experiments characterizing the current upper divertor in single-null (SN) configuration before and during detachment were carried out recently and interpreted by EMC3-EIRENE (Lunt et al., 2020) now including volumetric recombination. In continuation of these studies we here report on further simulations with this code extrapolating these conditions to the future upper divertor. For the same transport parameters, input power and upstream separatrix density for which the outer target (OT) of the SN is attached, the XD and SF show significant detachment at the OT accompanied by a reduction of the parallel heat flux by a factor of more than five. Despite the shallow field line incidence angles in the XD configuration the intrinsic 3D error fields from the current feeds only cause toroidal variations of the power fluxes of the order of 10%. With a hypothetical misalignment of the divertor coils by 3 cm substantial asymmetries in the power deposition profile are found, however, even those vanish when going to detached plasma conditions. In order to fully detach the plasma at the OT in the SF configuration impurities, here in the form of nitrogen, need to be puffed directly into the region of the secondary X-point in the simulation. • ASDEX Upgrade is preparing a hardware modification of its upper divertor in order to study alternative divertor configurations. • Simulations validated against recent detached ASDEX Upgrade H-mode discharges and extrapolated to X-divertor and snowflake divertor configurations. • XD easier access to detachment. • 3D error fields from artificially misaligned coils cause strong toroidal asymmetries of the power deposition pattern under attached divertor conditions that vanish in detachment. • First detached SF configurations with EMC3-EIRENE.

Geometrical effects of microchannel plates: Grazing incidence operation of time-of-flight mass spectrometry and comparison to standard carbon foil.

Direct detection of plasma composition in situ is integral to our understanding of ionospheric, magnetospheric, and heliospheric dynamics. Heavy, singly charged ions have been shown to contribute significantly to the mass and energy redistribution within the magnetosphere. However, most studies neglect the individual contributions of CNO group ions (instead attributing all effects to oxygen) due to the lack of mass separation of the spectrometer. In this paper, we investigate how the mass resolution of a traditional Time-of-Flight (ToF) mass spectrometer responds to different geometric applications of "grazing incidence" scattering. We apply this technique by utilizing standard Microchannel Plates (MCPs) to generate the "Start" signal, instead of the traditional carbon foil, reducing overall scattering via physically limiting plasma collisions to shallow angles of incidence. We considered only geometric differences, comparing LD 20 and LD 40 (length to diameter ratio) MCP effects on mass resolution observed by the ion composition and distribution function analyzer; traditional carbon foil spectra are contrasted against that produced by replacing one foil with these MCPs and subsequently illuminated with H+, H2 +, He+, N+, H2O+, N2 +, Ar+ ions over energies from ∼15 keV to 29 keV, similar to those seen in magnetospheric plasma. We found that among the MCP geometries, the increasing LD ratio corresponds to spectra that observe smaller FWHM and faster mean ToF, indicative of a significant drop in scattering. Additionally, the mass resolution is significantly better than when using the traditional carbon foil for masses above 4 amu. However, the geometry limits detection efficiencies due to the plasma's reduced cross section of collision with the surface that generates the "Start" signal.

Control pathway for an advanced divertor on ITER

Abstract This paper presents the development of a coupled shape and divertor controller for ITER with capabilities to control the flux expansion and an advanced divertor configuration, the x-divertor (XD), in which a secondary x-point is placed in the downstream scrape-off layer. Due to the high-performance nature of ITER and its relatively few shaping coils, satisfying constraints on the coil currents, power supplies, and plasma shape is a challenge for this configuration. To meet these constraints the controller uses the constrained linear quadratic regulator (CLQR) framework [1] , a variant of model predictive control (MPC). Previous work [2] has shown the existence of XD equilibria on ITER and in this study, we identify a control pathway for achieving the “pure” XD where the secondary x-point is placed at the outer strike point, representing a maximally flux-expanding scenario. Constraints are observed throughout the transition. One limitation is that at high flux expansion, the shallow angle of incidence of the magnetic field on the divertor results in a reduced plasma wetted area via tile shadowing. On the other hand, the high flux expansion and flaring characteristic of the XD facilitate enhanced detachment, which could negate the shadowing effect to some degree. In this case it would be desirable to operate as close to the flux expansion limit as possible. To this end we also demonstrate the controller's ability to track a trajectory for the flux expansion near the accepted angle of incidence limit.

An assessment of marine atmospheric boundary layer roll detection using Sentinel-1 SAR data

The ability of high-resolution synthetic aperture radar (SAR) to detect marine atmospheric boundary layer (MABL) roll-induced roughness modulation of the sea surface wave field is well known. This study presents SAR measurements of MABL rolls using global coverage data collected by the European Space Agency's C-band Sentinel-1A satellite in 2016–2017. An automated classifier is used to identify likely roll events from more than 1.3 million images that were acquired at two incidence angles of 23° and 36.5° in either VV or HH polarization. Characteristics of the detected rolls are examined for different wind speeds, polarizations, incidence and relative azimuth angles. Roll detection counts are much higher at the higher incidence angle and nearly equivalent for VV and HH polarizations. Detection depends strongly on the relative azimuth with roll detection rates at crosswind being 3–10 times lower than for up- or downwind. All data show a low wind speed threshold near 2 m s−1 and that rolls are most commonly observed at wind speeds near 9 m s−1. For all viewing configurations, we find that rolls induce a wide range of mean surface wind speed modulation with the most frequent value being 8% (±3.5%). Roll detection at crosswind is associated with stronger roll-induced surface wind enhancement. Dependencies of roll detection on the incidence and relative azimuth angles are consistent with rapid short-scale wind-wave adjustments to the roll-induced surface wind gusts. These cm-scale waves are highly directional and provide limited crosswind backscatter at shallower incidence angles. The same roll-induced surface forcing is thus not equally detectable at all viewing geometries or polarizaions. Stronger and possibly longer-duration wind forcing is likely needed to produce detectable roll-induced modulations at crosswind.

Directionally resolved measurements of momentum transport in sputter plumes as a critical test for simulations

A 50×50mm2 copper target is sputtered by an ion beam at angles of incidence from 0° to 90°. The resulting sputter plumes are characterized directionally resolved using a compact and maneuverable force probe. It measures the momentum flux of particles released due to sputtering or reflection inside the sputter plume. Argon ions at energies from 320 to 1220 eV are studied. As expected, the directional distributions are sensitive to the angle of incidence. The ion energy has significant influence only on the magnitude of forces and not their directional distribution. The experimental results are compared with simulations based on the “stopping and range of ions in matter” code (SRIM). The general trends of the directional distributions of forces match reasonably with simulations. However, the simulations overestimate the forces at normal incidence and shallow angles of incidence.

Cosmo-SkyMed and TerraSAR-X datasets for geomorphological mapping in the eastern of Marajó Island, Amazon coast

The Amazon coast is marked by the high discharge of sediments and freshwater, macrotidal influence, a wide continental shelf, extensive flood plains and lowered plateaus which make it unique as a delta and estuary landscape. Further, the tropical climate imposes heavy rains and incessant cloudiness that render microwave systems more suitable for cartography. This study proposed to recognize and map the Amazon coastal environments through the X-band Synthetic Aperture Radar, provided by Cosmo-SkyMed (CSK) and TerraSAR-X (TSX) systems. The SAR datasets consisted of interferometric and stereo pairs, restricted to single-revisit and obtained with small interval (1–11 days), under steeper (θ<35°) and shallow (θ≥35°) incidence angles, and during the rainy and dry seasons. From the 4 acquisitions of X-band SAR data, attributes such as the backscattering coefficient, coefficient of variation, texture, coherence, and Digital Surface Model (DSM) were derived, adding each variable in 5 scenarios. These combinations resulted in 20 models, which were submitted individually to the machine learning (ML) classification approach by Random Forest (RF). The backscattering and altimetry described the coastal environments which shared ambiguity and high dispersion, with the lowest separability for vegetated environments such as Mangrove, Vegetated Coastal Plateau and Vegetated Fluvial Marine Terrace. The coherence provided by interferometry was weak (<0.44), even during the dry season, in the other hand, the cross-correlation was strong (>0.60), during the rainy and dry season showing more suitability for radargrammetry on the Amazon coast. The RF models resulted in Kappa coefficient between 0.39 to 0.70, indicating that the use of X-band SAR images at an incidence angle greater than 44° and obtained in the dry season is more appropriated for the morphological mapping. The RF models given by TSX achieved the higher mapping accuracies per scenario of SAR products, in order of 0.48 to 0.63. Despite this, the best classification was carried out by 19 RF model with 0.70, provided by CSK in shallow incidence composed by intensity, texture, coherence and stereo DSM. The CSK and TSX data allowed to map the Amazon coast precisely, based on X-band at single polarization, high spatial resolution and revisit, which has demonstrated the support for detailed cartography scale (1:50,000) and frequent updating (monthly up to yearly).

Estimation of Level and Deformed First-Year Sea Ice Surface Roughness in the Canadian Arctic Archipelago from C- and L-Band Synthetic Aperture Radar

Utilizing synthetic aperture radar (SAR) data to understand and map sea ice roughness is an active area of research. An object-based image analysis is here used to quantitatively link the winter first-year sea ice surface roughness to C-band RADARSAT-2 and L-band ALOS-2 PALSAR-2 SAR backscatter measured in the Canadian Arctic. Results indicate: (1) C-band HH-polarization backscatter is correlated with roughness (r = 0.86) at a shallow incidence angle; and (2) L-band HH- and VV-polarization backscatter is correlated with roughness (r = 0.82) at a shallow incidence angle. Retrieval models for surface roughness are developed and applied to the imagery to demonstrate the potential of SAR for mapping deformation with a best case RMSE of 5 mm. Additionally, strong correlations between C- and L-band SAR backscatter acquired during the advanced melt period, and the winter measured surface roughness, are observed. These results are discussed in the context of SAR potentially providing significant sea ice structural information despite the occurrence of melt and presence of surface melt ponds.

2D and 3D studies of the X-divertor configuration in the future upper divertor of ASDEX upgrade

Abstract ASDEX Upgrade is planning the installation of a pair of in-vessel coils in order to study alternative divertor configurations, like an X-divertor (XD) or a Snowflake (SF) in a machine with a high heating power compared to its size. Two designs for the technical realization of these coils are currently proposed, one including z-shaped joints and one with a single spiral conductor. The magnetic fields produced by these conductors intrinsically have a non-axisymmetric component. In particular in the XD configuration, where very shallow field line incidence angles occur, these 3D fields have a strong impact on the power distribution on the outer divertor plate. The 2D and 3D plasma transport is studied by the SOLPS and initial EMC3-EIRENE code simulations, respectively, in order to support the decision making processes of the engineering design. Although several aspects still require further attention, this paper gives a status report. The strong toroidal asymmetries observed in the power deposition pattern for the spiral conductor seem to be strongly reduced, when highly dissipative divertor conditions are reached. The asymmetry for the z-joint design is substantially smaller than the spiral conductor even for low-dissipative conditions.

Optimal Compact Polarimetric Parameters and Texture Features for Discriminating Sea Ice Types during Winter and Advanced Melt

C-band synthetic aperture radar (SAR) is widely used for sea ice monitoring and operational activities. The RADARSAT Constellation Mission (RCM), with its anticipated launch in 2018, will provide hybrid compact polarimetric (CP) C-band SAR data offering near-polarimetric capabilities at large image acquisition widths suitable for achieving operational and scientific objectives in the Arctic. Although C-band SAR is effective for sea ice monitoring, it is difficult to implement during advanced melt, when the sea ice cover is melting and covered by melt ponds. Ice type separability during winter (pre-melt) and advanced melt conditions was assessed using Kolmogorov–Smirnov statistical separability analyses and Support Vector Machine supervised classifications of RCM parameters simulated from 2 winter and 2 advanced melt RADARSAT-2 scenes. Through a detailed analysis of the 2 advanced melt scenes, it was found that the steep incidence angle (22.3–24.2°) simulated RCM CP parameters provide improved ice type separability during the advanced melt period compared with shallow incidence angles (39.6–42.2°). With respect to classification, an overall accuracy of 77.06% was found for a scene comprising first-year and multiyear ice types, and a higher overall accuracy of 85.91% was achieved by including gray level co-occurrence matrix parameters in the classification.

Kinetic energy reconstruction with a single layer particle telescope

We demonstrate the implementation of a low-mass, single-layer pixel detector as a multi-component telescope for measuring the kinetic energy of charged particles. Rather than relying on several separate detection layers, we utilize the detector's individual pixels as the basis for multiple sampling of the stopping power of a particle under a shallow incidence angle. We present results from a single layer device corresponding to nearly 50 telescope layers. As the measured dE/dx response is highly stochastic, we utilize a maximum likelihood approach for which we calculate a probability function in the energy domain for each interacting particle. Using accelerator data, we show the limitations of the single-component approach and the advantages of our single-layer, multiple-sampling technique. Inferred energy spectra from accelerators show high accuracy and precision for protons with kinetic energies up to 400 MeV. For higher energies, the low energy limit is still very accurate.

Interaction between a surface quasi-geostrophic buoyancy anomaly jet and internal vortices

This paper addresses the dynamical coupling of the ocean’s surface and the ocean’s interior. In particular, we investigate the dynamics of an oceanic surface jet and its interaction with vortices at depth. The jet is induced by buoyancy (density) anomalies at the surface. We first focus on the jet alone. The linear stability indicates there are two modes of instability: the sinuous and the varicose modes. When a vortex in present below the jet, it interacts with it. The velocity field induced by the vortex perturbs the jet and triggers its destabilisation. The jet also influences the vortex by pushing it under a region of co-operative shear. Strong jets may also partially shear out the vortex. We also investigate the interaction between a surface jet and a vortex dipole in the interior. Again, strong jets may partially shear out the vortex structure. The jet also modifies the trajectory of the dipole. Dipoles travelling towards the jet at shallow incidence angles may be reflected by the jet. Vortices travelling at moderate incidence angles normally cross below the jet. This is related to the displacement of the two vortices of the dipole by the shear induced by the jet. Intense jets may also destabilise early and form streets of billows. These billows can pair with the vortices and separate the dipole.

“Feathered” fractal surfaces to minimize secondary electron emission for a wide range of incident angles

Complex structures on a material surface can significantly reduce the total secondary electron emission from that surface. The reduction occurs due to the capture of low-energy, true secondary electrons emitted at one point of the structure and intersecting another. We performed Monte Carlo calculations to demonstrate that fractal surfaces can reduce net secondary electron emission produced by the surface as compared to the flat surface. Specifically, we describe one surface, a “feathered” surface, which reduces the secondary electron emission yield more effectively than other previously considered configurations. Specifically, feathers grown onto a surface suppress secondary electron emission from shallow angles of incidence more effectively than velvet. We find that, for the surface simulated, secondary electron emission yield remains below 20% of its un-suppressed value, even for shallow incident angles, where the velvet-only surface gives reduction factor of only 50%.