Dose Deposition and Electrostatic Charging of Kapton Films Irradiated with Electrons

Kapton films are widely utilized in harsh radiation environments where radiation resistant insulating materials are required. For space applications, Kapton polymers are used on satellites as substrates for solar arrays and outer layers of thermal systems. Kapton is also used in nuclear power plants as wire insulation. Kapton materials can be exposed in nuclear reactors to a reactive chemical environment in addition to severe radiation. It is of utmost important to understand how Kapton materials behave under high irradiation conditions and mitigate radiation damage effects. High-energy electrons can deposit ionizing dose and electric charge deeply inside Kapton materials. The charge accumulation grows over time and may exceed the dielectric strength of Kapton resulting in the electrostatic discharge that may cause extensive material damage. The dose deposition and electrostatic charging of Kapton irradiated with electrons has been studied using the Monte Carlo stepping model implemented in the Geant4 software toolkit. The secondary radiation emission (photo-, Auger-, Compton-electrons, and fluorescence photons) generated by primary electrons is taken into account in the redistribution of dose and charge deposition within a Kapton film. The results of this study are the profiles of dose and charge deposited by primary and generated secondary electrons and photons within a thin film of Kapton as a function of its depth. The results also provide insights into distributions of dose and charge in Kapton films under various incidence angles and energies of electrons.https://scholarscompass.vcu.edu/gradposters/1103/thumbnail.jp

JxB FORCE EFFECTS ON BERYLLIUM MELT SPLASHING IN FUSION DEVICES

Instability and disruption of high-temperature plasma in fusion devices may result in the edge-localized modes (ELMs) and lead to melting of plasma facing components (PFCs) causing their damage. Beryllium (Be) is used as a first wall for PFCs due to its low density, high strength, and high thermal conductivity. However, melting of Be on the surface of first wall is of a great concern as splashing of a molten Be layer will result in the plasma contamination and termination of fusion reaction. Therefore, it is important to understand the physics mechanisms characterizing the splashing of Be from a pool under the plasma impact in a strong magnetic field as that in the International Thermonuclear Experimental Reactor (ITER). The computational model that combines the volume of fluid (VoF) and magneto-hydrodynamic (MHD) models is used to simulate the effects of thermal, viscous, gravitational and surface tension forces on the molten Be layer. The additional source terms representing the external and thermo-emission currents are also implemented. These currents are taken into consideration as they contribute to the electromagnetic JxB force and may result in faster melt motion, redistribution, and splashing. In this work, the effects of JxB forces on splashing of molten Be, development and growth of waves, and ejection of molten droplets are examined. The stimulation results show the motion of molten Be layer and development waves at the vapor-melt interface. Results may complement the experiments at Joint European Torus (JET) and studies of PFCs melt layer phenomenon for ITER program.https://scholarscompass.vcu.edu/gradposters/1087/thumbnail.jp

SPATIO-TEMPORAL EVOLUTION OF WARM DENSE PLASMAS: MOLECULAR DYNAMICS MODELING

SPATIO-TEMPORAL EVOLUTION OF WARM DENSE PLASMAS: MOLECULAR DYNAMICS MODELING Cole Wenzel and Gennady Miloshevsky Virginia Commonwealth University, Department of Mechanical and Nuclear Engineering, 401 West Main St, Richmond, VA 23284-3015 The exo-atmospheric detonation of nuclear device would be of great impact on the material integrity of orbiting satellites. The spectral energy distribution of high intensity X-ray flux, ~10 28 -10 35 photons/(cm 2 ∙s), originating from a nuclear blast is described by the Planck\u27s blackbody function with the temperature from 0.1 keV to 10 keV. Particular damage would occur to the multi-layered, solar cell panels of satellites. However, the X-ray flux incident upon the solar panels is inversely proportional to the square of the distance from a point where a weapon was detonated. For example, the X-ray flux is reduced by a factor of 10 -10 at the distance of 100 km. Even accounting for this geometric factor, the enormous power density, ~0.1 - 10 4 GW/cm 3 , absorbed within a few microns of a Ge slab of solar cells produces the extreme pressures and temperatures. The X-ray induced blow-off and Warm Dense Plasma (WDP) formation on the surface of materials, particularly in a gap between the unshielded Ge elements is initiated. In this work, the profiles of deposited energy and power density produced by cold X- rays (~ 1 keV) in the multi-layered materials are calculated using the Monte Carlo method within the Geant4 software toolkit. The power density is used as an input for the Molecular Dynamics (MD) modeling of WDP formation and expansion into vacuum. The MD computational model is implemented within the LAMMPS software toolkit. The spatio-temporal evolution of WDP as well as its temperature, stress, and mass density distribution are investigated for different X-ray irradiation conditions. Presenting author: Cole Wenzel This work is supported by Defense Threat Reduction Agency, Grant No. DTRA1‐19‐1‐0019.https://scholarscompass.vcu.edu/gradposters/1070/thumbnail.jp

The Impact of a Nuclear Disturbance on a Space-Based Quantum Network

Quantum communications tap into the potential of quantum mechanics to go beyond the limitations of classical communications. Currently, the greatest challenge facing quantum networks is the limited transmission range of encoded quantum information. Space-based quantum networks offer a means to overcome this limitation, however the performance of such a network operating in harsh conditions is unknown. This dissertation analyzes the capabilities of a space-based quantum network operating in a nuclear disturbed environment. First, performance during normal operating conditions is presented using Gaussian beam modeling and atmospheric modeling to establish a baseline to compare against a perturbed environment. Then, the DEfense Land Fallout Interpretive Code software and computational fluid dynamics study the effect of a nuclear explosion on the surrounding environment. Finally, these sources of noise are combined to estimate the degradation of quantum states being transmitted through a nuclear disturbed environment. It is found that the effects of a nuclear environment on a quantum network is a function of the height of blast, the explosive yield, and the network design. Debris lofted into the atmosphere during a surface blast dissipate after a couple of hours, yet the concentration is initially high and results in heavy signal loss. The nuclear fireball produced additional background light interference that scatters into the receiver\u27s detector from tens of seconds to a couple of minutes, causing excessive noise in the detector. All these effects are likely to impede a quantum network’s ability to distribute quantum information between a ground station and low Earth orbit satellite for approximately one transmission period. Afterwards, by the next satellite pass, normal operation is expected to resume. These results provide the operational capabilities of space-based optical quantum networks following a nuclear explosion. The model can be expanded to model satellite-based quantum networks in other harsh atmospheric environments

The Open State Gating Mechanism of Gramicidin A Requires Relative Opposed Monomer Rotation and Simultaneous Lateral Displacement

SummaryThe gating mechanism of the open state of the gramicidin A (gA) channel is studied by using a new Monte Carlo Normal Mode Following (MC-NMF) technique, one applicable even without a target structure. The results demonstrate that the lowest-frequency normal mode (NM) at ∼6.5 cm−1 is the crucial mode that initiates dissociation. Perturbing the gA dimer in either direction along this NM leads to opposed, nearly rigid-body rotations of the gA monomers around the central pore axis. Tracking this NM by using the eigenvector-following technique reveals the channel's gating mechanism: dissociation via relative opposed monomer rotation and simultaneous lateral displacement. System evolution along the lowest-frequency eigenvector shows that the large-amplitude motions required for gating (dissociation) are not simple relative rigid-body motions of the monomers. Gating involves coupling intermonomer hydrogen bond breaking, backbone realignment, and relative monomer tilt with complex side chain reorganization at the intermonomer junction

Simulation of Dust Grain Charging Under Tokamak Plasma Conditions

Dust grains in fusion devices may be radioactive, contain toxic substances, and may penetrate into the core plasma resulting in the termination of plasma discharges. Therefore, it is important to study the charging mechanisms of dust grains under tokamak\u27s plasma conditions. In this paper, the charging processes of carbon dust grains in fusion plasmas are investigated using the developed dust simulation (DS) code. The Orbital Motion Limited (OML) theory, which is a common tool when solving dust-charging problems, is used to study the charging of dust grains due to the collection of plasma ions and electrons. The secondary electron emission (SEE) and thermionic electron emission (TEE) are also considered in the developed model. The surface temperature of dust grains (Td) is estimated for different plasma parameters. Floating potentials have been validated against the data available from the dust simulation code package DUSTT. It is shown that the dust grains are negatively charged for relatively low plasma temperatures below 10 eV and plasma densities below 1019m−3 role= presentation style= box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; line-height: normal; font-size: 16.2px; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative; \u3e1019m−3. For higher plasma temperature and density, however, the charge on dust grains may become positive. The charging time depends not only on the grain\u27s size, but also on the plasma temperature

Materials Degradation in the Jovian Radiation Environment

The radiation environment of Jupiter represents a significant hazard for Europa Lander deorbit stage components, and presents a significant potential mission risk. The radiolytic degradation of ammonium perchlorate (AP) oxidizer in solid propellants may affect its properties and performance. The Monte Carlo code MONSOL was used for modeling of laboratory experiments on the electron irradiation of propellant samples. An approach for flattening dose profiles along the depth of irradiated samples is proposed. Depth-dose distributions produced by Jovian electrons in multi-layer slabs of materials are calculated. It is found that the absorbed dose in a particular slab is significantly affected by backscattered electrons and photons from neighboring slabs. The dose and radiolytic decomposition of AP crystals are investigated and radiation-induced chemical yields and weight percent of radical products are reported

Temperature Dependent Surface Modification of Tungsten Exposed to High-Flux Low-Energy Helium Ion Irradiation

Nuclear fusion is a great potential energy source that can provide a relatively safe and clean limitless supply of energy using hydrogen isotopes as fuel material. ITER (international thermonuclear experimental reactor) is the world first fusion reactor currently being built in France. Tungsten (W) is a prime candidate material as plasma facing component (PFC) due to its excellent mechanical properties, high melting point, and low erosion rate. However, W undergoes a severe surface morphology change when exposed to helium ion (He+) bombardment under fusion conditions. It forms nanoscopic fiber-form structures, i.e., fuzz on the surface. Fuzz is brittle and can easily contaminate the plasma, and therefore preventing the fusion chain reaction. In this study, we report on the effect of temperature on the surface morphology evolution of W coatings under low energy He+ ion irradiation, relevant to fusion conditions. Submicron thickness W films have been deposited on Si (100) at room temperature using RF sputtering deposition technique. Several samples were cut from the same wafer and exposed to 100 eV He+ ions having a constant flux of 1.2 × 1021 ions m−2 s−1 (total fluence of 4.3 × 1024 ions m−2) at several temperatures in the range of 1073 – 1273 K. During each ion irradiation experiments the applied sample temperature were constant throughout that experiment. Post ion-irradiation samples (including pristine) were characterized using field emission scanning electron microscopy (FE-SEM), X-ray photoelectric spectroscopy (XPS), and optical reflectivity measurements for monitoring the changes in surface morphology, chemical composition, and surface roughness/optical properties, respectively. Our analysis shows a sequential enhancement in W fuzz density, sharpness, and protrusions from the film surface, with increasing sample temperature, during helium ion irradiation. Ex-situ XPS study shows the evidence of W2O3 phase formation due to natural oxidation of W fuzz in the open atmosphere, for all irradiated samples. The study is significant in the understanding processes of fuzz formation on high-Z refractory metals for fusion applications. In addition, the observed W2O3 fuzz structure may have potential applications in solar power concentration technology and in water splitting for hydrogen production

Two-mode squeezing over deployed fiber coexisting with conventional communications

Squeezed light is a crucial resource for continuous-variable (CV) quantum information science. Distributed multi-mode squeezing is critical for enabling CV quantum networks and distributed quantum sensing. To date, multi-mode squeezing measured by homodyne detection has been limited to single-room experiments without coexisting classical signals, i.e., on ``dark'' fiber. Here, after distribution through separate fiber spools (5~km), $-0.9\pm0.1$-dB coexistent two-mode squeezing is measured. Moreover, after distribution through separate deployed campus fibers (about 250~m and 1.2~km), $-0.5\pm0.1$-dB coexistent two-mode squeezing is measured. Prior to distribution, the squeezed modes are each frequency multiplexed with several classical signals -- including the local oscillator and conventional network signals -- demonstrating that the squeezed modes do not need dedicated dark fiber. After distribution, joint two-mode squeezing is measured and recorded for post-processing using triggered homodyne detection in separate locations. This demonstration enables future applications in quantum networks and quantum sensing that rely on distributed multi-mode squeezing.Comment: 23 pages, 13 figures, 2 table