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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

An electron Bernstein wave heating scheme for the Proto-MPEX linear device

Linear plasma devices, such as the Prototype Material Plasma Exposure eXperiment (Proto-MPEX), are an economic method to study plasma-material interactions under high heat and particle fluxes. The Proto-MPEX device at Oak Ridge National Laboratory is a high-density helicon plasma generator with additional resonant electron heating to study plasma-material interactions in ITER-like conditions. Here, we present modeling results of an electron Bernstein wave (EBW) heating scheme for this device using a microwave-based system. A 28 GHz injection system is available to provide power levels up to 100 kW injected into low electron temperature and high electron density plasmas. A modified version of the GENRAY ray-tracing code, GENRAY-C, has been used to determine the EBW and electron cyclotron heating wave accessibility for these overdense plasmas. Modeling has shown that greater than 80% of mode conversion from the ordinary mode to the EBW is possible for 28 GHz injection in Proto-MPEX. Calculations show that by utilizing the effects of Doppler-shifted resonance absorption, power deposition near the core is possible. Finally, significant collisional damping, leading to edge absorption, is expected to occur for neutral pressures >1.0 mTorr; however, experiments can be designed to minimize these effects.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Development of a LIBS system for in situ surface measurements during plasma exposure in PISCES-A

A laser-induced breakdown spectroscopy (LIBS) system has been developed with a Q-switched Nd:YAG laser (wavelength = 1064 nm and pulse width ~5 ns) to conduct in situ surface measurements during plasma exposure in the PISCES-A linear divertor plasma simulator. The LIBS signal enhancement is obtained with both the magnetic field normal to the surface of a target and steady-state background plasma. Migration of sputtered Ta impurities onto the neighboring Cr surface is identified during He plasma exposure, only when cone structures are formed on the Cr surface. D retention in W during D plasma exposure is observed to decrease with increasing the sample temperature. The temporal evolution of D outgassing from W is measured in a time range of ~10-420 s right after D plasma exposure. A power law fit, t - α , yields α ~ 0.34 ± 0.09, which is nearly consistent with calculated and measured values.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

A free-standing wire scattering technique to monitor calibration variations of the DIII-D density profile reflectometer

Real-time phase calibration of the ITER profile reflectometer is essential due to the long plasma duration and expected waveguide path length changes during a discharge. Progress has been recently made in addressing this issue by employing a phase calibration technique on DIII-D that monitors calibration variations that occur during each plasma discharge. By installing a thin free-standing metallic wire (1 mm diameter) near the end of the overmoded waveguide transmission system (oriented perpendicular to the waveguide axis), the round-trip phase shift from the wire is detected simultaneously with the plasma phase shifts. Variations in the reflectometer round trip path length (-26 m) are then calculated after each DIII-D plasma discharge, allowing the calibration phase to be accurately monitored and updated. The round-trip reflectometer path length is observed to vary by -3 mm (root mean square value) during a typical DIII-D discharge. Using the variations in calibration phase, the density profile measurement accuracy can be improved. Since the wire retro-reflected power is -0.01 of the plasma signal, minimal effect is observed on the reflected signal from the plasma. Importantly, through a suitable choice in wire diameter, the calibration signal can be made approximately independent of the V-band reflectometer launch polarization. Furthermore, this is particularly important on DIII-D since orthogonal X- and O-mode polarized beams are coupled into the same transmission waveguide and launch antenna.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Time resolved detection of two-plasmon decay using three-halves harmonic emission on the National Ignition Facility

Supra-thermal (>100 keV) electrons generated by laser plasma interactions can be detrimental to the performance of ignition experiments conducted on the National Ignition Facility (NIF). On a NIF shot, the amount of electrons is estimated by measuring the hard X-rays passing through the hohlraum wall. The primary sources of hot electrons in a hohlraum are Stimulated Raman Scattering (SRS) and two plasmon decay (TPD). While SRS is well diagnosed on the NIF, there has been no diagnosis of TPD. Here we have designed and implemented a new diagnostic to characterize the time history of TPD on the NIF. The instrument provides a time resolved measurement of the 3/2 ω harmonic emission which is indicative of the presence of TPD. We describe the diagnostic setup, calibration, and the preliminary results obtained on NIF hohlraum experiments. Finally, we find evidence of a correlation between measured hard X-rays generated from the hot electron bremsstrahlung and the TPD emission.

47 OTHER INSTRUMENTATION↗
DOE OSTI2018

Scaling laws for dynamical plasma phenomena

A scaling and similarity technique is a useful tool for developing and testing reduced models of complex phenomena, including plasma phenomena. Similarity and scaling arguments will be applied to highly dynamical systems where the plasma is evolving from some initial to some final state, which may differ dramatically from each other in size and plasma parameters. A question then arises whether, in order to better understand the behavior of one such system, is it possible to create another system, possibly much smaller (or larger) than the original one, but whose evolution would accurately replicate that of the original one, from its initial to its final state. This would allow a researcher, by an experimental study of this second system, to make confident predictions about the behavior of the first one (which may be otherwise inaccessible, as is the case of some astrophysical objects, or too expensive and hard to diagnose, as in the case of fusion applications of pulsed plasma systems, or for other reasons). The scaling and similarity techniques for dynamical plasma systems will be presented as a set of case studies of problems from various domains of plasma physics, including collisional and collisionless plasmas. Finally, among the results discussed are similar for MHD systems with an emphasis on high-energy-density laboratory astrophysics, interference between collisionless and collisional phenomena in the context of shock physics, and similarity for liner-imploded plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Gyrokinetic GENE simulations of DIII-D near-edge L-mode plasmas

We present gyrokinetic simulations with the GENE code addressing the near-edge region of an L-mode plasma in the DIII-D tokamak. At radial position ρ = 0.80, simulations with the ion temperature gradient (ITG) increased by 40% above the nominal value give electron and ion heat fluxes that are in simultaneous agreement with the experiment. This gradient increase is consistent with the combined statistical and systematic uncertainty σ of the charge exchange recombination spectroscopy measurements at the 1.6σ level. Multiscale simulations are carried out with a realistic mass ratio and geometry for the first time in the near-edge. These multiscale simulations suggest that the highly unstable ion temperature gradient (ITG) modes of the flux-matched ion-scale simulations suppress electron-scale transport, such that ion-scale simulations are sufficient at this location. At radial position ρ = 0.90, nonlinear simulations show a hybrid state of ITG and trapped electron modes, which was not expected from linear simulations. The nonlinear simulations reproduce the total experimental heat flux with the inclusion of E × B shear effects and an increase in the electron temperature gradient by ∼23%. This gradient increase is compatible with the combined statistical and systematic uncertainty of the Thomson scattering data at the 1.3σ level. These results are consistent with previous findings that gyrokinetic simulations are able to reproduce the experimental heat fluxes by varying input parameters close to their experimental uncertainties, pushing the validation frontier closer to the edge region.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Dissociative recombination process of ammonium for HN-MAR in high density D-N plasmas

Ammonia molecules formed in a nitrogen-seeded D-fueled plasma are shown to catalyze volumetric recombination of hydrogen ions. This Hydronitrogen Molecular Assisted Recombination (HN-MAR) process occurs by charge and ion exchange reactions with ammonia (ND 3 ), leading to subsequent volumetric recombination with ND 3 + and ND 4 + molecular ions. When the plasma density is raised to the point where volumetric recombination dominates plasma losses to the wall [n e > 10 18 m -3 ], a calibrated electrostatic quadrupole plasma analyzer shows that the ND 4 + ammonium ion density fraction drops from 0.55 to 0.11. We show that this is consistent with a 0-d kinetics model that includes the HN-MAR process with rates that are proportional to the electron density. The results suggest that direct injection of ammonia would provide a more efficient pathway to achieve divertor detachment in future tokamak experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Reduction of ablated surface expansion in pulsed-power-driven experiments using an aerosol dielectric coating

The quality of warm dense matter samples created by magnetic compression can be largely affected by material ablation. When the ablated material carries currents, local instabilities can grow, which can lead to nonuniformities in the final magnetic pressure. Extending the previous work by Peterson et al., Awe et al., and Hutchison et al., the experiments reported here demonstrate that the expansion of the ablated material can be significantly reduced by using a simple aerosol spray technique. Coating the current-carrying surfaces with a 30–60-μm layer of polyurethane reduced the expansion of the ablated material by a factor of 2 and eliminated material ejections from sharp corners. Here, this technique, tested at the Michigan Accelerator for Inductive Z-Pinch Experiments pulsed power facility at the University of Michigan with currents up to 400 kA, could allow the production of homogeneous warm dense matter samples on pulsed-power drivers. Because of the simplicity of this method, this work brings forth an important contribution to pulsed-power-driven experiments designed to study nuclear fusion, material properties, and radiation science.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Investigation of wire-array Z-pinches by laser probing diagnostics

Laser diagnostics provides powerful tools for the investigation of dense Z-pinches. In this paper, wire-array Z-pinches are investigated at the 1 MA Zebra generator using laser diagnostics at different wavelengths coupled with x-ray diagnostics. Plasma dynamics during the ablation, implosion, and stagnation stages are observed by multiframe diagnostics. Cascading and nonprecursor implosions are studied in wire arrays. Ultraviolet diagnostics allows deep penetration into the Z-pinch plasma at stagnation. End-on probing reveals the complicated structure of the precursor. Strong magnetohydrodynamic instabilities are found in a dense pinch hidden in the trailing plasma. Small-scale instabilities are seen in the Z-pinch plasma with micrometer resolution. Probing of the pinch from four directions shows asymmetrical trailing plasma in some configurations of wire arrays. Faraday rotation diagnostics reveals the magnetic fields and the current distribution in the plasma of the precursor and Z-pinch. Redistribution of current in the trailing plasma is seen during kink and sausage instabilities in the stagnation stage. The formation of micropinches and hot spots in the Z-pinch is analyzed with coupled laser and x-ray diagnostics. Different laser diagnostics allow the study of Z-pinch plasmas in all stages, including fast dynamics and instabilities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Subcritical turbulence spreading and avalanche birth

In confined plasmas, a localized fluctuation in a marginal or weakly damped region will propagate and generate an avalanche if it exceeds a threshold. In this letter, a new model for turbulence spreading based on subcritical instability in the turbulence intensity is introduced. We derive a quantitative threshold for spreading from a seed in a stable region, based on a competition between diffusion and nonlinear growth of the turbulence intensity. The model resolves issues with the established Fisher equation model for turbulence spreading, which is supercritical and cannot support the stationary coexistence of multiple turbulence levels. Implications for turbulence spreading are discussed, including the dynamics of ballistic penetration of turbulence into the stable zone. Finally, tests of the theory are suggested.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Fast ion transport in the quasi-single helical reversed-field pinch

The reversed-field pinch (RFP) can spontaneously transition from an axisymmetric magnetic topology to a 3D-helical geometry. Investigations on fast ion transport associated with energetic particle driven Alfvén instabilities, tearing mode induced stochasticity, and neoclassical effects have been performed on the Madison Symmetric Torus. STELLGAP produced shear-Alfvén continua seeded with V3FIT 3D-equilibrium reconstructions describe the response of Alfvénic bursting activity as a direct consequence of the equilibrium change on the fast ion resonance. Furthermore, far infrared interferometry resolved electron density perturbations associated with the bursts provide a spatial measurement of the mode structure and support the reconstructions. The bursts produce no global resonant fast ion transport; however, their disappearance at a high core-resonant amplitude implies other transport mechanisms at play. Additionally, neutral particle analysis and neutron signals suggest fast ion losses at sufficient core tearing mode strength, supporting the lack of Alfvénic activity. The guiding-center code ORBIT corroborates rapid fast ion loss times in the helical state largely as a consequence of remnant tearing modes. Additionally, ORBIT simulations demonstrate little neoclassical enhancement of particle transport. While superbanana orbits may exist, the growth in the core-resonant fast ion island and the associated secondary mode overlap govern the largest transport process, leading to robust fast ion losses in the 3D-RFP.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Effects of plasmoid formation on sawtooth process in a tokamak

For realistic values of the Lundquist number in tokamak plasmas, 1/1 magnetic islands lead to the formation of a secondary thin current sheet, which breaks up into a chain of small magnetic islands, called plasmoids. The role of plasmoid dynamics during the sawtooth reconnection process in fusion plasmas remains an unresolved issue. In this study, systematic simulations are performed to investigate the resistive internal kink mode using the full resistive magnetohydrodynamics equations implemented in the NIMROD code in a simplified tokamak geometry. For Lundquist number S≥1.6×10 7 , the secondary current sheet is found to be unstable to plasmoids during the nonlinear resistive kink mode evolution with a critical aspect ratio of the current sheet of ~70. The merging of small plasmoids leads to the formation of a monster plasmoid that can significantly affect the primary island evolution. This may provide an explanation for the partial reconnection observed in sawtooth experiments.

Computer simulation↗
DOE OSTI2019

Global gyrokinetic simulation of microturbulence with kinetic electrons in the presence of magnetic island in tokamak

An electrostatic model has been formulated and implemented in the gyrokinetic toroidal code to study the nonlinear ion temperature gradient (ITG) turbulence in the presence of an n = 1, m = 2 magnetic island. The ions are described by the gyrokinetic equation while the electrons are treated with the drift-kinetic equation. In our simulation, an n = 1, m = 2 electrostatic mode is formed with the same vortex structure of the magnetic island. When the magnetic island flattening effect is turned on, the island vortex mode is well preserved and couples to the n = 0, m = 0 geodesic acoustic mode. Simulation shows that the magnetic island can suppress the ITG turbulence at the island O-point and strengthen it near the X-point. We show that the vortex mode can generate a substantial helical shear flow around the island. Finally, we also find that the turbulence and transport are suppressed inside the island and enhanced at the island X-point

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Study of Alfven eigenmodes stability in plasma with multiple NBI driven energetic particle species

The aim of this study is to analyze the destabilization of Alfven Eigenmodes (AEs) by multiple energetic particle (EP) species in DIII-D and LHD discharges. In this work, we use the reduced MHD equations to describe the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system, coupled with equations of density and parallel velocity moments for the energetic particle species, including the effect of the acoustic modes, diamagnetic currents, and helical couplings. Furthermore, we add the Landau damping and resonant destabilization effects using a closure relation. The simulations with multiple neutral beam injector (NBI) lines show three different regimes: the nondamped regime where the multibeam AE growth rate is larger compared to the growth rate of the AEs destabilized by the individual NBI lines, the interaction regime where the multibeam AE growth rate is smaller than the single NBI AEs, and the damped regime where the AEs are suppressed. Operations in the damped regime require EP species with different density profile flatness or gradient locations. In addition, the AE growth rate in the interaction regime is further reduced if the combined NBI lines have similar beam temperatures and the β of the NBI line with a flatter EP density profile increases. Additionally, optimization trends are identified in DIII-D high poloidal β and LHD low density/magnetic field discharges with multiple NBI lines as well as the configuration requirements to operate in the damped and interaction regimes. DIII-D simulations show a decrease in the n = 2 to 6 AE growth rate and n = 1 AE are stabilized in the LHD case. The helical coupling effects in LHD simulations lead to a transition from the interaction to the damped regime of the n = 2, –8, 12 helical family.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Development of a directly driven multi-shell platform: Laser drive energetics

Simulations predict that directly driven multi-shell targets can provide a robust alternative to conventional high-convergence implosion concepts by coupling two to three times more energy into the final igniting thermonuclear fuel assembly than indirect-drive concepts. The three-shell directly driven Revolver concept [K. Molvig, M. J. Schmitt, B. J. Albright, E.S. Dodd, N.M. Hoffman, G.H. McCall, and S.D. Ramsey, Phys. Rev. Lett. 116, 255003 (2016)] utilizes a design that maximizes laser energy conversion into inward kinetic energy of the outermost ablator shell (~9%) while minimizing the DT fuel convergence (~9) to reduce the mixing of material from the innermost shell into the fuel. Inherent in this design concept is the use of 192 narrow beams (with a 1/e laser beam-to-capsule diameter ratio of 0.33) from the National Ignition Facility laser pointed in a polar direct drive laser configuration. In this paper, we demonstrate that low average laser intensity at the capsule surface (≤300 TW/cm 2 ) limits the measured laser backscatter, indicating that a greater amount of laser energy is coupled into the target. Omega experiments have been performed to determine the coupling of laser energy to the outermost shell of a scaled Revolver target (i.e., the ablator shell) by measuring capsule implosion trajectories and scattered-light fractions for two different drive configurations. Comparisons of simulated shell trajectory and velocity profiles with experimental data obtained from self-emission images show good agreement and are consistent with measured scattered light data. Furthermore, the low levels of scattered light measured are consistent with post-shot simulation results that show high hydro-coupling efficiency. These results strengthen the case for using narrow beams at low intensity to drive large ablator capsules for future direct-drive, multi-shell ignition concepts.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Mapping of the HIDRA stellarator magnetic flux surfaces

The Hybrid Illinois Device for Research and Applications (HIDRA) is a classical stellarator designed for conducting plasma material interaction experiments and developing novel Plasma Facing Components (PFCs). Notably, the testing of two open-channel liquid lithium PFCs is imminent. Determining the shape of the plasma and its magnetic structure inside HIDRA is essential to carry out these tests. For this, electron traces were captured to build up the images of the HIDRA magnetic flux surfaces for several magnetic configurations, following the same procedure previously employed on the WEGA stellarator coupling an electron gun with a fluorescent detector. The FIELDLINES code has then been used to generate computational surfaces matching the experimental results. The obtained surfaces were found to be subject to a similar n = 1 error field as the one observed on WEGA, suggesting that the origin of this error field is inherent to the HIDRA vacuum vessel. Also, the effect of adding a vertical field was investigated, demonstrating the ability to radially shift the magnetic axis and move to a regime free of low-order rational resonances. Finally, this additional control over the HIDRA plasma and magnetic structure allows more freedom in setting up the PFC tests in the limiter and divertor regions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Kinetic particle simulations in a global toroidal geometry

The gyrokinetic toroidal code has been upgraded for global simulations by coupling the core and scrape-off layer regions across the separatrix with field-aligned particle-grid interpolations. A fully kinetic particle pusher for high frequency waves (ion cyclotron frequency and beyond) and a guiding center pusher for low frequency waves have been implemented using cylindrical coordinates in a global toroidal geometry. The two integrators correctly capture the particle orbits and agree well with each other, conserving energy and canonical angular momentum. As a verification and application of this new capability, ion guiding center simulations have been carried out to study ion orbit losses at the edge of the DIII-D tokamak for single null magnetic separatrix discharges. The ion loss conditions are examined as a function of the pitch angle for cases without and with a radial electric field. We report the simulations show good agreement with past theoretical results and with the experimentally observed feature in which high energy ions flow out along the ion drift orbits and then hit the divertor plates. A measure of the ion direct orbit loss fraction shows that the loss fraction increases with the ion energy for DIII-D in the initial velocity space. Finally, as a further verification of the capability of the new code, self-consistent simulations of zonal flows in the core region of the DIII-D tokamak were carried out. All DIII-D simulations were performed in the absence of turbulence.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Numerical study of transition between even and odd toroidal Alfvén eigenmodes on EAST

In this work, linear simulations of toroidal Alfvén eigenmodes (TAEs) driven by energetic particles (EPs) on EAST (Experimental Advanced Superconducting Tokamak) are performed using the hybrid-kinetic MHD model implemented in NIMROD code. Furthermore, the EAST equilibrium is reconstructed using the EFIT code based on the experimental measurement. The “slowing down” distribution is adopted for modeling the equilibrium distribution of the energetic ions from the deuterium neutral beam injection on EAST. The frequency, the dominant poloidal mode number, the radial location and the detailed 2D mode structure of the TAE/RSAE (Reverse Shear Alfvén Eigenmode)/EPM (energetic particle mode) modes are consistent between the eigenvalue analysis and the NIMROD simulation. As the β fraction of EP increases, a transition between even and odd TAEs occurs, along with that between the ballooning and antiballooning mode structures. It is demonstrated that when the β fraction of EP is close to the transition threshold, both types of TAEs coexist.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗