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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Advanced ion thruster research

A series of experiments conducted on a ring cusp magnetic field ion thruster; in which the anode, cathode and discharge chamber backplate were moved relative to the magnetic cusp; are described. Optimum locations for the anode, cathode and backplate which yield the lowest energy cost per plasma ion and highest extracted ion fraction are identified. The results are discussed in terms of simple physical models. The results of preliminary experiments into the operation of hollow cathodes on nitrogen and xenon over a large pressure range (0.1 to 100 Torr) are presented. They show that the cathode discharge transfers from the cathode insert to the exterior edge of the orifice plate as the interelectrode pressure is increased. Experimental evidence showing that a new ion extractor grid concept can be used to stabilize the plasma sheath at the screen grid is presented. This concept, identified by the term constrained sheath optics, is shown to hold ion beamlet divergence and impingement characteristics to stable values as the beamlet current and the net and total accelerating voltages are changed. The current status of a study of beamlet vectoring induced by displacing the accelerator and/or decelerator grids of a three grid ion extraction system relative to the screen grid is discussed.

Wilbur, P. J.↗
DOE OSTI2024

Enhancing the Performance of Plasma-facing Materials Through Solute-stabilized Nanostructured Tungsten Alloys

This report is a summary of the work completed under the auspices of the Department of Energy Early Career Award DE-SC0017899 entitled “Enhancing the Performance of Plasma-facing Materials Through Solute-stabilized Nanostructured Tungsten Alloys”. Summaries of initiatives on computational thermodynamics modeling, experimental alloy synthesis including traditional and additive manufacturing routes, several ion irradiation studies, and plasma exposures are reported.

36 MATERIALS SCIENCE↗
NASA NTRS1992

Inhibition of electron thermal conduction by electromagnetic instabilities

Heat flux inhibition by electromagnetic instabilities in a hot magnetized plasma is investigated. Low-frequency electromagnetic waves become unstable due to anisotropy of the electron distribution function. The chaotic magnetic field thus generated scatters the electrons with a specific effective mean free path. Saturation of the instability due to wave-wave interaction, nonlinear scattering, wave propagation, and collisional damping is considered. The effective mean free path is found self-consistently, using a simple model to estimate saturation level and scattering, and is shown to decrease with the temperature gradient length. The results, limited to the assumptions of the model, are applied to astrophysical systems. For some interstellar clouds the instability is found to be important. Collisional damping stabilizes the plasma, and the heat conduction can be dominated by superthermal electrons.

Levinson, Amir↗
DOE OSTI2023

Time dependent model for non-inductive ECH X-I current ramp-up for SHPD tokamak facility

Non-inductive (NI) plasma current start-up and ramp-up is an important research topic for spherical tokamak (ST) based reactors and fusion pilot plant (FPP). For a compact FPP, the OH flux availability is highly restricted due to its compact geometry. Efficient fundamental extraordinary mode (X-I) electron cyclotron heating (ECH) current start-up and ramp-up regime was identified for a reactor-like high toroidal magnetic field range which has more than a hundred times higher current drive efficiency compared to more conventional ECH methods for the relevant start-up temperature range. High current drive efficiency is possible due to the strong X-I fundamental ECH interaction only with unidirectional passing electrons constrained by the wave accessibility conditions. Here, we extend the X-I electron cyclotron current drive (ECCD) investigation to a time dependent model to simulate the non-inductive current ramp-up to 10 MA for the Sustained High-Power Density (SHPD) facility. As the X-I ECCD driven current I EC rises, due to the back EMF driven negative current, the net plasma current Ip rises more slowly with the current resistive time scale. For tokamak confinement time (both L-mode and H-Mode) which tends to rise with I p , a positive feedback results and even with constant applied ECH power, T e0 , I EC , and I p can continue to rise to very high values. However, in a realistic situation, the T e0 rise should saturate due to a number of factors such as enhanced core radiation and increased power loss at high temperature. To simulate this effect, we adopt a maximum T e0 model which would limit the temperature rise to certain T e0 . With this model, we investigated the current ramp-up for various ECH power levels and the maximum T e0 of 15, 20, and 25 keV. We find that while the power required is reduced with increasing T e0 limit due to increased current drive efficiency, the time to reach 10 MA tends to go up due to the reduced plasma resistivity for the higher T e0 limit. We also find for a given Te0 limit, the time to reach 10 MA tends to be reduced by increasing the applied ECH power by over driving the current ramp-up where I EC is driven at significantly higher level than 10 MA. While the current ramp-up time may not be an issue for the steady-state reactor systems, if it is desirable to minimize the current ramp-up time, it is prudent to have a sufficient ECH power for current over-drive and have some T e0 limiting tools such as impurity seeding for enhanced radiation. In conclusion, a well-controlled NI ECH start-up also has a potential of improving the tokamak start-up reliability and avoid run-away electrons while the NI off-axis current drive could enhance MHD stability and plasma performance improvements.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Plasma Exhaust in a Sheared-Flow-Stabilized Z Pinch

The fusion Z-pinch experiment device produces a 0.3 cm radius by 50 cm-long Z pinch between the end of the inner electrode of a coaxial plasma gun and an end wall 50 cm away. The plasma column is stabilized for thousands of instability growth times by an embedded radially sheared axial plasma flow. To investigate the effect of end wall design on Z-pinch plasma behavior, the solid end wall with a central exhaust port is replaced with a spoked end wall with a solid hub. While the Z-pinch plasma behavior was largely unaffected, the plasma exhausted from the Z-pinch provides an experimental platform to study magnetic confinement and detachment. Here, current and density signals are compared between two cases: a low energy and a high energy case. Plasma is found to be frozen-in flux in the Z-pinch assembly region. The exhaust of plasma from the flux conserving region is found to be dependent on the ratio of plasma ram and thermal pressures to the local magnetic pressure, calculated from an equilibrium model at the end wall. Plasma exhaust is observed to increase with the spoked end wall only for the high energy conditions as its ratio of ram and thermal pressures to the magnetic pressure increases.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Alfvén eigenmode stability in a JET afterglow deuterium plasma and projections to deuterium–tritium plasmas

Abstract The performance of fusion devices relies strongly on the good confinement of energetic particles (EPs). Therefore, the investigation of EP transport by magnetohydrodynamic instabilities is one of the key aspects in the development of plasma scenarios. Alfvénic instabilities in particular can lead to significant losses of alpha particles that are essential for plasma self-heating. A so-called afterglow scheme has been developed to study the destabilization of Alfvén eigenmodes (AEs) by alpha particles and associated EP transport in the JET tokamak. In this work, the linear stability of AEs is discussed for the partial afterglow phase in a JET deuterium plasma discharge and for the full afterglow phase in a projected deuterium–tritium (DT) plasma. Thanks to recent upgrades in the tokamak transport code TRANSP, one can account for the contributions of different EP species to mode stability. Analysis of deuterium plasmas shows that AE growth rates are extremely sensitive to the energy and distribution of fast ions. An increase in fast ion energy can lead to more unstable AEs. In the afterglow phase of projected DT plasmas, it is EPs that mostly drive the AEs. However, the drive by alpha particles is comparable to that by beam ions and their contribution to the net growth rate might be hard to separate. According to the discussed projections, the destabilization of AEs might be ineffective because the background plasma damping significantly exceeds the EP drive. In this case, the development of an alternative plasma scenario that allows us to overcome such damping would be required in future experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Determination of a macro- to micro-scale progression leading to a magnetized plasma disruption

We report here the observations of a plasma jet evolving through a macro- to micro-scale progression sequence. This leads to a fast magnetic reconnection that results in the jet breaking off from its originating electrode and forming a force-free state. A sausage-like pinching occurs first and squeezes an initially fat, short magnetized jet so that it becomes thin. The thin jet then becomes kink unstable. The lengthening of the jet by the kinking thins the jet even more since the kink is an incompressible instability. When the jet radius becomes comparable to the ion-skin depth, Hall and electron inertial physics become important and establish the environment for a fast magnetic reconnection. This fast reconnection occurs, disrupting the jet and establishing a force-free state. X-ray bursts and whistler waves, evidence of a magnetic reconnection, are observed when the plasma jet breaks off from the electrode. This experimentally confirmed sequence of successive thinning from pinching followed by kinking is reproduced in a three-dimensional ideal Magnetohydrodynamic (MHD) numerical simulation. The results of the experiment and the numerical simulation, together demonstrate a viable path from macro-scale MHD physics to micro-scale non-MHD physics where fast reconnection occurs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Experimental study of ECH pre-ionization on J-TEXT

An experimental study on electron cyclotron heating (ECH) pre-ionization has been conducted on J-TEXT in support of the joint experiment research for ITER plasma initiation. In this experiment, ECH power was injected to the vessel before the application of loop voltage ionize the neutral gas and form the initial plasma or so-called pre-plasma. The impact of several significant factors, such as magnetic field configuration, pre-fill gas pressure, ECH toroidal injection angle and ECH power on the evolution of pre-plasma are systematically studied, aiming to identify shared features, clarify their potential relationship and optimize the discharge parameters to generate a rather high pre-plasma density. By separating ECH power from the inductive start-up, the effect of pre-plasma on tokamak start-up can be observed. A dynamic magnetic configuration facilitates the transition of pre-plasma to tokamak plasma. To assess the influence of pre-plasma density on tokamak start-up, two kinds of magnetic field configurations are examined. While the effect of different pre-plasma densities on tokamak start-up is negligible, a significant difference is observed between pure ohmic start-up and start-up with pre-ionization. The studies presented here show evolutionary trends and threshold values needed to optimize ECH pre-ionization and also a feasible way to improve pre-plasma density and a configuration to stabilize pre-plasma for transition. Eventually, these results may contribute to multi-machine research and physics analysis that can assist ITER with its optimal preparation for first plasma operation.

ECH↗
DOE OSTI2026

Physics basis for the reference flat-top plasma scenario in the ST–E1 fusion power plant

As part of the U.S. Department of Energy’s Milestone-Based Fusion Energy Development Program, Tokamak Energy has completed the pre-concept design of the ST–E1 fusion power plant. ST–E1 is envisaged to operate in two phases: a pilot plant phase, targeting sustained net power production of 300 - 500 MWe for a duration >1 hr, followed by a commercial power plant phase targeting steady-state operations and a normalised overnight capital cost of ⩽12 000 $\$$/kWe. The design process adopted was highly iterative, integrating all major plant systems and progressing in a phased fidelity approach. At the pre-conceptual stage, the emphasis has been on exploring the design space, identifying the main system-level trade-offs, and making the key decisions that define the overall plant concept, rather than optimising a single operating point. This paper, part of a focused collection detailing the ST–E1 pre-concept design, addresses the development of a series of reference flat-top plasma operating points for the pilot plant phase. A modelling workflow was established to develop and assess candidate plasma design points and explore key dependencies. The workflow includes integrated core plasma modelling, magnetohydrodynamic (MHD) stability assessment, equilibrium generation, scrape-off-layer and exhaust modelling, heating & current drive design and optimisation, and turbulent transport modelling. Using this framework, the impact of several key parameters on the flat-top operating space was investigated, including the density limit, core radiation fraction and divertor power loading, level of external heating and curent drive power and assumed pedestal characteristics. The MHD stability, controllability and micro-stability characteristics of these plasmas were also analysed. These investigations informed the definition of a set of fully non-inductive, flat-top reference operating points that satisfy the high-level ST–E1 mission, including a low and high density case, a case that is stable to resistive wall modes and a case with reduced divertor power loading.

ST–E1↗
DOE OSTI2021

Quasisymmetric Optimization of Nonaxisymmetry in Tokamaks

Predictive 3D optimization reveals a novel approach to modify a nonaxisymmetric magnetic perturbation to be entirely harmless for tokamaks, by essentially restoring quasisymmetry in perturbed particle orbits as much as possible. Such a quasisymmetric magnetic perturbation (QSMP) has been designed and successfully tested in the KSTAR and DIII-D tokamaks, demonstrating no performance degradation despite the large overall amplitudes of nonaxisymmetric fields and strong response otherwise expected in the tested plasmas. The results indicate that a quasisymmetric optimization is a robust path of error field correction across the resonant and nonresonant field spectrum in a tokamak, leveraging the prevailing concept of quasisymmetry for general 3D plasma confinement systems such as stellarators. Finally, the optimization becomes, in fact, a simple eigenvalue problem to the so-called torque response matrices if a perturbed equilibrium is calculated consistent with nonaxisymmetric neoclassical transport.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Multiscale Chirping Modes Driven by Thermal Ions in a Plasma with Reactor-Relevant Ion Temperature

A thermal ion driven bursting instability with rapid frequency chirping, considered as an Alfv´enic ion temperature gradient mode, has been observed in plasmas having reactor-relevant temperature in the DIII-D tokamak. The modes are excited over a wide spatial range from macroscopic device size to micro-turbulence size and the perturbation energy propagates across multiple spatial scales. The radial mode structure is able to expand from local to global in ~ 0.1 ms and it causes magnetic topology changes in the plasma edge, which can lead to a minor disruption event. Since the mode is typically observed in the high ion temperature ≳ 10 keV and high-! plasma regime, the manifestation of the mode in future reactors should be studied with development of mitigation strategies, if needed. In conclusion, this is the first observation of destabilization of the Alfv´en continuum caused by the compressibility of ions with reactor-relevant ion temperature.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Neoclassical Tearing Mode Seeding by Nonlinear Three-Wave Interactions in Tokamaks

We report the experimental observation of seed magnetic island formation by nonlinear three-wave coupling of magnetic island triplets. In this experiment, disruptive 2,1 islands are seeded by the coupling of 4,3 and 3,2 tearing modes to a central 1,1 sawtooth precursor. Three-wave interactions between these modes are conclusively identified by bispectral analysis, indicating fixed phase relationships in agreement with theory. Furthermore, this new observation of this seeding mechanism has important implications for future reactors that must operate in stable plasma equilibria, free of disruptive 2,1 islands.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Self-Generated Plasma Rotation in a Z-Pinch Implosion with Preembedded Axial Magnetic Field

Using detailed spectroscopic measurements, highly resolved in both time and space, a self-generated plasma rotation is demonstrated using a cylindrical implosion with a preembedded axial magnetic field (B z0 ). The rotation direction is found to depend on the direction of B z0 and its velocity is found comparable to the peak implosion velocity, considerably affecting the force and energy balance throughout the implosion. Moreover, the evolution of the rotation is consistent with magnetic flux surface isorotation, a novel observation in a Z pinch, which is a prototypical time dependent system.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Ideal MHD Limited Electron Temperature in Spherical Tokamaks

It is well documented that the central electron temperature in the national spherical torus experiment (NSTX) remains largely unchanged as the external heating power, and hence the normalized volume averaged plasma pressure β increases [Stutman, Phys. Rev. Lett. 102, 115002 (2009)]. Herein we present a hypothesis that low n, pressure driven ideal magnetohydrodynamic (MHD) instabilities that are nondisruptive, can break magnetic surfaces in the central region and thereby flatten the electron temperature profiles. We demonstrate this mechanism in a 3D resistive MHD simulation of a NSTX discharge. By varying the toroidal magnetic field strength, and/or the heating power, we show that there is a critical value of β, above which the central temperature profile no longer peaks on axis.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Shifting and Splitting of Resonance Lines due to Dynamical Friction in Plasmas

We report a quasilinear plasma transport theory that incorporates Fokker-Planck dynamical friction (drag) and pitch angle scattering is self-consistently derived from first principles for an isolated, marginally unstable mode resonating with an energetic minority species. It is found that drag fundamentally changes the structure of the wave-particle resonance, breaking its symmetry and leading to the shifting and splitting of resonance lines. In contrast, scattering broadens the resonance in a symmetric fashion. Comparison with fully nonlinear simulations shows that the proposed quasilinear system preserves the exact instability saturation amplitude and the corresponding particle redistribution of the fully nonlinear theory. Even in situations in which drag leads to a relatively small resonance shift, it still underpins major changes in the redistribution of resonant particles. This novel influence of drag is equally important in plasmas and gravitational systems. In fusion plasmas, the effects are especially pronounced for fast-ion-driven instabilities in tokamaks with low aspect ratio or negative triangularity, as evidenced by past observations. The same theory directly maps to the resonant dynamics of the rotating galactic bar and massive bodies in its orbit, providing new techniques for analyzing galactic dynamics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Perturbed Ion Temperature and Toroidal Flow Profile Measurements in Rotating Neoclassical Tearing Mode Magnetic Islands

The perturbed ion temperature and toroidal flow were measured in rotating Neoclassical Tearing Modes (NTM) in a tokamak for the first time. These toroidally and radially resolved profiles were obtained by impurity ion spectroscopy in a 2,1 NTM in DIII-D. In agreement with drift-kinetic simulations, the electron temperature profile is flat, while the ion temperature gradient is restored across the magnetic island O-point in the presence of fast ions; the perturbed flow has minima in the O-points and maxima at the X-points. Furthermore, these measurements provide the first confirmation of the theoretically expected ion temperature and flow response to a magnetic island needed to predict the NTM on set threshold scaling for ITER and other future tokamaks.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗