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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Three-Dimensional Helical-Rotating Plasma Structures in Beam-Generated Partially Magnetized Plasmas

Azimuthal structures emerging in beam-generated partially magnetized plasmas are investigated using three-dimensional particle in cell-Monte Carlo collision simulations. Two distinct instability regimes are identified at low pressures. Furthermore, when the gas pressure is sufficiently high, quasineutrality is attained and 2D spiral-arm structures form as a result of the development of a lower-hybrid instability, resulting in enhanced cross-field transport. At lower pressures, quasineutrality is not achieved and a 3D helical-rotating plasma structure forms due to development of the diocotron instability. Analytical formulas are proposed for the critical threshold pressure between these regimes and for the rotation frequency of the helical structures. Preliminary experimental verification is provided.

Penning discharges↗
NASA NTRS1977

Plasma rotation by electric and magnetic fields in a discharge cylinder

A theoretical model for an electric discharge consisting of a spatially diverging plasma sustained electrically between a small ring cathode and a larger ring anode in a cylindrical chamber with an axial magnetic field is developed to study the rotation of the discharge plasma in the crossed electric and magnetic fields. The associated boundary-value problem for the coupled partial differential equations which describe the electric potential and the plasma velocity fields is solved in closed form. The electric field, current density, and velocity distributions are discussed in terms of the Hartmann number and the Hall coefficient. As a result of Lorentz forces, the plasma rotates with speeds as high as 1 million cm/sec around its axis of symmetry at typical conditions. As an application, it is noted that rotating discharges of this type could be used to develop a high-density plasma-ultracentrifuge driven by j x B forces, in which the lighter (heavier) ion and atom components would be enriched in (off) the center of the discharge cylinder.

Wilhelm, H. E.↗
DOE OSTI2021

Natural hot-ion modes in a rotating plasma

We report that in steady state, the fuel cycle of a fusion plasma requires inward particle fluxes of fuel ions. These particle flows are also accompanied by heating. In the case of classical transport in a rotating cylindrical plasma, this heating can proceed through several distinct channels depending on the physical mechanisms involved. Some channels directly heat the fuel ions themselves, whereas others heat electrons. Which channel dominates depends, in general, on the details of the temperature, density and rotation profiles of the plasma constituents. However, remarkably, under relatively few assumptions concerning these profiles, if the α particles, the by-products of the fusion reaction, can be removed directly by other means, then a hot-ion mode tends to emerge naturally.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Nonideal stability analysis of differentially rotating plasmas with global curvature effects

The linear stability of global nonaxisymmetric modes in differentially rotating, magnetized, nonideal plasma is critical to classifying turbulence and transport phenomena. We investigate the competition between the local magneto-rotational instability (MRI) and the magneto-curvature instability (MCI)—a distinct nonaxisymmetric low-frequency curvature-driven global branch that appears alongside MRI. Here, to accomplish this, we developed a nonideal global spectral method, which is validated against NIMROD code simulations. This spectral approach allows for the direct derivation of an extended effective potential formalism and a resistive Alfvénic resonance condition, providing a framework for direct analysis of energy contributions and confinement mechanisms. Our study reveals that the global, low-frequency MCI persists at low magnetic Reynolds numbers (Rm), whereas the localized, high-frequency MRI is stabilized by diffusive broadening of its structure around its Alfvénic resonances. Consequently, we identify the global MCI branch as the primary onset mechanism for nonaxisymmetric magnetohydrodynamic instability in systems with finite curvature, e.g., astrophysical rotators. We establish distinct parameter regimes for mode dominance: MCI prevails in geometrically moderate-thickness disks with intermediate curvature and radial gaps, while MRI dominates in thin, low-curvature disks with large radial gaps. Mode competition is also highly sensitive to the flow profile, particularly vorticity and its gradient, with nonuniform shear profiles exhibiting more robust instability due to flow curvature (i.e., the second derivative of the flow profile) and shear contributions. A key outcome is the development of spectral diagrams derived from the global spectral method. These diagrams comprehensively map dominant instabilities and their characteristics, offering a predictive tool for critical onset parameters (i.e., flow curvature, magnetic field, and Rm) and facilitating the interpretation of experimental and simulation results. Notably, these diagrams demonstrate that the global MCI is generally the sole unstable mode at the initial onset of nonaxisymmetric instability.

Haywood, Alexander [Princeton Univ., NJ (United St↗
DOE OSTI2026

Global Non-axisymmetric Hall Instabilities in a Rotating Plasma

Non-axisymmetric, flow-driven instabilities in the incompressible Hall-MHD model are studied in a differentially rotating cylindrical plasma. It is found that, in the Hall-MHD regime, both whistler waves and ion-cyclotron waves can extract energy from the flow shear, resulting in two distinct branches of global instability. The non-axisymmetric whistler modes grow significantly faster than non-axisymmetric, ideal MHD modes. A discussion of the global whistler instability mechanism is presented in the large-ion-skin-depth, “electron-MHD” limit. When the magnetic field is azimuthal, a subset of the whistler modes having zero axial wave number are uncovered to be destabilized by the “corotation amplifier” mechanism. It is observed that the effect of the Hall term on the non-axisymmetric modes can be appreciable when d i is on the order of a few percent of the width of the cylindrical annulus. Distinct global modes emerge in the strong Hall-MHD regime at significantly stronger magnetic fields than those required for unstable global MHD modes, as the Hall effect weakens the stabilizing “field-line bending” by decoupling ion motion from the magnetic field. These global non-axisymmetric modes may play an important role in weakly ionized accretion disks.

Alfven waves↗
DOE OSTI2022

Comparison of Doppler back-scattering and charge exchange measurements of E × B plasma rotation in the DIII-D tokamak under varying torque conditions

Measurements of the E × B toroidal angular velocity, $\omega_{E \times B}\,=\,E_\mathrm{r}/RB_\theta$ ($E_\mathrm{r}$ is the radial electric field, B θ is the poloidal magnetic field), are made using the Doppler back-scattering (DBS) and charge-exchange recombination (CER) spectroscopy diagnostics. DBS uses the Doppler shift of wavenumber-resolved density fluctuations while CER uses the Doppler shift of impurity emission lines to independently measure plasma parameters for calculating the local radial electric field. DBS and CER profiles of $\omega_{E \times B}$ as a function of normalized toroidal flux (ρ) are compared at various levels of neutral beam applied torque on the plasma. Under standard neoclassical theory $\omega_{E\times B}$ is a flux surface quantity, making it appropriate to compare across diagnostics. DBS and CER generally show good agreement when comparing $\omega_{E \times B}$ profiles at different levels of neutral beam injection-applied torque. Furthermore, the DBS values have close to the same precision as CER values when averaged over a similar time-scale and effects, such as prompt-torque are considered. DBS is able to observe the rapid ($\lt$10 ms) modification of the $E_\mathrm{r}$ profile by the diagnostic neutral beam 'blips'. This modification is most pronounced when the blip applies a large relative change in torque on the plasma. Overall, these results could have implications on transport analysis and suggests using DBS and CER in conjunction to constrain values of the E × B-shear (sometimes called $\gamma_{E \times B}$).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Impact of n > 1 neoclassical tearing modes on fast ions, plasma rotation, and the onset of disruptive tearing modes in DIII-D ITER baseline scenario

We present integrated TRANSP-kick analyses quantitatively characterizing how n > 1 magnetic islands redistribute fast ions (FIs) and modify momentum balance in low-torque DIII-D H-mode scenarios operated with the ITER normalized parameter set and shape (ITER baseline scenario, or IBS). In this plasma scenario, disruptive neoclassical tearing modes (NTMs) are most commonly seeded by nonlinear 3-wave coupling when the differential rotation between the q = 1 and q = 2 surfaces ($δf_{1, 2}$) approaches zero. Analysis of a DIII-D IBS database of shots unstable to the 2/1 NTM demonstrates that the flattening rotation profile is correlated with n > 1 NTM amplitude growth, with a strong drop in $δf_{1, 2}$ occurring when the radial magnetic perturbation amplitude at the rational surface ($δB$ r s ) is estimated to be about 20 G. The interpretive TRANSP-kick simulations show that the experimentally observed reduction in core rotation is due to the fast-ion kicks. Nonlinearity in momentum losses versus NTM amplitude is found to be correlated with overlap of FI island chains, occurring when $δB$ r s is about 35 G, in rough agreement with the experimental results. This agreement suggests the primary mechanism for the conductive momentum loss in the core is due to chaotic FI orbits and FI losses developing from overlapping resonant FI island chains.

ITER baseline scenario↗
DOE OSTI2024

Structure and dynamics of magneto-inertial, differentially rotating laboratory plasmas

We present a detailed characterization of the structure and evolution of differentially rotating plasmas driven on the MAGPIE pulsed-power generator (1.4 MA peak current, 240 ns rise time). The experiments were designed to simulate physics relevant to accretion discs and jets on laboratory scales. A cylindrical aluminium wire array Z pinch enclosed by return posts with an overall azimuthal off-set angle was driven to produce ablation plasma flows that propagate inwards in a slightly off-radial trajectory, injecting mass, angular momentum and confining ram pressure to a rotating plasma column on the axis. However, the plasma is free to expand axially, forming a collimated, differentially rotating axial jet that propagates at ${\approx }100\,{\rm km}\,{\rm s}^{-1}$ . The density profile of the jet corresponds to a dense shell surrounding a low-density core, which is consistent with the centrifugal barrier effect being sustained along the jet's propagation. We show analytically that, as the rotating plasma accretes mass, conservation of mass and momentum implies plasma radial growth scaling as $r \propto t^{1/3}$ . As the characteristic moment of inertia increases, the rotation velocity is predicted to decrease and settle on a characteristic value ${\approx }20\,{\rm km}\,{\rm s}^{-1}$ . We find that both predictions are in agreement with Thomson scattering and optical self-emission imaging measurements.

Valenzuela-Villaseca, V. (ORCID:0000000167721441)↗
DOE OSTI2024

Massive, long-lived electrostatic potentials in a rotating mirror plasma

Abstract Hot plasma is highly conductive in the direction parallel to a magnetic field. This often means that the electrical potential will be nearly constant along any given field line. When this is the case, the cross-field voltage drops in open-field-line magnetic confinement devices are limited by the tolerances of the solid materials wherever the field lines impinge on the plasma-facing components. To circumvent this voltage limitation, it is proposed to arrange large voltage drops in the interior of a device, but coexist with much smaller drops on the boundaries. To avoid prohibitively large dissipation requires both preventing substantial drift-flow shear within flux surfaces and preventing large parallel electric fields from driving large parallel currents. It is demonstrated here that both requirements can be met simultaneously, which opens up the possibility for magnetized plasma tolerating steady-state voltage drops far larger than what might be tolerated in material media.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Machine learning methods for probabilistic locked-mode predictors in tokamak plasmas

A rotating tokamak plasma can interact resonantly with the external helical magnetic perturbations, also known as error fields. This can lead to locking and then to disruptions. We leverage machine learning (ML) methods to predict the locking events. We use a coupled third-order nonlinear ordinary differential equation model to represent the interaction of the magnetic perturbation and the plasma rotation with the error field. This model is sufficient to describe qualitatively the locking and unlocking bifurcations. Here, we explore using ML algorithms with the simulation data and experimental data, focusing on the methods that can be used with sparse datasets. These methods lead to the possibility of the avoidance of locking in real-time operations. We describe the operational space in terms of two control parameters: the magnitude of the error field and the rotation frequency associated with the momentum source that maintains the plasma rotation. The outcomes are quan- tified by order parameters that completely characterize the state, whether locked or unlocked. We use unsupervised ML methods to classify locked/unlocked states and note the usefulness of a certain normalization of the order parameters. Three supervised ML classifiers are used in suite to estimate the probability of locking in the region of control parameter space with hysteresis, i.e., the set of control parameters for which both locked and unlocked states can exist. The results show that a neural network gives the best estimate of the locking probability. An analogy of the present locking model with the van der Waals equation of state is also provided.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Quasilinear theory of Brillouin resonances in rotating magnetized plasmas

Both spin and orbital angular momentum can be exchanged between a rotating wave and a rotating magnetized plasma. Through resonances the spin and orbital angular momentum of the wave can be coupled to both the cyclotron rotation and the drift rotation of the particles. It is, however, shown that the Landau and cyclotron resonance conditions which classically describe resonant energy–momentum exchange between waves and particles are no longer valid in a rotating magnetized plasma column. In this case a new resonance condition which involves a resonant matching between the wave frequency, the cyclotron frequency modified by inertial effects and the harmonics of the guiding centre rotation is identified. A new quasilinear equation describing orbital and spin angular momentum exchanges through these new Brillouin resonances is then derived, and used to expose the wave-driven radial current responsible for angular momentum absorption.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Stabilization of kink/peeling modes by coupled rotation and ion diamagnetic drift effects in quiescent H-mode plasmas in DIII-D and JT-60U

Magnetohydrodynamic stability at the edge pedestal in several quiescent H-mode (QH-mode) plasmas in DIII-D and JT-60U experiments was analyzed by considering plasma rotation and ion diamagnetic drift effects. It was identified that a kink/peeling mode, which is a prime candidate for a trigger of edge harmonic oscillation in QH-mode, is stabilized by plasma rotation when considering the ion diamagnetic drift simultaneously in both experiments. Furthermore, the stabilizing effect by rotation becomes more effective in case using the rotation profile of the main ion species evaluated by assuming radial force balance. In addition, when inverting the rotation direction, it was found that the kink/peeling mode is more stabilized when considering the rotation of the main ion species, though the mode is less stabilized by the rotation of impurity ion species. The result implies that the kink/peeling mode stability in QH-mode plasmas is sensitive to how the rotation profile is evaluated, but it is shown that a qualitative trend stabilizing the kink/peeling mode by rotation can be reproduced with the poloidal rotation profile of an impurity predicted numerically based on the neoclassical theory.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Image Rotation in Plasmas

Because of the speed of light compared to material motion, the dragging of light is difficult to observe under laboratory conditions. Furthermore we report on the first observation of image rotation, i. e. a dragging by the medium of the wave’s transverse structure, of Alfv´en waves in plasmas. Exploiting the naturally slow group velocity of these waves, significant wave rotation is achieved for modest angular frequency. Control over the rotation of the wave’s structure is demonstrated through the plasma rotation imposed by biased electrodes. Remarkably, experimental results are well reproduced by light dragging theory derived for isotropic media, even if magnetized plasmas are anisotropic. In addition to offering new insights into the fundamental issue of angular momentum coupling between waves and media, these findings also open possibilities for new remote rotation sensing tools.

Alfvén waves↗
NASA NTRS1992

The polarization electric field and its effects in an anisotropic rotating magnetospheric plasma

Spatial variations of density and temperature along a magnetic field line are evaluated for a plasma undergoing adiabatic motion in a rotating magnetosphere. The effects of centrifugal and gravitational forces are accounted for, as is anisotropy in the pitch angle distribution functions of individual species. A polarization electric field is invoked to eliminate the net electric charge density resulting from the aforementioned mass dependent forces and different anisotropies. The position of maximum density in a two-component, electron-ion plasma is determined both in the absence and in the presence of the polarization effect and compared. A scale height, generalized to include anisotropies, is derived for the density fall-off. The polarization electric field is also included in the parallel guiding center equation; equilibrium points are determined and compared in both individual and average senses with the position of density maximum. Finally a transverse (to magnetic field lines) electric component is deduced as a consequence of dissimilar charge neutralization on adjacent field lines. The E x B velocity resultant from such a 'fringing' electric field is calculated and compared with the magnitude of other drifts.

Huang, T. S.↗
DOE OSTI2025

Identification of a window for quiescent H-mode operation in MHD stability diagram of DIII-D plasmas

A window for quiescent H-mode (QH-mode) operation in DIII-D plasmas in the diagram of pedestal magnetohydrodynamic (MHD) stability was identified through linear MHD stability analysis, accounting for the effects of plasma rotation and ion diamagnetic drift. The operation window lies between the stability boundaries of the kink/peeling mode (K/PM) identified with and without the inclusion of plasma rotation effects. In this region, the mode remains unstable unless rotation effects are considered alongside the ion diamagnetic drift, which is consistently included in the analysis. The stabilization of the MHD mode, facilitated by the coupled effects of plasma rotation and ion diamagnetic drift, plays a crucial role in widening the window, enabling the attainment of the QH-mode state. Furthermore, the suppression of edge localized modes (ELM) can be achieved by controlling the pedestal structure to ensure the plasma state remains within the operation window. The location of the operation window in the stability diagram depends on the K/PM stability properties. Therefore, optimizing conditions for QH-mode requires adjustments based on changes in stability characteristics. A pressure pedestal and its associated bootstrap current density near the last closed flux surface are advantageous for situating the plasma state within the window. However, excessive current density can trigger ELMs. This trend was confirmed through comparisons of MHD stability diagrams between QH-mode and ELMy H-mode plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗