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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Accelerating the magnetic island by the modulated resonant magnetic perturbation for the disruption avoidance on J-TEXT

We report the acceleration of the magnetic island rotation by the modulated resonant magnetic perturbation (MRMP) has been studied in J-TEXT tokamak experiments. After applying the MRMP, the phase difference between the tearing mode (TM) and MRMP, Δξ, oscillated near the effective phase difference, Δξ eff , which was defined as the time averaged value of Δξ. When the Δξ eff was closed to the—π/2, the MRMP only contributed an accelerating torque on the TM. As the result, the TM rotation frequency was increased by a few kilohertz for the optimized relative phase by small RMPs of the order of 10 -5 of the toroidal field and the locked mode induced disruption was avoided. It is found that the TM rotation could be increased to a higher frequency by applying a stronger MRMP. There is a negative sinusoidal relationship between TM frequency and Δξ eff .

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Assessment of radial transport induced by Alfvénic resonances in tokamaks and stellarators

In a symmetric plasma confinement device, a perturbation such as an Alfvén mode produces a resonance island which flattens the local energy gradient due to induced rotation about the elliptic points of the resonance. The location of the resonance is mode frequency and particle energy dependent, and particles away from the resonance are not affected. Here, a projection onto the Poincaré subspace allows the evaluation of resonance widths and internal rotation rates. In a stellarator without symmetry because of the toroidal dependence of the equilibrium magnetic field, a mode instead produces chaos in all orbits passing where the amplitude is large and is, thus, much more effective in producing radial transport and loss. The mode growth rate and saturation are also affected by the local chaos.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Spatial channeling in toroidal plasmas: overview and new results

Energy and momentum transfer by eigenmodes in tokamaks and stellarators, which arises when eigenmodes are destabilized by superthermal ions in a region that does not coincide with the damping region, is considered. This phenomenon, named spatial channeling, may play an important role in toroidal plasmas. Furthermore, it may aect plasma energy connement, plasma rotation, heating of the electron and ion components, and features of eigenmodes. The work contains both an overview and new results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Plasma rotation and diamagnetic drift effects on the resistive wall modes in the negative triangularity tokamaks

Abstract It was found previously that the negative triangularity (NT) configuration is more MHD-unstable for low n modes than the positive triangularity (PT) case, although the situation is reversed for intermediate n modes and the NT configuration becomes more stable for intermediate n modes ( n = 3 − 10 ) (Zheng et al 2021 Nucl. Fusion 61 116014). Here, n is the toroidal mode number. In this work, we extend the studies to include the rotation effects, as well as the diamagnetic drift effects, to see how the resistive wall modes (RWMs) in the NT configuration are affected as compared with the PT configuration. This is particularly motivated by noting that the wall interface with the plasma is quite different between the NT and PT configurations. It affects the plasma rotation and diamagnetic drift effects on the low n RWM. We consider the DIII-D-NT-experiment equilibrium reconstructed by the EFIT code. Based on the equilibrium g-file, the extended equilibria are constructed with the VMEC code by varying the beta values while keeping the pressure and poloidal current flux profiles basically unchanged. The bootstrap current contribution to the equilibria is taken into account with the Sauter formula. The MHD stability is then computed using the AEGIS code with the rotation and diamagnetic drift effects taken into account. We found that, although the NT configuration is less stable for n = 1 MHD modes, the rotation and diamagnetic drift stabilization effects on RWMs are more effective in the NT configuration than in the PT one. Note that even in the PT case, the stabilization of RWMs by the rotation and kinetic effects is critical. Because the low-n RWMs in the regular NT case are more unstable, the rotation and diamagnetic drift stabilization effects found in this research are important for the NT tokamak concept.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
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 OSTI2023

Principal Challenges in Toroidal Magnetic Confinement Fusion Systems

Research efforts toward a practical and economical magnetically confined fusion power source have made steady progress over the years. Today, the principal approaches to confining a hot plasma of Deuterium and Tritium ions long enough for a significant frac/on to undergo thermonuclear burn employ toroidal geometries. Such geometries are attractive because charged particles are tightly bound around the field lines, undergoing locally helical orbits as they travel along those lines. In the core regions of the plasma, the field lines have no connection to the material surfaces making up the interior of the vacuum vessel, so that plasma confinement times can be long enough for significant thermonuclear burnup via fusion reactions to take place. For good confinement, the geometry of the field lines must be such that nested field lines (which lie on nested flux surfaces) twist around the toroidal axis (a quasi-circular line that approximately lies in the center of the confined plasma) at rates that vary based on their distance from that axis. The rotational transform (or field line pitch), iota/2π , is defined as the number of poloidal transits per single toroidal transit of a field line on a toroidal flux surface.

42 ENGINEERING↗
DOE OSTI2022

Power-transfer and fixed-point analysis of sawtooth simulations of a current-carrying stellarator

Power-transfer and fixed-point analysis of previous NIMROD simulations (Roberds et al. , Phys. Plasmas , vol. 23, issue 9, 2016, 092513) improved the understanding of the effect of 3D (non-axisymmetric equilibrium) magnetic fields on sawtooth oscillations in the Compact Toroidal Hybrid (CTH) experiment. Computing the locations of order-1 fixed points, their Greene's residues, and local values for the rotational transform results in a description of CTH sawteeth consistent with Kadomtsev's model. A power-transfer analysis quantifies the distribution of energy among toroidal Fourier modes and their nonlinear interactions. The Lorentz power transfer drives sawtooth growth, and it is unambiguously interpreted as the flow of energy from toroidal mode $n'$ to mode $n$ , catalysed by $\boldsymbol {B}_{n-n'}$ . It has been reported previously that the CTH sawtooth frequency increases with the 3D field strength. This is attributed to an increased growth rate of the internal kink that drives sawtooth oscillations. Here, 3D fields remove energy from the kink, eliminating the possibility that these fields are an additional energy source that drives growth. Instead, 3D fields catalyse energy transfer from large-to-small scales, where magnetic reconnection is stronger. It is proposed that this energy transfer increases the reconnection rate at small scales, which is consistent with the increased growth rate observed at higher 3D field strengths.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Bispectral analysis of broadband turbulence and geodesic acoustic modes in the T-10 tokamak

Local fluctuations of electrostatic potential, poloidal electric field, magnetic potential and electron density are simultaneously measured in the T-10 tokamak by a heavy ion beam probe (HIBP) having a five-slit energy analyser, which allows an estimate of the turbulent particle flux and $\boldsymbol {E}\times \boldsymbol {B}$ rotation velocity in the off-minor-axis gradient zone of the toroidal plasma column. The high spatial and temporal resolution of the modern multichannel HIBP makes it an effective tool to study plasma oscillations. Motivated by previous work that has documented time-resolved interactions between measured plasma parameters using correlation analysis (coherence of $E_{\textrm {pol}}$ and density $n_e$ , and cross-phase), a new result from bicorrelation analysis (bicoherence of magnetic potential $A_\zeta$ and density $n_e$ , and biphase) is reported for documenting the evidence of wave–wave coupling and energy transfer associated with the interaction between geodesic acoustic modes (GAM) and broadband, quasi-coherent modes.

Physics↗
DOE OSTI2021

Toroidal plasma acceleration due to NBI fast ion losses in LTX- β

The recent Lithium Tokamak Experiment-Beta (LTX- β ) upgrade includes the addition of neutral beam injection (NBI) in the same direction as the plasma current (co- I P ) and a new toroidal Mirnov array for MHD characterization. In initial NBI experiments, a spontaneously rotating n = 1 MHD mode is seen to accelerate during NBI in the counter-beam direction, accompanied by a rise in electron density consistent with the beam-injected inventory but without a clear increase in plasma pressure. Together with analytic and numerical modeling of beam optics and fast ion confinement, these observations indicate the prompt loss of all or nearly all beam ions. However, the same modeling also suggests that planned upgrades to the Ohmic heating system should provide the fast ion confinement necessary for beam heating and core fueling. Here, a simple analytic model relates the momentum confinement time τ Φ to the observed evolution of mode rotation due to the combination of NBI momentum coupling, fast ion loss $\vec{J}\times\vec{B}$, and anomalous viscous torques, yielding τ Φ values consistent with past measurements of electron energy confinement time τ E,e .

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Machine learning-based real-time kinetic profile reconstruction in DIII-D

Abstract Kinetic equilibrium reconstruction plays a vital role in the physical analysis of plasma stability and control in fusion tokamaks. However, the traditional approach is subjective and prone to human biases. To address this, the consistent automatic kinetic equilibrium reconstruction (CAKE) method was introduced, providing objective results. Nonetheless, its offline nature limits its application in real-time plasma control systems (PCSs). To address this limitation, we present RTCAKENN, a machine learning model that approximates 7 CAKE-level output profiles, namely pressure, inverse q , toroidal current density, electron temperature and density, carbon ion impurity temperature and rotation profiles, using real-time available inputs. The deep neural network consists of an encoder layer, where the scalars and interdependent inputs such as plasma boundary coordinates and motional Stark effect data are encoded using multi-layer perceptrons (MLPs), while profile inputs are encoded by 1D convolutional layers. The encoded data is passed through a MLP for latent feature extraction, before being decoded in the decoding layers, which consist of upsampling and convolutional layers. RTCAKENN has been implemented in the DIII-D PCS and our model achieves accuracy comparable to CAKE and surpasses existing real-time alternatives. Through clever dropout training, RTCAKENN exhibits robustness and can operate even in the absence of Thomson scattering data or charge exchange recombination data. It executes in under 8 ms in the real-time environment, enabling future application in real-time control and analysis.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Disruptive neoclassical tearing mode seeding in DIII-D with implications for ITER

New studies identify the critical parameters and physics governing disruptive neoclassical tearing mode (NTM) onset. An m/n = 2/1 mode in DIII-D that begins to grow robustly after a seeding event (edge localized mode ELM or sawtooth precursor and crash) causes the mode rotation to drop close to the plasma's Er = 0 rest frame; this condition opens the stabilizing ion-polarization current 'gate' and destabilizes an otherwise marginally stable NTM. Our new experimental and theoretical insights and novel toroidal theory-based modeling are benchmarked and scalable to ITER and other future experiments. Here, the nominal ITER rotation at q = 2 is found to be stabilizing ('gate closed') except for MHD-induced transients that could 'open the gate'. Extrapolating from the DIII-D ITER baseline scenario (IBS) discharges, MHD transients are much more likely to destabilize problematic robustly growing 2/1 NTMs in ITER; this makes predictions of seeding and control of both ELMs and sawteeth imperative for more than just minimizing divertor pulsed-heat loading.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Exploring nuclear exotica at the limits

The study of nuclear limits has been performed and new physical mechanisms and exotic shapes allowing the extension of nuclear landscape beyond the commonly accepted boundaries have been established. The transition from ellipsoidal to toroidal shapes plays an essential role in potential extension of nuclear landscape to hyperheavy nuclei. Moreover, rotational excitations leading to the birth of particle (proton or neutron) bound rotational bands provide a mechanism for an extension of nuclear landscape beyond spin zero proton and neutron drip lines.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Near-axis expansion of stellarator equilibrium at arbitrary order in the distance to the axis

A direct construction of equilibrium magnetic fields with toroidal topology at arbitrary order in the distance from the magnetic axis is carried out, yielding an analytical framework able to explore the landscape of possible magnetic flux surfaces in the vicinity of the axis. This framework can provide meaningful analytical insight into the character of high-aspect-ratio stellarator shapes, such as the dependence of the rotational transform and the plasma beta limit on geometrical properties of the resulting flux surfaces. The approach developed here is based on an asymptotic expansion on the inverse aspect ratio of the ideal magnetohydrodynamics equation. The analysis is simplified by using an orthogonal coordinate system relative to the Frenet–Serret frame at the magnetic axis. The magnetic field vector, the toroidal magnetic flux, the current density, the field line label and the rotational transform are derived at arbitrary order in the expansion parameter. Moreover, a comparison with a near-axis expansion formalism employing an inverse coordinate method based on Boozer coordinates (the so-called Garren–Boozer construction) is made, where both methods are shown to agree at lowest order. Finally, as a practical example, a numerical solution using a W7-X equilibrium is presented, and a comparison between the lowest-order solution and the W7-X magnetic field is performed.

Physics↗
DOE OSTI2025

Observation of pedestal mode spreading into SOL and broadening of divertor particle flux width on EAST tokamak

Pedestal turbulence spreading into a crape-off layer (SOL) can be used to explain the experimentally observed strong pedestal-SOL coupling and is expected to be important for the broadening of divertor deposition profiles in future devices (Xu et al 2019 Nucl. Fusion 59 126039). In the EAST tokamak, it is found that an electromagnetic (EM) mode in the pedestal region can spread into the SOL and broaden the divertor particle flux width. Multi-channel fluctuation reflectometry is used to measure the density fluctuations at the plasma edge. The EM mode rotates in the electron diamagnetic drift direction in the lab frame with a frequency range of [40–90] kHz, toroidal mode number n= 12–13 and poloidal wavenumber k θ = 0.41 cm −1 . The mode amplitude peaks around the maximum of the pedestal density gradient. As the mode amplitude increases, the reflectometry channel in the SOL can clearly capture the mode. This result suggests that the EM mode is excited in the pedestal gradient region and spreads into the SOL. It is further found that the particle flux deposition profile in the divertor is broadened as the EM mode appears.

Energy↗
DOE OSTI2020

Stellarators with Permanent Magnets

It is shown that the magnetic-field coils of a stellarator can, at least in principle, be substantially simplified by the use of permanent magnets. Such magnets cannot create toroidal magnetic flux but they can be used to shape the plasma and thus to create poloidal flux and rotational transform, thereby easing the requirements on the magnetic-field coils. As an example, a quasiaxisymmetric stellarator configuration is constructed with only 8 circular coils (all identical) and permanent magnets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

On the Geometry of the Near-core Magnetic Field in Massive Stars

It is well-known that the cores of massive stars sustain a stellar dynamo with a complex magnetic field configuration. However, the same cannot be said for the field's strength and geometry at the convective–radiative boundary, which are crucial when performing asteroseismic inference. In this Letter, we present 3D magnetohydrodynamic (MHD) simulations of a 7 M ⊙ mid-main-sequence star, with particular attention given to the convective–radiative boundary in the near-core region. Our simulations reveal that the toroidal magnetic field is significantly stronger than the poloidal field in this region, contrary to recent assumptions. Moreover, the rotational shear layer, also important for asteroseismic inference, is specifically confined within the extent of the Brunt–Väisälä frequency peak. These results, which are based on the inferred properties of HD 43317, have widespread implications for asteroseismic studies of rotation, mixing, and magnetism in stars. While we expect our results to be broadly applicable across stars with similar Brunt–Väisälä frequency profiles and stellar masses, we also expect the MHD parameters (e.g., Re m ) and the initial stellar rotation rate to impact the geometry of the field and differential rotation at the convective–radiative interface.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Nonlinear modeling of the scaling law for the m/n = 3/2 error field penetration threshold

The scaling law for the m/n=3/2 error field (EF) penetration threshold is predicted numerically based on nonlinear single-fluid and two-fluid modeling using the TM1 code. The simulated penetration threshold of radial magnetic field b r at the plasma edge is scaled to the electron density n e , temperature T e , viscous time τ μ , toroidal field B t and the natural frequency ω in the form of b r /B t ∝n e αn T e αT τ μ αμ B t αB ω αω by scanning these parameters separately. Here, α n , α T , α μ , α B and α ω are the scaling coefficients on n e , T e , τ μ , B t and ω, respectively. Single-fluid modeling shows that the 3/2 EF threshold scales as b r /B t ∝n e 0.56 T e 0.6 τ μ -0.59 B t -1.15 ω, which is similar with the analytical scaling law in both the Rutherford and visco-resistive regimes. Yet, two-fluid modeling shows that the scaling law differs significantly in particular regarding the dependence on plasma rotation. In detail, the scaling coefficient α n on density decreases from 0.67 to 0.56 and α T on temperature decreases from 0.67 to 0.32, while α μ on viscous time is around -0.45 and α B on toroidal field decreases slightly from -1.15 to -1, when the ratio |ω E /ω *e | between plasma rotation frequency ω E and diamagnetic drift frequency ω *e varies from 0 to 10. Scans of the plasma rotation reveals that the penetration threshold linearly depends on the perpendicular rotation frequency (or natural frequency) ω ⊥e =ω E +ω *e , and there is a minimum in the required field amplitude when ω ⊥e 0. In addition, the enduring mystery of non-zero penetration threshold at zero plasma natural frequency in EF experiments is resolved by two-fluid simulations. We report that the very small island and smooth bifurcation in EF penetration near zero frequency is hard to detect in the experiment, leading to a finite penetration threshold within the capability of the experimental measurements.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

On large-scale dynamos with stable stratification and the application to stellar radiative zones

Our understanding of large-scale magnetic fields in stellar radiative zones remains fragmented and incomplete. Such magnetic fields, which must be produced by some form of dynamo mechanism, are thought to dominate angular-momentum transport, making them crucial to stellar evolution. A major difficulty is the effect of stable stratification, which generally suppresses dynamo action. We explore the effects of stable stratification on mean-field dynamo theory with a particular focus on a non-helical large-scale dynamo (LSD) mechanism known as the magnetic shear-current effect. We find that the mechanism is robust to increasing stable stratification as long as the original requirements for its operation are met: a source of shear and non-helical magnetic fluctuations (e.g. from a small-scale dynamo). Both are plausibly sourced in the presence of differential rotation. Our idealized direct numerical simulations, supported by mean-field theory, demonstrate the generation of near equipartition large-scale toroidal fields. Additionally, a scan over magnetic Reynolds number shows no change in the growth or saturation of the LSD, providing good numerical evidence of a dynamo mechanism resilient to catastrophic quenching, which has been an issue for helical dynamos. These properties – the absence of catastrophic quenching and robustness to stable stratification – make the mechanism a plausible candidate for generating in situ large-scale magnetic fields in stellar radiative zones.

79 ASTRONOMY AND ASTROPHYSICS↗