arXiv subjects

E. V. Belova

Publications and source records attributed to E. V. Belova.

5 recordsLinked to original sources

arXiv2025

Hybrid simulations of FRC merging and compression

An improved understanding of Field Reversed Configuration (FRC) merging and stability in high acceleration and compression magnetic fields is needed to speed up the development of the pulsed fusion concept developed at Helion Energy. All previous theoretical and simulation work on FRC merging and compression was performed using 2D MHD models. The results of novel 2D hybrid simulations (fluid electrons and full-orbit kinetic ions) of FRC merging and compression are presented. Results of kinetic and MHD simulations, computed using the HYM code, are compared and analyzed. In cases without axial magnetic compression, both the MHD and hybrid simulations show a high sensitivity to the initial parameters (i.e. FRC separation, velocity, normalized separatrix radius, and plasma viscosity), showing that FRCs with large elongation and separatrix radius either do not merge or merge partially, forming a doublet FRC. Application of a mirror coil field at the FRC ends with increasing strength is shown to lead to fast and complete merging of the FRCs in MHD and kinetic simulations.

physics.plasm-ph↗

arXiv2020

Theory and Simulations of Compressional and Global Alfven Eigenmode Stability in Spherical Tokamaks

Neutral-beam-driven, sub-cyclotron compressional (CAE) and global (GAE) \Alfven eigenmodes are routinely excited in spherical tokamaks such as NSTX(-U) and MAST, have been observed on the conventional aspect ratio tokamak DIII-D, and may be unstable in ITER burning plasmas. Their presence has been experimentally linked to the anomalous flattening of electron temperature profiles at high beam power in NSTX, potentially limiting fusion performance. A detailed understanding of CAE/GAE excitation, therefore, is vital to predicting (and ultimately controlling) their effects on plasma confinement. To this end, hybrid kinetic-MHD simulations are complemented with an analytic study of the linear stability properties of CAEs and GAEs. Perturbative, local analytic theory has been used to derive new instability conditions for CAEs/GAEs driven by realistic neutral beam distributions. A comprehensive set of simulations of NSTX-like plasmas has been performed for a wide range of beam parameters, providing a wealth of information on CAE and GAE stability in spherical tokamaks. Linear simulations show that the excitation of CAEs vs GAEs has a complex dependence on the fast ion injection velocity and geometry, qualitatively described by the analytic theory developed in this thesis. Strong energetic particle modifications of GAEs are found in simulations, indicating the existence of a new type of high frequency energetic particle mode. A cross validation between the theoretical stability bounds, simulation results, and experimental measurements shows favorable agreement for both the unstable CAE and GAE spectra's dependence on fast ion parameters. The analytic results accurately explain the recent experimental discovery of GAE stabilization with small amounts of off-axis beam injection on NSTX-U and suggest new techniques for control of these instabilities in future experiments.

physics.plasm-ph↗

arXiv2019

Analytic stability boundaries for compressional and global Alfvén eigenmodes driven by fast ions. I. Interaction via ordinary and anomalous cyclotron resonances

Conditions for net fast ion drive are derived for beam-driven, sub-cyclotron compressional (CAE) and global (GAE) Alfvén eigenmodes, such as those routinely observed in spherical tokamaks such as NSTX(-U) and MAST. Both co- and counter-propagating CAEs and GAEs are investigated, driven by the ordinary and anomalous Doppler-shifted cyclotron resonance with fast ions. Whereas prior results were restricted to vanishingly narrow distributions in velocity space, broad parameter regimes are identified in this work which enable an analytic treatment for realistic fast ion distributions generated by neutral beam injection. The simple, approximate conditions derived in these regimes for beam distributions of realistic width compare well to the numerical evaluation of the full analytic expressions for fast ion drive. Moreover, previous results in the very narrow beam case are corrected and generalized to retain all terms in $ω/ω_{ci}$ and $k_\parallel/k_\perp$, which are often assumed to be small parameters but can significantly modify the conditions of drive and damping when they are non-negligible. Favorable agreement is demonstrated between the approximate stability criterion, simulation results, and a large database of NSTX observations of cntr-GAEs.

physics.plasm-ph↗

arXiv2019

Analytic stability boundaries for compressional and global Alfvén eigenmodes driven by fast ions. II. Interaction via Landau resonance

Conditions for net fast ion drive are derived for beam-driven, co-propagating, sub-cyclotron compressional (CAE) and global (GAE) Alfvén eigenmodes driven by the Landau resonance with super-Alfvénic fast ions. Approximations applicable to realistic neutral beam distributions and mode characteristics observed in spherical tokamaks enable the derivation of marginal stability conditions for these modes. Such conditions successfully reproduce the stability boundaries found from numerical integration of the exact expression for local fast ion drive/damping. Coupling between the CAE and GAE branches of the dispersion due to finite $ω/ω_{ci}$ and $k_\parallel/k_\perp$ is retained and found to be responsible for the existence of the GAE instability via this resonance. Encouraging agreement is demonstrated between the approximate stability criterion, simulation results, and a database of NSTX observations of co-CAEs.

physics.plasm-ph↗

arXiv2018

Energetic-particle-modified global Alfvén eigenmodes

Fully self-consistent hybrid MHD/particle simulations reveal strong energetic particle modifications to sub-cyclotron global Alfvén eigenmodes (GAE) in low-aspect ratio, NSTX-like conditions. Key parameters defining the fast ion distribution function -- the normalized injection velocity $v_0/v_A$ and central pitch -- are varied in order to study their influence on the characteristics of the excited modes. It is found that the frequency of the most unstable mode changes significantly and continuously with beam parameters, in accordance with the Doppler-shifted cyclotron resonances which drive the modes, and depending most substantially on $v_0/v_A$. This unexpected result is present for both counter-propagating GAEs, which are routinely excited in NSTX, and high frequency co-GAEs, which have not been previously studied. Large changes in frequency without clear corresponding changes in mode structure could indicate the existence of a new energetic particle mode, referred to here as an energetic-particle-modified GAE (EP-GAE). Additional simulations conducted for a fixed MHD equilibrium demonstrate that the GAE frequency shift cannot be explained by the equilibrium changes due to energetic particle effects.

physics.plasm-ph↗