Engineering topics

Carter, T. A.

Publications and source records attributed to Carter, T. A..

4 records

DOE OSTI2024

Linear gyrokinetic simulations of toroidal Alfvén eigenmodes in the Mega-Amp Spherical Tokamak

Linear gyrokinetic (GK) simulations using the Gyrokinetic Toroidal Code (GTC) have been performed to investigate Toroidicity-driven Alfvén Eigenmodes (TAEs) driven by the neutral beam injection (NBI) induced fast ions in the Mega-Amp Spherical Tokamak (MAST) to identify the non-perturbative and kinetic effects of thermal plasma. A specific TAE in MAST discharge 26887, with an on-axis NBI power of approximately 1.5 MW and plasma current around 800 kA, exhibited frequency chirping, and the tangential soft x-ray camera array resolved the radial mode structure peaked near |q|=1.5. Various excitation methods were used in the GTC linear simulations, illustrating this code's capability to realistically represent the mechanisms and behaviors of fast ion-driven TAEs in spherical tokamaks. The radial structures from these GK simulations closely match measurements and calculations performed using the NOVA ideal MHD code, though with the frequencies approximately 10 kHz lower, likely due to various kinetic and non-perturbative effects. The simulations measured the damping rates due to continuum damping, radiative damping, and ion Landau damping, revealing that ion Landau damping has the most significant contribution to the total damping rate of the TAE. A comparison of growth rates of TAEs excited by fast ion Maxwellian and slowing-down distributions shows that the TAEs excited by a fast ion anisotropic pitch distribution (as part of the slowing-down distributions) are more unstable compared to those excited by a Maxwellian distribution with an equivalent fast ion beta. This shows that the use of fast ion anisotropy alters the number of fast ions to be in shear Alfvén resonance, and hence, it can greatly affect the stability of TAEs. These tests can be performed with the GTC but impossible with ideal MHD simulations, highlighting the necessity of kinetic simulations such as the GTC for a precise prediction of the TAE stability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Reduction in RF sheath rectification with insulating antenna enclosure walls

Abstract Radiofrequency (RF) sheath rectification is one of the most prominent deleterious effects associated with ion cyclotron range of frequencies (ICRF) heating in fusion plasmas. RF sheaths and associated effects, such as impurity generation and convective cell generation, need to be mitigated to ensure that ICRF is a viable option for heating in future fusion devices. Experiments were performed on the Large Plasma Device (LAPD) at UCLA to explore the effects of using electrically-insulating antenna enclosures on RF rectified sheaths. Three different enclosure side-wall materials were used, including copper, MACOR (electrically insulating), and MACOR over copper. In the case of the MACOR–copper side walls, the non-conductive MACOR material was exposed to the bulk plasma but a layer of copper was added below to allow for image currents to flow. All three of the experiments had similar plasma density, temperature, and background magnetic field. In the case of the copper enclosure, RF rectified potentials, many times the local electron temperature, and associated formation of convective cells were observed and reported Martin M. et al (2017 Phys. Rev. Lett. 119 205002). In the experiments with MACOR and MACOR–copper enclosures, RF rectification was significantly reduced. Additionally, these latter two experiments showed no evidence of convective cell formation. Although the results from the MACOR experiment are reminiscent of the results obtained in ASDEX-U with a three-strap antenna optimized to reduce image currents on the antenna limiters Bobkov V. et al (2016 Nucl. Fusion 56 084001), the MACOR–copper experiment seems to suggest that insulating plasma facing materials have at least an equally strong impact on reducing potential rectification.

Physics↗

DOE OSTI2021

Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves

While the aurora has attracted attention for millennia, important questions remain unanswered. Foremost is how auroral electrons are accelerated before colliding with the ionosphere and producing auroral light. Powerful Alfvén waves are often found traveling Earthward above auroras with sufficient energy to generate auroras, but there has been no direct measurement of the processes by which Alfvén waves transfer their energy to auroral electrons. Here, we show laboratory measurements of the resonant transfer of energy from Alfvén waves to electrons under conditions relevant to the auroral zone. Experiments are performed by launching Alfvén waves and simultaneously recording the electron velocity distribution. Numerical simulations and analytical theory support that the measured energy transfer process produces accelerated electrons capable of reaching auroral energies. The experiments, theory, and simulations demonstrate a clear causal relationship between Alfvén waves and accelerated electrons that directly cause auroras.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

Stabilization of Alfvén Eigenmodes in DIII-D via Controlled Energetic Ion Density Ramp and Validation of Theory and Simulations

In this work, fast-ion driven Alfvén waves with frequency close to the ion cyclotron frequency (ƒ = 0:58ƒ ci ) excited by energetic ions from a neutral beam are stabilized via a controlled energetic ion density ramp for the first time in a fusion research plasma. The scaling of wave amplitude with injection rate is consistent with theory for single mode collisional saturation near marginal stability. The wave is identified as a shear-polarized global Alfvén eigenmode excited by Doppler-shifted cyclotron resonance with fast-ions with sub-Alfvénic energetic ions, a first in fusion research plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗