Jeff Lestz
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View article: Dependence of ion-cyclotron range of frequencies instabilities on species mix and fast-ion distribution (Part 3: Novel sub-cyclotron modes)
Dependence of ion-cyclotron range of frequencies instabilities on species mix and fast-ion distribution (Part 3: Novel sub-cyclotron modes) Open
Dedicated DIII-D experiments studied instabilities above and below the
ion cyclotron frequency by injecting thirteen different neutral beam populations into
L-mode plasmas with different thermal compositions of H, D, and 3He at fiv…
View article: Dependence of ion-cyclotron range of frequencies instabilities on species mix and fast-ion distribution (Part 1: Stability)
Dependence of ion-cyclotron range of frequencies instabilities on species mix and fast-ion distribution (Part 1: Stability) Open
Ion cyclotron emission (ICE) driven by fast ions has the potential to be a
reactor relevant diagnostic of confined and escaping fast ions; moreover, controlled
experiments in magnetic fusion devices can shed light on similar magnet…
View article: Dependence of ion-cyclotron range of frequencies instabilities on species mix and fast-ion distribution: II. Time evolution
Dependence of ion-cyclotron range of frequencies instabilities on species mix and fast-ion distribution: II. Time evolution Open
Dedicated DIII-D experiments studied instabilities below the ion cyclotron frequency f ci and at cyclotron harmonics by injecting thirteen different neutral beam populations into plasmas with different thermal compositions of H and D at fi…
View article: First Demonstration of Resonant Pitch-Angle Scattering of Relativistic Electrons by Externally-Launched Helicon Waves
First Demonstration of Resonant Pitch-Angle Scattering of Relativistic Electrons by Externally-Launched Helicon Waves Open
Helicon waves satisfying the normal wave-particle cyclotron resonance are observed to limit the growth and maximum energy of relativistic electrons (REs) in low-density Ohmic DIII-D tokamak plasmas. Following the application of helicon wav…
View article: Parametric raytracing modeling for NSTX-U scenario development with high harmonic fast waves and neutral beam injection
Parametric raytracing modeling for NSTX-U scenario development with high harmonic fast waves and neutral beam injection Open
High harmonic fast waves (HHFW) are a versatile heating and current drive tool for scenario development. Extensive modeling scans were performed to find optimal parameters for different uses of HHFW in National Spherical Tokamak Experiment…
View article: Role of perturbed parallel magnetic field effects in predicting turbulent transport in NSTX
Role of perturbed parallel magnetic field effects in predicting turbulent transport in NSTX Open
This study presents analysis of gyrokinetic simulations on the National Spherical Torus Experiment (NSTX) to investigate the effects of electromagnetic fields on plasma turbulence and transport. The simulations, performed with varying leve…
View article: Propagation of ion cyclotron emission in the DIII-D tokamak
Propagation of ion cyclotron emission in the DIII-D tokamak Open
Experiments in L- and H-mode plasmas performed on the DIII-D tokamak explored ion cyclotron emission (ICE) propagation via the recently upgraded ICE diagnostic. The distance between the plasma and the outer wall was scanned to alter the ev…
View article: Accelerating discoveries at DIII-D with the Integrated Research Infrastructure
Accelerating discoveries at DIII-D with the Integrated Research Infrastructure Open
DIII-D research is being accelerated by leveraging high performance computing (HPC) and data resources available through the National Energy Research Scientific Computing Center (NERSC) Superfacility initiative. As part of this initiative,…
View article: First high-power helicon results from DIII-D
First high-power helicon results from DIII-D Open
More than 0.6 MW of rf power at 476 MHz has been coupled to DIII-D plasmas by launching helicon (whistler) waves with a traveling-wave antenna (comb-line) in the fast-wave polarization (Van Compernolle et al 2021 Nucl. Fusion 61 116034) wh…
View article: NSTX-U research advancing the physics of spherical tokamaks
NSTX-U research advancing the physics of spherical tokamaks Open
The objectives of NSTX-U research are to reinforce the advantages of STs while addressing the challenges. To extend confinement physics of low- A , high beta plasmas to lower collisionality levels, understanding of the transport mechanisms…
View article: Shifting and Splitting of Resonance Lines due to Dynamical Friction in Plasmas
Shifting and Splitting of Resonance Lines due to Dynamical Friction in Plasmas Open
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 ener…
View article: Parametric study of Alfvénic instabilities driven by runaway electrons during the current quench in DIII-D
Parametric study of Alfvénic instabilities driven by runaway electrons during the current quench in DIII-D Open
To avoid or mitigate runaway electron (RE) beams in ITER, RE-driven instabilities are actively studied as a complimentary technique to massive material injection. In this work we report experimental dependencies of Alfvénic instabilities d…
View article: Mode structure measurements of ion cyclotron emission and sub-cyclotron modes on DIII-D
Mode structure measurements of ion cyclotron emission and sub-cyclotron modes on DIII-D Open
We report mode structure measurements of coherent ion cyclotron emission (ICE) and sub-cyclotron modes on DIII-D. Through a dedicated experiment, we aimed to characterize a variety of modes via the upgraded ICE diagnostic in both L- and H-…
View article: Application of simulations and theory of sub-cyclotron frequency modes to DIII-D
Application of simulations and theory of sub-cyclotron frequency modes to DIII-D Open
Theoretical and numerical study of Alfvén eigenmodes (AEs) in the sub-cyclotron frequency range explains the observed frequency scaling with beam parameters in experiments across different devices. Global Alfvén (GAEs) and compressional Al…
View article: NSTX-U theory, modeling and analysis results
NSTX-U theory, modeling and analysis results Open
The mission of the low aspect ratio spherical tokamak NSTX-U is to advance the physics basis and technical solutions required for optimizing the configuration of next-step steady-state tokamak fusion devices. NSTX-U will ultimately operate…
View article: Chirping ion cyclotron emission (ICE) on NSTX-U
Chirping ion cyclotron emission (ICE) on NSTX-U Open
In this work, we report on the discovery on NSTX-U of a qualitatively new type of non-thermal emission from plasmas in the ion-cyclotron frequency range (ICE). This new type of ICE is weakly damped, γdamp/ωICE ≈ 0.14%, low order eigenmodes…
View article: Stabilization of Alfvén Eigenmodes in DIII-D via Controlled Energetic Ion Density Ramp and Validation of Theory and Simulations
Stabilization of Alfvén Eigenmodes in DIII-D via Controlled Energetic Ion Density Ramp and Validation of Theory and Simulations Open
Fast-ion driven Alfvén waves with frequency close to the ion cyclotron frequency (f=0.58f_{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 researc…
View article: Hybrid simulations of sub-cyclotron compressional and global Alfvén Eigenmode stability in spherical tokamaks
Hybrid simulations of sub-cyclotron compressional and global Alfvén Eigenmode stability in spherical tokamaks Open
A comprehensive numerical study has been conducted in order to investigate the stability of beam-driven, sub-cyclotron frequency compressional (CAE) and global (GAE) Alfvén Eigenmodes in low aspect ratio plasmas for a wide range of beam pa…
View article: Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks
Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks Open
A comprehensive numerical study has been conducted in order to investigate the stability of beam-driven, sub-cyclotron frequency compressional (CAE) and global (GAE) Alfven Eigenmodes in low aspect ratio plasmas for a wide range of beam pa…
View article: Shifting and splitting of resonance lines due to dynamical friction in plasmas
Shifting and splitting of resonance lines due to dynamical friction in plasmas Open
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 ener…
View article: Numerical simulations of global Alfvén eigenmodes excitation and stabilization in NSTX-U
Numerical simulations of global Alfvén eigenmodes excitation and stabilization in NSTX-U Open
Three-dimensional nonlinear simulations of Alfvén eigenmodes in the subcyclotron frequency range show a robust physical stabilizing mechanism via modest off-axis beam injection, in agreement with experimental observations from the National…
View article: Energetic-particle-modified global Alfvén eigenmodes
Energetic-particle-modified global Alfvén eigenmodes Open
Fully self-consistent hybrid MHD/particle simulations reveal strong energetic particle modifications to sub-cyclotron global Alfvén eigenmodes (GAEs) in low-aspect ratio, NSTX-like conditions. Key parameters defining the fast ion distribut…
View article: Nonlinear simulations of beam-driven compressional Alfvén eigenmodes in NSTX
Nonlinear simulations of beam-driven compressional Alfvén eigenmodes in NSTX Open
Results of 3D nonlinear simulations of neutral-beam-driven compressional Alfvén eigenmodes (CAEs) in the National Spherical Torus Experiment (NSTX) are presented. Hybrid MHD-particle simulations for the H-mode NSTX discharge (shot 141398) …
View article: Energetic-particle-modified global Alfven eigenmodes
Energetic-particle-modified global Alfven eigenmodes Open
Fully self-consistent hybrid MHD/particle simulations reveal strong energetic particle modifications to sub-cyclotron global Alfven eigenmodes (GAE) in low-aspect ratio, NSTX-like conditions. Key parameters defining the fast ion distributi…
View article: Validating predictive models for fast ion profile relaxation in burning plasmas
Validating predictive models for fast ion profile relaxation in burning plasmas Open
The redistribution and potential loss of energetic particles due to MHD modes can limit the performance of fusion plasmas by reducing the plasma heating rate. In this work, we present validation studies of the 1.5D critical gradient model …