arXiv subjects

T. L. Rhodes

Publications and source records attributed to T. L. Rhodes.

11 recordsLinked to original sources

arXiv2026

Impurity-driven turbulence opens a pathway to ELM-free operation and enhanced pedestal stability in tokamaks

Edge-localized modes (ELMs) impose severe transient heat, and particle loads on plasma-facing components, posing a critical challenge for steady-state operation of tokamak fusion reactors. Existing ELM control techniques either rely on externally applied perturbations or operate within narrow parameter windows, raising concerns for reactor scalability. Here we demonstrate that controlled injection of a low-Z impurity can fundamentally modify pedestal transport and stability, enabling access to long ELM-free periods through impurity-driven turbulence. Using boron (B) powder injection in the DIII-D tokamak, we observe a progressive reduction of ELM frequency, culminating in long ELM-free phases. Pedestal stability analysis reveals a pronounced decoupling of peeling and ballooning stability boundaries at moderate B injection levels, opening a stability channel toward super-high confinement operation. At higher injection rates, long (~300 ms) ELM-free periods are achieved. Fluctuation measurements show that B injection selectively enhances low-frequency pedestal turbulence, increasing inter-ELM particle transport and regulating pedestal gradients. The establishment of a feedback loop between turbulence, particle transport, and the resulting modification of pedestal conditions, indicated by the observed hysteresis loop in the evolution of density fluctuations in response to the B injection rate, is presented.

physics.plasm-ph↗

arXiv2025

Disentangling Core and Edge Mechanisms of the Density Limit in DIII-D Negative Triangularity Plasmas

The density limit is investigated in the DIII-D negative triangularity (NT) plasmas which lack a standard H-mode edge. We find the limit may not be a singular disruptive boundary but a multifaceted density saturation phenomenon governed by distinct core and edge transport mechanisms. Sustained, non-disruptive operation is achieved at densities up to 1.8 times the Greenwald limit ($n_\mathrm{G}$) until the termination of auxiliary heating. Systematic power scans reveal distinct power scalings for the core ($n_e \propto P_\mathrm{SOL}^{0.27\pm0.03}$) and edge ($n_e \propto P_\mathrm{SOL}^{0.42\pm0.04}$) density limits. The edge density saturation is triggered abruptly by the onset of a non-disruptive, high-field side radiative instability that clamps the edge density below $n_\mathrm{G}$. In contrast, the core density continues to rise until it saturates, a state characterized by substantially enhanced core turbulence. Core transport evolves from a diffusive to an intermittent, avalanche-like state, as indicated by heavy-tailed probability density functions (kurtosis $\approx 6$), elevated Hurst exponents, and a $1/f$-type power spectrum. These findings suggest that the density limit in the low-confinement regime is determined by a combination of edge radiative instabilities and core turbulent transport. This distinction provides separate targets for control strategies aimed at extending the operational space of future fusion devices.

physics.plasm-ph↗

arXiv2024

Real-time steerable frequency-stepped Doppler Backscattering (DBS) System for local helicon wave electric field measurements on the DIII-D tokamak

A new frequency-stepped Doppler backscattering (DBS) system has been integrated with a real-time steerable electron cyclotron heating launcher to probe local background turbulence (f<10 MHz) and high-frequency (20-550 MHz) density fluctuations in the DIII-D tokamak. The launcher enables 2D steering (horizontal and vertical) over wide angular ranges to optimize probe location and wavenumber response, with vertical steering adjustable in real time during discharges. The DBS system utilizes a programmable frequency synthesizer with adjustable dwell time, capable of stepping across the E-band frequency range (60-90 GHz) in real time, launching either O or X-mode polarized millimeter waves. This setup facilitates diagnosis of the complex spatial structure of high-power (>200 kW) helicon waves (476 MHz) during current drive experiments. Real-time scans reveal broadband density fluctuations around the helicon frequency, attributed to backscattering of the DBS millimeter wave probe from plasma turbulence modulated by the helicon wave. These fluctuations appear as high-frequency sidebands in the turbulence spectrum, effectively 'tagging' the background turbulence with the helicon wave's electric field. This method allows for monitoring local helicon wave amplitude by comparing high-frequency signal amplitude to background turbulence. Coupled with real-time scanning of measurement location and wavenumber, this allows for rapid helicon wave power distribution determination during steady-state plasma operation, potentially validating predictive models like GENRAY or AORSA for helicon current drive in DIII-D plasmas.

physics.plasm-ph↗

arXiv2024

Broadening of the Divertor Heat Flux Profile in High Confinement Tokamak Fusion Plasmas with Edge Pedestals Limited by Turbulence in DIII-D

Multi-machine empirical scaling predicts an extremely narrow heat exhaust layer in future high magnetic field tokamaks, producing high power densities that require mitigation. In the experiments presented, the width of this exhaust layer is nearly doubled using actuators to increase turbulent transport in the plasma edge. This is achieved in low collisionality, high confinement edge pedestals with their gradients limited by turbulent transport instead of large-scale, coherent instabilities. The exhaust heat flux profile width and divertor leg diffusive spreading both double as a high frequency band of turbulent fluctuations propagating in the electron diamagnetic direction doubles in amplitude. The results are quantitatively reproduced in electromagnetic XGC particle-in-cell simulations which show the heat flux carried by electrons emerges to broaden the heat flux profile, directly supported by Langmuir probe measurements.

physics.plasm-ph↗

arXiv2024

Using Convolutional Neural Networks to detect Edge Localized Modes in DIII-D from Doppler Backscattering measurements

In H-mode tokamak plasmas, the plasma is sometimes ejected beyond the edge transport barrier. These events are known as edge localized modes (ELMs). ELMs cause a loss of energy and damage the vessel walls. Understanding the physics of ELMs and by extension, how to detect and mitigate them, is an important challenge. In this paper, we focus on two diagnostic methods $\unicode{x2013}$ D-alpha spectroscopy and Doppler backscattering (DBS). The former detects ELMs by measuring Balmer alpha emission while the latter uses microwave radiation to probe the plasma. DBS has the advantage of having higher temporal resolution and robustness to damage. These advantages of DBS diagnostics may be beneficial for future operational tokamaks and thus data processing techniques for DBS should be developed in preparation. In sight of this, we explore the training of neural networks to detect ELMs from DBS data, using D-alpha data as the ground truth. With shots found in the DIII-D database, the model is trained to classify each time step based on the occurrence of an ELM event. The results are promising. When tested on shots similar to those used for training, the model is capable of consistently achieving a high f1-score of 0.93. This score is a performance metric for imbalanced datasets that ranges between 0 and 1. We evaluate the performance of our neural network on a variety of ELMs $\unicode{x2013}$ grasssy, suppressed, and wide pedestal $\unicode{x2013}$ finding broad applicability. Beyond ELMs, our work demonstrates the wider feasibility of applying neural networks to data from DBS diagnostics.

physics.plasm-ph↗

arXiv2024

ELM-free Enhanced Dα H-mode with Near Zero NBI Torque Injection in DIII-D Tokamak

Enhanced $D_α$ H-mode (EDA H-mode), an ELM-free H-mode regime, is explored in neutral beam heated, lower single null plasmas with near zero torque injection. This regime exhibits a good energy confinement ($\mathrm{H}_{\mathrm{98y2}}$ $\sim 1$) with $β_N \sim 2$, high density, regime access at low input power, and no ELMs. This paper further presents the time-resolved measurements of electron and ion density, temperature, plasma rotation, and radial electric field during the EDA H-mode phase and examines the dynamics of the edge quasi-coherent mode (QCM). Measurements using multiple fluctuation diagnostics reveal the QCM to be a separatrix spanning mode, peaking just inside the separatrix, existing in a wide range of $k_{\perp}ρ_s \sim 0.1-1.2$ with multiple harmonics, and propagating with a very small phase velocity in the plasma frame, where $k_{\perp}$ is the binormal wavenumber and $ρ_s$ is the ion sound radius. Linear gyrokinetic simulations of an EDA H-mode discharge with CGYRO indicates that the trapped electron mode (TEM) and electron temperature gradient (ETG) are dominant instabilities in the region where QCM is unstable. Qualitative analysis indicates that the properties of TEM are consistent with the experimental observed characteristics of the QCM. These similarities suggest that the QCM might be a TEM instability existing in the edge region of the EDA H-mode plasmas.

physics.plasm-ph↗

arXiv2023

Characterization of mesoscopic turbulent transport events with long-radial-range correlation in DIII-D H-mode plasmas

A dimensionless collisionality scan has been performed in H-mode plasmas on DIII-D tokamak, with detailed measurements of intermediate-to-high wavenumber turbulence using Doppler backscattering systems. It is found that the shorter wavelength turbulence develops into spatially asymmetric turbulent structures with a long-radial-range correlation (LRRC) in the mid-radius region of high-collisionality discharges. Linear \textsc{cgyro} simulations indicate that the underlying turbulence is likely driven by the electron-temperature-gradient (ETG) mode. The LRRC transport events are highly intermittent and show a power spectrum of $S_{\tilde{n}}(k_\perp) \propto k^{-1}_\perp$ for density fluctuations, which is often associated with self-organized criticality. The magnitude and the radial scale of those turbulent structures increase significantly when the $E_{r}\times B$ mean flow shearing rate decreases. The enhanced LRRC transport events appear to be correlated with the degraded energy confinement time. The emergence of such LRRC transport events may serve as a candidate explanation for the degrading nature of \emph{H}-mode core plasma confinement at high collisionality.

physics.plasm-ph↗

arXiv2023

Observation of Long-Radial-Range-Correlation in Turbulence in High-Collisionality High-Confinement Fusion Plasmas

We report on the observation of spatially asymmetric turbulent structures with a long radial correlation length in the core of high-collisionality H-mode plasmas on DIII-D tokamak. These turbulent structures develop from shorter wavelength turbulence and have a radially elongated structure. The envelope of turbulence spans a broad radial range in the mid-radius region, leading to streamer-like transport events. The underlying turbulence is featured by intermittency, long-term memory effect, and the characteristic spectrum of self-organized criticality. The amplitude and the radial scale increase substantially when the shearing rate of the mean flow is reduced below the turbulent scattering rate. The enhanced LRRC transport events are accompanied by apparent degradation of normalized energy confinement time. These findings constitute the first experimental observation of long-radial-range turbulent transport events in high-collisionality H-mode plasmas, and demonstrate the role of mean shear flows in the formation and propagation of turbulence with long-radial-range correlation.

physics.plasm-ph↗

arXiv2023

A novel Doppler backscattering (DBS) system to simultaneously monitor radio frequency plasma fluctuations and low frequency turbulence

A novel quadrature Doppler Backscattering (DBS) system has been developed and optimized for the E-band (60-90GHz) frequency range using either O-mode or X-mode polarization in DIII-D plasmas. In general, DBS measures the amplitude of density fluctuations and their velocity in the lab frame. The system can simultaneously monitor both low-frequency turbulence (f < 10MHz) and radiofrequency plasma density fluctuations over a selectable frequency range (20-500 MHz). Detection of high-frequency fluctuations has been demonstrated for low harmonics of the ion cyclotron frequency (e.g., 2fci~23MHz) and externally driven high-frequency helicon waves (f = 476MHz) using an adjustable frequency down conversion system. Importantly, this extends the application of DBS to a high-frequency spectral domain while maintaining important turbulence and flow measurement capabilities. This unique system has low phase noise, good temporal resolution (sub-millisecond) and excellent wavenumber coverage (k_θ ~ 1-20cm^{-1} and k_r ~ 20-30cm^{-1}). As a demonstration, localized internal DIII-D plasma measurements are presented from turbulence (f <= 5MHz), Alfvenic waves (f~6.5MHz), ion cyclotron waves (f >= 20MHz) as well as fluctuations around 476MHz driven by an external high-power 476 MHz helicon wave antenna. In the future, helicon measurements will be used to validate GENRAY and AORSA modeling tools for prediction of helicon wave propagation, absorption and current drive location for the newly installed helicon current drive system on DIII-D.

physics.plasm-ph↗

arXiv2021

Ray-tracing Analysis for Cross-polarization Scattering Diagnostic on MAST-Upgrade Spherical Tokamak

A combined Doppler backscattering/cross-polarization scattering (DBS/CPS) system is being deployed on MAST-U, for simultaneous measurements of local density turbulence, turbulence flows, and magnetic turbulence. In this design, CPS shares the probing beam with the DBS and uses a separate parallel-viewing receiver system. In this study, we utilize a modified GENRAY 3D ray-tracing code, to simulate the propagation of the probing and scattered beams. The contributions of different scattering locations along the entire beam trajectories are considered, and the corresponding local $\tilde{\mathbf{B}}$ wavenumbers are estimated using the wave-vector matching criterion. The wavenumber ranges of the local $\tilde{\mathbf{B}}$ that is detectable to the CPS system are explored for simulated L- and H-mode plasmas.

physics.plasm-ph↗

arXiv2021

Observation of quasi-coherent density fluctuation in scrape-off layer enhancing boundary transport in high-betaN hybrid plasmas on DIII-D

We report the observation of a quasi-coherent density fluctuation (QCF) by the Doppler backscattering system in the scrape-off layer (SOL) region of the DIII-D tokamak. This QCF is observed in high-power, high-performance hybrid plasmas with near double-null divertor (DND) shape during the electron cyclotron heating period. This mode is correlated with a steepened SOL density profile and leads to significantly elevated particle and heat fluxes between ELMs. The SOL QCF is a long-wavelength ion-scale fluctuation and propagates in the ion diamagnetic direction in the plasma frame. Its radial expanse is about 1.5-2 cm, well beyond the typical width of heat flux on DIII-D. Also, the SOL QCF does not show any clear dependence on the effective SOL collisionality and thus may raise issues on the control of plasma-material interactions in low collisionality plasmas in which the blob-induced transport is reduced. A linear simulation using BOUT++ with a 5-field reduced model is performed and compared with experimental observations. In simulation results, an interchange-like density perturbation can be driven by the SOL density gradient, and its peak location and the radial width of the density perturbation are in agreement with the experimental observations.

physics.plasm-ph↗