arXiv subjects

W. A. Peebles

Publications and source records attributed to W. A. Peebles.

10 recordsLinked to original sources

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

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

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↗

arXiv2020

A Reliable Millimeter-Wave Quadrature Interferometer

A simple, low-cost millimeter-wave (70 GHz) interferometer with a phase accuracy better than plus or minur 2 degrees, and a response time of 10 ns is described. The simplicity of this interferometer makes it ideal for measurement of electron densities in laboratory or semiconductor processing plasmas. The relative high accuracy and low cost are attained by a homodyne system with a quadrature intermediate frequency (IF) mixer, now commercially available in millimeter-wave frequency bands. The design and construction of magnetic shielding for the system isolator, which may be required around magnetically-confined plasmas, is also described.

physics.ins-det↗

arXiv2015

Measurement of high-k density fluctuation wavenumber spectrum in MAST and Doppler backscattering for spherical tokamaks

The high-k ($7 \lesssim k_{\bot} ρ_i \lesssim 11$) wavenumber spectrum of density fluctuations has been measured for the first time in MAST [B. Lloyd et al, Nucl. Fusion 43, 1665 (2003)]. This was accomplished with the first implementation of Doppler backscattering (DBS) for core measurements in a spherical tokamak. DBS has become a well-established and versatile diagnostic technique for the measurement of intermediate-k ($k_{\bot} ρ_i \sim 1$, and higher) density fluctuations and flows in magnetically confined fusion experiments. A novel implementation with 2D steering was necessary to enable DBS measurements in MAST, where the large magnetic field pitch angle presents a challenge. We report on the scattering considerations and ray tracing calculations used to optimize the design and present data demonstrating measurement capabilities. Initial results confirm the applicability of the design and implementation approaches, showing the strong dependence of scattering alignment on toroidal launch angle. We also present comparisons of DBS plasma velocity measurements with charge exchange recombination and beam emission spectroscopy measurements, which show reasonable agreement over most of the minor radius, but imply large poloidal flows approaching the magnetic axis in a discharge with an internal transport barrier. The 2D steering is shown to enable high-k measurements with DBS, at $k_{\bot}>20\ \mathrm{cm}^{-1}$ ($k_{\bot} ρ_i>10$) for launch frequencies less than 75 GHz; this capability is used to measure the wavenumber spectrum of turbulence and we find $|n(k_{\bot})|^2 \propto k_{\bot}^{- 4.7 \pm 0.2}$ for $k_{\bot} ρ_i \approx 7-11$, which is similar to the expectation for the turbulent kinetic cascade of $|n(k_{\bot})|^2 \propto k_{\bot}^{- 13/3}$.

physics.plasm-ph↗

arXiv2015

Dependence of intrinsic rotation reversals on collisionality in MAST

Tokamak plasmas rotate even without external injection of momentum. A Doppler backscattering system installed at MAST has allowed this intrinsic rotation to be studied in Ohmic L-mode and H-mode plasmas, including the first observation of intrinsic rotation reversals in a spherical tokamak. Experimental results are compared to a novel 1D model, which captures the collisionality dependence of the radial transport of toroidal angular momentum due to the effect of neoclassical flows on turbulent fluctuations. The model is able to accurately reproduce the change in sign of core toroidal rotation, using experimental density and temperature profiles from shots with rotation reversals as inputs and no free parameters fit to experimental data.

physics.plasm-ph↗

arXiv2010

Interaction between Faraday rotation and Cotton-Mouton effects in polarimetry modeling for NSTX

The evolution of electromagnetic wave polarization is modeled for propagation in the major radial direction in the National Spherical Torus Experiment (NSTX) with retroreflection from the center stack of the vacuum vessel. This modeling illustrates that the Cotton-Mouton effect-elliptization due to the magnetic field perpendicular to the propagation direction-is shown to be strongly weighted to the high-field region of the plasma. An interaction between the Faraday rotation and Cotton-Mouton effects is also clearly identified. Elliptization occurs when the wave polarization direction is neither parallel nor perpendicular to the local transverse magnetic field. Since Faraday rotation modifies the polarization direction during propagation, it must also affect the resultant elliptization. The Cotton-Mouton effect also intrinsically results in rotation of the polarization direction, but this effect is less significant in the plasma conditions modeled. The interaction increases at longer wavelength, and complicates interpretation of polarimetry measurements.

physics.plasm-ph↗