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

Multimachine 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.

Highest fusion performance without harmful edge energy bursts in tokamak.

The path of tokamak fusion and International thermonuclear experimental reactor (ITER) is maintaining high-performance plasma to produce sufficient fusion power. This effort is hindered by the transient energy burst arising from the instabilities at the boundary of plasmas. Conventional 3D magnetic perturbations used to suppress these instabilities often degrade fusion performance and increase the risk of other instabilities. This study presents an innovative 3D field optimization approach that leverages machine learning and real-time adaptability to overcome these challenges. Implemented in the DIII-D and KSTAR tokamaks, this method has consistently achieved reactor-relevant core confinement and the highest fusion performance without triggering damaging bursts. This is enabled by advances in the physics understanding of self-organized transport in the plasma edge and machine learning techniques to optimize the 3D field spectrum. The success of automated, real-time adaptive control of such complex systems paves the way for maximizing fusion efficiency in ITER and beyond while minimizing damage to device components.

Tinkering with care: Implementing extended-release buprenorphine depot treatment for opioid dependence.

We examine how extended-release buprenorphine depot (BUP-XR) is put to use and made to work in implementation practices, attending to how care practices are challenged and adapted as a long-acting technology is introduced into service in opioid agonist treatment (OAT) in Australia. Our approach is informed by ideas in science and technology studies (STS) emphasising the irreducible entanglement of care practices and technology, and in particular the concept of 'tinkering' as a practice of adaptation. To make our analysis, we draw on qualitative interview accounts (n = 19) of service providers involved in BUP-XR implementation across five sites. Our analysis considers the disruptive novelty of BUP-XR. Tinkering to make a novel technology work in practice slows down the expectation of implementation in relation to transformative innovation, despite the promise of dramatic or rapid change. Tinkering allowed for more open relations, for new care practices that departed from the routine and familiar, opening potential for how BUP-XR could be put to use and made to work in its new situation, and as its situation evolved along-with its implementation. Flexibility and openness of altering relations was, however, at times, held in tension with inflexibility and closure. This analysis identifies a concern for what is made present and what is made absent in the altered care network affected by BUP-XR, with the multiple effects of supervised daily dosing practices thrown into relief as they become absented. Tinkering to implement BUP-XR locally connects with a broader assemblage of trial and movement in the constitution of treatment. The introduction of long-acting technologies prompts new questions about embedded implementation practices, including supervised dosing, urinalysis, the time and place of psychosocial support, and how other social aspects of care might be recalibrated in drug treatment.

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

A novel quadrature Doppler Backscattering (DBS) system has been developed and optimized for the E-band (60-90 GHz) 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 < 10 MHz) 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 ∼ 23 MHz) and externally driven high-frequency helicon waves (f = 476 MHz) 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 (kθ ∼ 1-20 cm-1 and kr ≲ 30 cm-1). As a demonstration, localized internal DIII-D plasma measurements are presented from turbulence (f ≤ 5 MHz), Alfvenic waves (f ∼ 6.5 MHz), ion cyclotron waves (f ≥ 20 MHz), as well as fluctuations around 476 MHz 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.

Design elements and first data from a new Doppler backscattering system on the MAST-U spherical tokamak.

A new Doppler backscattering (DBS) system has been installed and tested on the MAST-U spherical tokamak. It utilizes eight simultaneous fixed frequency probe beams (32.5, 35, 37.5, 40, 42.5, 45, 47.5, and 50 GHz). These frequencies provide a range of radial positions from the edge plasma to the core depending on plasma conditions. The system utilizes a combination of novel features to provide remote control of the probed density wavenumber, the launched polarization (X vs O-mode), and the angle of the launched DBS to match the magnetic field pitch angle. The range of accessible density turbulence wavenumbers (kθ) is reasonably large with normalized wavenumbers kθρs ranging from ≤0.5 to 9 (ion sound gyroradius ρs = 1 cm). This wavenumber range is relevant to a variety of instabilities believed to be important in establishing plasma transport (e.g., ion temperature gradient, trapped electron, electron temperature gradient, micro-tearing, kinetic ballooning modes). The system is specifically designed to address the requirement of density fluctuation wavevector alignment which can significantly reduce the SNR if not accounted for.

Analysis method for calculating radial correlation length of electron temperature turbulence from correlation electron cyclotron emission radiometer.

The radial correlation length (Lr) is one of the essential quantities to measure in order to more fully characterize and understand turbulence and anomalous transport in magnetic fusion plasmas. The analysis method for calculating Lr of electron temperature (Te) turbulence from correlation electron cyclotron emission (correlation ECE or CECE) radiometer measurements has not been fully developed partly due to the fact that the turbulent electron temperature fluctuations are generally imbedded in much larger amplitude thermal noise, which leads to a greatly reduced cross correlation coefficient (ϱ) between two spatially separated ECE signals. This work finds that this ϱ reduction factor due to thermal noise is a function of the local relative temperature fluctuation power and CECE system bandwidths of intermediate and video frequencies, independent of radial separations. This indicates that under the approximation of constant relative temperature fluctuation power for a small radial range of local CECE measurements, the original shape of ϱ as a function of radial separation without thermal noise is preserved in the CECE data with thermal noise present. For Te turbulence with a Gaussian radial wavenumber spectrum, a fit function using the product of Gaussian and sinusoidal functions is derived for calculating Lr. This analysis method has been numerically tested using simulated ECE radiometer data over a range of parameters. Using this method, the experimental temperature turbulence correlation length Lr in a DIII-D L-mode plasma is found to be ∼10 times the local ion gyroradius.

Improved Particle Confinement with Resonant Magnetic Perturbations in DIII-D Tokamak H-Mode Plasmas.

Experiments on the DIII-D tokamak have identified a novel regime in which applied resonant magnetic perturbations (RMPs) increase the particle confinement and overall performance. This Letter details a robust range of counter-current rotation over which RMPs cause this density pump-in effect for high confinement (H mode) plasmas. The pump in is shown to be caused by a reduction of the turbulent transport and to be correlated with a change in the sign of the induced neoclassical transport. This novel reversal of the RMP induced transport has the potential to significantly improve reactor relevant, three-dimensional magnetic confinement scenarios.

Evaluation of a new DIII-D Doppler backscattering system for higher wavenumber measurement and signal enhancement.

The high density fluctuation poloidal wavenumber, kθ (kθ > 8 cm-1, kθρs > 5, ρs is the ion gyro radius using the ion sound velocity), measurement capability of a new Doppler backscattering (DBS) system at the DIII-D tokamak has been experimentally evaluated. In DBS, wavenumber (k) matching becomes more important at higher wavenumbers, owing to the exponential dependence of the measured signal loss factor on wave vector mismatch. Wave vector matching allows for the Bragg scattering condition to be satisfied, which minimizes the signal loss at higher k's. In the previous DBS system, without toroidal wave vector matching, the measured DBS signal-to-noise ratio at higher kθ (>8 cm-1) is substantially reduced, making it difficult to measure higher kθ turbulence. The new DBS system has been optimized to access higher wavenumber, kθ ≤ 20 cm-1, density turbulence measurement. The optimization hardware addresses fluctuation wave vector matching using toroidal steering of the launch mirror to produce a backscattered signal with improved intensity. The probe's sensitivity to high-k density fluctuations has been increased by approximately an order of magnitude compared to the old system that has been in use at DIII-D. Note that typical measurement locations are above or below the tokamak midplane on the low field side with normalized radial ranges of 0.5-1.0. The new DBS probe system with the toroidal matching of fluctuation wave vectors is thought to be critical to understanding high-k turbulent transport in fusion-relevant research at DIII-D.

Validating and optimizing mismatch tolerance of Doppler backscattering measurements with the beam model (invited).

We use the beam model of Doppler backscattering (DBS), which was previously derived from beam tracing and the reciprocity theorem, to shed light on mismatch attenuation. This attenuation of the backscattered signal occurs when the wavevector of the probe beam's electric field is not in the plane perpendicular to the magnetic field. Correcting for this effect is important for determining the amplitude of the actual density fluctuations. Previous preliminary comparisons between the model and Mega-Ampere Spherical Tokamak (MAST) plasmas were promising. In this work, we quantitatively account for this effect on DIII-D, a conventional tokamak. We compare the predicted and measured mismatch attenuation in various DIII-D, MAST, and MAST-U plasmas, showing that the beam model is applicable in a wide variety of situations. Finally, we performed a preliminary parameter sweep and found that the mismatch tolerance can be improved by optimizing the probe beam's width and curvature at launch. This is potentially a design consideration for new DBS systems.

New methodology for measuring electron density perturbations caused by plasma coherent modes using profile reflectometry: Magnitudes and radial profiles in DIII-D.

New capabilities of fast-sweep frequency-modulated profile reflectometry are explored to measure electron density ne perturbation magnitudes and radial profiles due to plasma coherent modes in DIII-D. The first approach is based on the frequency analysis of phase perturbations associated with high frequency (∼MHz) Alfvén eigenmodes (AEs). The measurement of ∼5.5 MHz fast-ion-driven global Alfvén eigenmodes (GAEs) is demonstrated in a neutral beam-heated DIII-D plasma. The GAE induced a broad radial distribution of phase perturbations in the profile reflectometer data. Analysis of these data determined the effective cutoff location displacement and the estimated ne fluctuation profile. In the second approach, high resolution ne profiles are used directly to determine the radial structure of ne perturbations due to a neo-classical tearing mode. These new measurements broaden the application of profile reflectometry and advance the development of AE spectroscopy as a tool for non-invasive diagnosis of fast-ion-driven modes in DIII-D and burning plasmas such as ITER.

New, improved analysis of correlation ECE data to accurately determine turbulent electron temperature spectra and magnitudes (invited).

Turbulent electron temperature fluctuation measurement using a correlation electron cyclotron emission (CECE) radiometer has become an important diagnostic for studying energy transport in fusion plasmas, and its use is widespread in tokamaks (DIII-D, ASDEX Upgrade, Alcator C-Mod, Tore Supra, EAST, TCV, HL-2A, etc.). The CECE diagnostic typically performs correlation analysis between two closely spaced (within the turbulent correlation length) ECE channels that are dominated by uncorrelated thermal noise emission. This allows electron temperature fluctuations embedded in the thermal noise to be revealed and fluctuation level and spectra determined. We have demonstrated a new, improved CECE coherency-based analysis for calculating the temperature fluctuation frequency spectrum and level, which has been verified both numerically through the simulation of synthetic ECE radiometer data and through analysis of experimental data from the CECE system on DIII-D. The new formulation places coherency-based analysis on a firm foundational footing and corrects some currently published methodologies. This new method accurately accounts for bias error in the coherence function and correctly calculates noise levels for a fixed data record length. It provides excellent accuracy in determining temperature fluctuation level (e.g., <10% error) even for a small realization number in the ensemble average. The method also has a smaller uncertainty (i.e., error bar) in the power spectrum when compared to the more standard cross-power method when evaluated at low coherency. Direct calculation of system noise level using correlation between randomized intermediate frequency signals is recommended.

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 B̃ wavenumbers are estimated using the wavevector matching criterion. The wavenumber ranges of the local B̃ that are detectable to the CPS system are explored for simulated L- and H-mode plasmas.

Multiscale Chirping Modes Driven by Thermal Ions in a Plasma with Reactor-Relevant Ion Temperature.

A thermal ion driven bursting instability with rapid frequency chirping, considered as an Alfvénic ion temperature gradient mode, has been observed in plasmas having reactor-relevant temperature in the DIII-D tokamak. The modes are excited over a wide spatial range from macroscopic device size to microturbulence size and the perturbation energy propagates across multiple spatial scales. The radial mode structure is able to expand from local to global in ∼0.1 ms and it causes magnetic topology changes in the plasma edge, which can lead to a minor disruption event. Since the mode is typically observed in the high ion temperature ≳10 keV and high-ß plasma regime, the manifestation of the mode in future reactors should be studied with development of mitigation strategies, if needed. This is the first observation of destabilization of the Alfvén continuum caused by the compressibility of ions with reactor-relevant ion temperature.

Performance demonstration of vacuum microwave components critical for the operation of the ITER low-field side reflectometer.

Final design studies in preparation for manufacturing have been performed for functional components of the vacuum portion of the ITER Low-Field Side Reflectometer (LFSR). These components consist of an antenna array, electron cyclotron heating (ECH) protection mirrors, phase calibration mirrors, and vacuum windows. Evaluation of these components was conducted at the LFSR test facility and DIII-D. The antenna array consists of six corrugated-waveguide antennas for simultaneous profile, fluctuation, and Doppler measurements. A diffraction grating, incorporated into the plasma-facing miter bend, provides protection of sensitive components from stray ECH at 170 GHz. For in situ phase calibration of the LFSR profile reflectometer, an embossed mirror is incorporated into the adjacent miter bend. Measurements of the radiated beam profile indicate that these components have a small, acceptable effect on mode conversion and beam quality. Baseline transmission characteristics of the dual-disk vacuum window are obtained and are used to guide ongoing developments. Preliminary simulations indicate that a surface-relief structure on the window surfaces can greatly improve transmission. The workability of real-time phase measurements was demonstrated on the DIII-D profile reflectometer. The new automated real-time analysis agrees well with the standard post-processing routine.

Achievement of Reactor-Relevant Performance in Negative Triangularity Shape in the DIII-D Tokamak.

Plasma discharges with a negative triangularity (δ=-0.4) shape have been created in the DIII-D tokamak with a significant normalized beta (ß_{N}=2.7) and confinement characteristic of the high confinement mode (H_{98y2}=1.2) despite the absence of an edge pressure pedestal and no edge localized modes (ELMs). These inner-wall-limited plasmas have a similar global performance as a positive triangularity (δ=+0.4) ELMing H-mode discharge with the same plasma current, elongation and cross sectional area. For cases both of dominant electron cyclotron heating with T_{e}/T_{i}>1 and dominant neutral beam injection heating with T_{e}/T_{i}=1, turbulent fluctuations over radii 0.5<ρ<0.9 were reduced by 10-50% in the negative triangularity shape compared to the matching positive triangularity shape, depending on the radius and conditions.

Turbulence measurements on the high and low magnetic field side of the DIII-D tokamak.

In this paper, we address the challenging question of measuring turbulence levels on the high magnetic field side (HFS) of tokamak plasmas. Although turbulence measurements on the HFS can provide a stringent constraint for the turbulence model validation, to date only low magnetic field side (LFS) measured turbulence has been used in validation studies. To address this issue, an eight channel Correlation Electron Cyclotron Emission (CECE) system at DIII-D was modified to probe both LFS and HFS. In contrast to the second harmonic extraordinary mode electron cyclotron resonance emission that is typically used in CECE, we show that it is possible to probe the HFS using fundamental O-mode electron cyclotron resonance emission. The required hardware modifications for the HFS measurements are presented here, and the potential issues in this measurement are discussed.

Optimized quasi-optical cross-polarization scattering system for the measurement of magnetic turbulence on the DIII-D tokamak.

Simulations and laboratory tests are used to design and optimize a quasi-optical system for cross-polarization scattering (CPS) measurements of magnetic turbulence on the DIII-D tokamak. The CPS technique uses a process where magnetic turbulence scatters electromagnetic radiation into the perpendicular polarization enabling a local measurement of the perturbing magnetic fluctuations. This is a challenging measurement that addresses the contribution of magnetic turbulence to anomalous thermal transport in fusion research relevant plasmas. The goal of the new quasi-optical design is to demonstrate the full spatial and wavenumber capabilities of the CPS diagnostic. The approach used consists of independently controlled and in vacuo aiming systems for the probe and scattered beams (55-75 GHz).

A free-standing wire scattering technique to monitor calibration variations of the DIII-D density profile reflectometer.

Real-time phase calibration of the ITER profile reflectometer is essential due to the long plasma duration and expected waveguide path length changes during a discharge. Progress has been recently made in addressing this issue by employing a phase calibration technique on DIII-D that monitors calibration variations that occur during each plasma discharge. By installing a thin free-standing metallic wire (1 mm diameter) near the end of the overmoded waveguide transmission system (oriented perpendicular to the waveguide axis), the round-trip phase shift from the wire is detected simultaneously with the plasma phase shifts. Variations in the reflectometer round trip path length (∼26 m) are then calculated after each DIII-D plasma discharge, allowing the calibration phase to be accurately monitored and updated. The round-trip reflectometer path length is observed to vary by ∼3 mm (root mean square value) during a typical DIII-D discharge. Using the variations in calibration phase, the density profile measurement accuracy can be improved. Since the wire retro-reflected power is ∼0.01 of the plasma signal, minimal effect is observed on the reflected signal from the plasma. Importantly, through a suitable choice in wire diameter, the calibration signal can be made approximately independent of the V-band reflectometer launch polarization. This is particularly important on DIII-D since orthogonal X- and O-mode polarized beams are coupled into the same transmission waveguide and launch antenna.

First step toward a synthetic diagnostic for magnetic fluctuation measurements using cross-polarization scattering on DIII-D.

Cross-polarization scattering (CPS) provides localized magnetic fluctuation ( B ̃ ) measurements in fusion plasmas based on the process where B ̃ scatters electromagnetic radiation into the orthogonal polarization. The CPS system on DIII-D utilizes the probe beam of a Doppler backscattering (DBS) diagnostic combined with a cross-view CPS receiver system, which allows simultaneous density and B ̃ measurements with good spatial and wavenumber coverage. The interpretation of the signals is challenging due to the complex plasma propagation of the DBS probe beam and CPS receive beams. A synthetic diagnostic for CPS is therefore essential to interpret data and perform detailed validation tests of non-linear turbulence simulations. This work reports a first step toward a synthetic diagnostic for CPS utilizing GENRAY, a 3-D ray tracing code, to simulate the propagation of the probe and scattered rays. The local B ̃ wavenumber is calculated from the local O- and X-mode wavenumbers using the wave vector matching scattering condition. The CPS wavenumber values and spatial locations are determined by a complex consideration that includes the local density and B ̃ level, receive antenna pattern and orientation, scattering volume, wavenumber values detected at the various scattering centers, and alignment of the magnetic wave vector with the plane perpendicular to the magnetic field. The issue of a spurious CPS signal due to polarization mismatches for launch and receive is also discussed. It is suggested that simultaneous O- and X-mode DBS measurements should be utilized for better understanding of the CPS signal contamination when the cutoff locations for both polarizations are close.