Observation of edge instability limiting the pedestal growth in tokamak plasmas.

With fusion device performance hinging on the edge pedestal pressure, it is imperative to experimentally understand the physical mechanism dictating the pedestal characteristics and to validate and improve pedestal predictive models. This Letter reports direct evidence of density and magnetic fluctuations showing the stiff onset of an edge instability leading to the saturation of the pedestal on the Alcator C-Mod tokamak. Edge stability analyses indicate that the pedestal is unstable to both ballooning mode and kinetic ballooning mode in agreement with observations.

Edge scanning reflectometry for density profile measurement on the SPARC tokamak.

Edge scanning reflectometry (ESRL) on the SPARC tokamak aims to measure the electron density profile from the far scrape-off layer to the top of the typical H-mode pedestal and provide real-time data for plasma control. ESRL uses a standard frequency-modulated continuous wave technique from 18 to 90 GHz. By implementing both the O-mode and left-hand-cutoff X-mode, it covers densities from ∼4 × 1018 to ∼4 × 1020 m-3 at B0 ∼12 T. A voltage-controlled oscillator acts as the frequency sweep source. Phase-locked dielectric resonator oscillators and bandpass filters generate base signals ∼9-15 GHz. The signals are then frequency multiplied and amplified to reach the K (18-26 GHz), Ka (26-40 GHz), U (40-60 GHz), and E (60-90 GHz) bands. Multi-band signals are combined via the quasi-optical technique. ESRL plans to use oversized waveguides (∼20 m one-way) and a bi-static arrangement to minimize signal losses and distortions while allowing system flexibility. A COMSOL Multiphysics RF model in 2D has been set up to simulate the reflectometry process and help decide the layout of the horn antennas. Engineering analyses of the key parts of the system have been carried out in support of its preliminary design.

Performance predictions of the SPARC x-ray crystal spectrometers for ion temperature and toroidal rotation measurements.

SPARC will be outfitted with three systems of x-ray crystal spectrometer arrays. Two of these are designed using cylindrically bent crystals to achieve high spectral-resolution for ion temperature and toroidal velocity measurements via imaging He-like Kr and Ne-like Xe. The last acts as a spectral survey system to monitor Ne-like W and nearby H- and He-like emission from Cr, Fe, Co, Ni, and Cu. Line radiation intensities are calculated using the Flexible Atomic Code for atomic data and ColRadPy for collisional-radiative modeling, then convoluted with a Voigt line shape. Free-free, free-bound, and two-photon continuum radiation is also included. The ToFu code is used to perform volume-of-sight integration to produce synthetic detector images. In addition, presented is cross-validation performed using the XICSRT Monte Carlo ray-tracing code. Ion temperature and toroidal velocity profiles are reconstructed using ToFu via tomographic inversion.

Design of a multichannel vacuum ultraviolet spectroscopy system for SPARC.

The design of a vacuum ultraviolet spectroscopy system has been performed to monitor and provide feedback for impurity control in SPARC. The spectrometer, covering a wavelength range of 10-2000 Å through a flat-field configuration with diffraction gratings, incorporates five survey lines of sight. This allows for comprehensive impurity analysis across the core and four divertor regions (inner/outer and upper/lower). Its compact modular design facilitates vertical stacking of each spectrometer unit, significantly reducing space in the tokamak hall, where a dedicated radiation shielding bunker will be built. Safety features include a secondary helium enclosure to mitigate tritium permeation risks during deuterium-tritium (D-T) operations and shielding within the beamlines for enhanced radiation protection. The silicon carbide mirror design for divertor observation ensures its survivability in the in-vessel environment of SPARC, validated by thermal and electromagnetic analysis. Signal modeling and data acquisition testing results show that an exposure time of a few milliseconds is appropriate considering photon flux reaching the detector, demonstrating the system's capability for discharge control that includes disruption avoidance.

Development of a bolometry diagnostic for SPARC.

To control and optimize the power of the SPARC tokamak, we require information on the total radiated power of the plasma and its 2D and 3D spatial distribution. The SPARC bolometry diagnostic is being designed and built to measure the radiated power for controlling power balance, investigating the dissipation capabilities of various divertor concepts, and measuring the efficacy of the disruption thermal load mitigation. Proven resistive bolometer sensor technology will be used, with 248 lines of sight integrated into pinhole cameras in 20 different locations. This diversity of views will allow the bolometers to view the core, divertor, and particularly X-points of the plasma with high resolution. 14 of these camera locations are dedicated to 2D equilibrium radiated power, while the remaining six locations are designed to measure 3D radiated energy during disruptions. The bolometer sensor holders, pinhole camera boxes, and cabling have been designed to survive the high neutron flux (but low fluence) and up to 400 °C temperatures seen during operation and vacuum bake. The resistive bolometer sensors use Au absorbers with an Al heat conduction layer and C anti-reflective layer. These sensor chips are wire-bonded to an AlN circuit board, both of which are held inside a custom AlN and stainless steel bolometer holder. Design and optimization of the pinhole camera lines of sight are performed using Cherab. This work details the current state of the design of the SPARC bolometry diagnostic and its interfaces, as well as ongoing work to validate the design.

Formation and stability of impurity "snakes" in tokamak plasmas.

New observations of the formation and dynamics of long-lived impurity-induced helical "snake" modes in tokamak plasmas have recently been carried out on Alcator C-Mod. The snakes form as an asymmetry in the impurity ion density that undergoes a seamless transition from a small helically displaced density to a large crescent-shaped helical structure inside q<1, with a regularly sawtoothing core. The observations show that the conditions for the formation and persistence of a snake cannot be explained by plasma pressure alone. Instead, many features arise naturally from nonlinear interactions in a 3D MHD model that separately evolves the plasma density and temperature.

Effects of magnetic shear on toroidal rotation in tokamak plasmas with lower hybrid current drive.

Application of lower hybrid (LH) current drive in tokamak plasmas can induce both co- and countercurrent directed changes in toroidal rotation, depending on the core q profile. For discharges with q(0) <1, rotation increments in the countercurrent direction are observed. If the LH-driven current is sufficient to suppress sawteeth and increase q(0) above unity, the core toroidal rotation change is in the cocurrent direction. This change in sign of the rotation increment is consistent with a change in sign of the residual stress (the divergence of which constitutes an intrinsic torque that drives the flow) through its dependence on magnetic shear.

Overview and first measurements of the MAST Upgrade bolometer diagnostic.

A suite of multi-channel resistive bolometers has been implemented to measure the total radiation from Mega Amp Spherical Tokamak Upgrade plasmas, with cameras covering the core plasma and lower divertor chamber. Data are digitized and processed using novel field-programmable gate array-based electronics, offering improved compactness and new operational capabilities. A synthetic diagnostic has been developed to explore the quality of 2D reconstructions available from the system and to quantify the uncertainty on quantities such as the total radiated power. Measurements in the first campaign have demonstrated correct functioning of the diagnostic while also highlighting issues with electrical noise and some failure mechanisms of the detectors, as well as significant neutral beam fast-particle losses.

Rotation reversal bifurcation and energy confinement saturation in tokamak Ohmic L-mode plasmas.

Direction reversals of intrinsic toroidal rotation have been observed in diverted Alcator C-Mod Ohmic L-mode plasmas following electron density ramps. For low density discharges, the core rotation is directed cocurrent, and reverses to countercurrent following an increase in the density above a certain threshold. Such reversals occur together with a decrease in density fluctuations with 2 cm(-1)≤k(θ)≤11 cm(-1) and frequencies above 70 kHz. There is a strong correlation between the reversal density and the density at which the Ohmic L-mode energy confinement changes from the linear to the saturated regime.

Edge temperature gradient as intrinsic rotation drive in Alcator C-Mod tokamak plasmas.

Intrinsic rotation has been observed in I-mode plasmas from the C-Mod tokamak, and is found to be similar to that in H mode, both in its edge origin and in the scaling with global pressure. Since both plasmas have similar edge ∇T, but completely different edge ∇n, it may be concluded that the drive of the intrinsic rotation is the edge ∇T rather than ∇P. Evidence suggests that the connection between gradients and rotation is the residual stress, and a scaling for the rotation from conversion of free energy to macroscopic flow is calculated.

Methods to determine the radiated power in SPI-mitigated disruptions in JET.

This paper presents techniques for evaluating the radiated power in JET disruptions. Disrupting plasmas are shown to have non-axisymmetric radiation profiles, motivating the re-evaluation of the standard techniques for calculating the total radiated power at JET using bolometry. Four single-channel bolometers at different toroidal locations are exploited to quantify the radiation asymmetry. Toroidal radiation peaking factors integrated over the entire disruption of up to 1.5 have been observed when varying the quantity of neon in pellets used in disruptions mitigated by shattered pellet injection. Using synthetic bolometer diagnostics developed with the Cherab spectroscopy modeling framework, we can estimate the systematic error on total power calculations for relevant radiation profiles and improve estimates of the total radiated power. We show that the component of the systematic error on the total power due to the poloidal radiation profile can be reduced from 70% to 10% with suitable assumptions about the structure of the poloidal profile.

Integrated plasma facing component calorimetry for measurement of shot integrated deposited energy in the NSTX-U.

The upgrade to the National Spherical Torus eXperiment (NSTX-U) [J. Menard et al., Nucl. Fusion 52, 083015 (2012)] increases the injected neutral beam power up to 12 MW and the plasma current up to Ip = 2 MA for plasma durations up to 5 s. The graphite plasma facing components have been re-designed to handle greater heat and energy fluxes than were seen in NSTX using a castellated design. We present the experimental testing and validation of a castellated graphite target, similar to the prototype tile design, instrumented with thermocouples at various depths in the castellation. During testing, incident heat flux is provided by a programmed electron beam system and surface temperatures are measured via infrared thermography directly viewing the target surface. It was found that the thermocouple response scaled linearly with the measured surface temperature rise regardless of thermocouple depth in the castellation. A sensitivity of 14.3 °C/kJ of deposited energy was found when treating individual castellations as a semi-infinite solid.

Experimental tests of an infrared video bolometer on Alcator C-Mod.

A prototype of an infrared imaging bolometer (IRVB) was successfully tested on the Alcator C-Mod tokamak at the end of its 2016 campaign. The IRVB method interprets the power radiated from the plasma by measuring the temperature rise of a thin, ∼2 µm, Pt absorber that is placed in the torus vacuum and exposed, using a pinhole camera, to the full-spectrum of plasma's photon emission. The IRVB installed on C-Mod viewed the poloidal cross section of the core plasma and observed Ohmic and ion cyclotron range of frequency (ICRF)-heated plasmas. Analysis of total radiated power and on-axis emissivity from IRVB is summarized, and quantitative comparisons made to data from both resistive bolometers and AXUV diodes. IRVB results are clearly within a factor of two, but additional effort is needed for it to be used to fully support power exhaust research. The IRVB is shown to be immune to electromagnetic interference from ICRF which strongly impacts C-Mod's resistive bolometers. Results of the bench-top calibration are summarized, including a novel temperature calibration method useful for IRVBs.

Multi-energy x-ray detector calibration for T and impurity density (n) measurements of MCF plasmas.

Soft x-ray detection with the new "multi-energy" PILATUS3 detector systems holds promise as a magnetically confined fusion (MCF) plasma diagnostic for ITER and beyond. The measured x-ray brightness can be used to determine impurity concentrations, electron temperatures, ne2Zeff products, and to probe the electron energy distribution. However, in order to be effective, these detectors which are really large arrays of detectors with photon energy gating capabilities must be precisely calibrated for each pixel. The energy-dependence of the detector response of the multi-energy PILATUS3 system with 100 K pixels has been measured at Dectris Laboratory. X-rays emitted from a tube under high voltage bombard various elements such that they emit x-ray lines from Zr-Lα to Ag-Kα between 1.8 and 22.16 keV. Each pixel on the PILATUS3 can be set to a minimum energy threshold in the range from 1.6 to 25 keV. This feature allows a single detector to be sensitive to a variety of x-ray energies, so that it is possible to sample the energy distribution of the x-ray continuum and line-emission. PILATUS3 can be configured for 1D or 2D imaging of MCF plasmas with typical spatial energy and temporal resolution of 1 cm, 0.6 keV, and 5 ms, respectively.

Development of plasma bolometers using fiber-optic temperature sensors.

Measurements of radiated power in magnetically confined plasmas are important for exhaust studies in present experiments and expected to be a critical diagnostic for future fusion reactors. Resistive bolometer sensors have long been utilized in tokamaks and helical devices but suffer from electromagnetic interference (EMI). Results are shown from initial testing of a new bolometer concept based on fiber-optic temperature sensor technology. A small, 80 µm diameter, 200 µm long silicon pillar attached to the end of a single mode fiber-optic cable acts as a Fabry-Pérot cavity when broadband light, λo ∼ 1550 nm, is transmitted along the fiber. Changes in temperature alter the optical path length of the cavity primarily through the thermo-optic effect, resulting in a shift of fringes reflected from the pillar detected using an I-MON 512 OEM spectrometer. While initially designed for use in liquids, this sensor has ideal properties for use as a plasma bolometer: a time constant, in air, of ∼150 ms, strong absorption in the spectral range of plasma emission, immunity to local EMI, and the ability to measure changes in temperature remotely. Its compact design offers unique opportunities for integration into the vacuum environment in places unsuitable for a resistive bolometer. Using a variable focus 5 mW, 405 nm, modulating laser, the signal to noise ratio versus power density of various bolometer technologies are directly compared, estimating the noise equivalent power density (NEPD). Present tests show the fiber-optic bolometer to have NEPD of 5-10 W/m2 when compared to those of the resistive bolometer which can achieve <0.5 W/m2 in the laboratory, but this can degrade to 1-2 W/m2 or worse when installed on a tokamak. Concepts are discussed to improve the signal to noise ratio of this new fiber-optic bolometer by reducing the pillar height and adding thin metallic coatings, along with improving the spectral resolution of the interrogator.

Design and characterization of a prototype divertor viewing infrared video bolometer for NSTX-U.

The InfraRed Video Bolometer (IRVB) is a powerful tool to measure radiated power in magnetically confined plasmas due to its ability to obtain 2D images of plasma emission using a technique that is compatible with the fusion nuclear environment. A prototype IRVB has been developed and installed on NSTX-U to view the lower divertor. The IRVB is a pinhole camera which images radiation from the plasma onto a 2.5 µm thick, 9 × 7 cm2 Pt foil and monitors the resulting spatio-temporal temperature evolution using an IR camera. The power flux incident on the foil is calculated by solving the 2D+time heat diffusion equation, using the foil's calibrated thermal properties. An optimized, high frame rate IRVB, is quantitatively compared to results from a resistive bolometer on the bench using a modulated 405 nm laser beam with variable power density and square wave modulation from 0.2 Hz to 250 Hz. The design of the NSTX-U system and benchtop characterization are presented where signal-to-noise ratios are assessed using three different IR cameras: FLIR A655sc, FLIR A6751sc, and SBF-161. The sensitivity of the IRVB equipped with the SBF-161 camera is found to be high enough to measure radiation features in the NSTX-U lower divertor as estimated using SOLPS modeling. The optimized IRVB has a frame rate up to 50 Hz, high enough to distinguish radiation during edge-localized-modes (ELMs) from that between ELMs.

Preliminary design of a tangentially viewing imaging bolometer for NSTX-U.

The infrared imaging video bolometer (IRVB) measures plasma radiated power images using a thin metal foil. Two different designs with a tangential view of NSTX-U are made assuming a 640 × 480 (1280 × 1024) pixel, 30 (105) fps, 50 (20) mK, IR camera imaging the 9 cm × 9 cm × 2 µm Pt foil. The foil is divided into 40 × 40 (64 × 64) IRVB channels. This gives a spatial resolution of 3.4 (2.2) cm on the machine mid-plane. The noise equivalent power density of the IRVB is given as 113 (46) µW/cm2 for a time resolution of 33 (20) ms. Synthetic images derived from Scrape Off Layer Plasma Simulation data using the IRVB geometry show peak signal levels ranging from ∼0.8 to ∼80 (∼0.36 to ∼26) mW/cm2.

Energetic ion loss detector on the Alcator C-Mod tokamak.

A scintillator-based energetic ion loss detector has been successfully commissioned on the Alcator C-Mod tokamak. This probe is located just below the outer midplane, where it captures ions of energies up to 2 MeV resulting from ion cyclotron resonance heating. After passing through a collimating aperture, ions impact different regions of the scintillator according to their gyroradius (energy) and pitch angle. The probe geometry and installation location are determined based on modeling of expected lost ions. The resulting probe is compact and resembles a standard plasma facing tile. Four separate fiber optic cables view different regions of the scintillator to provide phase space resolution. Evolving loss levels are measured during ion cyclotron resonance heating, including variation dependent upon individual antennae.

X-ray imaging crystal spectroscopy for use in plasma transport research.

This research describes advancements in the spectral analysis and error propagation techniques associated with x-ray imaging crystal spectroscopy (XICS) that have enabled this diagnostic to be used to accurately constrain particle, momentum, and heat transport studies in a tokamak for the first time. Doppler tomography techniques have been extended to include propagation of statistical uncertainty due to photon noise, the effect of non-uniform instrumental broadening as well as flux surface variations in impurity density. These methods have been deployed as a suite of modeling and analysis tools, written in interactive data language (IDL) and designed for general use on tokamaks. Its application to the Alcator C-Mod XICS is discussed, along with novel spectral and spatial calibration techniques. Example ion temperature and radial electric field profiles from recent I-mode plasmas are shown, and the impact of poloidally asymmetric impurity density and natural line broadening is discussed in the context of the planned ITER x-ray crystal spectrometer.