Electron cyclotron emission detection of neoclassical tearing modes for control for ITER.

Successful operation of ITER requires control of magnetic instabilities including neoclassical tearing modes (NTMs) that can degrade confinement and lead to disruption. Low latency detection by electron cyclotron emission (ECE) diagnostics has been demonstrated in a few current experiments. Using a synthetic diagnostic, we demonstrate low latency NTM detection for ITER with plasmas described by ITER IMAS database scenarios and with realistic limitations imposed on the instrumentation by these high temperature scenarios. 2/1 NTMs are detected 430 ms after magnetic island seeding and before island locking. The radiometer configuration was optimized using simulation, and the smallest detectable island size was explored. Island sizes of ∼3 cm are detectable at the 2/1 surface. The simulated signals incorporate recent physics models for island growth and rotation, which show early locking and continued island growth after locking and before disruption. This work determines limits for ITER ECE spatial resolution imposed by relativistic broadening of channels, which informs hardware design. Real-time detection is demonstrated in hardware that is required by ITER, including on an NI PXI-7853R FPGA system. Development of a synthetic diagnostic and details of the hardware will be discussed.

Estimating the performance of lithium beam measurements of current density and electron density in an H-mode pedestal.

The lithium beam is an effective diagnostic tool for investigation of stability and particle transport in the pedestal. It was used successfully to measure edge current density on DIII-D, achieving qualitative agreement with neoclassical models. Electron density profiles were also measured. Proposed upgrades will continue these measurements with higher reliability as well as explore edge current measurements using spectroscopy. The optics will be redesigned to optimize throughput and aperture broadening and to replace the photomultiplier tubes with avalanche photodiodes. The new system will yield detailed measurements of the pedestal, complementing existing diagnostics for investigating pedestal stability, edge localized mode cycle, and particle transport through the pedestal.

Variable location channels to improve efficiency and precision for direct ∇T e measurements and high spatial resolution T e -profile measurements using electron cyclotron emission.

Electron cyclotron emission (ECE) diagnostics that use variable location channels based on yttrium iron garnet (YIG) bandpass filters improve the precision and the efficiency of measurements of electron temperature (T e ) profiles and fluctuations (δT e ). These variable frequency filters were substituted for fixed frequency filters in the intermediate frequency section to achieve the required higher resolution over a target radial range, just before the experiment. Here, we present the proof-of-principle for high temporal resolution measurement of the electron temperature gradient, via real-time slewing of a YIG filter for relocation of an ECE channel during a long pulse. The key component is the application of YIG tunable filters with their narrow bandwidth and capability for a high slew rate of their center frequency. This application permits fast relocation of the ECE channels for direct measurement of the gradient and close spacing of channels to investigate the magnetic island's dynamic behavior.

Four-dimensional calibration turntable of the motional Stark effect diagnostic on EAST.

The motional Stark effect (MSE) diagnostic is applied to measure the safety factor q and current density profile of a tokamak device, which are important parameters in realizing the high-performance and long-pulse steady state of a tokamak. A single-channel MSE diagnostic based on dual photoelastic modulators, whose sightline meets with the neutral beam injection at a major radius of R = 2.12 m, has been built for the D window of the Experimental Advanced Superconducting Tokamak (EAST). According to the requirements of MSE diagnostic polarimetric calibration, a high-precision four-dimensional calibration turntable, driven by four stepping motors and controlled by software running on the computer, was designed for EAST. The turntable allows us to rapidly calibrate the MSE diagnostic in a series of positions and angles during EAST maintenance. The turntable can move in four dimensions of translation, yaw, pitch, and roll of the polarizer and can create linearly polarized light at any given angle with accuracy of ∼0.05° for the MSE system offline calibration. The experimental results of the MSE diagnostic calibration in the laboratory show that the turntable has the advantages of high positioning accuracy, flexible spatial movement, and convenient control and fully meets the calibration requirements of an MSE diagnosis system.

Upgrade of the ECE diagnostic on EAST.

The electron cyclotron emission (ECE) diagnostic on the experimental advanced superconducting tokamak (EAST) was upgraded recently to provide electron temperature profile measurement with wider radial coverage and better precision. The lower limit of the ECE detection frequency band was extended from 104 GHz to 97 GHz by adding a new 8-channel heterodyne radiometer, which ensures capability for the measurement of the second harmonic ECE with toroidal magnetic field down to 1.75 T. Also, the existing 32-channel heterodyne radiometer has been upgraded, with the frequency interval for the lower frequency range up to 120 GHz reduced from 2 GHz to 1 GHz by introducing eight channels in the intermediate frequency part. In addition, a plan is presented to incorporate tunable yttrium iron garnet filters into the existing heterodyne radiometer to obtain detailed measurements of the electron temperature gradient scale length as well as finer spatial pinpointing of magnetohydrodynamic modes. Examples from DIII-D are provided where similar ECE diagnostic allowed precise measurement of the center and width of neoclassical tearing modes.

Design and first plasma measurements of the ITER-ECE prototype radiometer.

On ITER, second harmonic optically thick electron cyclotron emission (ECE) in the range of 220-340 GHz will supply the electron temperature (Te). To investigate the requirements and capabilities prescribed for the ITER system, a prototype radiometer covering this frequency range has been developed by Virginia Diodes, Inc. The first plasma measurements with this instrument have been carried out on the DIII-D tokamak, with lab bench tests and measurements of third through fifth harmonic ECE from high Te plasmas. At DIII-D the instrument shares the transmission line of the Michelson interferometer and can simultaneously acquire data. Comparison of the ECE radiation temperature from the absolutely calibrated Michelson and the prototype receiver shows that the ITER radiometer provides accurate measurements of the millimeter radiation across the instrument band.

Physics design of the in-vessel collection optics for the ITER electron cyclotron emission diagnostic.

Measurement of the electron cyclotron emission (ECE) is one of the primary diagnostics for electron temperature in ITER. In-vessel, in-vacuum, and quasi-optical antennas capture sufficient ECE to achieve large signal to noise with microsecond temporal resolution and high spatial resolution while maintaining polarization fidelity. Two similar systems are required. One views the plasma radially. The other is an oblique view. Both views can be used to measure the electron temperature, while the oblique is also sensitive to non-thermal distortion in the bulk electron distribution. The in-vacuum optics for both systems are subject to degradation as they have a direct view of the ITER plasma and will not be accessible for cleaning or replacement for extended periods. Blackbody radiation sources are provided for in situ calibration.

Temperature gradient scale length measurement: A high accuracy application of electron cyclotron emission without calibration.

Calibration is a crucial procedure in electron temperature (Te) inference from a typical electron cyclotron emission (ECE) diagnostic on tokamaks. Although the calibration provides an important multiplying factor for an individual ECE channel, the parameter ΔTe/Te is independent of any calibration. Since an ECE channel measures the cyclotron emission for a particular flux surface, a non-perturbing change in toroidal magnetic field changes the view of that channel. Hence the calibration-free parameter is a measure of Te gradient. BT-jog technique is presented here which employs the parameter and the raw ECE signals for direct measurement of electron temperature gradient scale length.

Respiratory sinus arrhythmia and the heart rate response to carotid baroreceptor activation after hard dynamic exercise in humans.

The aim of this study was to examine the influence of 20 min of hard exercise (HR>160 beats min(-1)) on the efficacy of the cardiac parasympathetic nervous control of heart rate in humans (20-31 years; of either sex). This intensity of exercise was chosen to produce strong activation of the cardiac sympathetic nerves. Using well-controlled stimulus parameters, the efficacy of cardiac parasympathetic control of heart rate was assessed by recording the heart rate response to carotid baroreceptor activation (CBR) and the amplitude of respiratory sinus arrhythmia (RSA). Measurements were made while the subject performed light exercise (100-135 beats min (-1)) before (Control 1) and after very brief (Control 2) and prolonged (20 min; post) periods of hard exercise. There was no difference in the CBR in the three different measurement periods; 0.33 +/- 0.17, 0.38 +/- 0.18 and 0.39 +/- 0.18 beats min(-1) mm Hg(-1) (mean +/- S.D., N=6) for Control 1, Control 2 and post, respectively. At a heart rate of 120 beats min (-1), amplitude of the RSA was 6.1 +/- 2.4, 5.6 +/- 2.4 and 3.3 +/- 2.1 beats min(-1) for Control 1, Control 2 and post, respectively (P<0.001 post vs. Control 1 and Control 2, N=8). The decrease in RSA amplitude following hard exercise may be attributable to an exercise-induced reduction in airway resistance and work of breathing. Overall, these results do not support the hypothesis that sustained hard exercise that produces strong activation of cardiac sympathetic nerves reduces cardiac parasympathetic efficacy.

A 16-channel heterodyne electron cyclotron emission radiometer on J-TEXT.

To study equilibrium temporal dynamics and the mechanisms of magnetohydrodynamic instabilities, a 16-channel heterodyne electron cyclotron emission (ECE) radiometer has been developed to view the J-TEXT tokamak from the low field side. The ECE radiometer detects second-harmonic extraordinary mode in the frequency band of 94-125 GHz which corresponds to resonances from 1.8 T to 2.2 T. This ECE system consists of an ECE transmission line, a radio frequency unit, and two 8-channel intermediate frequency units. An in situ blackbody calibration source is applied for system calibration by comparison of hot and cold sources in order to provide an absolute temperature measurement.

An integrated charge exchange recombination spectroscopy/beam emission spectroscopy diagnostic for Alcator C-Mod tokamak.

A novel integrated charge exchange recombination spectroscopy (CXRS)/beam emission spectroscopy (BES) system is proposed for C-Mod, in which both measurements are taken from a shared viewing geometry. The supplementary BES system serves to quantify local beam densities and supplants the common calculation of beam attenuation. The new system employs two optical viewing arrays, 20 poloidal and 22 toroidal channels. A dichroic filter splits the light between two spectrometers operating at different wavelengths for impurity ion and beam neutrals emission. In this arrangement, the impurity density is inferred from the electron density, measured BES and CXRS spectral radiances, and atomic emission rates.

Wide-view charge exchange recombination spectroscopy diagnostic for Alcator C-Mod.

This diagnostic measures temperature, density, and rotation for the fully stripped boron ion between the pedestal top and the plasma core with resolution consistent with the profile gradients. The diagnostic neutral beam used for the measurements generates a 50 keV, 6 A hydrogen beam. The optical systems provide views in both poloidal and toroidal directions. The imaging spectrometer is optimized to simultaneously accept 45 views as input with minimum cross-talk. In situ calibration techniques are applied for spatial location, spectral intensity, and wavelength. In the analysis, measured spectra are fitted to a model constructed from a detailed description of the emission physics. Methods for removal of interfering spectra are included. Applications include impurity and thermal transport.

Method to estimate the electron temperature and neutral density in a plasma from spectroscopic measurements using argon atom and ion collisional-radiative models.

We present a method to infer the electron temperature in argon plasmas using a collisional-radiative model for argon ions and measurements of electron density to interpret absolutely calibrated spectroscopic measurements of argon ion (Ar II) line intensities. The neutral density, and hence the degree of ionization of this plasma, can then be estimated using argon atom (Ar I) line intensities and a collisional-radiative model for argon atoms. This method has been tested for plasmas generated on two different devices at the University of Texas at Austin: the helicon experiment and the helimak experiment. We present results that show good correlation with other measurements in the plasma.