Electron Cyclotron Emission Data Research Articles

Refined interpretation of electron temperature response to neutral beam injection at DIII-D

Accurate particle and power deposition profiles of neutral beam injection (NBI) are essential to transport studies, and that information is usually acquired through Monte Carlo simulations with a given collisional model. The deposition process of the energetic beam particles leads to the informative electron temperature (Te) evolution trajectory, which can be captured by electron cyclotron emission (ECE) system due to its good spatial and temporal resolution. Previously, some work has been done to interpret the Te responses to the pulsed NBI as a linear heating source with Fourier-based techniques, although that approach fell short when the fast ion slowing-down time becomes significant (∼100 ms). It has been observed in DIII-D that the modulated NBI pulses (10–50 Hz) reduce local core Te values ∼0.1 keV through cold electron dilution in high-Te (>2 keV) plasmas alongside accumulative heating. Here, a novel approach to interpret the Te response to NBI was developed by linearizing and modeling the detailed Te evolution trajectory using coherently averaged ECE data based on the different time scales of the terms in the local power and particle balance equations. The technique does not require absolute calibrations of ECE and is independent of collisional models. The resulting beam deposition profiles show good consistency and reasonable agreement with Monte Carlo calculations based on the atomic data from the Atomic Data and Analysis Structure (ADAS). Local electron density response measured by Thomson scattering (TS) also suggests the same features when the beam pulse is large enough for that diagnostic to resolve. The remaining discrepancies are also discussed.

Diagnosing the pedestal magnetic field and magnetohydrodynamics radial structure with pedestal-scrape of layer electron cyclotron emission radiation inversion in H-mode plasma (invited).

Forward modeling is used to interpret inversion patterns of the pedestal-Scrape of Layer (SOL) Electron Cyclotron Emission (ECE) in DIII-D H-mode experiments. The modeling not only significantly improves the ECE data interpretation quality but also leads to the potential measurements of (1) the magnetic field strength |B| at the separatrix, (2) the pedestal |B| evolution during an inter-Edge Localized Mode (ELM) period, and (3) the pedestal Magnetohydrodynamics (MHD) radial structure. The ECE shine-through effect leads to three types of pedestal-SOL radiation inversions that are discussed in this paper. The first type of inversion is the non-monotonic Te,rad profile with respect to the major radius. Using the ECE frequency at the minimum Te,rad, the inversion can be applied to measure the magnetic field |B| at the separatrix and calibrate the mapping of the ECE channels with respect to the separatrix. The second type of inversion refers to the opposite phase between the radiation fluctuations δTe,rad at the pedestal and SOL. This δTe,rad phase inversion is sensitive to density and temperature fluctuations at the pedestal foot and, thus, can be used to qualitatively measure the MHD radial structure. The third type of inversion appears when the pedestal and SOL Te,rad evolve in an opposite trend, which can be used to infer the pedestal |B| field change during an inter-ELM period. The bandwidth effect on measuring δTe,rad due to pedestal MHD is also investigated in the radiation modeling.

Overview of the ECE measurements on EAST

Radiometer systems and a Michelson interferometer, have been operated routinely to detect the elec-tron cyclotron emission (ECE) from EAST plasmas for diagnosing the local electron temperature. A common quasi-optical antenna placed inside the vacuum vessel is employed to collect and focus the plasma emission, and the line of sight is along a radial chord. All of the systems are located in a diagnostic room where the plasma emission is transmitted by overmoded corrugated waveguide. In-situ absolute intensity calibration has been carried out for both the radiometer systems and the Michelson interferometer independently, to ensure that the ECE diagnostic provides an independent electron temperature measurement. In order to diagnose the small-amplitude electron temperature fluctuation, a correlation ECE (CECE) diagnostic has been designed and commissioned recently. The CECE diagnostic employs an independent antenna system which has improved poloidal resolution. A synthetic diagnostic is realized by using the simulation code SPECE to interpret the ECE data in plasmas with non-Maxwellian distribution, and preliminary results imply that the ECE data could be still useful as a localized measurement in plasmas with non-thermal electrons, such as the LHW-heated plasmas on EAST.

Numerical calculations of non-inductive current driven by microwaves in JET

Recent studies at JET focus on analysis of the lower hybrid (LH) wave power absorption and current drive (CD) calculations by means of a new ray tracing (RT)/Fokker–Planck (FP) package. The RT code works in real 2D geometry accounting for the plasma boundary and the launcher shape. LH waves with different parallel refractive index, , spectra in poloidal direction can be launched thus simulating authentic antenna spectrum with rows fed by different combinations of klystrons. Various FP solvers were tested most advanced of which is a relativistic bounce averaged FP code. LH wave power deposition profiles from the new RT/FP code were compared to the experimental results from electron cyclotron emission (ECE) analysis of pulses at 3.4 T low and high density. This kind of direct comparison between power deposition profiles from experimental ECE data and numerical model were carried out for the first time for waves in the LH range of frequencies. The results were in a reasonable agreement with experimental data at lower density, line averaged values of . At higher density, , the code predicted larger on-axis LH power deposition, which is inconsistent with the experimental observations. Both calculations were unable to produce LH wave absorption at the plasma periphery, which contradicts to the analysis of the ECE data and possible sources of these discrepancies have been briefly discussed in the paper. The code was also used to calculate the LH power deposition and CD profiles for the low-density preheat phase of JET’s advanced tokamak (AT) scenario. It was found that as the density evolves from hollow to flat and then to a more peaked profile the LH power and driven current move inward i.e. towards the plasma axis. A total driven current of about 70 kA for 1 MW of launched LH power was predicted in these conditions.

Plasma response measurements of external magnetic perturbations using electron cyclotron emission and comparisons to 3D ideal MHD equilibrium

The plasma response from an external n = 2 magnetic perturbation field in ASDEX Upgrade has been measured using mainly electron cyclotron emission (ECE) diagnostics and a rigid rotating field. To interpret ECE and ECE-imaging (ECE-I) measurements accurately, forward modeling of the radiation transport has been combined with ray tracing. The measured data is compared to synthetic ECE data generated from a 3D ideal magnetohydrodynamics (MHD) equilibrium calculated by VMEC.The measured amplitudes of the helical displacement around the outboard midplane are in reasonable agreement with the one from the synthetic VMEC diagnostics. Both exceed the predictions from the vacuum field calculations and indicate the presence of a kink response at the edge, which amplifies the perturbation. VMEC and MARS-F have been used to calculate the properties of this kink mode. The poloidal mode structure of the magnetic perturbation of this kink mode at the edge peaks at poloidal mode numbers larger than the resonant components , whereas the poloidal mode structure of its displacement is almost resonant . This is expected from ideal MHD in the proximity of rational surfaces. The displacement measured by ECE-I confirms this resonant response.

On the Validation of Gyrokinetic L-Mode Simulations

Over the last decade, plasma turbulence simulations based on gyrokinetic theory have reached an amazing degree of physical comprehensiveness and realism. In contrast to early gyrokinetic studies, which were restricted to qualitative statements, state-of-the-art investigations may now be compared quantitatively, therefore enabling validation and detailed analysis of their predictive capabilities. Here, particular attention is paid to outer-core L-mode discharges for which some previous gyrokinetic studies have found an underprediction of ion heat transport by almost one order of magnitude, the so-called shortfall. Carrying out radially local and nonlocal GENE simulations using actual plasma profiles and parameters and magnetohydrodynamic equilibria, and employing as much physics as available, only a mild underprediction is found, which can, furthermore, be overcome by varying the ion temperature gradient within the error bars associated with the experimental measurement. The significance and reliability of these simulations is furthermore demonstrated by extensive comparison with experimental measurements. The latter involve sophisticated synthetic beam emission spectroscopy and correlation electron cyclotron emission data analysis. The agreement found between the measurements and the state-of-the-art postprocessed simulation data confirms the high degree of realism.

Summary of EC-17: the 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (Deurne, The Netherlands, 7–10 May 2012)

An overview is given of the papers presented at the 17th Joint Workshop on Electron Cyclotron Emission (ECE) and Electron Cyclotron Resonance Heating (ECRH). The meeting covered all aspects of the research field ranging from theory to enabling technologies. From the workshop, advanced control by electron cyclotron heating and current drive is emerging as probably the main application of ECRH in fusion devices. Large progress is reported from various experiments on real-time control applications. At the same time ECE is developing into a multi-dimensional plasma diagnostic taking advantage of new technological developments. The resulting multi-dimensional ECE data reveal exciting new details of the complicated plasma dynamics in fusion devices.

Evaluating neoclassical tearing mode detection with ECE for control on ITER

Neoclassical tearing mode (NTM) control on ITER requires detection of the mode location to be accurate and with low latency. This paper presents a systematic way to evaluate mode detection algorithms for ITER using numerical simulations of electron cyclotron emission (ECE), taking into account the radial asymmetry in the temperature perturbation by a rotating magnetic island. Simulated ECE is detected using a synthetic radiometer, in the ITER equatorial port plug, and processed by two detection algorithms for the 2/1 and 3/2 NTMs for a burning H-mode ITER plasma. One of the algorithms also incorporates simulated Mirnov data. The video bandwidth is set at 2 kHz. This allows for intermediate frequency bandwidths of BIF = 400 MHz and BIF = 300 MHz for the two algorithms, respectively. The intermediate frequency bandwidth provides a trade-off between radial accuracy (low bandwidth) and low noise/latency (large bandwidth). 2/1 and 3/2 NTMs, seeded with widths up to 9 and 11 cm, are detectable with the required accuracy within 250 ms. With appropriate settings for the radiometer, the NTM detection using ECE is accurate and with low latency. The algorithm that incorporates both ECE and Mirnov data showed the lowest detection latencies.

ECE-imaging of the H-mode pedestal (invited)

A synthetic diagnostic has been developed that reproduces the highly structured electron cyclotron emission (ECE) spectrum radiated from the edge region of H-mode discharges. The modeled dependence on local perturbations of the equilibrium plasma pressure allows for interpretation of ECE data for diagnosis of local quantities. Forward modeling of the diagnostic response in this region allows for improved mapping of the observed fluctuations to flux surfaces within the plasma, allowing for the poloidal mode number of coherent structures to be resolved. In addition, other spectral features that are dependent on both T(e) and n(e) contain information about pedestal structure and the electron energy distribution of localized phenomena, such as edge filaments arising during edge-localized mode (ELM) activity.

Fast Sampling Upgrade and Real-Time NTM Control Application of the ECE Radiometer on ASDEX Upgrade

The ASDEX Upgrade tokamak employs a 60-channel electron cyclotron emission (ECE) radiometer diagnostic for the measurement of radial electron temperature profiles of the plasma. The data acquisition (DAQ) portion of the system has now been upgraded to sample at 1 to 2 MHz, and accordingly, electron temperature fluctuations from 500 kHz to 1 MHz may be measured. The high spatial resolution of [approximately]1 cm and flexible magnetic field coverage from 1.5 to 3.0 T remain unchanged. The system can now provide observations of plasma phenomena on the magnetohydrodynamic timescale, such as neoclassical tearing modes (NTMs) and toroidal Alfvén eigenmodes (TAEs). The upgraded and existing DAQ systems may be run in parallel for comparison, and some of the first plasma measurements using the two systems together are presented, along with an example of localization of [approximately]120-kHz TAEs in the fast ECE data. A principal planned application of the upgraded radiometer is integration into a real-time NTM stabilization loop using targeted deposition of electron cyclotron resonance heating (ECRH) or electron cyclotron current drive. For this loop, it is necessary to determine the locations of the NTM and ECRH deposition using ECE measurements. The NTM location is determined via correlation between ECE and Mirnov coil measurements, and results of this technique for (2,1) and (3,2) NTMs are presented. ECRH deposition is located by observing the modulation signature of the injected ECRH power in ECE measurements. Several additional applications enabled by the upgraded radiometer are also discussed.

Electron density measurements from right-hand cutoff of ECE in the TCABR tokamak

In tokamak machines with low toroidal magnetic fields and high plasma densities, the accessibility conditions impose restrictions to the detection of the Electron Cyclotron Emission (ECE). In these machines, the righthand cutoff condition can be used as an independent method to determine the local electron density from the ECE data in thermal discharges. In this paper is shown the results obatined from the detection of ECE radiation in the TCABR tokamak, in operation at the Institute of Physics of University of São Paulo. The effect of the ECE radiation cutoff was observed for different radial positions of the plasma column. To reach the ECE cutoff condition, the electron density was increased monotonically by the use of an external gas puffing system. For sufficient high densities, the emission at some frequencies is cutoff and the first and the last frequency to be cutoff depends on the shape of the density profile. These measurements do not require the plasma to be optically thick. It was observed that, for a toroidal field B O = 1:14T, the first cutoff of ECE occurs for a radial position r <FONT FACE=Symbol>@</FONT> 5 cm. From these measurements the radial electron density was determined. For a symmetric parabolic profile n e = n eo [1 - (r/a)²]alpha , values of alpha between 0.86 and 0.97 were experimentally obtained. A good agreement of these values with those obtained from the microwave interferometer measurements (<FONT FACE=Symbol>a »</FONT>0:85) was found. Therefore, the ECE right-hand cutoff constitutes an independent method to obtain information about the electron density profile.

Real time safety factor profile determination in JET

In this paper, we present the recent implementation at joint european torus (JET) of a real-time system to calculate the electron density and the safety factor profiles directly from the line-integrated density (LID) and the calibrated Faraday angle (FAR) measurements. Current density, electron temperature and density profiles are now available in real time using magnetic, interfero-polarimeter and electron cyclotron emission data, thus allowing real-time feedback control of these profiles with the heating and current drive actuators available at JET.

Magnetic island deformation due to sheared flow and viscosity

A partial differential equation for a two-dimensional magnetohydrodynamic (MHD) equilibrium with flow and viscosity is derived. The equation is used to describe the deformation of the magnetic island caused by the viscous drag of a sheared flow. The deformation is characterized by the phase gradient of the magnetic perturbation across a magnetic island. This phase gradient has been observed experimentally in electron cyclotron emission data from the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)].

Determination of the suprathermal electron distribution function during lower hybrid current drive in JET

The information provided by different diagnostics has been combined in order to characterize the fast electron distribution function in lower hybrid current drive experiments on JET. In particular, X-ray and electron cyclotron emission data are complementary in defining the fast electron energy contents in the directions parallel and perpendicular to the magnetic field. A numerical analysis chain has been developed which identifies the main moments of the distribution function of the current carrying fast electrons and allows simulations based on these moments to be compared with X-ray emission, electron cyclotron emission and magnetic data. The method of analysis and the associated diagnostics are described, and results are presented which have been obtained during the lower hybrid current drive campaign on JET

Nonthermal electron cyclotron emission from TFTR supershot plasmas

The Ohmic targets of supershot (Strachan et al., 1987) plasmas in the Tokamak Fusion Test Reactor (TFTR) are often characterized by the presence of nonthermal electron cyclotron emission (ECE) from a runaway or slideaway electron population. This nonthermal ECE decays away within the first 100-150 ms after the start of neutral beam injection (NBI) as the electron density rises and the loop voltage drops. After this usual slideaway-induced, nonthermal ECE is quenched a new nonthermal feature develops at frequencies just below both the optically thick, thermal emission of the ECE fundamental ordinary mode, and the second harmonic extraordinary mode. This paper presents an analysis of the ECE data which indicates a possible source of this nonthermal emission feature. In addition, correlation of the peak intensity of the nonthermal feature with various plasma parameters for 240 TFTR discharges from 1988 provides some insight toward identifying possible physical conditions which give rise to the feature.