Non-thermal Electron Cyclotron Emission Research Articles

Radiometry for the vertical electron cyclotron emission from the runaway electrons at the COMPASS tokamak.

Due to an increased interest in runaway electron (RE) phenomena in tokamak research, the need for diagnostics of runaway electron population in plasma has emerged. A novel diagnostic of the nonthermal electron cyclotron emission from runaway electrons can be utilized for this purpose. It was designed and installed at the COMPASS tokamak based on the available heterodyne radiometer. The vertical ECE (V-ECE) system uses a 16-channel heterodyne radiometer with a vertically placed E2-band horn antenna with a 76.5-90 GHz frequency range front-end. Simulations with the ray-tracing SPECE code have shown a measurement feasibility of the runaway electrons with energies up to 1 MeV. Due to a low optical depth of the plasma in COMPASS during RE discharges, reflected waves from the tokamak wall can be detected as well. First results show strong connection with other RE diagnostics at COMPASS. The V-ECE can obtain important information about RE population evolution and primary generation mechanism.

Low-frequency whistler waves in quiescent runaway electron plasmas

In quiescent runaway electron plasmas in the DIII-D tokamak, whistler waves with frequencies between 90 and 200 MHz are driven unstable in plasmas with appreciable hard x-ray and non-thermal electron cyclotron emission (ECE). Narrow (δf < 50 kHz) discrete modes with erratically spaced frequencies are observed. Unstable modes often extend over a range Δf ≃ 50 MHz but lower frequency unstable modes are usually most intense. The dependency of the frequency on field and density implies a wavenumber k ≃ 150 m−1 with parallel wavenumber k∥ ≪ k. Reducing the gap between the plasma and the wall increases the number of detected modes. Lowering the magnetic field promotes instability. Nonlinear limit-cycle-like oscillations in the whistler amplitude occur on a 10 ms timescale. The ECE signals often jump at whistler bursts, suggesting that the modes pitch-angle scatter the runaways. Sawteeth cause transient stabilization of the whistlers.

The effects of kinetic instabilities on the electron cyclotron emission from runaway electrons

In this paper we show that the kinetic instabilities associated with runaway electron beams play an essential role for the production of high-level non-thermal electron–cyclotron-emission (ECE) radiation. Most of the non-thermal ECE comes from runaway electrons in the low-energy regime with large pitch angle, which are strongly scattered by the excited whistler waves. The power of ECE from runaway electrons is obtained using a synthetic diagnostic model based on the reciprocity method. The electron distribution function is calculated using a kinetic simulation model including the whistler wave instabilities and the quasilinear diffusion effects. Simulations based on DIII-D low-density discharge reproduces the rapid growth of the ECE signals observed in DIII-D experiments. Unlike the thermal ECE where radiation for a certain frequency is strongly localized inside the resonance region, the non-thermal ECE radiation from runaway electrons is nonlocal, and the emission-absorption ratio is higher than that of thermal electrons. The runaway electron tail is more significant for ECE with higher frequencies, and the ECE spectrum becomes flatter as RE population grows. The nonlinear behavior of the kinetic instabilities is illustrated in the oscillations of the ECE waves. The good agreement with the DIII-D experimental observations after including the kinetic instabilities clearly illustrate the significance of the scattering effects from wave-particle interactions, which can also be important for runaway electrons produced in disruptions.

Electron cyclotron emission at the fundamental harmonic in GDT magnetic mirror

New electron cyclotron emission (ECE) diagnostics has been installed to facilitate the successful experiment of electron cyclotron plasma heating (ECRH) in a large open magnetic trap GDT at Budker Institute. The particularities of ECE in the vicinity of the ECRH frequency were studied experimentally for a broad range of discharge scenarios. The measured thermal emission has partly validated the existing physical conceptions about microwave plasma heating in the machine. Besides the expected emission of thermal electrons, a clearly resolved non-thermal ECE was observed which unambiguously confirmed the presence of suprathermal electrons driven by high-power microwave heating.

Runaway electron generation as possible trigger for enhancement of magnetohydrodynamic plasma activity and fast changes in runaway beam behavior

Peculiar phenomena were observed during experiments with runaway electrons: rapid changes in the synchrotron spot and its intensity that coincided with stepwise increases in the electron cyclotron emission (ECE) signal (cyclotron radiation of suprathermal electrons). These phenomena were initially observed in TEXTOR (Tokamak Experiment for Technology Oriented Research), where these events only occurred in the current decay phase or in discharges with thin stable runaway beams at a q = 1 drift surface. These rapid changes in the synchrotron spot were interpreted by the TEXTOR team as a fast pitch angle scattering event. Recently, similar rapid changes in the synchrotron spot and its intensity that coincided with stepwise increases in the non-thermal ECE signal were observed in the EAST (Experimental Advanced Superconducting Tokamak) runaway discharge. Runaway electrons were located around the q = 2 rational magnetic surface (ring-like runaway electron beam). During the EAST runaway discharge, stepwise ECE signal increases coincided with enhanced magnetohydrodynamic (MHD) activity. This behavior was peculiar to this shot. In this paper, we show that these non-thermal ECE step-like jumps were related to the abrupt growth of suprathermal electrons induced by bursting electric fields at reconnection events during this MHD plasma activity. Enhancement of the secondary runaway electron generation also occurred simultaneously. Local changes in the current-density gradient appeared because of local enhancement of the runaway electron generation process. These current-density gradient changes are considered to be a possible trigger for enhancement of the MHD plasma activity and the rapid changes in runaway beam behavior.

Modification of ordinary-mode reflectometry system to detect lower-hybrid waves in Alcator C-Mod

Backscattering experiments to detect lower-hybrid (LH) waves have been performed in Alcator C-Mod, using the two modified channels (60 GHz and 75 GHz) of an ordinary-mode reflectometry system with newly developed spectral recorders that can continuously monitor spectral power at a target frequency. The change in the baseline of the spectral recorder during the LH wave injection is highly correlated to the strength of the X-mode non-thermal electron cyclotron emission. In high density plasmas where an anomalous drop in the lower hybrid current drive efficiency is observed, the observed backscattered signals are expected to be generated near the last closed flux surface, demonstrating the presence of LH waves within the plasma. This experimental technique can be useful in identifying spatially localized LH electric fields in the periphery of high-density plasmas.

High field side measurements of non-thermal electron cyclotron emission on TCV plasmas with ECH and ECCD

Measurements of electron cyclotron emission (ECE) from the high field side of the TCV tokamak have been made on plasmas heated by second and third harmonic X-mode electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD). Suprathermal ECE, up to a factor of six in excess of thermal emission, is detected in the presence of second harmonic X-mode (X2) ECCD and of third harmonic X-mode (X3) ECH. The measured ECE spectra are modelled using a bi-Maxwellian describing the bulk and the suprathermal electron populations. Suprathermal temperatures between 10 and 50 keV and densities in the range 1×1017−6×1018 m−3 are obtained, and correspond to 3–15 bulk temperatures and 1–20% bulk densities. Good agreement between ECE suprathermal temperatures and energetic photon temperatures, measured by a hard x-ray camera, is found. For optically thin X3 low field side injection in the presence of X2 CO-ECCD, the suprathermal population partly explains the discrepancy between global and first pass absorption measurements.

Measurements of edge-localized-mode induced electron cyclotron emission bursts in DIII-D

Intense bursts of nonthermal electron cyclotron emission (ECE) have been observed in H-mode (high confinement) discharges in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], measured with heterodyne radiometers and a broadband far infrared (FIR) detector. These bursts are triggered by edge-localized modes (ELMs) and consist of single or several spikes, most lasting 5–150 μs. Radiation temperatures reach up to several MeV. Observed spectral bandwidths are typically less than 4 GHz. Bandwidths as low as 500 MHz, which is the limit of the instrumental filter spacing, have been found. Statistical analysis shows a steady decrease of the number of spikes with rising spike energy. Burst frequencies seem to depend linearly on the magnetic field. It is speculated that the bursts are coherent second harmonic ECE.

Multichannel heterodyne radiometers with fast-scanning backward-wave oscillators for ECE measurement on HT-7 tokamak

Two sets of fast-scanning heterodyne radiometer receiver systems employing backward-wave oscillators (BWOs) in 78–118 and 118–178 GHz were developed and installed for electron cyclotron emission (ECE) measurements on HT-7 superconducting tokamak. The double sideband (DSB) radiometer in 78–118 GHz measures 16 ECE frequency points with a scanning time period of 0.65 ms. The other radiometer in 118–178 GHz consists of one independent channel of DSB heterodyne receiver with intermediate frequency (IF) of 100–500 MHz and two channels of single sideband (SSB) heterodyne receiver that are sensitive to upper sideband and lower sideband individually; the IF frequency of the SSB channels are 1.5 GHz around the local oscillator frequencies with 1 GHz bandwidth. By employing a novel design, this unique radiometer measures 3 ECE frequency points at each of the 16 local oscillator frequency points in 118–178 GHz, and the full band can be swept in 0.65 ms period, thus the radiometer measures 48 ECE frequency points in 0.65 ms in principle. Each of the local oscillators’ frequency points can be preset by program to meet specific physics interests. Horizontal view of ECE was installed to measure electron temperature profiles; vertically viewing optics along a perpendicular chord was also installed to study nonthermal ECE spectra. Preliminary measurement results were presented during ohmic and pellet injection plasmas.

A fast-scanning heterodyne radiometer for electron cyclotron emission measurements in HT-7 superconducting tokamak

A fast-scanning heterodyne radiometer employing a backward-wave oscillator (BWO) in 78–118 GHz was developed and installed for electron cyclotron emission (ECE) measurements in HT-7 superconducting tokamak. The radiometer measures 16 ECE frequency points with a scanning time period of 0.65 ms, each of the frequency points can be preset by the program to meet specific interests in physics. The high scanning speed is achieved by carefully choosing a BWO, very finely adjusting the radiometer over the full waveband, and by eliminating some elements that are routinely included in a fast-scanning radiometer system. A horizontal view of the ECE optics was installed to measure electron temperature profiles. The spatial resolution is about 1 cm (Bt=2 T) in the center of the HT-7 minor cross section, determined by the intermediate frequency of 0.1–0.5 GHz in the radiometer. Vertically viewing optics along a perpendicular chord was also installed to study nonthermal ECE spectra. Preliminary measurement results in pellet injection plasma and lower-hybrid current drive plasma were presented.

Comparison of ECE spectra as observed from the high- and low-field side

Non-thermal electron cyclotron emission (ECE) spectra are measured by comparison of low-field side and high-field side observed ECE. The asymmetry in the respective spectra can be ascribed to the existence of non-thermal populations. Optical-depth effects are directly determined from electron cyclotron absorption measurements. Measurements are compared with theoretical ECE spectra, obtained on the basis of calculations with a quasi-linear bounce-averaged Fokker-Planck simulation code, including a simple model for radial diffusion. This yields information on the non-thermal population responsible for the measured non-thermal spectra. The good agreement between experiment and simulation corroborates the quasi-linear bounce-averaged Fokker-Planck model.

Thermal and nonthermal electron cyclotron emission by high-temperature tokamak plasmas

An analysis of the electron cyclotron emission (ECE) spectra emitted by a high-temperature tokamak plasma in the frequency range of the second and third harmonic of the electron cyclotron frequency is made, both in purely Maxwellian and in non-Maxwellian cases (i.e., in the presence of a current-carrying superthermal tail). The work is motivated mainly by the experimental observations made in the supershot plasmas of the Tokamak Fusion Test Reactor (TFTR), where a systematic disagreement is found between the Te measurements by second-harmonic ECE and Thomson scattering. We show that, by properly taking into account the overlap of superthermals-emitted third harmonic with second-harmonic bulk emission, the radiation temperature observed about the central frequency of the second harmonic may be enhanced up to 30%–40% compared to the corresponding thermal value. Moreover we show that, for parameters relevant to the International Thermonuclear Experimental Reactor (ITER) with Te(0)&amp;gt;7 keV, the overlap between the second and the downshifted third harmonic seriously affects the central plasma region, so that the X-mode emission at the second harmonic becomes unsuitable for local Te measurements.

Asymmetric electron cyclotron emission from non-thermal plasmas subject to a generalized Kirchhoff's law

The electron cyclotron emission (ECE) spectra characteristic of a non-thermal plasma comprising a current-carrying superthermal electron tail, for which a generalized Kirchhoff's law holds true, are evaluated for both the extraordinary (X) and ordinary (O) mode, for perpendicular as well as oblique propagation on the outboard side of the plasma; both higher order finite-Larmor-radius effects and harmonic overlap are accounted for. Besides a spectrally narrow non-thermal feature occurring at frequencies downshifted to both first (in X and O-modes) and second (X-mode only) harmonic cyclotron frequencies outside the plasma, the non-thermal ECE contributes a spectrally broad enhancement to the thermal emission, a salient characteristic of the non-thermal ECE being the asymmetry with espect to the parallel refractive index. The relevance of the upshifted second harmonic ECE of the X-mode is discussed along with the concomitant downshifted third harmonic emission for both optically thick and optically thin plasmas.

Transient nonthermal electron cyclotron emission phenomena in high β TFTR plasmas

Intense bursts of nonthermal electron cyclotron emission (ECE), measured with a grating polychromator, have been observed to coincide with edge localized modes in TFTR H-mode plasmas. These bursts contain random 10–150 μs emission spikes. Similar nonthermal ECE bursts or spikes have been observed just before the thermal quench during major disruptions in high β TFTR plasmas. The recent installation of a second fast ECE grating polychromator system, at a different toroidal location, has provided additional data on this bursting phenomenon which suggests that the source of the nonthermal ECE may be highly anisotropic and/or poloidally localized on a helical flux surface.

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.