Electron Cyclotron Emission Signals Research Articles

Identification of anomalous Doppler resonance effect during current ramp down in HT-7 tokamak

The abrupt steep jump of electron cyclotron emission (ECE) signals during current ramp-down has been observed and explained by an anomalous Doppler resonance effect (ADR). The identifying process of ADR was presented based on the fast Fourier transform (FFT) technique. The threshold value for triggering a steep jump on ECE signals has been identified under different discharge conditions.

Initial result of collective Thomson scattering using 77 GHz gyrotron for bulk and tail ion diagnostics in the Large Helical Device

The collective Thomson scattering (CTS) technique has been utilized with the backscattering configuration in the collective scattering regime to diagnose the velocity distribution functions in the Large Helical Device (LHD). The receiver was equipped with 16 channels and the first test has been carried out using the eight channels for scattered radiation and these channels cover a few GHz frequency shift from the 76.95 GHz probe beam. During the discharge, the electron density and temperature at the central region of the LHD are 1×1019m−3, and 1.0 keV, respectively. The probing beam with rectangular wave modulation is injected by 50 Hz in order to be distinct from the background electron cyclotron emission (ECE). The scattered radiation is resolved successfully at each channel of CTS receiver system. The detected signals of bulk ion and electron components are by a factor of 10 ~ 102 larger than the background ECE signal. We found that the measured spectra are in reasonably agreement with the theoretical spectra calculated by using the reliable measured electron temperature and density for input parameters. The CTS receiver system will be improved to obtain more accurate velocity distributions in high temperature plasmas.

Analysis of diffusive coefficient of electron heat pulse based on singular value decomposition in HT-7 tokamak

In this paper, singular value decomposition (SVD) as a filter-noise method is applied to electron cyclotron emission (ECE) diagnostic signals. The decomposed vectors contain the information about sawtooth such as the temporal vectors that show the sawtooth period and the spatial vectors that indicate the inverse radius. The propagation of electron heat pulse is investigated from electron cyclotron emission signals by using the perturbation method in HT-7 tokamak. The heat diffusivities are obtained at different densities in ohmic plasmas. The special result is that the heat diffusivity becomes larger as the heat pulse propagates outwards from the outside of the inverse radius.

Investigation of anomalous Doppler instability in ohmic and LHCD discharges in the HT-7 tokamak

We present the results of our experimental studies on a discharge in the Hefei Tokamak-7 (HT-7) device with a specific instability whose emergence coincides with a sudden jump in electron cyclotron emission (ECE) that shows the transverse energy of the plasma. It is shown that, in this slide-away discharge regime, the current is carried by a relatively small group of runaway electrons and that the period of high-frequency small-amplitude oscillations varies from 1 to 2 ms, which depends on the value of the loop voltage in ohmic discharges. In low hybrid current drive (LHCD) plasmas, the period of high-frequency small-amplitude oscillations in the ECE signal during the whole period of anomalous Doppler instability (ADI) is about 2 ms. It is also found that the ADI in ohmic and LHCD discharges can restrain the magnetic oscillations.

Extension of operational limits on EAST

The first plasma has been achieved successfully in the Experimental Advanced Superconducting Tokamak (EAST). Boronization by the glow discharge (GDC) method was studied in experiments. The plasma performance was obviously improved by GDC boronization. Extension of the operational region and improvement in the plasma performance were obtained. Sawtooth discharges were observed by means of soft x-ray signals, electron cyclotron emission signals and line averaged electron density after boronization. Lower qa and more stable operation were also achieved following GDC boronization. The plasma current ramp-up rate was also improved as a result of decreased impurity content and low averaged loop voltage due to boronization. PLEASE NOTE: THERE HAS BEEN A RETRACTION PUBLISHED FOR THIS ARTICLE.

Development of ECE Imaging System on LHD

Electron cyclotron emission (ECE) imaging has been an important tool to investigate behaviors of temperature perturbations in the specific spatial area. So far, we have applied ECE detection system to a LHD plasma to measure electron temperature fluctuations in core region, however, it was difficult to distinguish perturbed temperature signals to a noise, which is naturally consisted in radiometry features and so on. Recently, we have optimized an ECE detector array especially in sensitivity to increase signal to noise ratio. An intermediate frequency(IF) system composed of commercial components have been replaced by an integrated system utilizing millimeter(microwave) integrated circuit technology(MIC) to fabricate a compact system maintaining the performance of the commercial system. As a result of application to the LHD plasma, we have successfully obtained ECE signal even in comparably low temperature operation sequence.

Effect of the magnetic field structure on the intensity of electron cyclotron emission from the plasma of the L-2M stellarator

Results are presented from experimental studies of the influence of the stellarator magnetic field structure on the plasma behavior in electron-cyclotron resonance regimes with a high heating power per electron. The magnetic field structure was changed by varying the induction current Ip from −14 to +14 kA. The plasma electrons were heated at the second harmonic of the electron gyrofrequency by an X-mode microwave beam with a power of P ∼ 200 kW, the average plasma density being in the range ne = (0.5–2) × 1013 cm−3. At Ip = 0, the rotational transform varies from \(\rlap{--} \iota \)(0) = 0.2 on the magnetic axis to 0.8 at the plasma boundary. At a positive current of Ip = 13.5 kA, the rotational transform was \(\rlap{--} \iota \)(0) = 0.8 on the axis and \(\rlap{--} \iota \)(ap) = 0.9 at the plasma boundary. Experiments with a positive current have shown that the radiative temperature first increases with current. When the current increases to Ip = 11–14 kA, strong modulation appears in the electron cyclotron emission signals received from all the plasma radii, the emission spectrum changes, and the emission intensity decreases. At a negative current of Ip = −(6.5–13.5) kA, the rotational transform vanishes at r/ap = 0.4–0.6. In this regime, the number of suprathermal electrons is reduced substantially and the emission intensity decreases at both low and high plasma densities.

Electron cyclotron emission temperature fluctuations associated with magnetic islands and real-time identification and control system

An important focus of the present research on thermonuclear fusion based on magnetic confinement is the control of rotating magnetic islands of low helical mode numbers m, n that are the nonlinear stage of resistive instabilities developing at magnetic surfaces where q = m/n. These perturbations are often driven by external resonant ‘error fields’ or mode coupling effects and maintained by local distortions of the current profile. The islands can be reduced in width or completely suppressed by a current driven by electron cyclotron waves accurately located within the island. A requisite for an effective control action is the ability to identify the relevant state variables (radial location, island amplitude, frequency and phase) in ‘real time’ and vary accordingly the control variables (wave beam power modulation and direction). Traditionally magnetohydrodynamic instabilities are monitored by external magnetic signals that however give no information about the radial location of the perturbation within the plasma. Here we describe the design of a real-time diagnostics/control device using an original algorithm for processing electron cyclotron emission signals to monitor temperature fluctuations and EC wave power deposition.

Electron cyclotron emission imaging on a large helical device

In this article, we describe the electron cyclotron emission (ECE) imaging system applied to the Large Helical Device (LHD) at the National Institute for Fusion Science. The imaging system consists of focusing optics installed inside the vacuum chamber of LHD and planer-type detectors fabricated by monolithic microwave integrated circuit technology. The detector consists of the integration of a bowtie antenna, a down-converting mixer using a Schottky barrier diode, and heterojunction bipolar transistors (HBTs) on a GaAs substrate. The HBTs work as an intermediate frequency (IF) amplifier with a 10 GHz bandwidth and a 10 dB voltage gain. The ECE signal and local oscillator beam are irradiated from both sides of the detector. The ECE signals are down-converted at the mixers and the IF signal is fed to a filter bank with center frequencies of 1–8 GHz. The time evolution and the intensity of the ECE signals agree with those obtained by a conventional ECE heterodyne receiver. The cross-correlation spectra of the signals obtained with different IF frequencies (radial correlation) and different detectors (poloidal correlation) are obtained.

Thomson scattering upgrades on Alcator C-Mod

The Thomson scattering (TS) diagnostics set has been upgraded to deliver measurements of electron temperature (Te) and density (ne) with improved temporal and spatial resolution. Two 30 Hz, 1.3 J Nd-doped yttrium–aluminum–garnet (Nd:YAG) lasers share a vertical beam path through the tokamak plasma and are fired in an alternating fashion, allowing the collection of scattered light with a 60 Hz repetition rate. Eight compact polychromators have been added to the system, better than doubling the spatial resolution of core plasma TS measurements and allowing better diagnosis of core ne and Te gradients. Each fiber holder can be repositioned easily, making it possible to concentrate core measurements in the regions of particular interest with a spatial resolution of ∼1 cm. The newly designed mount can be positioned with submillimeter precision, and its alignment with respect to the Nd:YAG beam can be adjusted between discharges using remote controlled stepper motors. Cross calibration of TS ne measurements with electron cyclotron emission (ECE) diagnostics is performed using dynamic ramps of both plasma density and magnetic field to force ECE signal cutoffs at known values of ne. This technique supplements an absolute calibration based on Raman scattering in hydrogen and deuterium gas.

Characterization of internal MHD modes in Princeton beta experiment modification by ECE fluctuation radiometry

Localized measurements of internal magnetohydrodynamic (MHD) activity in the Princeton Beta Experiment-Modification (PBX-M) have been made using a heterodyne radiometer. The radiometer measures third harmonic optically gray electron cyclotron emission (ECE) from the center to the outer edge of the plasma on eight radially localized frequency channels from 112–128 GHz. The radiometer uses a focusing Gaussian optics system, which provides a vertical resolution of approximately 6 cm throughout the plasma. The radial resolution of the diagnostics is limited by the relativistic broadening to approximately 2–3 cm. Long wavelength (k<0.5 cm−1) temperature and density fluctuations can therefore be detected using this diagnostic. A MHD mode’s characteristics such as pressure fluctuations and mode amplitude can be derived from the coherent fluctuations it causes in the ECE signal using an analytical theory developed by Luckhardt et al. (MIT Plasma Fusion Center Report No. PFC-JA/91-16, 1991). Using this analysis, pressure fluctuations (δP/P) have been calculated for high n-number MHD modes in high-βp PBX-M plasmas.

Lower hybrid dynamics by ECE spectra in Tore Supra

The suprathermal electron cyclotron radiation is used to investigate fundamental phenomena governing electron tail formation during lower hybrid current drive (LHCD). To this end, an approximate analytical solution of the 2-D time-dependent Fokker–Planck equation is developed. Electron cyclotron emission (ECE) is analysed with this model. The pitch-angle scattering time, the characteristic time of the quasi-linear diffusion process, the perpendicular temperature and the quasi-linear diffusion coefficient are measured by fitting the temporal evolution of the ECE signal.

Three-dimensional fluctuation imaging diagnostic for TEXTOR (abstract)

After more than 4 decades of fusion research, plasma transport in tokamaks remains an outstanding issue. The standard hypothesis, that the observed anomalous transport is caused by small scale turbulence, is consistent with experimental observations, but it is neither based on a self-consistent theory of plasma turbulence nor on an exhaustive set of turbulence measurements. One of the major obstacles to the development of a satisfactory anomalous transport theory is the inability of standard fluctuation diagnostics to provide the full spectrum of turbulent fluctuations. The result is a nearly complete reliance on numerical simulations. Unfortunately, in spite of the enormous capabilities of today’s computers, this is extremely unsatisfactory since any theory or simulation of plasma turbulence can only be driven by direct experimental observations. The first step in achieving such data was the highly successful UC Davis Electron Cyclotron Emission (ECE) Imaging system, which was first developed for use on the TEXT-U tokamak and later modified for use on the Rijnhuizen Tokamak Project tokamak.1 Here, correlation techniques are applied to spatially resolved second harmonic ECE signals to provide detailed information about the microturbulence associated with Te fluctuations. The present work extends this work by developing a similar technique for spatially resolved ne fluctuations via microwave imaging reflectometry. The result is a diagnostic capable of the simultaneous measurement of both Te and ne fluctuations (both turbulent and coherent) and profiles on toroidal devices such as tokamaks and stellarators. A 16 channel ECE imaging system has recently been installed on the Torus Experiment for Technology Oriented Research (TEXTOR) tokamak. A prototype millimeter wave imaging reflectometry system will be installed on TEXTOR in July 2000, which will operate in V band (50–75 GHz) and reflect from the X-mode cutoff. Data collected with this system will be utilized in the design of the full three- dimensional (3D) imaging system, which is scheduled for installation on TEXTOR in April 2001. System details and laboratory characterization results of the prototype reflectometric imaging system will be presented, along with a preliminary design for the full 3D system.

Calibration of the Joint European Torus energetic ion and alpha particle collective Thomson scattering diagnostic receiver

The receiver of the Joint European Torus (JET) energetic ion and alpha particle collective Thomson scattering diagnostic is calibrated assuming blackbody emission from the torus vacuum vessel (VV) and using electron cyclotron emission (ECE). The 32 receiver channels are absolutely calibrated with a mechanical chopper in the quasioptical arm of the receiver, alternating the receiver view between the torus vacuum at 320 °C and room temperature. This calibration is noisy due to the small difference between torus and room temperatures. A more accurate relative calibration is achieved with the ECE during plasma shots. The intensity of the ECE is found to be a smooth function of frequency, which enables the combination of the ECE calibration with the VV calibration. The accuracy of the absolute VV calibration is hereby improved to nearly the same standard as the relative ECE calibration. ECE signals measured by the calibrated receiver agree well with standard JET ECE diagnostics. Based on mathematical considerations presented here, noise is injected into the receiver final intermediate frequency stage during VV calibrations to provide more bit transitions for accurate analog-to-digital (AD) conversion of the low level calibration signals. This yields a resolution which is better than 1% of an (AD) step.

Plasma diagnostics using millimetre wave two-dimensional imaging array

A millimetre wave two-dimensional detector array has been newly developed for application to a phase-imaging interferometer installed in the GAMMA 10 tandem mirror. The interferometer consists of two sources and a quasi-optical transmission system. The transmitted wave is mixed with the local oscillator (LO) wave to provide a 150 MHz intermediate frequency (IF) signal. The detector has 4 × 4 channels consisting of beam-lead GaAs Schottky barrier diodes bonded to bow-tie antennas fabricated using photolithographic techniques on a fused quartz substrate of size 38.1 mm × 38.1 mm. The interferometer successfully measures the time evolution of line density profiles with a phase resolution of less than 1 200 fringe and a spatial resolution of 16 mm in the vertical direction and 5 mm in the horizontal direction. The system is also applied to the measurement of electron cyclotron emission (ECE) as a heterodyne radiometer. The axial and radial dependence of the second-harmonic ECE is observed simultaneously. The fluctuation spectra of the ECE signals are compared with those obtained from the interferometer.

Multichannel electron cyclotron emission radiometry for magnetohydrodynamic mode studies in TEXTOR

Localized magnetohydrodynamic modes occur frequently as precursors to dangerous plasma disruptions in tokamaks. Fast electron cyclotron emission (ECE) radiometry permits a space-resolved study of the dynamic behavior of these instabilities. Two types of multichannel, heterodyne ECE systems working on TEXTOR are briefly described. Temperature profile perturbations caused by tearing modes are shown. Typical Te fluctuations generated by rotating magnetic islands are presented. An early disruption warning system was built and tested on TEXTOR discharges. A real-time cross correlation calculation of two ECE signals is used to identify an m=2 mode. An analog alarm signal is generated then early during the instability evolution. An experimental test result is presented from a beam-heated tokamak plasma.

Millimeter-wave two-dimensional imaging array for the GAMMA 10 tandem mirror

A millimeter-wave two-dimensional imaging array has been fabricated in order to diagnose the plug-cell plasma of the GAMMA 10 tandem mirror. The heterodyne system with two separate sources enables the measurement of density and electron cyclotron emission (ECE) in one plasma shot. The interferometer successfully measures the time evolution of the axial and radial line-density profiles with phase resolution less than 1/200 fringe and spatial resolution of 16 mm in the vertical direction and 5 mm in the horizontal one. It is shown that the Abel-inverted density profiles are changed due to the injection of electron-cyclotron-resonance heating at the plug/barrier cells and central cell. The axial and radial dependence of the second-harmonic ECE emitted from hot electrons produced in barrier region is observed simultaneously. The fluctuation spectra of the ECE signals are compared with those obtained from the far-forward scattering diagnostic system.

Correlation radiometry of electron cyclotron radiation in TEXT-U (invited)

Very low level plasma density and temperature fluctuations can be responsible for anomalous transport in thermonuclear magnetic traps. Measuring these fluctuations by detecting plasma electron cyclotron emission (ECE) might indicate where, in ω−k space, the most dangerous instabilities are located. The most attractive feature of ECE is the good localization of the source, unique to this range of frequencies among plasma radiation. However, to maintain spatial resolution, only a small number of radiation modes should be detected. The consequent poor statistics of the ECE measurement allows one to measure only the average value for the temperature fluctuation amplitude via correlation analysis of two ECE signals with suitable correlation properties. Initial measurements of this kind have been performed successfully. But, results have only been partially conclusive as to the relevance of electrostatic turbulence to heat transport, due to still too limited poloidal spatial resolution and the inability to correlate the measured signals with corresponding potential fluctuations. Suggestions of how to extend the present results to correct for these drawbacks are discussed.

Investigation of ballooning modes in high poloidal beta plasmas in the Tokamak Fusion Test Reactor*

Medium-n (toroidal mode number) instabilities with ballooning characteristics were investigated using x-ray and electron-cyclotron emission (ECE) imaging techniques in the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)]. The poloidal mode numbers were determined by fitting the data to soft x-ray signals simulated with a wave-packet model, and the toroidal mode numbers were determined from the phase difference between the soft x-ray and ECE signals. The modes are identified as ideal ballooning modes from the observation that the identified mode numbers are much higher than that of the usual kink or tearing modes, the mode has a strong ballooning characteristic, the growth rate is consistent with the theoretical prediction, and the equilibrium is theoretically predicted to be marginal to the infinite n ballooning mode. The ballooning mode accompanies a β collapse which happens near the Troyon limit in TFTR. The ballooning mode has also been observed just before a major disruption in TFTR.

Electron temperature fluctuation measurement from ECE on TEXT-U

The conceptual design and theory of diagnostics to measure electron temperature and density fluctuations using electron cyclotron emission (ECE) was proposed and a new, single-sightline ECE system with the heterodyne radiometer has been built for TEXT-U. This ECE system was designed to employ auto- and cross-correlation techniques. Calculations indicate that fluctuations as small as 0.2% in amplitude can be achieved from auto-correlation of the ECE signal. The cross-correlation technique, however, must be used to resolve the fluctuations in frequency. Auto- and cross-correlation between optically thick ECE channels will be used for our initial approach to electron temperature fluctuation measurement with the single-sightline ECE system. A description of the TEXT-U ECE system, investigation of the feasibility of this measurement, and the initial results of test, calibration, and operation are presented.