Electron Cyclotron Resonance Heating Experiments Research Articles

Development and testing of a fast Fourier transform high dynamic-range spectral diagnostics for millimeter wave characterization

A fast Fourier transform (FFT) based wide range millimeter wave diagnostics for spectral characterization of scattered millimeter waves in plasmas has been successfully brought into operation. The scattered millimeter waves are heterodyne downconverted and directly digitized using a fast analog-digital converter and a compact peripheral component interconnect computer. Frequency spectra are obtained by FFT in the time domain of the intermediate frequency signal. The scattered millimeter waves are generated during high power electron cyclotron resonance heating experiments on the TEXTOR tokamak and demonstrate the performance of the diagnostics and, in particular, the usability of direct digitizing and Fourier transformation of millimeter wave signals. The diagnostics is able to acquire 4 GHz wide spectra of signals in the range of 136-140 GHz. The rate of spectra is tunable and has been tested between 200,000 spectra/s with a frequency resolution of 100 MHz and 120 spectra/s with a frequency resolution of 25 kHz. The respective dynamic ranges are 52 and 88 dB. Major benefits of the new diagnostics are a tunable time and frequency resolution due to postdetection, near-real time processing of the acquired data. This diagnostics has a wider application in astrophysics, earth observation, plasma physics, and molecular spectroscopy for the detection and analysis of millimeter wave radiation, providing high-resolution spectra at high temporal resolution and large dynamic range.

A New Launcher for Real-Time ECRH Experiments on FTU

The development of electron cyclotron resonance heating (ECRH)-electron cyclotron current drive (ECCD) as a tool for suppression of plasma instabilities requires that the millimeter-wave beams used for testing magnetohydrodynamic (MHD) stabilization schemes for ITER be able to follow magnetic island position in real time. In the FTU tokamak, the design of a new ECRH fast-steerable launcher will enable a fast-controlled deposition at a precise poloidal location and the inclusion of the mirror motion in a feedback loop aimed at MHD stabilization. Two of the four existing transmission lines will be switched to the new launcher located in a different equatorial port. It will launch two independent beams with radius in the plasma changeable between 17 and 28 mm, in order to control the deposited power density. Real-time control of the poloidal steering requires high acceleration, speed, and positioning precision of the last mirror. Additionally, oblique toroidal injection at precise angles will allow current profile shaping through controlled ECCD and heating of overdense plasmas (ne > 2.4 × 1020 m-3) using electron Bernstein waves. For optimal O-X conversion, the required toroidal angle, estimated with dedicated beam-tracing calculations, is close to ±38.5 deg, near the upper limit in the toroidal steering angle. The launch requirements and their impact on the launcher design phase are presented in the paper.

Preliminary Results of Sawtooth Behaviour and Electron Heat Transport During ECRH Experiment on HL-2A

An electron cyclotron resonance heating (ECRH) system with a maximumoutput power of 1 MW, a frequency of 68 GHz and a duration of1s was operated on HL-2A tokamak. The temperature profile controland the sawtooth behaviour during ECRH experiments are investigated.`Density pumpout' during on-axis ECRH is analysed. Features ofconfinement and electron heat transport during ECRH are studied.

Electron Cyclotron Resonance Heating on TEXTOR

TEXTOR is equipped with two gyrotrons at 110 and 140 GHz, respectively. Both share a single power supply and a confocal quasi-optical transmission line. They cannot be operated simultaneously. The 110-GHz gyrotron with limited power and pulse length (300 kW; 200 ms) has been used in a first series of experiments on electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) and for collective Thomson scattering (CTS) diagnostics of energetic ions. In the future the 110-GHz gyrotron will be operated exclusively for CTS diagnostics, while for ECRH and ECCD, the newly installed 140-GHz, high-power (800-kW), long-pulse (>3-s) gyrotron is now available. The highlights of first ECRH experiments with the 110-GHz gyrotron are reported. These include observations of internal transport barriers with ECRH on various target plasmas: in the current plateau phase of both ohmic and radiation improved mode (RI-mode) discharges. In addition, sawtooth control by localized ECRH is demonstrated. First results on CTS include the observation of the slowing down of energetic ions and of the redistribution of energetic ions in sawtooth crashes.

Real time polarization monitor developed for high power electron cyclotron resonance heating and current drive experiments in large helical device

The polarization state of a wave is an important factor in electron cyclotron resonance heating (ECRH) and current drive (ECCD), for it strongly affects the propagation and absorption of the wave in the plasma. A real-time monitor of the polarization of the EC beam has been developed for use in ECRH/ECCD experiments in large helical device (LHD). Two orthogonal components of the wave field are measured in one of the miter-bends by use of a specially designed coupler and a waveguide circuit with a 0°–90° phase switch to deduce the polarization parameters: the polarization angle α and the ellipticity β. Since fast-response pin diodes are used for the switches, the polarization is determined every 3 ms, facilitating real time acquisition of the polarization. This article reports on the design and the principle of this monitor as well as on the algorithm used to calculate α and β. This article also reports on the method of calibration, for the accuracy of this measurement depends on it. Finally, a comparison is made between polarization parameters measured by a monitor actually installed on one of the ECRH transmission lines on LHD and the value set by the polarizer in ECRH experiments.

Electron cyclotron resonance heating on TEXTOR**This paper is an expanded version of the two papers presented on the TEXTOR ECRH system (J.A. Hoekzema et al) and experimental results (E. Westerhof et al) at the 12th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (Aix-en-Provence, France, 13–16 May 2002).

The 110 GHz and the new 140 GHz gyrotron systems for electron cyclotron resonance heating (ECRH) and ECCD on TEXTOR are described and results of ECRH experiments with the 110 GHz system are reported. Central ECRH on Ohmic plasmas shows the presence of an internal electron transport barrier near q = 1. This is confirmed by modulated ECRH experiments. A central barrier is also indicated by ECRH in radiatively improved (RI) mode discharges and up to two barriers are seen with ECRH during the current ramp phase. ECRH control of sawteeth is reported for both Ohmic and RI mode target plasmas.

The Design of an Elliptical Reflector in the Gaussian-like Antenna Used in HT-7 ECRH System

The structure of the antenna which will be used in HT-7 tokamak electron cyclotron resonance heating (ECRH) experiment and the design of its key component (the elliptical reflector) were introduced. A quasi-Gaussian TE11 wave sent from the TE01-TE11 mode converter was reflected by two mirrors from the waveguide aperture to the plasma center area. One of the two mirrors is a plane mirror and the other is an elliptical mirror. In this process, the wave beam will be converged by the two reflecting mirrors and the emissive direction of the wave is also controlled by them. As requested physically for ECRH experiment, the arrangement of the two mirrors can ensure the wave to be launched from the low-field side of the tour.

Directional Couplers-Polarimeters for High-Power Corrugated Waveguide Transmission Lines

Compact directional couplers-polarimeters were developed for the circular corrugated waveguide transmission lines of the joint IFP-ENEA Electron Cyclotron Resonance Heating experiment at 140 GHz, 2 MW on the Frascati Tokamak Upgrade in Frascati.A linear array of cutoff holes was drilled in the mirrors of the quasi-optical miter bends. The radiated pattern preserves the symmetry and polarization of the waveguide mode in the plane of the array. The direction of propagation is preserved too.Two pairs of standard gain horns with detectors are placed in the plane of the array along the propagation axes of incident and reflected radiation to detect both linear components. The whole assembly is enclosed in a shielded anechoic box of suitable geometry.

Microwave reflectometry for turbulence studies on ASDEX upgrade (abstract)

The reflectometry system installed in the tokamak ASDEX Upgrade has 11 channels primarily aimed at profile measurements and two dedicated channels operating at fixed frequency to study density fluctuations. The channels used for profile measurements can also be operated at fixed frequency. In this article we describe the potential of the system to study important characteristics of the plasma turbulence, from combined fixed frequency and profile measurements (to localize the reflecting layers). The numerical tools used to treat fixed frequency data are briefly described. Using a set of similar discharges and the ability of the microwave reflectometry system to probe simultaneously different layers with several channels, we obtain the time evolution of the integrated power spectra of the different signals versus the minor radius of the plasma. In plasma with H-mode edge and internal transport barrier we could identify a zone at the plasma core and another at the edge with reduced fluctuation levels, coinciding with the increase of central electron temperature and the L–H transition, respectively. Before each barrier is established, the turbulence is observed to decrease at the lower frequency range and is enhanced at the higher frequencies. In modulation electron cyclotron resonance heating experiments similar modifications in turbulence spectra are observed and may be correlated with the modulation period.

ECRH Antenna at 140 GHz on FTU Tokamak

The quasi-optical beam launching antenna for the Electron Cyclotron Resonance Heating (ECRH) experiment at 140 GHz, 1.6 MW to the plasma, on the Frascati Tokamak Upgrade (FTU) has been successfully exploited in experimental operations. It provides four beams 400 kW each, with independent poloidal and toroidal steering capability and a maximum power density of 60 kW/cm 2 at the plasma edge. The beam radius in the plasma is ≈20 mm, allowing a very high localisation of the absorbed power. The main characteristics of the antenna are: • Four movable launching mirrors under vacuum, far from the plasma edge (no movable parts near the plasma). • Oblique toroidal injection capability at fixed angles obtained with reflections on two fixed stainless steel plates, gold plated, inserted at the sides of the port. • Vacuum gate separating the main vacuum from the appendix containing the two matching mirrors and the barrier window (for safety and maintenance). • All movements are transferred outside the vacuum chamber through bellows and linear displacement. Actuators and encoders are in air. • Full capability to adjust small movements of the machine during cool-down and warm-up, preserving the alignment. Pattern measurements at low power before installation were performed to characterise the effects of the real system on the ideal shape and the polarisation of the launched beams, with particular attention to diffraction effects. Final control of the shape of the beams and the alignment of the mirrors in the system is performed after installation, under vacuum and with the tokamak at liquid nitrogen temperature, with the aid of a retractable probe and using a thermopile as the RF detector.

Investigation of the possibility of exceeding the Greenwald density limit during ECRH in T-10

In T-10 experiments, attempts were made to significantly exceed the Greenwald limit \(\bar n_{Gr} \) during high-power (Pab=750 kW) electron-cyclotron resonance heating (ECRH) and gas puffing. Formally, the density limit \((\bar n_e )_{\lim } \) exceeding the Greenwald limit \(({{(\bar n_e )_{\lim } } \mathord{\left/ {\vphantom {{(\bar n_e )_{\lim } } {\bar n_{Gr} }}} \right. \kern-\nulldelimiterspace} {\bar n_{Gr} }} = 1.8)\) was achieved for qL=8.2. However, as qL decreased, the ratio \({{(\bar n_e )_{\lim } } \mathord{\left/ {\vphantom {{(\bar n_e )_{\lim } } {\bar n_{Gr} }}} \right. \kern-\nulldelimiterspace} {\bar n_{Gr} }}\) also decreased, approaching unity at qL≈3. It was suggested that the “current radius” (i.e., the radius of the magnetic surface enclosing the bulk of the plasma current Ip), rather than the limiter radius, was the parameter governing the value of \((\bar n_e )_{\lim } \). In the ECRH experiments, no substantial degradation of plasma confinement was observed up to \(\bar n_e \sim 0.9(\bar n_e )_{\lim } \) regardless of the ratio \({{(\bar n_e )_{\lim } } \mathord{\left/ {\vphantom {{(\bar n_e )_{\lim } } {\bar n_{Gr} }}} \right. \kern-\nulldelimiterspace} {\bar n_{Gr} }}\). In different scenarios of the density growth up to \((\bar n_e )_{\lim } \), two types of disruptions related to the density limit were observed.

Charge-exchange neutral particle measurements on electron cyclotron resonance heating experiments in hot-ion-mode plasmas of the GAMMA 10 tandem mirror

Charge-exchange (C–X) neutral particle measurements have been carried out in hot-ion-mode plasmas of the GAMMA 10 tandem mirror. In the present experiment, a microwave power of 40 kW in 28 GHz is injected toward a second harmonic ECR layer located in the vicinity of the ICR layer at the central region and the radial profiles of ion temperatures determined from the energy spectrum of the C–X neutrals by using a neutral particle energy analyzer (NPA) are investigated from the viewpoint of ion energy balance. At the onset of the ECRH pulse, a remarkable increase of C–X neutral flux with high energy (few keV to few tens keV) is observed with NPA and the resultant ion temperature on the plasma axis is found to increase from 2.5 to 5.0 keV at the electron line density of 3×1013 cm−2. Based on the measured plasma parameters, radial profiles of ion-energy losses due to classical processes are evaluated and ECRH in the central region is confirmed to reduce the energy loss due to electron drag significantly in the core-plasma region.

Electron cyclotron emission diagnostic of high temperature electron cyclotron resonance heated plasmas on Frascati tokamak upgrade

The electron cyclotron emission (ECE) diagnostic on FTU tokamak is routinely performed with a Michelson interferometer with spectral range extending up to 1300 GHz. The diagnostic allowed accurate electron temperature measurements during the recent 140 GHz electron cyclotron resonance heating (ECRH) experiments on FTU. Very accurate measurements have been performed on a wide range of electron temperatures and profile peaking. The ECE measurements have been compared with Thomson scattering and with observations of x-ray spectra from highly stripped molybdenum ions. The suprathermal emission in these conditions has been studied.

An inexpensive instrument to detect radio-frequency leakage from transmission lines at millimeter wavelengths for personnel safety

Arrays of thermopile detectors arranged on flat panels were used to measure stray radiation from high-power transmission lines at 140 GHz in the FTU tokamak electron cyclotron resonance heating experiment. These instruments are insensitive to polarization and to the direction of propagation within ±40° off normal incidence. They have a time constant of 120 ms, therefore amplification and synchronous detection are required for the measurement of short radiation pulses. No special measurement techniques are required with long pulses or cw radiation. The typical sensitivity is 2 mV cm2/mW in long pulse operation and 47 μV cm2/mW for 1 ms pulses. The detectors are sensitive to light and adequate filtering should be added to suppress it if necessary. These characteristics are adequate for the measurement of human exposure levels to electromagnetic radiation in the millimeter wave range.

Propagation and radiation characteristics of the circular electric, circular magnetic and hybrid waveguide modes

The field distributions and propagation constants of the circular electric, circular magnetic and hybrid modes of oversized waveguides are expressed, taking the effects of walls into account. These effects are described by wall functions which depend on surface impedances of the wall, and are determined for different types of guides. The near and far field patterns are derived in the case of real wall functions. It is shown that, for very oversized waveguides, the terms containing wall functions can be ignored in the calculations, and it results that the expressions of fields and propagation constants become independent of the types of waveguides. An application to corrugated waveguides for Electron Cyclotron Resonance Heating experiments shows the variations of the radiation characteristics versus geometric parameters of the corrugations and determines the ranges of interest for these parameters.

Estimate of the Electron Power Deposition Profile in CHS

The electron power deposition profile has been estimated experimentally during the ion cyclotron range of frequency (ICRF) heating and the electron cyclotron resonance heating (ECRH) in the compact helical system (CHS). The time evolution of the local electron temperature is measured from the second harmonic electron cyclotron emission (ECE) using super heterodyne radiometer. The absorbed power by electrons has been derived from the change in the slopes of the local electron temperature just before and after the input power is turned off. The power deposition profiles of electrons are compared with results from the calculation code in ICRF experiment. Those results show good agreement. In the ECRH experiment the input power is modulated to reduce the power deposition profile. Those analyses give results that the input power is absorbed around {rho}=0.6. 4 refs., 4 figs.

The electron cyclotron resonance heating experiments on HL-1 tokamak

The electron cyclotron resonance heating (ECRH) experiments on HL-1 tokamak with 300 kW of power generated by a gyrotron are described. With ECE diagnostics, a 30% increase of electron temperature has been obtained. The following phenomena have been observed with various diagnostics during ECRH: increase of radiation intensity of soft x-ray; amplitude change and period elongation of sawtooth oscillation; MHD instability suppression; increase of Hα spectral line intensity and ionized impurity radiation from edge plasma; increase of the temperature and density of edge plasma. Although the total plasma energy content increases, the energy confinement time is decreased. Initial results of low voltage start-up experiments have also been obtained.

140 GHz electron cyclotron resonance heating experiments at the W7-AS stellarator

Abstract Electron cyclotron resonance heating (ECRH) at the W7-AS stellarator is performed two systems of comparable heating power at 70 GHz (0.8 MW, 3 s) and 140 GHz (0.9 MW, 0.4 s). Experiments with 140 GHz open a new parameter window with high density operation up to 1.1 × 1020 m−3. H mode transitions were observed in high density discharges at 2.5 T and at lower density at 1.25 T. The operational window for the stellarator H mode with emphasis on the density and power threshold and the influence of gas puffing on the H transitions are discussed. Perturbation experiments with modulated ECRH power were performed to determine the heat transport and the power deposition profile. Density control is achieved in combined heating with neutral beam injection (NBI) despite the beam particle fuelling, whereas with NBI alone a steady density rise is observed. The impact of combined heating on the impurity confinement was investigated. Electron cyclotron current drive (ECCD) was studied in different magnetic field configurations and the influence of trapped particles on the ECCD efficiency was examined experimentally and compared with theory.

Electron cyclotron resonance heating experiments in the GAMMA 10 tandem mirror

Abstract Two aspects of ECRH experiments in the GAMMA 10 tandem mirror are described. One is hot electron production in a minimum- B anchor cell by second harmonic ECRH. When a resonance region is located at the centre of the anchor cell, hot electrons are piled up near the centre region and the β-value attains more than 20%. For off-centre resonance, hot electrons have a spatial profile like a baseball seam. This profile is explained by a drift orbit of an electron in a minimum- B field. The other topic is observation of an intense axial electron flow with energy from hundreds of eV to a few keV generated by fundamental ECRH in the end cells. The force due to the fundamental ECRH driving heated electrons into the mirror loss cone is particularly strong. A bounce averaged Fokker-Planck code with a quasi-linear diffusion operator reproduces the electron flow with the same energies. The electron flow is a predominant ingredient for generation of the electrostatic potential in the GAMMA 10.

Electron cyclotron resonance heating on the TCA tokamak

Electron cyclotron resonance heating (ECRH) experiments at 39 GHz with power levels similar to those of Ohmic heating have been conducted in the TCA tokamak in either the X-mode or the O-mode, ECRH was observed to be more efficiently absorbed in low density and low qa discharges. At low density the central electron temperature increased by a factor of two over Ohmic values and the stored energy increased by 40% when up to 80% of the plasma energy was supplied by ECRH. During low density ECRH operation, the electron energy confinement time τEe was the same as during pure Ohmic heating, but at high density τEe degraded with ECRH power. The decrease in the loop voltage required to maintain a constant current during ECRH is a very reproducible quantity and can be attributed in almost all cases to the change in the electron temperature. Sawtooth stabilization occurred during ECRH, with a sawtooth period of up to ten times longer than that before ECRH. This stabilization depends on the ECRH toroidal injection angle and the total input power. High energy ions were observed and their production is attributed, as in earlier experiments, to a three-wave interaction process