Power Electron Cyclotron Resonance Heating Research Articles

Effect of deposition location of electron cyclotron resonance heating on active control of fishbone modes in the HL-2A tokamak

Experiment on suppressing fishbone activities is carried out in HL-2A tokamak by electron cyclotron resonance heating (ECRH). To achieve multiple deposition locations of ECRH, the magnetic field is in a range of 1.22–1.4 T from shot to shot. It is found that the fishbone modes exhibit different characteristics at different radial deposition locations. With the same injected power, the effect of off-axis ECRH is much better than that of on-axis heating. The fishbone modes can be completely suppressed when ECRH is deposited nearby the <i>q</i> = 1 rational surface, but would only mitigate in other cases. Further analysis indicate that injection of high power ECRH leads the electron temperature to increase, then the pressure gradient and plasma current density to change, finally safety factor to change and the minimum safety factor to reach a value larger than 1. Meanwhile, M3D-K simulation results show that the growth rate of fishbone mode declines with the increase of <i>q</i><sub>min</sub>. In other words, the growth of safety factor and disappearance of <i>q</i> = 1 rational surface induced by ECRH contribute to the suppression of fishbone activities. The experimental results reported here may not only help to better understand complex effects of ECRH on magnetohydrodynamic instability, but also provide a physics basis for actively controlling the energetic particle driven modes in the future magnetic confined fusion devices.

In-vessel piezoelectric actuation system for DTT ECRH launchers: Conceptual design

The Electron Cyclotron Resonance Heating (ECRH) system of Divertor Tokamak Test (DTT) facility will feature up to 35.2 MW installed power. At the time of its completion, it will be the most powerful ECRH system ever realized. In full power configuration, 4 sectors of the tokamak will be equipped with equatorial and upper antennas with respectively 6 and 2 launchers each, for a total of 32 launchers, fed by 16 1-MW, 170-GHz gyrotrons and 16 1.2-MW, 170-GHz gyrotrons. The last mirror of each ECRH launcher should be independently steerable about two axes to provide the required operational flexibility. Due to the high number of independent launchers in a relatively small space, a compact driving mechanism is required. In state-of-the-art ECRH systems, out-of-vessel drives are connected to the steering mirror through long shafts or bellows. In this paper, we present the conceptual design of an innovative driving mechanism, which relies on in-vessel piezoelectric actuators and compliant hinges to meet compactness requirements, minimize system’s inertia, friction and backlash, while maximizing positioning accuracy. The work includes a general overview of the system, the model of the semi-compliant transmission, the identification of candidate materials and the sizing of the driving mechanism.

Design of Electron Cyclotron Resonance Heating protection components for first plasma operations in ITER

Abstract The ITER first plasma operations will occur with no full blanket and divertor components installed. Machine protection is required and First Plasma Protection Components (FPPC) have been designed to shelter the vacuum vessel and other components from the plasma itself and from high power Electron Cyclotron Resonance Heating (ECRH) foreseen for first plasma breakdown. ECRH protection components will be installed to protect in-vessel structures from direct and stray radiation as EC beams will be used for plasma breakdown. This paper is focused on the design of these components including dedicated mirrors to shape and redirect the beams to the EC resonance location in the magnetic field null region and then into a beam dump located in an equatorial port, where exceeding EC radiation will be trapped and dumped. Two mirrors and one grating mirror have been designed to provide the required shaping and directions for the beams coming from the upper launcher towards EC resonance and dump. The beam dump consists of five large plates affixed in an equatorial port. Guidelines that drove the design of the quasi-optical system, characteristics of the mirrors, resulting launched beams and concept developed for the beam dump will be here described.

Progress of high power and long pulse ECRH system in EAST

In accordance with the long pulse objectives of the Experimental Advanced Superconducting Tokamak (EAST), an electron cyclotron resonance heating (ECRH) system with the feature of 4MW power for a pulse length up to 1000s at 140GHz, using second harmonic of the extraordinary mode (X2) is presently under construction at the institute of plasma physics, Chinese academy of sciences (ASIPP). The missions of the system are to provide central heating, current drive, plasma profile tailoring and control of magneto-hydrodynamic (MHD) instabilities. The continuous wave (CW) power is transmitted from the gyrotrons to EAST via low-loss evacuated waveguide transmission lines. Considering the diverse applications of the EC system, the front steering launcher is designed to inject four individually steered beams across nearly the entire plasma cross section. The beam's launch angles can be continuously varied with the optimized scanning range of over 30° in poloidal direction and ±25° in toroidal, as well as the polarization will be adjusted during the discharge by the orientations of a pair of polarizers in the transmission line to maintain the highest absorption for different operational scenarios. The commissioning of the first 2MW system will be commenced in the end of 2014.

Development of a Millimeter-Wave Beam Position and Profile Monitor for Transmission Efficiency Improvement in an ECRH System

In a high power Electron Cyclotron Resonance Heating (ECRH) system, a long-distance and low-loss transmission system is required to realize effective heating of nuclear fusion-relevant plasmas. A millimeter-wave beam position and profile monitor, which can be used in a high-power, evacuated, and cooled transmission line, is proposed, designed, manufactured, and tested. The beam monitor consists of a reflector, Peltier-device array and a heat-sink. It was tested using simulated electric heater power or gyrotron output power. The data obtained from the monitor were well agreed with the heat source position and profile. The methods of data analysis and mode-content analysis of a propagating millimeter-wave in the corrugated wave-guide are proposed.

An overview of the energetic electron induced instabilities with high-power ECRH on HL-2A

In this paper, an overview of the magnetohydrodynamic instabilities induced by energetic electrons on HL-2A is given and some new phenomena with high-power electron cyclotron resonance heating (ECRH) are presented. A toroidal Alfvén eigenmode with frequency from 200 to 350 kHz is identified during powerful ECRH. In the lower frequency range from 10 to 35 kHz, which is in the beta-induced Alfvén eigenmode frequency range, the coexistence of multi-mode is found during the high-power ECRH for the first time. The spectra become wide when the power is sufficiently high. The frequencies of the modes increase with and are much lower than the Alfvén frequency. The relationship between the mode frequency and (7/4 + Te/Ti)1/2 (Ti)1/2 can be obtained by statistical data analysis. Between the two previous frequency ranges, a group of new modes with frequencies from 50 to 180 kHz is observed with high-power ECRH and neutral beam injection heating together. The modes have clear frequency chirping within several milliseconds or several tens of milliseconds, which are identified as energetic particle mode like instabilities. The new features of the fishbone instability excited by energetic electrons are identified. It is interesting to find the frequency jump phenomena in the high-power ECRH. The difference between the low and high frequencies increases with ECRH power. The frequency jumps between 8 and 15 kHz within about 25 ms periodically, when the power is 1.2 MW.

Collective Thomson scattering of a high power electron cyclotron resonance heating beam in LHD (invited)

Collective Thomson scattering (CTS) system has been constructed at LHD making use of the high power electron cyclotron resonance heating (ECRH) system in Large Helical Device (LHD). The necessary features for CTS, high power probing beams and receiving beams, both with well defined Gaussian profile and with the fine controllability, are endowed in the ECRH system. The 32 channel radiometer with sharp notch filter at the front end is attached to the ECRH system transmission line as a CTS receiver. The validation of the CTS signal is performed by scanning the scattering volume. A new method to separate the CTS signal from background electron cyclotron emission is developed and applied to derive the bulk and high energy ion components for several combinations of neutral beam heated plasmas.

Features of ion and electron fishbone instabilities on HL-2A**The 11th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, 21–23 September 2009, Kyiv, Ukraine, OT16.

The features of ion and electron fishbone instabilities have been investigated during neutral beam injection (NBI) and electron cyclotron resonance heating (ECRH) on HL-2A. Some new phenomena, such as frequency jumps and V-font-style sweeping, have been presented in the paper. Three kinds of i-fishbones, including hybrid sawtooth–fishbone (sawbone), run-on fishbone and classical fishbone, have been identified during NBI. During high power (PECRH > 0.7 MW) ECRH, the experimental results indicate that the e-fishbone frequencies are higher than those during low power ECRH, and are provided with up- and down-chirping behaviours, and sometimes also with V-font-style sweeping. The periodic mode frequency jumps have also been detected by a soft x-ray array. It is possible to correlate these phenomena with the redistribution of energetic electrons.

Diplexers for Power Combination and Switching in High Power ECRH Systems

Electron Cyclotron Resonance Heating (ECRH) systems for next step large fusion-devices operate at a Continuous Wave (CW) power well beyond 10 MW generated by a large number of gyrotrons with typically 1 MW power per unit. The combination of the power of two (or more) gyrotrons and switching of the power between different launchers for different physics applications is an attractive feature for such systems. The combination of beams from different gyrotrons would reduce the number of transmission lines and the requirements on port space. Fast switching between two antennas synchronously with the Magneto-Hydro Dynamic (MHD) modes frequency would increase the efficiency of mode stabilization. Both combination and switching as well as power sharing between different ports can be performed with high-power four-port diplexers using small frequency differences or small frequency-shift keying of the gyrotrons, respectively. Fast directional switches (FADIS) and beam combiners (BC) can be designed on the basis of different physical mechanisms: some selected design variants were investigated and the results are presented. Considerations on the integration of FADIS/BC's into large ECRH systems and their use in test arrangements are presented.

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 line-of-sight electron cyclotron emission receiver for electron cyclotron resonance heating feedback control of tearing modes

An electron cyclotron emission (ECE) receiver inside the electron cyclotron resonance heating (ECRH) transmission line has been brought into operation. The ECE is extracted by placing a quartz plate acting as a Fabry-Perot interferometer under an angle inside the electron cyclotron wave (ECW) beam. ECE measurements are obtained during high power ECRH operation. This demonstrates the successful operation of the diagnostic and, in particular, a sufficient suppression of the gyrotron component preventing it from interfering with ECE measurements. When integrated into a feedback system for the control of plasma instabilities this line-of-sight ECE diagnostic removes the need to localize the instabilities in absolute coordinates.

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.

Transport studies with the ECH system on FTU tokamak

Experiments performed on the FTU tokamak, aimed at validation of physics-based transport models, which yield the electron temperature profile stiffness by using electron cyclotron resonance heating (ECRH), and at the implementation of ECRH-based techniques for plasma real-time control, are presented. ECRH is used to probe transport features, both in the steady state and in response to time-varied heating. The experiments clearly show stiffness in the electron temperature profile response to localized ECRH. The low gradient plasma region near the axis is characterized by low stiffness, in the sense that the temperature gradient can change with increasing heat flux, and low electron thermal diffusivity. Strong stiffness (in the sense that the temperature gradient length 1/LT = −∇T/T does not change significantly even with significantly different heating profiles and intensity) and high diffusivity are found in the confinement region (0.15 < r/a < 0.5). Particular attention is given to the experimental investigation of the transition layer between low and high diffusivity (and low and high stiffness) regions, which is located at the EC deposition radius rdep when powerful ECRH is applied. A transition layer, identified by heat waves launched by modulated ECH, is found also in plasmas with dominant Ohmic heating, showing that it is a local plasma feature, and not merely a consequence of a step-wise increase in the conducted heat flux at rdep. All observations fit well with a critical temperature gradient length modelling of local electron heat transport.The measurement of the rate of change of stored electron energy when ECRH is switched on (or off) is the basis of a technique for real-time and automatic detection of the absorption layer. Repetitive short pulses are used to increase the signal to noise ratio, with a very low duty cycle, so as not to waste ECRH power during the detection procedure.

Mirror development for the 140 GHz ECRH system of the stellarator W7-X

For the future stellarator W7-X in Greifswald, a powerful electron cyclotron resonance heating (ECRH) system at 140 GHz with a total RF power of 10 MW CW is under construction. The transmission of the radiation from the gyrotron hall to the machine will be realized via a fully optical system consisting of more than 160 water cooled mirrors. This paper describes the design of the mirrors and presents results of calculations of the thermo–mechanical properties. Experimental verification of the calculations showed that mirror deformation under application of a heat load equivalent to the absorbed power from a 1 MW mm-wave beam is in the tolerable range. Water cooled mirrors of the single-beam type have been used successfully during conditioning of a prototype gyrotron with a mm-wave power of 850 kW and a pulse length of up to 180 s. To test and optimize in-situ measurements of beam parameters and power of the CW ECRH system on W7-X, these mirrors are equipped with different diagnostics.

Performance of the 140 GHz prototype transmission system for ECRH on the stellarator W7-X

For the future stellarator W7-X of IPP at Greifswald, a powerful electron cyclotron resonance heating facility is being built which will include 10 gyrotrons at 140 GHz with a CW power of 1 MW each plus two optional gyrotrons at 70 GHz. The millimetre wave power will be transmitted fully optically via two multiple-beam wave guides (MBWG) with a length of about 50 m. This paper presents calculations and low-power measurements of the transmission characteristics of a prototype MBWG, which promises a high transmission efficiency for the ECRH system under construction. Furthermore, tests of prototype water-cooled mirrors are discussed.

Mirror development for the 140 GHz ECRH transmission system on the stellarator W7-X

For the future stellarator W7-X of IPP at Greifswald, a powerful electron cyclotron resonance heating (ECRH) facility is under design, which will include ten gyrotrons at 140 GHz with a CW power of 1 MW each plus two optional gyrotrons at 70 GHz. The millimetre wave power will be transmitted via two multi-beam waveguides (MBWG) with a length of about 50 m. This paper shows results for the optimum shape of the mirrors and the mode conversion of the MBWG transmission system. The design which will be used for water-cooled mirrors and cooling structures is discussed. Concepts for the back reflectors, which will be mounted at the inner side of the vacuum vessel of W7-X to redirect the non-absorbed part of the beam back into the plasma are presented. A test facility to test the transmission properties of quasi-optical lines, including a measurement system for ohmic loss of mirrors, a reflectometer to detect possible surface deformations and an alignment control system is described.

POLOIDALLY SPIN UP GENERATED BY ELECTRON CYCLOTRON RESONANCE HEATING IN CORE TOKAMAK PLASMAS

A new approach is presented to produce core plasma poloidal rotation during electron cyclotron resonance heating (ECRH) in Tokamaks. The physical mechanism employed here is as follows:in a Tokamak plasma with high power ECRH, the resonance particle localization causes poloidal electric field, which consequently induce a poloidally asymmetric ion profile. This kind of poloidal ion accumulation would overwhelm the damping of poloidal rotation from the magnetic pumping, and make the plasma spin up poloidally. Based on the fluid equations and the drift kinetic equation, we have derived the criterion of plasma spin up in the collisionless region. It is shown that the core plasma poloidal rotation can be generated by ECRH in the practical rf power level.

Theory of high power electron cyclotron resonance heating

A single particle model derived from a Lagrangian formalism has been used to describe electron cyclotron heating, with the view to investigating relativistic effects on the velocity diffusion coefficient. It is shown that large microwave fields lead to less diffusion in velocity space than anticipated by linear models and that ultimately the behaviour is not amenable to a diffusive treatment.