High Power Electron Cyclotron Heating Research Articles

Design and Analysis of 140-Degree Miter Bend for High Power Electron Cyclotron Heating Transmission Lines

The ITER Electron Cyclotron Heating (ECH) system Transmission Lines (TL) require highly polished copper mirrors on miter bends (both 90° and 140°) to direct microwaves from their origin to the tokamak. This will result in substantial heat dissipation on the miter bends and mirrors and will require water cooling in order to achieve long pulse operation. Analysis and optimization of the cooling design for the 140° miter bend assembly used ANSYS® Multiphysics™ software to develop and verify the fluid, thermal, and structural behavior of the mirror and miter bend assembly. Simulation model choices included a thermo-mechanical model of the mirror-only, a thermo-mechanical model of the miter bend assembly, and a thermo-mechanical model of the mirror with coolant. These analyses revealed an optimal solution that uses a major-axis cooling channel configuration for the 140° miter bend to meet the design criteria (e.g. structural stresses, mirror deflection, vacuum seal, coolant temperatures and pressures).

Neutral beam heating on the TCV tokamak

The TCV tokamak contributes to physics understanding in fusion reactor research by harnessing a wide experimental tool set: in particular flexible shaping and high power electron cyclotron heating. Plasma regimes with high plasma pressure, a wider range of temperature ratios and significant fast-ion population are now attainable with a TCV heating system upgrade. In a first stage, a 1MW neutral beam was installed (2015) and is reported in this paper.The installation of the NB injector required modifications of the vacuum vessel and considerable work on the machine infrastructure, resulting in a shutdown from late 2013 to mid-2015. TCV is now operating partly as a European Medium-Size Tokamak (MST) facility under the auspices of the EUROfusion consortium. The NBI was intensively operated in the February–July 2016 phase of the MST campaign. Record ion temperatures of 2.0–2.5keV and toroidal rotation velocities up to 160km/s were promptly attained in the first few L-mode discharges with NB injection. Ion temperatures up to 3.5keV were subsequently achieved in ELMy H-mode. The injector produces a focused deuterium neutral beam with 25keV energy, 1MW neutral power and 2s duration.

Integral staggered point-matching method for millimeter-wave reflective diffraction gratings on electron cyclotron heating systems

The reflective diffraction gratings are widely used in the high power electron cyclotron heating systems for polarization strategy. This paper presents a method which we call “the integral staggered point-matching method” for design of reflective diffraction gratings. This method is based on the integral point-matching method. However, it effectively removes the convergence problems and tedious calculations of the integral point-matching method, making it easier to be used for a beginner. A code is developed based on this method. The calculation results of the integral staggered point-matching method are compared with the integral point-matching method, the coordinate transformation method and the low power measurement results. It indicates that the integral staggered point-matching method can be used as an optional method for the design of reflective diffraction gratings in electron cyclotron heating systems.

A Multifrequency Notch Filter for Millimeter Wave Plasma Diagnostics based on Photonic Bandgaps in Corrugated Circular Waveguides

Sensitive millimeter wave diagnostics need often to be protected against unwanted radiation like, for example, stray radiation from high power Electron Cyclotron Heating applied in nuclear fusion plasmas. A notch filter based on a waveguide Bragg reflector (photonic band-gap) may provide several stop bands of defined width within up to two standard waveguide frequency bands. A Bragg reflector that reflects an incident fundamental TE11 into a TM1n mode close to cutoff is combined with two waveguide tapers to fundamental waveguide diameter. Here the fundamental TE11 mode is the only propagating mode at both ends of the reflector. The incident TE11 mode couples through the taper and is converted to the high order TM1n mode by the Bragg structure at the specific Bragg resonances. The TM1n mode is trapped in the oversized waveguide section by the tapers. Once reflected at the input taper it will be converted back into the TE11 mode which then can pass through the taper. Therefore at higher order Bragg resonances, the filter acts as a reflector for the incoming TE11 mode. Outside of the Bragg resonances the TE11 mode can propagate through the oversized waveguide structure with only very small Ohmic attenuation compared to propagating in a fundamental waveguide. Coupling to other modes is negligible in the non-resonant case due to the small corrugation amplitude (typically 0.05·λ0, where λ0 is the free space wavelength). A Bragg reflector for 105 and 140 GHz was optimized by mode matching (scattering matrix) simulations and manufactured by SWISSto12 SA, where the required mechanical accuracy of ± 5 μm could be achieved by stacking stainless steel rings, manufactured by micro-machining, in a high precision guiding pipe. The two smooth-wall tapers were fabricated by electroforming. Several measurements were performed using vector network analyzers from Agilent (E8362B), ABmm (MVNA 8-350) and Rohde&Schwarz (ZVA24) together with frequency multipliers. The stop bands around 105 GHz (- 55dB) and 140 GHz (-60dB) correspond to the TE11-TM12 and TE11-TM13 Bragg resonances. Experiments are in good agreement with theory.

Soft x-ray intensity profile measurements of electron cyclotron heated plasmas using semiconductor detector arrays in GAMMA 10 tandem mirror.

Temporally and spatially resolved soft x-ray analyses of electron cyclotron heated plasmas are carried out by using semiconductor detector arrays in the GAMMA 10 tandem mirror. The detector array has 16-channel for the measurements of plasma x-ray profiles so as to make x-ray tomographic reconstructions. The characteristics of the detector array make it possible to obtain spatially resolved plasma electron temperatures down to a few tens eV and investigate various magnetohydrodynamic activities. High power electron cyclotron heating experiment for the central-cell region in GAMMA 10 has been started in order to reduce the electron drag by increasing the electron temperature.

Development of High Power ECH Systems for High and ELM-like Heat Flux in GAMMA 10 Tandem Mirror

Development of high power gyrotrons and electron cyclotron heating (ECH) systems for the power modulation experiments in GAMMA 10 have been started in order to generate and control the high heat flux and to make the ELM (edge localized mode) like intermittent heat load pattern for divertor simulation studies. ECH for potential formation at the plug region (P-ECH) produces the electron flow with high energy along the magnetic filed line. From the power scaling of confining potential and electron flux in GAMMA 10, the higher ECH power generates the higher confinement potential and the higher electron flux. The peak heat-flux of more than 10 MW/m 2 has been obtained during the ECH injection within the available power of ECH. This value almost corresponds to the steady-state heat load of the divertor plate of ITER. However, we need substantial upgrade of the heating power to approach the ITER level ELM energy density. For this purpose, MW power gyrotrons at 28 GHz have been started and the development is undergoing efficiently in the ECH system upgrade program with the collaboration of Japan Atomic Energy Agency (JAEA). c

High Power ECH Experiments for Direct Bulk Electron Heating in Central-Cell Region Plasmas on GAMMA 10 Tandem Mirror

High power electron cyclotron heating (ECH) experiment for the central-cell region in GAMMA 10 has been started in the hot ion mode operation (Ti = several keV). Transmission system for the central-cell ECH, including a taper, mirrors and polarizers, has been designed and installed for efficient direct bulk electron heating. ECH system with a twister and a circular polarizer for central-cell region has the advantages for controllable polarization of incident wave. Recently, a high power gyrotron of ~500 kW at 28 GHz has been developed in collaboration with Japan Atomic Energy Agency (JAEA) and this high power gyrotron was installed in central-cell region of GAMMA 10. This high power and efficient ECH system was applied to central-cell electron heating and significant increment of soft X-ray and diamagnetism signals during a pulse of ECH operation was obtained. In this paper, central ECH experimental results in the hot ion mode operation are reported.

Study of Efficient Electron Cyclotron Wave Coupling with Grooved Mirror Polarizer in the GAMMA 10 Tandem Mirror

An improvement of electron cyclotron heating (ECH) system in the central cell of the GAMMA 10 tandem mirror is carried out by the use of two grooved mirrors. Two grooved mirrors consist of a twister and a circular polarizer. In case of the ECH system in the central cell region, fundamental ECH is examined and launched microwave beam from the antenna propagates through the two parabolic mirrors. The incident microwave is radiated from the second mirror at a position with magnetic field intensity B=2.0 T and with an incident angle of 35.1 degrees. Two grooved mirrors can generate suitable elliptical polarizations for both of ordinary mode (O-mode) and extraordinary mode (X-mode) in oblique launch with a slanted magnetic field line. The first results of the polarization control of the incident wave for high power ECH experiments are reported.

Progress of Electron Cyclotron Heating/Current Drive Technology in JT-60U

A key issue in attaining high plasma performance in the Tokamaks is the optimization of current profile, involving such functions as stabilizing the neo-classical tearing mode (NTM) and control of the internal transport barrier (ITB). In the JT-60U, a high power electron cyclotron heating (ECH) system at 110 GHz has been developed for local plasma heating and current drive. We have demonstrated the technical feasibility of an antenna which can scan EC beam in both the toroidal and poloidal directions. The toroidal scan enabled co- and counter- current drive and also pure plasma heating. Automatic stabilization of the NTM was demonstrated by means of current drive at the magnetic island employing feedback control of the poloidal beam angle. The total injected power has been extended to 2.8 MW for 3.6 sec and each gyrotron delivers ˜ 1 MW for 5 sec. This world record regarding the injection energy was attained by an upgrade of the gyrotron using an RF absorber in the beam tunnel to suppress the parasitic oscillation and to improve the transmission efficiency of the waveguide system.

High-power ECH and fully non-inductive operation with ECCD in the TCVtokamak

Experiments with high-power electron cyclotron heating (ECH) and current drive(ECCD) in the TCV tokamak are discussed. Power up to 2.7 MW from six gyrotronsis delivered to the tokamak at the second-harmonic frequency (82.7 GHz) inX-mode. The power is transmitted to the plasma by six independent launchers,each equipped with steerable mirrors that allow a wide variety of injectionangles in both the poloidal and toroidal directions. Fully non-inductiveoperation of the tokamak has been achieved in steady state, for the full 2 sgyrotron pulse duration, by co-ECCD with a highest current to date of 210 kAat full power. The experimentally measured ECCD efficiency agrees well withpredictions obtained from linear modelling. We have observed that the highestglobal efficiency attainable at a given power is limited by stabilityconstraints. While the efficiency is maximum on the magnetic axis, adisruptive MHD instability occurs when the width of the deposition profile islower than a minimum value, which increases with total power. Many ECCDdischarges display a high level of electron energy confinement, enhanced by upto a factor of two over the Rebut-Lallia-Watkins (RLW) scaling law, which bycontrast is well satisfied in ohmic conditions. The longest confinement times(up to four times RLW) are observed with central counter-ECCD. Centralelectron heat diffusivities comparable to ohmic levels are obtained in thesescenarios, with electron temperatures in excess of 10 keV.

Antenna and transmission system for high power electron cyclotron heating in a compact helical system

An electron cyclotron heating (ECH) system for the compact helical system using two types of high power gyrotron as a power source is designed. In order to utilize fully the advantage of the ECH, the antenna system is designed so as to inject the microwave power as an elliptic gaussian beam, which allows well-defined high power local heating. This antenna and associated transmission system from gyrotron are designed with the quasi-optical concept. This design concept and the results of the cold test for the transmission and antenna system are described. The resultant beam parameter of the cold test for the antenna system shows fairly good agreement with the designed value, which indicates the validity of this design concept. The power transmission efficiency of about 90% was estimated from the measured power at both ends.

106-GHz Electron Cyclotron Heating System for Heliotron-E

A new high-power electron cyclotron heating system has been installed for the Heliotron-E helical device. This system is designed to operate at 106-GHz frequency with a half-megawatt output power. The system consists of a pulse gyrotron with TE[sub 12,2] whispering gallery mode (WGM) output, conversion system of the WGM into the Gaussian-like beam, transmission line for HE[sub 11] mode, and launching system. From measurement of radiation patterns, it was confirmed that the WGM was effectively converted into the Gaussian-like beam, and the emergent radiation profile from the tubular oversized corrugated waveguide was close to a circular Gaussian one even when the beam coupled to the HE[sub 11] mode had the side lobes before the transmission. This indicates that the oversized corrugated waveguides act as a mode filter. The launching system effectively focuses the Gaussian beam in the free space to a 2-cm (poloidal) [times] 3-cm (toroidal) e-folding power spot size. These are small enough compared with the plasma minor radius ([approximately]15 cm). It is expected that the power deposition can be well localized in the plasma central region. 21 refs., 10 figs.

Fokker–Planck studies of high power electron cyclotron heating in tokamaks

A Fokker–Planck formalism has been developed to interpret experimental data from high power electron cyclotron heating of tokamak plasmas in which the electron distribution function can be substantially distorted. The Fokker–Planck equation is solved using a 2-D code which incorporates electron trapping, a steady Ohmic electric field and a bounce averaged electron cyclotron heating term. The quasilinear RF diffusion coefficient is calculated using a single particle model which includes the mildly relativistic resonance condition and takes into account localized RF power injection and tokamak rotational transform effects. Comparisons are presented of the calculated and observed soft X-ray spectra from 60 GHz second harmonic electron cyclotron heating experiments on the CLEO tokamak. Good agreement between theory and experiment is found. In addition, predicted RF current drive efficiencies for the COMPASS tokamak are presented.

Observation of the parametric decay instability during electron cyclotron resonance heating on the Versator II tokamak

Observations are reported on a nonlinear, three-wave interaction process occurring during high-power electron cyclotron heating in the Versator II tokamak. The measured spectra and the threshold power are consistent with a model in which the incident power in the extraordinary mode of polarization decays at the upper-hybrid resonance layer into a lower-hybrid wave and an electron Bernstein wave.