GHz Electron Cyclotron Resonance Heating Research Articles

Short-pulse frequency stabilization of a MW-class ECRH gyrotron at W7-X for CTS diagnostic

At the Wendelstein 7-X stellarator, a 174 GHz Collective Thomson Scattering (CTS) diagnostic will be implemented. One of the 140 GHz Electron Cyclotron Resonance Heating (ECRH) gyrotrons will be operated at around 174 GHz in a higher cavity mode, using it as source for the CTS mm-wave probing beam. To prevent any damage to the CTS receiver, a notch filter cuts out the high-power gyrotron signal at the entrance of the receiver. The bandwidth of the gyrotron signal determines the notch filter bandwidth. First proof-of-principle experiments on frequency stabilization were conducted on W7-X ECRH gyrotrons employing Phase-Locked Loop techniques. The gyrotron output frequency was controlled with the accelerating voltage, which is applied between the anode and cathode of the gyrotron diode-type Magnetron Injection Gun. Frequency stabilization experiments with 10 ms pulses were conducted at the gyrotron nominal frequency of 140 GHz as well as at 174 GHz. It is concluded that the gyrotron frequency could be stabilized for at least 3 ms at 140 GHz and 8 ms at 174 GHz. In the frequency spectrum, a clear main peak of the gyrotron frequency at 140 GHz with a full -15 dB linewidth of below 500 Hz was achieved.

Commissioning of Electron Cyclotron Resonance Heating (ECRH) system on tokamak Aditya-U

The Aditya tokamak has been upgraded to Aditya-U by changing it’s vacuum vessel from rectangular to circular to accommodate the diverter coil for shaped plasma. The 42 GHz ECRH (Electron Cyclotron Resonance Heating) system has been integrated with the tokamak. The system is capable to deliver 500 kW power for 500 ms duration. The ECRH system with modified layout has been integrated successfully on Aditya-U; the burn pattern close to tokamak port ensures good Gaussian beam. Earlier the Aditya Tokamak used to operate at lower magnetic field (Max. 1.0 T) and the ECRH assisted low-loop voltage (˜6 V) plasma start-up experiments were carried out at second harmonic. Now the upgraded Aditya is operated at higher magnetic field up to 1.5 T. Initial ECRH experiments have been carried out at fundamental harmonic at magnetic field from 1.2 T to 1.35 T. Approximately 150 kW to 250 kW power in fundamental O-mode is launched from low field side. The ECH pulse of ˜40 ms is launched at the flat-top of plasma current. The effect of ECR heating is seen as some increase in soft X-ray signal. In some small plasma shots, the EC power is launched after the shot and EC breakdown effect has been observed.

Test of polarizers for the 105 GHz ECRH system on J-TEXT

To match the electron cyclotron wave with the plasma efficiently, we design two polarizers including a linear polarizer and an elliptical polarizer for the 105 GHz electron cyclotron resonance heating system on J-TEXT. The linear polarizer is mainly used to change the rotation angle of the wave, while the ellipticity of the wave is regulated by the elliptical polarizer. The sinusoidal grooves are applied to improve power capability. The polarizers were manufactured, and we set up a low power test platform to measure polarization characteristics of the wave resulting from polarizer reflection. To improve accuracy of measurement, the detector was calibrated with the standard wave source. The test results agree well with the design parameters. Therefore, we think the designed polarizers can meet requirements of the electron cyclotron resonance heating system on J-TEXT.

Modification of argon impurity transport by electron cyclotron heating in KSTAR H-mode plasmas

Experiments with a small amount of Ar gas injection as a trace impurity were conducted in the Korea Superconducting Tokamak Advanced Research (KSTAR) H-mode plasma ( = 2.8 T, = 0.6 MA, and = 4.0 MW). 170 GHz electron cyclotron resonance heating (ECH) at 600 and 800 kW was focused along the mid-plane with a fixed major radial position of = 1.66 m. The emissivity of the Ar16+ (3.949 ) and Ar15+ (353.860 ) spectral lines were measured by x-ray imaging crystal spectroscopy (XICS) and a vacuum UV (VUV) spectrometer, respectively. ECH reduces the peak Ar15+ emission and increases the Ar16+ emission, an effect largest with 800 kW. The ADAS-SANCO impurity transport code was used to evaluate the Ar transport coefficients. It was found that the inward convective velocity found in the plasma core without ECH was decreased with ECH, while diffusion remained approximately constant resulting in a less-peaked Ar density profile. Theoretical results from the NEO code suggest that neoclassical transport is not responsible for the change in transport, while the microstability analysis using GKW predicts a dominant ITG mode during both ECH and non-ECH plasmas.

Operation and control of high power Gyrotrons for ECRH systems in SST-1 and Aditya

The Electron Cyclotron Resonance Heating (ECRH) system is an important heating system for the reliable start-up of tokamak. The 42GHz and 82.6GHz ECRH systems are used in tokamaks SST-1 and Aditya to carry out ECRH related experiments. The Gyrotrons are high power microwave tubes used as a source for ECRH systems. The Gyrotron is a delicate microwave tube, which deliver megawatt level power at very high voltage ∼40–50kV with the current requirement ∼10A–50A. The Gyrotrons are associated with the subsystems like: High voltage power supplies (Beam voltage and anode voltage), dedicated crowbar system, magnet, filament and ion pump power supplies, cooling, interlocks and a dedicated data acquisition & control (DAC) system. There are two levels of interlocks used for the protection of Gyrotron: fast interlocks (arcing, beam over current, dI/dt, anode voltage and anode over current etc.) operate within 10μs and slow interlocks (cooling, filament, silence of Gyrotron, ion pump and magnet currents) operate within 100ms.Two Gyrotrons (42GHz/500kW/500ms and 82.6GHz/200kW/1000s) have been commissioned on dummy load for full parameters. The 42GHz ECRH system has been integrated with SST-1 & Aditya tokamak and various experiments have been carried out related to ECRH assisted breakdown and start-up of tokamak at fundamental and second harmonic. These Gyrotrons are operated with VME based data acquisition and control (DAC) system. The DAC system is capable to acquire 64 digital and 32 analog signals. The system is used to monitor & acquire the data and also used for slow interlocks for the protection of Gyrotron. The data acquired from the system are stored online on VME system and after the shot stored in a file in binary format. The MDSPlus, a set of software tools has been used for data visualization & management for after shot analysis.

Development and Preliminary Commissioning Results of a Long Pulse 140 GHz ECRH System on EAST Tokamak (Invited)**supported by the National Magnetic Confinement Fusion Science Program of China (Nos. 2011GB102000, 2012GB103000 and 2015GB103000)

A long pulse electron cyclotron resonance heating (ECRH) system has been developed to meet the requirements of steady-state operation for the EAST superconducting tokamak, and the first EC wave was successfully injected into plasma during the 2015 spring campaign. The system is mainly composed of four 140 GHz gyrotron systems, 4 ITER-Like transmission lines, 4 independent channel launchers and corresponding power supplies, a water cooling, control & inter-lock system etc. Each gyrotron is expected to deliver a maximum power of 1 MW and be operated at 100-1000 s pulse lengths. The No.1 and No.2 gyrotron systems have been installed. In the initial commissioning, a series of parameters of 1 MW 1 s, 900 kW 10 s, 800 kW 95 s and 650 kW 753 s have been demonstrated successfully on the No.1 gyrotron system based on calorimetric dummy load measurements. Significant plasma heating and MHD instability suppression effects were observed in EAST experiments. In addition, high confinement (H-mode) discharges triggered by ECRH were obtained.

Threefold Increase of the Bulk Electron Temperature of Plasma Discharges in a Magnetic Mirror Device.

This Letter describes plasma discharges with a high temperature of bulk electrons in the axially symmetric high-mirror-ratio (R=35) open magnetic system gas dynamic trap (GDT) in the Budker Institute (Novosibirsk). According to Thomson scattering measurements, the on-axis electron temperature averaged over a number of sequential shots is 660±50 eV with the plasma density being 0.7×10^{19} m^{-3}; in few shots, electron temperature exceeds 900 eV. This corresponds to at least a threefold increase with respect to previous experiments both at GDT and at other comparable machines, thus, demonstrating the highest quasistationary (about 1 ms) electron temperature achieved in open traps. The breakthrough is made possible by application of a new 0.7 MW/54.5 GHz electron cyclotron resonance heating system in addition to standard 5 MW heating by neutral beams, and application of a radial electric field to mitigate the flute instability.

Improved beta (local beta >1) and density in electron cyclotron resonance heating on the RT-1 magnetosphere plasma

This study reports the recent progress in improved plasma parameters of the RT-1 device. Increased input power and the optimized polarization of electron cyclotron resonance heating (ECRH) with an 8.2 GHz klystron produce a significant increase in electron beta, which is evaluated by an equilibrium analysis of the Grad–Shafranov equation. The peak value of the local electron beta βe is found to exceed 1. In the high-beta and high-density regime, the density limit is observed for H, D and He plasmas. The line-averaged density is close to the cutoff density for 8.2 GHz ECRH. When the filling gas pressure is increased, the density limit still exists even in the low-beta region. This result indicates that the density limit is caused by the cutoff density rather than the beta limit. From the analysis of interferometer data, we found that inward diffusion causes a peaked density profile beyond the cutoff density.

The conceptual design of an electron cyclotron emission imaging system for studying ITER-like high temperature plasmas

The design of an electron cyclotron emission imaging (ECEI) system for two-dimensional (2D) observation of the magnetohydrodynamical modes in high temperature ITER (from ‘International Thermonuclear Experimental Reactor’) H-mode-like plasmas (5.3 T and 25 keV) based on fundamental ordinary mode (O1-mode) and second-harmonic extraordinary mode (X2-mode) measurements is explored conceptually. For studying the spatial resolution in high temperature plasmas, the relativistic broadening and inward shift of the emission layer in the mid-plane are calculated. The radial spatial resolution is significantly degraded in the range R < 5.1 m for the O1-mode and in the range R < 6.9 m for the X2-mode. The region with R < 6.5 m is inaccessible for X2-mode study. The emission layer width is enlarged in a narrow region of the pedestal due to the magnetic field being modified by the large pressure gradient. The broadening and shift in the poloidal plane are also calculated, to investigate their effects on 2D measurements. The frequency range of electron cyclotron emission measurements is selected to protect the system from stray radiations of the 170 GHz electron cyclotron resonance heating source and to avoid harmonic overlap. The frequency ranges of 115–160 GHz for the O1-mode and 230–320 GHz for the X2-mode provide radial coverage of 5.9 < R < 8.2 m or −0.15 < r/a < 1.The ECEI system utilizes a dual-array detection technique which provides a simultaneous measurement at two radial positions, and each array has 8 by 16 (radial by vertical) channels. The radial image size with 8 channels is ∼41–76 cm for the O1-mode and ∼19–36 cm for the X2-mode, with sufficient resolution. The front-end optics, which focuses the electron cyclotron emission to the low loss corrugated transmission waveguides, is designed with two flat mirrors and two focusing mini-lens arrays. The vertical image size with 16 channels is ∼150 cm and the spot size of each channel is 8–15 cm in the plasma region, taking into account the sensitivity pattern of the waveguide. The refraction effect due to inhomogeneous plasma enlarges the vertical image size up to 20% and 5% for the O1-mode and X2-mode cases, respectively. The horizontal distortion due to the relativistic inwards shift is reduced by the increased toroidal field in the core region.

Design of the Transmission Lines for 140 GHz ECRH System on HL-2A

A new 140 GHz/2 MW/3 s electron cyclotron resonance heating (ECRH) system composed of two units is now being constructed on HL-2A. As a part of the system, two transmission lines marked No.7 & 8 play the role of carrying microwave power from two gyrotrons to the tokamak port. Based on the oversized circular corrugated waveguide technology, an evacuated transmission system with high power capability and high transmission efficiency is designed. Details are presented for the design of the corrugated waveguide, the layout of the proposed lines and the vacuum pumping system. Then mode conversion losses due to coupling, misalignment, bends and gaps are discussed to serve as a reference for analyzing the transmission efficiency and alignment. Finally, a dual-modes propagation case consisting of the HE11 and LP11 even modes is discussed.

An overview of FTU results

Since the 2010 IAEA-FEC Conference, FTU has exploited improvements in cleaning procedures and in the density control system to complete a systematic exploration of access to high-density conditions in a wide range of plasma currents and magnetic fields. The line-averaged densities at the disruptive limit increased more than linearly with the toroidal field, while no dependence on plasma current was found; in fact, the maximum density of 4.3 × 1020 m−3 was reached at B = 8 T even at the minimum current of 0.5 MA, corresponding to twice the Greenwald limit. The lack of plasma current dependence was due to the increase in density peaking with the safety factor. Experiments with the 140 GHz electron cyclotron resonance heating (ECRH) system were focused on the sawtooth (ST) period control and on the commissioning of a new launcher with real-time steering capability that will act as the front-end actuator of a real-time system for ST period control and tearing mode stabilization. Various ECRH and electron cyclotron current-drive modulation schemes were used; with the fastest one, the ST period synchronized with an 8 ms modulation period. The observed period variations were simulated using the JETTO code with a critical shear model for the crash trigger. The new launcher was of the plug-in type, allowing quick insertion and connection to the transmission line. Both beam characteristics and steering speed were in line with design expectation. Experimental results on the connection between improved coupling of lower hybrid waves in high-density plasmas and reduced wave spectral broadening were interpreted by fully kinetic, non-linear model calculations. A dual-frequency, time-of-flight diagnostic for the measurement of density profiles was developed and successfully tested. Fishbone-like instabilities driven by energetic electrons were simulated by the hybrid MHD-gyrokinetic XHMGC code.

Extension and characteristics of an ECRH plasma in LHD

One of the main objectives of LHD is to extend the plasma confinement database for helical systems and to demonstrate such extended plasma confinement properties to be sustained in the steady state. Among the various plasma parameter regimes, the study of confinement properties in the collisionless regime is of particular importance. Electron cyclotron resonance heating (ECRH) has been extensively used for these confinement studies of LHD plasma from the initial operation. The system optimizations including the modification of the transmission and antenna system are performed with special emphasis on the local heating properties. As a result, a central electron temperature of more than 10 keV with an electron density of 0.6 × 1019 m−3 is achieved near the magnetic axis. The electron temperature profile is characterized by a steep gradient similar to those of an internal transport barrier observed in tokamaks and stellarators. The 168 GHz ECRH system demonstrated efficient heating at densities more than 1.0 × 1020 m−3. The continuous wave ECRH system is successfully operated to sustain a 756 s discharge.

Ray Tracing Calculation of ECRH Power Absorption Profile in Heliotron J

Power absorption profiles of electron cyclotron resonance heating (ECRH) are calculated for a helical-axis heliotron device, Heliotron J, by a ray tracing code, TRECE. The three-dimensional structure of flux surfaces and magnetic field is considered to evaluate the power absorption profile accurately. The calculation results show that the power absorption profile can be generally controlled under the experimental conditions of the 70 GHz ECRH system by setting the appropriate launching condition. The well localized single pass power absorption can be realized when the ECRH power is launched with a tokamak-like B contour. The single pass absorption efficiency is as high as 92 % of the injected power in the case of moderate toroidally oblique launch because of the long path along the resonance region. On the other hand, the power is fully absorbed when ECRH power is perpendicularly launched at the poloidal cross section with a saddle-type B contour, although the absorption profile is wider. This is because the magnetic field gradient is gentle at the resonance region. These indicate that the control of power absorption is possible with the appropriate launching condition in the spatially winding plasmas. The calculation is compared with the experimental results on the ECRH plasma on Heliotron J.

Test and Commissioning of 82.6 GHz ECRH system on SST-1

Electron Cyclotron Resonance Heating (ECRH) system will play an important role in plasma formation, heating and current drive in the Superconducting Steady state Tokamak (SST-1). Commissioning activity of the machine has been initiated. Many of the sub-systems have been prepared for the first plasma discharge. A radial and a top port have been allotted for low field side (LFS) and high field side (HFS) launch of O and X- modes in the plasma.The system is based on a gyrotron source operating at a frequency of 82.6±0.1GHz (GLGD-82.6/0.2) and capable of delivering 0.2 MW / 1000s with 17% duty cycle. The transmission line consisting of ∼15 meters length 63.5mm corrugated wave guide, DC break, wave guide switch, mitre bend, polariser, bellows that terminates with a vacuum barrier CVD window. A beam launching system used to steer the microwave beam in the plasma volume is connected between the end of the transmission line and the tokamak radial and top ports. A VME based real time data acquisition and control (DAC) system is used for monitoring, acquisition and control. Hard-wired interlock operates a rail-gap based crowbar system in less than 10µs under any fault condition. Burn patterns are recorded at various stages in the transmission line. The gyrotron is tested for ∼200 kW / 1000s operation on a water dummy load. Transmission line is tested at various power levels for long pulse operation. The paper highlights the experimental results of successful commissioning of the system.

The design of an ECRH system for JET-EP

An electron cyclotron resonance heating (ECRH) system has been designed for JET in the framework of the JET enhanced performance project (JET-EP) under the European fusion development agreement. Due to financial constraints it has been decided not to implement this project. Nevertheless, the design work conducted from April 2000 to January 2002 shows a number of features that can be relevant in preparation of future ECRH systems, e.g. for ITER. The ECRH system was foreseen to comprise six gyrotrons, 1 MW each, in order to deliver 5 MW into the plasma (Verhoeven A.G.A. et al 2001 The ECRH system for JET 26th Int. Conf. on Infrared and Millimeter Waves (Toulouse, 10–14 September 2001) p 83; Verhoeven A.G.A. et al 2003 The 113 GHz ECRH system for JET Proc. 12th Joint Workshop on ECE and ECRH (13–16 May 2002) ed G. Giruzzi (Aix-en-Provence: World Scientific) pp 511–16). The main aim was to enable the control of neo-classical tearing modes. The paper will concentrate on: the power-supply and modulation system, including series IGBT switches, to enable independent control of each gyrotron and an all-solid-state body power supply to stabilize the gyrotron output power and to enable fast modulations up to 10 kHz and a plug-in launcher that is steerable in both toroidal and poloidal angles and able to handle eight separate mm-wave beams. Four steerable launching mirrors were foreseen to handle two mm-wave beams each. Water cooling of all the mirrors was a particularly ITER-relevant feature.

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.

First results from the multichannel interferometer system on HSX

Measuring the equilibrium electron density distribution and its response to perturbations provides important information to magnetically confined plasma research. A multichannel interferometer system is now routinely operating on the new quasihelically symmetric stellator (HSX) to measure the equilibrium profile and electron density dynamics. The interferometer system has nine viewing chords with 1.5 cm spacing. The source is a bias-tuned Gunn diode at 96 GHz with passive solid-state tripler providing output at 288 GHz (8 mW). The HSX plasma is produced by 28 GHz electron cyclotron resonance heating and first results of the interferometer measurement are reported. The density spatial distribution is reconstructed from the measured line-integrated density. At high density [n̄e&amp;gt;2×1012 cm−3], an m=1 density oscillation with frequency of 1–2 kHz is observed. Plans to determine the radial particle flux and transport coefficients will be discussed.

Ray Tracing Calculation of ECRH Power Absorption for Heliotron J.

A ray tracing code, TRECE, of electron cyclotron resonance heating (ECRH) has been developed for the helical-axis heliotron, Heliotron J. The accurate power absorption profile can be obtained for the three dimensional structure of flux surfaces and magnetic field in Heliotron J. The calculation shows that the launching condition of the 70 GHz ECRH system assures a well localized power deposition.

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.

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.