Electron Cyclotron Heating System Research Articles

Development of a dummy load and waveguide components for 1 MW CW gyrotron

A dummy load dissipates the radiofrequency power and is required for the electron cyclotron heating system to test and adjust a gyrotron or a transmission line in ITER and JT60SA. The dummy load is comprised of a water-cooled vacuum chamber and a reflector. The vacuum chamber has intensive cooling and the inside surface has a ceramic coating layer. It is important that the dummy load has reliable durability and long lifetime. Linear movement is used as a main mechanism for the reflector to mitigate the heat deposition concentration instead of rotating reflector. The vacuum boundary for the linear movement is simply made with bellows and the bellows is accessible from the outside. The distribution of the ceramic coating thickness is carefully optimized considering the heat deposition rate. Another critical issue is to minimize the back reflection of the RF power from the dummy load. However, it is desired to design the dummy load without the pre-dummy-load. A prototypical dummy load will be manufactured and tested, following the design phase.

Study of the Polarization Strategy for Electron Cyclotron Heating Systems on HL-2M

As important components integrated in transmission lines of electron cyclotron heating systems, polarizers are mainly used to obtain the desired polarization for highly efficient coupling between electron cyclotron waves and plasma. The polarization strategy for 105-GHz electron cyclotron heating systems of HL-2M tokamak is studied in this paper. Considering the polarizers need high efficiency, stability, and low loss to realize any polarization states, two sinusoidal-grooved polarizers, which include a linear polarizer and an elliptical polarizer, are designed with the coordinate transformation method. The parameters, the period p and the depth d, of two sinusoidal-grooved polarizers are optimized by a phase difference analysis method to achieve an almost arbitrary polarization. Finally, the optimized polarizers are manufactured and their polarization characteristics are tested with a low-power test platform. The experimental results agree well with the numerical calculations, indicating that the designed polarizers can meet the polarization requirements of the electron cyclotron heating systems of HL-2M tokamak.

DIII-D Electron Cyclotron Heating System Status and Upgrades

DIII-D Electron Cyclotron Heating System Status and Upgrades

Analysis of ITER ECH Transmission Line Waveguide Couplings

The ITER Electron Cyclotron Heating (ECH) system will produce a high-intensity beam of electromagnetic radiation for plasma heating. A total of 20 MW of power will be transferred from 170 GHz gyrotrons through multiple transmission lines. The transmission lines consist of evacuated, aluminum, circularly corrugated waveguides that will each transmit up to 1.5 MW for up to 3600 seconds. The waveguides, as well as mirror and polarizer components, will be actively water cooled in order to support the heat load from the long-pulse high-power radiation. Transmission lines will be as long as 200 meters, made up of individual lengths of 2 to 4 meter pieces that are joined by couplings. These couplings must retain high vacuum during operation, and maintain a very high degree of straightness between adjacent waveguide pieces. Analyses have been performed to examine various parameters of the design of these couplings, and confirm that stringent criteria are met during installation and operation. Further couplings are used to join the waveguide to other transmission line components, such as miter bends, expansion units, and switches. All of these are analyzed to confirm structural integrity during operation.

Development of 28 GHz Gyrotron for Cooperative ECH Study

At the Plasma Research Center at University of Tsukuba, development of megawatt gyrotrons is being performed as a collaborative electron cyclotron heating (ECH) study with some research organizations. A 28 GHz 1 MW 1 s gyrotron has been developed to upgrade the GAMMA 10/PDX ECH systems. To improve the oscillation efficiency in high current regions, the magnetron injection gun (MIG) of the 28 GHz gyrotron has been modified. Output power of 1.25 MW has been achieved with this gyrotron. For the first step of the collaborative research between Tsukuba University and Kyushu University, the Tsukuba 28 GHz gyrotron was adapted to the Q-shu University Experiment with Steady-State Spherical Tokamak (QUEST) ECH system, and the plasma heating and current drive effect were demonstrated. We obtained successful results, including an electron cyclotron–driven plasma current of 66 kA in the QUEST plasma experiment. For the next step of the collaborative research, the design targets of a 2 MW 3 s and 0.4 MW continuous wave have been achieved in a design study of a new 28 GHz gyrotron.

Supply equipment to the new NBH system for the TCV tokamak

The TCV tokamak is equipped with a high power real time-controllable Electron Cyclotron Heating (ECH) system, made of 9 gyrotrons capable of 500kW RF power. One of the major scientific challenge, the TCV upgrades aimed to, will be the access to a wider range of temperature ratios up to Te/Ti∼1. For this purpose, the TCV heating system will be upgraded by the integration of a Neutral Beam Heating (NBH) system impacting directly on the ion temperature (Ti), while gyrotrons are used to vary mainly the electron temperature (Te). This NB Injector (NBI) will provide 1MW of neutral power during 2s into the TCV plasma, at a nominal energy of 30keV, for an electrical power installed of 2.2MVA. This paper describes the infrastructure required to implement successfully this novel NBI at CRPP premises.

Performance and Upgrades for the Electron Cyclotron Heating System on DIII-D

The electron cyclotron heating (ECH) system on the DIII-D fusion reactor consists of six 110-GHz gyrotrons with 6 MW installed power for pulses limited administratively to 5 s in length. The transmission coefficient is better than -1.1 dB for four of the transmission lines, which is close to the theoretical value. A new depressed collector gyrotron was recently installed and is injecting up to 720 kW of power into DIII-D during 2013 tokamak operations. Three of the four dual waveguide launchers, which can steer the RF beams ±20° both poloidally and toroidally, were used for real-time neoclassical tearing mode control and suppression with increased poloidal scanning speed up to 60°/s and positioning accuracy of the beams of ±2 mm at the plasma center. The ECH capabilities on DIII-D are being steadily updated, leading to increased experimental flexibility and high reliability of the system. In the past year, the ECH system reliability reached 87% for 2352 successful individual gyrotron shots into DIII-D. Planning is under way for the addition of two new depressed collector gyrotrons, one at 110 GHz, 1.2 MW and another at 117.5 GHz, 1.5 MW generated power, both of which are in the test stage at Communications and Power Industries.

Physics and Experimental Results of KSTAR ECH

Since the first plasma in Korea Superconducting Tokamak Advanced Research (KSTAR), the electron cyclotron heating (ECH) system has been an essential tool for ECH-assisted start-up using second-harmonic 84-GHz and 110-GHz ECH and for experimental studies of other physics issues such as edge-localized mode control, rotation control, sawtooth control, tearing mode control, and core impurity control for long-pulse discharge and noninductive start-up. The loop voltage in KSTAR is limited by superconducting ohmic coils and a thick vacuum vessel. The ECH-assisted start-up was useful to overcome burn-through during the ramp-up phase with limited loop voltage by the electron cyclotron beam injection before or after the onset of the inductive loop voltage to reduce resistive power consumption. Later, the 170-GHz ECH system is installed as a main electron heating and local current drive for the control of magnetohydrodynamic modes such as sawteeth and neoclassical tearing modes. The 170-GHz gyrotron is an ITER preprototype gyrotron developed by the Japan Atomic Energy Agency. Second-harmonic 170-GHz ECH-assisted start-up was also attempted with a raised toroidal magnetic field of 3 T in the 2011 KSTAR campaign, and flux saving in the ramp-up phase was observed. This lecture describes the physics issues and experimental results of the KSTAR ECH system. The present status and an upgrade plan of the KSTAR ECH system is also described.

Assessment of plasma parameters for the low activation phase of ITER operation

An assessment of ITER plasma parameters is carried out for the low activation phase that is required for commissioning the basic ITER systems including plasma control, heating and current drive. Such an operation is analysed for hydrogen, helium and deuterium plasmas for full field and current, as well as with magnetic field and plasma current reduced to half of their design values, B0 = 2.65 T, Ip = 7.5 MA. Both hydrogen and deuterium neutral beam injection (NBI) are considered. We assess the possible domain for safe operation, and the possible target plasmas for commissioning the NBI, electron cyclotron heating (ECH) and ion cyclotron heating (ICH) systems, taking into account the constraints imposed by NB shine-through loss, Greenwald limit and access to H-mode operation. Simulations with the Automated System for Transport Analysis (ASTRA) show that for 33 MW of NBI with 20 MW of ECH, H-mode access is marginal for hydrogen plasmas. Good H-mode confinement, expected at PNB + PEC + PIC > 1.5 PL–H, is more likely for the helium and deuterium cases. It is found that plasma parameters, such as normalized beta, plasma density and current flat-top duration, for full power/half field/half current operation can be similar to those required for the DT long pulse operation. Preliminary assessment is also made of the maximum of tritium and neutron yield achievable in a single shot at the deuterium phase of ITER operation.

Steady-state operation using a dipole mode ion cyclotron heating antenna and 77 GHz electron cyclotron heating in the Large Helical Device

The steady-state operation of high-performance plasmas in the Large Helical Device (LHD) has progressed since the 2010 IAEA Conference in Korea by means of a newly installed ion cyclotron heating (ICH) antenna (HAS antenna) and an improved electron cyclotron heating (ECH) system. The HAS antenna can control the launched parallel wave number and heat the core plasma efficiently in the case of dipole mode operation. Understanding of the physics and technology of wave heating, particle and heat flow balances, and plasma–wall interactions in LHD has also improved. The heating power of steady-state ICH and ECH exceeded 1 MW and 500 kW, respectively, and a higher density helium plasma with minority hydrogen ions was maintained using the HAS antenna and new 77 GHz gyrotrons. As a result, plasma performance improved, e.g. electron temperature of more than 2 keV at a density of more than 2 × 1019 m−3 became possible for more than 1 min. Heat flow balance and particle flux balance of steady-state operation were evaluated. Particle balance analysis indicated that externally fed helium and hydrogen particles were mainly absorbed by the chamber wall and divertor plates, even after the 54 min operation.

Electron Bernstein wave heating by electron cyclotron wave injection from the high-field side in LHD

In the Large Helical Device (LHD), evident electron Bernstein wave (EBW) heating was successfully performed. The experiment was carried out using the electron cyclotron heating (ECH) system that was upgraded by installation of high-power, long-pulse 77 GHz gyrotrons. The EBW heating was achieved by a mode conversion from injected EC wave to EBW, by the so-called slow-XB technique where an X-mode wave is injected to the plasma from the high magnetic field side. The specific magnetic configuration of LHD provides a good opportunity to realize the slow-XB technique, which is generally difficult for tokamaks. With the slow-XB technique, increases in kinetically evaluated electron energy Wpe and electron temperature Te were observed in overdense plasmas. An electron heating in the so-called super dense core plasma in LHD, which is characterized with an internal diffusion barrier and a steep density gradient at the plasma core, was successfully demonstrated in the plasma core region where the central electron density ne0 of 17 × 1019 m−3 was about 1.2 times higher, at the beginning of the EC-wave injection, than the left-hand cut-off density of applied 77 GHz EC waves.

Gyrotron and power supply development for upgrading the electron cyclotron heating system on DIII-D

An upgrade of the electron cyclotron heating system on DIII-D to almost 15MW is being planned which will expand it from a system with six 1MW 110GHz gyrotrons to one with ten gyrotrons. A depressed collector 1.2MW 110GHz gyrotron is being commissioned as the seventh gyrotron. A new 117.5GHz 1.5MW depressed collector gyrotron has been designed, and the first article will be the eighth gyrotron. Two more are planned, increasing the system to ten total gyrotrons, and the existing 1MW gyrotrons will subsequently be replaced with 1.5MW gyrotrons.Communications and Power Industries completed the design of the 117.5GHz gyrotron, and are now fabricating the first article. The design was optimized for a nominal 1.5MW at a beam voltage of 105kV, collector potential depression of 30kV, and beam current of 50A, but can achieve 1.8MW at 60A. The design of the collector permits modulation above 100Hz by either the body or the cathode power supply, or both, while modulation below 100Hz must use only the cathode power supply.General Atomics is developing solid-state power supplies for this upgrade: a solid-state modulator for the cathode power supply and a linear high voltage amplifier for the body power supply. The solid-state modulator has series-connected insulated-gate bipolar transistors that are switched at a fixed frequency by a pulse-width modulation regulator to control the output voltage. The design of the linear high voltage amplifier has series-connected transistors to control the output voltage, which was successfully demonstrated in a proof-of-principle test at 2kV. The designs of complete power supplies are progressing.The design features of the 117.5GHz 1.5MW gyrotron and the solid-state cathode and body power supplies will be described and the current status and plans are presented.

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

Difraction theory of quasi-optical diplexers

We propose a description of the diplexer, a device for control of high-power beams of millimeterwave radiation in electron-cyclotron heating systems for controlled fusion facilities. The diplexer is a four-mirror quasi-optical traveling-wave resonator, in which the transmission lines are connected by a pair of corrugated mirrors forming the resonator. In this paper, we develop a theory of such systems. An integral equation describing the beams of electromagnetic waves in the resonator in paraxial approximation and defining the diplexer characteristics with allowance for losses is deduced. The losses are due to the ohmic dissipation, diffraction, and limited size of the mirrors, as well as the difference between the widths and wave fronts of the diplexer resonator modes and the transmission lines. The integral equation is numerically solved by the iteration method using software with imaging. The eigenfrequencies of such a system with allowance for the vector character of the wave beams are found. Conditions under which the diplexer can be operated irrespective of the polarization of the incident radiation are determined.

Recent Results from the Development of the Electron Cyclotron Heating System for JT-60SA toward High-Power Long-Pulse Operations

Development of an electron cyclotron (EC) wave system was conducted in an effort to achieve the capability required in JT-60SA. Pulse duration at 1 MW output was extended to 31 s. Transmission line components with a diameter of 60.3 mm were installed in 2011 to reduce temperature rise during a gyrotron oscillation. Development of a dual-frequency gyrotron was started to enable heating and current drive in the core region of the JT-60SA plasma for the toroidal field of 2.3 T. The EC wave frequency was chosen to be 138 GHz to meet the requirements of physics experiments and gyrotron design. Fabrication of the gyrotron began in 2011.

Overview of FTU results

New FTU ohmic discharges with a liquid lithium limiter at IP = 0.7–0.75 MA, BT = 7 T and ne0 ⩾ 5 × 1020 m−3 confirm the spontaneous transition to an enhanced confinement regime, 1.3–1.4 times ITER-97-L, when the density peaking factor is above a threshold value of 1.7–1.8. The improved confinement derives from a reduction of electron thermal conductivity (χe) as density increases, while ion thermal conductivity (χi) remains close to neoclassical values. Linear microstability reveals the importance of lithium in triggering a turbulent inward flux for electrons and deuterium by changing the growth rates and phase of the ion-driven turbulence, while lithium flux is always directed outwards. A particle diffusion coefficient, D ∼ 0.07 m2 s−1, and an inward pinch velocity, V ∼ 0.27 m s−1, in qualitative agreement with Bohm–gyro-Bohm predictions are inferred in pellet fuelled lithized discharges. Radio frequency heated plasmas benefit from cleaner plasmas with edge optimized conditions. Lower hybrid waves penetration and current drive effects are clearly demonstrated at and above ITER densities thanks to a good control of edge parameters obtained by plasma operations with the external poloidal limiter, lithized walls and pellet fuelling. The electron cyclotron (EC) heating system is extensively exploited in FTU for contributing to ITER-relevant issues such as MHD control: sawtooth crash is actively controlled and density limit disruptions are avoided by central and off-axis deposition of 0.3 MW of EC power at 140 GHz. Fourier analysis shows that the density drop and the temperature rise, stimulated by modulated EC power in low collisionality plasmas are synchronous, implying that the heating method is the common cause of both the electron heating and the density drop. Perpendicularly injected electron cyclotron resonance heating is demonstrated to be more efficient than the obliquely injected one, reducing the minimum electric field required at breakdown by a factor of 3. Theoretical activity further develops the model to interpret high-frequency fishbones on FTU and other experiments as well as to characterize beta-induced Alfvén eigenmodes induced by magnetic islands in ohmic discharges. The theoretical framework of the general fishbone-like dispersion relation is used for implementing an extended version of the HMGC hybrid MHD gyrokinetic code. The upgraded version of HMGC will be able to handle fully compressible non-linear gyrokinetic equations and 3D MHD.

ECH system developments including the design of an intelligent fault processor on the DIII-D tokamak

A new generation fault processor is in development which is intended to increase fault handling flexibility and reduce the number of incomplete DIII-D shots due to gyrotron faults. The processor, which is based upon a field programmable gate array device, will analyze signals for aberrant operation and ramp down high voltage to try to avoid hard faults. The processor will then attempt to ramp back up to an attainable operating point. The new generation fault processor will be developed during an expansion of the electron cyclotron heating (ECH) areas that will include the installation of a depressed collector gyrotron and associated equipment. Existing systems will also be upgraded. Testing of real-time control of the ECH launcher poloidal drives by the DIII-D plasma control system will be completed. The ECH control system software will be upgraded for increased scalability and to increase operator productivity. Resources permitting, all systems will receive an extra layer of interlocks for the filament and magnet power supplies, added shielding for the tank electronics, programmable filament boost shape for long pulses, and electronics upgrades for the installation of the advanced fault processor.

Modeling of Electron Cyclotron Heating in Large Tokamaks and ITER with the 3-D Full-Wave Code STELEC

We present modeling results of basic electron cyclotron heating (ECH) scenarios in several tokamaks and ITER performed with the most recent version of the three-dimensional (3-D) full-wave STELEC code (stellarator ECH, including tokamaks as a special case). This code includes all basic wave physics such as interference, diffraction, wave tunneling, mode conversion to electron Bernstein waves at the upper hybrid resonance (UHR), and appropriate boundary conditions. The code solves the wave equations in real 3-D magnetic geometry and thanks to the use of massive parallel teraflop computers, it is the first to provide full-wave solutions of the problem in toroidal plasmas. Several important new results are thus obtained that cannot be predicted with codes based on ray-tracing techniques, such as the influence of diffraction effects and the importance of the UHR for both X- and O-mode antenna excitation at fundamental harmonic. This last result also shows that the so-called “O and X” modes are coupled solutions. The coupling of these modes, partly supported by experiments in the DIII-D tokamak showing similar heating efficiencies for both radiated modes, leads to different power deposition profiles and spatial distribution, compared to results from ray-tracing codes. Coupling between the O-mode and the X-mode (launched at the low-field side) reveals the importance of electron Bernstein waves in ECH calculations for high-density ITER plasmas. These results not only could influence the predictions for neoclassical tearing mode suppression for ITER using electron cyclotron current drive but could also lead to important simplifications in ECH hardware (converters, polarization, etc.) and to a reduced cost of the ECH system on ITER. The code also allowed investigation of the urgent issue of the O-X-B ECH scenario for overdense tokamak/stellarator plasmas.

Recent and future upgrades to the DIII-D tokamak

Research on the DIII-D tokamak focuses on support for next-generation devices such as ITER by providing physics solutions to key issues and advancing the fundamental understanding of fusion plasmas. To support this goal, the DIII-D facility is planning a number of upgrades that will allow improved plasma heating, control, and diagnostic measurement capabilities. The neutral beam system has recently added an eighth ion source and one of the beamlines is currently being rebuilt to allow injection of 5 MW of off-axis power at an angle of up to 16.5° from the horizontal. The electron cyclotron heating (ECH) system is adding two additional gyrotrons and is using new launchers that can be aimed poloidally in real-time by an improved plasma control system. The fast wave heating system is being upgraded to allow two of the three launchers to inject up to 2 MW each in future experiments. Several diagnostics are being added or upgraded to more thoroughly study fluctuations, fast ions, heat flux to the walls, plasma flows, rotation, and details of the plasma density and temperature profiles.

Electron cyclotron heating power supplies on DIII-D

The electron cyclotron heating system on DIII-D has been supporting experiments with six gyrotrons. The gyrotrons are connected to three power supplies: two have single modulators, each energizing two gyrotrons, while the third has three modulators that can energize three gyrotrons asynchronously. However, only two gyrotrons can be run synchronously due to the limitation of the high voltage dc power supply providing the input voltage to the modulators. These two configurations mimic proposed architectures of power systems for multiple gyrotron systems and demonstrate their advantages or disadvantages, which will be discussed in more detail. A fourth power system is being built as part of an upgrade to eight gyrotrons. The two future gyrotrons will have depressed collectors. A 1.2 MW gyrotron will be delivered in mid-2011 and the design of a 1.5 MW gyrotron has been initiated. This fourth power supply will have two modulators to independently energize the cathode of each gyrotron. Commercially available high voltage amplifiers will provide the body voltage. It will also have a solid-state crowbar instead of an ignitron crowbar. This power supply is described in more detail, as well as the testing performed on the solid-state crowbar to prove its performance.