Electron Cyclotron Heating System Research Articles

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.

Development of 28 GHz and 77 GHz, Mega-Watt Gyrotrons for Fusion Devices

A 28 GHz 1 MW with TE8,3 cavity for GAMMA10 tandem mirror and a 77 GHz 1.5 MW gyrotron with TE18,6 for Large Helical Device (LHD) have been developed to upgrade their Electron Cyclotron Heating (ECH) systems. In the 28 GHz gyrotron, the maximum power of 1.05 MW was obtained, which is in agreement with its design target value. And the high efficiency of 40% without collector potential depression (CPD) was obtained with 0.8 MW. In the first and second 77 GHz gyrotrons, 0.8 MW 3.6 sec., 0.3 MW 60 sec operations have been achieved, but several issues due to the stray RF and velocity dispersion have been found, too. In consideration of these, the design improvement was performed to aim at 1.5 MW in the third tube. In the short pulse test, the maximum output power of 1.6 MW and the maximum total efficiency of 49.4% with CPD were obtained. In the long pulse test, the pulse length extended to 5 sec with 1 MW, 1800 sec with 0.1 MW, 1.6 sec with 1.5 MW. Total injection power of 3.1 MW to LHD plasma has been achieved. It is shown that the electron beam pitch factor α decreases with increasing of beam current, comparing with the experimental and calculation results.

Recent Upgrades and Extensions of the ASDEX Upgrade ECRH System

The multi-frequency Electron Cyclotron Heating (ECRH) system at the ASDEX Upgrade tokamak employs depressed collector gyrotrons, step-tunable in the range 105–140 GHz. The system is equipped with a fast steerable launcher allowing for remote steering of the ECRH RF beam during the plasma discharge. The gyrotrons and the mirrors are fully integrated in the discharge control system. The polarization can be controlled in a feed-forward mode. 3 Sniffer probes for millimeter wave stray radiation detection have been installed.

Progress Toward Steady-State Operation in LHD Using Electron Cyclotron Waves

Trials of steady-state operation (SSO) in the Large Helical Device (LHD) were started when a continuous wave (cw) gyrotron with the output power up to 0.2 MW was introduced to the electron cyclotron heating (ECH) system on LHD in 2003. During the first trial of SSO in the seventh LHD experimental campaign in 2004, severe temperature increase on the waveguide transmission line and, as a result, intense pressure increase in the evacuated waveguide occurred, which terminated the operation at 756 s. Additional pumping sections and cooling structures on the transmission line worked well, allowing a 3900-s sustainment of plasma with ne = 0.15 × 1019 m−3 and Te0 = 1.7 keV by 0.1 MW injection power in 2005. The improvement of the ECH system by introducing cw gyrotrons with higher power for further improvement of plasma performance in SSO is in progress.Investigations on electron cyclotron current drive (ECCD) physics have been advanced a few years after the proof of ECCD in LHD. By obliquely injecting second-harmonic X-mode EC waves in toroidal direction, stable current up to 5.5 kA is driven, which was evaluated as a difference in plasma currents of the co- and counter-ECCD discharges with 0.1-MW EC wave power. It takes a few seconds for the driven current to saturate. Change in profile of rotational transform by ECCD and profile of driven current density are investigated by use of motional Stark effect measurement. Peaked and localized driven current profile at the plasma core region was confirmed for on-axis second-harmonic ECCD discharges.

Mode Content Study of Propagating Waves Using Burn Patterns in the KSTAR 84-GHz ECH System

In order to transmit a wave efficiently in an electron cyclotron heating (ECH) system, it is important to suppress mode conversion loss caused by coupling in the matching optics unit and misalignment in the transmission line. To understand the cause of mode conversion loss, it is necessary to analyze the mode content in an oversized circular corrugated waveguide. For mode analysis of the propagating wave in the corrugated waveguide, several methods based on the phase-retrieval process and the iterative process are suggested. But, in the Korea Superconducting Tokamak Advanced Research 84-GHz ECH transmission line, a well-known method using burn patterns was used for better coupling of the output beam from the gyrotron onto the axis of the corrugated waveguide by adjusting a large ellipsoidal mirror in an L-shaped chamber, a so-called L-box. During the adjustment of the mirror in the L-box, evidence of the existence of higher modes other than HE11 was found. For the mode content study, the radiation intensity distribution was measured using thermal paper as a function of the distance along the waveguide at a high power level. The mode content of the wave was estimated by comparing the measured burn patterns and calculated patterns at different locations. This paper describes the results of mode content estimation using burn pattern images as a function of the mode mixture ratio.

Calorimetric Measurements of the Radio-Frequency Power and of the Transmission Line Losses on the DIII-D ECH System

The measurement of the power injected by the electron cyclotron heating (ECH) system in the DIII-D tokamak is a critical requirement for analysis of experiments, for tuning the gyrotrons for maximum power and efficiency, for tracking long-term operational trends, and for providing a warning of problems with the system. The ECH system at General Atomics consists of six 110-GHz, 1-MW-class gyrotrons. The radio-frequency (rf) power generated by each gyrotron is determined from calorimetry, using the relevant temperature and flow measurements from the cooling circuits of the cavity, matching optics unit, and dummy loads (DLs). The rf pulse length and time dependence are measured using an rf monitor at the first miter bend in the transmission line. The cavity power loading measured directly gives the generated rf power using a previously determined relationship between cavity loading and rf production. The direct measurement of the efficiencies of four of the transmission lines was performed using a high-power DL placed alternately in two positions of each DIII-D waveguide line, at accessible points close to the beginning and the end of each line. Total losses in the transmission lines range from 21.2 to 30.7%. Experimental results are compared to theoretical predictions of the performance of the components and waveguide lines.

Commissioning of KSTAR 84-GHz ECH Transmission System

An 84-GHz electron cyclotron heating (ECH) system has been installed to assist plasma start-up by preionization in the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The KSTAR 84-GHz ECH system consists of a 500-kW gyrotron, a transmission line, and an antenna system. The wave power is transmitted from the gyrotron to the antenna through an evacuated corrugated circular waveguide of 31.75-mm inner diameter and six miter bends, which include a pair of polarizer miter bends for polarization control. The maximum permitted vacuum pressure without radio-frequency (rf) breakdown in the 31.75-mm waveguide at 84 GHz, 500 kW was calculated to be ~0.1 torr. The pumping time to reach the vacuum pressure of 1 × 10-3 torr in the KSTAR ECH system was ~2 h by two turbomolecular pumps. The transmission efficiency of ~93% from the output of the mirror optical unit to the torus window was measured using a low-power rf source. The wave polarization by a pair of polarizer miter bends with grooved mirrors was tested using the low-power system, and it showed good agreement with numerical calculations. In this paper, we present the design and commissioning results of the KSTAR 84-GHz transmission line.

Power Combination of Two 140-GHz Gyrotrons and Fast Switching of the Combined Beam

Experiments on the combination of the high-power wave beams from two gyrotrons and fast switching of the combined beam between two transmission channels are described. The measurements were performed using a high-power resonator diplexer in the optical transmission line of the electron cyclotron heating system for W7-X. The principle and the engineering design of the prototype four-port quasi-optical diplexer is presented. The wave beams from two gyrotrons with output powers of 370 and 560 kW, respectively, have been combined for pulse lengths up to 10 s, limited only by the uncooled mirrors used in the diplexer. By modulating the gyrotron frequency using a fast high-voltage body modulator, controlled toggling of the combined power between the two outputs of the diplexer was demonstrated with switching frequencies of up to 20 kHz.The experiments are compared to theory, showing good agreement when the limited stability of the free-running gyrotrons is taken into account.

Real-time feedback control of millimeter-wave polarization for LHD

Electron cyclotron heating (ECH) is widely used in magnetic fusion devices, and the polarization of the injected millimeter-wave beams plays a crucial role in the propagation and absorption of the beam energy by the plasma. This polarization can be adjusted by grating mirror polarizers placed in the transmission lines which carry the microwaves from the power source to the plasma. In long-pulse devices such as the Large Helical Device (LHD) and ITER, it is desirable to track changes in the plasma and adjust the polarization of the ECH in real time such as to keep the absorption as high as possible and avoid shine-through which may lead to overheating of vessel components. For this purpose a real-time feedback control scheme is envisioned in which a measure of the absorption efficiency can be used to adjust the orientation of the polarizing mirrors toward an optimum. Such a setup has been tested in a low-power test stand as preparation for future implementation in the LHD ECH system. It is shown that a simple search algorithm is efficient and can in principle be used to control either the absorption efficiency or the linear polarization angle.

Experience in integrated control of the multi-megawatt electron cyclotron heating system on the TCV tokamak: the first decade

The ECH system on the TCV tokamak consists of six gyrotrons (82.6 GHz/0.5 MW/2 s) used for X2 and electron Bernstein wave (EBW) ECH/ECCD with individual low-field-side launchers. Three additional gyrotrons (118 GHz/0.5 MW/2 s) are used for X3-ECH in a top-launch configuration to provide central heating of high-density plasmas, at nearly 3 times the cutoff density of X2. The X2 subsystem was installed by the end of 1999 and the X3 subsystem by the end of 2003, making 4.2 MW available for experiments. The installation work provides data related to testing, repair and reliability of a complex ECH system designed to allow the highest possible degree of automation, integration and flexibility in the experimental programme. Its effective integration into the TCV plant is evidenced by the fact that the mean time between shots when operating with ECH increases roughly in proportion to the increase in the resources required to prepare, monitor and record the experimental sessions. Each of the X2 and X3 subsystems is routinely individually operated by one person. This gives confidence that with proper layout, planning and integration, the EC systems of future fusion experiments, such as ITER, can routinely provide reliable actuators, on demand.

Progress in the 10-MW 140-GHz ECH System for the Stellarator W7-X

During the last years, electron cyclotron heating (ECH) was proven to be one of the most attractive heating schemes for stellarators because it provides net-current-free plasma startup and heating. Both the stellarator Wendelstein 7-X (W7-X), which is under construction at the Max-Planck-Institut fu umlr Plasmaphysik, Greifswald, Germany, and the International Thermonuclear Experimental Reactor (ITER) tokamak, which will be built in Cadarache, France, will be equipped with a strong ECH and current-drive system. Both systems are comparable in frequency and have continuous-wave capability (140 GHz, 10 MW for W7-X and 170 GHz, 24 MW for ITER). The commissioning of the ECH plant for W7-X is well underway; the status of the project and the first integrated full-power test results from two modules are reported and may provide valuable input for the ITER plant. The ten gyrotrons at W7-X will be arranged in two subgroups symmetrically to a central beam duct in the ECH hall. The RF wave of each subgroup will be combined and transmitted by a purely optical multibeam-waveguide (MBWG) transmission line from the gyrotrons to the torus. The combination of five 1 MW gyrotron beams to one beam line with a power of 5 MW reduces the complexity of the system considerably. The single- and MBWG mirrors and the polarizers have been manufactured. Cold tests of a full-size uncooled prototype line delivered an efficiency exceeding 90%. The microwave power will be launched to the plasma through ten chemical-vapor-deposited-diamond barrier windows and in-vessel quasi-optical plug-in launchers, allowing each 1-MW RF beam to be steered independently. The polarization, as well as the poloidal and toroidal launch angles, will be adjusted individually to provide optimum conditions for different heating and current-drive scenarios.

High-Power Millimeter-Wave Calorimetric Beam Absorbers

Beam absorbers play an important role both in electron cyclotron heating systems at high power and in millimeter-wave diagnostics that need a low level of stray or reflected power. In the first case short- and long-pulse loads are used, whose back-reflection can be kept within a few percent with proper techniques. In the second case, absorbers or scramblers are envisaged, to be put in hostile environments. At Istituto di Fisica del Plasma in Milan, a number of calorimetric loads have been developed, adopting several techniques for overall reflectivity reduction, which are suitable for beam sinking with calorimetric capability. They achieve a low overall reflectivity and high-power capability by a properly chosen power distribution in the absorbing wall provided by a dispersing mirror, by a smooth geometrical shape, by heat-resistant absorbing coatings of optimized thickness, and by accurate trapping of most of the escaping radiation with preload structures. Fundamental, when it becomes impossible to diffuse the incoming beam by the mirror alone, mostly because of side lobes at large angles, is the use of a newly developed phase-scrambling surface presented in this paper. It provides the necessary spreading, complementing all the other techniques into a set that can be helpful in designing millimeter-wave systems and diagnostics, in order to reduce spurious or unwanted signals.

Dynamic Transport Study of Electron Thermal Energy in Nonlinear Fusion Plasma

In nuclear fusion plasmas, both thermal energy and particle transports governed by plasma turbulence are anomalously enhanced above neoclassical levels. Plasma turbulence induces various complex phenomena in transport processes, such as nonlinearity and nonlocality. Therefore, it is very important to clarify the relationship between plasma turbulence and anomalous transports. We have approached these complicated problems by analyzing the dynamics, which are recognized as temporal trajectories in a flux-gradient space, rather than using conventional power balance. In particular, in fusion research, it is critical to elucidate the mechanism of electron thermal energy transport, because the incoming burning plasmas are sustained by the heating of alpha particles. In Large Helical Device (LHD), the dynamic relationships between electron thermal fluxes and electron temperature gradients are investigated using modulated electron cyclotron heating and modern electron cyclotron emission diagnostic systems. Some trajectories, such as a hysteresis loop and a line segment with a steep slope, are observed in high-temperature LHD plasmas. Strong nonlinear properties in the transport are revealed by studying the dynamics.

Possible Improvements to a Remote Steering Launcher for Localized Electron Cyclotron Current Drive

The remote steering (RS) system (C. P. Moeller, Proc. 23rd Int. Conf. Infrared and Millimeter Waves, September 7–11, 1998, University of Essex, pp. 116–118) provides a method of steering a millimeter-wave beam for electron cyclotron heating (ECH) and current drive (CD) applications without having moveable mirrors close to the plasma. The input beam is coupled into a square corrugated waveguide having a length such that the phase and amplitude profile of the input beam is repeated at the waveguide output. In the most basic implementation, by injecting the input beam at an angle relative to the waveguide axis, the output beam is radiated at that same angle. The steering range (typically to ± deg) and the focusing ability are strongly limited because of the restricted space for the launcher in a fusion device, which results in a large deposition profile in the plasma. However, the waveguide and optical arrangement can be modified to either increase the steering range and/or focus the RS system. For example, if a converging beam is injected into the waveguide, the output beam’s waist will be projected far from the waveguide aperture. Likewise, a tapered square waveguide can be used to increase the scanning range of the RS system beyond that of ± deg. This paper will investigate such hybrid designs of the RS launcher, providing alternative configurations for optimizing the launching configuration, depending on the requirements of a given ECH and CD system.

Transmission Line Technology for Electron Cyclotron Heating

We briefly review the history of transmission line technology for electron cyclotron heating (ECH) applications and then survey the major developments over the past few years. These developments are grouped by function. Papers in this special issue are highlighted. We concentrate on the transmission from a matching optics unit near a gyrotron microwave source to the location of a vacuum window near the plasma, without review of window and launcher technology. Prospects for components handling more than 1-MW continuous wave are reviewed. While both waveguide and free-space propagation are considered, a greater emphasis is placed on corrugated waveguide components in preparation for the ITER ECH system.

Electrical Powering Concept for ITER Electron Cyclotron Radio-Frequency Sources

The ITER electron cyclotron heating (ECH) baseline scheme (2001) is composed of 24 gyrotrons, each generating 1 MW of radio-frequency power at 170 GHz in addition to 3 gyrotrons (1 MW) at 120 GHz for breakdown assist. Considering an efficiency of ≥45%, which can be achieved with the depressed-collector–type gyrotron (CPD), the amount of electrical power would be in the range of 55 MW. This paper has two purposes. First, it presents requirements that will be imposed on the electrical power supplies with regard to the updated physics needs for ITER presently being discussed. Demanding parameters (like modulation capability, transient margin, and fault clearing) will be described. In this context, the consequences of those new requirements on the technical choices and the impact on the complexity of the power supplies will be discussed. Second, two possible schemes for the ITER reference power supply design for the ECH system will be compared. The advantages (and respective disadvantages) of each solution will be highlighted taking care of the requirements previously presented. In conclusion, a proposal is presented for a revised ECH power supply structure.

Study on Pre-Ionization Using Second-Harmonic Electron Cyclotron Waves for the KSTAR First Plasma

Korea Superconducting Tokamak Advanced Research (KSTAR) is adopting electron cyclotron heating (ECH)-assisted startup for the flexibility and reliability of KSTAR operation through preionization by using EC waves. An 84-GHz ECH system is being installed on the KSTAR tokamak, and the maximum available ECH power is presently 500 kW with a maximum duration of 2 s. A plasma breakdown voltage (or loop voltage) in the range of 4 V to 5 V is needed when fundamental harmonic EC power is used. Currently, second-harmonic ECH-assisted start-up is under consideration because a low toroidal field of BT ∼ 1.5 T is desirable for safety and because the high-beta experiments will be done in the initial operation phase. There have been limited studies of the efficacy of plasma breakdown by second-harmonic pre-ionization. The studies reported in this paper address the effectiveness of second-harmonic pre-ionization by 84-GHz EC-waves and alternatives to it for KSTAR. The optimum and threshold conditions for its successful start-up are calculated using a 0-dimentional time-dependent plasma-evolution code. The feasibility of second-harmonic breakdown of the KSTAR plasma is studied based on the recent experimental results in DIII-D tokamak on second-harmonic breakdown where 60 GHz and 110 GHz EC power was used. In this feasibility study, the ionization growth rate in G. L. Jackson’s theoretical model, which was used for the comparison with International Thermonuclear Experimental Reactor (ITER) second-harmonic pre-ionization, is also compared with KSTAR second-harmonic pre-ionization based on the DIII-D experimental results.

Development of a feedback system to control MHD instabilities in ASDEX Upgrade

An automatic feedback control system (FCS) is presently being developed for the tokamak ASDEX Upgrade. The FCS will be used to control and/or suppress magneto-hydro-dynamic instabilities, in particular neoclassical tearing modes and sawteeth, using its new, multi-frequency, electron cyclotron heating system, which allows the on-line fast steering of the poloidal injection angle. The FCS is conceived to couple with different time scales. The general features, strategy and first tests for a reliable controller are presented.

Simulation of the hybrid and steady state advanced operating modes in ITER

Integrated simulations are performed to establish a physics basis, in conjunction with present tokamak experiments, for the operating modes in the International Thermonuclear Experimental Reactor (ITER). Simulations of the hybrid mode are done using both fixed and free-boundary 1.5D transport evolution codes including CRONOS, ONETWO, TSC/TRANSP, TOPICS and ASTRA. The hybrid operating mode is simulated using the GLF23 and CDBM05 energy transport models. The injected powers are limited to the negative ion neutral beam, ion cyclotron and electron cyclotron heating systems. Several plasma parameters and source parameters are specified for the hybrid cases to provide a comparison of 1.5D core transport modelling assumptions, source physics modelling assumptions, as well as numerous peripheral physics modelling. Initial results indicate that very strict guidelines will need to be imposed on the application of GLF23, for example, to make useful comparisons. Some of the variations among the simulations are due to source models which vary widely among the codes used. In addition, there are a number of peripheral physics models that should be examined, some of which include fusion power production, bootstrap current, treatment of fast particles and treatment of impurities. The hybrid simulations project to fusion gains of 5.6–8.3, βN values of 2.1–2.6 and fusion powers ranging from 350 to 500 MW, under the assumptions outlined in section 3. Simulations of the steady state operating mode are done with the same 1.5D transport evolution codes cited above, except the ASTRA code. In these cases the energy transport model is more difficult to prescribe, so that energy confinement models will range from theory based to empirically based. The injected powers include the same sources as used for the hybrid with the possible addition of lower hybrid. The simulations of the steady state mode project to fusion gains of 3.5–7, βN values of 2.3–3.0 and fusion powers of 290 to 415 MW, under the assumptions described in section 4. These simulations will be presented and compared with particular focus on the resulting temperature profiles, source profiles and peripheral physics profiles. The steady state simulations are at an early stage and are focused on developing a range of safety factor profiles with 100% non-inductive current.

Status of KSTAR Electron Cyclotron Heating System

An 84-GHz, 500-kW electron cyclotron (EC) heating (ECH) system is under installation for ECH-assisted start-up in the Korea Superconducting Tokamak Advanced Research (KSTAR) facility. An 84-GHz, 500-kW gyrotron, and 1.5-MVA power supply system have been installed at KSTAR, and the initial test of the gyrotron has been carried out with a short-pulse condition of 20 μs and maximum beam parameters of 80 kV and 25 A that generate an output radio-frequency (rf) power of 500 kW. The planned 2-s-long operation with 500-kW rf output power is beginning with a long-pulse test of the gyrotron power supply. The launcher system was fabricated in collaboration with Princeton Plasma Physics Laboratory. It will inject 500-kW rf power into the KSTAR plasma with a highly flexible steering mirror system, allowing toroidal and poloidal beam deposition scans. KSTAR will employ 170-GHz EC current drive (CD) in ITER-relevant experiments such as the suppression of the neoclassical tearing modes and the creation of an electron internal transport barrier. The Japan Atomic Energy Agency will provide a 170-GHz, 1-MW gyrotron on loan in 2008 in accordance with a Korea-Japan fusion collaboration agreement, and it will be used for the 170-GHz, 1-MW ECCD system in 2010. This paper describes the current status of the installation and initial conditioning tests of the 84-GHz gyrotron system as well as the development plan of the 170-GHz ECH and CD system. Also, this paper discusses the CD efficiency and the steering range of the second-harmonic X-mode injection at 170 GHz and 5 MW from an equatorial launcher.