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Electron cyclotron (EC) resonance heating in the T-15MD tokamak is calculated using the GENRAY code. The injection schemes without current generation for heating plasma using an extraordinary wave with a frequency of 140 GHz at the third harmonic of the fundamental EC frequency are considered. The cases of the wave injected through vertical, upper inclined, and equatorial port-plugs for three scenarios of the T-15MD operation—the baseline configuration and two configurations with different moderate values of plasma elongation and triangularity—are analyzed. A comparison of calculations performed using the GENRAY code with the published calculations based on the OGRAY code shows that the efficiency of EC heating in single-pass absorption and the spatial position of the absorbed power maximum are in good agreement, while the shape of the spatial profile of the absorbed power can differ significantly. [ABSTRACT FROM AUTHOR]

This article presents the first results of calculations of power contribution of electron cyclotron heating power and driven current in the DEMO-S and DEMO-FNS facilities for their design parameters and model distributions of temperature, density, and effective plasma charge. The calculations were based on OGRAY code for in-axis and off-axis power contribution. It has been demonstrated that, for electron cyclotron heating and current drive in the central part of plasma thread in the DEMO-FNS, the wave sources are required to operate in the frequency range available in modern experiments: ~140–160 GHz. The off-axis power contribution in the DEMO-FNS in calculations is provided by displacement of resonance region toward the high field side. It has been shown that, in order to generate EC current at ρ ~ 0.65 in the equatorial plane at the high field side, the wave frequency should be increased to ~185 GHz. In order to heat plasma and to generate current using EC waves in the DEMO-S the EC radiation sources are required to operate in the range of 200–230 GHz. In the regimes with low plasma temperature (Te(0) < 10 keV) in both facilities, the fraction of single-pass absorption upon off-axis power contribution has been lower than 50%, which requires further optimization of the power contribution and, probably, discharge scenario. [ABSTRACT FROM AUTHOR]

Results of experiments carried out with a quasi-stationary L-2M stellarator in the regime of electron cyclotron resonance heating by means of two gyrotrons are presented. The data on increase in the plasma energy lifetime by means of modulation (profiling) of the microwave pulse are obtained. The first gyrotron operating at fixed power was used for ionization and initial heating of plasma, while the second gyrotron initiated a stationary discharge with a duration of 10 ms. It is demonstrated that the lifetime can be increased by varying the second-gyrotron power in the range of 50–200 kW. A fourfold increase in the lifetime was obtained when the second-gyrotron power was reduced to 50 kW. This work is of interest as a method of studying hot plasma confined in the toroidal magnetic system of a stellarator. [ABSTRACT FROM AUTHOR]

• The work in the paper is the investigation of microwave transmission modes in the transmission line on HL-3 tokamak relevant the ECRH system. • A specific high power test platform with about 40-meter transmission line is established and the mode contents in the middle and the end of the transmission line are obtained through phase retrieval method from intensity measurement. • In the middle of the transmission line, the proportion of LP 01 mode is about 97 % and other high-order modes contents are <3 %. In the end of the transmission line, the main mode purity is about 94 %. • The work indicates that the designed and machined corrugated waveguides transmission line, and the alignment method for long distance transmission line installation is highly useful for high-purity wave mode transmission for the ECRH system on HL-3. The transmission modes of high-power millimeter wave in corrugated waveguides transmission line (TL) are crucial, especially for the long-pulse operation for long-distance TL. The microwave transmission mode in the TL on HL-3 tokamak relevant the electron cyclotron heating system are studied for the purpose according to the phase retrieval method based on intensity distribution measured by an infrared camera at several different locations away from the outlet of TL. A specific high power test platform with about 40-meter TL is established and the mode contents in the middle and the end of the TL are analyzed. In the middle of the TL, the proportion of main transmission mode LP 01 mode is about 97 % and other high-order modes contents are <3 %. In the end of the TL, the main mode purity is about 94 %, and the high-order mode, LP 11 even mode is excited about 4.6 %. These mode analysis results indicate that the high-power and long-pulse millimeter wave corrugated waveguides TL are designed and machined well, and the alignment method for long distance transmission line installation is highly useful for high-purity wave mode transmission. [ABSTRACT FROM AUTHOR]

The plasma current start-up experiment is conducted through electron cyclotron (EC) heating in the QUEST spherical tokamak. During the EC heating, the application of a toroidal electric field in the opposite direction to the plasma current effectively inhibits the growth of energetic electrons. Observations show rapid increases in plasma current and hard x-ray count immediately following the cancellation of the retarding electric field. When a compact tokamak configuration maintains equilibrium on the high field side, along with the retarding field, it leads to effective bulk electron heating. This heating achieved an electron temperature of T _e ≈ 1 keV at electron density n _e > 1.0 × 10 ^18 m ^−3 . Ray tracing of the EC wave verifies that more power absorption into plasma through a single-pass occurs around the second resonance layer with higher values of electron density and temperature.

Electron cyclotron resonance heating is one of the methods of additional plasma heating in the T-15MD tokamak. The article considers the possibility of plasma heating using an extraordinary wave at the third harmonic of electron cyclotron resonance at a frequency of 140 GHz. It has been shown that, in accordance with the calculations by the quasi-linear OGRAY code, a high fraction of single-pass absorption (up to 96%) can be expected even when the ECR is turned on with an ohmic background. Increase in this fraction with the increase in plasma temperature agrees with the theoretical predictions. Vertical power launch seems to be the most attractive. Possible engineering solutions for implementation of such a scheme are proposed. [ABSTRACT FROM AUTHOR]

Transient coaxial helicity injection (CHI) is a promising candidate for solenoid-free plasma current start-up in a low-aspect-ratio tokamak in support of developing fully non-inductive scenarios. The aim of the transient CHI research on QUEST is to develop a reactor-relevant CHI design. On QUEST, a CHI discharge is initiated by driving current along magnetic field lines that connect an electrically insulated electrode plate (which is referred to as a bias electrode) to a vessel component at the bottom region of the spherical tokamak. In the first application of CHI on QUEST, the electrically insulated electrode plate was biased with respect to the outer vessel in a configuration referred to as low-field-side (LFS) injection. To maintain a narrow injector flux footprint width throughout the discharge, QUEST is now developing a high-field-side (HFS) injection configuration, in which the electrically insulated electrode plate is biased with respect to the inner vessel components. Through the implementation of a CHI-dedicated gas injection system, studies in the HFS injection configuration have now demonstrated good magnetic flux evolution into the vacuum vessel. Toroidal currents up to 43 kA have been achieved, and the generated current has increased in proportion to the magnetic flux connecting the electrodes. These results which show agreement with the CHI-scaling suggest that much higher levels of toroidal current can be generated on QUEST in an optimized CHI system in which the magnetic flux connecting the CHI electrodes is further increased. [ABSTRACT FROM AUTHOR]

Electron cyclotron heating is one of the methods of additional plasma heating and noninductive current drive in the T-15MD tokamak. One to eight gyrotrons with a total power up to 7 MW will be used. The frequency of the microwave used depends on the parameters of the experimental scenarios in which the heating will be used and on the physical tasks for which it is intended. This article provides justification for the choice of the gyrotron frequency f1 ≅ 82.6 GHz for the start-up phase and for the first plasma experiments and considers the possibility of using this frequency at the stage of full-scale experiments. In order to provide flexibility of the Т-15MD ECH system, it is proposed to consider the possibility of the development of a gyrotron complex based on two-frequency gyrotrons with f1 ~ 82.6 GHz and f2 in the range of ~102–110 GHz. [ABSTRACT FROM AUTHOR]

Theoretical and experimental investigation of broadband step-frequency tuning in a prototype sub-terahertz gyrotron for advanced plasma fusion research is presented. The possibilities of successive excitation of various operating modes in a conventional gyrotron cavity are considered. Restrictions of the gyrotron capability due to properties of the electron beam formation and the radiation output systems are determined and discussed. Successful excitation of modes with frequencies range of 133–250 GHz with output power not less than 35 kW was demonstrated in experiments. [ABSTRACT FROM AUTHOR]

The Materials Plasma Exposure eXperiment (MPEX) is a steady-state linear plasma device that will address plasma-material interaction (PMI) science and enable testing of fusion reactor-relevant divertor plasma-facing materials. The MPEX source concept consists of a helicon plasma source to generate the plasma, electron cyclotron heating (ECH) for electron heating, and ion cyclotron heating (ICH) for ion heating. The MPEX source plasma is then transported axially to the PMI material target region to test material samples in fusion reactor-relevant divertor conditions. This paper will summarize the physical and technical basis of MPEX. The paper will first define the MPEX parameters and scenarios at the target relevant to PMI science for various fusion reactor-relevant divertor conditions and show plasma transport modeling results to set the MPEX source parameters. Recent experimental and modeling results from Proto-MPEX, a short-pulse experiment to develop the plasma production, heating, and transport physics for MPEX, will be shown. From these results, it will be shown that MPEX can reach its desired scenarios. The MPEX physical and technical basis will also determine important functional requirements for magnetic field, radiofrequency (RF) power, RF frequency, and neutral pressure in the helicon, ECH, ICH, and PMI regions that are required to achieve the desired MPEX scenarios. The necessity for key in-vessel components such as skimmers, limiters, and microwave absorbers will also be highlighted.

• After a thorough research on the underlying physics, mathematical formulations and Computational Electromagnetic Methods, a complete radiofrequency electromagnetic analysis methodology has been developed to cover the specific needs of the ITER Electron Cyclotron Upper Launchers. • A key aspect of this methodology is its effective integration with the Upper Launcher end-to-end analysis process, enabling coupling the electromagnetic analysis with the thermo-structural analysis in order to consider the impact of the mirrors deformation on the deviations and distortions of the electromagnetic beams in subsequent analysis loops. • The proposed methodology covers all the aspects that require specific assessment – the main optical path analysis (ohmic losses), the spillover and stray radiation, together with the effect of the High Order Modes (HOM) – and is aimed to obtain the parameters of the electromagnetic beams injected into the plasma, but also to verify the structural integrity of mirrors and structures against the thermal loading. Framed in the Final Design of ITER Electron Cyclotron Upper Launchers, a methodology has been developed for performing robust, reliable and efficient electromagnetic analysis of the millimeter waves and their interaction with the quasi-optical elements, launcher structures and finally the plasma. After a thorough research on the physics, mathematical formulations and Computational Electromagnetic Methods, a complete RF-EM Analysis procedure has been defined, tailored to the specific needs of this project. Special focus has been placed on the integration of this electromagnetic analysis in the end-to-end analysis cycle of the Launcher. The integrated procedure allows coupling the electromagnetic analysis with the thermo-structural analysis, so perturbations of the electromagnetic beams due to mirrors deformation can be accounted for. This methodology is aimed to obtain the parameters of the electromagnetic beams injected into the plasma, but also to verify the structural integrity of mirrors and structures against ohmic heating and stray radiation. [ABSTRACT FROM AUTHOR]

The design of the Transmission Line (TL) as a part of the Electron Cyclotron Heating (ECH) system for Divertor Tokamak Test facility (DTT) is approaching the conceptual design maturity. With an ECH system of 16 MW installed for the first phase and with a total of 32 gyrotrons (170 GHz, ≥ 1 MW, 100 s) the TL design is undertaking the challenge of an evacuated Multi-Beam TL (MBTL) concept to deliver the large number of beam lines from the gyrotron hall to the torus hall buildings. The system is organized in 4 clusters, each of them including 8 beamlines. The routing consists of single-beam TL section used to connect the gyrotron output to a beam-combiner mirror unit for each cluster, a common MBTL running in a suspended corridor reaching the Tokamak building and a beam-splitter mirror unit to connect to the ex-vessel optics and launchers sections located in the equatorial and upper ports of one sector, for a total of 4 DTT sectors. The TL mirrors will be actively water cooled to cope with the heat load in long pulses due to the high power incident radiation, with the possibility to include advanced concepts for the cooling design compatible with additive manufacturing technology. The characteristics of the system and its components are presented, showing both the progress of the adopted solutions and the current design. Since the main challenge of this TL is to maintain the overall losses below 15%, in this paper we present the expected ohmic and spillover losses, including beam coupling simulations evaluating losses given by high order Transverse Electro-Magnetic modes (i.e. aberrations). We describe how the effects have been estimated with electromagnetic simulations and how losses could be mitigated, since TL efficiency could significantly drop due to the presence of non-idealities, like the deformations of mirrors surface ascribed to the microwaves heat loads and possible misalignments and aberrations effects occurring along the line. [ABSTRACT FROM AUTHOR]

In anticipation of an Electron Cyclotron Resonance Heating system upgrade for the stellarator Wendelstein 7-X, a 1.5 MW – 140 GHz continuous-wave gyrotron is under development. In order to provide a first experimental verification of the scientific RF and electron beam optics design of the gyrotron with ms pulses, the Karlsruhe Intitule of Technology has developed a short-pulse pre-prototype gyrotron. In this work, we present details regarding the construction of the pre-prototype as well as measurements from the first experimental campaign delivering up to 1.6 MW in short pulses. [ABSTRACT FROM AUTHOR]

In the T-10 tokamak, the results of plasma heating at the first and second electron cyclotron (EC) harmonics differ substantially. However, the problem is multi-parametric, so it is impossible to estimate the heating quality by sight. In this work, the analysis of heating at both harmonics is provided with two transport models, which were constructed separately for simulation in each mode. For the heating on the second harmonic, we solve the inverse problem of determining the profile of the absorbed power from the experimental electron temperature profile. In this mode, we are able to determine both the deposited power profile and the ratio of absorbed to deposited power. It is shown that at low plasma densities, this ratio is small, which, apparently, is a consequence of the low absorption of the heating waves during their single pass though the plasma. During the heating at the second harmonic, the absorbed power profile is much flatter than predicted by ray tracing calculations of EC waves. Multi-pass wave absorption with random reflections from the corrugated chamber wall leads to such absorption profiles. At high plasma densities, the absorbed power is close to the input power. However, in this case, the results at the second harmonic are worse than the results obtained at the first harmonic due to the flatter absorbed density profile. [ABSTRACT FROM AUTHOR]

• New high voltage power supply (PS) for a klystron has been developed. • This PS is used for the electron cyclotron heating (ECH). • 8.56 GHz-RF with the PS will be a promising ECH system in QUEST spherical tokamak. • An IGBT and a reactor can shut down the PS quickly to protect the klystron. • AC switching has been set up for the overload and overheat of the PS. A high voltage DC power supply for the cathode of the 8.56 GHz CW klystron has been set up for electron cyclotron heating (ECH) in steady state tokamak operation on QUEST spherical tokamak. The power supply is equipped with an IGBT array and a reactor for fast shutoff of the voltage in 10 μs, where the influx of electric energy at the short circuit is limited to 5 J. AC switches also have been installed in the three-phase power lines. High voltage relays are useful to save electric energy consumption. Fast three-phase AC switching by IGBT-stack is applicable to reduce the electric load of the components of the klystron power supply. [ABSTRACT FROM AUTHOR]

The ITER ECRH system consists of 24 gyrotrons with up to 24 MW installed millimeter wave heating power at 170 GHz, power supplies, control system, transmission lines, one Equatorial and the four Upper Launchers. With its high frequency and small beam focus the ECRH has the unique capability of driving locally current. While the Equatorial Launcher mainly acts for central heating and current profile shaping, the Upper Launchers aim on suppressing MHD instabilities, especially neoclassical tearing modes (NTM) triggering plasma disruptions. The Upper Launchers inject millimeter waves through a quasi-optical section consisting of three fixed and the front steering mirror set. The eight overlapping beams have focal points optimized for suppression of the q = 3/2 and q = 2/1 NTMs. Several project change requests required the redesign of the Upper Launchers and the connected ex-vessel system. This redesign includes a new boundary geometry of the launchers as well as a newly designed cooling system for the Blanket Shield Module (BSM), a modified flange of the BSM to the structural main frame and a refined optical design. Additionally shield blocks with integrated in-vessel waveguides were added and the closure plate with waveguide and supply line feedthroughs was adapted. Further changes, not all caused by PCRs, include newly designed ex-vessel waveguide components with a reduced aperture and redesigned ultra low-loss CVD diamond windows. Finally several components originally foreseen as off-the-shelf components have become part of the design scope. The new launcher design status is presented with selected results on numerical design validation. [ABSTRACT FROM AUTHOR]

A new code framework, MORAMC, is presented which model transmission line (TL) systems consisting of overmoded circular waveguide and other components including miter bends and transmission line gaps. The transmission line is modeled as a set of mode converters in series where each component is composed of one or more converters. The parametrization of each mode converter can account for the fabrication tolerances of physically realizable components. These tolerances as well as the precision to which these TL systems can be installed and aligned gives a practical limit to which the uncertainty of the microwave performance of the system can be calculated. Because of this, Monte Carlo methods are a natural fit and are employed to calculate the probability distribution that a given TL can deliver a required power and mode purity. Several examples are given to demonstrate the usefulness of MORAMC in optimizing TL systems. [ABSTRACT FROM AUTHOR]

The interaction cavity of the European 170 GHz, 1 MW Continuous Wave (CW) gyrotron for ITER, which could also be water-cooled using mini-channels as recently proposed, experiences during operation a very large heat load (>15 MW/m 2 ) localized on a very short (<1 cm) axial length. Such heat loads are typical for high power gyrotrons. As the thermal deformation of the cavity influences the electromagnetic field structure and consequently the gyrotron operation, the analysis of the cavity performance requires the simultaneous solution of the coupled thermal-hydraulic, thermo-mechanic and electro-magnetic fields. In this paper, the thermal behaviour of the cavity under nominal heat load is computed first by CFD. Then a 3D thermo-mechanical model of the cavity is developed, based on the temperature maps computed by CFD, to evaluate the resulting deformation of the inner cavity surface. Finally the deformation is used to compute the updated heat load coming from the electromagnetic field generated by the electron beam in the deformed cavity, which becomes the input for a new iteration of the thermal-hydraulic, thermal-mechanical and electromagnetic analyses. It is shown that this iterative procedure converges to a self-consistent heat-load/temperature-field/deformation-field picture in nominal operating conditions, without exceeding a temperature of ∼230 °C on the inner surface of the cavity. [ABSTRACT FROM AUTHOR]

• Circuit design to double the toroidal magnetic field on the QUEST spherical tokamak is in progress. • Electron cyclotron (EC) resonances in QUEST changes with a high field. Fundamental resonance for 28 GHz EC waves emerges on the high field side. • The toroidal coil current to double the field is 100 kA. The current source consists of the 125 power supply modules in parallel. • A module comprises a bank of lithium-ion capacitors and a DC–DC buck converter. • In the bench test of a single module, the current increased linearly and maintained the flattop as programmed. Various scenarios of electron cyclotron heating can be explored in a spherical tokamak with a high toroidal field. On the QUEST spherical tokamak, doubling the toroidal field results in the emergence of the fundamental resonance layer for 28 GHz electron cyclotron waves. The second-harmonic resonance layer shifts toward the plasma central area. To increase the coil current from 50 to 100 kA, a current source is designed with 125 modules in parallel. The module power supply comprises a bank of lithium-ion capacitors and a DC–DC buck converter. In the bench test of a single module, the current was found to increase linearly and maintained the flat-top as programmed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The ITER electron cyclotron heating transmission lines will consist of 63.5-mm-diameter corrugated waveguides, each carrying 1 MW of 170 GHz microwaves. These transmission lines must include expansion units to accommodate expansion and contraction along the path from the gyrotron microwave sources to the tokamak. A numerical mode matching code has been developed to calculate power losses due to mode conversion of the operating mode, HE, to higher order modes as a result of the radial discontinuities in a sliding joint. Two expansion unit designs were evaluated, a simple gap expansion unit and a more complex tapered expansion unit. The gap expansion unit demonstrated loss that oscillated rapidly with expansion length, due to trapped modes within the unit. The tapered expansion unit has been shown to effectively suppress these trapped modes at the expense of increased fabrication complexity. In a gap expansion unit, for a waveguide step size of 2.5 mm, loss can be kept below 0.1 % to a maximum expansion length of 17 mm. Expansion units without corrugation on interior walls were also evaluated. Expansion units that lack corrugations are found to increase mode trapping within the units, though not beyond useful application. The mode matching code developed in this paper was also used to estimate mode conversion loss in vacuum pumpouts for the ECH lines; the estimated loss was found to be negligibly small. [ABSTRACT FROM AUTHOR]

The commissioning of the 28 GHz power transmission line of the TJ-II stellarator, designed for the excitation of electron Bernstein waves (EBW) through the O-X-B mode conversion process, is presented in this paper. Based upon a comprehensive set of thermal measurements, its purpose is to go into details about the several problems that arouse during the whole process, namely higher order modes excitation because of the wider beam size and alignment mismatches at the waveguide mouth. All these drawbacks may have prevented the correct O-X mode conversion, thus providing a reasonable explanation for the unsuccessful EBW heating experiments. [ABSTRACT FROM AUTHOR]

Highlights: [•] Front steering mirror design. [•] Plasma facing blanket shield module/first wall panel design. [•] Fixed frequency torus CVD diamond window serving as first tritium barrier. [•] Prototypes and tests of the above key components in the Launcher Handling and Testing Facility. [Copyright &y& Elsevier]

A directional coupler array with 5 ports integrated into a mitre bend for corrugated waveguide transmission lines at 140 GHz has been manufactured. The design is reviewed, and calculations on the performance for in-situ power measurement and mode discrimination are shown. Emphasis is given on detection of errors in alignment of the transmission system. Experimental tests are performed to benchmark the calculations. The results confirm the predictions and show that such a coupler is a viable tool for power measurement and basic mode analysis in high-power transmission systems. [ABSTRACT FROM AUTHOR]