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Using the open-source code SMILEI [J. Derouillat et al., Comput. Phys. Commun. 222, 351-373 (2018)], we perform one-dimensional full-f particle-in-cell (PIC) simulations of parasitic electrostatic wave excitation in the Ion Cyclotron Range of Frequencies (ICRF) and High Harmonic Fast Wave (HHFW) regimes in an inhomogeneous plasma. We first study direct coupling from the fast wave to electrostatic waves at the lower hybrid (LH) resonance (S=0). In the ICRF regime, we show that the fast wave can couple to the Ion Bernstein Wave (IBW), which propagates beyond the LH resonance layer. On the other hand, in the HHFW regime, no direct coupling to the IBW is observed, but electrostatic waves, likely to be Hot Ion Plasma Waves (HIPW or HPW), are seen on the low-density side of the LH resonance layer. The coupling efficiency to electrostatic waves is seen to increase with ion temperature. Parametric decay instabilities (PDIs) are then investigated in both regimes. In the ICRF regime, both resonant and non-resonant decay channels are observed and compared with theory. In the HHFW regime, we observe multiple sidebands separated by the ion cyclotron frequency, as measured experimentally on NSTX [J. R. Wilson et al., AIP Conf. Proc. 787, 66 (2005)]. The nature of these waves is discussed. Perpendicular ion heating is also found in the region where PDIs occur, consistent with experimental observations. [ABSTRACT FROM AUTHOR]

New integral kernels describing the full-wave dielectric response of Maxwellian tokamak plasmas are presented. They realistically account for the rotational transform and for wave dispersion in presence of equilibrium magnetic field parallel gradients. These kernels rely on special functions of three variables that generalize the standard plasma dispersion function; their main analytical properties are given, leading to efficient evaluation. This approach is free from the poloidal Fourier mode expansion of the HF fields which appears in earlier formulations and gives complete freedom for the numerical resolution of the wave equation: it will typically be applied to 2D finite element discretizations, allowing local mesh refinements as required near cyclotron resonance layers and in regions of rapid HF field variations. This first presentation is to lowest order in the Larmor radius for the sake of clarity but will readily generalize to all orders in (ρLT /λ⊥). [ABSTRACT FROM AUTHOR]

Scikit-rf is an open-source Python package developed for RF/Microwave engineering. The package provides a modern, object-oriented library for network analysis and calibration which is both flexible and scalable. Besides offering standard microwave network physics and operations, it is also capable of advanced operations such as interpolating between an individual set of networks or deriving network statistical properties. The package also allows direct plotting of rectangular plots, Smith Charts or automated uncertainty bounds. In this paper, the scikit-rf package is used to simulate the WEST ICRH antennas. The antenna is modelled by connecting the various elements that compose it, separately full-wave modelled. Tunable elements, such as the matching capacitors, can be either created from ideal lump components or from interpolating full-wave calculations performed at various capacitance configurations. Finally, a numerical antenna model of a WEST ICRH antenna can be used offline or online by using operating-system-level virtualization services. [ABSTRACT FROM AUTHOR]

We present a comprehensive overview of the use of ICRF (ion cyclotron range of frequency) scheme from its beginnings, with specific attention paid to the progress that has been achieved on the understanding of the interaction between the plasma edge and the ICRF heating. The developments and the progress made in overcoming the challenges faced are emphasized. We also describe, based on personal experiences, some of the dead-end roads, the sound deductions that sometimes were prematurely discarded or the intuitive conclusions that were later confirmed by more experimental data. We end with a vision for the future. [ABSTRACT FROM AUTHOR]

KSTAR has decided to develop six ECH systems for the advance TOKAMAK operation. To install six ECH systems in the limited space, ECH equipment room was completely newly constructed by steel beam-type deck. The existing two ECH system was totally disassembled and rearranged and cooling system were re-designed and newly developed, so two 105/140GHz ECH systems including the new gyrotron were commissioned and operated in 2018 KSTAR campaign. Other two 105/140GHz gyrotrons will be delivered in 2019 and one more ECH system will be operated in incoming 2019 campaign with newly designed power supply and control system. The remaining two gyrotron sources were also decided to GYCOM ITER 170GHz gyrotrons, but the order was postponed due to budget issues. The transmission line system has been using 63.5mm circular corrugated waveguides and there is no window in TL and antenna also, so it is kept in a tokamak vacuum by the main window of the gyrotron. The final positions of the antenna were decided considering the current drive and heating efficiency. However, due to the poloidal limiters installed inside vacuum vessel, in 2018 ECH couldn't operate in Ctr-CD. To solve this problem, the pivot position of all ECH launchers will approach 350 mm to the plasma in the radial direction. A 2-channel antenna for 3rd and 4th ECH system is in design which will be identical to the mechanism of the existing launcher. In this paper, the development status and the design parameters of ECH system will be presented and discussed and the operation results of ECH system in 2018 will be presented. [ABSTRACT FROM AUTHOR]

A grid-based ECH ST/Tokamak start-up model is being developed where plasma parameters, generated plasma currents, and resulting poloidal magnetic fields are evolved from the vacuum fields. Initially, the current generated is purely pressure driven since the electron temperature is too low for ECCD. The grad-B drift driven current together with the processional currents can then create a closed flux surface configuration and then the bootstrap current in a closed configuration can further enhance the plasma current. After the flux closure, the 2nd harmonic ECH heating efficiency increases with plasma current since the confinement is increased and resulting electron temperature rise would further increase the ECH absorption and plasma current. Once the temperature of hot minority component becomes sufficiently high ∼1 keV, the ECCD becomes significant for full current ramp-up. [ABSTRACT FROM AUTHOR]

The real-time power monitoring of gyrotron is one of the key issues in the operation of electron cyclotron resonance heating system. The detector can be used for real-time power monitoring. We analyzed the principle of diode detection and designed a D-band wideband detector based on Schottky diode in this paper. The detector includes a waveguide-to-microstrip transition, a matching circuit, a diode, and a low pass filter. A novel waveguide-to-microstrip transition was developed based on probe coupling. A wideband lossy matching circuit was developed based on tapered-line and series matching resistor. The simulation results show that when the input power is -30dBm at 140 GHz, the detection sensitivity is about 1600V/W. [ABSTRACT FROM AUTHOR]

Experiments in DIII-D have measured the width of the power deposition and current drive profiles for electron cyclotron (EC) waves by modulating the gyrotrons and fitting the periodic response of the electron temperature profile or motional Stark effect signals to a linearized diffusion equation. The measured EC power deposition widths are scattered around the predicted values, including both narrower and wider cases. The average width for single gyrotron modulation is ≈88% of the theoretical value from ray tracing, while the average width for multiple gyrotron modulation is ≈127% of theory; aiming misalignment between the different EC launchers may explain why multi-gyrotron measurements are wider than for single gyrotrons. The measured EC current drive widths have nearly the same amount of profile broadening (≈131% of theory, on average) as does multi-gyrotron power deposition. Finally, while the measured locations of EC current drive are well centered on the expected values, the measured heating locations are skewed to larger radius. This difference may be due to rapid heat pulse propagation causing an inherent outward bias to the deduced power deposition profile, whereas magnetic flux diffusion has less effect on current drive measurements. [ABSTRACT FROM AUTHOR]

High-power plasma waves in the lower hybrid range of frequencies (LHRF) are being used for non-inductive current drive in experiments around the world. In this range, plasmas can support waves with two different polarizations, a quasi-electrostatic wave known as the "slow wave" or simply the lower hybrid wave, and a quasi-electromagnetic wave referred to variously as the "fast wave in the LHRF", "whistler", or "helicon". An improved qualitative understanding of how fast and slow waves in the LHRF propagate can be gained by studying the "inverse wave normal surface" or IWNS. It is shown that the two branches propagate in a qualitatively similar way, with, however, important quantitative differences. [ABSTRACT FROM AUTHOR]

The China Fusion Engineering Test Reactor (CFETR) will require significant lower hybrid current drive (LHCD) power to supplement the bootstrap current in steady-state. A scoping study was performed using GENRAY/CQL3D ray-tracing/Fokker-Planck models to determine the appropriate antenna parameters for optimum current drive performance in the CFETR hybrid scenario. CFETR is a large (R0 = 7.2 m, a = 2.2 m) steady-state tokamak with a toroidal magnetic field of 6.5 T, plasma current of 13 MA, core density of 1.2×1020 m−3, and core temperature of 30 keV. The key parameters scanned were the launched parallel refractive index of the primary lobe of the LH launcher (n||) and poloidal angle (θ) of the antenna, with a LH source frequency of 4.6 GHz. A comparison of high field side (HFS) versus low field side (LFS) launch points shows similar current drive efficiency (1300 kA per 20 MW net power) from both the HFS and LFS. HFS launch yields a broad current profile in the outer part of the plasma (r/a = 0.6−0.9) as desired for supplementing the bootstrap current, while LFS launch yields a very narrow current profile near the top of the pedestal (r/a∼0.9). A 5-D parameter scan (n||1, n||2, θ1, θ2, P1/P2) of synergy between HFS and LFS antennas at fixed total power of 20 MW shows a modest (∼5%) increase in efficiency when only cases with single-pass absorption are considered. Here the subscripts 1 and 2 refer to HFS and LFS respectively. Synergy simulations with very low n|| (n|| 1.375) show higher current drive efficiency, but these scenarios suffer from poor wave accessibility, the multi-pass nature of the rays, and extremely far off-axis absorption (r/a≥0.95). Parametric scans in these studies were enabled by the πScope workflow. [ABSTRACT FROM AUTHOR]

In this paper a one-dimensional approach to the lower hybrid current drive (LHCD) modelling in the presence of inductive electric field is applied to calculate LHCD for deuterium plasmas at the FT-2 tokamak. The simulation results are compared to experimental data. [ABSTRACT FROM AUTHOR]

Plasma start-up experiments using the lower hybrid wave (LHW) were performed on the TST-2 spherical tokamak. Two capacitively-coupled combline antennas (outboard-launch and top-launch) are presently being used. Bottom-launch can be simulated using the top-launch antenna with reversed toroidal field. The unexpectedly good result of the simulated bottom-launch case was investigated by a combined ray-tracing and Fokker-Planck analysis. In the case of simulated bottom-launch the parallel index of refraction (n||) downshifts initially, but because of the plateau formed by the upshifted n|| LHW after reflecting from the cut-off layer at the bottom of the plasma, strong absorption of the low n|| LHW occurs during the first pass through the plasma, generating a distribution function which extends to very high energies. [ABSTRACT FROM AUTHOR]

The EAST tokamak relies on Lower Hybrid (LH) heating and current drive for long pulse steady-state operation [1]. The combination of the two antennas—operating at 2.45 and 4.6 GHz—offers a unique opportunity to understand the synergy between LH waves and how to best exploit it for optimization of plasma performance towards demonstration of sustained steady-state. Because the DC electric field affects the fast electron distribution function, it is important that the magnetic equilibrium and the LH current drive are evolved self-consistently [2]. For this reason, time-dependent modeling is a necessary tool to guide experiments on EAST dedicated to access and sustainment of steady-state. The time-dependent evolution of an EAST plasma discharge with lower hybrid power has been modelled using the TRANSP code in combination with GENRAY and CQL3D. Analytical profiles for electron temperature and density were fitted to the measured profiles to remove noise from experimental measurements, as well as for ease of scaling studies. This enables quick scans of density to identify an optimal range for high LH current drive and maximum plasma performance. Preliminary results from scaling studies suggest there is a density threshold, beyond which current drive is significantly reduced. Additionally, there is some indication that the electron heating and the driven current do not scale similarly with density and temperature, indicating a complex nonlinearity in the absorption process. Future work will address the current challenges in time-dependent LH simulations and explore the synergy between the two LH antennas during simultaneous power injection. [ABSTRACT FROM AUTHOR]

Nonlinear interactions of electrons with lower hybrid waves are studied by a particle simulation code. The code is based on the gyro-kinetic electron and fully-kinetic ion (GeFi) model [Lin et al., Plasma Phys. Control. Fusion 47, 657 (2005)]. The evolution of the wave spectrum, as well as the evolution of the distribution of the electrons in phase space, is investigated. For a double-peak spectrum, when the resonances of the two peak waves slightly overlap, two resonant plateaus with two different levels in the velocity distribution function are formed due to the wave damping if the resonances are non-overlapping. When the resonances of the two peak waves strongly overlap, even though the resonant waves are damped to lower levels, two resonant plateaus merge into an extended plateau in the velocity distribution function due to the growths the waves with intermediate mode numbers. [ABSTRACT FROM AUTHOR]

Comparative modeling of the coupling properties and the power spectra with different plasma densities and different phasing were performed for 2.45 GHz and 4.6 GHz waves respectively by using the ALOHA code. The modeling results show that the electron density for optimum coupling is ∼3 × 1017/m3 and ∼10 × 1017/m3 for 2.45 GHz and 4.6 GHz LH waves respectively. For both launchers the power directivity can be high as 76% and reflection coefficient less than 1% around the optimum density. When the density increases above the optimum value, the wave coupling becomes poor and simultaneously the power directivity decreases for both waves, hence reducing the current drive efficiency. However, the coupling property shows a lower sensitivity to plasma density for 4.6 GHz wave. By adjusting the phasing between main waveguides, the parallel refractive index (N//) can be varied from 1.7 to 2.5 for 4.6 GHz wave and from 1.9 to 2.9 for 2.45 GHz. It is found that the power directivity shows weak dependence on the phase difference between main waveguides for both of the waves. Comparison between ALOHA and experimental LH coupling results on EAST are presented. The power spectra with some modules powered off is also calculated. The results show that some lobes arise from left and right sides of the first main lobe for both of 2.45 GHz and 4.6 GHz waves. [ABSTRACT FROM AUTHOR]

A 4MW/3.7GHz/3s LHCD system will be developed on HL-2M tokamak. Up to now, half of the system has been constructed, including a 4MW full active multijunction (FAM) concept antenna. The antenna consisted of 6×32 grills. 1-3 power divider in poloidal direction based on TE10-TE30 mode converter is used for 6-row realization. The peak refractive index is designed as n||0=2.25, with high directivity D=0.75 and low reflection coefficient RC<1%. The maximum power density is 41MW/m2, which is acceptable for HL-2M LHCD system. The whole antenna body is separated to 8 modules, all made of stainless steel. The RF characteristic of the antenna is tested via special designed measurement components. The test result has shown that the reflection coefficient, transmission characteristic and the phase shift of the grills match the design, indicating the antenna is well manufactured. [ABSTRACT FROM AUTHOR]

WEST represents the ideal device to test a reactor relevant TWA due to its long pulse capability and full metal wall configuration. Moreover, the already installed ICRF high power launchers will allow a direct comparison between a classical in-port antenna and the TWA. This paper reports the progress in the design of an actively cooled high power mock-up of the WEST TWA antenna that will be tested in the TITAN facility. The main objective of the test is to assess the voltage stand-off of the antenna at a power level relevant for future operation. In order to be installed and operated in WEST, the antenna design has to comply with the specific machine requirements, in particular active cooling, magnetic configuration and interface with existing auxiliary heating systems. Here several aspects are taken into account: toroidal magnetic field ripple, field line inter-connection and thermal loads on the antenna. An initial assessment of mechanical compatibility in case of VDE is performed. In conclusion, the next steps and implications of the project are outlined. [ABSTRACT FROM AUTHOR]

Lower hybrid (LH) wave has been widely used for non-inductive current drive in modern tokamak experiments with high power injection. LH parametric instability (PI) has been observed in many LH experiments and considered as the most likely candidate which causes the decrease of LH current drive efficiency. Traditional LH PI theories are mostly based on the electrostatic model given the fact that the slow wave branch in LH frequency range is a quasi-electrostatic wave. However, electrostatic description is not accurate for the plasma parameters of scraped-off layer(SOL) region where LH PIs are observed in current fusion experiments. Thus, in this work, we include the electromagnetic correction for slow wave and build up the corresponding nonlinear dispersion relation of PI. The electromagnetic effects on two major decay channels, i.e., ion sound quasi-mode (ISQM) and ion cyclotron quasi-mode (ICQM) are discussed. [ABSTRACT FROM AUTHOR]

The parallel wavenumber k|| of lower hybrid waves is a key parameter that determines the wave propagation, absorption and the current drive efficiency in lower hybrid current drive (LHCD) experiments. However, at present, this parameter has not been measured during experiments. The design, simulation and test of the magnetic probe for the measurement parallel wavenumber spectrum of lower hybrid waves at 2.45GHz in EAST (Experimental Advanced Superconducting Tokamak) have been performed. Using the software COMSOL Multiphysics 5.2, some key dimensions that affect the coupling performance of the magnetic probe are optimized. The simulation and test results show that the magnetic probe has good coupling performance for 2.45GHz lower hybrid waves and has the ability to identify wave polarization. This work provides important reference for the establishment of parallel wave-number measurement diagnosis system in EAST, which is critical for further high-density LHCD experiments. [ABSTRACT FROM AUTHOR]

The aim of WEST [1] [2] is to master long pulses (> 30s and up to 1000s) and submit ITER like tungsten divertor tiles to power fluxes of 10 MW/m2. To reach these goals, the Tore Supra Lower Hybrid Current Drive system (LHCD) constituted by two launchers with an injection power capability of 7MW/1000s has been upgraded to meet WEST new environment (diverted plasma, new launchers positions). In particular, the LH1 Full Active Multi-junction launcher (FAM) has been toroidally reshaped to match the new magnetic configuration. The poloidal curvature of the plasma has also been optimized to guarantee good coupling properties of the waves. During the last experimental campaign (October-December 2018), we obtained 30s repetitive pulses with a LH power of 2.7 MW (more than 2 hours of plasma during the experimental campaign) and 43 discharges with a total LH power from 4-5.3 MW. This paper describes the results obtained in term of coupling for both launchers. Thanks to Infra-Red (IR) monitoring it also assesses the reshaping of LH1 in terms of thermal loads due to supra-thermal electrons on the launcher edges. During the upcoming experimental campaign, the IR coverage will further improve allowing assessing the LH2 Passive Active Multi-junction launcher (PAM) thermal loads. [ABSTRACT FROM AUTHOR]

Lower hybrid current drive(LHCD) is a candidate to achieve non-inductive plasma current operations on China Fusion Engineering Test Reactor(CFETR). [1] In this work, parametric decay instability(PDI) near the edge for different operation scenarios is discussed. The growth rates of the child waves are obtained by solving nonlinear dispersion relation, while the relevant amplification coefficients are also calculated varying the parallel refractive index n||. As a result of numerical calculations, the dominance of the two common decay channels: ion sound quasi-mode(ISQM) and ion cyclotron quasi-mode(ICQM), is determined by plasma parameters and the primary convective loss mechanism. The dependency of the PDI broadened n|| spectrum and the edge plasma parameters is investigated preliminarily, indicating the parameter regime for PDI mitigation. [ABSTRACT FROM AUTHOR]

The ITER ICRF antenna is a port-plug antenna. There is a clearance gap surrounding the antenna between the antenna plug walls and the vacuum vessel, creating a cavity in which resonant modes can be excited. The frequency response of the antenna array is perturbed by the excitation of a TE0,1 (p=1) mode in the ITER frequency band (around 45MHz) in the clearance gap in absence of any additional grounding. Different grounding options have been proposed and both numerically studied and tested on a scaled low-power mock-up of the ITER antenna. These studies however did not take the Faraday screen into account. In view of the technical difficulties to implement the proposed grounding options the present contribution revisits the need for grounding based on new gap voltage measurements on a reduced-scale mock-up of the ITER ICRH antenna and numerical modeling, including the presence of the Faraday screen. [ABSTRACT FROM AUTHOR]

Ion Cyclotron Range of Frequencies (ICRF) strap antennas are routinely adopted in most of the existing nuclear fusion experiments, even though their main goal, i.e. to couple high power to the plasma (MW), is often limited by rather severe drawbacks due to high fields on the antenna itself and on unmatched part of the feeding lines directly connected to the antenna. In this work, we propose, describe, prototype and measure an ICRF strap antenna based on the high impedance surfaces concept that is matched at a specific tunable frequency. The adopted high-impedance structure, positioned between the strap and the backwall, is a metallic patch displaced on top of a dielectric block and grounded by means of a vertical post, in a mushroom-like shape. This structure presents a high impedance, within a given very narrow frequency band, such that the image currents are in-phase with the currents of the strap itself, thus determining a significant efficiency increase. After a general description on the properties of high impedance surfaces applied to ICRF antennas, we describe the optimization steps, carried on by means of numerical codes, to define an antenna configuration suitable for a nuclear fusion experiment. The antenna has been then manufactured and measured; strengths and weaknesses of the proposed solution are outlined. [ABSTRACT FROM AUTHOR]

On an RF driven inductively coupled plasma (ICP) torch device for thermal spraying, a measurement system based on voltage and current probes was installed on the transmission line between the matching system and the ICP torch to measure the input impedance of the RF coil. The amplitude and phase difference of voltage and current probes signals are measured by oscilloscope. So the intensity of input impedance of the coil can be calculated. In addition, the input power was used to extract the phase to calculate the input impedance. The optimized coil input impedance was used to evaluate RF ICP coupling characteristics, to improve coatings fabrication process. [ABSTRACT FROM AUTHOR]

The Divertor Tokamak Test (DTT) facility will be built to study a solution to the issue of power exhaust in conditions relevant for DEMO. The Italian DTT tokamak, by coupling to plasma up to 45 MW of additional power, will reach the needed condition of power flow to the divertor of 15 MW/m. The selected Heating Systems to achieve this goal are Electron Cyclotron Heating (ECH), Ion Cyclotron Heating (ICH) and Negative Neutron Beam Injector (NNBI). The power will be installed in two stages: a day-1 configuration with a coupled power of 25 MW and a second step where the completion of the 45 MW will be realized in 4 years from the day-1. At first stage 16 MW of ECH power and 4 MW of ICH will be installed, making the DTT plasma dominated by RF heating. The EC system is based on 170 GHz, 1 MW gyrotron, while for the transmission line a Quasi Optical approach has been chosen, with the feature to install the multi- beam mirrors (8 beams on each one) under vacuum. The goal is to reduce the overall losses at ∼10% avoiding atmospheric absorption and selecting the proper polarization for the longest section. The power will be injected into the tokamak using front steering individual antennas and capable to real time steer all the beams for the tasks assigned to EC waves. The first module of the ICH systems will be based on transmitters, capable of a wide frequency range (60-90 MHz), connected, though standard coaxial cables and RF components, to two movable antennas inserted in the equatorial ports of DTT. The selected range is done to exploit different heating schemes. The choice of the antenna type will be based on reliability (i.e. power density) rather than on its performance in term of peak coupled power. This led to choose a two-strap antenna with a power density of 3.5 MW/m2, shaped to fit the DTT scrape-off plasma and with an adjustable radial position. An external matching system is envisaged to cope with fast variation of antenna loading, e.g. due to edge localized modes. [ABSTRACT FROM AUTHOR]

An important test for W7-X is to demonstrate confinement of fast trapped ions at volume averaged beta values up to 5%, corresponding to plasma densities above 1020 m−3. Energetic ions in W7-X with energies 50 < E < 100 keV mimic alphas in a reactor. To generate such a population is a challenging task in high-density plasmas but this can be efficiently realized with the H- (3He)-D three-ion heating ICRH scenario, foreseen for f∼25 MHz in W7-X. An ICRH system is prepared for W7-X, expected to deposit RF powers up to ∼1.5 MW (depending on the coupling) at frequencies between 25-38 MHz in pulses up to 10s. A two-strap ICRH antenna for W7-X is under construction. Each strap of the antenna is on one side connected to a tuning capacitor and grounded to the antenna box at the other end. A prematching has been implemented by connecting the RF transmission lines at an intermediate position on each strap. The main dimensions of straps and antenna box have been optimized to maximise the power delivered to the plasma, using the reference plasma density profile in front of the antenna, provided by the W7-X team. A dedicated test stand is under construction to check the main functional tests of the full ICRH antenna system. [ABSTRACT FROM AUTHOR]