Electron Cyclotron Heating Research Articles

Expansion of the Operation Region of Tokamak Plasmas Using the Stationary Direct Current of a Central Solenoid

A stationary direct current of a central solenoid (DCCS) can expand the operation region of tokamak plasmas. For tokamak plasma formation experiments using electron cyclotron heating (ECH) alone, the minimum ECH power and plasma current necessary for tokamak plasma formation were reduced using the DCCS, which was applied in the counter rotational direction to the plasma current. In TST-2, the minimum ECH power necessary for the formation of a tokamak plasma was reduced from 3.3 to 1.6 kW when the DCCS was changed from 0 to 184 A. Simultaneously, the plasma current that was needed to sustain the tokamak plasma configuration was reduced to 0.6 kA.

Evolution of ELMs, pedestal profiles and fluctuations in the inter-ELM period in NBI- and ECH-dominated discharges in DIII-D

In DIII-D, it has been observed that ELM frequency decreases by 40% and ELM spacing becomes more regular in time when heating is changed from pure neutral beam injection (NBI) to predominantly electron cyclotron heating (ECH) in ITER similar shape plasmas. In comparison with the pure NBI discharges, pedestal fluctuations in magnetics and density increase in the ECH-dominated discharges. Recovery of the pedestal profiles like electron density (n e), temperature (T e) and pressure (p e) shows marked differences for these two heating schemes. Average profiles in the last 30% of the ELM cycle show higher T e, lower n e, and similar p e at the pedestal top for the ECH discharge when compared to the NBI discharge. The gradient of T e (∇T e) is also steeper at the pedestal in the ECH discharge. Magnetic fluctuations show three distinct modes at 13–116 kHz in the ECH discharges only. n e fluctuations show two modes evolving in the inter-ELM period of the ECH discharge, a low-frequency (400 kHz) quasi-coherent mode (LFQC) and high-frequency (∼2 MHz) broadband (HFB) fluctuations. Evolution of these modes has marked correspondence with the inter-ELM ∇T e recovery. A sharp decrease in the D α baseline is observed whenever the LFQC weakens and the HFB grows, prior to each large ELM. Transport coefficients obtained from TRANSP show that MTM and/or TEM are plausible candidates for the observed fluctuations. Linear gyrofluid simulation (TGLF) corroborates this characterization. TGLF shows that the linear growth rate of the most dominant mode peaks at ion-scale (k θ ρ s ∼ 0.4) at the pedestal steep gradient and the frequency is in the electron diamagnetic direction. It is proposed that increased fluctuations in the ECH-dominated case, due to increased ∇T e, caused an increase in fluctuation-driven transport in the pedestal and slowed the pedestal recovery between ELMs, leading to a reduction in the ELM frequency.

CVD diamond photodetectors for FTU plasma diagnostics

We report on the results achieved by installing two Chemical Vapor Deposition (CVD) single crystal diamond detectors in one of the equatorial ports of the Frascati Tokamak Upgrade (FTU) tokamak, during the last six months of operation of the machine. The devices were fabricated at the University of Rome “Tor Vergata” in a metal/instrinsic/p-type diamond layered structure, allowing them to be used as Schottky photodiodes for VUV and Soft-X ray detection. Both detectors were placed inside the machine and operated in current mode under high vacuum conditions.The fast response capabilities of diamond detectors allowed to observe several plasma events, like the so-called Anomalous Doppler instabilities, the pellet injection and ablation, and Multifaceted Asymmetric Radiation From the Edge (MARFE). Diamond detectors often but not always followed the Magnetohydrodynamics (MHD) activity, depending on their localization relative to the emitting region. Core temperature oscillations following Electron Cyclotron Heating system (ECH) modulation were also observed. In addition, the diamond signals were compared to selected channels of the FTU bolometry system with similar line-of-sight: the encouraging results have launched an R&D program for the development of diamond-based bolometers.

Advances in physics understanding of high poloidal beta regime toward steady-state operation of CFETR

Experimental and modeling investigations of high βp scenarios on DIII-D and EAST tokamaks show advantages in high energy confinement, avoidance of n = 1 MHD, and core-edge integration with reduced heat flux, making this scenario an attractive option for China Fusion Engineering Test Reactor steady-state operation. Experiments show that plasmas with high confinement and high density can be achieved with neutral beam injection on DIII-D (βp ∼ 2.2, βN ∼ 3.5, fBS ∼ 50%, fGw ∼ 1.0, and H98y2 ∼ 1.5) and pure RF power on EAST (βP ∼ 2.0, βN ∼ 1.6, fBS ∼ 50%, fGw ∼ 0.8, and H98y2 > 1.3). By tailoring the current density profile, a q-profile with local (off-axis) negative shear is achieved, which yields improved confinement and MHD stability. Transport analysis and simulation suggest that the combination of a high density gradient and high Shafranov shift allows turbulence stabilization and higher confinement. Using on-axis Electron Cyclotron Heating injection, tungsten accumulation is avoided on EAST, and this is reproduced in modeling. Reduced heat flux (by > 40%) and maintenance of high core confinement is achieved with active feedback control of the radiated divertor, an important result for long pulse operation in tokamaks. The improved physics understanding and validated modeling tools are used to design a 1 GW steady-state scenario for CFETR.

Prototype mitre bends of the ex-vessel waveguide system for the ITER upper launcher: Thermal hydraulic simulations and experiments with off-center mm-wave beams

On ITER, long pulse gyrotrons are required as a power source for electron cyclotron heating (ECH) and current drive (CD). The microwaves are guided from the gyrotrons, which are placed far from the Tokamak, into the plasma by transmission lines (TLs) and a launching antenna (launcher). Each of the four ECH Upper launchers features eight waveguide (WG) TLs, with at least 95% of the power from the gyrotrons coupled into in the main HE11 mode of the TLs. In the ex-vessel portion of the system between the port-plug closure plate and the isolation valve and diamond window, there are miter bends (MBs) that change the direction of the TL by reflecting the millimeter waves (mm-waves) using mirrors; the mirrors must handle 1.31 MW, 170 GHz, 3600 s pulses. Various MBs perform these reflections at an angle of 90 degrees, or nearly 100 degrees. As a result of the ohmic dissipation, an intensive peaked heat flux appears near the center of the MB mirror and thus, a dedicated cooling system is present to ensure the temperature control of the mirror and housing e.g. [1]. The power that is not found in the HE11 mode can cause the beam to be not perfectly centered, resulting in an off-centered heat flux on the mirror surface; as is the case for the experiments described here. This study presents new finite element modeling of such beams, created in CFX of ANSYS Workbench [2], compared to the experimental findings for pulses of 170 GHz, 0.5 MW and 240 s. Monitor points are placed in the same positions as TCs that have been fitted in the mirror close to the heated surface and direct comparison of the temperature values is performed. Through transient simulation the time constants are calculated and compared with those of the experiments.

Magnetic Configuration and Heating Location Dependences of Toroidal Torques by Electron Cyclotron Heating in LHD

Magnetic Configuration and Heating Location Dependences of Toroidal Torques by Electron Cyclotron Heating in LHD

Radial electric field and density fluctuations measured by Doppler reflectometry during the post-pellet enhanced confinement phase in W7-X

Radial profiles of density fluctuations and the radial electric field, E r, have been measured using Doppler reflectometry during the post-pellet enhanced confinement phase achieved, under different heating power levels and magnetic configurations, during the 2018 W7-X experimental campaign. A pronounced E r-well is measured with local values as high as −40 kV m−1 in the radial range ρ ∼ 0.7–0.8 during the post-pellet enhanced confinement phase. The maximum E r intensity scales with both the plasma density and electron cyclotron heating power level, following a similar trend to the plasma energy content. A good agreement is found when the experimental E r profiles are compared to simulations carried out using the neoclassical codes, the drift kinetic equation solver (DKES) and kinetic orbit-averaging solver for stellarators (KNOSOS). The density fluctuation level decreases from the plasma edge toward the plasma core and the drop is more pronounced in the post-pellet enhanced confinement phase than in reference gas-fuelled plasmas. Besides, in the post-pellet phase, the density fluctuation level is lower in the high iota magnetic configuration than in the standard one. To determine whether this difference is related to the differences in the plasma profiles or to the stability properties of the two configurations, gyrokinetic simulations have been carried out using the codes stella and EUTERPE. The simulation results point to the plasma profile evolution after the pellet injection and the stabilization effect of the radial electric field profile as the dominant players in the stabilization of the plasma turbulence.

Performance demonstration of vacuum microwave components critical for the operation of the ITER low-field side reflectometer.

Final design studies in preparation for manufacturing have been performed for functional components of the vacuum portion of the ITER Low-Field Side Reflectometer (LFSR). These components consist of an antenna array, electron cyclotron heating (ECH) protection mirrors, phase calibration mirrors, and vacuum windows. Evaluation of these components was conducted at the LFSR test facility and DIII-D. The antenna array consists of six corrugated-waveguide antennas for simultaneous profile, fluctuation, and Doppler measurements. A diffraction grating, incorporated into the plasma-facing miter bend, provides protection of sensitive components from stray ECH at 170GHz. For in situ phase calibration of the LFSR profile reflectometer, an embossed mirror is incorporated into the adjacent miter bend. Measurements of the radiated beam profile indicate that these components have a small, acceptable effect on mode conversion and beam quality. Baseline transmission characteristics of the dual-disk vacuum window are obtained and are used to guide ongoing developments. Preliminary simulations indicate that a surface-relief structure on the window surfaces can greatly improve transmission. The workability of real-time phase measurements was demonstrated on the DIII-D profile reflectometer. The new automated real-time analysis agrees well with the standard post-processing routine.

Design optimization of the Upper Steering Mirror Assembly (USMA) for ITER ECHUL in view of disruptive events

Four Electron Cyclotron Heating Upper Launchers (ECHUL) will be used at ITER to counteract magneto-hydrodynamic plasma instabilities by targeting them with up to 20 MW of mm-wave power at 170 GHz. The millimeter waves are guided through a set of fixed mirrors (M1, M2 and M3) and the front steering mirror set (M4), aiming at the correct location in the plasma for suppression of the q = 3/2 and q = 2/1 Neoclassical Tearing Modes (NTMs). At the M4 reflecting mirror surfaces, part of the mm-wave power is converted into heat by ohmic dissipation, totaling ca. 25 kW of absorbed power and reaching a peak power density of up to 1.8 MW/m2 in each of the 4 beam center spots.The latest in-vessel mm-wave mirrors Components Load Specification (CLS) data imposes an increase of the electromagnetically induced loads relative to those anticipated in earlier designs, resulting in higher mechanical load on the Crossed Flexure Pivot (CFP) due to Vertical Displacements Events (VDEs). The present paper reports the main design optimizations as well as the finite elements analyses carried out with the objective to: 1) reduce the electromagnetic loads on the components due to induced Eddy currents, 2) dissipate the thermal loads coming from the beams themselves and the plasma following the design requirements in terms of coolant temperature rise, pressure drop and admissible corrosion rate values, 3) assure the components structural integrity enforcing the ITER Structural Design Code for the In-Vessel Components (SDC-IC).

Quasi-optical design of EC H&CD launcher for CFETR

The China Fusion Engineering Test Reactor (CFETR) is the next-generation device for China magnetic controlled fusion program. A high power (36 MW) and continuous millimeter-wave injected by two Upper Launchers (ULs) are planned for Electron Cyclotron Heating and Current Drive (EC H&CD) in CFETR. A 3D visual quasi-optical design tool for the UL has been developed based on MATLAB that is capable of accurate visualization of beam propagations and the evaluations of quasi-optical system layout in the UL. One UL has two identical sets of quasi-optical modules, each one is mainly composed of nine fixed focusing mirrors and one flat steering mirror. By using the design tool to optimize the quasi-optical system for the UL, the minimum concentration radius (∼ 110 mm) of the nine mm-wave beams is obtained at the resonance layer. The evaluation results of the peak heat load on the FMs and SM obtained were 0.71 MW/m2 and 1.46 MW/m2, respectively.

On the Possibility of Anomalous Scattering of an Extraordinary Microwave by Oscillations of an Intermediate Frequency Range Localized in the Edge Transport Barrier of a Fusion Installation

The parametric decay instability of microwaves with extraordinary polarization is analyzed that leads to their anomalous scattering by oscillations of the intermediate frequency range, localized in the edge transport barrier of a fusion installation. This effect can occur during electron cyclotron heating of the plasma at the second harmonic of the resonance frequency by multi megawatt microwave beams.

Fluid-dynamic and thermo-mechanical analyses of the Monoblock Mitre Bend for the ITER electron cyclotron Upper launcher

The ITER Electron Cyclotron Heating (ECH) system will be used to counteract magneto-hydrodynamic plasma instabilities by aiming up to 20 MW of mm-wave power at 170 GHz. The primary vacuum boundary at the Electron Cyclotron Upper Launcher (EC UL) extends into the port cell region through eight beamlines, defining the so-called First Confinement System (FCS). Each beamline, designed for the transmission of 1.31 MW, is delimited by the closure plate at the port plug back-end and by a diamond window in the port cell. The FCS essentially consists of a Z-shaped set of straight corrugated waveguides (WG) connected by Mitre Bends (MB) with a nominal inner diameter of 50 mm. Due to the space restrictions, the eight MBs at the last FCS section are grouped into two monoblocks. Each Monoblock Mitre Bend (MBMB) consists of a body with corrugated feedthroughs defining a specific angle for each beamline and four mirrors attached by bolted connection to reflect the mm-wave power. The thermal expansion arising from the ohmic losses, the cooling pressure, the bolt pre-tension and the imposed displacements coming from the connection with the transmission lines are the primary design loads for the MBMBs. The fluid-dynamic analyses performed for both upper and lower MBMBs show that highest temperature takes place in the MB mirrors, reaching a maximum value of 203 °C at the beam center. The thermo-mechanical analyses demonstrate that the peak stress also occurs at this location with a maximum value of 324 MPa. These stresses are categorized and compared with the allowable limits defined in the ASME code, what probes that the current design of both upper and lower MBMBs are able to withstand the loads taking place during the mm-wave normal operation.

Superconducting Magnet Qualification Methodology for the Material Plasma Exposure Experiment

The Material Plasma Exposure eXperiment (MPEX), currently under design, is a new linear plasma device to advance the understanding of plasma-material interactions through the generation and delivery of plasmas as they are expected in future fusion reactor divertors. MPEX will be a steady-state device to study high-fluence exposures of plasma-facing materials and components. The requirements for the magnetic field at the target and the heating stages make the application of superconducting coils necessary. Conceptual designs for the superconducting magnets have been developed, and multiple cryostats with warm bore diameters of either 65 cm or 156 cm are envisioned to facilitate their integrated and timely assembly with other systems such as vacuum, water cooling, and RF power. Although design, fabrication, and testing for the magnets as stand-alone units are straightforward, challenges will arise during the integration of the system. Two different field profiles will be used during operation. The magnetic field where the electron cyclotron heating occurs needs to operate at both 1.25 and 2.5 T. It is critical that the magnets all share the same magnetic axis and alignment. The mutual inductance between cryostats will affect the quench behavior of the system. Also, cryostat-to-cryostat forces can be as large as 700 kN, and the magnitude and direction will change depending on which coils are energized. The design of the system must take those characteristics into account along with the quench scenarios. This paper describes the qualification approach that will be used to determine whether stand-alone tests can be used to ensure the success of the integrated system. Fiducials will be used to define the location of the magnetic axis for each cryostat to ensure proper alignment. Quench tests of a single magnet will be performed at a current above the normal operating current to account for additional stored energy from the mutual inductance to adjacent cryostats. Also, a 1018 steel plate will be mounted on either end of a cryostat to simulate the cryostat-to-cryostat forces. Requirements for the size and location of the steel plates are described.

Russian Gyrotrons: Achievements and Trends

The last decade has contributed to the rapid progress in the gyrotron development. Megawatt-class, continuous wave gyrotrons are employed as high-power millimeter (mm)-wave sources for electron cyclotron heating (ECH) and current drive in the tokamaks and stellarators. The progress in gyrotron development pushes ECH from a minor to a major heating method. Also gyrotron based technological complexes successfully applied in electron cyclotron resonance ion sources, for microwave ceramic sintering and diamond disk production. The paper describes the main features of high frequency gyrotrons. Some data about pulsed and CW tubes, working in the terahertz frequency range, are given. These gyrotrons operate (in some specific combinations) at very low voltage and beam current, demonstrate an extremely narrow frequency spectrum or wide frequency tuning. Although in comparison with the classical microwave tubes the gyrotrons are characterized by greater volume and weight due to the presence of bulky parts (such as superconducting magnets and massive collectors where the energy of the spent electron beam is dissipated) they can easily be embedded in a sophisticated laboratory equipment (e.g., spectrometers, technological systems, etc.). All these advantageous features have opened the road to many novel and prospective applications of gyrotrons.

Observation of line emissions from Ni-like W46 + ions in wavelength range of 7–8 Å in the Large Helical Device

Tungsten W46+ lines were successfully observed in the extreme ultraviolet (EUV) wavelength range of 7 ∼ 8 Å in the Large Helical Device (LHD). Tungsten ions are distributed in the neutral beam injection (NBI) heated LHD plasma by injecting a pellet consisting of a small piece of tungsten metal wire enclosed by a carbon tube. While the electron temperature has a sudden drop due to the pellet injection, it can be recovered by applying electron cyclotron heating (ECH) together with continuous NBI heating. It is found that a W46+ line at 7.93 Å is emitted when the central electron temperature ranges around 3.4 keV with relatively high intensity and is isolated from other intrinsic impurity lines. The 7.93 Å line consists of two lines of forbidden transitions which are blended with each other; an electric quadrupole (E2) transition at 7.928 Å and a magnetic octupole (M3) transition at 7.938 Å. Observation of W46+ lines in the stellarator experiments is reported for the first time in the present study while the lines have been already observed in several tokamak experiments. The electron temperature dependence of the emission intensity of the 7.93 Å line agreed well with that of the fractional abundance of W46+ ions calculated using the ionization and recombination rate coefficients available in the ADAS database under the assumption of the collisional ionization equilibrium. The 7.93 Å line observed in this study will be used as tools for further spectroscopic researches, such as the measurements of spatial profile of W46+ ions using a space-resolved EUV spectrometer developed in LHD.

‘BAAE’ instabilities observed without fast ion drive

The instability that was previously identified (Gorelenkov 2009 Phys. Plasmas 16 056107) as a fast-ion driven beta-induced Alfvén-acoustic eigenmode (BAAE) in DIII-D was misidentified. In a dedicated experiment, low frequency modes (LFMs) with characteristic ‘Christmas light’ patterns of brief instability linked to the safety factor evolution occur in plasmas with electron temperature T e ≳ 2.1 keV but modest beta. To isolate the importance of different driving gradients on these modes, the electron cyclotron heating (ECH) power and 80 keV, sub-Alfvénic neutral beams are altered for 50–100 ms durations in reproducible discharges. Although beta-induced Alfvén eigenmodes and reversed-shear Alfvén eigenmodes stabilize when beam injection ceases (as expected for a fast-ion driven instability), the LFMs that were called BAAEs persist. Data mining reveals that characteristic LFM instabilities can occur in discharges with no beam heating but strong ECH. A large database of over 1000 discharges shows that LFMs are only unstable in plasmas with hot electrons but modest overall beta. The experimental LFMs have low frequencies (comparable to diamagnetic drift frequencies) in the plasma frame, occur near the minimum of the safety factor q min, and appear when q min is close to rational values. Theoretical analysis suggests that the LFMs are a low frequency reactive instability of predominately Alfvénic polarization.

Magnetic shear effect on plasma transport at T e/T i ∼ 1 through electron cyclotron heating in DIII-D plasmas

The effect of magnetic shear on plasma transport for an electron to ion temperature ratio (T e/T i) near unity has been explored in DIII-D utilizing electron cyclotron heating (ECH). Previous reports showed that significant confinement degradation occurred at T e/T i ∼ 1 in positive shear (PS) plasmas in DIII-D, whereas reduced confinement degradation was observed in negative central shear (NCS) plasmas. In this study, plasma transport in weak magnetic shear (WS) plasmas with ECH is investigated and compared with that in NCS and PS plasmas. Here the magnetic shears () are > 0.5, ∼0 and <-0.1 in the core region (ρ∼ 0.3–0.4) of PS, WS and NCS plasmas, respectively, and flat or negative inside ρ∼ 0.4 in the WS and NCS plasmas. Weak magnetic shear is found to be effective in minimizing degradation of ion thermal confinement as T e/T i increases through ECH application, and an improved confinement factor of H 98y2 ∼ 1.2 is maintained, similar to NCS plasmas. At T e/T i ∼ 1, the ion thermal diffusivity around an internal transport barrier decreases when changing the magnetic shear from positive to weak or negative shear. Also, reduced local particle and momentum transport was indicated by steeper density and toroidal rotation profiles in the weak and negative shear regimes. Linear gyrokinetic simulations predict little change in growth rates of low-k turbulence with ECH application in the WS and NCS plasmas, which is consistent with the transport and profile analyses.

FEM analyses of the ITER EC H&amp;CD torus diamond window unit towards the prototyping activity

The chemical vapour deposition (CVD) diamond torus window unit is a sub-component of the ITER Electron Cyclotron Heating and Current Drive (EC H&CD) system used for a diverse range of applications including plasma heating and control of plasma magneto-hydrodynamic (MHD) instabilities. It consists of an ultra-low loss polycrystalline diamond disk brazed to copper cuffs and then enclosed by a metallic structure. The diamond disk with 1.11 mm thickness already passed successfully the ITER final design review (FDR) in 2018. In view of the complete window assembly FDR, prototyping activities of the window are essential and, therefore, they shall start soon in order to check the feasibility of the proposed manufacturing and assembling sequence of the component. In this perspective, as the design of the systems surrounding the window is currently in development phase, the paper describes the finite element method (FEM) analyses of the window to carry out with the aim to prove the soundness of the design used for the prototyping and also to define requirements for the surrounding systems. Specific methodologies are adopted such as the limit analysis approach for the external loads acting on the window unit. Several combinations of forces and moments are applied to the window unit to find the maximum loads, i.e. the limits loads, which generate stresses in the unit equal to the allowable ones, according to the selected design criteria.

Time–Space Evolution of the Parameters of Turbulent Density Fluctuations During Pulsed EC Heating of the Plasma at the L-2M Stellarator

The study is continued of the temporal changes of the parameters of turbulent plasma density fluctuations during multi-pulse electron cyclotron (EC) heating of the plasma on the L-2M stellarator. The plasma density fluctuations with different wave vectors (k = 30, 20, 2, and 1 cm –1) in different spatial regions of the plasma column were recorded by the method of collective scattering of microwave radiation. The time dependence of plasma density fluctuations, which has a bursty shape, was analyzed, including the discharges in which the gyroresonance region was shifted by 3–4 cm from the center of the plasma column toward the inside of the torus. A comparison was made between the specificities of the evolution of plasma density fluctuations and the evolution of their statistical characteristics. It was found that the kurtosis (M4) of the scattering signals deviates from the value 3, which also occurs in bursts. During a EC heating pulse, three groups of different bursts (of signal intensity, its kurtosis and their increments) were observed. Autocorrelation analysis of density fluctuations and their increment was carried out and it showed that the half-width of the autocorrelation functions changes during multi-pulse EC heating. The probability of the initiation of bursty processes by perturbations in the near-wall plasma region is discussed.

Experimental investigation of magnetic islands growth in tokamak plasmas by applying emissive biased limiter and resonant helical field

At the present time, Magnetic Confinement Fusion (MCF) is considered as a way to produce energy. In this work, one of the Magnetohydrodynamic (MHD) limitations has been discussed. As closed magnetic surfaces, the Magnetic Islands (MIs) which are generated due to pressure effects, need to be surrounded by a separatrix which separate them from the other parts. External Magnetic Fields (EMFs), the safety factor (q) and the pressure profiles would be used to take the MIs under control. This could be achieved through an exterior medium, exclusively the Electron Cyclotron Heating (ECH) as well as the Current Drive (ECCD). Study of the magnetic flux surfaces and the effect of magnetic perturbation on tokamak plasmas, inform us about the formation of the MIs and their locations. In this work, together with the comprehensive review of the MIs and their importance, the conventional methods for improving the magnetic confinement has been introduced and discussed. In this regards, the Hot Limiter Biasing (HLB) method and the Resonant Helical Field (RHF) which is produced by external Helical Coils (HCs) were introduced and used. Then, the plasma current (), the Loop voltage, and the MO were obtained for different states. Finally, the Magnetic Islands Width (W) and their Growth Rate (GR) were calculated and compared with the experimental results.