Electron Bernstein Wave Heating Research Articles

MAST Upgrade microwave heating and current drive system – engineering design overview

MAST-Upgrade (MAST-U) is undergoing several enhancements to deliver increased performance and functionality. One such enhancement is the design, development, and implementation of an Electron Bernstein Wave (EBW) Heating and Current Drive (HCD) System. The MAST-U EBW System aims to provide experimental data for model validation, and to provide a greater understanding of EBW physics and its capabilities. The MAST-U EBW System provides up to 1.8 MW of microwave power into the plasma, through a system comprising: high voltage power supplies; two gyrotrons; evacuated transmission lines; a steerable in-vessel launching system; and associated control and ancillary systems. The gyrotrons from Kyoto Fusioneering have a 0.9 MW output power capability at the dual frequencies of 28GHz and 34.8GHz, allowing start-up and current drive studies to be carried out at their respective optimum frequencies. Additional diagnostics, termed interceptor plates, are proposed to sit in the path of the first reflection. These will measure the reflected power from the plasma, to both act as an interlock if the reflected power is too high, and provide key information on the coupling efficiency.

Benchmarking full-wave codes for studying the O-SX mode conversion in MAST Upgrade

Three full-wave codes for simulating microwave propagation and O-SX mode conversion in magnetized plasma are described and compared. Their feasibility to investigate mode conversion processes and obtain conversion efficiencies for parameters relevant for a potential MAST Upgrade 28 GHz electron Bernstein wave heating scenarios is explored.

Electron Bernstein wave excitation and heating by nonlinear interactions of Laguerre and Hermite Gaussian laser beams in a magnetized plasma

In this paper, we have studied the nonlinear interaction of Laguerre-Gaussian (LG) and Hermite-Gaussian (HG) laser beams in magnetized plasma, and electron Bernstein wave (EBW) excitation and heating of electron is evaluated. The beat wave with frequency ω=(ω1-ω2) and wave vector k→=k→1+k→2, exert a pondermotive force on electrons. This force has potential to drive electron Bernstein wave in both homogeneous and inhomogeneous plasma. The convective loss of waves in inhomogeneous plasma is discussed. An analytical theory is also developed for the anomalous heating of electrons. The variation of normalized potential and power distribution with normalized frequency and beam direction is demonstrated to excite EBWs under resonance conditions. The laser parameters such as beam width, modes index, and frequency affect the EBWs excitation and rate of electron heating. In addition, the spatial tuned shape of beams and the optimum laser beams parameter is proposed for much more exaction and heating.

The Materials Plasma Exposure eXperiment: Status of the Physics Basis Together With the Conceptual Design and Plans Forward

The Materials Plasma Exposure eXperiment (MPEX) is a linear plasma device that will address the plasma–material interaction (PMI) science for future fusion reactors and will enable testing of plasma-facing components (PFCs). It is designed as a steady-state device eventually delivering an ion fluence of up to 1031 m−2 to the target. The device will be designed to handle neutron-activated materials. These capabilities should push the technical readiness level of PFCs up to six for some end-of-life aspects. In order to achieve the relevant plasma conditions, as they are expected in future fusion reactor divertors, MPEX will utilize a novel plasma source concept. This plasma source concept uses a high-power helicon (200 kW, 13.56 MHz), an electron cyclotron heating (ECH) system, which will heat electrons via electron Bernstein wave (EBW) heating (about 400 kW, 70 GHz), and an ion cyclotron heating (ICH) system (400 kW, 6–9 MHz). The physics basis for this plasma source concept including the heating physics and transport is based on experiments on Proto-MPEX. An overview of the experimental results and the physics basis will be given. The physics basis directly translates into functional requirements of MPEX and the conceptual design. The status of the conceptual design of MPEX will be shown.

Induced scattering of a pump wave during electron Bernstein wave heating in the high poloidal beta regime

In the present paper we investigate induced scattering of the pump microwave during electron Bernstein wave heating in the regimes with the high poloidal beta. It occurs in the upper hybrid resonance vicinity and leads to the nonlinear excitation of electron Bernstein waves confined within a finite 3D region limited in the radial direction by a potential well at the LFS and in the poloidal and toroidal directions by the dimensions of the spot on the magnetic flux surface illuminated by the pump beam.

Electron heating of over-dense plasma with dual-frequency electron cyclotron waves in fully non-inductive plasma ramp-up on the QUEST spherical tokamak

A 28 GHz system with a high-power gyrotron tube has been used for the QUEST spherical tokamak to form an over-dense plasma for electron Bernstein wave (EBW) heating and current drive with an 8.2 GHz-wave. Non-inductive high-density plasma ramp-up experiments with dual-frequency (dual-f ) electron cyclotron (EC) (8.2 GHz and 28 GHz) waves were conducted. A spontaneous density jump (SDJ) to an over-dense state was first observed as a bifurcation phenomenon in the dual-f wave experiment. The over-dense plasma on the 8.2 GHz wave was non-inductively ramped up to 25 kA, and was maintained for 0.4 s under stable plasma equilibrium after two such jumps in one shot. Heating to mildly energetic electrons and bulk electrons was observed even in the over-dense region. The electrostatic EBW heating effect on the mildly energetic electrons in the over-dense region is assessed following a dispersion analysis of the 8.2 GHz wave. The bulk electron heating effect observed is explained as heat exchange from mildly energetic electrons heated by the electrostatic EBW. Remarkably, a high hard-x-ray-radiation temperature of ∼500 keV was also observed in tangential viewing for current-carrying electrons in the over-dense core region. Synergetic heating from the overlap of different 28 GHz EC harmonic resonances as well as higher harmonic heating is discussed for maintaining the highly energetic electrons in the over-dense core region. In addition, the SDJ process and mechanism are considered based on the discussion of the electron heating effects with the 8.2 GHz wave.

Development of the calibration method for a fast steering antenna for investigating the mode conversion window used in EBW heating in the LHD plasma

In this study, we developed a calibration method for a fast steering antenna for investigating the mode conversion window used in electron Bernstein wave heating in the large helical device. The calibration was carried out in under-dense plasma against a line-of-sight with an optical thickness which varied spatially. Although multi-reflected background radiation becomes dominant in optically thin lines-of-sight, we succeeded in calibrating the fast steering antenna by including the effect of multi-reflected background radiation in the solution of the radiation transfer equation as the constant by which the temperature of the center of the plasma is multiplied. In addition, we report the initial results of experiments investigating the mode conversion window in over-dense plasma using the calibrated antenna.

Construction of an EBW Heating System for the MST RFP

The first observation of rf heating in a reversed field pinch (RFP) using the electron Bernstein wave (EBW) has been demonstrated on Madison Symmetric Torus (MST). The design of the RF heating system used in this experiment is described. Klystron and antenna limitations are examined during launch into the plasma.

An electron Bernstein wave heating scheme for the Proto-MPEX linear device

Linear plasma devices, such as the Prototype Material Plasma Exposure eXperiment (Proto-MPEX), are an economic method to study plasma-material interactions under high heat and particle fluxes. The Proto-MPEX device at Oak Ridge National Laboratory is a high-density helicon plasma generator with additional resonant electron heating to study plasma-material interactions in ITER-like conditions. Here, we present modeling results of an electron Bernstein wave (EBW) heating scheme for this device using a microwave-based system. A 28 GHz injection system is available to provide power levels up to 100 kW injected into low electron temperature and high electron density plasmas. A modified version of the GENRAY ray-tracing code, GENRAY-C, has been used to determine the EBW and electron cyclotron heating wave accessibility for these overdense plasmas. Modeling has shown that greater than 80% of mode conversion from the ordinary mode to the EBW is possible for 28 GHz injection in Proto-MPEX. Calculations show that by utilizing the effects of Doppler-shifted resonance absorption, power deposition near the core is possible. Significant collisional damping, leading to edge absorption, is expected to occur for neutral pressures >1.0 mTorr; however, experiments can be designed to minimize these effects.

Excitation of half-integer up-shifted decay channel and quasi-mode in plasma edge for high power electron Bernstein wave heating scenario

Electron Bernstein waves (EBW) consist of promising tools in driving localized off-axis current needed for sustained operation as well as effective selective heating scenarios in advanced over dense fusion plasmas like spherical tori and stellarators by applying high power radio frequency waves within the range of Megawatts. Here some serious non-linear effects like parametric decay modes are highly expect-able which have been extensively studied theoretically and experimentally. In general, the decay of an EBW depends on the ratio of the incident frequency and electron cyclotron frequency. At ratios less than two, parametric decay leads to a lower hybrid wave (or an ion Bernstein wave) and EBWs at a lower frequency. For ratios more than two, the daughter waves constitute either an electron cyclotron quasi-mode and another EBW or an ion wave and EBW. However, in contrast with these decay patterns, the excitation of an unusual up-shifted frequency decay channel for the ratio less than two is demonstrated in this study which is totally different as to its generation and persistence. It is shown that this mode varies from the conventional parametric decay channels which necessarily satisfy the matching conditions in frequency and wave-vector. Moreover, the excitation of some less-known local non-propagating quasi-modes (virtual modes) through weak-turbulence theory and their contributions to energy leakage from conversion process leading the reduction in conversion efficiency is assessed.

Observation of Electron Bernstein Wave Heating in a Reversed Field Pinch.

The first observation of rf heating in a reversed field pinch (RFP) using the electron Bernstein wave (EBW) is demonstrated on the Madison Symmetric Torus. Propagation across and heating in a stochastic magnetic field is observed. Novel techniques are required to measure the suprathermal electron tail generated by EBW heating in the presence of intense Ohmic heating. rf-heated electrons directly probe the edge transport properties in the RFP; measured loss rates imply a large noncollisional radial diffusivity.

Developing the science and technology for the Material Plasma Exposure eXperiment

Linear plasma generators are cost effective facilities to simulate divertor plasma conditions of present and future fusion reactors. They are used to address important R&D gaps in the science of plasma material interactions and towards viable plasma facing components for fusion reactors. Next generation plasma generators have to be able to access the plasma conditions expected on the divertor targets in ITER and future devices. The steady-state linear plasma device MPEX will address this regime with electron temperatures of 1–10 eV and electron densities of . The resulting heat fluxes are about 10 MW . MPEX is designed to deliver those plasma conditions with a novel Radio Frequency plasma source able to produce high density plasmas and heat electron and ions separately with electron Bernstein wave (EBW) heating and ion cyclotron resonance heating with a total installed power of 800 kW. The linear device Proto-MPEX, forerunner of MPEX consisting of 12 water-cooled copper coils, has been operational since May 2014. Its helicon antenna (100 kW, 13.56 MHz) and EC heating systems (200 kW, 28 GHz) have been commissioned and 14 MW was delivered on target. Furthermore, electron temperatures of about 20 eV have been achieved in combined helicon and ECH heating schemes at low electron densities. Overdense heating with EBW was achieved at low heating powers. The operational space of the density production by the helicon antenna was pushed up to at high magnetic fields of 1.0 T at the target. The experimental results from Proto-MPEX will be used for code validation to enable predictions of the source and heating performance for MPEX. MPEX, in its last phase, will be capable to expose neutron-irradiated samples. In this concept, targets will be irradiated in ORNL’s High Flux Isotope Reactor and then subsequently exposed to fusion reactor relevant plasmas in MPEX.

X-ray analysis of electron Bernstein wave heating in MST.

A pulse height analyzing x-ray tomography system has been developed to detect x-rays from electron Bernstein wave heated electrons in the Madison symmetric torus reversed field pinch (RFP). Cadmium zinc telluride detectors are arranged in a parallel beam array with two orthogonal multi-chord detectors that may be used for tomography. In addition a repositionable 16 channel fan beam camera with a 55° field of view is used to augment data collected with the Hard X-ray array. The chord integrated signals identify target emission from RF heated electrons striking a limiter located 12° toroidally away from the RF injection port. This provides information on heated electron spectrum, transport, and diffusion. RF induced x-ray emission from absorption on harmonic electron cyclotron resonances in low current (<250 kA) RFP discharges has been observed.

Excitation of ion Bernstein waves as the dominant parametric decay channel in direct X-B mode conversion for typical spherical torus

The electron Bernstein wave (EBW) is typically the only wave in the electron cyclotron (EC) range that can be applied in spherical tokamaks for heating and current drive (H&CD). Spherical tokamaks (STs) operate generally in high-β regimes, in which the usual EC ordinary (O) and extraordinary (X) modes are cut off. As it was recently investigated the existence of EBWs at nonlinear regime thus the next step would be the probable nonlinear phenomena study which are predicted to be occurred within the high levels of injected power. In this regard, parametric instabilities are considered as the major channels for losses at the X-B conversion. Hence, we have to consider their effects at the UHR region which can reduce the X-B conversion efficiency. In the case of EBW heating (EBH) at high power density, the nonlinear effects can arise. Particularly at the UHR position, the group velocity is strongly reduced, which creates a high energy density and subsequently a high amplitude electric field. Therefore, a part of the input wave can decay into daughter waves via parametric instability (PI). Thus, via the present research, the excitations of ion Bernstein waves as the dominant decay channels are investigated and also an estimate for the threshold power in terms of experimental parameters related to the fundamental mode of instability is proposed.

Pre-conceptual design activities for the materials plasma exposure experiment

The development of next step fusion facilities such as DEMO or a Fusion Nuclear Science Facility (FNSF) requires first closing technology gaps in some critical areas. Understanding the material-plasma interface is necessary to enable the development of divertors for long-pulse plasma facilities. A pre-conceptual design for a proposed steady-state linear plasma device, the Materials Plasma Exposure Experiment (MPEX), is underway. A helicon plasma source along with ion cyclotron and electron Bernstein wave heating systems will produce ITER divertor relevant plasma conditions with steady-state parallel heat fluxes of up to 40MW/m2 with ion fluxes up to 1024/m2s on target. Current plans are for the device to use superconducting magnets to produce 1–2T fields. As a steady-state device, active cooling will be required for components that interact with the plasma (targets, limiters, etc.), as well as for other plasma facing components (transport regions, vacuum tanks, diagnostic ports). Design concepts for the vacuum system, the cooling system, and the plasma heating systems have been completed. The device will include the capability for handling samples that have been neutron irradiated in order to consider the multivariate effects of neutrons, plasma, and high heat-flux on the microstructure of divertor candidate materials. A vacuum cask, which can be disconnected from the high field environment in order to perform in-vacuo diagnosis of the surface evolution is also planned for the facility.

Comparison between Full Wave and Ray-Tracing Calculations to Examine Scenarios for Electron Bernstein Wave Heating in LHD

Comparison between Full Wave and Ray-Tracing Calculations to Examine Scenarios for Electron Bernstein Wave Heating in LHD

Cryogenic considerations for superconducting magnet design for the material plasma exposure experiment

In order to determine long term performance of plasma facing components such as diverters and first walls for fusion devices, next generation plasma generators are needed. A Material Plasma Exposure eXperiment (MPEX) has been proposed to address this need through the generation of plasmas in front of the target with electron temperatures of 1-15 eV and electron densities of 1020 to 1021 m-3. Heat fluxes on target diverters could reach 20 MW/m2. To generate this plasma, a unique radio frequency helicon source and heating of electrons and ions through Electron Bernstein Wave (EBW) and Ion Cyclotron Resonance Heating (ICRH) has been proposed. MPEX requires a series of magnets with non-uniform central fields up to 2 T over a 5-m length in the heating and transport region and 1 T uniform central field over a 1-m length on a diameter of 1.3 m. Given the field requirements, superconducting magnets are under consideration for MPEX. In order to determine the best construction method for the magnets, the cryogenic refrigeration has been analyzed with respect to cooldown and operational performance criteria for open-cycle and closed-cycle systems, capital and operating costs of these system, and maturity of supporting technology such as cryocoolers. These systems will be compared within the context of commercially available magnet constructions to determine the most economical method for MPEX operation. The current state of the MPEX magnet design including details on possible superconducting magnet configurations is presented.

Physical program and diagnostics of the T-15 upgrade tokamak (brief overview)

Kurchatov Institute is upgrading now the T-15 tokamak to the machine with D-shaped plasma and copper magnetic system, capable for realizing lower and upper single-null and double-null magnetic configurations. The heating and current drive (CD) system consisting of the neutral beam injection (NBI), electron cyclotron resonance heating (ECRH/CD), electron Bernstein waves (EBW) heating and CD, ion cyclotron resonance heating (ICRH/CD), helicon and Lower Hybrid (LH) waves heating and CD is aiming to provide an effective heating of both electrons and ions, and on- and off-axis CD. The main research topics foreseen are the features of the confinement at high magnetic field and low aspect ratio, Advanced Tokamak regimes, steady-state operation, effects of turbulence with an emphasis on the role of the radial electric field Er, Geodesic Acoustic Modes (GAM) and Zonal Flows (ZF) in transport and confinement (including plasma self-organization, profile resiliency, influence of the q-profile), investigations of MHD effects and disruptions, Alfvén Eigenmodes (AE) and fast particles. Extended set of advanced diagnostics with identical equipment located at two toroidal positions will contribute to the 3D reconstruction of various types of the plasma structures like quasicoherent modes and long-range correlations.

Influence of upper hybrid resonance localized oscillation on X-B mode conversion efficiency for high-β National Spherical Torus Experiment in nonlinear regime

Ever increasing needs and capabilities in high power radio frequency waves heating and current drive scenarios of present and future magnetic confined fusion plasmas motivate expansion of understanding for vast variety of ever upcoming nonlinearities in such levels of power. Among many motivating experiments, one of the most relevant and actively studied in the regime for electron Bernstein wave (EBW) heating is high-β National Spherical Torus Experiment. A very special type of large amplitude electron plasma oscillations known as localized upper hybrid (UH) mode is demonstrated. It is shown that the mutual synergetic interaction of EBW and the localized UH mode can significantly shift the resonance layer about △x∼0.9 mm compared to the prediction of linear theory and consequently can explain the considerable reduction of conversion value around 35% observed in our modelling. This reduction is due to scale up of density scale length, Ln, at the new UH resonance (UHR) location followed by the increase of Budden parameter, η, which varies from 0.18 predicted by linear aspect to 0.40 in new position of UHR layer obtained by our modelling. Moreover, the parametric instabilities in the form of ion decays and dispersion of localized UH mode, approximately 7 mm due to the finite electron temperature account, are also observed which have an important contribution in reduction of conversion efficiency.

Commissioning of the 28 GHz ECRH power transmission line for the TJ-II stellarator

The commissioning of the 28GHz 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.