Lower Hybrid Frequency Range Research Articles

The Lower Hybrid Frequency Range Wave Emission in the Ohmic Discharge of the FT-2 Tokamak

The Lower Hybrid Frequency Range Wave Emission in the Ohmic Discharge of the FT-2 Tokamak

Resonance cones in cold plasma: Origin, singularities, and power flow

In magnetized tenuous plasma, typical at the plasma edge of fusion devices, a nearly electrostatic wave mode with relatively enhanced electric field can propagate along a specific angle with the magnetic field. For this characteristic, it is known as a “resonance cone.” For instance, these waves can be excited by radio frequency antennas in the ion-cyclotron and lower-hybrid range of frequencies. We consider the resonance cones emitted by idealized spatially extended sources. In 2D, we use a novel geometric construction which generalizes the d'Alembert solution to curved boundaries/moving sources, and show, for the first time, that singular electric fields arise under these conditions, thereby bringing the resonance cones in line with the other resonances of the cold plasma theory. Still in 2D, we give an expression for the amount of power radiated by resonance cones in terms of surface quantities on the source, which is finite despite the singular electric field. We generalize the conclusions regarding the presence and location of singular electric fields to the 3D electromagnetic case.

Electrostatic lower hybrid turbulence due to magnetic islands and particle heating in magnetopause reconnection regions

This investigation presents a model to understand the magnetopause turbulence in the lower hybrid frequency range at reconnection regions as observed by the magnetospheric multiscale mission. We have modeled 3D electrostatic lower hybrid wave (LHW) in cold magnetized plasma in the presence of magnetic islands. In this model, we have developed the modified nonlinear Schrödinger equation, contemplating that nonlinear effects are due to ponderomotive force. The dynamical equation of the system thus obtained is solved using numerical simulation techniques. The simulation results illustrate the formation and evolution of localized structures and the current sheets. Due to the nonlinear effects and magnetic reconnection, the current sheets become turbulent with time. A semi-analytical model is also given to illustrate the scale size of current sheets. The turbulent power spectra have also been studied. The spectral index of the power spectrum is found to be −2.7 (approximately) in the perpendicular direction and −2.2 (approximately) in the parallel direction, indicating anisotropy. Using the Folker–Plank diffusion equation with the new velocity space diffusion coefficient, we find the distribution function of energetic electrons due to these turbulent structures. We propose that LHW may be responsible for plasma heating in magnetopause.

The lower hybrid frequency range wave emission in the ohmic discharge of the FT-2 tokamak

The paper presents observations of plasma intermediate frequency range wave emission ((200--1000) MHz) performed in ohmic discharge of the FT-2 tokamak. The dependencies of the RF signal amplitude and spectrum on the plasma parameters and on the location of the probes are presented. Two possible explanations for this effect is proposed. One of them is related to the distortion of the electron distribution function due to the strong ripple of the magnetic field in the FT-2 tokamak. Second one is related to the existence of a beam of run-away electrons, the buildup of a fan instability responsible for the generation of synchrotron radiation in the range (10-80) GHz, and a complex chain of nonlinear decay instabilities leading to such a strong decrease in the frequency of the observed oscillations. Keywords: tokamak, lower hybrid current drive, high frequency oscillations, parametric decay instabilities.

Ion‐Ion Cross‐Field Instability of Lower Hybrid Waves in the Inner Coma of Comet 67P

AbstractWe show that an ion‐ion cross‐field streaming instability between cold newborn cometary ions and heated heavy ions that were picked up upstream is likely a contributing source of observed lower hybrid (LH) waves in the inner coma of comet 67P/Churyumov‐Gerasimenko. Electric field oscillations in the LH frequency range are common here, and have previously been attributed mainly to the lower‐hybrid drift instability, driven by gradients associated with observed local density fluctuations. However, the observed wave activity is not confined to such gradients, nor is it always strongest there. Thus, other instabilities are likely needed as well to explain the observed wave activity. Several previous works have shown the existence of multiple populations of cometary ions in the inner coma of 67P, distinguished by differences in mass, energy and/or flow direction. We here examine two selected time intervals in October and November 2015, with substantial wave activity in the LH frequency range, where we identify two distinct cometary ion populations: a bulk population of locally produced, predominantly radially outflowing ions, and a more tenuous population picked up further upstream and accelerated back toward the comet by the solar wind electric field. These two populations exhibit strong relative drifts (∼20 km/s, or about five times the pickup ion thermal velocity), and we perform an electrostatic dispersion analysis showing that conditions should be favorable for LH wave generation through the ion‐ion cross‐field instability.

Measurement of electromagnetic waves from runaway electrons.

Electromagnetic waves emitted during a tokamak discharge can be partially ascribed to coupling with plasma waves. In particular, in the presence of runaway electrons, the electromagnetic waves deliver information, otherwise inaccessible, about kinetic instabilities excited by the fast particles. Experiments aimed at studying radio frequency emissions from runaway electron scenarios during different stages of plasma discharge have been carried out at the Frascati Tokamak Upgrade. Frequencies in the range of lower hybrid and whistler waves have been explored, in the presence of relativistic electrons with different energies, ranging from a few to tens of MeV. A pronounced sensitivity of the radio frequency measurements in detecting driven instabilities is observed, providing the possibility to exploit this kind of technique as a monitor of the instability processes and for studies of the fast electron activity. In particular, in this work, we propose a simplified analysis of the frequency scaling of a specific family of kinetic instabilities arising at the lower hybrid frequency range during the current ramp-up stage. The study is performed with respect to the density profile and the wave vector coupling conditions and is aimed at obtaining a rough estimate of the most likely radial location of the interaction between the runaway electron beam and plasma waves at the emission times of the observed signals.

Particle simulations on propagation and resonance of lower hybrid wave launched by phased array antenna in linear devices

In this work, we performed first-principles electromagnetic-kinetic simulations to study a phased antenna array and its interaction with deuterium plasmas within the lower hybrid range of frequency. We first gave wave accessibility and resonance results, which agree well with theoretical prediction. In addition, we further investigated the antenna power spectrum with different antenna phases in the presence of the plasma and compared it with that in a vacuum, which directly indicates wave coupling and plasma absorption. Furthermore, for the case with zero phasing difference, our simulation results show that, albeit the launch is away from the accessibility region, tunneling effect and mode conversion occurred, which enhanced coupling and absorption. Moreover, consistent interactions between the injected wave and the plasma concerning various antenna phase differences are shown. We presented the inchoate response of the plasma in terms of the launching directions. Our results could be favorable for the engineering design of wave heating experiments with a tunable phased antenna array in linear devices, such as simple magnetic mirrors or tandem mirrors.

Overview of GLOBUS-M2 spherical tokamak results at the enhanced values of magnetic field and plasma current

The paper provides an overview of the results obtained on the spherical tokamak Globus-M2 in 2019–2020. The experiments were performed with the toroidal magnetic field up to 0.8 T and plasma current up to 0.4 MA (80% of the design values). The temperature of electrons 1 keV and ions 800 eV at the plasma density of 1020 m−3 were recorded at neutral beam injection (850 kW, 28.5 keV). Heat conductivity analysis was made by means of the codes ASTRA 7.0, NCLASS, SPIDER, NUBEAM, 3D fast ion tracking algorithm on the basis of the experimental data. A scaling for spherical tokamaks, which demonstrates strong τ E dependence on magnetic field and moderate dependence on plasma current, has been confirmed for the magnetic field up to 0.8 T. For Globus-M/M2 it is . The dependence of the normalized energy confinement time (B T τ E) on collisionality (ν*) in a wide range 0.02 < ν* < 0.2 was determined as . A non-inductively driven current was recorded during the launch of the electromagnetic waves of the lower hybrid frequency range (2.45 GHz) with the help of a toroidally oriented grill. The fraction of noninductively driven current has exceeded 70% in the discharge with a total current of 0.2 MA. The achieved values of efficiency η = (0.15–0.4) × 1019 A m−2 W−1 are comparable with the results obtained on conventional tokamaks. This paper presents the results of experiments on the study of Alfvén modes. The resulting scaling for the loss of fast ions caused by toroidal Alfvén eigenmodes demonstrates their decrease with increasing magnetic field and plasma current. Observation of Alfvén cascades made it possible to apply the method of MHD spectroscopy to determine the evolution of q min in a discharge. Also presented are the results of SOL investigation. Attention is also paid to the development of diagnostics.

Statistical investigation of electric field fluctuations around the lower-hybrid frequency range at dipolarization fronts in the near-earth magnetotail

Statistical investigation of electric field fluctuations around the lower-hybrid frequency range at dipolarization fronts in the near-earth magnetotail

Statistical investigation of electric field fluctuations around the lower-hybrid frequency range at dipolarization fronts in the near-earth magnetotail

Dipolarization fronts (DFs) are thin magnetic boundary structures, embedded in short-duration, fast earthward-moving plasma flows, so-called bursty bulk flows. Previous case studies have shown that the density gradient at DFs can excite the lower-hybrid drift instability (LHDI) and resulting kinetic-scale waves. These waves feature strong electric field fluctuations perpendicular to the ambient magnetic field and associated effective particle acceleration/heating. In this study, we statistically examine electric field fluctuations in the lower-hybrid (LH) frequency range of 61 DF events, using data from the Magnetospheric Multiscale (MMS) Mission between 2017 and 2018 when the MMS apogee was in the magnetotail. We observed that all DF events exhibit enhanced power in the electric field fluctuations in the LH frequency range. Among the observed events, the power can vary by several orders of magnitude. The waves are detected for both high and low values of the perpendicular electron density or pressure gradient. In addition, the peak wave power within the DF is often observed at the time of steepest density gradient within the DF. The results also reveal that the wave power correlates with the magnetic flux transport rate of the DFs. These findings suggest that enhanced density and pressure gradients, which can be formed by large-scale flux transport at DFs, lead to LHDI-related kinetic-scale wave signatures at the DFs, and this may modify the original gradient layer profile of the DF in the course of its propagation from the source to the observation point.

The high-power helicon program at DIII-D: gearing up for first experiments

Helicon current drive (CD), also called fast wave CD in the lower hybrid range of frequencies, has long been regarded as a promising CD tool for reactor grade plasmas. A newly installed MW-level system at DIII-D will be the first test of this technology in reactor-relevant plasmas, in the sense that full single-pass absorption is expected. A 30-module traveling wave antenna has been installed and optimized in-vessel in early 2020. The linear electromagnetic characteristics of the unloaded module array have been extensively tested both on the bench and in the vessel at instrumentation power levels. Excellent performance has been achieved, ∼2% reflected power and ∼1.5% dissipated power per module in air, in a 10 MHz band around 476 MHz. Stripline feeds on both ends of the antenna allow either co or counter CD. The installation of a 1.2 MW klystron and associated high-power electronics was completed in Fall 2020. Commissioning of the antenna is ongoing. An important goal of this experiment is to validate the helicon CD physics basis using an extensive set of new and upgraded diagnostics.

Studies of Spectral Broadening of the Lower Hybrid Wave Line in the Current-Drive Regimes and Ion Heating at the FT-2 Tokamak

The experiments at the FT-2 tokamak are described that were focused on clearing up the role of the parametric decay instabilities in decreasing the generation efficiency of the non-inductive current excited by the electromagnetic waves in the lower hybrid frequency range. The most discussed instability of such kind is the decay of the pump wave into the daughter high-frequency waves and the low-frequency ion–acoustic quasi-modes. The studies performed have shown that, under conditions of the FT-2 experiment, the ion–acoustic instability has no decisive effect on the decrease in the efficiency of the lower hybrid current drive.

Radio frequency emissions driven by energetic ions from neutral beam in KSTAR low confinement mode plasma

The tangential neutral beam injection in KSTAR low confinement mode plasma is rapidly accompanied by the electromagnetic emissions in radio frequency (RF) range (0.1–1 GHz). The RF emission is initially onset within 1 ms from the beam injection, at discrete frequencies with steadily increasing intensity. The frequency spacing for these discrete emission lines corresponds to the deuteron cyclotron frequency, at a location midway between the magnetic axis and the edge. Further, the observed discrete frequencies lie in the lower hybrid frequency (fLH) range in a broad region on the low field side (LFS). As the initial RF emission becomes saturated, there is another onset of intense RF bursts occurring at discrete frequencies, broadening the emission frequency range further, either at higher or lower frequencies. In some cases, the time interval of the intense RF bursts at the dominant frequency is comparable with the toroidal rotation period at the radial location where fLH ∼ dominant frequency. The rapid rise and saturation of RF emission intensity in a broad frequency range indicate that a small population of fast ions is sufficient for the growth of energetic particle driven instabilities on the LFS. The multiple onsets in RF emission and the intense RF burst repetition frequency comparable with toroidal rotation frequency indicate the possibility that reorganization in the anisotropic fast ion population results in localized growth of the above instabilities. A gradual decay of RF emission intensity over few tens of milliseconds indicate that enhanced population of fast ions has damping effect on these instabilities.

Observation of Parametric Decay of a Pumping Wave in Lower Hybrid Wave Heating and Current Drive Experiments at the FT-2 Tokamak

The systematization of experimental data related to development of the parametric decay instability in tokamak plasma during RF power input within the lower hybrid frequency range was performed, which allows its characteristics to be compared with theoretical representations. The spectra of pumping waves and radiation from the plasma are obtained, which illustrate the appearance of satellites that are shifted from a pump frequency by a value that is a multiple of the ion cyclotron frequencies, toward the “red” and “blue” sides. It is noted that with a rise in density, a considerable broadening of the line of a pumping wave occurs. Values of the RF power and plasma density are observed, which are threshold values for the onset of the development of parametric decay instability. Using the diagnostic complex, the analysis of the low-frequency region of the plasma oscillation spectrum was carried out. The conditions of the development of instability were analyzed within the existing theoretical representations.

A New Perspective for Dipolarization Front Dynamics: Electromagnetic Effects of Velocity Inhomogeneity

AbstractThe stability of a quasi‐static near‐Earth dipolarization front (DF) is investigated with a two‐dimensional electromagnetic particle‐in‐cell model. Strongly localized ambipolar electric fields self‐consistently generate a highly sheared dawnward electron drift on the kinetic scale in the DF. Electromagnetic particle‐in‐cell simulations based on the observed DF thickness and gradients of plasma/magnetic field parameters reveal that the DF is susceptible to the kinetic electron‐ion hybrid (EIH) instability driven by the strong velocity inhomogeneity. The excited waves show a broadband spectrum in the lower hybrid (LH) frequency range, which has been often observed at DFs. The wavelength is comparable to the shear scale length, and the growth rate is also in the LH frequency range, which are consistent with the EIH theory. As a result of the LH wave emissions, the velocity shear is relaxed, and the DF is broadened. When the plasma beta increases, the wave mode shifts to longer wavelengths with reduced growth rates and enhanced magnetic fluctuations although the wave power is mostly in the electrostatic regime. This study highlights the role of velocity inhomogeneity in the dynamics of DF which has been long neglected. The EIH instability is suggested to be an important mechanism for the wave emissions and steady‐state structure at the DF.

Avoidance of impurity-induced current quench using lower hybrid current drive

This work reports observations of a tokamak plasma that experienced a thermal quench due to a large, transient high-Z influx but avoided a current quench. This is argued to be caused by the presence of lower hybrid range of frequency (LHRF) waves that sustain a non-thermal, current-carrying electron population. In Alcator C-Mod L-mode plasmas at kA, m−3, nearly all of the current can be sustained non-inductively by injecting 700 kW of LHRF power at 4.6 GHz and . A sudden influx of a large amount of tungsten, , triggers a cooling wave that propagates at 2–3 m s−1 all the way into the core, dropping on-axis Te from 3 keV to temperatures less than measurement floor of 50 eV. An off-axis reheat begins after 100 ms, but Te profiles remain hollow for 300–350 ms after the injection. Throughout this temperature evolution, the plasma density, current and shape remain unchanged to within 10%. Following the expulsion of the tungsten, the plasma returns to its baseline conditions and the plasma ends as planned with a controlled current ramp-down. Energy balance analysis shows the LHRF power continues to be absorbed in the plasma after the thermal quench, as a significant fraction of it is needed to be consistent with radiated power measurements. Examination of current relaxation time, , and fast-electron slowing down time, , indicate the LHRF must contribute to driving current, despite the low temperatures, as the current remains nominally stationary despite ms and ms for relativistic electrons. These measurements represent an important existence proof of a possible technique for avoidance of disruptions caused by sudden, unplanned influx of impurities in the form of dust or flakes of high-Z wall material. Implications and suggestions for future experimental and modeling and simulation work are summarized.

Tests of advanced RF off-axis current drive techniques on DIII-D

The establishment of reactor-relevant radiofrequency heating and current drive techniques is a focus of work on DIII-D in the next five-year period. This paper gives an overview of the planned experimental work in the areas of (1) nearly vertically launched ECCD, (2) ‘helicon’ (whistlers or fast waves in the lower hybrid range of frequencies) current drive, and (3) high-field-side-launch (HFS) lower hybrid (slow wave) current drive. Each of these techniques addresses the need for efficient off-axis current drive for a steady-state tokamak reactor to supplement the bootstrap current and to provide current profile control, and each will be experimentally assessed at a coupled power level of ~1 MW on DIII-D in the next few years.

Discharge Oscillations in Morozov’s Stationary Plasma Thruster as a Manifestation of Large-Scale Modes of Gradient Drift Instability

The phenomenon of large-scale discharge oscillations in Morozov’s stationary plasma thruster (SPT) is physically interpreted by analyzing global modes of gradient drift instability. The problem is solved using an ideal two-fluid hydrodynamic plasma model that includes the effects of stationary electron flow, electron inertia, and spatial inhomogeneities of the magnetic field and plasma density along the accelerating channel. The frequencies and axial structure of unstable eigenmodes are calculated for typical parameters of the SPT-100 thruster. The obtained spectrum is characterized by a finite set of long-wavelength azimuthal modes in the lower hybrid frequency range, which are predominantly localized in the near-anode region of the thruster. It is shown that the eigenmodes can form wave packets the main characteristics of which in the linear stage of instability coincide with the parameters of the experimentally observed large-scale azimuthal spoke-like structures. The influence of the thruster geometry (the length and width of the accelerating channel) on the frequency characteristics of oscillations and formation of beatings is investigated.

High field side lower hybrid wave launch for steady state plasma sustainment

This paper explores how the use of waves in the lower hybrid range of frequencies (LHRF) alleviates many unresolved current drive issues when launched from the high field side (HFS) of a tokamak. Efficient, robust, reliable, steady-state current drive is a requirement for a viable tokamak fusion reactor. Launching LHRF waves from the HFS opens the ‘accessibility window’, allowing for use of a lower parallel refractive index, , for a given plasma density and on-axis magnetic field. The lower waves damp in a region of higher temperature, resulting in increased current drive efficiency and a damping profile shifted closer to the mid-radius as compared to low field side launch. Moving the LHRF antenna to the quiescent scrape off layer on the HFS reduces the likelihood of detrimental wave scattering from density fluctuations, as well as ameliorating the plasma-material-interaction and wave coupling challenges. Additional benefits with respect to fast particle and neutron fluxes can be realized by moving to the HFS. An engineering assessment of HFS launch shows that it is feasible to implement an HFS LHRF system on existing tokamaks, with fewer hurdles anticipated for future tokamaks built with HFS antenna systems integrated in the design from the start.

Semi-analytical inspection of the quasi-linear absorption of lower hybrid wave in presence of -particles in tokamak reactor

In a reactor plasma like demonstration power station (DEMO), when using the radio frequency (RF) for heating or current drive in the lower hybrid (LH) frequency range (Frankeet al.,Fusion Engng Des., vol. 96–97, 2015, p. 46; Cardinaliet al.,Plasma Phys. Control. Fusion, vol. 59, 2017, 074002), a large fraction of the ion population (the continuously born$\unicode[STIX]{x1D6FC}$-particle, and/or the neutral beam injection (NBI) injected ions) is characterized by a non-thermal distribution function. The interaction (propagation and absorption) of the LH wave must be reformulated by considering the quasi-linear approach for each species separately. The collisional slowing down of such an ion population in a background of an electron and ion plasma is balanced by a quasi-linear diffusion in velocity space due to the propagating electromagnetic wave. In this paper, both propagations are considered by including the ion distribution function, solution of the Fokker–Planck equation, which describes the collisional dynamics of the$\unicode[STIX]{x1D6FC}$-particles including the effects of frictional slowing down, energy diffusion and pitch-angle scattering. Analytical solutions of the Fokker–Planck equation for the distribution function of$\unicode[STIX]{x1D6FC}$-particles with a background of ions and electrons at steady state are included in the calculation of the dielectric tensor. In the LH frequency domain, ray tracing (including quasi-linear damping), can be analytically solved by iterating with the Fokker–Planck solution, and the interaction of the LH wave with$\unicode[STIX]{x1D6FC}$-particles, thermal ions and electrons can be accounted self-consistently and the current drive efficiency can be evaluated in this more general scenario.