Lower Hybrid Waves Research Articles

Lower-Hybrid Drift Waves Driving Electron Nongyrotropic Heating and Vortical Flows in a Magnetic Reconnection Layer.

We report measurements of lower-hybrid drift waves driving electron heating and vortical flows in an electron-scale reconnection layer under a guide field. Electrons accelerated by the electrostatic potential of the waves exhibit perpendicular and nongyrotropic heating. The vortical flows generate magnetic field perturbations comparable to the guide field magnitude. The measurements reveal a new regime of electron-wave interaction and how this interaction modifies the electron dynamics in the reconnection layer.

Diagnostic development for parallel wave-number measurement of lower hybrid waves in EAST.

An eight-channel magnetic probe diagnostic system has been designed and installed adjacent to the 4.6 GHz lower hybrid (LH) grill antenna in the low-field side of the Experimental Advanced Superconducting Tokamak (EAST) in order to study the n∥ evolution of LH waves in the first pass from the launcher to the core plasma. The magnetic probes are separated by 6.6 mm, which allows measurement of the dominant parallel refractive index n∥ up to n∥ = 5 for 4.6 GHz LH waves. The magnetic probes are designed to be sensitive to the magnetic field component perpendicular to the background magnetic field with a slit on the casing that encloses the probe. The intermediate frequency stage, which consists of two mixing stages, down-coverts the frequency of the measured wave signals at 4.6 GHz to 20 MHz. A bench test demonstrates the phase stability of the magnetic probe diagnostic system. By evaluating the phase variation of the measured signals along the background magnetic field, the dominant n∥ of the LH wave in the scrape-off layer has been deduced during the 2019 experimental campaign. In the low density plasma, the measured dominant n∥ of the LH waves is about 2.1, corresponding to the main peak 2.04 of the launched n∥ spectrum. n∥ deduced by the least-squares linear fit method remains near this value in the low density plasma with a high spatial correlation magnitude of 0.9. With an eight-channel probe system, a wave-number spectrum has also been deduced, which has a peak near to the measured dominant n∥.

Discrete Alfvén eigenmodes excited by energetic particles in China Fusion Engineering Test Reactor

In burning plasmas, discrete Alfven eigenmodes ( α TAEs, where α denotes a parameter of plasma pressure gradient) can be readily destabilized by energetic particles under the wave-particle resonance condition. In this study, we theoretically analyze the physical features of α TAEs in the China Fusion Engineering Test Reactor (CFETR), which is viewed as the next generation device in the Chinese magnetic confinement fusion program. One of the scientific objectives of the device is to achieve steady-state operation. There are three reference schemes for baseline steady-state operation, namely Case 1: NB+EC, Case 2: NB+EC+LH, and Case 3: EC+LH, where NB, EC, and LH represent neutral beam, electron cyclotron wave, and lower hybrid wave, respectively. Within the ideal magnetohydrodynamic (MHD) description, we focus on the physical characteristics of α TAEs and correlation between the radial profile of total current density and α TAEs for the three scenarios of CFETR operation with the help of a MHD eigenvalue code. The numerical results indicated that multiple branches of α TAEs can be trapped in potential wells of the ballooning drive under the CFETR operation condition. In the inner region, the bootstrap current density is large, and the total current density profile, defined by the peak of the radial profile of total plasma current density is relatively flat. Therefore, there are abundant α TAEs in this region. However, in the outer region, the bootstrap current is relatively small, and the total current density decreases gradually. So, α TAEs are hardly seen in this region. The α TAEs in the inner region are quasi-marginally stable within the ideal MHD description. In the CFETR operations, the energetic particles generated can destablize the MHD α TAEs under the wave-particle resonance conditions. We study the stability features of α TAEs interacting with energetic particles by employing linear gyrokinetic-MHD hybrid eigenvalue codes. The multiple branches of α TAEs are excited under the various resonance conditions. Moreover, the multiple branches of α TAEs can also be excited by energetic particles with virial energy. Furthermore, higher-frequency α TAEs may be destabilized by energetic particles with higher energy. These α TAEs could change the distribution of energetic particles in phase space and cause the loss of a large number of particles, which may affect the plasma confinement.

Studies of a Lower-Hybrid Wave Driven Plasma Equilibrium with a Hybrid-MHD Model on the TST-2 Spherical Tokamak

Studies of a Lower-Hybrid Wave Driven Plasma Equilibrium with a Hybrid-MHD Model on the TST-2 Spherical Tokamak

Mechanism of the active divertor flux control by the supersonic-molecular-beam-injection with lower hybrid wave-induced magnetic perturbations on the EAST tokamak

The redistribution of the divertor flux caused by the synergy of the supersonic-molecular-beam-injection (SMBI) and the magnetic perturbations induced by lower hybrid waves (LHWs), has been observed on the Experimental Advanced Superconducting Tokamak (EAST) (Li et al 2013 Nature Phys. 9 817). To reveal the physical mechanism, simulations with good agreements to the experimental findings are first performed by utilizing a self-consistent fluid 3D edge plasma Monte-Carlo code coupled to a kinetic neutral particle transport code. The ions and electrons originating from the ionization of injected neutral particles in the plasma edge flow along the magnetic flux tube towards to the divertor, thus directly increasing the divertor flux on the split strike lines in the footprint. Combining this with the multi-lobe structure of the edge magnetic topology, actively controlling the divertor flux can be realized by adjusting the SMBI position or the phase of the magnetic perturbations.

Landau quantization effects on hybrid waves in semiconductor plasmas

The impact of Landau quantization on the lower hybrid waves (LHWs) exciting in the semiconductor plasmas, due to classical electron beam, is studied. The quantum hydrodynamic model is employed to derive the dispersion relation of low-frequency electrostatic waves for instability analysis of the LHWs. The quantum effects include Fermi degenerate pressure, Bohm potential, Landau quantization, and exchange-correlation potential for semiconductor plasma species. The dispersion relation is elaborated both quantitatively and qualitatively for the special case of GaAs in the presence of external magnetic field $$\mathbf{B}_0$$. The impact of different plasma parameters such as density ratios of electrons and holes $$\alpha $$, beam temperature $$T_{\mathrm{b}}$$, speed of beam electrons $$v_0$$, propagation angle $$\theta $$, cyclotron frequency $$\omega _{\mathrm{ce}}$$, temperature of electrons and holes $$T_{\mathrm{e}}$$, $$T_{\mathrm{h}}$$ and Landau quantization $$\eta $$, on the dispersion relation of LHWs, is studied. The instability of the LHWs remarkably increases by increasing the concentration of the semiconductor electrons, propagation angle of the wave vector and the beam speed, for the magnetic quantization parameter. However, the instability decreases by increasing the temperature of beam. It is noticed that at $$(T_{\mathrm{b}}>v_0)$$, the $$\eta $$ has no effect on the instability.

Kinetic theory of parametric instabilities of lower hybrid waves in tokamaks in the electromagnetic framework

Parametric instabilities during the injection of lower hybrid waves into tokamak edge plasmas have received considerable attention for affecting the current drive at high density, mainly through spectral broadening. Here, kinetic theory of parametric instabilities was revisited in the electromagnetic framework. From Vlasov–Maxwell equations in the guiding center formalism, the theory of parametric instabilities in the electromagnetic framework is presented with appropriate nonlinearity treatment. Numerical calculations are performed under realistic lower hybrid current drive experimental parameters. Comparison between the electrostatic and electromagnetic results shows that the electromagnetic corrections have a stabilizing effect on the parametric process in all decay channels. The quantitative differences in growth rates, as well as in the most unstable regions, might have a non-negligible impact on the amplification factor of the parametric instabilities. However, with the dependence of parametric instabilities on the local plasma parameters, calculations show the same tendency between the electromagnetic and electrostatic model in a wide parameter range.

Electron Bernstein wave conversion of high-field side injected X-modes in QUEST

This paper presents a detailed design of the Q-shu University experimental steady state spherical tokamak’s (QUEST’s) high-field side (HFS) injection system for electron Bernstein wave (EBW) excitation and the results of an experimental comparison of the HFS eXtraordinary X-mode and low-field side (LFS) ordinary O-mode injection of 8.2 GHz radio frequency (RF) power. Waveguides, as an alternative to mirror polarizers for transmitting RF X-mode power from LFS to HFS for EBW conversion, were used instead of the installation of an RF mirror. Testing of LFS-to-HFS RF power transmission at 8.2 GHz, using an RG-50-type vacuum waveguide in a bench-scale device filled with SF6 gas at 0.03 Mpa, revealed that an RF power of 10.8 kW could traverse the fundamental electron cyclotron resonance layer for 60 s without breakdown. The short-length, open-ended waveguide antenna used in the HFS injection-induced wave diffraction reduced the efficiency of power delivery to the upper hybrid resonance (UHR) by approximately 7% at an electron temperature of 50 eV. The HFS injection was able to produce brighter camera images than the standard LFS injection. The location of the UHR, as estimated by measuring the density with an interferometer, agreed with its location as measured by plasma radiation low-field, side-edge positions shown by fast camera imaging. This indicates that the plasma was produced by mode-converted EBW. The HFS injection had an absorption efficiency of 96%, compared to 40% for LFS. A greater fluctuation of floating potential adjustable to the lower hybrid wave (LHW) was observed in the HFS case by installing a Langmuir probe, confirming that EBW conversion efficiency was higher in the HFS case. Moreover, after setting the poloidal field to BPF = 7.6 mT, plasma current (IP) in the HFS peaked at 1.3 kA, as opposed to 0.3 kA for LFS, despite LFS injection having a total power of 55 kW, compared to 40 kW for HFS. However, as the impurity level was comparatively high, it is believed that this IP is dominated by pressure-drive, which makes it difficult to analyze EBWCD. Finally, the line-integrated density in the HFS injection peaked at 1.6 × 1018 m−2, compared to 8 × 1017 m−2 in the LFS one.

Full-wave model for the lower hybrid wave electric field vector with synthetic turbulence on Alcator C-Mod

The effects of synthetic turbulence on lower hybrid (LH) wave field magnitude and polarization are studied on Alcator C-Mod. Three synthetic turbulence models are evaluated to assess the impact of filaments, holes, and periodic fluctuations on the LH wave electric fields. For all three cases, the synthetic turbulence can greatly impact the LH wave magnitude and polarization. Back scattering, interference and edge/SOL absorption of the LH wave is observed. This impact is shown to depend on the wavelength and amplitude of the fluctuations. The strongest effect appears for fluctuations at high amplitudes and wavelengths comparable to the LH wavelength. This synthetic turbulence model can be used as inputs into a synthetic diagnostic to calculate the lower hybrid wave field magnitude and direction measured by dynamic Stark effect spectroscopy. It will be shown that fluctuations at these high amplitudes and wavelengths may explain the experimentally measured results. The effect of fluctuations on modifying LH polarization are also shown to be anti-correlated with SOL power losses, similar to recently observed experimental trends on Alcator C-Mod. These SOL losses may have detrimental impact on LHCD efficiency.

Early Time Evolution of Turbulence in the Space Environment by Neutral Beam Injection

AbstractThe Space Measurement of A Rocket‐released Turbulence mission will demonstrate the production and evolution of turbulence in the space environment via a cascade of plasma physics processes. A sounding rocket will inject neutral barium atoms into the upper ionosphere at high speed across the magnetic field. The barium atoms photoionize, and the magnetic field confines them, leading to an ion ring distribution in velocity space that is unstable to electrostatic lower hybrid waves. Nonlinear induced scattering of lower hybrid waves produces electromagnetic magnetosonic and whistler waves that rapidly escape the source region and propagate into the radiation belts. In this paper, we present models and simulations of early time parts of this cascade until the lower hybrid waves are generated. We study the neutral atom injection, expansion, and ionization analytically and computationally. In a realistic scenario, the velocity distribution function is found to be nongyrotropic instead of a perfect ring in the plane perpendicular to the magnetic field. Using these results, we examine a representative region of the barium cloud and perform particle‐in‐cell simulations of the excitation of the lower hybrid waves. Of particular interest is the energy extracted from the kinetic energy of the barium ions, as this will ultimately affect the amplitude of the electromagnetic waves in the radiation belts. We find that for velocity distribution functions that deviate from an ideal ring distribution, the energy extracted from the fast barium ions increases.

Experimental investigation of lower hybrid current drive induced plasma rotation on the experimental advanced superconducting tokamak

Rotation and its shear can reduce the magnetohydrodynamic instabilities and enhance the confinement. The LHCD has been proposed as a possible means of rotation driving on a future fusion reactor. Exploring the mechanisms of LHCD rotation driving on the current tokamaks can provide important reference for future reactors. On EAST, it was previously shown that 2.45 GHz LHCD can drive plasma toroidal rotation and the change of edge plasma rotation leads the co-current core rotation to increase. At higher frequency, 4.6 GHz lower hybrid wave can more effectively drive co-current plasma toroidal rotation. On EAST, at the lower current, the effects of different LHCD power on plasma toroidal rotation are analyzed. Higher power LHCD has a better driving efficiency. The effect of safety factor (<i>q</i>) profile on toroidal rotation is also presented. The LHCD can change the profile of safety factor due to current drive. It is found that when the power exceeds 1.4MW, the <i>q</i> profile remains unchanged and the rotation changes only very slightly with LHCD power, suggesting that the current profile is closely related to rotation. In order to further analyze the dynamic process of plasma toroidal rotation driven by lower hybrid current drive on EAST, the toroidal momentum transport due to LHCD is deduced by using the modulated LHCD power injection. Based on the momentum balance equation, the toroidal momentum diffusion coefficient (<i>χ</i><i><sub>φ</sub></i>) and the toroidal momentum pinch coefficient (<i>V</i><sub>pinch</sub>) are obtained by the method of separation of variables and Fourier analysis for the region where the external momentum source can be ignored. It is found that the momentum diffusion coefficient (<i>χ</i><sub><i>φ</i></sub>) and momentum pinch coefficient (<i>V</i><sub>pinch</sub>) tend to increase from the core to the outer region. This is consistent with the characteristic that the toroidal rotation velocity first changes in the outer region and then propagates to the core when the toroidal rotation is driven by LHCD.

Progress of physics understanding for long pulse high-performance plasmas on EAST towards the steady-state operation of ITER and CFETR

Recently, the first ever 100 s long, steady-state H-mode discharge with good control of impurities, core and edge MHD stabilities, and heat exhaust was demonstrated in the Experimental Advanced Superconducting Tokamak (EAST) using the ITER-like (International Tokamak Experimental Reactor) tungsten upper divertor. Using both radio frequency (RF) power and neutral beam injection (NBI) heating, EAST has demonstrated fully non-inductive scenarios with an extension of fusion performance at high density and low rotation: β P ∼ 2.5, β N ∼ 2.0, H98,y2 ∼ 1.2, bootstrap current fraction fBS ∼50% at q95 ∼ 6.8. With pure RF power heating, plasmas have been maintained for up to 21 s (over 40 times the current relaxation time) with zero loop voltage and small edge localized modes (ELMs) at high density (ne/nGW ∼ 0.6–0.8), β P ∼ 2.0, β N ∼ 1.6, and ƒBS ∼47%. Experimental investigations show how plasma current profiles, turbulent transport and radiation properties self-consistently evolve toward fusion relevant steady state conditions. Modeling and physics experiments have confirmed the synergistic effects between electron cyclotron heating (ECH) and low hybrid wave (LHW), where ECH enhances the heating and current drive from LHW injection, enabling fully non-inductive operation at higher density. Small/no ELMs facilitate the RF power coupling in the H-mode phase and reduce divertor erosion. A low tungsten concentration was observed at high β P with a hollow profile in the core. Reduction of the peak divertor heat flux with f rad of up to 40% was compatible with the high β P scenario by using active radiation feedback control. With features such as dominant electron heating, zero/low NBI torque and an ITER-like tungsten divertor, fully non-inductive high-performance experiments on EAST offer unique contributions towards the succesful operation of ITER and CFETR (the Chinese Fusion Engineering Testing Reactor).

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.

Integrated infrared and visible tangential wide-angle viewing systems for surface temperature measurement and discharge monitoring in EAST

Two newly developed integrated infrared and visible tangential wide-angle (WA) viewing systems (VS) have been mounted in the Experimental Advanced Superconducting Tokamak (EAST) to provide temperature measurement and hot spot monitoring of the plasma facing components (PFC) for machine protection and further physics studies of various plasma parameters such as the scrap-off layer power fall-off length λq, the peak heat flux on the divertor target qpeak etc. The systems use endoscopes with reflective optics to obtain a wide-angle view (49.5°×62°). Via endoscope, the 3-5 μm middle wave infrared camera and the visible camera with spectral range of 400−700 nm, the WAVS could provide simultaneous real time infrared and visible image data of the first wall components in the same field of view (FOV). Some key components e.g. the 4.6 GHz lower hybrid wave (LHW) antenna, the ion cyclotron resonance frequency (ICRF) antenna, the high field side region of neutral beam injection in port F, and approximately 78° more toroidal coverage of outer divertor regions with 27° more toroidal coverage of inner divertor regions are firstly covered by the camera system. This paper presents the diagnostic description of the newly developed WAVS and preliminary experimental results observed including fast-ion losses to the inner wall during neutral beam injection (NBI), single edge localized mode (ELM) observation on upper outer divertor, hot spots on antenna guide limiters and temperature and heat flux distribution on the tungsten and graphite divertor targets in EAST.

Universality of Lower Hybrid Waves at Earth's Magnetopause

AbstractWaves around the lower hybrid frequency are frequently observed at Earth's magnetopause and readily reach very large amplitudes. Determining the properties of lower hybrid waves is crucial because they are thought to contribute to electron and ion heating, cross‐field particle diffusion, anomalous resistivity, and energy transfer between electrons and ions. All these processes could play an important role in magnetic reconnection at the magnetopause and the evolution of the boundary layer. In this paper, the properties of lower hybrid waves at Earth's magnetopause are investigated using the Magnetospheric Multiscale mission. For the first time, the properties of the waves are investigated using fields and direct particle measurements. The highest‐resolution electron moments resolve the velocity and density fluctuations of lower hybrid waves, confirming that electrons remain approximately frozen in at lower hybrid wave frequencies. Using fields and particle moments, the dispersion relation is constructed and the wave‐normal angle is estimated to be close to 90° to the background magnetic field. The waves are shown to have a finite parallel wave vector, suggesting that they can interact with parallel propagating electrons. The observed wave properties are shown to agree with theoretical predictions, the previously used single‐spacecraft method, and four‐spacecraft timing analyses. These results show that single‐spacecraft methods can accurately determine lower hybrid wave properties.

4.6 GHz lower hybrid wave power control system for EAST.

In order to achieve steady state operation in experimental advanced superconducting tokamak (EAST), a 6 MW/4.6 GHz lower hybrid current drive system with twenty-four 250 kW/4.6 GHz high power klystron amplifiers (model VKC7849A) has been developed. A 4.6 GHz lower hybrid wave (LHW) power control system (4.6 GHz LPCS) with continuous wave mode has been set up, which was used to control the LHW power as needed. This paper introduces the architecture and software design of LPCS system, including microwave preamplifier subsystem, power measurement subsystem, power control computers, and so on. Proportional integral differential (PID) closed-loop control time is less than 100 μs due to the use of high-speed acquisition cards and user-programmable Field Programmable Gate Arrays. So far, high power CW operation and power control experiments with PID feedback mode by plasma parameters (beta, loop voltage, and so on) were performed in detail. Results show that 4.6 GHz LPCS is stable and reliable, suggesting that it meets the needs of EAST.

A Survey of Plasma Waves Appearing Near Dayside Magnetopause Electron Diffusion Region Events

AbstractOne of the major unresolved questions regarding the magnetic reconnection phenomenon is how plasma waves impact the process. In 2015, the National Aeronautics and Space Administration launched the four‐satellite Magnetospheric Multiscale Mission to study magnetic reconnection, especially on the electron scale. Since launch, it has identified several wave modes below the electron plasma frequency that occur near the dayside reconnection X‐line. These include large‐amplitude parallel electrostatic waves, whistler mode waves, lower hybrid waves, and turbulence associated with a corrugated current structure. We survey 23 electron diffusion region events observed by Magnetospheric Multiscale Mission at the dayside magnetopause to help understand how these wave modes impact the reconnection process. Common wave modes are identified, as well as their typical location within the reconnection layer (e.g., electron diffusion region, ion diffusion region, separatrix, and inflow and outflow jets). We find that, with a few exceptions, electromagnetic whistlers are most commonly confined to the separatrices of the exhaust boundary. Lower hybrid waves are found on the magnetosphere side of the current layer and do not make it to the X‐line. The wave modes that typically occur closest to the dissipation region are the corrugated current structures and large‐amplitude parallel electrostatic waves.

Electron acceleration at comet 67P/Churyumov-Gerasimenko

We report the observation by the Ion and Electron Sensor (IES) of energetic (>1 keV) electrons in the plasma environment of comet 67P Churyumov-Gerasimenko (67P). Most of the electrons in the cometary coma are expected to be of solar wind, photoionization, or electron impact origin and are therefore not expected to exceed some hundreds of eV in energy. During the Vega flybys of comet Halley, 1 keV electrons were also observed, and these are explained as having been accelerated by lower hybrid (LH) waves resulting from the two-stream instability involving the solar wind and pickup-ion flows. These waves resonate with the cyclotron motion of the ions and the longitudinal motion of electrons and are on the order of several Hz, at least in the case of 67P. We postulate that the energetic electrons we have observed intermittently during December 2015 through January 2016 are also the result of such a process and that Landau damping causes the acceleration and subsequent abrupt decrease in this energy (also seen at Halley). We show from this study an event on 19 January 2016 when IES simultaneously observed accelerated electrons, solar wind protons, water ions, and LH waves. A dispersion analysis shows that the ion–ion two-stream instability has positive growth rates for such waves during the observation period.

The Wave Energy Up Conversion of Plasma Wave in Inhomogeneous Ionosphereic Plasma

Different types of instabilities are observed in the thermodynamically nonequilibrium Earth's ionosphere. Effective energy exchange process among waves may takes place through nonlinear interaction modes because of availability of free energy. We consider gradients in density and magnetic field is present in the system which support drift wave turbulence. In this study we concern on the wave energy up conversion of electrostatic nonresonant lower hybrid wave through plasma maser instability in the mid-altitude ionospheric region. We have formulated the growth rate of lower hybrid wave by Vlasov-Poisson mathematical frame and estimated its value by observational data.

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.