Lower Hybrid Waves Research Articles

Effect of chirp on stimulated-forward Raman scattering in a magnetised plasma with rippled density for fusion applications

Stimulated Raman scattering (SRS) is one of the mechanisms limiting power scaling in inertial confinement fusion (ICF). In this work, we demonstrate the effective suppression of SRS by the combined effects of static density fluctuations and an azimuthal magnetic field with a propagating chirped laser pulse. In the presence of an azimuthal magnetic field, chirped laser pulse propagates through a density-rippled plasma and undergoes stimulated-forward Raman scattering (SFRS), resulting in two radially localized electromagnetic sidebands waves and a lower-hybrid wave. Absolute and growing modes saturate due to ion density fluctuations, which then suppress instability growth through mode coupling. The modes modified by the combined effect of chirp and azimuthal magnetic field are effectively damped after saturation. As a result, the overall growth rate of the instability reduces. The comparison of positive and negative chirp demonstrated that when a positive chirp is being used, instability is more effectively suppressed. Based on non-local theory, we have analyzed the growth of the SFRS for positive and negative chirp and estimated it for ICF-relevant parameters and observed the effect of the growth rate.

Dust density effects on electron density gradient driven lower hybrid waves in magnetized plasma

AbstractElectrostatic lower hybrid waves propagating transverse to the external magnetic field in magnetized dusty plasma are investigated, using the fluid equations and also including the effect of density gradient in plasma. Lower hybrid modes are considered important in laboratory, astrophysical as well as space plasmas in interpreting low frequency noises. Unlike other electrostatic waves, lower hybrid waves can resonantly interact with electrons as well as ions in plasma. The dust and gradient terms lead to novel growth of these waves in dusty plasma. A theoretical model is developed to investigate density gradient effects on lower hybrid waves in plasma comprised of electrons, ions and dust particles. The dependence of plasma components' density, strength of magnetic field, temperature and charge of electrons, ions and dust grains on the frequency and growth rate of electrostatic lower hybrid waves has been investigated and presented graphically. It has been observed that the number density of each plasma components directly affects the growth rate. The electrostatic waves are found to grow along the positive electron gradient and the dust effect depends on the gradient length chosen. Above a certain threshold value of gradient length, the waves are found to stabilize. A correlation of the obtained theoretical results with the experimental observations is also discussed.

Quantitative analysis of tungsten in steel by one-point calibration laser-induced breakdown spectroscopy in vacuum

Laser-induced breakdown spectroscopy (LIBS) has been proposed as a promising in-situ diagnostic approach for the elemental analysis of the co-deposition impurities on plasma-facing components (PFCs) of the experimental advanced superconducting tokamak (EAST) as well as ITER tokamak. The EAST has been equipped with an ITER-like tungsten (W) divertor, and impurities of W originating from divertor erosion can be transported into core plasma as impurities, which seriously reduces the plasma performance with power radiation losses and fuel dilution. In the present work, to achieve a more reliable quantitative analysis, a variation of the CF-LIBS algorithm, the so-called one-point calibration LIBS (OPC-LIBS), is examined for major and minor constituents in a set of W-Mo-Fe-Cr steel samples (chosen as a proxy of the co-deposition layers on the lower hybrid wave gas puffing pipe of EAST) at 5 × 10−5 mbar. The OPC-LIBS essentially employs one matrix-matched standard sample of known composition to evaluate the comprehensive correction factors of crucial experimental and spectroscopic parameters. These are essential for empirically correcting the LIBS line intensities to achieve a more precise quantitative analysis compared with the conventional CF-LIBS. In the case of OPC-LIBS measurement in vacuum, the relative error of major elements such as Fe (85.24–88.69a.t.%) is below 1.64%, while that of minor elements like W (1.91–6.27a.t.%) is within 24.79%. It is substantially better than the predicted results of CF-LIBS, in which the relative error of Fe and W in order are within 10.75% and 202.03%. The obtained results confirm the quantitative analysis capability of the OPC methodology in a tokamak environment for the in-situ LIBS system.

Long-pulse high-performance H-mode plasmas achieved on EAST

A record duration of a 310 s H-mode plasma (H98y2 ∼ 1.3, ne/nGW ∼ 0.7, fBS > 50%) has been recently achieved on experimental advanced superconducting tokamak (EAST) with metal walls, exploiting the device's improved long-pulse capabilities. The experiment demonstrates good control of tungsten concentration, core/edge MHD stability, and particle and heat exhaust with an ITER-like tungsten divertor and zero injected torque, establishing a milestone on the path to steady-state long-pulse high-performance scenarios in support of ITER and CFETR. Important synergistic effects are leveraged toward this result, which relies purely on radio frequency (RF) powers for heating and current drive (H&CD). On-axis electron cyclotron heating enhances the H&CD efficiency from lower hybrid wave injection, increasing confinement quality and enabling fully non-inductive operation at high density (ne/nGW ∼ 70%) and high poloidal beta (βP ∼ 2.5). A small-amplitude grassy edge localized mode regime facilitates the RF power coupling to the H-mode edge and reduces divertor sputtering/erosion. The high energy confinement quality (H98y2 ∼ 1.3) is achieved with the experimental and simulated results pointing to the strong effect of Shafranov shift on turbulence. Transport analysis suggests that trapped electron modes dominate in the core region during the record discharge. The detailed physics processes (RF synergy, core-edge integration, confinement properties, etc.) of the steady-state operation will be illustrated in the content. In the future, EAST will aim at accessing more relevant dimensionless parameters to develop long-pulse high-performance plasma toward ITER and CFETR steady-state advanced operation.

Simulation of parametric decay instability excited by oscillating pump near lower hybrid frequency

Parametric decay instability of lower hybrid waves plays an important role in plasma heating. Simulation of such instability requires inclusion of both electron and ion kinetics with a realistic mass ratio. In this work, the gyrokinetic electron and fully kinetic ion (GeFi) particle simulation model is used to study parametric instabilities in uniform, magnetized plasmas. A pump electric field near the lower hybrid frequency is imposed, which is treated in the dipole approximation. Three types of parametric decay processes are obtained, including purely growing modulational instability, resonant decay instability, and non-resonant decay instability. Dependences of parametric growth rates on the wavenumber, pump strength, and pump frequency are systematically investigated. The simulation results agree well with those in the linear growth stage of parametric decay instability obtained in an existing theory. Whether electron or ion nonlinearity dominates in the parametric decay process is assessed, and the importance of the electron E×B drift in the three-wave interaction process is confirmed.

Development of an arch antenna for LHCD in a spherical tokamak

A lower-hybrid frequency surface wave antenna called arch is designed and fabricated in this paper for the spherical tokamak named EXL-50. It is used to drive plasma current and heat plasma at 2.45 GHz. The arch antenna consists of 27 antenna elements. The left and right rectangular waveguides integrated with the antenna are used for microwave input and output, respectively. The refraction parallel index of this proposed antenna is , which is favorable for the lower hybrid current drive. The measurement results in the absence of plasma show that the reflection and transmission coefficients ( and ) are and , respectively. The antenna was installed and tested on EXL-50. Up to a plasma current was achieved using the arch antenna with a microwave of .

3D simulation of lower-hybrid drift waves in strong guide field asymmetric reconnection in laboratory experiments

The magnetic reconnection experiment has recently seen short wavelength ( k ρ e ∼ 1) lower-hybrid waves near the electron diffusion region in strong guide field reconnection. Based on plasma parameters from the experiment, we perform a three-dimensional fully kinetic simulation in order to investigate the generation of the lower-hybrid waves and their effects on the reconnection process. We find that the low-beta regions around the reconnection site are unstable to the lower-hybrid drift instability propagating in the outflow direction, driven by the difference between the electron and ion outflows. The waves modify the electron distributions, leading to periodic opening and closing of gaps in electron velocity space, and provide a small contribution to the anomalous resistivity. Finally, the simulation results are discussed in the context of space observations and laboratory experiments.

Lower hybrid wave nonlinear saturation and turbulence in the magnetopause

Lower hybrid wave nonlinear saturation and turbulence in the magnetopause

Hot spots induced by RF-accelerated electrons in the scrape-off layer on Experimental Advanced Superconducting Tokamak

Preventing impurity emission from hot spots on plasma-facing materials is a critical issue in the maintenance of high-performance plasma on the Experimental Advanced Superconducting Tokamak (EAST). In this study, experimental and theoretical analyses were performed to investigate the mechanism of hot spot formation. In the upper single null magnetic configuration of the EAST, two separatrices were connected to the upper (primary) and lower (secondary) X-points. Experiments on plasma configuration control indicated that the reduction in the gap between the lower (secondary) separatrix and lower hybrid antenna is effective in preventing hot spot formation on the lower divertor, which frequently emits impurities in long-duration discharges. This effectiveness was quantitatively confirmed by magnetic field lines tracking simulation and calorimetric measurement of divertors in the experiment. Two-frequency power modulation of the lower hybrid wave (LHW) was conducted to evaluate power deposition on the scrape-off layer (SOL) during propagation from the LHW antenna to the main plasma. This experiment clarified that LHW-accelerated electrons in the SOL via collision damping deliver their energies to hot spots along the magnetic field line. These findings help alleviate or even eliminate the formation of hot spots and maintain the performance of plasma.

Plasma heating by electron cyclotron wave and the temperature effects on lower hybrid current drive on EAST

This paper presents the progress in the long—pulse operation of the electron cyclotron (EC) system and the achievements in high—electron temperature plasmas by the combined EC and lower hybrid (LH) waves heating since the EC system was built in 2015. An electron temperature of up to 12 keV with a duration over 100 s was realized by the simultaneous heating of EC and LH waves at the line-averaged density ∼ 1.8 × 1019 m−3. The plasma heating effect strongly depends on the location of EC power deposition. H-mode plasmas with solely EC wave auxiliary heating have been obtained on EAST for the first time. These H-mode discharges show an enhanced confinement factor H 98(y,2) around 1.0, which is higher than the previous H-mode using LH power alone (Xu et al 2011 Nucl. Fusion 51 072001). The total heating power is very close to the threshold value for L–H transition according to the international tokamak scaling. In addition to the temperature effects inside the separatrix, higher electron temperature produced by EC wave is found to reduce the LH power loss in the scrape-off layer due to collisional absorption, which is beneficial to further increase the LH current drive efficiency. Ray-tracing/Fokker–Planck modeling results indicate that higher electron temperature can shorten the LH wave propagation on EAST in a multiple-pass regime, thus decreasing the collisional dissipation.

Harmonic Structure of Lower Hybrid Waves Driven by Energetic Ions at 4000 km Altitude: PIC Simulation

AbstractPast satellite observations in the polar region at about 4,000 km altitude have reported lower hybrid waves (LHWs) and their harmonics, but the excitation mechanism of the harmonic LHWs has not yet been clarified. We perform one‐dimensional, electromagnetic, full particle‐in‐cell simulations by setting parameter values in the ranges observed by the satellite. The results from the two runs, that is, for ωpe/Ωe = 0.1 and 0.2, show that the energetic ions can generate the harmonic LHWs in both cases. However, in the former case, more harmonic LHWs with larger amplitudes are created and stronger background‐ion acceleration is observed. Thus, our present simulations have solved a previously unsolved problem, revealing the possibility that harmonic LHWs are involved in ion acceleration phenomena commonly observed in the polar region, such as ion outflow.

Rocket‐Released Neutral Clouds in the Ionosphere: Formation, Evolution, and Detection

AbstractReleasing diffuse artificial clouds into the space environment using a rocket or spacecraft can modify the natural plasma state. This uses space itself as a laboratory for studying plasma phenomena that cannot be reproduced on the ground. The Space Measurement of A Rocket‐released Turbulence (SMART) mission will inject a beam of barium neutral vapor into the ionosphere and perpendicular to the Earth's magnetic field. Barium atoms will be photoionized, forming an ion ring distribution that is unstable to lower hybrid waves. Large amplitude lower hybrid waves nonlinearly scatter to whistler and magnetosonic waves that can propagate out to the magnetosphere. This paper details the theory, modeling, and simulation we used to design this release experiment to optimize the energy in the plasma waves under a variety of constraints. A product of this analysis is quantitative predictions for some in situ and remote measurements. Hydrocode simulations of barium vapourization and acceleration via shaped charge provide the initial state of the neutral beam. We optimize the apogee, orientation, and position of the payload and instruments. Cloud dynamics are simulated with a direct simulation Monte Carlo technique, which includes photoionization with metastable barium states, collisions with the neutral background atmosphere, optical line emission, gravity, and electromagnetic forces. We predict the plasma density measured by the instrument payload and the remotely measured optical intensity. We also examine how the plasma wave growth differs at the measurement location compared to the bulk of the barium cloud.

Conceptual design of multichannel fast electron bremsstrahlung detection system to study fast electron dynamics during lower hybrid current drive in ADITYA-U tokamak

To drive plasma current non inductively, a Lower Hybrid Current Drive (LHCD) system with a passive-active multijunction (PAM) antenna for injecting lower hybrid wave (LHWs) has been designed, fabricated and to be integrated with ADITYA-U Tokamak. A fast electron population in the energy range of a few keV to several hundreds of keV is generated with the injection of the LHWs. These fast electrons interacting mainly with ions and electrons result in Hard X-Ray (HXR) emission called Fast Electron Bremsstrahlung acronym as FEB emission. A single channel FEB detection system is being used in the earlier experiment of LHCD in ADITYA-U tokamak. However, the single channel detection system can't provide a radial emissivity profile of HXR intensity distribution. In order to determine a radial emissivity profile, for the first time, a multichannel FEB detection system is designed for ADITYA-U tokamak. This paper describes the detailed conceptual design for the selection of suitable detectors, optimization of the collimators, shielding geometry, and low energy filtering cut-off. In the present design analysis, the soller collimator concept is considered over a simple pinhole camera system due to its several advantages. In order to optimize the multichannel FEB system parameters for the best possible performance, a forwarded modelling code has been used. The signal to background ratio at each detector location has been estimated for a few system parameters and reported herein.

Statistical Properties of Lower Hybrid Waves in the Magnetopause Reconnection Exhaust Region

AbstractLower hybrid waves (LHWs) are frequently observed associated with magnetic reconnection. A general picture of the statistical properties of LHWs in magnetopause reconnection, however, has not yet been established. Here we report such a statistical study of LHWs using magnetospheric multiscale mission data in the magnetopause reconnection exhaust region. LHWs are identified in reconnection outflow regions over a broad range of magnetic local time. Most of the LHWs are near strong density gradients located on the low‐density magnetospheric side of the reconnection layer. The wave amplitude is typically several mV/m but can occasionally reach a few tens of mV/m. Compared with intervals that do not exhibit LHWs, intervals with LHWs show a ∼50% increase in parallel electron temperature, providing statistical evidence that LHWs contribute to parallel electron heating in reconnection exhausts and the dayside low‐latitude boundary layer.

Design of Probe to Investigate Energetic Electrons in Lower Hybrid Wave Plasmas in the TST-2 Spherical Tokamak

The plasmas whose current is driven by the lower-hybrid wave (LHW) alone have been investigated on the TST-2 spherical tokamak. We are developing a probe to obtain the information about energetic electrons generated by the LHW. We investigated a detector, motivated by the lost fast-ion probe (LIP) or fast ion loss detector (FILD), which can identify the energy and pitch angle of captured ions. However, the pitch angle of electrons of interest is expected to be so small that it was found that a conventional configuration with the normal vector of an entrance-orifice surface almost perpendicular to the magnetic field line was not applicable. Thus, we accessed the feasibility of the configuration with two tiny orifices by which orbits, or the energy and pitch angle,of detected electrons can be well specified. We carried out numerical calculations by using calculation conditions and geometries which are manufacturable. The results suggest that it is difficult to identify the energy of detected electrons, but that it is possible to identify their pitch angle well. Based on the results, we designed the diagnostic system with 3 channels which will detect energetic electrons with the pitch angle of 3 - 5, 5 - 9, and 9.5 - 12.5 degrees, respectively.

Lower Hybrid Waves in High Altitude Echoes of the Topside Equatorial Ionosphere

AbstractWe investigate the mechanism underlying lower hybrid waves associated with high altitude echoes recently detected in the post‐sunset equatorial topside ionosphere and inner plasmasphere by the Jicamarca VHF radar. These waves are visible as prominent sidebands in the echo Doppler spectra. New experimental results and newly processed incoherent scatter radar (ISR) datasets are presented that provide clues as to the conditions in which the echoes and associated waves occur. Numerical simulations are presented which demonstrate the feasibility of an inverse energy cascade coupled with a short wavelength instability, that is, the lower hybrid drift instability, in explaining the waves. An inverse cascade is required for short wavelength lower hybrid waves to extend to the 3 m wavelengths measured by the Jicamarca radar. The simulations were able to reproduce some features of the measurements including the lower hybrid sidebands at 3 m wavelengths, asymmetry in the sidebands, and the damping effect of higher densities and lower altitudes.

Kinetic theory of parametric decay instabilities near the upper hybrid resonance in plasmas

Parametric decay instabilities (PDIs) near the upper hybrid resonance layer are studied with a 1D framework. In a uniform plasma, the kinetic nonlinear dispersion relation of PDI is numerically calculated for parameters corresponding to electron cyclotron heating experiments at the ASDEX-U tokamak, in which O-mode radiation was converted to X-mode radiation by reflection from the high-field sidewall. The forward scattering processes driven by X-mode and linearly converted electron Bernstein waves (EBWs) are investigated and found to lead to a primary PDI where the pump waves decay into lower hybrid waves and sideband EBWs. A frequency shift of 930 MHz is obtained for the sideband EBWs in the primary PDIs. Subsequently, the sideband EBWs can decay into a low-frequency ion Bernstein quasi-mode (IBQM) and a secondary EBW, where the dominant forward scattering channel is the first-order IBQM with a frequency close to twice the ion cyclotron frequency. The decay channels obtained by numerical calculation can explain the characteristics of the signal observed in ASDEX-U experiments. The threshold of the pump electric field strength required to excite the primary PDI in the presence of plasma inhomogeneity is also estimated.

Localized absorption of laser energy in X-mode configuration of magnetized plasma

The heating of ions via lower hybrid (LH) waves has been observed in several astrophysical as well as laboratory plasmas. We have conducted particle-in-cell simulations to demonstrate absorption of the incident laser pulse at a chosen localized point in the plasma target by manipulating its density profile. We show that a part of the incident laser propagates inside the plasma target when its frequency lies below the LH resonance frequency. Thereafter, as it experiences a negative density gradient, it approaches the resonance point where its group velocity approaches zero. This is where the electromagnetic (EM) energy prominently gets converted into the electrostatic and eventually the kinetic energy of ions. Thus, by tailoring the plasma density profile one can have the absorption of incident EM wave energy at a designated location inside the plasma. This may be important in various applications where energy deposition/heating of plasma in a localized region is desirable.

The scalings of the thermal energy confinement time in EAST H-mode plasmas

This paper describes scalings of the H-mode confinement database of neutral beam injection (NBI)-lower hybrid wave (LHW) and electron cyclotron resonant heating-LHW plasmas in the EAST tokamak. Fundamental information of the EAST H-mode database and the details of calculation of the thermal energy confinement time in EAST are presented. The result of the scaling model of the thermal energy confinement time in NBI-LHW plasmas with root-mean-square-error , compared with the multi-machine scaling model derived from the multi-machine database (mainly NBI heating) with , is obtained. The scaling presents a similar dependency on plasma current and power loss and a lower dependency on plasma density and elongation, but stronger dependency on toroidal field than in . The scaling demonstrates a similar dependency on plasma current and power loss and lower dependency on plasma density, but a negative dependency with large error bars on elongation compared with in NBI-LHW plasmas. The comparison of EAST models with the more recent multi-tokamak scaling ITPA20-IL shows a similar engineering dependency on plasma current, power loss and plasma density, but opposite dependency on toroidal field.

Dataset for "3D simulations of lower-hybrid drift waves in strong guide field asymmetric reconnection in laboratory experiments"

Dataset for "3D simulations of lower-hybrid drift waves in strong guide field asymmetric reconnection in laboratory experiments"