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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Suprathermal electron transport studies in radio-frequency-heated tokamak plasmas

This contribution reports on the study of suprathermal electron radial transport enhancement in the presence of electron-heating radio-frequency waves, such as electron-cyclotron waves or lower-hybrid waves. Since suprathermal electrons emit hard X-rays from Bremsstrahlung, experimental data from hard X-ray measurements are used to follow their dynamics. In particular, electron-cyclotron power modulation experiments performed in TCV show that this transport depends on the total electron-cyclotron power. These results are corroborated by experimentally constrained Fokker-Planck simulations [J. Cazabonne et al, Plasma Phys. Control. Fusion 65, 104001 (2023)]. Similar experiments have been performed in WEST with lower-hybrid waves, and are reported for the first time. Preliminary analyses show potential evidence of suprathermal electron transport, but no clear dependence on wave power nor photon energy at this stage.

Cazabonne, Jean [CEA, IRFM, Saint Paul Lez Durance↗
NASA NTRS1987

Electron cyclotron harmonic wave acceleration

A nonlinear analysis of particle acceleration in a finite bandwidth, obliquely propagating electromagnetic cyclotron wave is presented. It has been suggested by Sprangle and Vlahos in 1983 that the narrow bandwidth cyclotron radiation emitted by the unstable electron distribution inside a flaring solar loop can accelerate electrons outside the loop by the interaction of a monochromatic wave propagating along the ambient magnetic field with the ambient electrons. It is shown here that electrons gyrating and streaming along a uniform, static magnetic field can be accelerated by interacting with the fundamental or second harmonic of a monochromatic, obliquely propagating cyclotron wave. It is also shown that the acceleration is virtually unchanged when a wave with finite bandwidth is considered. This acceleration mechanism can explain the observed high-energy electrons in type III bursts.

Karimabadi, H.↗
NASA NTRS1992

Stochastic gyroresonant electron acceleration on a low-beta plasma. II - Implications of thermal effects in a solar flare plasma

We consider the thermal damping of R and L waves under typical solar flare conditions. We find that the thermal particles cause rapid dissipation of the cyclotron waves, but do not affect whistlers and Alfven waves. The dissipation of electron cyclotron waves in particular leads to a threshold energy for acceleration which is about 10 times the thermal energy. Therefore, in the absence of an instability that excites these HF waves, a second mechanism is required in order to inject a sufficient number of electrons above the threshold energy and account for solar flare gamma-ray bremsstrahlung emission. We also find that the comoving gyroresonance, which occurs when the electron is in gyroresonance with an R wave whose group velocity equals the parallel electron velocity, can be realized by relativistic electrons if the plasma temperature is less than or approximately equal to 5,000,000 K.

Miller, James A.↗
NASA NTRS2013

THEMIS Observations of the Magnetopause Electron Diffusion Region: Large Amplitude Waves and Heated Electrons

We present the first observations of large amplitude waves in a well-defined electron diffusion region based on the criteria described by Scudder et al at the subsolar magnetopause using data from one Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite. These waves identified as whistler mode waves, electrostatic solitary waves, lower hybrid waves, and electrostatic electron cyclotron waves, are observed in the same 12 s waveform capture and in association with signatures of active magnetic reconnection. The large amplitude waves in the electron diffusion region are coincident with abrupt increases in electron parallel temperature suggesting strong wave heating. The whistler mode waves, which are at the electron scale and which enable us to probe electron dynamics in the diffusion region were analyzed in detail. The energetic electrons (approx. 30 keV) within the electron diffusion region have anisotropic distributions with T(sub e(right angle))/T(sub e(parallel)) > 1 that may provide the free energy for the whistler mode waves. The energetic anisotropic electrons may be produced during the reconnection process. The whistler mode waves propagate away from the center of the "X-line" along magnetic field lines, suggesting that the electron diffusion region is a possible source region of the whistler mode waves.

magnetopause and boundary layers↗
NASA NTRS2020

The Rapid Variability of Wave Electric Fields Within and Near Quasiperpendicular Interplanetary Shock Ramps: STEREO Observations

We present STEREO observations within 1500 proton gyroradii of 12 quasiperpendicular interplanetary shocks, with long-duration burst mode electric field acquisition by S/WAVES enabling observation of the evolution of waves throughout the entire ramp of interplanetary shocks. The shocks are low Mach number (Mf ~1-5), with beta ( ) ~0.2-1.8. High variability in frequency, amplitude, and wave mode is observed upstream, downstream, and in shock ramps. Observations in every region include ion acoustic-like waves, electron cyclotron drift instability driven waves, electrostatic solitary waves, and high frequency whistler mode waves. We also show for the fi rst time the existence of electrostatic waves with frequencies in the ion acoustic range which are frequency dispersed in time and the first observations of electron cyclotron drift instability (ECDI) driven waves at interplanetary shocks. The waves are bursty, large amplitude (~5 to > 200 mV/m), and seen in all three regions. All wave modes are more commonly observed downstream than upstream.

Z. A. Cohen↗
NASA NTRS1987

Electron cyclotron harmonic waves observed by the AMPTE-IRM plasma wave experiment following a lithium release in the solar wind

An unexpected occurrence following the second lithium release by the AMPTE-IRM spacecraft in the solar wind on September 20, 1984, was the appearance of electron cyclotron harmonic emissions. These emissions began about 50 s after the release and continued for several minutes. Narrow-band emissions polarized perpendicular to the magnetic field with amplitudes of approximately 0.00001 V/m were observed in each of the first five harmonic bands. The diffuse emissions extended from below the lowest measured frequency channel to above the highest narrow-band emission with a maximum below the electron cyclotron frequency. It will be shown that these observations are inconsistent with their generation by several ion beam instabilities.

Roeder, J. L.↗
NASA NTRS1994

Investigation of plasma instabilities in the polar cusp

During the last six months, considerable progress was made in studying the excitation of electromagnetic waves in the whistler frequency range by an anisotropic or gyrating electron beam. A paper entitled 'Electron Cyclotron Wave Generation by Relativistic Electrons' was published in the Journal of Geophysical Research. Another paper entitled 'Electron Beam Excitation of Upstream Waves in the Whistler Mode Frequency Range' was submitted for publication in Journal of Geophysical Research. This paper is in collaboration with Dr. C. W. Smith at Bartol Research Institute. In this paper, it was shown that an anisotropic electron beam (or gyrating electron beam) is capable of generating both left-hand and right-hand polarized electromagnetic waves in the whistler frequency range. Our earlier paper 'Electromagnetic Components of Auroral Hiss and Lower Hybrid Waves in the Polar Magnetosphere' was accepted for publication in the AGU Chapman Conference on Micro and Meso Scale Phenomena in Space Plasmas. Electromagnetic waves in the lower hybrid and whistler waves regime were identified and a mechanism of how these waves are generated was suggested.

Wong, H. K.↗
NASA NTRS2023

Diffuse Auroral Intensities Produced By Whistler Mode and Electron Cyclotron Harmonic Waves

Whistler mode waves and ECH waves as observed by the THEMIS-D and THEMIS-E satellites have been analyzed. It is observed that ECH waves are very weak for pitch-angle diffusion whereas whistler mode waves are more efficient. Bounce-averaged pitch-angle diffusion rates at the edge of atmospheric loss-cone have been calculated for both waves. Further, these are used to obtain electron precipitation flux inside the loss-cone. The electron flux at the edge of the loss-cone is represented by the kappa distribution. Numerical calculation of precipitation flux is performed by varying the fitting parameters (which determine the shape of electron precipitation flux) appearing in the kappa distribution. Precipitation flux is used to obtain volume excitation rates and height-integrated volume excitation rates for the seven excitation states. Calculations have been performed using two standard atmosphere models corresponding to mean and high solar and geomagnetic activities. It has been observed from studies that the excitation rates produced by the ECH wave are several orders of magnitude smaller than the rates produced by whistler mode waves. We show that the scattering efficiency of whistler Mode is higher than that of ECH waves. The oblique whistler mode waves allow efficient scattering of electrons in diffuse aurora. Outcomes are discussed.

Arvind K. Tripathi↗
NASA NTRS2014

Quantified Energy Dissipation Rates in the Terrestrial Bow Shock: Waves and Dissipation - 2

We present the first quantified measure of the energy dissipation rates, due to wave-particle interactions, in the transition region of the Earth's collision-less bow shock using data from the Time History of Events and Macro-Scale Interactions during Sub-Storms spacecraft. Our results show that wave-particle interactions can regulate the global structure and dominate the energy dissipation of collision-less shocks. In every bow shock crossing examined, we observed both low-frequency (less than 10 hertz) and high-frequency (approximately or greater than10 hertz) electromagnetic waves throughout the entire transition region and into the magnetosheath. The low-frequency waves were consistent with magnetosonic-whistler waves. The high-frequency waves were combinations of ion-acoustic waves, electron cyclotron drift instability driven waves, electrostatic solitary waves, and whistler mode waves. The high-frequency waves had the following: (1) peak amplitudes exceeding delta B approximately equal to 10 nanoteslas and delta E approximately equal to 300 millivolts per meter, though more typical values were delta B approximately equal to 0.1-1.0 nanoteslas and delta E approximately equal to 10-50 millivolts per meter (2) Poynting fluxes in excess of 2000 microWm(sup −2) (micro-waves per square meter) (typical values were approximately 1-10 microWm(sup −2) (micro-waves per square meter); (3) resistivities greater than 9000 omega meters; and (4) associated energy dissipation rates greater than 10 microWm(sup −3) (micro-waves per cubic meter). The dissipation rates due to wave-particle interactions exceeded rates necessary to explain the increase in entropy across the shock ramps for approximately 90 percent of the wave burst durations. For approximately 22 percent of these times, the wave-particle interactions needed to only be less than or equal to 0.1 percent efficient to balance the nonlinear wave steepening that produced the shock waves. These results show that wave-particle interactions have the capacity to regulate the global structure and dominate the energy dissipation of collision-less shocks.

Terrestrial bow 2↗
NASA NTRS1994

Plasma and radio waves from Neptune: Source mechanisms and propagation

This report summarizes results obtained through the support of NASA Grant NAGW-2412. The objective of this project is to conduct a comprehensive investigation of the radio wave emission observed by the planetary radio astronomy (PRA) instrument on board Voyager 2 as if flew by Neptune. This study has included data analysis, theoretical and numerical calculations, ray tracing, and modeling to determine the possible source mechanism(s) and locations of the Neptune radio emissions. We have completed four papers, which are included in the appendix. The paper 'Modeling of Whistler Ray Paths in the Magnetosphere of Neptune' investigated the propagation and dispersion of lighting-generated whistler in the magnetosphere of Neptune by using three dimensional ray tracing. The two papers 'Numerical Simulations of Bursty Radio Emissions from Planetary Magnetospheres' and 'Numerical Simulations of Bursty Planetary Radio Emissions' employed numerical simulations to investigate an alternate source mechanism of bursty radio emissions in addition to the cyclotron maser instability. We have also studied the possible generation of Z and whistler mode waves by the temperature anisotropic beam instability and the result was published in 'Electron Cyclotron Wave Generation by Relativistic Electrons.' Besides the aforementioned studies, we have also collaborated with members of the PRA team to investigate various aspects of the radio wave data. Two papers have been submitted for publication and the abstracts of these papers are also listed in the appendix.

Wong, H. K.↗
NASA NTRS1989

Auroral plasma waves

A review is given of auroral plasma wave phenomena, starting with the earliest ground-based observations and ending with the most recent satellite observations. Two types of waves are considered, electromagnetic and electrostatic. Electromagnetic waves include auroral kilometric radiation, auroral hiss, ELF noise bands, and low-frequency electric and magnetic noise. Electrostatic waves include upper hybrid resonance emissions, electron cyclotron waves, lower hybrid waves, ion cyclotron waves and broadband electrostatic noise. In each case, a brief overview is given describing the observations, the origin of the instability, and the role of the waves in the physics of the auroral acceleration region.

Gurnett, Donald A.↗
DOE OSTI2022

Advances in physics and applications of 3D magnetic perturbations on the J-TEXT tokamak

In the last two years, three major technical improvements have been made on J-TEXT in supporting of the expanded operation regions and diagnostic capabilities. (1) The successful commission of the 105 GHz/500 kW/1 s electron cyclotron resonance heating (ECRH) system increasing the core electron temperature from 0.9 keV up to around 1.5 keV. (2) The poloidal divertor configuration with an X-point in the high-field side has been achieved. In particular, the 400 kW electron cyclotron wave has also been successfully injected into the diverted plasma. (3) A 256-channel electron cyclotron emission imaging diagnostic system and two sets of four-channel Doppler backscattering diagnostics have been successfully developed on J-TEXT, allowing detailed measurement of the electron temperature and density fluctuations for turbulence and MHD research. The locked mode (LM), especially the 2/1 LM, is one of the biggest threats to the plasma operation. Both the thresholds of 2/1 and 3/1 LM are observed to vary non-monotonically on electron density. The electrode biasing was applied successfully to unlock the LM from either a rotating or static resonant magnetic perturbation (RMP) field. In the presence of 2/1 LM, three kinds of standing wave (SW) structures have been observed to share a similar connection to the island structure, i.e. the nodes of the SWs locate around the O- or X-points of the 2/1 island. The control and mitigation of disruption is essential to the safe operation of ITER, and it has been systematically studied by applying a RMP field, massive gas injection (MGI) and shattered pellet injection on J-TEXT. When the RMP-induced 2/1 LM is larger than a critical width, the MGI shutdown process can be significantly influenced. If the phase difference between the O-point of LM and the MGI valve is +90° (or –90°), the penetration depth and the assimilation of impurities can be enhanced (or suppressed) during the pre-thermal quench (TQ) phase and result in a faster (or slower) TQ. A secondary MGI can also suppress the runaway electron (RE) generation, if the additional high-Z impurity gas arrives at the plasma edge before TQ. When the secondary MGI has been applied after the formation of the RE current plateau, the RE current can be dissipated, and the dissipation rate increases with the injected impurity quantity but saturates with a maximum of 28 MA s –1 . The non-local transport is experimentally observed in the ion transport channel. The electron thermal diffusivity significantly increases with the ECRH power. Theoretical work shows that significant intrinsic current can be driven by electromagnetic turbulence, and the robust formation mechanism of the E × B staircase is identified from the Hasegawa–Wakatani system.

divertor↗
NASA NTRS1983

Experimental evidence for the acceleration of thermal electrons by ion cyclotron waves in the magnetosphere

Experimental evidence is presented for the acceleration of thermal electrons by large amplitude ion cyclotron waves (ICWs). The wave power in the ULF range near the helium gyrofrequency is compared with the distribution function of low energy electrons measured by GEOS satellite instruments. This comparison shows that electrons are accelerated near the geomagnetic equator along field lines, at times when the ICW energy is large and the cold plasma density is below a threshold value. It is suggested that these accelerated electrons can account for the ELF emissions, modulated at the ICW frequency, observed by Wehrlin (1981). A very efficient acceleration of thermal electrons along field lines results from other ULF events having frequencies close to the proton gyrofrequency. Evidence for this lies in the fact that medium energy protons having large temperature anisotropies in the 100-500 eV range are responsible for the ICW wave generation.

Norris, A. J.↗
DOE OSTI2024

Toroidal injection angle dependence of EC assisted plasma initiation at DIII-D

An experimental scan of the electron cyclotron waves (EC) toroidal injection angle in plasma breakdown is performed at the DIII-D tokamak. The second harmonic, extraordinary mode EC is used for the study. The dependence of n e and T e on the EC injection angle cannot be conclusively drawn from this study due to the large error bars in the n e and T e measurements. On the other hand, high T e data points are observed in some discharges which can be explained by nonlinear heating. The D α emission measurement shows a clear relation between the breakdown time and the injection angle. An experimental investigation of the cause of the dependence of breakdown delay on the EC injection angle suggests that when the injection angle is oblique, the EC heating after the reflection at the inboard wall may become ineffective and cause the breakdown delay even when the EC heating before and directly upon injection remains effective. A preliminary run of the heat and transport balance code DYON indicates that the obtained dataset is suitable for a quantitative validation of EC absorption models.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1985

Wave/particle interactions in the plasma sheet

The most frequent and intense waves observed in the geomagnetic tail region are short wavelength electrostatic broadband emissions. Early observations of these waves were made by using IMP 7 and 8 respectively. A broad frequency range for the waves was found, from about 10Hz to several kHz, with an average r.m.s. electric field amplitude of about 1mV/m was found. Other less frequent and less intense wave modes were also identified: magnetic noise bursts and electrostatic electron-cyclotron waves. The highest frequency of occurrence of the electrostatic noise as found in the region near the plasma sheet boundary when anisotropic fluxes of ions streaming either earthward or antiearthward were present. It was confirmed, by using ISEE data that the waves were usually observed during times of ion streaming and that the intensity and frequency range was maximum in the plasma sheet boundary layer and fell off dramatically in the tail lobe and central plasma sheet regions. It is suggested that increases in the electron and ion temperature in going from the tail lobe to the central plasma sheet are a result of scattering of resonant, boundary layer plasma.

Dusenberry, P. R.↗
DOE OSTI2021

Scattering of radio frequency waves by randomly modulated density interfaces in the edge of fusion plasmas

In the scrape-off layer and the edge region of a tokamak, the plasma is strongly turbulent and scatters the radio frequency (RF) electromagnetic waves that propagate through this region. It is important to know, whether used for diagnostics or for heating and current drive, the spectral properties of these scattered RF waves. The spectral changes influences the interpretation of the diagnostic-data obtained and the current and heating profiles. A full-wave, 3D electromagnetic code ScaRF (see Papadopoulos et al. 2019) has been developed for studying the RF wave propagation through turbulent plasma. ScaRF is a finite-difference frequency-domain (FDFD) method for solving Maxwell's equations. The magnetized plasma is defined through the cold plasma, anisotropic permittivity tensor. As a result, ScaRF can be used to study the scattering of any cold plasma RF wave. It can be for the study of scattering of electron cyclotron waves in ITER-type and medium-sized tokamaks such as TCV, ASDEX-U, DIII-D. For the case of medium-sized tokamaks, there's experimental evidence that drift waves and rippling modes are present in the edge region (see Ritz et al. 1984). Hence, we study the scattering of RF waves by periodic density interfaces (plasma gratings) in the form of a superposition of spatial modes with varying periodicity and random amplitudes (see Papadopoulos et al. 2019). The power reflection coefficient (a random variable) is calculated for different realizations of the density interface. In this work, the uncertainty of the power reflection coefficient is rigorously quantified by use of the Polynomial Chaos Expansion (see Xiu & Karniadakis 2002) method in conjunction with the Smolyak sparse grid integration (see Papadopoulos et al. 2018) (PCE-SG). The PCE-SG method is proven accurate and much more efficient (roughly 2-orders of magnitude shorter execution time) compared to alternative methods such as the Monte Carlo (MC) approach.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Direct preemptive stabilization of $m,n$ = 2,1 neoclassical tearing modes by electron cyclotron current drive in the DIII-D low-torque ITER baseline scenario

We report the first preemptive direct stabilization of $m,n$ = 2,1 neoclassical tearing modes (NTMs) in DIII-D low-torque ITER baseline scenario plasmas by electron cyclotron waves. These experiments strongly support that the observed stabilization is achieved by replacing the missing bootstrap current in the island by electron cyclotron current drive (ECCD). These plasmas (with and without ECCD) are stable to 2,1 NTMs when the differential rotation between the q = 1 and q = 2 surfaces (Δf 1,2 ) is sustained above a critical value (Δf CRIT ≈ 1 kHz), but those evolving to low differential rotation (Δf 1,2 < Δf CRIT ) routinely develop a disruptive 2,1 island at the time and frequency of a sawtooth precursor. Preemptive, local and direct stabilization by ECCD is tested by scanning the current drive amplitude and location inside and outside of the q = 2 rational surface shot by shot. Analysis of only low differential rotation time windows, i.e. when stabilization from Δf 1,2 is absent, shows focused ECCD at q = 2 prevents the onset of 2,1 NTMs. These results are important, as they give the first demonstration of disruptive NTM control by ECCD in the plasma scenario planned for ITER.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗