Ion Cyclotron Frequency Research Articles

Chirping ion cyclotron emission (ICE) on NSTX-U

We report here on the discovery on NSTX-U of a qualitatively new type of non-thermal emission from plasmas in the ion-cyclotron frequency range (ICE). This new type of ICE is weakly damped, γdamp/ω ICE ≈ 0.14%, low order eigenmodes, excited by a fast ion population radially located near an internal transport barrier. The ICE appears as repetitive bursts with durations of ≈1 ms, and the bursts exhibit frequency chirps upwards and/or downwards with maximum δf/f ≈ 1%. We refer to this chirping ICE as ch-ICE. The bursts are longer than the typical ICE bursts of ≈100 μs reported previously for NSTX and NSTX-U, for simplicity, NSTX(-U). The chirping resembles that seen in modeling of weakly unstable fast particle driven instabilities (cf, Berk et al 1997 Phys. Lett. A 234 213). These data also provide some preliminary indications of non-linear coupling between the ch-ICE harmonics. Despite extensive experimental and theoretical studies of ICE, theoretical understanding of ICE remains qualitative at best. The detailed experimental observations of ICE from NSTX(-U) find a wide variation in the ICE characteristics. The new observations of ch-ICE presented here, combined with the characteristics of ICE on NSTX(-U) previously reported (Fredrickson et al 2019 Phys. Plasmas 26 032111), could guide the development of more complete theoretical models of ICE.

Helicon wave plasma generated by a resonant birdcage antenna: magnetic field measurements and analysis in the RAID linear device

This paper describes a helicon-wave-sustained plasma generated by a novel birdcage antenna with cosine azimuthal current distribution. The resonant birdcage source maintains a stable intense plasma column from low (300 W) to high (10 kW) steady-state RF power in hydrogen or argon, enabling plasma diagnostic measurements over the whole volume. The plasma density, 1018 to 1019 m−3, and uniform static magnetic field, 200–800 G, are typical of high density, low B-field helicon sources where the RF excitation frequency, 13.56 MHz here, is far above the ion cyclotron frequency and much lower than the electron cyclotron and plasma frequencies. Magnetic field measurements of the unbounded plasma column show a helicon wave propagating from the birdcage source to the target 1.5 m away. The axial wavelength, measured for a range of plasma density and magnetic field, is described by the whistler dispersion relation; it is not determined by the antenna length but varies smoothly as a function of electron density irrespective of the gas type, and shows no discontinuous transitions. This wavelength was compared with semi-analytical and purely numerical models, showing a good fit with the m = +1 helicon eigenvalue of shortest axial wavelength. The birdcage resonant antenna is a suitable source for continuous helicon-sustained plasma experiments, from fundamental wave studies up to high power applications in hydrogen or deuterium.

Gyrokinetic theory of low-frequency electromagnetic waves in finite-β anisotropic plasmas

We present a gyrokinetic theory for the electromagnetic waves and instabilities with frequencies much lower than the ion cyclotron frequency in finite-β anisotropic uniform plasmas. Here, β is the ratio between plasma and magnetic pressures. Kinetic effects due to both the finite Larmor radii and wave-particle resonances are fully kept in the analysis. Corresponding linear dispersion relation and wave polarizations, valid for general β value and perpendicular wavelength, are then specifically derived for a bi-Maxwellian plasma. Analytic expressions for the criteria of kinetic firehose and mirror instabilities are also given. The mode frequency, stability, and wave polarization of a broad spectrum of normal modes are then investigated numerically in a systematic study over a set of parameters. Our study clearly demonstrates that, due to the finite ion Larmor radius effect, the ion-sound wave, mirror mode, and shear Alfvén wave are intrinsically coupled.

Measurement of axial phase difference of density fluctuations owing to spontaneously excited waves by using microwave reflectometer on GAMMA 10/PDX.

In the GAMMA 10/PDX tandem mirror, plasma with strong ion-temperature anisotropy is produced by using the ion cyclotron range of frequency waves. This anisotropy of ion temperature causes several Alfvén-Ion-Cyclotron (AIC) waves to spontaneously excite in the frequency range just below the ion cyclotron frequency. In addition, difference-frequency (DF) waves are excited in the radial inner region of the plasma by wave-wave coupling among the AIC waves. The radial density profiles were measured at multi-axial positions using a frequency-modulation reflectometer with an axial array of microwave antennas, and an axial variation of the density was found to be significant. In addition, a relative phase difference of the DF wave between axially separated two points was first obtained by finely choosing the probing frequency of the reflectometers with a maximum coherence used as a measure, indicating that the DF wave is a propagating wave, while the pump AIC waves are standing waves in the axial region of measurement.

Stabilization of Alfvén Eigenmodes in DIII-D via Controlled Energetic Ion Density Ramp and Validation of Theory and Simulations.

Fast-ion driven Alfvén waves with frequency close to the ion cyclotron frequency (f=0.58f_{ci}) excited by energetic ions from a neutral beam are stabilized via a controlled energetic ion density ramp for the first time in a fusion research plasma. The scaling of wave amplitude with injection rate is consistent with theory for single mode collisional saturation near marginal stability. The wave is identified as a shear-polarized global Alfvén eigenmode excited by Doppler-shifted cyclotron resonance with fast ions with sub-Alfvénic energetic ions, a first in fusion research plasmas.

Alfvénic modes excited by the kink instability in PHASMA

Magnetic flux ropes have been successfully created with plasma guns in the newly commissioned PHAse Space MApping (PHASMA) experiment. The flux ropes exhibit the expected m = 1 kink instability. The observed threshold current for the onset of this kink instability is half of the Kruskal–Shafranov current limit, consistent with predictions for the non-line tied boundary condition of PHASMA. The helicity, paramagnetism, and growth rate of the observed magnetic fluctuations are also consistent with kink instability predictions. The observed fluctuation frequency appears to be a superposition of a real frequency due to a Doppler shift of the kink mode arising from plasma flow (∼2 kHz) and a contribution from a wave mode (∼5 kHz). The dispersion of the wave mode is consistent with an Alfvén wave. Distinct from most previous laboratory studies of flux ropes, the working gas in PHASMA is argon. Thus, the ion cyclotron frequency in PHASMA is quite low and the frequency of the Alfvénic mode plateaus at ∼0.5 of the ion gyro frequency with increasing background magnetic field strength.

A Gyrokinetic simulation model for low frequency electromagnetic fluctuations in magnetized plasmas

We present a new model for simulating the electromagnetic fluctuations with frequencies much lower than the ion cyclotron frequency in plasmas confined in general magnetic configurations. This novel model (termed as GK-E&B) employs nonlinear gyrokinetic equations formulated in terms of electromagnetic fields along with momentum balance equations for solving fields. It, thus, not only includes kinetic effects, such as wave-particle interaction and microscopic (ion Larmor radius scale) physics; but also is computationally more efficient than the conventional formulation described in terms of potentials. As a benchmark, we perform linear as well as nonlinear simulations of the kinetic Alfvén wave; demonstrating physics in agreement with the analytical theories.

DISTRIBUTION OF 0.2...4.5 keV PLASMA IONS IN SET OF U-2M DISCHARGES

Charge exchange (CX) neutral fluxes were measured by neutral particle analyzer (NPA) in plasma discharges sustained by the W7-X-like radio frequency (RF) antenna in the Uragan-2M (U-2M) stellarator. CX fluxes in pure hydrogen discharge (B0 = 0.36 T, f = 4.926 MHz) in stellarator configuration (effective perpendicular ion temperature TꞱ ≈ 450 eV) is less energetic in comparison with U-2M hybrid configuration (TꞱ ≈ 800 eV). RF discharge in stellarator configuration with helium and hydrogen mixture (B0 = 0.351 T; f = 5.156 MHz, P = 6·10-4Torr) shows more energetic CX fluxes (TꞱ ≈ 1 keV). The ion cyclotron frequency distribution across the U-2M plasma has been studied numerically. These calculations are accompanied by direct measurement of the RF frequency by magnetic sensor. The ion cyclotron frequency is present in plasma bulk of all discharges under consideration.

MODELLING OF RADIO-FREQUENCY WALL CONDITIONING IN SHORT PULSES IN A STELLARATOR

Results of numerical modelling of electron cyclotron (EC) short pulse wall conditioning discharge are presented. The analysis carried out with the usage of a self-consistent model that simulates radio-frequency (RF) plasma production in stellarator type machines in the ion cyclotron and EC frequency ranges. The discussion of the results of the calculations is presented.

Ion flow driven by low frequency Alfvén waves in a low-beta plasma

In a low-beta plasma, the ion flow in the parallel direction along the background magnetic field is investigated when ions are accelerated by low-frequency polarized Alfvén waves with the finite amplitude propagating along the magnetic field due to nonresonant interactions. The results indicate that the magnitude of the ion flow is closely related to not only the amplitude but also the frequency and the polarization of the wave, which is verified by a test particle simulation. The ion flows driven by the nondispersive and dispersive Alfvén waves are proportional to vAα21±ε2 and vAα21±ε3/2, respectively, where α is the ratio of the magnetic field component of the Alfvén wave to the background magnetic field, ε is the ratio of the wave frequency to the ion cyclotron frequency, and the positive sign corresponds to the right-handed Alfvén wave and the negative sign corresponds to the left-handed Alfvén wave, respectively. For a wave with finite frequency, the ion flow is different from different polarized waves in low beta plasmas. In particular, the saturation value of ion flow has a minimum threshold for the left-handed polarized Alfvén wave and a maximum threshold for the right-handed polarized Alfvén wave. If the frequency is less than 1/10 of the ion cyclotron frequency, the Alfvén wave can be seen as propagating in nondispersive medium. When the frequency of the Alfvén wave is far less than the frequency of the ion cyclotron, the ion flows driven by the left-handed and right-handed polarized Alfvén waves with and without wave dispersion tend to be the same.

Экспериментальное определение дисперсионного соотношения излучения плазмы на частотах ионного циклотронного резонанса и его гармоник в токамаке

The study of the spectral properties of the electromagnetic radiation in the ion cyclotron frequency range from the TUMAN-3M tokamak plasma in the neutral beam injection (NBI) heating mode has been performed. The spectrum of this emission consists of several (up to four) narrow lines corresponding to different harmonics of the ion cyclotron resonance of the injected fast ions in the center of the plasma. Wave vectors corresponding to individual spectral lines are determined from the delays of signals from spatially separated probes. It is shown that under the assumption that the measured frequencies and wave vectors are described by a common dispersion relation, the observed emission can be explained by the appearance in the plasma an unstable fast magnetosonic wave propagating almost normally to the magnetic field.

Origin of the zebra structure in the Jovian decameter radio emission

Context.We discuss the origin of quasi-harmonic emission bands that have been observed in the dynamic spectra of the Jovian decameter emission.Aims.We aim to show that the interpretation of the observed structure can be based on the effect of double plasma resonance (DPR) at ion cyclotron harmonics.Methods.According to the proposed model, in the extended source in the upper ionosphere of Jupiter, where the DPR condition is satisfied for one of the ion cyclotron frequency harmonics, the ion cyclotron waves are effectively excited at the frequency of the lower hybrid resonance. The observed electromagnetic radiation with a quasi-harmonic structure arises due to scattering of ion cyclotron waves by supra-thermal electrons.Results.Based on the VIP4 magnetic field model, we determine the longitudes at which the source of the considered radiation can be located. The obtained estimates of the plasma density and its height distribution in the source, as well as the energies of emitting ions and scattering electrons provide information about the plasma parameters in the upper ionosphere of Jupiter. Furthermore, these estimates are in good agreement with the observational data.

Optimized phasing conditions to avoid edge mode excitation by ICRH antennas

An ion cyclotron resonance heating (ICRH) antenna system must launch radio frequency (RF) power with a wavenumber spectrum which maximizes the coupling to the plasma. It should also ensure good absorption while minimizing the wave interaction with the plasma edge. Such interactions lead to impurity release, whose effect has been measured far from the antenna location (Klepper et al. 2013; Wukitch et al. 2017; Perkins et al. 2019) and can involve the entire scrape-off layer. In the normal heating scenario, for which the frequency of the waves launched by the antenna is larger than the ion cyclotron frequency of the majority ions $\omega > \omega _{\textrm {ci},\textrm {maj}}$ , release of impurities due to ICRH can be affected by minimizing the low $|k_{\parallel }| < k_0$ power spectrum components of the antenna. Impurity release can be the result of low central absorption of the waves or power transfer from the fast to the slow wave due to the presence of a confluence in the plasma edge. In ASDEX Upgrade (AUG), a reduction of heavy impurity release by ICRH in the plasma was qualitatively well correlated to the parallel electric field and RF currents flowing around the antenna (Bobkov et al. 2017). In this article, we first show a correlation between the reduction in impurity release by ICRH in AUG and the rejection of the low $|k_{\parallel }| < k_0$ region of the antenna power spectrum. We show that the same correlation holds for results obtained in the Alcator C-Mod tokamak. Finally, using this idea, we reproduce ICRH induced impurity release behaviour in a not yet published experiments of JET, and make predictions for ITER and DEMO.

Modulational instability of the coupled waves between fast magnetosonic wave and slow Alfvén wave in the laser–plasma interaction

By performing modulational instability analysis of the the nonlinear coupled dimensionless equations between a fast magnetosonic wave (FMSW) propagating obliquely with the magnetic field and a low-frequency slow Alfvén wave (SAW), we obtain the dispersion relation of the perturbation wave. The growth rate of the perturbation wave is obtained. It is found that the growth rate increases as background magnetic field increases, which is in agreement with that reported by Tiwary et al (2016 Phys. Plasmas 23 122307). A critical perturbation wave number is found. When the perturbation wave number is greater than or equal to the critical value, the growth rate is positive and it increases as the perturbation wave number increases, while the wave is stable. The maximum growth rate is reached when the frequency of the FMSW is half of the ion cyclotron frequency. The minimum growth rate is reached when the propagation direction of the perturbation wave is the same as that of the FMSW.

Beam-driven ion-cyclotron modes in the scrape-off layer of a field-reversed configuration

In the scrape-off-layer (SOL) of a field-reversed configuration, neutral beam injection can drive modes in the vicinity of the ion-cyclotron frequency. Depending on their properties, these modes can have differing macroscopic effects on the plasma. Using particle-in-cell simulations and semi-analytical methods, this work examines the properties of the various categories of ion-cyclotron modes that can be excited in the SOL environment, taking an ion beta of 10%. Propagation angle and beam injection angle are scanned from 0° to 90° with respect to the external magnetic field. The linear physics of these modes is treated foremost, but the non-linear physics, particularly regarding ion acceleration with consequences for plasma stability and fusion reactivity, is also examined. Near perpendicular propagation and beam injection are found to give the greatest fusion enhancement. This result derives from an extension of wakefield acceleration physics to the case of ion acceleration in a magnetized fusion plasma. The non-linear physics of this regime is thus given particular attention.

A Strong Instability in the Ion Cyclotron Frequency Range of the Upward Sheared Flow of the Oxygen Ions in Aurora

The dispersion relation of a new electrostatic ion cyclotron (IC) instability of a plasma with a sheared current of the oxygen ions along the magnetic field is investigated analytically and solved by numerical analysis. The considered case corresponds to the upward sheared flow of the oxygen ions in the aurora where the velocity shearing rate of ion current may be larger than the cyclotron frequency of O+ ion. In such a plasma, we found a new instability in the IC frequency range which develops in the sheared flow and can have a growth rate larger than O+ cyclotron frequency. This instability develops jointly with the known IC instability for the uniform current with a growth rate much less than O+ cyclotron frequency

Linear theory of current driven ion acoustic instability at the distance 0.3 to 1 AU from the Sun

Several kinds of instabilities are often observed in the solar wind. We studied the linear theory of the current-driven electrostatic ion acoustic instability at frequencies near the ion cyclotron frequency at distances from the Sun over 0.3 to 1 AU. The properties of current driven electrostatic ion-acoustic instability are investigated. The instability parameters, such as angular wave frequency, the critical drift velocity and the growth rate are calculated. We used the dispersion relation in hot magnetized plasma to calculate the current driven ion acoustic instability properties. To determine the critical drift velocity and the growth rate a full numerical solution of the dispersion relation equation is carried out. Since the growth rate of instability depends on the wave normal angle, the drift velocity of the electron along the magnetic field and the ion thermal cyclotron radius, the calculation is carried out for ranging of the wave normal angles, drift velocities and ion thermal cyclotron radius. The results show that the growth rate of instability increases with increasing distance from the Sun. The growth rate of instability increases with decreasing the wave normal angle, but the critical drift velocity decreases with decreasing the wave normal angle.

Performance improvement of particle-in-cell method for numerical modelling of open magnetic system

The work is connected with numerical simulation of plasma dynamics in open magnetic trap in diamagnetic regime. The hybrid particle-in-cell model we develop allows to perform numerical experiments for the high ratio of Larmor radius of the ions and the one of the electrons due to the combining of the kinetic description for the ions and MHD description for the electron plasma component. The disadvantage of the model is the stability condition and the corresponding requirements for the time step. In practice a doubling of the grid nodes in each direction leads to the decrease of the time step x6 times. For the characteristic times of the plasma processes 102 reciprocal ion cyclotron frequencies the computations required few days for grids 100x500. The processing of the particle data takes more than 85% of the total computation time, thus its effective realization yields significant gain in performing. In our algorithm we combine the dynamic load balance and vectorized computations of densities and current densities. We present the results of numerical experiments on its basis including the highly non-uniform particle distribution in the domain and the increasing of the particle number due to the beam injection.

Narrowband Modulation Two-Dimensional Mass Spectrometry and Label-Free Relative Quantification of Histone Peptides.

Two-dimensional mass spectrometry (2D MS) on a Fourier transform ion cyclotron resonance (FT-ICR) mass analyzer allows for tandem mass spectrometry without requiring ion isolation. In the ICR cell, the precursor ion radii are modulated before fragmentation, which results in modulation of the abundance of their fragments. The resulting 2D mass spectrum enables a correlation between the precursor and fragment ions. In a standard broadband 2D MS, the range of precursor ion cyclotron frequencies is determined by the lowest mass-to-charge (m/z) ratio to be fragmented in the 2D MS experiment, which leads to precursor ion m/z ranges that are much wider than necessary, thereby limiting the resolving power for precursor ions and the accuracy of the correlation between the precursor and fragment ions. We present narrowband modulation 2D MS, which increases the precursor ion resolving power by reducing the precursor ion m/z range, with the aim of resolving the fragment ion patterns of overlapping isotopic distributions. In this proof-of-concept study, we compare broadband and narrowband modulation 2D mass spectra of an equimolar mixture of histone peptide isoforms. In narrowband modulation 2D MS, we were able to separate the fragment ion patterns of all 13C isotopes of the different histone peptide forms. We further demonstrate the potential of narrowband 2D MS for label-free quantification of peptides.

Estimating Effective Collision Frequency and Kinetic Entropy Uncertainty in Particle-in-Cell Simulations

A kinetic entropy diagnostic was systematically developed for fully kinetic collisionless particle-in-cell (PIC) simulations in Liang et al., Phys. Plasmas 26, 082903 (2019). Here, we first show that kinetic entropy can be used to quantitatively evaluate numerical dissipation in the PIC simulation. Assuming numerical effects can be treated using a relaxation time approximation collision operator, the rate of increase of the kinetic entropy is related to the kinetic entropy. The effective collision frequency due to numerical effects is then easy to evaluate in a collisionless PIC simulation. We find an effective collision frequency of approximately a tenth of the ion cyclotron frequency. This could have important implications for collisionless PIC simulation studies of magnetic reconnection, plasma turbulence, and collisionless shocks. Then, we analyze the uncertainty of the local kinetic entropy density at different locations as a function of the chosen velocity space grid. We find that although the numerically obtained kinetic entropy density varies significantly for small or large velocity space grids, there is a range for which the kinetic entropy density is only weakly sensitive to the velocity space grid. Our analysis of the uncertainty suggests a velocity space grid close to the thermal velocity is optimal, and the uncertainty introduced is significantly less than the physical change in kinetic entropy density.