Parallel Ion Temperatures Research Articles

Effect of ion temperature anisotropy on ITG mode in reversed field pinch plasmas

Abstract The Ion Temperature Gradient (ITG) mode instability in the Reversed-Field-Pinch (RFP) plasmas with anisotropies in ion temperature and its gradient, is studied for the first time using the gyrokinetic integral eigen-mode equation. Detailed numerical results indicate that ITG instability is reduced by ion temperature anisotropy, specifically when the temperature is higher in the direction perpendicular to the magnetic field or sufficiently high in the direction parallel to the magnetic field, with Landau damping playing a significant role. Moreover, the driving force of the ion temperature gradient in the parallel direction for the ITG mode is stronger than that in the perpendicular direction, particularly when the parallel ion temperature is higher. In addition, the study provides detailed insights into the synergistic effects of ion temperature anisotropy with magnetic shear, poloidal wave number, safety factor, and density gradient on ITG mode. It also elucidates the threshold of the ion temperature gradient necessary for mode excitation.

Determination of Parallel and Perpendicular Ion Temperatures Based on Selective Ion Transmission in a Retarding Field Analyzer

Determination of Parallel and Perpendicular Ion Temperatures Based on Selective Ion Transmission in a Retarding Field Analyzer

On the possibility of mode coupling for low frequency electromagnetic waves in plasmas with anisotropy of temperature

We study the dispersion relation for low frequency electromagnetic waves propagating along the ambient magnetic field and investigate the possibility of occurrence of coupling between waves in the ion cyclotron branch and waves in the whistler branch. The results obtained show that the coupling may occur in the case of conditions leading to the ion cyclotron instability, for sufficiently high value of the ratio between perpendicular and parallel ion temperature, and does not occur in the case of conditions leading to the ion firehose instability. The results also show that the decrease in the value of the plasma beta may lead to the disappearance of the mode coupling conditions. Regarding the effect of the electron population, it is shown that the change in the shape of the electron velocity distribution, from Maxwellian to bi-Kappa form, does not change the results obtained, as long as the electron temperatures are isotropic, but the increase in anisotropy in the electron temperatures may lead to the disappearance of the coupling between the different waves. The consequences of the frequency dependency of the mode coupling conditions are discussed considering wave propagation in an inhomogeneous medium, leading to the conclusion that the energy of a packet of waves of a given mode can be absorbed or mode converted over an extended region of space. These findings can be of relevance for the analysis and understanding of processes related to the conversion between ion cyclotron waves and whistler waves.

Impact of selective ion transmission on measurement by retarding field analyzer

The impact of selective ion transmission on the use of retarding field analyzers (RFAs), which is attributed to the ion Larmor motion, was investigated. Depending on the ion Larmor radius and ion guiding center position, selective transmission limits the parallel and perpendicular ion velocities inside the RFA volume. The velocity limitation flattens I–V curve, leading to an overestimation of the parallel ion temperature. However, the overestimation can be reduced to an acceptable level using the slope in a large grid potential region. The influence of selective transmission depends on both parallel and perpendicular ion temperatures. This nature likely allows the simultaneous determination of two ion temperatures by the best fit of the experimental data, even from a single I–V curve. Applicability of this method was investigated using a radio frequency plasma.

Cross-code comparison of the edge codes SOLPS-ITER, SOLEDGE2D and UEDGE in modelling a low-power scenario in the DTT

As reactor-level nuclear fusion experiments are approaching, a solution to the power exhaust issue in future fusion reactors is still missing. The maximum steady-state heat load that can be exhausted by the present technology is around 10 MW m−2. Different promising strategies aiming at successfully managing the power exhaust in reactor-relevant conditions such that the limit is not exceeded are under investigation, and will be tested in the Divertor Tokamak Test (DTT) experiment. Meanwhile, the design of tokamaks beyond the DTT, e.g. EU-DEMO/ARC, is progressing at a high pace. A strategy to work around the present lack of reactor-relevant data consists of exploiting modelling to reduce the uncertainty in the extrapolation in the design phase. Different simulation tools, with their own capabilities and limitations, can be employed for this purpose. In this work, we compare SOLPS-ITER, SOLEDGE2D and UEDGE, three state-of-the-art edge codes heavily used in power exhaust studies, in modelling the same DTT low-power, pure-deuterium, narrow heat-flux-width scenario. This simplified, although still reactor-relevant, testbed eases the cross-comparison and the interpretation of the code predictions, to identify areas where results differ and develop understanding of the underlying causes. Under the conditions investigated, the codes show encouraging agreement in terms of key parameters at both targets, including peak parallel heat flux (1%–45%), ion temperature (2%–19%), and inner target plasma density (1%–23%) when run with similar input. However, strong disagreement is observed for the remaining quantities, from 30% at outer mid-plane up to a factor 4–5 at the targets. The results primarily reflect limitations of the codes: the SOLPS-ITER plasma mesh not reaching the first wall, SOLEDGE2D not including ion-neutral temperature equilibration, and UEDGE enforcing a common ion-neutral temperature. Potential improvements that could help enhance the accuracy of the code models for future applications are also discussed.

The role of B T-dependent flows on W accumulation at the edge of the confined plasma

Near-separatrix impurity accumulation between the crown and the outer midplane of tokamaks is a common feature in results from codes such as SOLPS-ITER and DIVIMP; however, experimental evidence of accumulation has only recently been obtained and is reported here. The codes find that the poloidal distribution of impurity ions in the scrape-off layer (SOL) depends primarily on toroidal field (B T)-dependent parallel flow patterns of the background plasma and the parallel ion temperature gradient (∇‖ T ion) force. Experimentally, Mach probes used in L-mode plasmas with favorable (for H-mode access) B T measure fast (M ∼ 0.3–0.5) inner-target-directed (ITD) background plasma flows at the crown of single-null discharges. This study reports a set of DIVIMP simulations for two similar H-mode discharges from the DIII-D W metal rings campaign differing primarily in B T-direction to assess the effect that fast ITD flows have on the distribution of W ions in the SOL. It is found that for imposed ITD flows of M = 0.3, W ions that otherwise accumulate due to the ∇‖ T ion-force are largely flushed out. It is also found that doubling the radial diffusion coefficient from 0.3 to 0.6 m2 s−1 prevents accumulation due to rapid cross-field transport into the far-SOL, where background plasma flows drain W ions to the divertors. Far-SOL W distributions from DIVIMP are then used to specify input to the impurity transport code 3DLIM, which is used to interpretively model collector probe (CP) deposition patterns measured in the ‘wall-SOL’. It is demonstrated that the deposition patterns are consistent with the DIVIMP predictions of near-SOL accumulation for the unfavorable-B T direction, and little/no accumulation for the favorable-B T direction. The wall-SOL CPs have thus provided the first experimental evidence, albeit indirect, of near-SOL W accumulation—finding it occurs for the unfavorable-B T direction only. For the favorable-B T direction, fast flows can largely prevent accumulation from occurring.

Impact of ion temperature anisotropy on 2D edge-plasma transport

Abstract A model of ion temperature anisotropy for 2D plasma transport in the scrape-off layer (SOL) of tokamaks is described and implemented in the UEDGE fluid transport code. Two ion energy equations are used to describe the evolution of the separate parallel and perpendicular ion temperatures. The temperature anisotropy generates viscous forces in both parallel and perpendicular directions that modify the parallel force balance equation and add an additional cross-magnetic-field drift velocity. Using the full set of UEDGE plasma and neutral equations (particle continuity, momentum, and energy), simulations are performed for both a 1D poloidal case and a 2D (radial and poloidal) single-null tokamak geometry case to highlight the 2D effects. The results show that ion parallel flows near the magnetic X-point in a comparatively low collisionality regime can be overestimated by the standard isotropic Braginskii model. The 2D ion temperature anisotropy varies substantially near the X-point and also near the divertor target plates, due to ionization sources. Moving radially outwards at the outer midplane, the anisotropy decreases between the core boundary and the magnetic separatrix and then it increases while moving across the SOL to the chamber wall.

Effect of Hot He+ Ions on the Electron Pitch Angle Scattering Driven by H+, He+, and O+ Band EMIC Waves

AbstractIn this paper, we find that hot He+ ions significantly alter the kinetic dispersion relation of electromagnetic ion cyclotron (EMIC) waves and thus strongly affect the wave‐driven electron pitch angle scattering rates. Increasing hot He+ ion parallel temperature, temperature anisotropy, or abundance raises the pitch angle scattering rates of MeV electrons driven by the H+ band EMIC waves. During the electron resonance with the He+ and O+ band EMIC waves, the electron pitch angle scattering rates are more sensitive to the variation of temperature anisotropy and abundance of hot He+ ions than their parallel temperature. The increase of hot He+ ion abundance (temperature anisotropy or abundance) generates a reduction of the energy range of electrons interacting with the He+ (O+) band EMIC waves. Therefore, the effect of hot He+ ions on the EMIC wave‐driven electron pitch angle scattering should be involved in the radiation belt modeling.

Hybrid Simulation Study on Anisotropic Response of Ions to a Low-Frequency Wave in a Field-Reversed Configuration

Using a hybrid simulation code, we studied the application of low-frequency wave to a field-reversed configuration plasma. The wave application antenna was arranged to sandwich the separatrix, and the antenna current was varied sinusoidally with a maximum of 30 kA and a frequency of 80 kHz. The simulation revealed that when the ion pressure is regarded as a tensor, the parallel ion temperature estimated from the axial velocity distribution near the midplane exceeds the perpendicular temperature. That is, the plasma ions respond anisotropically to a low-frequency wave.

Flows, Fields, and Forces in the Mars‐Solar Wind Interaction

AbstractWe utilize suprathermal ion and magnetic field measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, organized by the upstream magnetic field, to investigate the morphology and variability of flows, fields, and forces in the Mars‐solar wind interaction. We employ a combination of case studies and statistical investigations to characterize the interaction in both quasi‐parallel and quasi‐perpendicular regions and under high and low solar wind Mach number conditions. For the first time, we include a detailed investigation of suprathermal ion temperature and anisotropy. We find that the observed magnetic fields and suprathermal ion moments in the magnetosheath, bow shock, and upstream regions have observable asymmetries controlled by the interplanetary magnetic field, with particularly large asymmetries found in the ion parallel temperature and anisotropy. The greatest temperature anisotropies occur in quasi‐perpendicular regions of the magnetosheath and under low Mach number conditions. These results have implications for the growth and evolution of wave‐particle instabilities and their role in energy transport and dissipation. We utilize the measured parameters to estimate the average ion pressure gradient, J × B, and v × B macroscopic force terms. The pressure gradient force maintains nearly cylindrical symmetry, while the J × B force has larger asymmetries and varies in magnitude in comparison to the pressure gradient force. The v × B force felt by newly produced planetary ions exceeds the other forces in magnitude in the magnetosheath and upstream regions for all solar wind conditions.

Distribution of water-group ion cyclotron waves in Saturn\u2019s magnetosphere

The water-group ion cyclotron waves (ICWs) in Saturn’s magnetosphere were studied using the magnetic field data provided by the MAG magnetometer on board the Cassini satellite. The period from January 2005 to December 2009, when the Cassini radial distance is smaller than 8 RS, was used. ICWs were identified by their left-hand circularly polarized magnetic perturbations and wave frequencies near the water-group ion gyrofrequencies. We obtained the spatial distribution of ICW amplitude and found that the source region of ICWs is mostly located in the low-latitude region, near the equator and inside the 6 RS radial distance. However, it can extend beyond 7 RS in the midnight region. In general, the wave amplitude is peaked slightly away from the equator, for all local time sectors in both the Northern and Southern Hemispheres. By assuming that the water-group ions are composed of pickup ions and background thermal ions, we obtained the local instability condition of the ICWs and estimated their growth rate along the field lines. If the wave amplitude is correlated with the growth rate, the observed latitudinal dependence of the wave amplitude can be well explained by the local stability analysis. Also, latitudinal location of the peak amplitude is found to depend on the local time. This implies a local time dependence for the water-group ion parallel temperature T|, as determined from the theoretical calculations. Graphical abstractFigure 7. The upper panel shows the projected orbit of Cassini in the cylindrical R–Z plane across the L-shell from the Northern Hemisphere to the Southern Hemisphere. The red line denotes Cassini’s orbit and the blue lines denote the L-shell field lines going through points A and B, respectively. The lower panel shows the power spectrum of delta B_{phi }, the blue dotted line shows the local W+ gyrofrequency (flocal), and the black line is the W+ gyrofrequency at the equator (feq) of the dipole L-shell of the Cassini orbit, which moves from A to B

Ion heating and flows in a high power helicon source

We report experimental measurements of ion temperatures and flows in a high power, linear, magnetized, helicon plasma device, the Resonant Antenna Ion Device (RAID). Parallel and perpendicular ion temperatures on the order of 0.6 eV are observed for an rf power of 4 kW, suggesting that higher power helicon sources should attain ion temperatures in excess of 1 eV. The unique RAID antenna design produces broad, uniform plasma density and perpendicular ion temperature radial profiles. Measurements of the azimuthal flow indicate rigid body rotation of the plasma column of a few kHz. When configured with an expanding magnetic field, modest parallel ion flows are observed in the expansion region. The ion flows and temperatures are derived from laser induced fluorescence measurements of the Doppler resolved velocity distribution functions of argon ions.

Electromagnetic ion-cyclotron instability in a dusty plasma with product-bi-kappa distributions for the plasma particles

We study the dispersion relation for parallel propagating ion-cyclotron (IC) waves in a dusty plasma, considering situations where the velocity dispersion along perpendicular direction is greater than along the parallel direction, and considering the use of product-bi-kappa (PBK) velocity distributions for the plasma particles. The results obtained by numerical solution of the dispersion relation, in a case with isotropic Maxwellian distributions for electrons and PBK distribution for ions, show the occurrence of the electromagnetic ion-cyclotron instability (EMIC), and show that the decrease in the kappa indexes of the PBK ion distribution leads to significant increase in the magnitude of the growth rates and in the range of wavenumber for which the instability occurs. On the other hand, for anisotropic Maxwellian distribution for ions and PBK distribution for electrons, the decrease of the kappa index in the PBK electron distribution contributes to reduce the growth rate of the EMIC instability, but the reduction effect is less pronounced than the increase obtained with ion PBK distribution with the same $\kappa$ index. The results obtained also show that, as a general rule, the presence of a dust population contributes to reduce the instability in magnitude of the growth rates and range, but that in the case of PBK ion distribution with small kappa indexes the instability may continue to occur for dust populations which would eliminate completely the instability in the case of bi-Maxwellian ion distributions. It has also been seen that the anisotropy due to the kappa indexes in the ion PBK distribution is not so efficient in producing the EMIC instability as the ratio of perpendicular and parallel ion temperatures, for equivalent value of the effective temperature.

Critical role of electron heat flux on Bohm criterion

Bohm criterion, originally derived for an isothermal-electron and cold-ion plasma, is often used as a rule of thumb for more general plasmas. Here, we establish a more precise determination of the Bohm criterion that are quantitatively useful for understanding and modeling collisional plasmas that still have collisional mean-free-path much greater than plasma Debye length. Specifically, it is shown that electron heat flux, rather than the isothermal electron assumption, is what sets the Bohm speed to be kB(Te∥+3Ti∥)/mi with Te,i∥ the electron and ion parallel temperature at the sheath entrance and mi the ion mass.

Laser induced fluorescence measurements of axial velocity, velocity shear, and parallel ion temperature profiles during the route to plasma turbulence in a linear magnetized plasma device.

We report experimental measurements of the axial plasma flow and the parallel ion temperature in a magnetized linear plasma device. We used laser induced fluorescence to measure Doppler resolved ion velocity distribution functions in argon plasma to obtain spatially resolved axial velocities and parallel ion temperatures. We also show changes in the parallel velocity profiles during the transition from resistive drift wave dominated plasma to a state of weak turbulence driven by multiple plasma instabilities.

Geodesic acoustic mode in toroidally rotating anisotropic tokamaks

Effects of anisotropy on the geodesic acoustic mode (GAM) are analyzed by using gyro-kinetic equations applicable to low-frequency microinstabilities in a toroidally rotating tokamak plasma. Dispersion relation in the presence of arbitrary Mach number M, anisotropy strength σ, and the temperature ration τ is analytically derived. It is shown that when σ is less than 3 + 2τ, the increased electron temperature with fixed ion parallel temperature increases the normalized GAM frequency. When σ is larger than 3 + 2τ, the increasing of electron temperature decreases the GAM frequency. The anisotropy σ always tends to enlarge the GAM frequency. The Landau damping rate is dramatically decreased by the increasing τ or σ.

ALFVÉN WAVE SOLAR MODEL (AWSoM): CORONAL HEATING

We present a new version of the Alfven Wave Solar Model (AWSoM), a global model from the upper chromosphere to the corona and the heliosphere. The coronal heating and solar wind acceleration are addressed with low-frequency Alfven wave turbulence. The injection of Alfven wave energy at the inner boundary is such that the Poynting flux is proportional to the magnetic field strength. The three-dimensional magnetic field topology is simulated using data from photospheric magnetic field measurements. This model does not impose open-closed magnetic field boundaries; those develop self-consistently. The physics includes: (1) The model employs three different temperatures, namely the isotropic electron temperature and the parallel and perpendicular ion temperatures. The firehose, mirror, and ion-cyclotron instabilities due to the developing ion temperature anisotropy are accounted for. (2) The Alfven waves are partially reflected by the Alfven speed gradient and the vorticity along the field lines. The resulting counter-propagating waves are responsible for the nonlinear turbulent cascade. The balanced turbulence due to uncorrelated waves near the apex of the closed field lines and the resulting elevated temperatures are addressed. (3) To apportion the wave dissipation to the three temperatures, we employ the results of the theories of linear wave damping and nonlinear stochastic heating. (4) We have incorporated the collisional and collisionless electron heat conduction. We compare the simulated multi-wavelength EUV images of CR2107 with the observations from STEREO/EUVI and SDO/AIA instruments. We demonstrate that the reflection due to strong magnetic fields in proximity of active regions intensifies the dissipation and observable emission sufficiently.

Charge-to-mass-ratio-dependent ion heating during magnetic reconnection in the MST RFP

Temperature evolution during magnetic reconnection has been spectroscopically measured for various ion species in a toroidal magnetized plasma. Measurements are made predominantly in the direction parallel to the equilibrium magnetic field. It is found that the increase in parallel ion temperature during magnetic reconnection events increases with the charge-to-mass ratio of the ion species. This trend can be understood if the heating mechanism is anisotropic, favoring heating in the perpendicular degree of freedom, with collisional relaxation of multiple ion species. The charge-to-mass ratio trend for the parallel temperature derives from collisional isotropization. This result emphasizes that collisional isotropization and energy transfer must be carefully modeled when analyzing ion heating measurements and comparing to theoretical predictions.

Ion temperature anisotropy effects on threshold conditions of a shear-modified current driven electrostatic ion-acoustic instability in the topside auroral ionosphere

Abstract. Temperature anisotropies may be encountered in space plasmas when there is a preferred direction, for instance, a strong magnetic or electric field. In this paper, we study how ion temperature anisotropy can affect the threshold conditions of a shear-modified current driven electrostatic ion-acoustic (CDEIA) instability. In particular, this communication focuses on instabilities in the context of topside auroral F-region situations and in the limit where finite Larmor radius corrections are small. We derived a new fluid-like expression for the critical drift which depends explicitly on ion anisotropy. More importantly, for ion to electron temperature ratios typical of F-region, solutions of the kinetic dispersion relation show that ion temperature anisotropy may significantly lower the drift threshold required for instability. In some cases, a perpendicular to parallel ion temperature ratio of 2 and may reduce the relative drift required for the onset of instability by a factor of approximately 30, assuming the ion-acoustic speed of the medium remains constant. Therefore, the ion temperature anisotropy should be considered in future studies of ion-acoustic waves and instabilities in the high-latitude ionospheric F-region.

Recent results of divertor simulation research using an end-cell of a large tandem mirror device

Abstract This paper describes the results of characterization of high heat and particle fluxes produced at the end-cell of the large tandem mirror GAMMA 10 and of the initial plasma-irradiation experiments. In the case of ICRF plasmas, the heat flux of 0.8 MW/m2 and the particle flux of 4 × 1022/m2 s were achieved at the end-mirror exit. The heat flux increases with the ICRF power and has a linear relationship with the stored energy. Direct ion energy analysis clarified that the parallel ion temperature can be controlled from 100 eV to 400 eV by changing the ICRF power. Additional plasma heating using another ICRF system in the anchor-cell significantly increases the particle flux, which gives a clear prospect of generating the higher particle flux by applying additional ICRF heating in the neighboring cells. The initial results of the plasma–gas–material interactions on a new V-shaped tungsten target were also reported.