Lower Hybrid Waves Research Articles

Wave-driven torques to drive current and rotation

In the classic Landau damping initial value problem, where a planar electrostatic wave transfers energy and momentum to resonant electrons, a recoil reaction occurs in the nonresonant particles to ensure momentum conservation. To explain how net current can be driven in spite of this conservation, the literature often appeals to mechanisms that transfer this nonresonant recoil momentum to ions, which carry negligible current. However, this explanation does not allow the transport of net charge across magnetic field lines, precluding E × B rotation drive. Here, we show that in steady state, this picture of current drive is incomplete. Using a simple Fresnel model of the plasma, we show that for lower hybrid waves, the electromagnetic energy flux (Poynting vector) and momentum flux (Maxwell stress tensor) associated with the evanescent vacuum wave become the Minkowski energy flux and momentum flux in the plasma and are ultimately transferred to resonant particles. Thus, the torque delivered to the resonant particles is ultimately supplied by the electromagnetic torque from the antenna, allowing the nonresonant recoil response to vanish and rotation to be driven. We present a warm fluid model that explains how this momentum conservation works out locally, via a Reynolds stress that does not appear in the one-dimensional initial value problem. This model is the simplest that can capture both the nonresonant recoil reaction in the initial-value problem, and the absence of a nonresonant recoil in the steady-state boundary value problem, thus forbidding rotation drive in the former while allowing it in the latter.

A model investigation of the impact of lower hybrid wave scattering angle on current drive profile in EAST and Alcator C-Mod

Lower hybrid current drive (LHCD) is beneficial for developing a steady-state operation scenario in a tokamak. This paper conducts a modeling investigation to identify an optimum rotation angle of the initial lower hybrid perpendicular (to the background magnetic field ) wavevector for best matching the experimental RF current profile. It is hypothesized that central RF power deposition widely observed in the present-day LHCD experiments arises from wave scattering by turbulence. In a standard model without considering such interactions, the predicted power deposition profile is generally broad with off-axis peaking, not in agreement with experimental observations. A heuristic approach is adopted by introducing a spectral broadening mechanism by modifying the initial orientation of the perpendicular wavevector. The ray-tracing/Fokker–Planck solver GENRAY/CQL3D is utilized within the python-based π-scope framework. A focus is given to identify the perpendicular wavenumber orientation angle with respect to the magnetic surface normal vector at the initial ray location. Our modeling study shows that rotating the perpendicular wavevector in such a way as to increase the initial poloidal component is effective in reproducing the centrally peaked current profile observed in normal shear plasmas on both EAST and C-Mod. These waves can readily be absorbed to the central plasma, which reduces the sensitivity of the power deposition profile to a slight change of the plasma condition. The same approach is also found to help broaden the off-axis power deposition profile in a reverse-shear EAST plasma, leading to a better agreement with the experiment. The results presented here suggest that spectral modification arising from edge density fluctuations in a tokamak may need to be considered in understanding wave propagation and absorption. A further experimental and theoretical/modeling study is vital as a reverse approach is adopted in this study. Our work suggests that mitigation or control measures are critical for parasitic effects occurring on the first pass in a reactor regime.

Modulated Upper‐Hybrid Waves Coincident With Lower‐Hybrid Waves in the Cusp

AbstractDuring the Twin Rockets to Investigate Cusp Electrodynamics (TRICE‐2) High‐Flyer rocket's passage through the cusp the high frequency (HF) radio wave receiver observed three intervals of banded upper‐hybrid (UH) waves. The bands begin at the UH frequency (1.2–1.3 MHz), descending to as low as 1.1 MHz, with amplitudes of hundreds of mV/m. The spacing of the bands are 4.5–6 kHz and the number of bands ranges from 3 to 10. Simultaneously, the very low frequency (VLF) radio wave receiver observed lower‐hybrid (LH) waves with amplitudes ranging from 1 to 10 mV/m and frequencies of 4.5–6 kHz. Slight variations of the spacings of the bands in the UH waves were closely correlated with variations in the LH peak frequencies. Two possible wave‐wave interactions are explored to explain this phenomenon: decay of an UH wave into a lower frequency UH wave and a LH wave, and coalescence of independent UH waves and LH waves that spawn UH waves. Using a dispersion relation calculator with electron and ion distribution functions based off those observed by the particle instruments suggests that UH waves, and to a lesser degree LH waves, can be excited by linear instabilities. Kinematic analysis of the waves dispersion relations and the wave matching conditions show that wave‐wave interactions linking UH and LH modes are possible through either decay or coalescence. This analysis along with comparisons of the energy densities of the waves, and the ratio of their occupation numbers suggest that the decay process is more likely than coalescence.

Study of the mechanism of ITB formation and sustainment with optimized q profiles in ELMy H mode discharges on the EAST

Recently, improved confinement plasma with the internal transport barrier (ITB) on the Experimental Advanced Superconducting Tokamak has been achieved through extensive experiments in high β N scenario development. ELMy H-mode plasmas are heated by lower hybrid wave and neutral beam injection, with B T = 1.6 T, I p = 400/450 kA and q 95 in the range 3.7–4.6. ITB with optimized reversal q profiles and central flat q profiles are observed in these H-mode plasmas, which lead to better confinement. The electron temperature ITB, ion temperature ITB, and electron density ITB exhibit different characteristics. In this paper, the formation and sustainment of ITB on n e channel has great relationship with magnetohydrodynamics (MHD) behaviors. For T e channel, q profile has a dominating effect. Reversal magnetic shear and MHD behaviors are both effective in T i ITB formation. Suppression of turbulence in the ITB region is the common reason on all the channels. In addition, the power threshold of the ITB formation should be concentrated on electrons and ions separately. The results of this study can widen the understanding of ITB and be helpful in exploring better candidate operational scenarios for future fusion devices.

The achievement of the T e,div feedback control by CD4 seeding on EAST

A multi-function divertor feedback control system has been built on Experimental Advanced Superconducting Tokamak (EAST) to treat the divertor heat load issue. With the real-time data of the Langmuir probes and the impurity seeding, the divertor electron temperature (T e,div) is well controlled to achieve the partial detachment phase. The first trial of the CD4 seeding for the T e,div reduction has been achieved on EAST long-pulse discharge. In the seeding phase, the T e,div was maintained close to 5 eV, and the surface temperature of the target plate (T surface,div) had a reduction of about 150 °C. The plasma stored energy had a reduction in the control phase, so it is necessary to find a way to keep the good plasma confinement in the next step. The CD4 injection also mitigated the low hybrid wave coupling rate in some degree. The big volume of the CD4 injection lifted the Greenwald density fraction from ∼0.4 to ∼0.7, which made the SOL into high recycling state. Most of the injected carbon particles were in the high ionized state, and with the lower of the T e,div, the tungsten line emission was suppressed obviously.

Modeling very high electron heating by radio frequency waves on EAST

In the 2018 EAST experimental campaign, a very high central electron heating, fully-non-inductive discharge with the core electron temperature over 9 keV has been achieved. Such high central electron heating was realized by injecting radio frequency waves, including 1.8 MW lower hybrid wave (LHW) and 0.8 MW electron–cyclotron waves (ECW). Experimental diagnosis indicates two different time scales characterizing the electron heating process, a rapid and a slow rise of the central electron temperature after the injection of ECW. In this work, integrated modeling is performed to investigate the physical mechanisms of such high electron heating. Five characteristic phases during the increase of the electron temperature are chosen for modeling. In phase 1, the electron heating is by LHW alone. The modeling confirms that the LHW can only sustain the core electron temperature T e ≈ 5.5 keV, which is consistent with the experiment. In phase 2, the electron temperature increases rapidly after the first 0.4 MW ECW is injected. The result shows that the rapid increase of the electron temperature is due to the interaction between the ECW and the electrons. With the increase of the electron temperature, the electron flux induced by the trapped electron modes (TEMs) and the electron temperature gradient driven modes (ETGs) is enhanced in the core region. In phase 3, the electron temperature increases slowly after phase 2. It is found that the slow increase is mainly due to the flattening of the density profile. The flattening of the density profile can decrease the thermal diffusivity of the electrons mainly induced by the TEMs leading to a higher electron temperature for a given heating source. In phase 4, the electron temperature again increases rapidly after the second 0.4 MW ECW is injected. The physical mechanism is similar to that in phase 2. In phase 5, the LHW power deposition of the LHW remains almost unchanged compared to that in phase 4 since the electron temperature is sufficiently high. The slow rise of the electron temperature is caused by the improvement of the electron energy confinement as thermal diffusivity of the electrons is decreased due to the flattening of the electron density profile, which is similar to the main reason in phase 3.

Modeling the spectral modification of lower hybrid wave in the presence of drift-wave type density fluctuation in the scrape-off-layer of the EAST tokamak

The spectrum change of the lower hybrid (LH) waves caused by a low-frequency density fluctuation in the scrape-off-layer is studied by applying the wave scattering model developed by Bonoli and Ott [Phys. Fluids 25, 359 (1982)] via a Monte Carlo method. Due to the influence of the density fluctuation, the perpendicular component of the LH wave-vector can be rotated in the 2D perpendicular space, which will further change the ray trajectory of the LH wave. A ray-tracing model specific to this purpose is developed to evaluate the probability distribution of both the poloidal refractive index (Nθ) and the parallel refractive index ( N∥) of the LH wave at the last closed flux surface (LCFS). The scattering probability and the rotation scattering angle are determined by the combined effect from the geometric optics approximation term and the E×B drift term in the LH tensor elements. The probability distributions of N∥ and Nθ at the LCFS are studied using the EAST parameters as a function of wave frequency, the initial N∥, and the polar injection position, which may influence the lower hybrid current drive efficiency.

Error Analysis of Directional Coupler for Lower Hybrid Wave Power Measurement

As one of the important parameters for lower hybrid wave (LHW) systems with 24 4.6 GHz/250 kW continuous-wave klystron amplifiers, the microwave power needs to be accurately acquired in real time for the experimental advanced superconducting tokamak (EAST) experiment. A new 4.6 GHz/250 kW directional coupler was developed for power measurements in an LHW system. The design principle and characteristic parameters of the coupler are discussed based on transmission line theory and the system requirements. The calculations and simulations indicate that the measurement error of the incident power is less than 5% when the isolation is greater than 25 dB. The directional coupler was also applied to the 4.6 GHz LHW system. The results show that the device can meet the requirements of LHW system operations. Comprehensive Research Facility for Fusion Technology(CRAFT)is being built. This work is beneficial to the design of power measurement system for LHW system.

Comparison of natural grassy ELM behavior in favorable/unfavorable B t in EAST

The properties of grassy edge-localized modes (ELMs) in EAST in the favorable and unfavorable B t are statistically studied. Statistical analysis indicates that there is no systematical difference in the frequencies of grassy ELMs under the two different magnetic configurations in the similar parameter spaces. The high-frequency grassy ELM (f ELM > 1 kHz) in unfavorable B t is dependent on the high poloidal beta β p and high triangularity δ u, while the high-frequency grassy ELM (f ELM > 1 kHz) in favorable B t appears to rely on the high plasma density. A frequently occurring phenomenon in favorable B t defined as ‘clustered ELM’ seems to be the most evident difference in ELM behavior between favorable and unfavorable B t. Statistical analysis shows that larger plasma-wall outer gap, longer plasma elongation, lower low-hybrid wave heating power and electron cyclotron resonance heating power favor the occurrence of clustered grassy ELMs. Further studies indicate that the generation of clustered grassy ELMs could be correlated with the lower electron temperature in the bulk plasma.

Observations of Short‐Period Ion‐Scale Current Sheet Flapping

AbstractKink‐like flapping motions of current sheets are commonly observed in the magnetotail. Such oscillations have periods of a few minutes down to a few seconds and they propagate toward the flanks of the plasma sheet. Here, we report a short‐period ( s) flapping event of a thin current sheet observed by the Magnetospheric Multiscale spacecraft in the dusk‐side plasma sheet following a fast Earthward plasma flow. We characterize the flapping structure using the multi‐spacecraft spatiotemporal derivative and timing methods, and we find that the wave‐like structure is propagating along the average current direction with a phase velocity comparable to the ion velocity. We show that the wavelength of the oscillating current sheet scales with its thickness as expected for a drift‐kink mode. The decoupling of the ion bulk motion from the electron bulk motion suggests that the current sheet is thin. We discuss the presence of the lower hybrid waves associated with gradients of density as a broadening process of the thin current sheet.

Characteristics of double-peaked particle deposition at divertor target plates in the EAST tokamak

Increasing the heat and particle deposition on the divertor target plates is an effective approach for reducing the extremely high heat load. Future tokamak fusion devices are expected to have a deposition width on the order of a millimeter. Recently, a double-peaked distribution (DPD) pattern of particle deposition at the divertor target plates has been observed in both deuterium and helium plasma discharges in the Experimental Advanced Superconducting Tokamak, which can significantly spread the deposition width of heat and particle fluxes, and clearly shows a toroidal symmetric distribution. It has been found that the DPD behavior occurs not only in plasma discharges with lower hybrid wave (LHW) heating, but also with electron cyclotron resonance heating or neutral beam injection (NBI) heating alone. In addition, the DPD shows an obvious in–out asymmetry between the divertor target plates, which strongly depends on the toroidal field direction, i.e. it distinctly appears on the outer divertor plate in unfavorable B t conditions, while shifting to the inner divertor plate under favorable B t conditions. Meanwhile, the transition between the single-peaked distribution and the DPD is normally correlated with the line-averaged plasma density and the auxiliary heating power. The statistical results for the pure LHW heating discharge scheme show that the DPD behavior is significantly affected by both the plasma density and the heating power, i.e. the lower the ratio of the plasma density to the heating power, the more likely it is that DPD behavior will occur. The critical boundary lines between the DPD and non-DPD patterns are obtained with the line expressions ∼ 2P LHW,abs + 1.4 and ∼ 3.33P LHW,abs + 1 for the unfavorable and favorable B t cases, respectively. Finally, the underlying physics is also discussed, suggesting that the electric field drifts and the local sheath boundary condition may exert a strong influence on the DPD.

Observation of synergy between lower hybrid waves at two frequencies in EAST

Synergistic effects between two frequencies of lower hybrid (LH) waves—operating at 2.45 and 4.6 GHz—were observed in experiment on EAST for the first time. At low density (ne,lin≈2.0×1019 m−3), simultaneous injection of a 65/35 mix of 2.45/4.6 GHz power achieved an lower hybrid current drive efficiency that was 25% higher than what should be expected from the linear combination of the two sources. The experiment was interpreted with time-dependent simulations, using the equilibrium and transport solver TRANSP, coupled with the ray-tracing code GENRAY and the Fokker–Planck solver CQL3D. For each discharge, profiles of current and hard x-rays from simulation and measurement agree within uncertainties. An examination of the electron distribution function indicates that the LH synergy is supported by the increased width of the LH resonance plateau in the simultaneous injection case compared to independent injection.

Wave Activity in a Dynamically Evolving Reconnection Separatrix

AbstractWaves driven by electron beams are frequently observed within the narrow separatrices of magnetic reconnection. Strong plasma instabilities can grow from mixing between the reconnection inflow and outflow, resulting in electron heating which ultimately influences the reconnection process. Observations of a separatrix encounter by the Magnetospheric MultiScale mission on July 11, 2017 feature an anti‐parallel electron beam accelerated in a sub‐ion‐scale layer containing high amplitude (>50 mV/m) electrostatic waves. In two consecutive separatrix crossings, one exhibits faster electron beams, greater magnetic shear, higher amplitude waves, and more electron heating than the other. The observations suggest that variable reconnection outflow pressure is associated with these changes. Coincident with lower hybrid waves are Buneman and beam‐mode instabilities, likely responsible for parallel heating of electrons. Dissipation (J⋅E) within the mixing layer is found to be negative where wave activity is strongest, indicating a slowing of electrons and growth of electromagnetic fields likely related to local thinning of the separatrix. Calculation of momentum balance via the generalized Ohm's law indicates an overall imbalance of >40 mV/m in the more extreme separatrix crossing, requiring a significant dissipative term or error in the electron inertial term to make up the remainder. Finally, an analysis of quasi‐viscous heating shows that active heating of electrons is more prevalent in the presence of a high speed electron inflow and increased magnetic shear.

Lower-hybrid drift waves and their interaction with plasmas in a 3D symmetric reconnection simulation with zero guide field

We investigate lower-hybrid drift waves (LHDW) in symmetric magnetic reconnection with zero guide field using three-dimensional particle-in-cell simulations. The long-wavelength mode with kρiρe∼1 develops in the bifurcated electron current layer around the X-line within the width of the electron meandering motion from the mid-plane, where ρi(e) is the ion (electron) gyroradius. The short-wavelength mode with kρe∼1 develops in the separatrix region downstream of the electron outflow jet, producing electron vortices in the background flow frame. Electrons follow the E × B drift with corrections from the diamagnetic drift and are heated inside the vortices with diverging electric fields. In the vortices, ions have comparable E × B and inertia drifts, which together mostly cancel the diamagnetic drift. Toward the center of diverging field vortices, ions are decelerated, leading to a decrease in the perpendicular temperature, while the loss of low-energy ions results in an increase in the parallel temperature. Parallel electric fields exist as a combination of the LHDW wave field projected to the magnetic field direction and the penetration of whistler waves that are mainly outside of the LHDW layer. The magnetic flux tube is twisted in the vortices. The twist may potentially lead to slippage reconnection, as indicated by the non-uniform parallel potential variation across field lines, while the periodic variations of the twisting directions are a limiting factor.

Electron Heat Flux Instabilities in the Inner Heliosphere: Radial Distribution and Implication on the Evolution of the Electron Velocity Distribution Function

Abstract This Letter investigates the electron heat flux instability using the radial models of the magnetic field and plasma parameters in the inner heliosphere. Our results show that both the electron acoustic wave and the oblique whistler wave are unstable in the regime with large relative drift speed (ΔV e ) between electron beam and core populations. Landau-resonant interactions of electron acoustic waves increase the electron parallel temperature that would lead to suppressing the electron acoustic instability and amplifying the growth of oblique whistler waves. Therefore, we propose that the electron heat flux can effectively drive oblique whistler waves in an anisotropic electron velocity distribution function. This study also finds that lower-hybrid waves and oblique Alfvén waves can be triggered in the solar atmosphere, and that the former instability is much stronger than the latter. Moreover, we clarify that the excitation of lower-hybrid waves mainly results from the transit-time interaction of beaming electrons with resonant velocities v ∥ ∼ ω/k ∥, where ω and k ∥ are the wave frequency and parallel wavenumber, respectively. In addition, this study shows that the instability of quasi-parallel whistler waves can dominate the regime with medium ΔV e at the heliocentric distance nearly larger than 10 times of the solar radius.

Dependence of upstream SOL density shoulder on divertor neutral pressure observed in L-mode and H-mode plasmas in the EAST superconducting tokamak

Upstream density profiles in the scrape-off layer (SOL) have been examined in low-confinement mode (L-mode) and high-confinement mode (H-mode) plasmas in the EAST superconducting tokamak. A weak density shoulder forms in the near SOL region in upper single-null configurations when the neutral pressure measured at the lower divertor exceeds a threshold value of 2 × 10−2 Pa in L-mode plasmas. When the neutral pressure is below this threshold, the weak density shoulder is absent and the sidebands of the lower hybrid waves associated with SOL parametric instabilities are reduced. Active detachment control with neon–deuterium seeding demonstrate that the weak density shoulder can form before the onset of the outer divertor detachment as long as the neutral pressure is above the threshold. Furthermore, no remarkable expansion of a shoulder is observed during divertor detachment, suggesting that divertor detachment is not a necessary condition for the formation or growth of a density shoulder. Through the increase in neutral pressure in the lower divertor by an order of magnitude, the weak shoulder was observed to expand into the far SOL and reach the leading edge of the limiter. The results in L-mode discharges identified the neutral pressure in the lower divertor as a primary factor for the formation of an SOL density shoulder in the upper single-null discharges. For the type-I ELMy H-mode plasmas, a similar density shoulder was detected during the inter-ELM phase when the neutral pressure in the lower divertor exceeded a threshold value of 4 × 10−2 Pa. On the other hand, the shoulder was absent when the divertor neutral pressure went below this threshold even though the plasma discharge was conducted with a higher core line-averaged density and divertor collisionality. This is consistent with the observations in L-mode plasmas. The neutral particle ionization of the working gas is thus believed to play a key role during the formation of the SOL density shoulder in the EAST tokamak.

The effects of E r × B t drift on LH wave coupling with divertor configuration at EAST

The coupling characteristics of the 4.6 GHz lower hybrid (LH) grill and the factors affecting LH wave coupling, mainly drift, with divertor configuration in anti-clockwise/clockwise magnetic field direction are studied. Based on the Langmuir probe measurement, the relationship between the reflection coefficient (RC) of LH power and the edge density at the grill mouth () measured by Langmuir probes is investigated. The optimum LH wave coupling with RC less than 2% is achieved when is in the range of –, which is quite close to the theoretically predicted value. In addition, the experimental results show that drift in the divertor scrape off layer plays a dominant role in affecting and LH wave coupling. Studies show that the discrepancy in and LH wave coupling in different magnetic field directions is mainly attributed to the different directions of drift, whereas the corresponding discrepancy with different plasma currents is mainly ascribed to the change in drift intensity. Therefore, for the upper single null divertor configuration, a lower/higher plasma currents is more favorable for increasing and improving LH wave coupling in anti-clockwise/clockwise . The above studies are of great significance for optimizing and improving LH wave coupling in the extension of the plasma operation regime.

The Whistler Traveling Wave Parametric Amplifier Driven by an Ion-Ring Beam Distribution from a Neutral Gas Injection in Space Plasmas

A new parametric amplifier model describes the observations of intensified whistler waves produced by a dedicated burn of the BT-4 engine on the Cygnus spacecraft during the NG-13 mission. Ground very low frequency (VLF) radio emissions at 25.2 kHz from a Navy NML transmitter in North Dakota were amplified by 20-30 dB during the Cygnus burn at 480-km altitude and recorded at 1060 km by the e-POP/RRI plasma wave receiver on the SWARM-E satellite. The amplification process starts with charge exchange between the exhaust molecules and the ambient O+ in the ionosphere to produce water vapor ions that spiral around the earth's magnetic field lines. This ion-velocity-ring distribution generates broadband, oblique-lower-hybrid (OLH) waves, which act as a pump for the parametric amplifier. The nonlinear ponderomotive force on the electrons causes the high-amplitude OLH pump to mix with the input whistler signal, yielding an OLH idler wave. Resonant mixing of the pump and idler electric fields promotes temporal growth in the amplitude of the whistler waves as they propagate through the exhaust cloud. The key features to rocket exhaust-driven amplification (REDA) process are broadband gain, bi-directionality along the magnetic field, pump depletion, phasing, and feedback. Pump depletion limits the intensity of the output whistler waves by wave energy conservation. The total wave energy is the electrostatic energy of the pump and idler lower hybrid waves plus the energy extracted by the propagating whistler signal. The whistler traveling wave parametric amplifier provides an efficient mechanism for active amplification of signals in space.

A Statistical Study of Lower Hybrid Waves in the Earth's Magnetosphere by Van Allen Probes

AbstractThe lower hybrid (LH) waves are electrostatic emissions near the LH resonant frequency. They propagate perpendicularly with a small wavelength comparable to Larmor radius of thermal particles and can be capable of heating both ions and electrons. In this study, we statistically study the global distribution of LH waves in the inner magnetosphere by using Van Allen Probes observation from 2012 to 2018. We find that (a) LH waves are commonly observed in the inner magnetosphere. Most LH waves are confined near the magnetic equator with typical amplitudes of 0.02–0.2 mV/m and occurrence rates up to 10%. (b) LH waves extend to inner L regions with increasing wave amplitudes as AE* increases. (c) Weak LH waves occur at the nightside inside the plasmapause. Moderate and strong LH waves occur at the nightside and noon inside the plasmapause. As AE* increases, they extend to all magnetic local times inside the plasmapause and dawnside outside the plasmapause.

Growth of the lower hybrid drift instability in the plume of a magnetically shielded Hall thruster

Analytical and numerical investigations have been carried out that show the lower hybrid drift instability is active in a region of low-β plasma outside the discharge channel of a magnetically shielded Hall thruster. The region is located downstream of the inner front pole magnet and is contained by the thruster ion beam and hollow cathode plume. It is found that large enough E × B and diamagnetic drifts in the out-of-plane (azimuthal) direction are produced in this region to destabilize lower hybrid waves, even in the presence of electron collisions. The results have been obtained by solving an electrostatic dispersion relation in which the density inhomogeneity is accounted for in the velocity distribution function of the strongly magnetized electrons. Ions in the azimuthal direction have been treated as stationary and unmagnetized. The plasma properties in the dispersion have been provided independently by large-scale 2D multi-fluid/particle-in-cell simulations. The growth rates are found to be even greater than those associated with the modified two-stream instability, which was also predicted recently to be active in this region. The implication of the findings is that ions may be heated anomalously by these instabilities, in both the out-of-plane and in-plane directions perpendicular to the applied magnetic field, thereby altering the velocity field and enhancing the energy of those ions that strike nearby thruster surfaces.