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Noninvasive measurements of electron properties in a Hall effect thruster (HET) are needed to understand the physical processes in the acceleration region and to validate simulations. This paper presents spatially resolved laser Thomson scattering (LTS) measurements across the entire acceleration region of a HET. The test article is the H9, a 9 kW class magnetically shielded HET. The H9 is operated on krypton at a facility pressure of 1.2 × 10−5 Torr Kr (1.6 mPa). The thruster is operated at three discharge conditions: 171 V, 35 A, an inner coil current (Iic) of 4.11 A, and an outer coil current (Ioc) of 2.27 A as the baseline 6 kW condition; 154 V, 34.8 A, an Iic of 4.11 A, and an Ioc of 2.27 A to vary the discharge voltage; and 171 V, 34.4 A, an Iic of 4.52 A, and an Ioc of 2.5 A to vary the magnetic field strength. At each discharge condition, we measured axial profiles of electron density and electron temperature along the channel centerline from 5% to 95% of a channel length downstream of the channel exit plane. At the baseline condition, we also measure the axial profile of the azimuthal electron drift velocity. We measure a minimum electron density of 1.3 × 1017 m−3, peak electron temperatures around 40 eV, and a peak azimuthal electron drift velocity around 680 km/s. The results suggest the presence of anomalous electron heating and demonstrate that low discharge voltages allow LTS to access the entire acceleration region of a HET. [ABSTRACT FROM AUTHOR]

This paper established a three-dimensional transient MHD model of a high-current multi-component vacuum arc under a transverse magnetic field (TMF) to simulate the dynamic characteristics of plasma parameters in the uniform motion mode of a contracted arc under lower TMF. The model considered the ionization–recombination process among the multi-components and used the P1 model to calculate the thermal radiation losses. The simulation results show that the ion number density of The TMF arc is on the order 1024 m−3, and when the plasma temperature reaches 4.3 eV, the effects of thermal radiation cannot be ignored. The majority of the plasma from the cathode and anode undergo acceleration and then deceleration before finally intersecting at the center of the arc, creating an extreme value of the density and the pressure, where a large amount of kinetic energy is converted into internal energy. The arc will be bent and deflected under the action of the ampere force, and the deflection of Cu3+ is especially obvious. The TMF affects the collision strength of the jet in the intersection area and, thus, affects the ion density, in which the change of Cu2+ is dominant. The ionization–recombination process of ions is mainly determined by the electron temperature, which is affected by the arc current. As the current decreases, Cu1+ increases and Cu2+ and Cu3+ decrease, and the change in ionization rate is the main reason for the change in the proportion of each ionic component; the heat flux density to the anode and cathode is on the order of 1010 W/m2, which heats the front of the arc and ensures the stable movement of the arc. Meanwhile, due to the high ion number density, the ion heat flux accounts for the main part of the heat flux, and the anode exhibits a higher proportion of ion heat flux. [ABSTRACT FROM AUTHOR]

We develop an analytical model for estimating the equilibrium quantities, such as electron temperature and number density, in an electron beam–plasma interaction system. This model provides a convenient way to calculate the effective electron temperature and density by considering the energy balance of the bulk cold electrons. Six energy sources/losses terms relevant to the cold electrons are accounted for, where quasi-linear theory is applied for estimating wave heating at equilibrium. We compare this calculation with the particle-in-cell (PIC) simulation results and find good agreement. Based on these results, we then consider two situations where we can simplify our model. The first is dominated by the balance between electron–electron Coulomb collisions and loss to the anode, which is mostly relevant to the conduction phase of plasma switches. The second is dominated by wave heating balanced by the anode loss, relevant to the electron beam–plasma discharge systems. We then couple our simplified energy balance model with the ion diffusion model and solve both the number density and the electron temperature as functions of the current density, electrode distance, pressure, and applied voltage, where a nice agreement is also obtained when comparing to PIC simulations. [ABSTRACT FROM AUTHOR]

A reduced pressure glow discharge is produced by passing a high-power pulsed DC source of 0–500 W with a frequency of 50 Hz across two parallel disk electrodes. A hydrogen peroxide aqueous solution is used as a flowing gas for discharge generation. Optical emission spectroscopy is employed to diagnose the discharge generated at a reduced pressure of 0.2 mbar with an electrode gap of 4 cm. The spectra are recorded at a power density of 9.4 mW/cm3 and typically lie in the visible wavelength range of 380–880 nm. The spectra are analyzed using the line intensity ratio method to estimate electron temperature and density. The results indicated that the electron temperature and density are, respectively, 0.87 eV and 6.4 × 1014 cm−3. [ABSTRACT FROM AUTHOR]

We investigated the magnetic properties of Sendust (Fe-Al-Si) alloys not only at 0 K but also at finite temperatures by means of the first-principles calculations assuming A2, B2, and D 0 3 structures. We confirmed that the itinerant characteristics of 3 d electrons of Fe are not negligible and a significantly small exchange stiffness constant exists at zero temperature in a B2 structure. However, the calculated Curie temperatures are in the same order for all structures; this indicates that the Curie temperature cannot be determined only by the exchange interactions at zero temperature in itinerant electron systems. Temperature dependence of the exchange interaction, namely, spin configuration dependence, also might be important for determining it. In addition, this property might also be related to the unique behavior of the temperature dependence of the exchange stiffness constant for the B2 structure, which does not decrease monotonically as temperatures increase, contrary to the behavior expected from the Heisenberg model. In addition, we investigated composition dependence on the exchange stiffness constant at zero temperature and confirmed that the substitution of Si with Al could improve the amplitude of the exchange stiffness constant at zero temperature for all structures. [ABSTRACT FROM AUTHOR]

Density Functional Theory (DFT) has become a cornerstone in the modeling of metals. However, accurately simulating metals, particularly under extreme conditions, presents two significant challenges. First, simulating complex metallic systems at low electron temperatures is difficult due to their highly delocalized density matrix. Second, modeling metallic warm-dense materials at very high electron temperatures is challenging because it requires the computation of a large number of partially occupied orbitals. This study demonstrates that both challenges can be effectively addressed using the latest advances in linear-scaling stochastic DFT methodologies. Despite the inherent introduction of noise into all computed properties by stochastic DFT, this research evaluates the efficacy of various noise reduction techniques under different thermal conditions. Our observations indicate that the effectiveness of noise reduction strategies varies significantly with the electron temperature. Furthermore, we provide evidence that the computational cost of stochastic DFT methods scales linearly with system size for metal systems, regardless of the electron temperature regime. [ABSTRACT FROM AUTHOR]

The unstable plume mode of hollow cathodes should be avoided in practical applications because it severely degrades the overall cathode lifetime. In this study, we investigate the spot-plume transition and plasma stability characteristics of an unused segmented lanthanum hexaboride emitter. The expansion of the unstable plume mode region is observed during a discharge experiment. Subsequently, the segmented emitter is retrieved, the inner surface of the emitter is observed, and the work function on the surface is measured at room temperature. The emitter surface exhibits color variations with oxygen and carbon detection. The downstream edge shows the original purple color and almost no degradation in the work function. The high temperature in this region promotes the desorption of carbon and oxygen. In the spot mode, this region mainly contributes to thermionic electron emission; therefore, the discharge voltage in the spot mode does not change during the discharge experiment. Carbon or carbide is detected in the middle of the axial direction on the emitter surface, where the surface temperature is not sufficiently high to desorb carbon during discharge. Based on the surface analysis results, the dominant substance in the region where carbon is detected was lanthanum carbide. An increase in the work function is indicated in the region, which appears to increase the plasma instability. According to previous studies, an increase in the work function results in a rise in the potential in the emitter, and an increase in the electron temperature in the outside plume region induces the plasma instability. Further investigation is needed to understand the mechanism connecting the rise in the work function and the rise in the electron temperature in the plume region. [ABSTRACT FROM AUTHOR]

We use a 2.5D hybrid simulation to study the breathing mode (BM) dynamics in Hall thrusters (HTs). This involves a 1D Euler fluid simulation for neutral dynamics in the axial direction, coupled with a 2D axial–azimuthal Particle-in-Cell (PIC) simulation for charged species. The simulation also includes an out-of-plane virtual dimension for wall losses. This setup allows us to replicate the BM's macroscopic features observed in experiments. A comprehensive analysis of plasma parameters in BM's phases divides it into two growth and two decay sub-phases. Examining 1D axial profiles of electron temperature, gas and plasma densities, and particle creation rate shows that an increase in electron temperature alone cannot sustain ionization. Ionization seems to be influenced by the spatial correlation between electron and gas densities and the ionization rate coefficient. Investigating ion back-flow reveals its impact on modulating neutral flux entering the ionization region. The hybrid simulation's outcomes let us assess the usual 0D predator–prey model's validity and identify its limitations. The ionization and ion convection term approximations hold, but the gas convective term approximation does not. Introducing an alternative gas convective term approximation involving constant density ejection from the ionization region constructs an unstable BM model consistent with simulation results. In addition, this paper explores how varying the imposed voltage and mass flow rate impacts the BM. The BM frequency increases with imposed voltage, aligning with theoretical predictions. The mass flow rate variation has a limited effect on BM frequency, following the theoretical model's trend. [ABSTRACT FROM AUTHOR]

We report the first ever measurements of the thermalization length of low-energy electrons injected into solid para-hydrogen at a temperature T ≈ 2.8 K. The use of the pulsed Townsend photoinjection technique has allowed us to investigate the behavior of quasi-free electrons rather than of massive, slow negative charges, as reported in all previous literature. We have found an average thermalization length ⟨z0⟩ = 26.1 nm, which is three to five times longer than that in liquid helium at the same temperature. [ABSTRACT FROM AUTHOR]

This paper presents the results of spectroscopic measurements of plasma in a linear simulator of a plasma-beam installation (PBI) for conducting materials science research. For the first time, non-contact optical diagnostic methods were employed to measure the plasma parameters in the PBI. These measurements were allowed for the clarification of the PBI plasma parameters and the identification of transition zones in different operational modes. Analysis of the plasma emission spectrum enabled the identification of impurity spectral lines and the estimation of key plasma parameters under varying experimental conditions. The spectra were recorded using two optical spectrometers covering a wavelength range of 200–800 nm. The experimental conditions varied in terms of electron beam accelerating voltage (1–5 kV), working gas pressure (1–6 mTorr), and target bias voltage (from −500 to −100 V). The Boltzmann plot method and Stark broadening of the Balmer lines were used to estimate electron temperature and density. Based on the obtained spectroscopic data, the electron density and the electron temperature were determined. The methods described in this study are applicable to linear plasma devices. [ABSTRACT FROM AUTHOR]

We describe the design of a Thomson scattering (TS) diagnostic to be used on the SMall Aspect Ratio Tokamak (SMART). SMART is a spherical tokamak being commissioned in Spain that aims to explore positive triangularity and negative triangularity plasma scenarios at a low aspect ratio. The SMART TS diagnostic is designed to operate at high spatial resolution, 6 mm scattering length in the low-field side and 9 mm in the high-field side regions, and a wide dynamic range, electron temperature from 1 eV to 1 keV and density from 5 × 1 0 18 m − 3 to 1 × 1 0 20 m − 3 , to resolve large gradients formed at the plasma edge and in the scrape-off layer (SOL) under different triangularities and low aspect ratios. A 2 J @ 1064 nm laser will be used that is capable of operating in the burst mode at 1, 2, and 4 kHz to investigate fast phenomena and at 30 Hz to study 1 s (or more) long discharges. The scattered light will be collected over an angular range of 60 ° – 120 ° from 28 spatial points in the midplane covering the entire plasma width and the outer midplane SOL. Each scattering signal will be spectrally resolved on five wavelength channels of a polychromator to obtain the electron temperature measurement. We will also present a method to monitor in situ laser alignment in the core during calibrations and plasma operations. [ABSTRACT FROM AUTHOR]

Laser-collisional induced fluorescence is used to study the plasma generated by a split-ring resonator discharge under an external cusp shaped magnetic field created by permanent magnets. The electron density and electron temperature are measured for a helium plasma at different pressures, powers, and magnet field strengths. It is found that the magnetic fields produce higher electron temperatures with peak temperatures of ∼3 eV, while the no magnet case has peak temperatures of ∼0.8 eV. Conversely, the peak electron density is obtained in the no magnet case at a value of ∼1.9 × 1011 cm−3. This indicates that the cusp-field did magnetize the electrons, but contrary to expectations, it resulted in a decrease in electron density. This is believed to be due to the magnetic field having negative effects on the resonance of the plasma source. [ABSTRACT FROM AUTHOR]

A semi-empirical global model for a nitrogen discharge in a strong magnetic field is developed. The model is based upon experimental data from high-resolution Doppler and extreme-ultraviolet vacuum spectroscopy, which establish the plasma composition, discharge parameters, and, most importantly, electronic transitions. This allows the number of required molecular systems and atomic/ionic states to be reduced, thereby retaining only the essential plasma chemistry reactions. The set of 35 stiff non-linear ordinary differential equations is numerically integrated using an unconditionally stable adaptive method. Simulations show the existence of two solution branches with low and high electron temperature, respectively. A distinct hysteresis is exhibited by the discharge and illustrated for three typical N2 mass flow rates. The dependencies of the plasma parameters on the applied power are presented and discussed in detail, including in the vicinity of the bifurcation points. The efficiency of operation in the opposing limits of N2 discharge behavior as either a source of plasma or light emission is examined, with special emphasis on electric propulsion capabilities. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate the transition of fast electron generation mechanism from J × B heating to stochastic heating by varying preplasma scale length in the interaction of ultrashort (∼25 fs) high intensity (∼3–4 × 1019 W/cm2) laser with thin foil. At sharp plasma density laser interaction (contrast ∼2 × 10−10 at 1 ns, L/λ ≪ 1), fast electrons were observed along the laser propagation direction demonstrating J × B heating. Interestingly, fast electron temperature in this case was less than ponderomotive scaling. The reasons were identified to be the small excursion length of electron compared to laser wavelength in sharp density interaction along with energy loss while escaping through the rear surface. A simplistic model has been proposed to understand the energy loss mechanism from the rear surface. Next, preplasma was introduced gradually by varying the amplified spontaneous emission contrast and additional picosecond prepulse at different delays. It resulted in an increase in the energy and temperature of fast electrons. Most importantly, at larger scale length (L/λ ≫ 1), fast electron temperature beyond the ponderomotive limit was observed. The temperature scales with scale length as T ∝ L 0.59 and shows a saturation effect at longer scale length. The results indicate a gradual change in the fast electron generation mechanism to stochastic heating producing superponderomotive energy. Particle-in-cell simulation also very well reproduces our experimental findings. [ABSTRACT FROM AUTHOR]

While the electron transport barrier remains in its final form before an edge-localized mode crash, edge turbulence manifests as fluctuations in electron temperature. Because edge turbulence is closely related to the evolution and collapse of pedestal, the microscopic spatial structure and dynamics of electron temperature fluctuations during the electron temperature pedestal evolution phase are studied using broadband electron cyclotron emission measurements. The cross phase between the electron temperature and potential fluctuations is evaluated using a velocimetry technique to identify the nature of turbulence. A comprehensive comparison of the properties of various instabilities confirms that the micro-tearing mode is a leading candidate associated with the electron temperature pedestal evolution and collapse. The quadratic transfer function reveals that the energy within the pedestal is nonlinearly transferred to the interior of the electron temperature pedestal before the pedestal collapse, resulting in radial change in the mode structure and dynamics. [ABSTRACT FROM AUTHOR]

An experimental study is conducted on macro- and microscopic characteristics of low to high (LH) transition acquired by electron cyclotron resonance heating (ECRH) in KSTAR. A scan of pre-transition density demonstrates the existence of the characteristic minimum density, which requires minimum power to make LH transition. The minimum density in KSTAR shows a tendency to be lower than that evaluated from an empirical scaling law proposed by Ryter et al. [Nucl. Fusion 54, 083003 (2014)]. A comprehensive study of the evolution of low wavenumber electron temperature fluctuations is carried out using electron cyclotron emission imaging diagnostics. An analysis of the fluctuation amplitude in L-mode in terms of the electron collisionality and the electron temperature scaling length suggests that the dissipative trapped electron mode is likely to be a dominant instability before the LH transition. The fluctuation amplitude reduces first as the transition starts. A coherent mode that chirps down from 60 to 20 kHz emerges when the transition further develops. This coherent mode is then shown to revive turbulence that has been reduced significantly during the initial stage of the LH transition. The revival of turbulence could be a possible origin of the absence of the large edge localized mode when the coherent mode is persistent in ECRH-induced H-mode plasmas. [ABSTRACT FROM AUTHOR]

Results from a laboratory experiment are presented in which, for the first time, a shear Alfvén wave is launched using an antenna in a current-carrying plasma column that is tailored to be either stable or unstable to the kink oscillation. As the plasma is driven kink unstable, the frequency power spectrum of the Alfvén wave evolves from a single peak to a peak with multiple sidebands separated by integer multiples of the kink frequency. The main sidebands (one on either side of the launched wave peak in the power spectrum) are analyzed using azimuthal wavenumber matching, perpendicular and parallel wavenumber decomposition, and bispectral time series analysis. The dispersion relation and three-wave matching conditions are satisfied, given each sideband is a propagating Alfvén wave that results from the interaction of the pump Alfvén wave and the co-propagating component of a half-wavelength, standing kink mode. The interaction is shown to generate smaller perpendicular wavelength Alfvén waves that drive energy transport to scales that will approach the dissipation scale of k ⊥ ρ s = 1 , with k ⊥ being the perpendicular wavenumber and ρs being the ion gyroradius at the electron temperature. [ABSTRACT FROM AUTHOR]

Magnetic field configurations extending over macroscopic scale distances are shown to be generated in rarefied collisionless plasmas when non-thermal and spatially inhomogeneous electron distributions in phase space emerge. The analyzed representative case is where, in the presence of a significant spatial gradient of the electron pressure and of a sheared magnetic field configuration, an alternating magnetic field reconnection process can develop associated with the anisotropy of the fluctuating electron temperature. The relevant process is intrinsically different from that known as the "Biermann Battery," which does not involve magnetic reconnection and requires that the unperturbed spatial gradient of the (isotropic) electron temperature be misaligned relative to that of the density gradient. [ABSTRACT FROM AUTHOR]

Head-on collision of the two small-amplitude electron-acoustic (EA) solitons is studied in an unmagnetized collisionless plasma in the presence of superthermal (hot) trapped electrons. For this purpose, using a well-known extended Poincare–Lighthill–Kuo (PLK) method, a pair of the trapped Korteweg–de Vries (tKdV) equations is derived to investigate the soliton trajectories and phase shifts. The latter are found dependent on amplitudes of the interacting solitons, effectively altering with hot-electron superthermality and plasma parameters. Typical parameters for the electron diffusion region (EDR) and day-side auroral zone have been selected to examine the impact of hot-electron superthermality, trapping parameter, hot-to-cold electron number density ratio, and cold-to-hot electron temperature ratio on the profiles of potential excitations and phase shifts of interacting solitons. It is found that phase speed of the EA waves becomes altered by varying the κ – parameter, strongly modifying the nonlinearity and dispersive coefficients in a superthermal trapped plasma. However, particle trapping phenomenon does not affect the linear phase speed but introduces a fractional nonlinearity in the tKdV equations of two interacting solitons. The impact of the adiabatic and isothermal pressures is also highlighted to show new modifications in the propagation characteristics of two interacting solitons. [ABSTRACT FROM AUTHOR]

In hypersonic computational fluid dynamics, the two-temperature (2-T) model is widely used to simulate thermochemical nonequilibrium. The 2-T model incorporates translational-rotational and electron-electronic-vibrational energies, assuming that the integrated energies have the equivalent temperature. In this study, multi-T models are constructed to accurately predict the effects on heat flux and free electrons due to the separation of energy modes under hypersonic environments. The three-temperature (3-T) model separates the electron-electronic energy from the electron-electronic-vibrational energy of the 2-T model. The 3-T model can accurately predict the distribution and temperature of free electrons by separating the energy of free electrons, which has different characteristics from heavy particles. The four-temperature model treats rotational energy as a nonequilibrium energy mode, distinct from translational-rotational energy. While the rotational temperature reaches equilibrium rapidly at low temperatures, at high-temperature regime rotational temperature shows a relaxation time similar to that of vibrational temperature, which cannot be ignored. To develop multi-T models, electron-vibrational relaxation and translational-rotational relaxation, which are omitted in the 2-T model, are considered. Various flight test and ground facility conditions are analyzed to verify the effects of electron and heat flux under circumstances that include shock, expansion, and shock wave boundary layer interaction. The results of the multi-T models show significant differences in electron temperature and distribution caused by electron-electronic nonequilibrium. Additionally, rotational nonequilibrium increases the shock standoff distance and alters the electron distribution at high altitudes. The heat flux difference across multi-T models is found to be negligible, except in the high degree of ionization condition. [ABSTRACT FROM AUTHOR]

Cryogenic confocal microscopy is a powerful method for studying solid state quantum devices such as single photon sources and optically controlled qubits. While the vast majority of such studies have been conducted at temperatures of a few Kelvin, experiments involving fragile quantum effects often require lower operating temperatures. To also allow for electrical dynamic control, microwave connectivity is required. For polarization-sensitive studies, free space optical access is advantageous compared to fiber coupling. Here we present a confocal microscope in a dilution refrigerator providing all the above features at temperatures below 100 mK. The installed high frequency cabling meets the requirements for state-of-the-art spin qubit experiments. As another unique advantage of our system, the sample fitting inside a large puck can be exchanged while keeping the cryostat cold with minimal realignment. Assessing the performance of the instrument, we demonstrate confocal imaging, sub-nanosecond modulation of the emission wavelength of a suitable sample, and an electron temperature of 76 mK. While the instrument was constructed primarily with the development of optical interfaces to electrically controlled qubits in mind, it can be used for many experiments involving quantum transport, solid state quantum optics, and microwave-optical transducers. [ABSTRACT FROM AUTHOR]

A pioneering 4-channel, high-k poloidal, millimeter-wave collective scattering system has been successfully developed for the Experimental Advanced Superconducting Tokamak (EAST). Engineered to explore high-k electron density fluctuations, this innovative system deploys a 270 GHz mm-wave probe beam launched from Port K and directed toward Port P (both ports lie on the midplane and are 110° part), where large aperture optics capture radiation across four simultaneous scattering angles. Tailored to measure density fluctuations with a poloidal wavenumber of up to 20 cm−1, this high-k scattering system underwent rigorous laboratory testing in 2023, and the installation is currently being carried out on EAST. Its primary purpose lies in scrutinizing ion and electron-scale instabilities, such as the electron temperature gradient (ETG) mode, by furnishing measurements of the kθ (poloidal wavenumber) spectrum. This advancement significantly bolsters the capacity to probe high-k electron density fluctuations within the framework of EAST. Beam tracing and data interpretation modules developed for both EAST and NSTX-U high-k scattering diagnostics are described. [ABSTRACT FROM AUTHOR]

The mission of the JT-60SA project is to complement ITER's capabilities by addressing the fundamental physics and engineering challenges necessary to develop a practical and reliable fusion power plant. Diagnostics play a pivotal role in achieving this mission, especially the Thomson Scattering (TS) diagnostic systems developed by a collaborative Japan–EU team. The edge Thomson scattering of JT-60SA is tailored to measure the low field side outer region of the plasma, in particular, to resolve the electron temperature Te and density ne. The collection optics of the edge TS system have a critical role in meeting the required field of view and spatial resolution despite the limited space. This work presents a comprehensive optomechanical design of the optics assembly, whose main features are telecentricity and compactness, highlighting its capabilities. The tests undertaken to verify its performance: focal plane identification, thermal cycle, and magnification, are described. [ABSTRACT FROM AUTHOR]

The Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak is capable of operating at a wide range of toroidal magnetic fields up to 3.5 T at the major radius. The electron cyclotron emission (ECE) diagnostic on KSTAR is required to cover a broad frequency range for electron temperature profile measurements in both the low and high field sides. To meet these broadband requirements, the ECE system consists of W-band (78–110 GHz) and D-band (110–162 GHz) heterodyne radiometers. The two radiometers are connected to 28 and 48 detection channels, respectively. However, since the absolute ECE calibration based on the hot–cold calibration has been very challenging, an alternative method of calibration was performed using Thomson scattering measurements and varying toroidal magnetic fields. As the toroidal magnetic field is scanned from 1.6 to 3.2 T in steps of 0.2 T, most of the 76 ECE channels are calibrated relatively by the electron temperature values of Thomson scattering in a narrow region (0.2 < r/a <0.6). In this article, the methodological details of the ECE calibration are described. In addition, to demonstrate the robustness of the ECE calibration factors, the calibrated electron temperature profiles from ECE measurements are compared with the ion temperature profiles in terms of the plasma position as the plasma positon shifts outward. [ABSTRACT FROM AUTHOR]

A set of new millimeter-wave diagnostics will deliver unique measurement capabilities for National Spherical Torus Experiment-Upgrade to address a variety of plasma instabilities believed to be important in determining thermal and particle transport, such as micro-tearing, global Alfvén eigenmodes, kinetic ballooning, trapped electron, and electron temperature gradient modes. These diagnostics include a new integrated intermediate-k Doppler backscattering (DBS) and cross-polarization scattering (CPS) system (four channels, 82.5–87 GHz) to measure density and magnetic fluctuations, respectively. The system can access reasonably large normalized wavenumbers kθρs ranging from ≤0.5 to 15 (where ion sound gyroradius ρs = 1 cm and kθ is the binormal density turbulence wavenumber). The system addresses the challenges for making useful DBS/CPS measurements with a remote control of launch polarization (X- or O-mode), probed wavenumber, polarization match of the launch beam with the edge magnetic field pitch angle, and beam steering of the launched beam for wave-vector alignment. In addition, a low-k DBS system consisting of eight fixed frequencies (34–52 GHz) and four tunable frequencies (55–75 GHz) for low-k density turbulence and fast ion physics will be located at a nearby port location. The combined systems cover the near LCFS and pedestal regions (34–52 GHz), the pedestal or mid-radius (50–75 GHz), and core plasmas (82.5–87 GHz). [ABSTRACT FROM AUTHOR]

To validate nonlinear gyrokinetic simulations of electron temperature turbulence, the experimental correlation electron cyclotron emission (CECE) measurements are to be compared using a synthetic CECE diagnostic, which generates modeled CECE measurement quantities by implementing realistic measurement parameters (e.g., spatial and wavenumber resolutions, radial location, etc.) to nonlinear gyrokinetic simulations. In this work, we calculate the radial and vertical spatial and wavenumber transfer functions, which are defined by the electron cyclotron emission emissivity radial profile and vertical probing antenna pattern, respectively. These transfer functions are applied to nonlinear gyrokinetic simulations of electron temperature turbulence using the continuum gyrokinetic code. A simultaneous comparison of the experimental electron temperature turbulence power spectrum and root-mean-square (RMS) level, as well as the radial correlation length with the new synthetic CECE diagnostic at a core location ρ ∼ 0.75 in an L-mode DIII-D tokamak plasma, is presented. The preliminary result shows that the synthetic CECE output underestimates the RMS level by ∼42% and overestimates the radial correlation length by ∼40%. [ABSTRACT FROM AUTHOR]

A novel formulation of the electron energy relaxation terms is presented here, which is applicable to plasma flows and discharges wherein the electron temperature could be higher or lower than the gas temperature. It is demonstrated that the electron energy losses due to inelastic collisions can be expressed as a function of only two species-dependent parameters: the reduced electric field and the reduced electron mobility. This formulation is advantageous over previous ones, being simpler to implement and more accurate when experimental data of the reduced electric field and reduced mobility are available. Curve fits to empirical data of these two properties are outlined here for all important air molecular species. The approach accounts for all inelastic electron energy relaxation processes without needing individual cross sections or rates, reducing potential errors associated with independently handling each process. Several test cases are presented to validate the proposed electron energy source terms including reentry plasma flows for which the electron temperature is less than the gas temperature, as well as discharges in which the electron temperature reaches values in excess of 30 eV. In all cases, the agreement with experimental data is observed to be very good to excellent, significantly surpassing prior electron energy models for plasma flows. [ABSTRACT FROM AUTHOR]

Force-free current sheets, characterized by field-aligned electric currents and approximately uniform plasma pressures, have been widely observed in the planetary magnetosphere and throughout the heliosphere. Recent observations of force-free current sheets have clearly shown the presence of anisotropic electron distributions with different temperatures perpendicular and parallel to the local magnetic field. In most of the kinetic models for one-dimensional, force-free current sheets, however, the electron distributions are nearly isotropic, which necessitates the construction of new models accounting for the electron temperature anisotropy. In this paper, we develop a model for anisotropic force-free current sheets, by incorporating the magnetic moment as an additional invariant of motion into the nearly isotropic electron distribution function of a previous model. Despite the different electron distributions, the electromagnetic profiles of the new model are often close to those in the nearly isotropic model. The applicability of our model is then validated via a comparison to a typical force-free current sheet in the Jovian magnetodisk, which shows good agreement between the model and the observations. [ABSTRACT FROM AUTHOR]

In this study, we have investigated the production of negative ions by mixing electronegative oxygen gas with electropositive argon gas in a geometrically asymmetric cylindrical capacitively coupled radio frequency plasma discharge. The plasma parameters such as density (electron, positive, and negative ion), negative ion fraction, and electron temperature are investigated for fixed gas pressure and increasing axial magnetic field strength. The axisymmetric magnetic field creates an E × B drift in the azimuthal direction, leading to the confinement of high-energy electrons at the radial edge of the chamber, resulting in decreased species density and negative ion fraction in the plasma bulk. However, the electron temperature increases with the magnetic field. It is concluded that low magnetic fields are better suited for negative ion production in such devices. Furthermore, in addition to the percentage ratio of the two gases, the applied axial magnetic field also plays a vital role in controlling negative ion fraction. [ABSTRACT FROM AUTHOR]

In fusion plasmas, essential plasma parameters can be inferred from x-ray and neutron measurements. The analysis of emitted neutron spectra can give access to the fuel ion ratio and ion temperature, while the local x-ray plasma emissivity contains information about electron temperature, magnetic axis, and concentration of impurities that can be estimated with the help of dedicated tomographic inversion and synthetic diagnostic tools. We report on ongoing efforts dedicated to x-ray and neutron diagnostics of tokamak plasmas. A methodology to determine the core impurity concentration and electron temperature from multiple line-integrated x-ray measurements in different energy bands is introduced. It is first validated with synthetic measurements and then applied on WEST experimental data. Regarding neutron measurements, a thin-foil proton recoil spectrometer using a gas electron multiplier (GEM) detector is being developed to measure fast neutron energy spectra. The modeling of the energy response function of this NS-GEM prototype, intended for future applications of fusion plasma spectrometry, is presented and discussed. [ABSTRACT FROM AUTHOR]

A set of experiments using controlled, skin depth-sized plasma pressure filaments in close proximity have been carried out in a large linear magnetized plasma device. Two- and three-filament configurations have been used to determine the scale of cross field nonlinear interaction. When the filaments are separated by a distance of approximately five times the size of a single filament or less, a significant transfer of charge and energy occurs, leading to the generation of inter-filament electric fields. This has the effect of rotating the filaments and influencing the merging dynamics. Nonlinear gyrokinetic simulations using seeded filaments confirm the presence of unstable drift-Alfvén modes driven by the steep electron temperature gradient. When the filaments are within a few collisionless electron skin depths (separations twice the size of a single filament), the unstable perturbations drive the convective mixing of the density and temperature and rearrange the gradients such that they maximize in the region surrounding the filament bundle. [ABSTRACT FROM AUTHOR]

Linear plasma kinetic theory for a non-thermal, magnetized, homogeneous, and collisionless plasma is incorporated to study Alfvén cyclotron instability (ACI) driven by the ion/proton perpendicular temperature anisotropy (T ⊥ p / T ∥ p > 1) (⊥ , ∥ symbols designate directions perpendicular and parallel to ambient magnetic field, respectively), and the wave propagation is considered in the direction of the ambient magnetic field ( k → ∥ B → 0 ) with left hand circular polarization. We consider that electron–proton plasma with kappa distributed electrons and protons is taken to be Vasyliunas–Cairns distributed. We, further, validated our model distributions for the electrons and ions against the observations of solar wind at various heliocentric distances. The transverse dielectric response function of ACI is calculated and numerically solved to study its dispersion and growth characteristics under the influence of pertinent parameters, i.e., non-thermal parameters of protons and electrons αp, κp, and κe, proton and electron temperature anisotropy ratios τ p , e = T ⊥ p , e / T ∥ p , e , and plasma beta of protons ( β ∥ p (V C) and β ∥ p (M) ). A concept of the non-thermality dependent effective temperature model is invoked, which updates plasma beta and makes it a non-thermality dependent quantity. The dispersion and growth rates of ACI are found appreciably and significantly augmented in the case of non-thermal protons as compared to Maxwellian protons previously presented by [Gary et al., J. Geophys. Res. 117, A08201 (2012); 122, 464–474 (2017)]. The increase in the magnitude of proton parameters enhances the growth rate of the instability, whereas the increment in electron parameters inhibits the growth rate. This study is advantageous to understand the plasma dynamics of natural environments, such as magnetosphere and solar wind, where the excessive non-thermal populations are present that cannot be modeled by the Maxwellian distribution. [ABSTRACT FROM AUTHOR]

We present results from x-ray imaging of high-aspect-ratio magnetic reconnection experiments driven at the National Ignition Facility. Two parallel, self-magnetized, elongated laser-driven plumes are produced by tiling 40 laser beams. A magnetic reconnection layer is formed by the collision of the plumes. A gated x-ray framing pinhole camera with micro-channel plate detector produces multiple images through various filters of the formation and evolution of both the plumes and current sheet. As the diagnostic integrates plasma self-emission along the line of sight, two-dimensional electron temperature maps ⟨ T e ⟩ Y are constructed by taking the ratio of intensity of these images obtained with different filters. The plumes have a characteristic temperature ⟨ T e ⟩ Y = 240 ± 20 eV at 2 ns after the initial laser irradiation and exhibit a slow cooling up to 4 ns. The reconnection layer forms at 3 ns with a temperature ⟨ T e ⟩ Y = 280 ± 50 eV as the result of the collision of the plumes. The error bars of the plumes and current sheet temperatures separate at 4 ns, showing the heating of the current sheet from colder inflows. Using a semi-analytical model, we survey various heating mechanisms in the current sheet. We find that reconnection energy conversion would dominate at low density ( n e ≲ 7 × 10 18 cm−3) and electron-ion collisional drag at high-density (≳ 10 19 cm−3). [ABSTRACT FROM AUTHOR]

An experimental study on the role of an external longitudinal magnetic field on the characteristics changes of pulsed plasma stream, produced in argon medium, is carried out at different time spans of its evolution. The spectroscopic observations are time integrated and are carried out at different times of plasma formation for pulsed discharge. This study gives insights into the recombination and diffusion phase of the plasma species in the presence of the magnetic field. The transition of plasma species from a dominant recombination phase to a diffusional phase is well revealed by the density profile during the time evolution. Moreover, the decrease in electron temperature and the increase in electron excitation temperature explain the energy transfer to electrons due to metastable quenching, and the system gradually approaches equilibrium. The magnetic field also affects the transitions of the ionized argon population between different energy levels. It is found that faster decay occurs for transitions of different plasma species to non-metastable states, while the populations of metastable states exist for a longer time. In addition, the time-resolved morphology changes of the plasma stream are also observed by high speed imaging, which shows the flow structure of the plasma stream at different time frames. The imaging of the plasma stream evolution shows the initial ejection of the plasma sheet from the electrode assembly, its detachment, the steady flow, and gradually its nature of dying out. [ABSTRACT FROM AUTHOR]

Near-surface layers of the mercury chalcogenide (HgSe and n-HgTe) single crystals with the electron concentration in the range of 7 × 1014–4.2 × 1018 сm−3 were studied using the laser pump-probe thermoreflectance (TR) technique combined with the Fabry–Perot (FP) interferometer in the wide temperature interval of 10–300= K. The TR–FP measurements were carried out toward samples with a freshly etched reflective surface. As a result, a minimum in the time dependence of the TR–FP signal and sign inversion of the TR–FP signal were revealed both for HgSe and n-HgTe within the specified temperature range. Noticeably, both anomalies were observed for n-HgTe at substantially lower temperatures than that for HgSe. The proposed qualitative interpretation of the observed anomalies is based on the hypothesis of two types of Weyl nodes located in the Brillouin zone of mercury chalcogenides involving specific for the Weyl semimetals energy barriers, separating the bulk Weyl fermion helical states from the topologically protected Fermi-arc surface states. [ABSTRACT FROM AUTHOR]

The excitation energies, transition rates, collisional-excitation cross sections, and collisional-excitation rate coefficients of N II ions have been calculated by the relativistic configuration-interaction method with the implementation of the flexible atomic code. The population rate coefficients were subsequently investigated and shown that the net population rate coefficients of the 2p3d configuration are greater than those of the 2p3p configuration at an identical electron temperature. A reasonable collisional-radiative model has been constructed to simulate the 2p3s–2p3p and 2p3p–2p3d spectra, which were in reasonable agreement with the lightning data presently obtained in experimental measurements. A further application was implemented in effectively determining the lightning parameters, with the aid of a certain area surrounded by the intersections of contours of multiple line-intensity ratios on the two-dimensional plane of electron temperature and electron density. [ABSTRACT FROM AUTHOR]

In this work, as a promising laboratory-based extreme ultraviolet (EUV) radiation source, microwave (MW)-induced helium discharge is studied and analyzed based on spectroscopic measurements at low pressures. The helium emission spectrum in the EUV wavelength range is presented to show all the relatively strong EUV lines. A Maxwellian shape is assumed for the electron energy distribution, and a corona model is applied to evaluate the plasma parameters under low-pressure conditions. The intensities of a pair of emission lines at 30.38 and 58.43 nm, which are the strongest in the spectrum and of great astrophysical interest, are studied under gradient discharge powers and pressures. After correcting for plasma reabsorption, the intensity ratio of the two lines is used to obtain the electron temperature (Te), which is found to vary within the range of 3.7–5.6 eV. Electron density (ne) is deduced from a global discharge model, which is of the order of magnitude of 1010 cm−3. From experimental determination and mechanism analyses, the optimal discharge pressure is found to be within 1.45–2.18 mbar for the 34.38 nm line and the vicinity of 3.45 mbar for the 58.43 nm line. This work explores the dependency of physical behavior of discharge on different working conditions based on a EUV spectroscopic study and theoretical analyses and determines the optimal working condition to produce the strongest EUV emission lines of the low-pressure MW-induced helium discharge. [ABSTRACT FROM AUTHOR]

The micro-discharge with a hollow cathode and a second anode with different geometries is experimentally and numerically studied. Effects of the second anode on the micro-hollow cathode sustained discharge (MCSD) in helium are investigated. Three kinds of electrodes with different geometries are used as the second anode. The results show that when needles are inserted into the plate to be used as the second anode, the formation of MCSD can be promoted, and MCSD can be obtained at a low cathode current. Moreover, the distribution of needles has an important influence on the formation of MCSD. The cathode breakdown threshold currents for the three cases of the second anode from low to high are as follows: 13 needle-plate, five needle-plate, and plate with no needles. At the same cathode current, a stronger MCSD and a higher second anode current can be obtained when needles are inserted into the second anode plate compared with that when only a plate is used as the second anode. In the present experiment, the volume of MCSD reaches approximately 10 cm3 with an electron density of 4–5 × 1017 m−3 and an electron temperature of 2–3 eV. The experimental and simulated results show that the MCSD originates from cooperative formation between the micro-hollow cathode discharge in the cavity and the discharge around the second anode. Compared with that when only a plate is used as the second anode, when needles are inserted into the second anode, the electric field, electron temperature, and electron production rates near the second anode increase, and a stronger discharge is generated near the second anode. When the needle-plate is used as the second anode, the electron density in both the axial and radial directions in the MCSD region is higher than that when the plate without needles is used as the second anode. Before the formation of MCSD, the electrons generated by the strong discharge near the second anode extend toward the first anode, thus promoting the formation of MCSD. [ABSTRACT FROM AUTHOR]

This study determined the electron temperature and density in atmospheric pressure argon plasma using optical emission spectroscopy. The analysis combined continuum and line spectral data. Visible bremsstrahlung inversion was used to derive a partial electron energy probability function (EEPF) from the continuum spectrum. Subsequently, electron temperature was estimated assuming a two-temperature distribution based on the derived EEPF. Electron density was obtained by fitting a collisional-radiative (CR) model to the line spectrum, incorporating the obtained EEPF instead of assuming a Druyvesteynian EEPF. Comparative analysis revealed that the electron densities determined using the approach were approximately one order of magnitude lower than those derived from the CR model with the Druyvesteynian EEPF. However, they exhibited strong agreement with the results obtained by the CR model using a two-temperature distribution. This approach demonstrated favorable performance in reproducing both continuum and line spectra, revealing its high reliability and accuracy for atmospheric pressure argon plasma diagnosis. [ABSTRACT FROM AUTHOR]

Nanoscale thermal imaging and temperature detection are of fundamental importance in diverse scientific and technological realms. Most nanoscale thermometry techniques focus on probing the temperature of lattice or phonons and are insensitive to nonequilibrium electrons, commonly referred to as "hot electrons." While terahertz scanning noise microscopy (SNoiM) has been demonstrated to be powerful in the thermal imaging of hot electrons, prior studies have been limited to room temperature. In this work, we report the development of a cryogenic SNoiM (Cryo-SNoiM) tailored for quantitative hot electron temperature detection at low temperatures. The microscope features a special two-chamber design where the sensitive terahertz detector, housed in a vacuum chamber, is efficiently cooled to ∼5 K using a pulse tube cryocooler. In a separate chamber, the atomic force microscope and the sample can be maintained at room temperature under ambient/vacuum conditions or cooled to ∼110 K via liquid nitrogen. This unique dual-chamber cooling system design enhances the efficacy of SNoiM measurements at low temperatures. It not only facilitates the pre-selection of tips at room temperature before cooling but also enables the quantitative derivation of local electron temperature without reliance on any adjustable parameters. The performance of Cryo-SNoiM is demonstrated through imaging the distribution of hot electrons in a cold, self-heated narrow metal wire. This instrumental innovation holds great promise for applications in imaging low-temperature hot electron dynamics and nonequilibrium transport phenomena across various material systems. [ABSTRACT FROM AUTHOR]

We develop a method to use the mixed third and second harmonic electron cyclotron emission (ECE) signal in the DIII-D tokamak to reconstruct the electron temperature profile of a rotating magnetic island. The third harmonic ECE is removed by extracting the rotating-island-associated fluctuations in the mixed signal, and the extracted fluctuation is combined with the equilibrium temperature obtained from other diagnostics after correcting for the third harmonic reabsorption. The accuracy of the reconstruction is studied by considering a DIII-D shot where an unmixed signal from an island is available on the low field side of the plasma and a mixed signal from the same island is available from the high field side. It is found that the reconstruction method successfully reproduces the island shape and temperature perturbation magnitude without the distortion caused by third harmonic ECE mixing. However, the radial location of the reconstructed island is somewhat displaced relative to the location of the q = 2 surface in the axisymmetric equilibrium reconstruction, resulting in a corresponding inaccuracy in the absolute temperature of the island. It is conjectured that this may arise from an inaccuracy of the reconstructed axisymmetric equilibrium in this region. [ABSTRACT FROM AUTHOR]

In many fusion devices, such as tokamaks or stellarators like Wendelstein 7-X (W7-X), soft x-ray pulse height analysis (PHA) system diagnostics are routinely used during the experiments. The PHA system is dedicated to providing information about the impurity content, and average along line-of-sight electron temperature in the plasma conditions. Moreover, it is also able to estimate impurity density and an average effective charge from the comparison of experimental spectra with the modeled ones. However, the experimental x-ray spectra can be interpreted in terms of interesting plasma parameters only when the theoretical radiation models first identify and then take into account all the relevant factors that affect the spectrum. Therefore, for this purpose, a theoretical model has been applied. Flexible Atomic Code, which allows for calculation of various atomic properties such as energy levels, cross sections for excitation and ionization by electron impact, transition probabilities for radiative transitions and autoionization, and any others as needed in the collisional–radiative approximation. The chosen spectra collected during the W7-X campaign (OP1.2b) were examined, trying to obtain an agreement between the observed and simulated spectra. The analysis carried out allowed for a reliable interpretation of experimental x-ray spectra, estimation of the electron temperature, and obtaining information on the content of tungsten impurities. [ABSTRACT FROM AUTHOR]

Stimulated Brillouin scattering (SBS) has complex dependence on the plasma electron temperature via the Landau damping, particle trapping, and consequent nonlinear frequency shift. It is found from our numerical simulation that the SBS reflectivity in its saturation stage tends to increase with the plasma electron temperature within a certain range, although the linear growth rate of SBS normally reduces with the increasing electron temperature. This is because the phase velocity of an ion acoustic wave (IAW) increases with the electron temperature, which tends to reduce the Landau damping of the IAWs and hence reduce ion trapping. In the nonlinear saturation stage, ion trapping will modify the ion distribution function and induce a negative frequency shift in the IAW. This nonlinear frequency shift will break the three-wave coupling, thereby causing saturation of the SBS. With further increase in the electron temperature, however, electron trapping will dominate over ion trapping, which induces a positive frequency shift in the IAW and can lead to the SBS saturation as well. As a result, the SBS reflectivity first increases and then decreases with increase in the electron temperature. At around the peak of the SBS reflectivity, the positive frequency shift of IAW induced by electron trapping roughly offsets the negative frequency shift induced by ion trapping. [ABSTRACT FROM AUTHOR]

For atmospheric argon RF dielectric barrier discharges, a self-consistent one-dimensional fluid model based on the drift-diffusive approximations of the particles is established to investigate the role of the neutral gas temperature on the discharge process and the plasma characteristics. A finite difference method is used to solve numerically the model, and the numerical results are obtained for the cases that the neutral gas temperature varies from 300 to 600 K. It shows that an increase in the neutral gas temperature causes a decrease in the ionization rate peak and a decrease in the plasma density, but the electric field and the electron temperature do not change very much. Moreover, the discharge mode transition from α mode to α-γ mode occurs because the growing ion flux induces more secondary electron flux, even if the ions entering the sheaths decrease. In addition, the ground state ionization and the ground state excitation are the main collisions in the argon discharges. When metastable atoms are focused on, the three-body quenching is also an important collision progress. [ABSTRACT FROM AUTHOR]

The utilization of HBr/Cl2 mixed gas discharge in semiconductor etching processes has been a subject of analysis both experimentally and through simulations to understand its discharge characteristics. In this study, we have developed a model that extends the previous global model of the HBr/Cl2 plasma. The electron temperature and densities are solved in a self-consistent manner, while previous global model uses the measured electron temperature and electron density. Additionally, we have included further data on electron collision reactions to enhance accuracy. This model was then compared with experimental results obtained from pure HBr, pure Cl2, and HBr/Cl2 plasmas. The calculated results align well with the experimental findings within the margin of error. One notable observation from our study is the occurrence of an unusual phenomenon: as the HBr partial concentration increased, the Br+ ion flux initially increased until the ratio reached 0.5, after which it decreased. This behavior can be attributed to Br+ ions being predominantly produced through collisions between Br atoms and electrons. The dominant mechanisms for Br atom generation involve dissociations by Cl radicals, such as Br2 + Cl → Br + BrCl. Consequently, there exists an optimal flow rate at which the Br+ ion flux is maximized. [ABSTRACT FROM AUTHOR]

Microtearing modes (MTMs) are suggested as a candidate for anomalous thermal transport in tokamak H-mode discharges. This study investigates MTMs in tokamak plasmas, employing simulations in the BOUT++ framework. It simplifies and linearizes the governing equations in detailed linear simulations. The study meticulously evaluates various conductivity models under diverse plasma conditions and collision regimes. The research thoroughly assesses different conductivity models across a range of plasma conditions and collision regimes. A unified dispersion relation that includes both MTM and Drift-Alfvén Wave (DAW) instabilities is derived, showing that DAW and MTM instabilities occur at varying distances from the rational surface. Specifically, MTMs become unstable near the rational surface but stabilize farther away, while drift-Alfvén instability appears farther from the rational surface. The study also re-derives MTM dispersion relations using Ohm's law and the vorticity equation, providing a thorough analysis of electromagnetic and electrostatic interactions in tokamaks. Global simulations demonstrate an inverse correlation between MTM growth rates and collisionality, and a direct correlation with temperature gradients. The nonalignment of the rational surface with the peak ω * e stabilizes the MTMs. Nonlinear simulations highlight electron temperature relaxation as the primary saturation mechanism for MTMs, with magnetic flutter identified as the dominant mode of electron thermal transport. [ABSTRACT FROM AUTHOR]

Most in situ plasma diagnostics are only reliable for low-pressure systems, e.g., systems where the collision frequency, ν e n , is small compared to the plasma frequency, ω p e . In this article, we describe a novel diagnostic technique for simultaneous measurement of the plasma density, n e ; electron temperature, T e ; and ν e n in systems in which ν e n approaches ω p e . We illustrate the operational setup of our probe system along with an analytical framework for ascertaining n e , T e , and ν e n . Furthermore, we show experimental results from argon capacitively coupled plasma operating in the 0.5–1 Torr pressure range. [ABSTRACT FROM AUTHOR]