Peak Electron Temperature Research Articles

Soft x-ray diagnostics system for electron temperature measurement in the integrated commissioning phase of JT-60SA.

A soft x-ray (SX) diagnostic system has been designed and installed in JT-60SA for the first plasma. For quantitative measurement, the etendue of each viewing chord was evaluated analytically and numerically. The electron temperature is evaluated from the detected bremsstrahlung emission ratio between two detector arrays with two different thicknesses of Be filters. The two filter thicknesses were optimized to be 7 and 50μm for the expected electron temperature range of 0-3keV. The one-dimensional profile of SX emission is reconstructed from line integrated emission by the elliptic Abel inversion scheme. For a plasma discharge with a plasma current of 1 MA, a peaked electron temperature profile with about 800eV at the center is obtained. The total bremsstrahlung power was also evaluated using the electron temperature profile and the absolutely evaluated etendue of each viewing chord. In this evaluation, the bremsstrahlung power is around 10% of ohmic heating power.

The impact of moderate solar flare activity on ionospheric response from august 5th to 7th, 2023

This study investigates the ionospheric response to a period of heightened solar flare activity from August 5th to August 7th, 2023, by analysing ground-based observations of various ionospheric parameters. The data includes detailed information on solar flare events, including their class, start time, maximum time, and end time, along with solar activity indicators such as sunspot numbers, solar radio flux, and solar zenith angles. On August 5th, multiple C-class, M-class, and an X1.63 flare were recorded, with the X-class flare being the most intense event. Ionospheric measurements revealed enhancements in electron temperatures, reaching a peak of 2104K at 6 UT, coinciding with the onset of an M1.6 flare. Additionally, a pronounced increase in total electron content (TEC) was observed, peaking at 34 TEC units at 6 UT, suggesting increased ionization due to the flare's influence. On August 6th, an M5.51 flare was the most significant event. Notably, the peak electron temperature of 2097K and the TEC maximum of 35.9 units were recorded at 6 UT, several hours before the flare's maximum phase, indicating potential preconditioning effects from the flare's preparatory stages. August 7th witnessed an X1.51 flare, along with multiple M-class flares. The peak electron temperature of 2088K and the TEC maximum of 35.3 units were observed at 7 UT, again preceding the X1.51 flare's peak, suggesting the influence of precursor effects. Throughout the observation period, the data exhibited typical diurnal patterns in ion temperatures and TEC, consistent with regular ionospheric behavior driven by solar radiation intensity variations. The study's findings provide evidence of the ionosphere's responsive nature to intense solar flare activity, with significant enhancements in electron temperatures and TEC observed during flare events. Notably, the effects were evident not only during the peak flare phases but also several hours prior, potentially due to the arrival of energetic particles or precursor electromagnetic radiation, indicating the influence of preconditioning processes. Furthermore, the data suggests that solar flares can impact the ionosphere's chemistry and dynamics, as inferred from changes in the "Ion Percentage" parameter, representing relative ionospheric compositions. Continuous monitoring and analysis of such events across different geophysical conditions are crucial for advancing the understanding of solar-terrestrial interactions and their impacts on space-based technologies, contributing to the development of improved space weather prediction and mitigation strategies.

Laser Measurement of Anomalous Electron Diffusion in a Crossed-Field Plasma.

The anomalous diffusion of particles and energy in magnetized plasma systems is a widespread phenomenon that can adversely impact their operation and preclude predictive models. In this Letter, this diffusion is characterized noninvasively in a low-temperature, Hall-type plasma. Laser-induced fluorescence and incoherent Thomson scattering measurements are combined with a 1D generalized Ohm's law to infer the time-averaged inverse Hall parameter, a transport coefficient that governs cross-field diffusion. While the measured diffusion profile agrees with model-based estimates in magnitude, the measurements do not exhibit the steep "transport barrier" typically imposed in models. Instead, these results show that the electric field is primarily driven by a diamagnetic contribution due to the large peak electron temperature exceeding 75eV. This finding motivates a reconsideration of nonclassical energy transport across field lines in low-temperature plasmas.

Observation of fast electron redistribution during saturated kink mode in high βp H-mode discharge with central heating in EAST tokamak

In the EAST tokamak, we have developed an internal transport barrier (ITB) high-confinement mode (H-mode) scenario characterized by dominant electron heating and centrally peaked electron temperature profiles, facilitated primarily through the combustion of lower hybrid current drive and electron cyclotron radio heating (ECRH). Hard x-ray diagnostics reveal a marked increase in the population of fast electrons with energy from 30 keV to 80 keV, concurrent with augmented ECRH power during H-mode plasma operations. This surge in fast electron population precedes the formation of the electron temperature ITB (Te-ITB). Within the Te-ITB H-mode discharge, a mild and long-lived m/n = 1/1 mode (where m and n denote the toroidal and poloidal mode numbers, respectively) emerges proximal to the ITB region. This mode precipitates a redistribution of fast electrons, contributing to an increase in the safety factor near the magnetic axis and thereby promoting the stability of the Te-ITB. Furthermore, we explore the influence of fast electrons on plasma pressure and examine the effects of the profile of fast electrons on the central Te. Strategies to maintain the m/n = 1/1 mode at a moderate amplitude are also discussed, highlighting their significance in the sustained management of Te-ITB.

Improvement of core heat transport in NBI plasmas of heliotron J using high-intensity gas puffing

Controlling the heat transport profile is important for high performance in magnetically confined fusion plasmas. In this study, improved electron heat transport was achieved in neutral beam injection plasmas by applying high-intensity gas puffing (HIGP) on a stellarator/heliotron device called Heliotron J. Compared with conventional gas puffing (GP) fueling discharge, a higher and more peaked electron temperature profile was obtained, and the core ion temperature was slightly higher but similarly shaped. Using similar parameters, the electron density profile for HIGP remained similar and differed from the hollow density profile observed in electron cyclotron heating-eIBT plasma. Transport analysis using the FIT3D and TR-snap codes showed a clear reduction in the effective electron heat transport coefficient in the plasma core region. However, more detailed experiments are required to understand the mechanisms underlying this improvement fully.

Erosion estimates for the divertor and main wall components from STEP

The tungsten erosion within Spherical Tokamak for Energy Production (STEP) assuming tungsten main wall and tungsten divertor has been estimated with ERO at the inner and outer divertor, at the inner and outer midplane and at the outboard baffle entrance. Plasma parameters are based on SOLPS simulations applying argon puffing for edge cooling. The plasma parameter range covers peak electron temperatures T e between 3 and 25 eV in the divertor. At the inner midplane T e ∼ 13 eV, at the outer midplane ∼7 eV and at the outboard baffle entrance between 1 eV and 4 eV. The modelled peak gross erosion is highest in the divertor with up to 1E19 W m−2 s−1 within the inner and 7E19 W m−2 s−1 in the outer one for the plasma parameter range studied. At the main wall the gross erosion is about 2E18 W m−2 s−1 at the inner midplane and 1.3E17 W m−2 s−1 at the outer one. However, tungsten deposition within the divertor is much larger with amounts between 88% and 98% and only between 10% and 60% at the midplane. At all locations studied, tungsten erosion due to deuterium ions is negligibly small compared to the erosion by argon ions. Erosion due to deuterium atoms has been studied for the outer midplane and is there at least four times smaller than the erosion due to argon ions. The simulations have been performed considering singly ionised Ar. However, according to the SOLPS runs the mean charge of Ar impinging the surfaces is about two at the locations of largest erosion, which leads to an increase of the gross erosion by a factor between 1.5 and 5 with the largest increase occurring at the outer divertor target.

Tunable ultrafast thermionic emission from femtosecond-laser hot spot on a metal surface by control of laser polarization and angle of incidence: A numerical investigation

Ultrafast laser induced thermionic emission from metal surfaces has several applications. Here, we investigate the role of laser polarization and angle of incidence on the ultrafast thermionic emission process from laser driven gold coated glass surface. The spatio-temporal evolution of electron and lattice temperatures are obtained using an improved three-dimensional (3D) two-temperature model (TTM) which takes into account the 3D laser pulse profile focused obliquely onto the surface. The associated thermionic emission features are described through the modified Richardson-Dushman equation, including dynamic space-charge effects and are included self-consistently in our numerical approach. We show that temperature-dependent reflectivity influences laser energy absorption. The resulting peak electron temperature on the metal surface monotonically increases with the angle of incidence for the P polarization, while for the S polarization it shows the opposite trend. We observe that thermionic emission duration shows a strong dependence on the angle of incidence and contrasting polarization dependent behavior. The duration of the thermionic current shows strong correlation to the intrinsic electron-lattice thermalization time, in a fluence regime well below the damage threshold of gold. The observations and insights have important consequences in designing ultrafast thermionic emitters using a metal based architecture.

Analysis of a cusped helicon plasma thruster discharge

Experiments and simulations are used to analyze a compact helicon plasma thruster with a cusp in its internal magnetic field. The former rely on a compensated Langmuir probe and a Faraday cup, while the latter employ a hybrid PIC/fluid transport model combined with a frequency-domain electromagnetic field model. Measurements serve to tune the anomalous transport parameters of the model and overall show the same trends as the numerical results, including a secondary peak of electron temperature downstream in the magnetic nozzle, where electron cyclotron resonance conditions for the 13.56 MHz excitation frequency are met. The cusp plays a central role in determining the plasma losses to the walls and the profile of electron temperature, which in turn defines the excitation and ionization losses. While losses to the rear wall are reduced, losses to the lateral wall are increased, which, together with the low production efficiency, limit the performance of the device.

Electron Heating in Shocks: Statistics and Comparison

AbstractSupernova remnant (SNR) shocks are the highest Mach number non‐relativistic shocks in electron‐ion plasmas. These shocks are the most efficient particle accelerators in space. SNR shock parameters are inferred from measurements of electromagnetic radiation from heated and accelerated particles. Temperature of the shock heated electrons is one of the most important parameters in supernova remnant shocks. Knowledge of the downstream electron‐to‐ion temperature ratio or of the ratio of the downstream electron temperature to the incident ion energy is crucial for understanding physics of the very high‐Mach number SNR shocks. Heliospheric shocks have substantially lower Mach numbers than SNR shocks but can be extensively studied in in situ observations with further extrapolation of the findings to higher Mach numbers. Magnetospheric Multiscale mission observations of the Earth bow shock are used to analyze dependence of the electron heating on the shock Mach number. It is found that the ratio of the downstream electron temperature to the incident ion energy decreases with the increase of the Mach number. At high Mach numbers this ratio and stabilizes at about 2.5%. The electron‐to‐ion temperature ratio stabilizes at about 10%. The peak electron temperature occurs at the overshoot maximum, further downstream electrons cool down. The mean ratio of the 4.5 s averages of the downstream and maximum electron temperatures is 0.85. Electron heating does not follow the thermodynamic adiabatic law. The heating and cooling behavior implies that the energy is provided by the overall cross‐shock potential while small‐scale electric fields rapidly isotropize the electron distribution.

Real-time control applications of JET heterodyne radiometer electron temperature profiles using ICRH

The Real Time ECE data processing has been recently implemented at JET, by using a cross calibration between the Martin-Puplett interferometer (also known as Michelson) measurements and the Heterodyne faster data. The applicability of RT control with ICRH has been discussed in recent works. In particular, in case of a hollow electron temperature profile, central ICRF heating could be applied to restore temperature peaking. In this work a study about the possibility of controlling the electron temperature profile by using ICRHas an actuator, with inputs from ECE measurements, is described. The ICRH is here used for the optimization of the plasma ramp-down, in order to correct the end of the discharge and avoid the plasma disruption, though a similar approach could also be proposed to improve the plasma performances during the flat top. Preliminary results have been obtained on the physical and logical feasibility of using ICRH in a real-time control loop. A database of good pulses in identifiable conditions has been compiled and validated by covering the main experiments both in the baseline and hybrid scenarios. Such scenarios have been simulated, for a selected numbers of discharges, using the full-wave propagation solver TORIC-SSQLFP, in order to establish the ICRH power deposition profiles to be used to locally restore a peaked electron temperature profile.

ELM temperature in JET and COMPASS tokamak divertors

Analysis of the divertor edge localized mode (ELM) electron temperature at a uniquely high temporal resolution (10−5 s) was reported at the JET tokamak (Guillemaut et al 2018 Nucl. Fusion 58 066006). By collecting divertor probe data obtained during many dozens of ELMs, the conditional-average (CAV) technique yields surprisingly low peak electron temperatures, far below the pedestal ones (70%–99% reduction!) which we, however, question. This result was interpreted through the collisional free-streaming kinetic model of ELMs, by a transfer of most of the electron energy to ions, implying a high tungsten sputtering for unmitigated ELMs in future fusion devices like ITER. Recently, direct microsecond temperature measurements on the COMPASS tokamak, however, showed that the electron temperature peak of ELM filaments measured in the divertor is reduced by less than a third with respect to the pedestal one. This was further confirmed by a dedicated 1D particle-in-cell (PIC) simulation and tends to prove that the pedestal electrons can transfer only their parallel energy to ions (due to low collisionality), thus less than a third, as is predicted by the collisionless free-streaming model. This finding strongly contradicts the JET observations. We have therefore compared the CAV to the direct (microsecond) ball-pen and Langmuir probes measurements in COMPASS and found very good agreement between them. Revisiting the aforementioned JET CAV analysis indeed shows that the electron temperatures are much higher than previously reported, close to those predicted by the PIC simulation, and thus the ion energy seems to not significantly increase in the scrape-off layer.

Simulation of a helicon plasma source in a magnetoplasma rocket engine

The helicon plasma source, which generates high thrust and high impulse, is of vital importance for magnetoplasma rocket engines. In this work, a multi-component, two-dimensional, axisymmetric fluid model coupled with an electromagnetic field was developed to model the helicon discharge. The simulation results demonstrate that: (i) the discharge mode changes twice—each conversion is accompanied by a plasma density jump and an electron temperature peak in the discharge; (ii) when the input current increases, the plasma density increases, and ionization occurs faster; (iii) the background magnetic field clearly enhances the discharge; (iv) the plasma density may be smaller if the discharge has not entered the wave mode.

A numerical study of picosecond laser micro-grooving of single crystalline germanium: Mechanism discussion and process simulation

The temperature field evolution and material removal process during picosecond (ps) laser micro-grooving of single crystalline germanium (Ge) are explored by developing a 2D two-temperature model (TTM). The transient temperature-dependent thermal and optical properties of electrons and lattice are incorporated for accurate calculations, and the electron-lattice coupling strength is estimated considering the electron transition from valence band (VB) to conduction band (CB) mainly caused by one photon absorption. Different material removal mechanisms, i.e. the thermal way caused by vaporization and the athermal manner resulted from covalent bonds breakage, are proposed and compared with experimental results. The variation trends of the thermal model predicted groove depths are similar and consistent with the measured ones within selected ranges, while the athermal model leads to groove depth far from reality, demonstrating that the vaporization-caused mechanism is the proper one. Employing the verified model, simulation study is conducted to explore the evolution of temperature field under different fluences. Under a low fluence of 0.071 J/cm 2 , the peak electron temperature shows a decreasing trend with pulse width, and the temperatures of electron and lattice evolve in a similar pattern when the pulse duration is wider than 200 ps. For a higher fluence of 7.074 J/cm 2 , lattice temperature increases up to melting and boiling point within 1 ps and 3 ps, respectively, and the groove depth produced by a 12 ps pulse heating is about 0.65 μm. Afterwards, residual heat diffusion during pulse interval and further material removal during following pulse heating will occur, leading to a deeper groove. Moreover, the effects of laser power and critical free electron density on the machining process are also analysed, demonstrating that an increase in any of these two parameters results in a faster element removal. • Free electron density variance in Ge under laser irradiation is considered. • Comprehensive temperature-dependent properties are incorporated. • The thermal coupling strength depends on temperature and free electron density. • Material removal is confirmed to be a thermal process via model verification. • Shorter pulse width leads to larger electron-lattice temperature gap. • The groove formation process under high fluence is detailed simulated.

Simulation of tungsten target erosion and tungsten impurity transport during argon seeding on EAST

The upgrade of experimental advanced superconducting tokamak lower divertor to use tungsten (W) target is undertaken, however the target erosion and W impurity accumulation in the core pose a great challenge during high power discharge with external impurity seeding. This paper aims to study the influence of argon (Ar) seeding on the target erosion and the W impurity transport on steady-state operation. The scrape-off layer/divertor plasma as well as W impurity are self-consistently simulated by using SOLPS-ITER code package. With the constant input power, the Ar seeding rate is varied to study the changes on divertor plasma, incident Ar flux, the target erosion and W impurity transport. The simulation results indicate that the external Ar impurity influences W target erosion through both reducing the plasma temperature and introducing heavier incident particles, which significantly complicates the erosion process. The eroded W flux has great impact on the W core accumulation, and the divertor plasma condition influences W impurity transport remarkably. A correlation between peak electron temperature (T e) at the outer target and the W concentration (C W) at the core-edge-interface is found. Moreover, the influence of the W impurity on the background boundary plasma is also evaluated.

Experimental Observation of Magnetic Island Heteroclinic Bifurcation in Tokamaks.

We report empirical observations of magnetic island heteroclinic bifurcation for the first time. This behavior is observed in interacting coupled 2/1 tearing modes in the core of a DIII-D tokamak plasma. Poincaré maps constrained by measured magnetic amplitudes and phasing show bifurcation from heteroclinic to homoclinic topology in the 2/1 island as the 4/2 relative amplitude (R_{4/2}) decreases. Initially, the local electron temperature peak in the 2/1 island splits, consistent with two O points. As R_{4/2} decreases a single peak forms, consistent with one O point. These call for developing tearing stability theory and control solutions for heteroclinic islands in tokamaks.

Anomalous Electron Temperature Increases in the Evening Equatorial Ionosphere

AbstractWe use novel radar observations at the Jicamarca Radio Observatory to study the first observations of highly enhanced plasma temperatures in the evening equatorial ionosphere. These short‐lived solstice evening events occurred following large upward plasma drifts and sharply reduced plasma densities. The electron temperatures increase during the August 5 and 6, 2011 moderate solar event reached peak values of about 1,000 K near 350 km. Smaller electron temperature increases were observed to altitudes up to about 500 km. There were also smaller concurrent ion temperature enhancements. During the very low solar flux January 20 and 21, 2020 event, a peak electron temperature increase of about 700 K occurred at an altitude of about 270 km. We also show that SAMI2‐PE simulations using the measured vertical plasma drifts reproduce the main characteristics of the measured plasma densities and temperatures up to 300 km, but not at higher altitudes. These simulations indicate that the evening anomalous plasma heating below about 300 km is due to the decrease in the electron‐ion cooling rate resulting from upward plasma drift driven decrease in the plasma density. Plasma transport and reduced cooling rate is a potential source of higher altitude heating.

A computational study of heat transfer and material removal in picosecond laser micro-grooving of copper

A 2D two-temperature model (TTM) is developed for picosecond laser micro-grooving of copper (Cu) and discretized using the finite difference scheme, to explore the temperature evolution and groove formation process. For lattice and electron subsystems, as well as the coupling strength between them, comprehensive temperature-dependent properties are employed and updated in real time to make the calculation more accurate. The model verification is then conducted and a reasonable good agreement has been found between predicted and measured groove depths. Using the verified model, computational studies of temperature field evolution are carried out in different fluence levels. For low fluence of around 0.283 J/cm2, phase change can be neglected, and the temperature difference between two subsystems decreases with an increase in pulse width and can be neglected when the pulse width is over 200 ps. In contrast, for high fluence of about 10.61 J/cm2, the maximum lattice temperature increases rapidly to boiling point within 3 ps, and the peak electron temperature is around 10 times higher, leading to material removal via evaporation. Under this high fluence level, groove depth around 0.25 μm is obtained within a 12 ps single pulse irradiation. In addition, thermal diffusion occurs from the residual hot zone adjacent to groove edge towards surrounding area during the off-pulse period, as the heat affected zone depth is up to 1 μm after 500 ps and over 4 μm after 6 ns, and the peak residual lattice temperature drops to about 396 K from boiling point within 53 ns. Moreover, the groove depth and groove profile show a periodically evolving manner with respect to scanning times, and for each scanning, a slow-fast-slow pattern has been observed in groove depth increase and profile development.

Evidence for electron energization accompanying spontaneous formation of ion acceleration regions in expanding plasmas

We report experiments conducted in an expanding argon plasma generated in the inductive mode of a helicon source in the Hot hELIcon eXperiment–Large Experiment on Instabilities and Anisotropies facility. As the neutral gas pressure increases, the supersonic ion acceleration weakens. Increasing neutral pressure also alters the radial profile of electron temperature, density, and plasma potential upstream of the plasma expansion region. Langmuir probe measurements of the electron energy probability function (EEPF) show that heating of electrons at the plasma edge by RF fields diminishes with increasing gas pressure, yielding a plasma with a centrally peaked electron temperature, and flat potential profiles at higher neutral pressures. For neutral pressures at which ion acceleration regions develop in the expanding plasma plume, EEPFs reveal electrons with two temperature components.

Resonant dual-pulse laser ignition technique based on oxygen REMPI pre-ionization

This contribution investigates a novel laser ignition method based on a dual-pulse resonant pre-ionization scheme. The first laser pulse efficiently creates initial gas ionization (seed electrons) through a 2 + 1 resonantly-enhanced multiphoton ionization (REMPI) scheme targeting molecular oxygen (λ ~ 287.6 nm). This pulse is followed by a second non-resonant near-infrared pulse (λ = 1064 nm) for energy addition into the gas via inverse bremsstrahlung absorption. The sequence of two pulses creates a laser induced plasma that exhibits high peak electron number density and temperature (ne ~ 8 × 1017 cm-3 at t = 100 ns and T ~ 8000 K at t = 10 μs, respectively). These plasma parameters are similar to those attained for typical single-pulse near-infrared laser plasmas but with the advantage of substantially lower pulse energy (by factor of ~ 2.5) in the dual-pulse REMPI case. A combustion study focusing on ignition of propane/air mixtures shows that the dual-pulse REMPI method leads to an extension of the lean flammability limit, and an increase in combustion efficiency near the lean limit, as compared to laser ignition with a single NIR pulse. The measurement results and observed gas dynamics are discussed in the context of their impact on combustion applications.

First Report of Electron Measurements During a Europa Footprint Tail Crossing by Juno

AbstractWe report the first in situ observations of electron measurements at a Europa footprint tail (FPT) crossing in the auroral region. During its 12th science perijove pass, Juno crossed magnetic field lines connected to Europa's FPT. We find that electrons in the range ~0.4 to ~25 keV, with a characteristic energy of 3.6 ± 0.5 keV, precipitate into Jupiter's atmosphere to create the footprint aurora. The energy flux peaks at ~36 mW/m2, while the peak ultraviolet (UV) brightness is estimated at 37 kR. We estimate the peak electron density and temperature to be 17.3 cm−3 and 1.8 ± 0.1 keV, respectively. Using magnetic flux shell mapping, we estimate that the radial width of the interaction at Europa's orbit spans roughly 3.6 ± 1.0 Europa radii. In contrast to typical Io FPT crossings, the instrument background caused by penetrating energetic radiation (> ~5–10 MeV electrons) increased during the Europa FPT crossing.