Time Evolution Of Electron Density Research Articles

On learning latent dynamics of the AUG plasma state

In this work, we demonstrate the utility of state representation learning applied to modeling the time evolution of electron density and temperature profiles at ASDEX-Upgrade (AUG). The proposed model is a deep neural network, which learns to map the high dimensional profile observations to a lower dimensional state. The mapped states, alongside the original profile's corresponding machine parameters, are used to learn a forward model to propagate the state in time. We show that this approach is able to predict AUG discharges using only a selected set of machine parameters. The state is then further conditioned to encode information about the confinement regime, which yields a simple baseline linear classifier, while still retaining the information needed to predict the evolution of profiles. We, then, discuss the potential use cases and limitations of state representation learning algorithms applied to fusion devices.

Comparison Between Air and SF6 Breakdown by Microwaves at High Pressure

A model coupling Maxwell's equations with electron fluid equations is used to simulate air and SF 6 breakdown at high pressure by microwaves radiated from a circular waveguide. The modeling for microwaves radiated from a circular waveguide is validated by comparing its results with those from the commercial software FEKO. Under a fixed initial growth rate of electron density in the maximum radiated field, the space and time evolution of electron density in SF 6 breakdown is similar to that in air breakdown. However, SF 6 plasma causes less change in the radiated field than air plasma. This is because the saturation electron density playing an important role in absorption and scattering by plasma is lower in SF 6 than in air. The ratio of critical radiated power in SF 6 to that in air increases as pulsewidth or gas pressure increases. The breakdown threshold from simulations as a function of pressure shows the same trend as the experiment.

Time-resolved electron temperature and electron density measurements in a nanosecond pulse filament discharge in H2–He and O2–He mixtures

Time evolution of electron density and electron temperature in a nanosecond pulse, diffuse filament electric discharge in H2–He and O2–He mixtures at a pressure of 100 Torr is studied by Thomson/pure rotational Raman scattering and kinetic modeling. The discharge is sustained between two spherical electrodes separated by a 1 cm gap and powered by high voltage pulses ~150 ns duration. Discharge energy coupled to the plasma filament 2–3 mm in diameter is 4–5 mJ/pulse, with specific energy loading of up to ~0.3 eV/molecule. At all experimental conditions, a rapid initial rise of electron temperature and electron density during the discharge pulse is observed, followed by the decay in the afterglow, over ~100 ns–1 µs. Electron density in the afterglow decays more rapidly as H2 or O2 fraction in the mixture is increased. In He/H2 mixtures, this is likely due to more rapid recombination of electrons in collisions with and ions, compared to recombination with ions. In O2/He mixtures, electron density decay in the afterglow is affected by recombination with and ions, while the effect of three-body attachment is relatively minor. Peak electron number densities and electron temperatures are ne = (1.7–3.1) · 1014 cm−3 and Te = 2.9–5.5 eV, depending on gas mixture composition. Electron temperature in the afterglow decays to approximately Te ≈ 0.3 eV, considerably higher compared to the gas temperature of T = 300–380 K, inferred from O2 pure rotational Raman scattering spectra, due to superelastic collisions. The experimental results in helium and O2–He mixtures are compared with kinetic modeling predictions, showing good agreement.

Three-Dimensional Fully-Coupled Electrical and Thermal Transport Model of Dynamic Switching in Oxide Memristors

Since researchers from the HP Labs published “The missing memristor found” [1] in Nature in 2008, memristors have become a new research frontier, as they have shown many potential applications, including non-volatile memory, hybrid CMOS/memristor 3D circuits, and brain-inspired computing. Oxide-based memristors reported so far have shown a wide spectrum of complex switching current-voltage characteristics [2], ranging from bipolar nonlinear, bipolar linear, unipolar bistable, to the unipolar threshold switching. It is generally recognized that the complex switching behavior in memristors is governed by temporally and spatially intertwined electrical and thermal processes of electrons, holes, and mobile vacancies, including field drift, Fick diffusion, Soret effect, and Joule heating. In order to facilitate the understanding of switching physics in memristors, a high-fidelity model is necessary and should be able to simulate the entire switching spectrum. However, no existing models in literature have captured all these processes and their couplings for electrons, holes, and mobile vacancies on an equal footing in realistic 3D device structures. In this work, we present a fully-coupled electrical and thermal model that includes all the important processes and treats the transport of electrons, holes, and vacancies together with the Joule heating on an equal footing. Namely, we solve simultaneously the five coupled partial differential equations: the time-dependent continuity equations for electrons, holes, and vacancies, the time-dependent lattice heat equation, and the Poisson equation for all the charged species in three spatial dimensions. The current density for each species includes contributions from the field drift due to potential gradient, Fick diffusion due to concentration gradient, and the Soret effect due to temperature gradient. The heat generation in the temperature equation includes Joule heating due to all carriers. The fully-coupled equations are implemented in Charon [3], an open-source TCAD code developed at Sandia National Labs. This fully-coupled model allows us to investigate the microscopic interplay of field drift, Fickian diffusion, Soret effect, and Joule heating, and to facilitate the understanding of physical switching mechanisms in all relevant oxide memristors. We apply the coupled model to simulate the switching process in a 3D filamentary tantalum oxide memristor shown in Fig.1. Since the Ta electrode and conduction filament (CF) are the active regions, we solve the coupled electrical and thermal equations there, and allow the heat to conduct in the Ta2O5 and Pt regions. Voltage is applied to the top of Ta electrode and the bottom of CF is grounded. The top and bottom surfaces of the structure are set to 300 K. The rigid point ion model [4] is used to describe the movement of vacancies, which exponentially depends on the temperature and the electric field when the field exceeds a critical value. Thermal and electrical parameters are being calibrated with experimental data as a function of temperature and vacancy density, and reasonable constant values are currently used for the results shown here. Figure 2 shows center view of the simulated time evolution of vacancy density, electron density, and lattice temperature in the Ta and CF regions during ON switching, when 1V is applied to the top of Ta electrode, and the Ta and CF regions have an initial uniform vacancy density of 1x1021 cm-3 and 5x1020 cm-3 respectively. With the positive voltage applied to the Ta electrode, Joule heating causes an increase of temperature, which is the strongest near the Ta/CF interface, causing vacancies from the Ta electrode to flow into the CF, as seen at 1ms. As temperature further increases, vacancies continue filling in the CF, and by 10ms, the CF is nearly completely filled (CF is assumed to be less conductive than Ta). Time evolution of electron density shows a process similar to vacancies, indicating that vacancies indeed act as n-type mobile dopants. Radial expansion of CF is not observed here, which may be due to a combination of small radius, uniform initial vacancy density, and constant physical parameters used. [1] D. B. Strukov, G. S. Snider, D. R. Stewart, and R. S. Williams, Nature 453, 80-83 (2008). [2] J. J. Yang, D. B. Strukov, and D. R. Stewart, Nature Nanotechnology 8, 13-24 (2013). [3] G. Hennigan, D. Fixel, J. Castro, and P. Lin, “Charon Parallel Semiconductor Device Simulator”, SAND2010-3905, June 2010. [4] N. Cabrera and N. F. Mott, Rep. Prog. Phys. 12, 163-184 (1948). This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. Figure 1

Time-evolution of electron density in plasma measured by high-order harmonic generation

We propose a new method of measuring the electron density in plasma by high-order harmonic generation (HHG) of intense two-color femtosecond (fs) laser. As the 800 nm fundamental beam is introduced after its second harmonic generation (SHG) beam, the recovery of HHG by the fundamental pulses at a delay of ~40 ps indicates the decay time of the generated plasma. The electron-ion recombination rate and electron density decay are revealed by fitting the harmonic emission to the model that accounts for depletion of neutral atoms, phase mismatch and re-absorption of HHG.

Microwave measurements of the time evolution of electron density in the T-11M tokamak

Unambiguous diagnostics intended for measuring the time behavior of the electron density and monitoring the line-averaged plasma density in the T-11M tokamak are described. The time behavior of the plasma density in the T-11M tokamak is measured by a multichannel phase-jump-free microwave polarization interferometer based on the Cotton-Mouton effect. After increasing the number of simultaneously operating interferometer channels and enhancing the sensitivity of measurements, it became possible to measure the time evolution of the plasma density profile in the T-11M tokamak. The first results from such measurements in various operating regimes of the T-11M tokamak are presented. The measurement and data processing techniques are described, the measurement errors are analyzed, and the results obtained are discussed. We propose using a pulsed time-of-flight refractometer to monitor the average plasma density in the T-11M tokamak. The refractometer emits nanosecond microwave probing pulses with a carrier frequency that is higher than the plasma frequency and, thus, operates in the transmission mode. A version of the instrument has been developed with a carrier frequency of 140 GHz, which allows one to measure the average density in regimes with a nominal T-11M plasma density of (3–5)×1013 cm−3. Results are presented from the first measurements of the average density in the T-11M tokamak with the help of a pulsed time-of-flight refractometer by probing the plasma in the equatorial plane in a regime with the reflection of the probing radiation from the inner wall of the vacuum chamber.

Field-induced electron transport and phonon dynamics in a GaAs-basedp-i-nnanostructure: A subpicosecond time-resolved Raman probe

Electron transport and phonon dynamics in a GaAs-based $p$-$i$-$n$ nanostructure under the application of an electric field have been studied by time-resolved Raman spectroscopy at $T=80$ K. The time evolution of electron density, electron distribution, electron drift velocity, and LO-phonon population has been directly measured with subpicosecond time resolution. Our experimental results show that, for a photoexcited electron-hole pair density of $n\ensuremath{\cong}{10}^{17}$ cm${}^{\ensuremath{-}3}$, the effects of the drifting of electrons and electron intervalley scattering processes govern electron transport properties as well as the LO-phonon dynamics. All of the experimental results are compared with ensemble Monte Carlo simulations and satisfactory agreement is obtained.

Electrical characterization and modeling of a dust forming plasma in a radio frequency discharge

In this article we focus on the time evolution of macroscopic and microscopic properties of a dust forming plasma in a silane–argon low pressure radio frequency (rf) discharge. The time evolution of electron density, ion density, dust particle concentration, and the current density were recorded in a low pressure (0.1 Torr) rf Ar–SiH4 discharge. From these data and optical emission spectroscopy simultaneous measurements, the time evolution of the electron mean energy and the particle mean charge are determined. Together with previous experimental data concerning the nucleation and growth of dust particles, a complete experimental set of microscopic and macroscopic data is available and it is compared to a theoretical model including both particle growth and dusty discharge parameters. A clear representation of this closely coupled time evolution of the particles’ formation and plasma properties is given. It includes the nucleation and coagulation phenomena for particles, and the strong increase of the rf electric field in the plasma volume and the increase of the excitation rates of electrons.

The thermal ionization phase during the plasma formation in TM-1-MH tokamak

The start-up phase of discharge in a TM-1-MH tokamak has been investigated. We present a simple model describing the time evolution of electron density in a time interval where the bulk of electron distribution function is already Maxwellian and “thermal” ionization of neutral gas dominates. Comparison of the computed results with experimental data proved the validity of this model and allowed us to determine the electron temperature during this period of the discharge. The role of the run-away electron production during the start-up phase is discussed.