Electron Drift Direction Research Articles

Consequences of the application of the streamer fluid model to the study of the sprite inception mechanism

We present the results of a streamer-fluid model used to investigate the electrodynamical coupling between the troposphere and upper atmosphere due to the penetration of lightning electric fields into the mesosphere and the lower ionosphere, generating sprites. The model solves the continuity equation for electrons and ions coupled to Poisson equation. The dominant physical response of the atmosphere is the formation of a screening-ionization wave. The wave shields the atmosphere above it from the action of the lightning field and, together with the conductivity reduction below it due to attachment, the wave amplifies the total field below it, allowing for the penetration of intense electric fields in the mesosphere as it propagates downwards into regions of higher density that compress the wave. This is the key physical mechanism for sprite inception. We evaluated the effects of the thundercloud charge geometry, lightning current waveshape, atmospheric conductivity, via different electron density profiles, and the effect of ionization, attachment and electron mobility coefficients in the electrical breakdown process, related to halo production, and sprite streamer initiation. The results showed that electrons with higher mobility are more efficient in shielding the lightning electric field before breakdown, causing delay, and they contribute to the formation of the streamer seed after breakdown, anticipating the sprite streamer inception. Similarly, a higher effective ionization rate, produced by modifications in the attachment and ionization coefficients, anticipates sprite inception. The simulations with 6 different electron density profiles, and therefore conductivities, spanning 4 orders of magnitude, showed that the altitude of breakdown and sprite initiation, as well as their time delays from the lightning discharge are directly related to atmospheric conductivity: higher conductivities produce halo and sprite inception at lower altitudes with longer delays and may hinder sprite formation. We document that variations of 30 times in the lightning current leads to sprite initiation altitudes in the range 66.0–73.5km, with delays between 1.550 and 34.500ms, while variations of 4 orders of magnitude in the conductivity profile lead to initiation altitudes 61.0–70.6km, with delays in the range 3.825–9.825ms. Consequently, we suggest that lightning characteristics dominate over atmospheric parameters in determining sprites’ initiation altitude and delay. The simulation of a −CG, with a constant current of 30kA, did not produce a sprite seed, confirming an asymmetry in the response of the atmosphere to positive and negative lightning. This is due to the free electron drift direction that is away from the screening ionization wave, preventing the formation of the streamer seed for the great majority of −CGs. The same does not apply to halos, which depend on the occurrence of breakdown and can be produced by discharges of both polarities.

A phenomenological explanation for the anomalous ion heating observed in the JET alpha-heating experiment of 1997

In the so-called ‘alpha-heating’ experiment performed on the JET tokamak during the deuterium–tritium campaign of 1997, the ion temperature was found to be far exceeding (both in absolute value and in its rise time) the level that could have been expected from direct collisional heating by the fusion-born alpha particles themselves and energy equipartition with the electrons. To date, no explanation has been put forward for this long standing puzzle, despite much work having been performed on this subject in the early 2000s.Two analysis methods that have recently become available have been employed to re-analyse these observations of an anomalous ion heating. First, an algorithm based on the sparse representation of signals has been used to analyse magnetic, reflectometry and electron-cyclotron emission measurements of the turbulence spectra in the drift-wave range of frequencies. This analysis has then been complemented with turbulence simulations performed with the GENE code.We find, both experimentally and in the simulations, that the presence of a minority, but sufficiently large, population of fusion-born alpha particles that have not yet fully thermalized stabilizes the turbulence in the ion-drift direction, but practically does not affect the turbulence in the electron-drift direction. We link such stabilization of the ion-drift-wave turbulence to the increase in the ion temperature above the level achieved in similar discharges that did not have (at all or enough) alpha particles. When the fusion-born alpha particles have fully thermalized, the turbulence spectrum in the ion-drift direction reappears at somewhat larger amplitudes, which we link to the ensuing reduction in the ion temperature. This phenomenological dynamics fully corresponds to the actual experimental observations. By taking into account an effect of the alpha particles that had not been previously considered, our new analysis finally presents a phenomenological explanation for the so-far-unexplained anomalous ion heating observed in the JET alpha-heating experiment of 1997.Through the formulation of an empirical criterion for ion-drift-wave turbulence stabilization by fusion-born alpha particles, we also show why similar observations were not made in the other deuterium–tritium experiments run so far in JET and TFTR. This allows assessing the operational domain for this stabilization mechanism for ion-drift-wave turbulence in future burning plasma experiments such as ITER, which may open a new path towards the sustainment of a high energy gain in such forthcoming devices.

Zonal Flows Driven by Small-Scale Drift-Alfven Modes

Generation of zonal flows by small-scale drift-Alfven modes is investigated by adopting the approach of parametric instability with the electron polarization drift included. The zonal mode can be excited by primary modes propagating at both electron and ion diamagnetic drift directions in contrast to the assertion in previous studies that only primary modes propagating in the ion diamagnetic drift directions can drive zonal instabilities. Generally, the growth rate of the driven zonal mode is in the same order as that in previous study. However, different from the previous work, the growth rate is no longer proportional to the difference between the diamagnetic drift frequencies of electrons and ions.

Characterization of edge profiles and fluctuations in discharges with type-II and nitrogen-mitigated edge localized modes in ASDEX Upgrade

Edge localized modes (ELMs) with high frequency and low power loss (type-II ELMs) occur in high triangularity, near double null configurations in ASDEX Upgrade with full tungsten plasma facing components. The transition from type-I to type-II ELMs is shown to occur above a collisionality threshold. For the first time the characteristic MHD fluctuations around 40 kHz have been localized. The fluctuations are observed in a wide region extending from the pedestal inward to normalized poloidal radius ρpol = 0.7. Their amplitudes on the low-field side of the plasma exhibit maxima above and below the mid-plane. The fluctuations move in the electron drift direction and lead to a reduced edge electron temperature gradient. The reduction in the edge pressure gradient is connected with these MHD fluctuations, which affect the electron temperature but not the electron density profiles. A comparison with nitrogen-mitigated type-I ELMs in the same plasma shape shows that core profiles are also affected. The electron temperature profile is self-similar for type-I and nitrogen-mitigated type-I ELMs but is not self-similar in the case of type-II ELMs.

Spatial gradient of dynamic nuclear spin polarization induced by breakdown of the quantum Hall effect

We studied spatial distribution of dynamic nuclear polarization (DNP) in a Hall-bar device in a breakdown regime of the quantum Hall effect (QHE). We detected nuclear magnetic resonance (NMR) signals from the polarized nuclear spins by measuring the Hall voltage $V_{xy}$ using three pairs of voltage probes attached to the conducting channel of the Hall bar. We find that the amplitude of the NMR signal depends on the position of the Hall voltage probes and that the largest NMR signal is obtained from the pair of probes farthest from the electron-injecting electrode. Combined with results on pump-and-probe measurements, we conclude that the DNP induced by QHE breakdown develops along the electron-drift direction.

Maximum Entropy Analysis of the 2D Density Turbulence Measured by MIR in TPE-RX

Two-dimensional (2D) turbulence measurement by using microwave imaging reflectometry (MIR) has been performed in the reversed field pinch (RFP) plasma in TPE-RX. In this work, the maximum entropy method (MEM) is used to estimate the 2D k-spectrum. The turbulences between the pulsed poloidal current drive (PPCD) and the high Θ plasmas are studied. The m = 1 modes are dominant in PPCD plasma, while in high Θ plasma the k spectrum expands to the high k range and the turbulence propagates in the electron drift direction. The preliminary result shows that PPCD plasma switches off the turbulence, and the high Θ plasma has strong turbulence.

Classical modeling of non-sequential double ionization: Recollision, time-delayed ionization, drift, and possible reattachment

We employ 3d classical ensembles to study Non-Sequential Double Ionization (NSDI) of atoms by strong laser fields at visible and infrared wavelengths. We consider in particular the wavelength dependence of final electron drift directions and the net momentum spectra parallel to the laser polarization. We find that as the wavelength is increased the spectrum transitions from a singlet to a doublet, then to a triplet, and finally back to a doublet. We also show that electrons that escape the central potential-energy well during the pulse may nonetheless be bound to the nucleus after laser turnoff.

Electron drift directions in strong-field double ionization of atoms

Longitudinal momentum spectra and electron drift directions are considered for several laser wavelengths in non-sequential double ionization of helium using three-dimensional classical ensembles. In this model, the familiar doublet for wavelength 800 nm and intensities of order 5 × 1014 W cm−2 becomes a triplet for wavelength 1314 nm, then a doublet for 2017 nm. The results are explained based on whether the post-ionization impulse from the laser results in backward drift for one or both electrons.

Electromagnetic effects on quasilinear turbulent particle transport

It is well known that a nonadiabatic part of the electron density response is needed for particle transport in tokamaks. Such main reactive effects are electron trapping and electromagnetic induction. Although electron trapping has been studied rather extensively, electromagnetic effects have hardly been studied at all although they are already included in transport codes. Here the electromagnetic effects have been analyzed and parameter studies have been performed, showing that an electromagnetic particle pinch may appear in the flat density regime, just as for the case of electron trapping although the conditions are more restrictive. The particle pinch is particularly sensitive to the direction of propagation of the eigenmode. The electromagnetic particle flux is found to be outward for modes propagating in the ion drift direction and inward for modes propagating in the electron drift direction. A pinch may be obtained rather close to the axis for International Thermonuclear Experimental Reactor simulation data.

HF radar observations of high-aspect angle backscatter from the E-region

Abstract. We present evidence for the observation of high-aspect angle HF radar backscatter from the auroral electrojets, and describe the spectral characteristics of these echoes. Such backscatter is observed at very near ranges where ionospheric refraction is not sufficient to bring the sounding radio waves to orthogonality with the magnetic field; the frequency dependence of this propagation effect is investigated with the Stereo upgrade of the CUTLASS Iceland radar. We term the occurrence of such echoes the "high-aspect angle irregularity region" or HAIR. It is suggested that backscatter is observed at aspect angles as high as 30°, with an aspect sensitivity as low as 1dB deg–1. These echoes are distinguished from normal electrojet backscatter by having low Doppler shifts with an azimuthal dependence that appears more consistent with the direction of the convection electric field than with the expected electron drift direction. This is discussed in terms of the linear theory dispersion relation for electrojet waves. Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; auroral ionosphere)

Plasma residual rotation in the TCABR tokamak

This paper reports the first results on the measurement of the radial profiles of plasma poloidal and toroidal rotation performed on the TCABR tokamak, in the collisional regime (Pfirsch–Schluter), using Doppler shift of carbon spectral lines, measured with a high precision optical spectrometer. The results for poloidal rotation show a maximum velocity of (4.5 ± 1.0) × 103 m s−1 at , (a—limiter radius), in the direction of the diamagnetic electron drift. Within the error limits, reasonable agreement is obtained with calculations using the neoclassical theory for a collisional plasma, except near the plasma edge, as expected. For toroidal rotation, the radial profile shows that the velocity decreases from a counter-current value of (20 ± 1) × 103 m s−1, at the plasma core, to a co-current value of (2.0 ± 0.9) × 103 m s−1 near the limiter. An agreement within a factor 2, for the plasma core rotation, is obtained with calculations using the model proposed by Kim, Diamond and Groebner (1991 Phys. Fluids B 3 2050).

Electrical rectification by magnetic edge states

We investigate electron channelling by inhomogeneous magnetic fields in hybrid semiconductor/ferromagnet structures. A novel type of rectifying behaviour is observed in the current–voltage characteristics measured along the drift path of magnetic edge states. This rectification effect is switched on by the application of the magnetic potential and changes sign with its polarity. We ascribe this non-linear behaviour to the anisotropic electron–phonon interaction arising from the asymmetry of the energy dispersion curves of snake orbits that results in electrons thermalising by emitting phonons along the electron drift direction. We present a theory to evaluate the electromotive force due to this phonon drag effect and compare it to the non-linear behaviour.

Extension of drift magnetic island theory beyond the common profile function approximation

The approach of several profile functions for studying drift magnetic islands is developed. This approach yields more favorable predictions for the role of the polarization current than the less adequate approach of a common profile function adopted in preceding studies. It is shown that the polarization current is stabilizing not only for subdrift islands propagating in the ion drift direction but also for islands propagating in the electron drift direction with a sufficiently low rotation frequency.

Observation of Multiple Modes of Interior Density Turbulence in the TEXT-U Tokamak

Two distinct broadband modes of density turbulence propagating in the electron drift direction are simultaneously observed, in the first complete measurement of the wave number frequency spectrum $S(k,\ensuremath{\omega})$ in the interior of the TEXT-U tokamak plasma. A drift wave-type mode with a peak in the wave number spectrum at ${k}_{\ensuremath{\theta}}\ensuremath{\sim}4.5\mathrm{cm}{}^{\ensuremath{-}1}$ is found on both sides of the magnetic axis. A long-wavelength mode with ${k}_{\ensuremath{\theta}}\ensuremath{\le}2\mathrm{cm}{}^{\ensuremath{-}1}$ is found only on the low field side of the tokamak. The results resolve apparent discrepancies in previous turbulence measurements. Confinement properties of TEXT-U are consistent with the transport predicted by the observed drift wave-type ${\overline{k}}_{\ensuremath{\theta}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3\mathrm{cm}{}^{\ensuremath{-}1}$.

Performance study of liquid xenon detector for PET

The coincidence time resolution and the intrinsic position resolution along the electron drift direction of a liquid xenon multiwire ionization chamber for PET were measured, being, respectively, 1.5 ns (fwhm) and 0.5 mm (fwhm). Data on the low temperature performance of a photomultiplier tube operating on the VUV are presented.

Studies on basic phenomena during the pellet injection into high-temperature plasmas

The injection of hydrogen/deuterium pellets into a tokamak leads to a sudden increase in the electron density, and subsequently to a profile peaking of the density and an increase in the stored energy. Immediately after the injection, different types of oscillations are excited. On TEXTOR, the first type immediately follows the injection and the second one is excited with a delay of more than 10 ms. The oscillations show a `snake-like' structure and occur close to the q = 1 surface with a frequency of 0.7 - 2 kHz. The radial location of the second oscillation is slightly shifted with respect to the first one. A fast-cooling phenomenon (`pre-cooling') in the core region of a plasma is often observed at pellet ablation phase. A study on the relation between the `pre-cooling' and sawtooth oscillations suggests that the central value of safety factor of plasmas, q(0), is kept sufficiently below unity even just after the sawtooth crash. During pellet injection, the ablation rate is strongly modulated; these modulations cause so-called `striations' in the ablation cloud. One model relates the striations to the energy reservoir on the plasma flux surfaces and describes the possibility of deriving the q-profile; the question of whether this method provides reliable results cannot yet be answered conclusively. The trajectory of the pellet in the plasma is in general not straight but deflected in the electron drift direction (OH discharges) or in the ion drift direction (CO-NBI discharges). The cloud develops a helically structured tail in the electron flow direction (toroidally) and in the electron diamagnetic drift direction (poloidally). The tail structure is attributed to charge-exchange processes and to plasma rotation.

Use of a quartz microbalance for plasma-wall interaction studies

This paper reports on the first use of quartz crystal microbalances as a useful diagnostic for plasma-wall interaction studies in tokamaks. Two quartz microbalance systems were installed recently in the TdeV tokamak. Reliable mass measurements are obtained when a correction is applied to the frequency response of the crystal, owing to its temperature rise during the discharges. Particle fluxes in the scrape-off layer (SOL), perpendicular and parallel to the magnetic field lines along the ion drift or electron drift directions were measured. As expected, a larger erosion rate was measured in the ion drift direction, while deposition was observed on the quartz surfaces facing up and facing down but not facing the plasma (i.e. field lines at grazing incidence). Radial displacement of the set-up permitted the measurement of the radial dependence of the erosion-deposition processes. An increase of the plasma current induces a slight increase of the deposit thickness. A detachment of the plasma in the divertor doubles the thickness of the deposit. The quartz microbalance showed that the deposition rate is strongly increased for a few discharges following a disruption. This diagnostic method is also sensitive to the atomic number of injected impurities.

Saturation of the Farley‐Buneman instability via nonlinear electron E×B drifts

The Farley‐Buneman instability is a collisional two‐stream instability observed in the E region ionosphere at altitudes in the range 90–120 km. While linear theory predicts the dominant wavelengths, it cannot fully describe the behavior of this nonlinearly saturated instability, as observed by radar and rocket measurements. This paper explores the nonlinear behavior of this phenomenon and the resulting waves through simulations and theory. Our two‐dimensional simulations model wave behavior in the plane perpendicular to the Earth's magnetic field, applying a fluid model to describe the electron dynamics and either a particle or a fluid model to describe ion behavior. The results show the growth, saturation, and nonlinear behavior of the instability for a much longer period of time than was possible with the pure particle codes used in previous studies. These simulations show (1) growth of Farley‐Buneman waves, (2) the development of secondary waves which propagate along the extrema and perpendicular to the Farley‐Buneman waves, (3) turning of the primary waves away from the electron drift direction, (4) a saturated wave phase velocity below the one predicted by linear theory but above the acoustic speed and (5) nonlinear electron E×Bo drifting dominates the behavior of the saturated waves. This paper describes both the simulation techniques and fundamental results. Additionally, this paper outlines a theory explaining the dominant nonlinear process seen in this instability.

Hybrid simulations of the saturated Farley‐Buneman instability in the ionosphere

Numerical simulations of the Farley‐Buneman instability in 2–1/2 dimensions using particle ions and fluid electrons show the growth, saturation and nonlinear behavior of two‐stream waves. This hybrid technique models the saturated state of the instability for a much longer period of time than the pure particle codes that preceded it. While focusing principally on modeling the topside E region equatorial electrojet, many of these results apply to the auroral two‐stream instability as well. The following features are seen in all our hybrid simulations: (1) wave growth at an angle offset from the electron drift direction where the angle depends on the strength of the driving electric field, (2) nonlinear coupling to waves traveling perpendicular to the propagation direction of the principal two‐stream waves, (3) a saturated wave phase velocity at or above the sound speed but well below the velocity predicted by linear theory and (4) phase velocities which remain almost constant as a simulated radar sweeps from a horizontal direction to nearly vertical. The nonlinear electron motion dominates the behavior of these waves. Further, these simulations indicate that ion kinetic effects are not essential for the saturation of the instability and that electron temperature effects have a minor impact on the final saturated state.

Toroidal phase locking phenomena and nonaxisymmetric flux perturbations in the Extrap T1 reversed-field pinch

The structure of the magnetic fluctuations in the Extrap T1 reversed-field pinch [Phys. Scr. 44, 358 (1991)] has been studied over the pinch parameter range 1.6<Θ=Bθ(a)/〈Bφ〉<2.2. It is found that nonaxisymmetric perturbations of the toroidal flux and edge field dominate the global dynamics at all Θ. These perturbations are formed by phase locking of the dominant m=1 modes and m=0 modes due to strong nonlinear interaction. This m=1/m=0 mode locking rotates toroidally in the electron drift direction, with an angular rotation velocity of typically 3.3×104 rad/s. The toroidal mode numbers of the m=0 modes are consistent with being nonlinearly driven by the m=1 modes. Evidence is found that the behavior of the leading m=1 mode governs the degree of phase locking. The role of this phenomenon in mean field maintenance is discussed.