Ambipolar Drift Research Articles

Formation of fluctuations in a molecular slab via isobaric thermal instability

Frictional heating by the ion-neutral drift is calculated and its effect on the isobaric thermal instability is studied. Ambipolar drift heating of a one-dimensional self-gravitating magnetized molecular slab is used under the assumptions of quasi-magnetohydrostatic and local ionization equilibrium. We see that ambipolar drift heating is inversely proportional to density and its value in some regions of the slab can be significantly larger than the average heating rates of cosmic rays and turbulent motions. The results show that isobaric thermal instability can occur in some regions of the slab, and thus it may produce slab fragmentation and formation of astronomical unit scale condensations.

Electric field cumulation in dissipative structures of gas-discharge plasmas

The process of cumulation of the dynamic order parameters (E/N and ne/N) in dissipative structures of a gas-discharge plasma is simulated using the model of ambipolar drift and ionization. The model is applicable for explaining the onset of electric energy cumulation (for E2/8π ≫H2/8π) at the periphery of electric arcs; beaded, ball, and streak lightning; cathode spots; and other spherical, cylindrical, conical, and planar dissipative plasma structures, viz., the plasmoids previously observed in “enigmatic” phenomena and in experiments with nonequilibrium gas-discharge plasmas. It is shown that, in contrast to Turing regular dissipative structures (1952), the nonlinear profiles of the dynamic order parameters in cumulative-dissipative structures (Vysika\(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l} \)lo, 1996) are described not by diffusion processes, but by convective processes of ambipolar drift focusing the electric field density (E2/8π).

Kinetics of ambipolar diffusion and drift currents of nonequilibrium carriers in semiconductors

A radically new method is suggested for drift mobility determination in semiconductors; this method is based on the measurement of how much time it takes to attain a peak value of the diffusion-drift current of nonequilibrium charge carriers excited by short pulses of light from a high-absorption region through the one of the contacts. The estimations show that the method is applicable if the drift flow exceeds the diffusion flow. This condition is satisfied at voltages exceeding those corresponding to the highest rate of tmax(U) decay. Mobility μ can be calculated by the formula μ=d2(2Utmax)−1, where d is the distance between contacts, U is the applied voltage, and tmax is the time the peak value of the photocurrent is attained.

Ambipolar Drift in a Turbulent Medium

The interstellar magnetic field strength and gas density are observed to be correlated, but there is a large dispersion in this relation. In particular, the magnetic field is often observed to be weaker than expected. It is usually assumed that the field is frozen into the gas, except at very low ionization fraction, in which case ion-neutral drift, or ambipolar diffusion, leads to appreciable slip of the field and tends to make it more uniform. We show that turbulence enhances the rate of ambipolar drift, and that this may explain the surprisingly low fieldstrengths sometimes seen in dense interstellar gas.

Space charge solution of Rittner photoconductor equation for a HgCdTe detector

The steady-state charge continuity equations are linearized to derive a space charge field that accompanies the ambipolar diffusion and drift described by the Rittner equation. The space charge field is evaluated for a typical 14.2 μm cutoff wavelength HgCdTe detector operating at 85 K. It is found that the space charge density is ∼10 −5 times the hole and electron population density generated by photon flux. This corroborates that Rittner's equation gives an accurate solution for the hole and electron densities. But, at relatively high photon flux levels that are found in some Geostationary Operational Environment Satellite instrument channels, the small average space charge field can have a noticable effect on the linearity of detector response. Divergence of electric field terms in the continuity equations, which are absent from the Rittner equation, can also contribute a non-linearity to detector response.

Turbulent Flows within Self‐Gravitating Magnetized Molecular Clouds

Self-gravitating magnetized flows are explored numerically in slab geometry. In this approximation, the derivatives are computed only in one dimension but all three components of vector fields are retained. This is done for a range of fiducial values for the interstellar medium at the scale of molecular clouds. The overall characteristic scale of the turbulence, its Mach number, and the initial ratio of longitudinal to transverse turbulent velocities, as well as the extent of the initial density bulges within the fluid, are the main parameters of the study. Simulations have been performed with and without ambipolar drift. No external forcing is included. Velocity, density, and magnetic perturbations develop self-consistently to comparable levels in all cases. This includes those cases where the medium is initially static. However, a fully random flow produces substantially more density contrast with nested substructures. Collapse eventually occurs after typically three free-fall times. The magnetic field slows down the collapse as expected. For higher Mach numbers, the collapse is faster, and yet the peak densities reached in the final collapsed objects are lower. We have also modeled the effects of ambipolar drift in the presence of cosmic ray ionization and far-ultraviolet ionization. Because the turbulent timescales are shorter than the ambipolar drift timescales, we find that ambipolar drift does not play a significant role in gravitational collapse in a turbulent medium of the type modeled in our simulations.

Ambipolar drift of spatially separated electrons and holes

We report on the ambipolar transport process of photogenerated, spatially separated charge carriers in the doping layers of a $p\ensuremath{-}i\ensuremath{-}n$ diode under the influence of lateral electric fields. By including externally applied electric fields into the theory of ambipolar diffusion of spatially separated electrons and holes, we show that the transport of excess carriers can be described as a combined drift and diffusion process. Compared to the well-known ambipolar transport in bulk material, the ambipolar diffusion process is enhanced by several orders of magnitude, whereas the ambipolar drift can be described by the same ambipolar mobility as in bulk material if the electric fields in both doping layers are identical. One major difference of the ambipolar drift of electrons and holes propagating in different layers in comparison to bulk material is the possibility to control the ambipolar mobility dynamically by changing the dark carrier densities by varying the reverse bias applied to the $p\ensuremath{-}i\ensuremath{-}n$ structure. Most interesting however, is the fact that the ambipolar drift can be controlled by different external fields for electrons and holes. In order to verify the predictions of our theoretical description of the ambipolar transport of spatially separated electrons and holes, we have developed a new pump-and-probe technique that allows for a direct temporally and spatially resolved investigation of the various transport scenarios. The results agree very well with the theoretical simulations.

Threshold Frequency of Helical Electron–Hole Plasma Instability

Experimental evidence on the dependence of the threshold frequency of silicon oscillistors on the threshold electric field strength, magnetic induction, temperature, and injecting-contact separation is presented. In the temperature interval, where the weak magnetic field criterion is roughly satisfied, the experimental results are shown to be adequately explained by the classical theory of the bulk helical instability of an extrinsic plasma. The threshold frequency in this temperature interval is determined by the sum of two components. One component is due to the ambipolar drift of helical plasma perturbations, and the other results from the presence of the charge-carrier concentration gradient in a direction normal to the vectors of the electric field strength and magnetic induction. In short oscillistors (0.85·10–3, 2.38·10–3 m) at 77 K, a semiconductor plasma, wherein the helical instability is excited, approximates an intrinsic plasma, and the threshold frequency is determined by the rotation rate of helical perturbations.

Ambipolar Drift Heating in Turbulent Molecular Clouds

Although thermal pressure is unimportant dynamically in most molecular gas, the temperature is an important diagnostic of dynamical processes and physical conditions. This is the first of two papers on thermal equilibrium in molecular clouds. We present calculations of frictional heating by ion-neutral (or ambipolar) drift in three-dimensional simulations of turbulent, magnetized molecular clouds. We show that ambipolar drift heating is a strong function of position in a turbulent cloud, and its average value can be significantly larger than the average cosmic ray heating rate. The volume averaged heating rate per unit volume due to ambipolar drift, H_AD ~ |JxB|^2 ~ B^4/L_B^2, is found to depend on the rms Alfvenic Mach number, M_A, and on the average field strength, as H_AD ~ M_A^2 ^4. This implies that the typical scale of variation of the magnetic field, L_B, is inversely proportional to M_A, which we also demonstrate.

Fragmentation Instability of Molecular Clouds

Turbulent motions observed in molecular clouds are thought to reflect initial conditions associated with cloud formation and may be sustained over the cloud lifetime by mechanical energy sources associated with star formation. This paper demonstrates that free energy stored in the magnetic fields of clouds represents another source of turbulent energy, which can be released through an instability driven by ambipolar drift. The instability operates even in cases in which the cloud would be dynamically stable if the magnetic field were completely frozen to the gas. The instability has a weak form, to which clouds are generally susceptible, and a strong form, which appears if the cloud is within about 30% of critical. In the strong form, the instability grows at a rate intermediate between the slow rate of ambipolar drift and the more rapid rates associated with dynamical processes. In the weak form of the instability the growth rate is close to the ambipolar drift rate. The instability drives turbulent motions, both compressive and vortical, and may accelerate the fragmentation of a molecular cloud into substructures.

Can the turbulent galactic dynamo generate large-scale magnetic fields?

Large-scale magnetic fields in galaxies are thought to be generated by a turbulent dynamo. However the same turbulence also leads to a small-scale dynamo which generates magnetic noise at a more rapid rate. The efficiency of the large-scale dynamo depends on how this noise saturates. We examine this issue taking into account ambipolar drift, which obtains in a galaxy with significant neutral gas. We argue that, (1) the small-scale dynamo generated field does not fill the volume, but is concentrated into intermittent rope like structures. The flux ropes are curved on the turbulent eddy scales. Their thickness is set by the diffusive scale determined by the effective ambipolar diffusion; (2) For a largely neutral galactic gas, the small-scale dynamo saturates, due to inefficient random stretching, when the peak field in a flux rope has grown to a few times the equipartition value; (3) The average energy density in the saturated small-scale field is sub equipartition, since it does not fill the volume; (4) Such fields neither drain significant energy from the turbulence nor convert eddy motion of the turbulence on the outer scale into wavelike motion. The diffusive effects needed for the large-scale dynamo operation are then preserved until the large-scale field itself grows to near equipartition levels.

Can the turbulent galactic dynamo generate large-scale magnetic fields?

Large-scale magnetic fields in galaxies are thought to be generated by a turbulent dynamo. However, the same turbulence also leads to a small-scale dynamo which generates magnetic noise at a more rapid rate. The efficiency of the large-scale dynamo depends on how this noise saturates. We examine this issue, taking into account ambipolar drift, which obtains in a galaxy with significant neutral gas. We argue as follows. (i) The small-scale dynamo generated field does not fill the volume, but is concentrated into intermittent rope-like structures. The flux ropes are curved on the turbulent eddy scales. Their thickness is set by the diffusive scale determined by the effective ambipolar diffusion. (ii) For a largely neutral galactic gas, the small-scale dynamo saturates, as a result of inefficient random stretching, when the peak field in a flux rope has grown to a few times the equipartition value. (iii) The average energy density in the saturated small-scale field is subequipartition, since it does not fill the volume. (iv) Such fields neither drain significant energy from the turbulence nor convert eddy motion of the turbulence on the outer scale into wave-like motion. The diffusive effects needed for the large-scale dynamo operation are then preserved until the large-scale field itself grows to near equipartition levels.

Bias formation in a pulsed radiofrequency argon discharge

A one dimensional (1D) particle-in-cell (PIC) computer simulation has been used in conjunction with a small experimental plasma reactor, to investigate the effects of pulsing on a low pressure, capacitively coupled, rf argon plasma. In particular this article investigates the time-constants involved in the development and evolution of the bias voltage in asymmetric reactor geometry. Surprisingly, the charging time for the blocking capacitor does not occur on electron time scales, but is influenced primarily by the ambipolar drift of ions to the earthed electrode. It is shown that following plasma breakdown there is a net current flow in the system which charges the blocking capacitor in the external matching circuit and produces the bias voltage. Both the PIC simulation and the experimental measurements show that a net current flow is produced by a delay in the onset of the electron current to the earthed electrode, which is correlated to the charging time of the capacitor. From the simulation it can be seen that during this period the plasma potential in the center of the discharge is higher than one would expect, preventing electrons from reaching the earthed electrode.

Current Sheet Formation in the Interstellar Medium

There is phenomenological evidence that magnetic reconnection operates in the interstellar medium, and magnetic reconnection is also necessary for the operation of a galactic dynamo. The extremely long ohmic diffusion times of magnetic fields in typical interstellar structures suggest that reconnection occurs in two stages, with thin current layers that have relatively short resistive decay times forming by magnetohydrodynamical processes first, followed by reconnection of the fields in the layers. We propose that ambipolar drift can lead to the formation of these thin sheets in weakly ionized interstellar gas and can delineate the parameter regime in which this occurs by means of a numerical model: we find that the magnetic field cannot be too large and the medium cannot be too diffusive. Both limits are imposed by the requirement that the field be wound up about 1 time by the eddy.

Mechanism of particle transport in magnetized silane plasmas

Particle transport phenomena were investigated in silane plasmas in the presence of a magnetic field B perpendicular to a discharge electric field E. From the experimental results, it was concluded that silicon particles were transported in the opposite direction to the drift, and the particle density decreased with increasing applied magnetic flux density. Theoretical calculations on particle drift show that negatively charged particles can be transported in the opposite direction to drift and the drift velocity increases with B for the present experimental conditions. Both experimental and theoretical results suggest that transport by modified ambipolar drift can eliminate particles from discharge space.

Characteristic lengths for transport in illuminated intrinsic a-Si:H

The five coupled differential equations that describe one-dimensional transport in semiconductor are linearised, using a small-signal approach. The general homogeneous solution of this system of differential equations has four characteristic lengths. Considering diffusion and drift separetely, they reduce to two characteristic lengths L + and L. Furthermore, a material parameter allows one to distinguish between two different behaviors: the “relaxation” and the “lifetime”-type material. In “lifetime”-type material, it is the ambipolar drift or diffusion length that dominates transport. The dielectric relaxation lengths governs space-charge, if any. These theoretical results are discussed in the experimental framework of hole/ambipolar μτ measurements such as Steady State Photcarrier Grating (SSPG), Surface Photo Voltage (SPV), and Steady-State Hecht Plot (SSHP). Of these, SSPG is considered the only reliable method to measure small-signal μτ.

Ambipolar diffusion drifts and dynamos in turbulent gases

Ambipolar drift in turbulent fluids are considered. Using mean-field electrodynamics, a two-scale theory originally used to study hydromagnetic dynamos, it is shown that magnetic fields can be advected by small-scale magnetosonic (compressional) turbulence or generated by Alfvenic (helical) turbulence. A simple dynamo theory is made and is compared with standard theories in which dissipation is caused by turbulent diffusion. The redistribution of magnetic flux in interstellar clouds is also discussed.

Stability of a high-frequency glow discharge in the normal combustion regime

A high frequency glow discharge in its high current form differs from a dc discharge in that there is a significant decrease in the role of the pre-anode region in plasma generation [1], thus leading to greater stability [2]. In a dc discharge the pre-anode current-voltage characteristic (CVC) is falling [3], which causes electrodynamic instability of the plasma column and leads to its contraction over times much shorter than the thermal times [4]. It is characteristic that conductivity in the volume of a dc discharge at moderate pressures is caused by drift of ions from the pre-anode region. In an hf discharge the plasma distribution is symmetric about the midpoint of the interelectrode gap and the space charge zones near the electrodes are separated from the volume by narrow zones with high conductivity. Under such conditions, together with volume ionization processes a noticeable contribution to maintenance of conductivity can be produced by ambipolar diffusion and plasma drift due to disruption of quasineutrality [5, 6]. In order to study the stability of an hf discharge plasma column it is of interest to find the current-voltage characteristic of this part of the discharge under conditions of high longitudinal inhomogeneity and low values of E/p. In connection with the experimentally observed weak current form of the discharge [1, 2], which is characterized by a unique value of the normal current density and lacks a proper theoretical explanation, there has been increased interest in the properties of the hf discharge which are produced by phenomena in the pre-electrode regions. In particular, it is necessary to theoretically confirm the similar properties of the dc discharge and the hf discharge in the normal current density regime. The present study will present results of a numerical calculation of an hf discharge in nitrogen with consideration of space charge effects within the framework of a two-dimensional model and calculated the CVC of a plasma column with the diffusion-drift mechanism for maintenance of conductivity.

Ambipolar drift-length measurement in amorphous hydrogenated silicon using the steady-state photocarrier grating technique

The steady-state photocarrier grating technique, which yields the ambipolar diffusion length in photoconductive insulators, can also yield the ambipolar drift length if the measurements are carried out as a function of electric field. This characteristic length in turn gives a lower bound on the experimental value of the minority-carrier drift length. From measurements of both the diffusion and drift length on the same sample of amorphous hydrogenated silicon it is found that the ratio between the ambipolar diffusion coefficient and the ambipolar mobility is equal to about twice the classical Einstein value.

Charge Collection by Drift during Single Particle Upset

An improved analytical model for the drift charge collection during a single particle upset is developed. The model is based on the assumption that initially the plasma column created by an energetic particle is spread by the ambipolar diffusion. Then the collection occurs via the ambipolar drift process at the outer radius of the plasma column where the plasma density drops to a value comparable to the substrate doping. In agreement with experimental data the model predicts that the collected charge increases with the decrease in the substrate doping. A good fit to experimental data for beryllium, oxygen and copper ions is obtained for Si n+p and p+n samples.