Showing papers in "Plasma Physics Reports in 2000"

Sterilization of Medical Products in Low-Pressure Glow Discharges

Abstract: Results are presented from experimental and theoretical studies of the sterilization of medical products by the plasmas of dc glow discharges in different gas media. The sterilization efficiency is obtained as a function of discharge parameters. The plasma composition in discharges in N2 and O2 is investigated under the operating conditions of a plasma sterilizer. It is shown that free surfaces of medical products are sterilized primarily by UV radiation from the discharge plasma, while an important role in sterilization of products with complicated shapes is played by such chemically active particles as oxygen atoms and electronically excited O2 molecules.

Discharge dynamics and the production of active particles in a cathode-directed streamer

Abstract: The emission spectroscopy technique is used to analyze a cathode-directed streamer discharge in air at atmospheric pressure in point-plane geometry at interelectrode distances of up to 100 mm and a high-voltage pulse amplitude of 18 kV. The densities of molecules in the N2(C3Πu, v=0), N2+(B2Σu+, v=0) and NO(A2Σ+, v=0) states are determined, and the reduced electric field in the streamer head is estimated. It is shown that the increase in the average electric field in the discharge gap substantially intensifies the production of active particles in the discharge plasma and makes the plasma more homogeneous. This effect is only related to the increase in the fraction of regions with a high electric field in the discharge gap and, as a result, the reduction of the discharge energy losses via rapidly thermalized degrees of freedom. The active particles are only produced in the streamer head, including the case in which the interelectrode gap is bridged by the streamer channel.

Transition of a multipin negative corona in atmospheric air to a glow discharge

Abstract: It is commonly accepted that, as the current increases, a diffuse negative corona inevitably goes over to a strongly nonuniform and nonsteady spark discharge. In this paper, a new effect—the transition of a negative corona to a diffuse glow discharge at atmospheric pressure—is studied experimentally and numerically. The evolution of the corona parameters during the transition to the regime of a glow discharge is traced.

Study of plasma fluctuations in the Tuman-3m tokamak using microwave reflectometry with an obliquely incident probing beam

Abstract: Plasma fluctuations in the Tuman-3M tokamak are studied experimentally by analyzing backscattered radiation for different angles of incidence of the probing beam from the normal to the cut-off surface. The poloidal rotation velocity of the plasma fluctuations is determined from the Doppler shift of the reflected radiation spectrum measured on the edge of the tokamak during the transition to the H-mode. It is shown that, before the transition to the H-mode, the rotation velocity can be estimated quantitatively from the spectral shift or from the rate at which the phase of the reflected signal grows. The experimental data obtained during the transition to the H-mode provide evidence for the onset of a sheared poloidal flow. The shear makes it difficult to correctly estimate the poloidal rotation velocity in the improved confinement regime. The main mechanisms responsible for the broadening of the backscattered radiation spectra are considered. The turbulent diffusion coefficients determined under the assumption that the spectral broadening is diffusive in character are found to be close to those determined from the charged-particle balance.

Experimental Study of a Low-Pressure Glow Discharge in Air in Large-Diameter Discharge Tubes: I. Conditions for the Normal Regime of a Glow Discharge

Abstract: The initiation and characteristics of a low-pressure glow discharge in air in large-diameter discharge tubes are studied. A deviation from the Paschen law is observed: the breakdown curves Udc(pL) shift toward the higher values of Udc and pL as the interelectrode distance L increases. It is shown that the normal regime of a glow discharge is accompanied by gas ionization in the anode sheath. This takes place only for pL values lying to the right of the inflection point in the breakdown curve. The cathode-sheath characteristics in the normal and abnormal regimes of an air discharge for a duralumin cathode are determined. The axial profiles of the ion density, electron temperature, and plasma potential, as well as the anode voltage drop, are measured at various air pressures.

Ion Larmor radius effects in collisionless reconnection

Abstract: The nonlinear evolution of collisionless magnetic field line reconnection is investigated numerically in plasma regimes where the effects of the electron and ion temperatures are important. These effects modify the structure of the current and vorticity layers that are formed during the onset of the reconnection instability. The results of investigations in a two-dimensional periodic configuration including ion Larmorradius effects to all orders are presented and compared with the results obtained in regimes with a large sound Larmor radius. It is found that, while the roles of the sound Larmor radius and the ion Larmor radius are inter-changeable as far as the nonlinear reconnection rate is concerned, the structure of the vorticity and current density layers is different in the two cases.

Effect of continuous current during pauses between successive strokes on the decay of the lightning channel

Abstract: A one-dimensional model is used to study the dynamics of the hydrodynamic parameters of the lightning channel in the return stroke and after the pulse current is damped. The effect of the continuous residual electric current during pauses between the successive strokes on the plasma cooling in the channel is analyzed. It is shown that a continuous electric current, which is several orders of magnitude lower than the peak current in the return stroke, is capable of maintaining the channel conductivity. This effect cannot be explained merely by Joule heating but is largely governed by the fact that the turbulent heat transport is substantially suppressed. In this case, even a continuous current as low as 50–100 A is capable of maintaining the conductivity of the lightning channel at a level at which only M-components can develop in the channel rather than the dart leader of the subsequent stroke.

One-dimensional hydrodynamic model of the atom and ion dynamics in a stationary plasma thruster

Abstract: A one-dimensional hydrodynamic model of the atom, ion, and electron dynamics in the channel of a stationary plasma thruster is developed. The relevant set of integrodifferential equations is derived and investigated both analytically (steady-state solutions) and numerically (dynamic regimes). It is shown that adjusting only one parameter (the channel resistivity) makes it possible to achieve a good agreement between the calculated global parameters and experimental data. The general features of oscillations revealed with the help of the model are also found to agree fairly well with the experiment.

Ionization instability and dusty plasma structurization

Abstract: In the presence of ionization processes, a homogeneous equilibrium dust distribution often appears as a balance between plasma generation by ionization and plasma absorption by dust particles. It is shown that such equilibrium, often present in laboratory plasmas, is generally unstable against the formation of dust clumps separated by dust-free regions (dust voids). The driving force that separates an initially homogeneous dusty plasma into dust clumps and dust voids is the drag force produced by ions flowing out from the regions with reduced dust density. The lower the dust density, the lower the electron absorption by dust particles and the larger the ionization rate proportional to the electron density. An increase in the ion drag force leads to a further decrease in the dust density and, thus, drives the instability. In the nonlinear stage, the instability creates structures—dust clouds separated by dust voids. The dependence of the instability growth rate on the wavenumber (or, in other words, on the size of the dust-free and dust-containing regions) is investigated. It is shown that, for sufficiently small wavenumbers, a homogeneous distribution is always unstable. An analogy with a gravitational-like instability related to shadowing of the plasma flux by dust particles is pointed out. This effect, which is due to collective shadowing of the plasma flux, dominates the shadowing by individual dust particles discussed previously. Similar to the usual gravitational instability, perturbations with the largest scales are always unstable. Contrary to the usual gravitational instability, the largest growth rate corresponds not to the largest possible scale but to the size close to the mean free path of plasma particles colliding with dust particles. A special investigation is undertaken to determine the influence of the ion-neutral collisions on the growth rate of the instability.

One-Dimensional Hybrid Model of a Stationary Plasma Thruster

Abstract: A one-dimensional hybrid model of the dynamics of atoms, ions, and electrons in the channel of a stationary plasma thruster is developed. The relevant set of integrodifferential equations is studied numerically. The results obtained are compared with the results of previous calculations based on a hydrodynamic model. It is shown that, with the use of one fitting parameter (the channel resistance), the calculated integral characteris-tics agree well with the experimental ones. The current-voltage characteristic is obtained. The general features of low-frequency oscillations that have been revealed in numerical simulations using the model proposed are also in fairly good agreement with experimental results. The value of the electron thermal conductivity is estimated.

Nonlocal Nature of the Electron Energy Spectrum in a Glow-Discharge in Pure O2: II. Actinometry of O(3P) Atoms in a Plasma at Low Gas Pressures

Abstract: Results are presented from measurements of the density of oxygen atoms in the positive column of a dc discharge in pure oxygen by the actinometric technique using Ar atoms. Based on the excitation rate constants calculated using two different approaches (namely, the two-term approximation and the Monte Carlo method) to solving the Boltzmann equation for a spatially inhomogeneous electron distribution function, the applicability of the actinometric technique is analyzed. The effects of the discharge kinetics and the nonlocal character of the electron energy spectrum on the accuracy of actinometric measurements are studied. It is shown that the results of measurements depend only slightly on the accuracy with which the electron energy distribution function is described. Over a wide range of the reduced electric field E/N≈40–250 Td, the oxygen atom density calculated using the spatially homogeneous distribution function differs by several percent from that calculated accurately, taking into account the nonlocal character of the electron energy spectrum. It is shown that using the actinometric technique to measure the absolute concentration of oxygen atoms in a plasma requires a detailed description of the discharge plasmochemical kinetics, including a thorough analysis of all possible processes (particularly, surface heterogeneous reactions) that determine the density of active particles at low pressures. At the same time, the use of the actinometric technique for monitoring the behavior of the density of oxygen atoms in a plasma is justified over a wide range of reduced electric fields up to ∼200 Td when the O(3p3P-3p3S) transition (λ=844.6 nm) is used and the degree of dissociation is [O]/[O2]>0.02.

Present status of the theory of relativistic plasma microwave electronics

Abstract: Theoretical research on high-power microwave sources based on stimulated emission from relativistic election beams in plasma waveguides and resonators is reviewed. Both microwave amplifiers and oscillators are investigated. Two mechanisms for stimulated emission—resonant Cherenkov emission from a relativistic electron beam in a plasma and nonresonant Pierce emission arising from the onset of a high-frequency Pierce instability—are studied theoretically. The theory developed here is motivated by recent experiments carried out at the Institute of General Physics of the Russian Academy of Sciences and is aimed at creating high-power pulsed plasma microwave sources [both narrowband (Δω/ω<0.1) and broadband (or noisy, Δω/ω≈1)] based on high-current relativistic electron beams. Although the paper is devoted to theoretical problems, all analytic estimates and numerical calculations are made with real experiments in mind and theoretical results are compared with reliable experimental data. Special attention is paid to the opportunity to progress to short (millimeter) and long (decimeter) wavelength ranges. Some factors that influence the formation of the wave spectra excited by relativistic electron beams in plasma sources are discussed.

Generation of a Wakefield during Gas Ionization

Abstract: One-dimensional equations are derived that describe the hydrodynamic and electrodynamic properties of a plasma created through gas ionization by a short intense laser pulse. Different approaches (in particular, the particle-in-cell method) are used to show that, with ionization processes included, the excitation of a wakefield by an intense laser pulse can be described by the method of slowly varying amplitudes. It is shown that ionization processes enhance the wakefield excited by a moderate-intensity laser by about 10% in the case of a linearly polarized laser and by about 50% in the case of a circularly polarized laser. Ionization processes in light gases irradiated with high-intensity laser pulses have essentially no effect on the wakefield during the resonant excitation of a plasma wave by the ponderomotive force and play a governing role far from the resonance.

Microexplosion of a hot point in an X-pinch constriction

Abstract: The dynamics of an X-pinch in the diode of a high-power nanosecond current generator is studied experimentally and theoretically. The X-ray backlighting technique with subnanosecond time resolution and micron space resolution made it possible to trace both the formation of the constriction before the X-ray burst and the subsequent breaking and decay of the constriction. The radiative MHD model allowed simulation of the main characteristics of the process, including the formation of a minidiode and constriction, microexplosion of the hot point, and the generation of shock waves, followed by breaking of the constriction.

Ordered dusty structures in the plasma of an RF electrodeless gas discharge

Abstract: Results are presented from the experimental studies and numerical simulations of the behavior of dust grains in the plasma of an inductive RF discharge. The experiments were carried out with neon at a pressure of 25–500 Pa and with 1.87-µm melamine formaldehyde grains. The discharge was excited by a ring inductor supplied from a generator operating at a 100-MHz frequency. The effective dust-grain interaction potential used in numerical simulations involved the spatial dependence of the grain charge on the plasma floating potential, grain-interaction anisotropy resulting from the focusing of the drift ion current by the negatively charged grains, and specific features of the shielding of the dust grains by the plasma electrons and ions recombining both in the plasma bulk and on the grain surface. The results of Monte Carlo simulations show that the dust grains form specific filament structures observed experimentally in the plasma of an inductive electrodeless discharge.

Emission spectra of a cherenkov plasma relativistic maser

Abstract: The spectra of a plasma relativistic maser are measured. It is shown that the microwave frequency can be varied from 4 to 28 GHz by varying the plasma density from 4×1012 to 7×1013 cm−3 at a power of 30–50 MW. The relative width of the emission spectrum is within 50–80% for low plasma densities and 15–30% for high densities. Experimental results are compared with calculations.

Reversed-shear experiments in the T-10 tokamak

Abstract: Results from T-10 experiments in regimes with nonmonotonic plasma current profiles are presented. The possibility of controlling the current profile j(r) by electron-cyclotron current drive is demonstrated experimentally. Nonmonotonic q profiles with the reversed shear are obtained in which the qmin value varies in a wide range, qmin=1–2.3. It is shown that the current profiles with qmin∼2 (in this case, there are two resonant magnetic surfaces q=2 in the plasma) can cause the onset of MHD instabilities. The possibility of the formation of an internal transport barrier in reversed-shear discharges in the T-10 tokamak is analyzed. In T-10, electron transport is governed by short-wavelength electron turbulence. As a result, there is no clear evidence of the formation of an inner transport barrier in these experiments.

Spatial Structure of Emission from an Electrode Microwave Discharge in Hydrogen

Abstract: The structure of electrode microwave (2.45 GHz) discharges in hydrogen with electrodes of various shapes and sizes at pressures of 1–8 torr and incident powers of 2–150 W is studied. It is found that the discharges exhibit a common feature that is independent of the antenna-electrode design: near the electrode surface, there is a thin bright sheath surrounded by a less bright, sharply bounded region, which is usually shaped like a sphere. It is suggested that the structure observed arises because the microwave field maintaining the discharge is strongly nonuniform. Near the electrode, there exists a thin dense plasma sheath with a high electron density gradient. A strong dependence of the electron-impact excitation coefficient on the electric field makes the effect even more pronounced. As the electron density decreases due to dissociative recombination, the microwave field gradient decreases and the discharge emission intensity tends to a nearly constant value. Presumably, in the boundary region of the discharge, there exists a surface wave, which increases the emission intensity at the periphery of the discharge.

Capillary discharges for guiding of laser pulses

Abstract: The dynamics of a capillary discharge is studied to achieve optimum conditions for the guiding of ultrashort intense laser pulses. A dynamic regime is revealed in which, after a short transient process, the discharge plasma is in dynamic and thermal equilibrium. Such plasma configuration is stable against MHD perturbations. It is shown that the radial inhomogeneity of the discharge plasma composition can provide the improvement of the focusing properties of a plasma waveguide. The radius of the region where electromagnetic radiation is localized is governed by a contact discontinuity between the plasma that initially fills the channel and the plasma that is produced due to ablation of the capillary wall material.

Mechanism for electric breakdown in a chemically nonequilibrium system and the influence of the chain oxidation reaction in an H2-air mixture on the breakdown threshold

Abstract: A mathematical model is developed and a numerical analysis is performed for an electric breakdown in a hydrogen-air mixture with a low concentration of H2. It is shown that, at sufficiently low pressures p<10−2 atm, a small molecular-hydrogen additive (η=5×10−5–5×10−3) decreases the reduced field of an electric breakdown in air by a factor of more than 2 because of the appearance of an additional detachment process associated with the chain hydrogen-oxidation reaction. Detailed calculations are performed for the mean number density of negative oxygen ions [O 2 − ]=103 cm−3 and the hydrogen concentration in air [H2]=0.5, 0.05, and 0.005%. It is found that, for [H2]=0.005%, the breakdown can develop under the action of a geoelectric field of 1.3 V/cm at p≃10−4 atm.

ECE measurements via B-X-O mode conversion: A proposal to diagnose the q profile in spherical tokamaks

Abstract: Generation of electron Bernstein waves by the ordinary-extraordinary-Bernstein (O-X-B) mode conversion process has been successfully demonstrated on W7-AS. According to Kirchoff’s law, the inverse process of plasma EC emission by B-X-O mode conversion at particular angles must take place in tokamak plasmas. The optical depth at electron cyclotron harmonics is generally very high for electron Bernstein waves in tokamak plasmas. Consequently the O-mode ECE spectrum measured below the plasma frequency will show steps in the emitted power when each EC harmonic coincides with the upper hybrid resonance zone, where the mode conversion occurs, giving a local measurement of the relationship between the total magnetic field and plasma density. In a spherical tokamak, there are several EC harmonics below the plasma frequency, so several such steps can be observed via the B-X-O mode conversion mechanism. This is a very promising way to get information about the q profile in ST plasmas.

Passage of electromagnetic waves through the critical surface

Abstract: A study is made of the passage of electromagnetic waves through the critical surface at small angles between the plasma density gradient and the magnetic field. Expressions are derived for the transmission and reflection coefficients of electromagnetic oscillations that are periodic in the direction transverse to the density gradient. The penetration of wave beams is also analyzed. In the case of a wide beam, the incident and transmitted ray trajectories are shown to be mirror-image about the resonance surface. Behind the resonance surface, a narrow incident wave beam generates a beam propagating along the magnetic field.

Effect of the transverse magnetic field on turbulence and parameters of a plasma column in the L-2M stellarator

Abstract: The influence of magnetic configurations with magnetic hills or wells on the parameters of a plasma column and turbulence characteristics were studied in experiments in which the plasma was created and heated by a microwave beam at the second harmonic of the electron cyclotron frequency. Calculations show that, for 〈β〉=(1.5−2)×10−, a configuration with a magnetic well takes place and the Mercier criterion for stability of the ideal MHD modes is satisfied. It is shown that the compensation of the Shafranov shift of the plasma column by a transverse (vertical) field (B v /B 0 =5×10−3) leads to a configuration with a magnetic hill in which the Mercier stability criterion is violated in the central region of the plasma column. It is experimentally shown that the stored plasma energy in the magnetic-hill configuration is reduced by one-half in comparison with the magnetic-well configuration. In the case of a magnetic hill, the energy of fluctuations increases both in the plasma core and near the separatrix, and the quasi-regular components of the wavelet spectra grow. When the Shafranov shift is compensated only partially (B v/B 0∼3×10−3) and the system is near the instability threshold, the stored plasma energy and the central electron temperature are somewhat higher, and the radiation power of fast electrons from non-Maxwellian tails at the second harmonic of the electron gyrofrequency decreases. It is found that the wavelet spectra of fluctuations change, the coherence coefficient for spectral components increases, and the radial electric field near the separatrix decreases.

Nonlocal Nature of the Electron Energy Spectrum in a Glow Discharge in Pure O 2 : I. Nonlocal Character of the Electron Distribution Function

Abstract: A study is made of the nonlocal nature of the electron energy distribution function in the positive column of a glow discharge in a tube filled with pure oxygen. The distribution function and the axial (E z) and radial (E r) electric fields as functions of radius are measured using an array of mobile probes. The experimentally obtained spatial profiles of the distribution function are used to test the applicability of the two-term approximation to the distribution function of the electrons with a nonlocal energy spectrum. The distribution function in a specified electric field E=E z +E r (where E z ⊥ E r) is calculated by solving the coordinate-dependent Boltzmann equation in the two-term approximation and by directly integrating the equations of electron motion using the Monte Carlo method. A comparison between the experimental data and the results of simulations carried out for a broad parameter range shows that, in the case of a highly nonlocal electron energy spectrum, the two-term approximation makes it possible to calculate the electron distribution function with a fairly good accuracy, in which case, however, in imposing the boundary conditions, the electron losses at the plasma surface should be treated in the kinetic approximation. It is shown that using the reflection coefficient of the plasma surface for electrons instead of the loss cone in space makes it possible to accurately calculate the electron energy distribution function over the entire parameter range under consideration, including the transient region in which the electron-energy relaxation length is comparable to the characteristic plasma dimension.

Ion Separation by a Curved Magnetic Field in a Multicomponent Plasma

Abstract: It is shown that a curved magnetic field can be used to separate ions in a multicomponent plasma. Without selective ion preheating, the separation over one cycle is inefficient: the separated ion fractions will only be enriched with ions of the corresponding isotopes. Selective ion cyclotron resonance heating makes it possible to achieve essentially a complete separation of the ions.

Effect of the Magnetic Field on the Resonant Particle Acceleration

Abstract: The acceleration of charged particles trapped by a potential wave in a magnetic field is investigated as applied to the problem of the generation of fast particles in a laser plasma The conditions for unlimited particle acceleration are determined, and the spectra of fast particles are found

Three-temperature model for the dynamics of a plasma produced by exploding metal wires

Abstract: An MHD model of the implosion of a dense hot plasma column is developed. Because of a better description of radiation transport, this model has a higher spatial resolution compared to the previously developed, simpler, two-temperature model. The new model is applied to calculating the load (a single metal wire or an X-pinch, in particular, a heterogeneous corona-core structure with a sharp boundary) of a nanosecond high-voltage generator. An algorithm of the type previously used to solve the problem in the two-temperature model is supplemented by the iteration procedure for calculating the quasisteady radiation under the assumption that the plasma is optically thick.

Adiabatic separation of motions and reduced MHD equations

Abstract: A variational method for separating fast and slow motions in quasi-Lagrangian continuous media is proposed, which makes it possible to discard fast stable collective degrees of freedom and to derive simpler (reduced) nonlinear equations describing the adiabatic dynamics of quasi-Lagrangian systems. The method is applied to derive an improved version of the reduced Kadomtsev-Pogutse-Strauss MHD equations that describe the dynamics of a tokamak plasma with steady-state sheared flows, as well as adiabatic equations for two-dimensional modeling of MHD plasma convection near the threshold for flute instability in systems like compact tori.

Wakefield excitation by a relativistic electron bunch in a magnetized plasma

Abstract: The excitation of a wake wave by a relativistic electron beam in an unbounded magnetized plasma and a plasma waveguide is studied theoretically. It is shown that, in a waveguide partially filled with a plasma, the energy that the electrons of the accelerated beam can gain is 37 times higher than the energy of the electrons of the beam generating wakefield.