Estimation of the electron density and radiative energy losses in a calcium plasma source based on an electron cyclotron resonance discharge

The parameters of a calcium plasma source based on an electron cyclotron resonance (ECR) discharge were calculated. The analysis was performed as applied to an ion cyclotron resonance system designed for separation of calcium isotopes. The plasma electrons in the source were heated by gyrotron microwave radiation in the zone of the inhomogeneous magnetic field. It was assumed that, in such a combined trap, the energy of the extraordinary microwave propagating from the high-field side was initially transferred to a small group of resonance electrons. As a result, two electron components with different transverse temperatures—the hot resonance component and the cold nonresonance component—were created in the plasma. The longitudinal temperatures of both components were assumed to be equal. The entire discharge space was divided into a narrow ECR zone, where resonance electrons acquired transverse energy, and the region of the discharge itself, where the gas was ionized. The transverse energy of resonance electrons was calculated by solving the equations for electron motion in an inhomogeneous magnetic field. Using the law of energy conservation and the balance condition for the number of hot electrons entering the discharge zone and cooled due to ionization and elastic collisions, the density of hot electrons was estimated and the dependence of the longitudinal temperature T _e∥ of the main (cold) electron component on the energy fraction β lost for radiation was obtained.     

Parametric turbulence-sustained gas discharges

A brief review is given of papers on the RF production of a plasma whose electrons are heated due to the parametric turbulence driven by an alternating electric pump field and maintain the discharge by ionizing the working gas atoms. Results are summarized from studies of low-frequency parametric turbulence, specifically, ion-acoustic plasma turbulence in a magnetic field, ion-cyclotron turbulence associated with the excitation of ion Bernstein modes, and lower hybrid turbulence in a plasma with ions of one or two species. The turbulence level and the rate of turbulent heating of the electrons and ions are presented, and the results of modeling of these phenomena are described. Attention is focused on experiments in which low-frequency parametric turbulence may be observed.     

Low-pressure discharge induced by microwave radiation with a stochastically jumping phase

Results are presented from experimental studies on the unique beam-plasma generator of microwave radiation with a stochastically jumping phase (MWRSJP). To interpret the experimental results, a computer code was developed that allows one to simulate the process of gas ionization by electrons heated in the MWRSJP field and the behavior of plasma particles in such a field. The conditions for ignition and maintenance of a microwave discharge in air by MWRSJP are found both experimentally and theoretically, and the pressure range in which the power required for discharge ignition and maintenance is minimum are determined.     

Investigation of the electron energy distribution function in hollow-cathode glow discharges in nitrogen and oxygen

The mechanism for the formation of the inverse electron distribution function is proposed and realized experimentally in a nitrogen plasma of a hollow-cathode glow discharge. It is shown theoretically and experimentally that, for a broad range of the parameters of an N₂ discharge, it is possible to form a significant dip in the profile of the electron distribution function in the energy range ε=2–4 eV and, accordingly, to produce the inverse distribution with df(ε)/d?>0. The formation of a dip is associated with both the vibrational excitation of N₂ molecules and the characteristic features of a hollow-cathode glow discharge. In such a discharge, the applied voltage drops preferentially across a narrow cathode sheath. In the main discharge region, the electric field E is weak (E<0.1 V/cm at a pressure of about p～0.1 torr) and does not heat the discharge plasma. The gas is ionized and the ionization-produced electrons are heated by a beam of fast electrons (with an energy of about 400 eV) emitted from the cathode. A high-energy electron beam plays an important role in the formation of a dip in the profile of the electron distribution function in the energy range in which the cross section for the vibrational excitation of nitrogen molecules is maximum. A plasma with an inverted electron distribution function can be used to create a population inversion in which more impurity molecules and atoms will exist in electronically excited states.     

One-dimensional model of a helicon discharge

A simplified model describing the steady state of a helicon discharge in a low-pressure plasma is considered. The electron Langmuir frequency of the plasma produced by the discharge is shown to be much higher than the electron gyrofrequency. It is found that the gas medium is ionized and the electrons are heated primarily by the extraordinary mode. The calculated electron density depends nonmonotonically on the magnetic field, in agreement with the results of numerous experiments.     

Spatial distribution of the RF power absorbed in a helicon plasma source

The spatial distributions of the RF power absorbed by plasma electrons in an ion source operating in the helicon mode (ω _ci < ω < ω _ce < ω _pe ) are studied numerically by using a simplified model of an RF plasma source in an external uniform magnetic field. The parameters of the source used in numerical simulations are determined by the necessity of the simultaneous excitation of two types of waves, helicons and Trivelpiece-Gould modes, for which the corresponding transparency diagrams are used. The numerical simulations are carried out for two values of the working gas (helium) pressure and two values of the discharge chamber length under the assumption that symmetric modes are excited. The parameters of the source correspond to those of the injector of the nuclear scanning microprobe operating at the Institute of Applied Physics, National Academy of Sciences of Ukraine. It is assumed that the mechanism of RF power absorption is based on the acceleration of plasma electrons in the field of a Trivelpiece-Gould mode, which is interrupted by pair collisions of plasma electrons with neutral atoms and ions of the working gas. The simulation results show that the total absorbed RF power at a fixed plasma density depends in a resonant manner on the magnetic field. The resonance is found to become smoother with increasing working gas pressure. The distributions of the absorbed RF power in the discharge chamber are presented. The achievable density of the extracted current is estimated using the Bohm criterion.     

Creation of a long magnetized plasma column in a metal chamber

A method for creation of a long magnetized column of dense hydrogen plasma in a metal chamber by means of a high-current linear discharge is considered. It is the main method for the formation of preliminary plasma in the GOL-3 multimirror trap, in which a plasma column with a length of up to 12 m and diameter of 8 cm, suitable for conducting experiments on the injection of a relativistic electron beam, was obtained. Conditions for stable discharge operation in the density range of 3 × 10¹⁹–10²² m^?3 are determined, including a discharge with a uniform longitudinal plasma density profile and incomplete initial ionization of hydrogen. It is demonstrated that the system is capable of operating in a magnetic field with a variable configuration and strength of up to 6 T in the solenoidal section and up to 12 T in the end mirrors. It is shown that an important role in the development of a discharge is played by fast electrons with energies corresponding to the initial applied voltage (about 25 kV), which provide primary gas ionization. The properties of low-temperature plasma in such a discharge are discussed.     

Plasma Parameters Controlled by a Movable Ion Sheath

It is shown that plasma parameters, such as the electron density, electron temperature, and plasma potential, in multidipole discharge plasma can be controlled by a negatively biased movable metal plate. Here, plasma is produced in the target region by a flux of energetic electrons coming from the source region of a double plasma device. Further, the thickness of the ion sheath formed in front of the biased metal plate varies depending on its axial position inside the cage.     

Double layer in a cylindrical hollow-cathode discharge

A dc cylindrical coaxial glow discharge with an inner grid anode has been studied. The region between the two electrodes is seen dark, while a brightly glowing region forms inside the grid anode up to the center. The current-voltage characteristic of a dc cylindrical glow discharge in nitrogen is similar to that of a normal glow discharge, while the normal glow discharge voltage decreases with increasing pressure. The minimum plasma potentials are observed in the hollow cathode region due to the accumulation of electrons at the back of the grid anode. At the center, some of the passed electrons are converged, so their potential is decreased. These electrons have a sufficient time to be redistributed to form one group with a Maxwellian electron energy distribution function. The electron temperature measured by electric probes varies from 1.6 to 3.6 eV, while the plasma density varies from 3.9 × 10¹⁶ to 7 × 10¹³ m⁻³, depending on the discharge current and probe position. The plasma density increases as the electrons move radially from the grid toward the central region, while their temperature decreases.     

Plasma electron-emitting source in a low-pressure beam-plasma discharge in a stationary plasma thruster

A new type of plasma electron-emitting source capable of increasing the temperature of plasma electrons behind the edge of a stationary plasma thruster (SPT) to 7–15 eV has been developed and investigated experimentally. For the same parameters of the main discharge, the thrust, the thrust efficiency, the mass use factor, and the lifetime of the “SPT anode unit-plasma electron-emitting source” assembly are found to increase substantially as compared to a thruster equipped with a conventional cathode compensator. Simultaneously, the neutral particle pressure required for the existence of self-consistent distributions of the electric field and charged particle density in the drift space of the neutralized ion beam decreases appreciably. It is shown that the volume of the region in which primary slow ions are produced increases with increasing ionization frequency. Three additional channels for discharge control are implemented. The ranges in which the discharge parameters can be controlled are extended.     

Investigations of high-energy electrons of the microwave discharge plasma at configuration of the “Magnetor” Bi-dipole magnetic confinement system by X-ray radiation analyses

The results of the investigations of a group of fast electrons in a microwave discharge plasma in the “Magnetor” magnetic trap are presented. The data on the presence and location of this group of electrons is important for estimating the total plasma pressure taking the previous probe measurements into account. Fast electrons are found to be localized within the magnetic separatrix in the region of confinement of the main plasma. The maximal energy of fast electrons is higher than 25 keV.     

Interaction of microwave radiation undergoing stochastic phase jumps with plasmas or gases

New types of beam-plasma devices generating intense stochastic microwave radiation in the interaction of electron beams with hybrid plasma waveguides were developed and put into operation at the National Science Center Kharkov Institute of Physics and Technology (Ukraine). The objective of the paper is to discuss the results of theoretical and experimental studies and numerical simulations of the normal and oblique incidence of linearly polarized electromagnetic waves on an interface between a vacuum and an overcritical plasma. The main results of the reported investigations are as follows: (i) for the parameter values under analysis, the transmission coefficient for microwaves with a stochastically jumping phase is one order of magnitude greater than that for a broadband regular electromagnetic wave with the same spectral density; (ii) the electrons are heated most efficiently by obliquely incident waves with a stochastically jumping phase and, in addition, the electron distribution function has a high-energy tail; and (iii) necessary conditions for gas breakdown and for the initiation of a microwave discharge in stochastic fields in a light source are determined. The anomalously large transmission coefficient for microwaves, the anomalous character of the breakdown conditions, the anomalous behavior of microwave gas discharges, and the anomalous nature of collisionless electron heating, are attributed to stochastic jumps in the phase of microwave radiation.     

Beam discharge excited by distributed virtual cathode

A new type of beam discharge, i.e., beam discharge with a distributed virtual cathode (VC) is proposed and considered by numerical simulation. The discharge is established during counter motion of high-current electron beams in a gas-filled equipotential cavity and is characterized by a state of hot dense electron plasma of primary electrons. The discharge temporal dynamics is studied. It is shown that the VC lifetime depends linearly from this sum in a wide range of the sum of beam currents, from the boundary current of two-beam instability to the critical current of Pierce instability. Generation of nonlinear electrostatic structures shaped as phase bubbles in the discharge is detected, and their dynamics is studied. The parameters are determined, at which the multiple coexistence of phase bubbles and their coalescence during collisions is observed.     

Model for calculating low-current moderate-pressure RF discharges with photon-driven secondary electron photoemission from the electrode surface

A model is developed for calculating a low-current moderate-pressure RF discharge with allowance for an electron cloud that is formed by electrons produced during the preceding periods of the RF field and fills the electrode sheath at regular intervals in accordance with the phase of the RF voltage applied to the electrodes. The cloud arises due to a phase shift of π/2 between the voltage across the sheath and that across the column of a low-current RF discharge plasma. The photon generation mechanism is as follows: as the cloud electrons fill the sheath, they acquire energy in superelastic collisions with metastables produced by the sheath electrons during the preceding periods of the RF field and then excite the metastable states to emitting levels. The discharge sheath forms due to the overlap of the secondary electron avalanches triggered by electron photoemission from the electrode surface. The parameters of the sheath in a low-current RF discharge are determined by the conditions under which the electron photoemission current in the sheath is self-sustaining, but the capacitive susceptance of the sheath is substantially higher than its active electrical conductance. The results of calculations are compared with the experimental data.     

Optical characteristics and parameters of the plasma of a barrier discharge excited in a mixture of mercury dibromide vapor with nitrogen and helium

Results are presented from experimental and theoretical studies of the optical characteristics and parameters of the plasma of an atmospheric-pressure barrier discharge excited in a HgBr₂: N₂: He mixture, which was used as the working medium of a small-size (with a radiation area of 8 cm²) exciplex gas-discharge radiation source. The mean radiation power of 87 mW was achieved at the radiation wavelength λ_max = 502 nm. The electron energy distribution function, the transport characteristics, the specific energy lost in the processes involving electrons, the electron temperature and density, and the rate constants of elastic and inelastic electron scattering by the components of the working mixture were calculated as functions of the reduced field E/N. The plasma of a discharge excited in a HgBr₂: N₂: He mixture can be used as the working medium of a small-size blue-green radiation source. Such a source can find application in biotechnology, photonics, and medicine and can also be used to manufacture gas-discharge display panels.     

Energy characteristics of beam-plasma interaction in a closed volume

Energy exchange between an electron beam and plasma during a beam-plasma discharge in a closed cavity excited by the electron beam is analyzed using computer simulations by the KARAT code. A method allowing one to analyze the beam-plasma interaction in the quasi-steady stage of the discharge is proposed. Qualitative characteristics of energy exchange (such as beam energy losses and the energy distributions of beam electrons and plasma particles leaving the discharge) both during spontaneous discharge excitation and in the presence of initial beam modulation by regular or noiselike signals are determined. The results obtained enable one to estimate the energy characteristics of a plasma processing reactor based on a beam-plasma discharge.     

Possible mechanism for radial ion acceleration in a beam-plasma discharge

A mechanism is proposed that can lead to radial ion acceleration in a plasma discharge excited by an electron beam in a relatively weak longitudinal magnetic field. The mechanism operates as follows. The beam generates an azimuthally asymmetric slow potential wave, which traps electrons. Trapped magnetized electrons drift radially with a fairly high velocity under the combined action of the azimuthal wave field (which is constant for them) and a relatively weak external longitudinal magnetic field. The radial electron flux generates a radial charge-separation electric field, which accelerates unmagnetized plasma ions in the radial direction. The ion flux densities and energies achievable in experiments with kiloelectronvolt electron beams in magnetic fields of up to 100 G are estimated.     

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

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. __________ Translated from Fizika Plazmy, Vol. 26, No. 5, 2000, pp. 445–454. Original Russian Text Copyright ? 2000 by Zobnin, Nefedov, Sinel’shchikov, Sinkevich, Usachev, Filinov, Fortov.     

On the longitudinal distribution of electric field in the acceleration zones of plasma accelerators and thrusters with closed electron drift

The long-term experience in controlling the electric field distribution in the discharge gaps of plasma accelerators and thrusters with closed electron drift and the key ideas determining the concepts of these devices and tendencies of their development are analyzed. It is shown that an electrostatic mechanism of ion acceleration in plasma by an uncompensated space charge of the cloud of magnetized electrons “kept” to the magnetic field takes place in the acceleration zones and that the electric field distribution can be controlled by varying the magnetic field in the discharge gap. The role played by the space charge makes the mechanism of ion acceleration in this type of thrusters is fundamentally different from the acceleration mechanism operating in purely electrostatic thrusters.