Beams of fast neutral atoms and molecules in low-pressure gas-discharge plasma

Fast neutral atom and molecule beams have been studied, the beams being produced in a vacuum chamber at nitrogen, argon, or helium pressure of 0.1–10 Pa due to charge-exchange collisions of ions accelerated in the sheath between the glow discharge plasma and a negative grid immersed therein. From a flat grid, two broad beams of molecules with continuous distribution of their energy from zero up to e(U + U _c ) (where U is voltage between the grid and the vacuum chamber and U _c is cathode fall of the discharge) are propagating in opposite directions. The beam propagating from the concave surface of a 0.2-m-diameter grid is focused within a 10-mm-diameter spot on the target surface. When a 0.2-m-diameter 0.2-m-high cylindrical grid covered by end disks and composed of parallel 1.5-mm-diameter knitting needles spaced by 4.5 mm is immersed in the plasma, the accelerated ions pass through the gaps between the needles, turn inside the grid into fast atoms or molecules, and escape from the grid through the gaps on its opposite side. The Doppler shift of spectral lines allows for measuring the fast atom energy, which corresponds to the potential difference between the plasma inside the chamber and the plasma produced as a result of charge-exchange collisions inside the cylindrical grid.     

Thermospray liquid chromatography—mass spectrometry with a quadrupole ion trap mass spectrometer

A thermospray ion source using corona discharge ionization was interfaced to a quadrupole ion trap mass spectrometer via a multi-element lens system. Ions were injected into the trap periodically where they were stabilized by collisions with helium bath gas. Mass spectra were recorded on the trapped ions using the mass-selective instability scan mode. Data are shown for a peptide and a nucleoside and the effects of some experimental variables on the spectra are explored.     

Some aspects of ion sheath dynamics under the action of pulsed voltage

The time evolution of an unsteady cathode ion sheath in plasma has been studied numerically. The calculations demonstrate the presence of spikes in the ion current density and electric field strength at the cathode. The amplitudes of these spikes significantly exceeds the steady-state level, which agrees with data from other studies. Approximate formulas are derived that describe the spike amplitudes as functions of the rise time and amplitude of the applied voltage pulse, the mass and charge of ions, and the density and temperature of plasma electrons.     

Kinetic theory of a gas-discharge positive column and wall sheath

The positive column and wall sheath in a gas discharge are studied with allowance for ion collisions in a plasma and ion reflection from a solid surface under conditions of incomplete ion neutralization. The kinetic equation for ions in a positive column is reduced to a Fredholm equation of the second kind. This makes it possible to solve the kinetic equation using a resolvent and thereby derive a single integrodifferential equation for the potential, which is referred to as a generalized plasma-sheath equation. Specific versions of the plasma-sheath equation are obtained that take into account charge exchange of the ions in a plasma and the thermal spread in velocities of the ionization-produced ions.     

Ion flows from a beam-plasma discharge

The results of measurements of the energy distribution function of ions escaping from a beam-plasma discharge are compared with the data from probe measurements in the discharge region. It is shown that, on the discharge axis, there is a region with a higher degree of ionization, whose position depends on the external parameters, in particular, on the gas pressure. The mean energy of the ions that leave the plasma from the outside of this region is determined by the potential of the plasma column. Inside the region with a higher degree of ionization, there is an additional mechanism for ion acceleration; as a result, the energy of the ions that leave the plasma from this region is higher than the energy of the electrostatically accelerated ions by a factor of 1.5 to 5. The results obtained show promise for creating a plasma-processing reactor with controlled ion parameters for the purposes of treating materials for microelectronics.     

EUV spectroscopy of plasmas created in the final anode-cathode gap of the Z-Machine high-current pulsed generator (SNL)

The effect of short-circuit across the final anode-cathode gap of powerful pulsed current generators could hamper efficient power delivery to the Z-pinch plasma. To study this effect, a novel EUV diagnostics of plasmas created in the final section of the transmission line (the anode-cathode gap near the main load) of the Z-Machine high-current generator (Sandia National Laboratories, United States) was developed. The work included developing spectroscopic instruments, theoretical and experimental studies of EUV spectra of iron ions in well-diagnosed laser-produced plasmas, and a comparison of these spectra with those of plasmas created in the final anode-cathode gap of the transmission line. The EUV spectra of highly charged Fe ions in the spectral range λ ～ 20–800 Å were investigated. In experiments performed at Sandia National Laboratories, spectra of FeXIII-FeXVII ions were observed. A comparison of the measured and calculated spectra shows that the electron plasma temperature in the anode-cathode gap is T _e ～ 200 eV.     

Bis(trifluoromethyl)aryl derivatives for drug analysis by gas chromatography electron capture negative ion chemical ionization mass spectrometry. Application to the measurement of low levels of clonidine in plasma

A gas chromatographic mass spectrometric assay for clonidine in plasma with a detection limit of a few picograms per ml was required. The p-trifluoromethylbenzyl, pentafluorobenzyl and pentafluorobenzoyl derivatives of clonidine were synthesized and the electron capture negative ion chemical ionization mass spectra of these compounds show extensive fragmentation with prominent ions at m/z 35 and 37 due to the two chlorine atoms in the clonidine molecule. Selected ion monitoring of specific high mass ions in these mass spectra indicated that the required sensitivity could not be obtained with these derivatives. Several bis(trifluoromethyl)pyrimidines were synthesized and these compounds were found to give an intense negative ion current under conditions of resonance electron capture. Consequently, a derivative of clonidine containing a bis(trifluoromethyl)aryl group was synthesized by reacting the drug with 3,5-bis(trifluoromethyl)benzoyl chloride. The negative ion mass spectrum of the reaction product has a base peak at m/z 673 and, when this ion is specifically monitored, an amount of derivative equivalent to 1 picogram of clonidine can be detected. This allowed the development of an assay for clonidine in plasma with a precision of 8% (SD) at 50 pg ml-1, 22% (SD) at 20 pg ml-1 and a lower limit for quantitative determination of 10 pg ml-1. Plasma concentrations of clonidine in 10 subjects given a single 50 micrograms oral dose are reported.     

Self-sustained low pressure glow discharge with a hollow cathode at currents of tens of amperes

Self-sustained glow discharge with a hollow cathode was studied at high discharge currents (up to 30 A). Using a grid analyzer placed on the side flange of the hollow cathode, the ion and electron currents flowing in the cathode sheath were measured. At a discharge current of 30 A, pressure of 0.2?C2 Pa, and plasma density of 10¹¹ cm^?3, the coefficient of secondary ion-electron emission ?? calculated from the experimental data is found to be 0.1?C0.15. The dependences of the plasma parameters on the area of the small anode placed inside the larger hollow cathode are determined.     

Experimental study of the ion flux parameters and anode plasma dynamics in the Angara-5-1 facility

Results are presented from experimental studies of the anode plasma dynamics and measurements of the ion flux ejected along the axis of a high-current Z-pinch. Pinch discharges were formed by the implosion of tungsten wire arrays in the Angara-5-1 facility. It is shown that the ion energy spectrum depends on the mass and configuration of wire arrays, as well as on the diameter of the anode aperture. The shape of the ion spectrum indicates that the plasma propagates in the form of a compact plasmoid. Shadow and X-ray images of the plasma show that the axial velocity of the plasma outflowing through the anode aperture is comparable with the velocity of radial plasma compression and, for tungsten ions, can reach a value corresponding to an energy of 100 keV. The experimental data indicate that the ion energy spectrum mainly forms due to the electrodynamical acceleration of the plasma and cumulative jets. A possible mechanism for the production of compact plasma formations in the course of electrodynamic plasma acceleration is discussed.     

Simulation of oscillatory processes in a beam-plasma system with a virtual cathode in gas-filled interaction space

Physical processes occurring in an intense electron beam with a virtual cathode in an interaction space filled with neutral gas are studied in a two-dimensional model. A mathematical model is proposed for investigating complicated self-consistent processes of neutral gas ionization by the beam electrons and the dynamics of an electron beam and heavy positive ions in the common space charge field with allowance for the two-dimensional motion of charged particles. Three characteristic dynamic regimes of the system are revealed: complete suppression of oscillations of the virtual cathode as a result of neutralizing its space charge by positive ions; the pulsed generation regime, in which the ions dynamics repeatedly suppresses and restores the virtual cathode oscillations; and the continuous generation regime with an anomalously high level of noisy oscillations.     

Experimental study of the dynamics of neutron emission from the GOL-3 multimirror trap

In experiments on the plasma heating and confinement in the GOL-3 multimirror trap, a deuterium plasma with a density of ～10¹⁵ cm^?3 and an ion temperature of 1–2 keV is confined for more than 1 ms. The plasma is heated by a relativistic electron beam. The ion temperature, which was measured by independent methods, reached 1.5–2 keV after the beginning of the beam injection. Since such a fast ion heating cannot be explained by the classical energy transfer from electrons to ions through binary collisions, a theoretical model of collective energy transfer was proposed. In order to verify this model, a new diagnostics was designed to study the dynamics of neutron emission from an individual mirror cell of the multimirror trap during electron beam injection. Intense neutron bursts predicted by this model were detected experimentally. Periodic neutron flux modulation caused by the macroscopic plasma flow along the solenoid was observed. The revealed mechanism of fast ion heating can be used to achieve fusion temperatures in the multimirror trap.     

Investigation of plasma characteristics in an unbalanced magnetron sputtering system

Results are presented from experimental studies of a magnetron sputtering system for different configurations of the magnetic field above the cathode surface. The current-voltage characteristics of a magnetron discharge at different working gas pressures (0.08–0.3 Pa) and currents in the unbalancing coil were studied. The production and transport of charge carriers in a magnetron discharge with an unbalanced magnetic field was investigated by means of probe measurements of plasma characteristics and ion energies in the region between the substrate and the magnetic trap at the cathode surface. The radial distributions of the ion current density, plasma potential, and floating potential in the unbalanced operating mode are found to have pronounced extrema at the magnetron axis. It is shown that the plasma density near the substrate can be increased considerably when the axial magnetic field is high enough to efficiently confine plasma electrons and prevent their escape to the chamber wall.     

Plasma Decay in the Afterglow of High-Voltage Nanosecond Discharges in Unsaturated and Oxygenated Hydrocarbons

Results of experimental and theoretical study of plasma decay in the afterglow of high-voltage nanosecond discharges in gaseous ethylene and dimethyl ether at room temperature and pressures from 2 to 20 Torr are presented. Using a microwave interferometer, the time behavior of the electron density in the range from 2 × 10¹⁰ to 3 × 10¹² cm^–3 during plasma decay is investigated. By processing the experimental data, the effective coefficients of electron–ion recombination as functions of the gas pressure are obtained. It is found that these coefficients substantially exceed the recombination coefficients of simple hydrocarbon ions. This distinction, as well as the increase in the effective recombination coefficient with pressure, is explained by the formation of cluster ions in three-body collisions, which recombine with electrons more efficiently than simple molecular ions. The coefficients of three-body conversion of simple molecular ions into cluster ions in the plasmas of ethylene and dimethyl ether, as well as the coefficients of recombination of electrons with cluster ions in these gases, are determined by analyzing the experimental data.     

Electric-field enhancement and ion-flow focusing at the multiwire cathode of a high-current plasma-filled diode

The time evolution of an unsteady ion sheath in the presence of cylindrical protrusions at the cathode is studied numerically in a two-dimensional model. The calculations demonstrate the enhancement of the electric field and the focusing of the ion flow at the protrusion vertices. It is shown that, in the stage of the sheath formation, in addition to the electrostatic focusing of the ion flux, there is also focusing caused by the curvature of the plasma boundary around a protrusion. The results of calculations agree satisfactorily with the experimental data on the delay time of explosive emission at the multiwire cathode of a high-current plasma-filled diode.     

Self-consistent modeling of radio-frequency plasma generation in stellarators

A self-consistent model of radio-frequency (RF) plasma generation in stellarators in the ion cyclotron frequency range is described. The model includes equations for the particle and energy balance and boundary conditions for Maxwell’s equations. The equation of charged particle balance takes into account the influx of particles due to ionization and their loss via diffusion and convection. The equation of electron energy balance takes into account the RF heating power source, as well as energy losses due to the excitation and electron-impact ionization of gas atoms, energy exchange via Coulomb collisions, and plasma heat conduction. The deposited RF power is calculated by solving the boundary problem for Maxwell’s equations. When describing the dissipation of the energy of the RF field, collisional absorption and Landau damping are taken into account. At each time step, Maxwell’s equations are solved for the current profiles of the plasma density and plasma temperature. The calculations are performed for a cylindrical plasma. The plasma is assumed to be axisymmetric and homogeneous along the plasma column. The system of balance equations is solved using the Crank-Nicholson scheme. Maxwell’s equations are solved in a one-dimensional approximation by using the Fourier transformation along the azimuthal and longitudinal coordinates. Results of simulations of RF plasma generation in the Uragan-2M stellarator by using a frame antenna operating at frequencies lower than the ion cyclotron frequency are presented. The calculations show that the slow wave generated by the antenna is efficiently absorbed at the periphery of the plasma column, due to which only a small fraction of the input power reaches the confinement region. As a result, the temperature on the axis of the plasma column remains low, whereas at the periphery it is substantially higher. This leads to strong absorption of the RF field at the periphery via the Landau mechanism.     

A phenomenological model of the current filamentation instability driven by cathode processes in the Livermore spheromak

The current density on the open field lines of the Livermore spheromak (SSPX) typically exceeds the saturation current density of the bulk plasma. We assume that the mechanism that provides conditions for that is associated with the formation of a thin layer near the cathode surface, where both the plasma and the neutral density are higher than in the bulk plasma and where intense ionization occurs. The ions formed in this layer fall back onto the cathode, whereas electrons contribute to the high current density in the bulk plasma. The particle balance in the ionizing layer is determined by the recycling coefficient, which, in turn, depends on the cathode temperature and the sheath voltage. As it turns out, these dependences give rise to an instability that leads to the current filamentation and the formation of hot spots on the cathode surface. The instability can be characterized in a phenomenological manner without going into the details of the structure of the ionizing layer, whose effect on the instability shows up in the form of a couple of numerical coefficients of the order of one. We predict the characteristic size and the shape of the filaments (and the hot spots), which are in a general agreement with discoloration patterns on the surface of the cathode in the SSPX. If the magnetic field is tilted to the surface, the footpoints of the filaments move with a significant velocity, whose direction depends on the ratio of the ion gyroradius and the thickness of the ionizing layer. This instability, although primarily considered in conjunction with the SSPX experiment, may play a role in spherical tokamaks and other systems with coaxial helicity injection.     

Effect of ion collisions on the parameters of dust-plasma structures

A study is made of how collisions of ions with gas atoms affect the parameters of an ion flow and the interaction between dust grains, as well as their interaction with the flow. The ion velocity distribution in a gas discharge is analyzed with allowance for both resonant charge exchange of the ions with parent gas atoms and polarizing collisions. The interaction forces between a dust grain and an ion flow and among the grains due to the charge exchange of ions with gas atoms near the grain are examined.     

Simulations of a surface glow discharge in a supersonic gas flow in the presence of external ionization

Results of two-dimensional hydrodynamic simulations of a surface glow discharge operating at pressures of 0.2–0.5 Torr in a nitrogen flow propagating with a velocity of 1000 m/s in the presence of external ionization are presented. The effect of the external ionization rate on discharge operation is analyzed. The current-voltage characteristics of the discharge are calculated for different intensities of external ionization in both the presence and absence of secondary electron emission from the cathode. The discharge structure and plasma parameters in the vicinity of the loaded electrode are considered. It is shown that, when the discharge operates at the expense of secondary emission from the cathode, the discharge current and cathode sheath configuration are insensitive to external ionization. It is also demonstrated that, even at a high rate of external ionization, the discharge operates due to secondary emission from the cathode.