Mechanism for ion acceleration along the normal to the axis of a beam-plasma discharge in a weak magnetic field

The mechanism responsible for the previously discovered phenomenon of acceleration of an ion flow along the normal to the axis of a beam-plasma discharge in a weak magnetic field is investigated. It is suggested that the ions are accelerated in the field of a helicon wave excited in the discharge plasma column. It is shown theoretically that, under actual experimental conditions, a helicon wave can be excited at the expense of the energy of an electron beam. The spectral parameters and spatial structure of the waves excited in a beam-plasma discharge in the frequency ranges of Langmuir and helicon waves are studied experimentally and are shown to be related to the parameters of the ion flow. Theoretical estimates are found to agree well with the experimental results.     

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.     

Control of power characteristics of ion flow in plasma-etching reactor based on beam-plasma discharge

It is shown that on the basis of the earlier revealed effect of generating the ion flow in the beam-plasma discharge from the discharge axis, a plasma processing reactor can be created for low-energy etching of semiconductor structures. The possibility of easily controlling the density and energy of ion flow by means of varying the potential of the discharge collector is demonstrated. The charge compensation of the ion flow incident on the nonconducting surface is implemented using the modulation of the potential of the substrate holder as well as the plasma-potential modulation.     

Acceleration of ions in a beam-plasma discharge in a low magnetic field: Interrelation between the electron and ion energy distributions

Previous experiments revealed the effect of stable acceleration of ions in a plasma-beam discharge in a low magnetic field to energies one order of magnitude higher than the electron thermal energy. To verify the previously proposed mechanisms for this effect, the velocity distribution function of the electrons arriving at the collector and the energy distribution of the ions escaping from the discharge transversely to the axis were measured. It is found that ion acceleration is accompanied by significant electron heating near the discharge axis. The time behavior and longitudinal profile of the intensity of the excited high-frequency oscillations in the frequency range ω ～ ω _pe were studied. The accumulation of regular oscillations in the beam-injection region and their stochastization during the propagation along the system axis were observed. The experimental results correlate qualitatively with the data of previous numerical simulations.     

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.     

Ion flux onto a nonconducting surface in a beam-plasma discharge: Simulation and experiment

Computer simulations of a plasma sheath formed near a nonconducting surface in a beamplasma discharge (BPD) at a low magnetic field in the reactor chamber are performed. The results of computer simulations are compared with experimental data. The optimal mode of GaAs etching in a BPD-based plasma processing reactor is chosen using results of computer simulations. Simulations for carbon (DLC) film sputtering modes in the framework of a similar model are also performed.     

Effect of an external RF field on the beam-plasma discharge in a magnetic field

The effect of an RF field on a steady-state beam-plasma discharge with a plane electrode placed parallel to a sheetlike electron beam is studied experimentally. The plasma parameters were measured by a single probe, and the electron distribution function was determined with the use of an electrostatic analyzer. The energy and current of the electron beam were E _B=2.5 keV and J _B=0.05–1.5 A, respectively. The working pressure was p=2×10^?5–10^?3 torr. The frequency of the external RF field was 13.56 MHz. Both the steady-state regimes in which the RF field had no effect on the plasma parameters and regimes with a pronounced effect of the RF field were observed. The experiments show that the regime of the discharge depends strongly on the plasma density and the magnetic field. The parametric instability is studied theoretically in the weak-turbulence approximation. It is shown that, due to the decay nature of the spectrum of plasma oscillations, the onset of instability is accompanied by the transfer of the energy of fluctuations over the spectrum, from the pump frequency toward its harmonics.     

Electron beam relaxation in nonuniform plasma of a steady-state beam-plasma discharge at moderately low gas pressures

Electron beam relaxation in plasma under conditions typical of laboratory plasma devices based on a steady-state beam-plasma discharge was investigated. It is shown that the measured dependences of the beam loss factor in a discharge operating at a moderately low gas pressure disagree with theoretical dependences calculated for a longitudinally uniform plasma. Analytic dependences obtained in the framework of quasilinear theory with allowance for longitudinal plasma inhomogeneity agree with experimental data. Some effects caused by the influence of the main discharge parameters on electron beam relaxation are analyzed.     

Ion flux onto conducting and isolated surfaces in the beam-plasma discharge: Computer simulation

A physical model which allows the use of the program code KARAT for simulating the quasisteady state of the beam-plasma discharge with plasma regeneration from a neutral gas is developed. The results of simulation of the modes of discharge at different potentials at the discharge collector are reported. The results obtained for isolated and grounded ion collectors are compared.     

Electromagnetic beam-plasma interactions in a magnetic field

The excitation of plasma oscillations in a thin-walled annular plasma by an annular electron beam in a cylindrical waveguide is considered in the linear approximation. The instability growth rates and spatial amplification coefficients in the beam-plasma system under the conditions of the Cherenkov and anomalous Doppler resonances are obtained and compared with those in a transversely homogeneous system. The contributions from different instability mechanisms are analyzed.     

Peculiarities of the beam-plasma discharge development in high-pressure gas and prospects of its application

The dynamics of a stationary beam-plasma discharge at the neutral gas pressure of 10⁻³-1 Torr is investigated. Three discharge modes have been revealed: two of them are characterized by novel properties and have not been studied as of yet. Dissipative instabilities with different increments have been found. The generation of ion flows has been studied and the experiments on “dry” cleaning of the metal specimen surfaces using them have been carried out. The pressure ranges in a neutral gas at which the discharge can be efficiently applied in nonequilibrium plasma chemistry are given.     

PADC nuclear track detector for ion spectroscopy in laser-plasma acceleration

The transparent polymer polyallyl-diglycol-carbonate (PADC), also known as CR-39, is widely used as detector for heavy charged particles at low fluence. It allows for detection of single protons and ions via formation of microscopic tracks after etching in NaOH or KOH solutions. PADC combines a high sensitivity and high specificity with inertness towards electromagnetic noise. Present fields of application include laser-ion acceleration, inertial confinement fusion, radiobiological studies with cell cultures, and dosimetry of nuclear fragments in particle therapy. These require precise knowledge of the energy-dependent response of PADC to different ion species. We present calibration data for a new type of detector material, Radosys RS39, to protons (0.2–3 MeV) and carbon ions (0.6–12 MeV). RS39 is less sensitive to protons than other types of PADC. Its response to carbon ions, however, is similar to other materials. Our data indicate that RS39 allows for measuring carbon ion energies up to 10 MeV only from the track diameters. In addition, it can be used for discrimination between protons and carbon ions in a single etching process.     

Electric potential of a macroparticle in beam-plasma systems

Dusty plasma research is one of the most extensively developed areas of modern plasma physics. In spite of the large number of papers devoted to determining the electric potential of dust grains in gas discharge plasmas, this problem is still far from being resolved completely. In this paper, the behavior of the floating electric potential of a macroparticle in an electron beam-plasma system is studied in the framework of the orbit motion limited model taking into account secondary electron emission.     

Collective ion acceleration during the decay of a high-current Z-pinch

A study is made of the Z-pinch plasma expansion after the current is switched off. Measurements were carried out in experiments on the implosion of tungsten wire arrays in the Angara-5-1 facility. It is found experimentally that, at a distance of 2 m from the pinch, the ion velocity in the expanding Z-pinch plasma is about (2.5–4.0) × 10⁷ cm/s, which substantially exceeds the thermal velocity of tungsten ions. A model describing the plasma expansion process is proposed that is based on the ambipolar acceleration mechanism. The results of numerical simulations are compared with the experimental data.     

Possible mechanism of nitric oxide production from N-hydroxy- -arginine or hydroxylamine by superoxide ion

It has been speculated that N^G-hydroxy- -arginine (OH- -Arg), which is an intermediate in NO production from -arginine, may be converted to NO by superoxide ion. However, there is still no direct evidence for this conversion. In the present study this was investigated using superoxide ion generated either in acellular or cellular systems. It was found that OH- -Arg and hydroxylamine were converted to nitrite and nitrate apparently via NO by superoxide ion in aqueous solution. Arginine remained unaffected. These changes were observed during reaction of chemical substances as well as in a biological system (zymosan-activated macrophages in culture). Superoxide dismutase prevented this transformation. OH- -Arg was also spontaneously hydrolysed to hydroxylamine and -citrulline, however this occurred at pH> 9 only. Activated microsomes (containing different isoforms of cytochrome P450) were unable to replace NO-synthase in its ability to produce OH- -Arg from -arginine. These data support the hypothesis that a pathway alternative to the well-known synthesis of NO by NO-synthase via OH- -Arg exists. This pathway may involve the production of OH- -Arg by NO-synthase and decomposition of OH- -Arg to NO by the action of superoxide ion. Alternatively, hydrolysis of OH- -Arg to hydroxylamine may occur followed by its oxidation to NO, again by superoxide ion.     

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.