Alexey A. Goncharov

High-current plasma lens

The static and dynamic characteristics of a high-current plasma lens (PL) with a two-component quasi-neutral plasma medium formed by a wide-aperture, repetitively pulsed ion beam and secondary electron emission are investigated experimentally. Such a system provides magnetic isolation of electrons and ion beam focusing by means of electric fields due to uncompensated space charge. It is shown that careful selection of the magnetic field geometry, the number of lens electrodes, and the external potential distribution applied to them according to theoretical plasma optics principles allows the radial electric potential within the PL volume to be tailored to widely varying profiles. It is shown that there is a significant influence of the ion beam itself on the potential shape. Lens collective processes due to nonremovable radial gradients of the magnetic field in the ion beam focusing direction are investigated. It is shown that the conductivity mechanism and lens electrode leakage are connected with observed small-scale turbulence. A theoretical analysis of the linear growth phase of an instability is presented, and estimates are made of the nonlinear variable potential amplitude. >

Focusing and control of multiaperture ion beams by plasma lenses

The characteristics of focusing and control of well-formed low-divergence multiaperture ion beams by a plasma lens (PL) are investigated. It is shown that spherical aberrations introduced by the departure of the radial electric potential profile from parabolic, significantly influence the focusing within the PL volume. It is established that consistent use of theoretical plasma optics principles allows the reduction of spherical aberrations and evaluation of the maximum density of the beam focused. It is shown experimentally and theoretically that the controlled introduction of spherical aberrations permits control of the radial beam profile at the target and, in particular, of the beam homogeneity. >

Focusing of high-current, large-area, heavy-ion beams with an electrostatic plasma lens

We report on measurements of the focusing of high-current, large-area beams of heavy metal ions using an electrostatic plasma lens. Tantalum ion beams were formed by a repetitively pulsed vacuum arc ion source, with energy in the 100 keV range, current up to 0.5 A, initial beam diameter 10 cm, and pulse length 250 μs. The plasma lens was of internal diameter 10 cm and length 20 cm, and had nine electrostatic ring electrodes with potential applied to the central electrode of up to 7 kV, in the presence of a pulsed magnetic field of up to 800 G. The current-density profile of the downstream, focused, ion beam was measured with a radially moveable, magnetically suppressed, Faraday cup. The tantalum ion-beam current density at the focus was compressed by a factor of up to 30. The results are important in that they provide a demonstration of a means of manipulating high-current ion beams without associated space-charge blowup.

Manipulating large-area, heavy metal ion beams with a high-current electrostatic plasma lens

We describe some experimental investigations of the manipulation of high-current, large-area beams of heavy metal ions using a high-current electrostatic plasma lens. Beams of carbon, copper, zinc, and tantalum ions (separately) were formed by a repetitively pulsed vacuum arc ion source, with energy in the range about 10-140 keV, beam current up to 0.5 A, initial beam diameter 10 cm, and pulse length 250 ms. The plasma lens was of 10-cm internal diameter and 20-cm length and had nine electrostatic ring electrodes with potential applied to the central electrode of up to 7 kV, in the presence of a pulsed magnetic field of up to 800 Gauss. The current density profile of the focused beam was measured with a radially moveable, magnetically suppressed Faraday cup. The results show that the focal length of the lens and the profile of the transported beam can be controlled by variation of the lens magnetic field and electrode potential distribution. Under optimum operating conditions the ion beam current density at the focus was compressed by a factor of up to 30. We also carried out some preliminary observations of the effect of the plasma lens on the ion charge state distribution of the beam. Here we outline the principles underlying plasma lens operation and summarize the experimental observations we have made of the lens performance in this large-area. high-current regime.

Role of aberrations in high-current plasma lenses

The influence of spherical and momentum aberrations on the focusing characteristics of intense ion beams in the electrostatic plasma lens (PL) is analyzed. The dependence of the ion beam focus on the uncompensated beam space charge and the finite azimuthal velocity at the PL exit is examined. Minimum focused beam radii are calculated for different system parameters and are shown to depend on the ion charge-to-mass ratio. The radial profiles of focused, multicharged, inhomogeneous ion beams were measured experimentally and compared with the theoretical predictions. We show that the PL, in principle, can be used for ion charge state analysis.

Permanent magnet plasma lens

We have designed and fabricated, for the first time, a simple, compact, and low-cost electrostatic plasma lens based on the use of permanent magnets rather than an electrically driven solenoid to establish the magnetic field. Characteristics of the focused ion beam passed through the lens have been measured. Some of the beam characteristics depend strongly on the applied magnetic field strength and the precise form of the external potential distribution applied along the lens electrodes. The experimental results obtained at the Institute of Physics (Kiev) and at the Lawrence Berkeley National Laboratory (Berkeley) show that this plasma optical device can be used beneficially for focusing and manipulating moderate energy, large area, heavy metal ion beams.

Some characteristics of moderate energy metal ion beam focusing by a high current plasma lens

The results of experiments on moderate energy (4–25 keV) metal ion beam focusing by a high current plasma lens (PL) are presented. The ion beam was produced by a two-chamber vacuum–arc metal vapor vacuum arc (MEVVA)-type ion source. Characteristics of the beam passing through the lens have been measured and the PL focusing properties determined for a wide range of ion beam energy and current. Distributions have been determined of the external potential along the lens electrodes that provide maximum increase of the beam current density on axis. Certain features of metal ion beam focusing by the PL are discussed. Some of these characteristics depend on the method of application of the external electric potential to the lens electrodes.

Plasma Devices Based on the Plasma Lens—A Review of Results and Applications

We review some novel developments of the electrostatic plasma lens and some new plasma devices based on the plasma-optical idea of magnetic insulation of electrons and equipotentialization along magnetic field lines. The plasma lens configuration of crossed electric and magnetic fields provides an attractive and simple method in establishing a stable plasma discharge at low pressure and has been used to develop some cost-effective low-maintenance plasma devices for ion cleaning, surface activation, and polishing of substrates prior to film deposition. Recent embodiments of these devices use permanent magnets and possess considerable flexibility with respect to spatial configuration.

Moderate energy metal ion beam focusing by a high-current plasma lens

Some features of the electrostatic plasma lens as applied to the focusing of moderate-energy, high-current, wide-aperture metal ion beams have been investigated and are described here. Static characteristics within the plasma lens volume have been explored. It is shown that when the potentials applied to the lens electrodes are held constant, the electric field increase with increasing ion beam current is limited by the increase of the potential in the paraxial region of the beam. Some estimates are made of the limiting static electric fields that can be obtained in the plasma lens, based on a quasi-neutral, two-component plasma model with anomalous electron mobility, where it is assumed that the anomalous mobility is caused by the absence of electron correlations when vortex-like structures appear in the hydrodynamically unstable plasma. The results of a theoretical analysis are compared with experimental data. Some results of experiments on the focusing of high-current, large area, heavy metal ion beams are also presented.

Recent advances in plasma devices based on plasma lens configuration for manipulating high-current heavy ion beams.

We describe new results of development of novel generation cylindrical plasma devices based on the electrostatic plasma lens configuration and concept of electrons magnetic insulation. The crossed electric and magnetic fields plasma lens configuration provides us with the attractive and suitable method for establishing a stable plasma discharge at low pressure. Using plasma lens configuration in this way some cost-effective plasma devices were developed for ion treatment and deposition of exotic coatings and the effective lens was first proposed for manipulating high-current beams of negatively charged particles. Here we describe operation and features of these plasma devices, and results of theoretical consideration of mechanisms determining their optimal operation conditions.