Efim M. Oks

Ion charge state distributions of pulsed vacuum arc plasmas in strong magnetic fields

Vacuum arc plasmas with discharge currents of 300 A and duration 250 μs have been produced in strong magnetic fields up to 4 T. Ion charge state distributions have been measured for C, Al, Ag, Ta, Pt, Ho, and Er with a time-of-flight charge-mass-spectrometer. Our previous measurements have been confirmed which show that ion charge states can be considerably enhanced when increasing the magnetic field up to about 1 T. The new measurements address the question of whether or not the additional increase continues at even higher magnetic field strength. It has been found that the increase becomes insignificant for field strengths greater than 1 T. Ion charge state distributions are almost constant for magnetic field strengths between 2 and 4 T. The results are explained by comparing the free expansion length with the freezing length. The most significant changes of charge state distributions are observed when these lengths are similar.

Hollow-cathode plasma electron gun for beam generation at forepump gas pressure

The characteristics, performance, and design feature of a filamentless plasma-cathode electron gun for beam generation in the forepump gas pressure range are presented. The plasma cathode is based on a hollow-cathode direct current (dc) discharge. Using the method of “grid stabilization” it was possible to generate an e beam at a background gas pressure as high as about 10−1 Torr. This pressure can be easily obtained by using mechanical pump only. The operation of the gun with a magnetic field up to 0.1 T was investigated. The presence of a magnetic field (B field) is often required, for instance in plasma chemistry and surface treatment processes. The effect of the B field both on discharge and emission parameters of the gun are observed. The results obtained can be explained based on the concept of electron confinement and motion across the B field. With the accelerating voltage up to 8 kV, the gun is able to generate an electron beam of about 0.7 A dc.

Electron-beam enhancement of ion charge state fractions in the metal-vapor vacuum-arc ion source

We report demonstrations of ion charge-state enhancement for an electron-beam metal-vapor vacuum-arc (E-MEVVA) ion source. Results with a lead cathode yielded a maximum ion charge state of Pb7+, which implies an ionization potential of at least 130 eV. Electron current densities j=70 A/cm2 and ionization times τ≅100 μs produced jτ=9.2×10−3 C/cm2 (5.8×1016 electrons/cm2). Standard analysis for these conditions indicates—somewhat surprisingly—that successive single (stepwise) ionization accounts for the present observations, even though the charge states are substantially higher than most previous results with MEVVA-based ion sources.

Vacuum arc ion sources: recent developments and applications

The vacuum arc ion source has evolved over the past 20 years into a standard laboratory tool for the production of high current beams of metal ions, and is now used in a number of different embodiments at many laboratories around the world. The primary application of this kind of source has evolved to be ion implantation for material surface modification. Another important use is for injection of high current beams of heavy metal ions into the front ends of particle accelerators, and much excellent work has been carried out in recent years in optimizing the source for reliable accelerator application. The source also provides a valuable tool for the investigation of the fundamental plasma physics of vacuum arc plasma discharges. As the use of the source has grown and diversified, at the same time, the ion source performance and operational characteristics have been improved in a variety of different ways also. Here we review the growth and status of vacuum arc ion sources around the world and summarize some of the applications for which the sources have been used.

Electron beam extraction from a broad-beam vacuum-arc metal plasma source

We describe experiments demonstrating the formation of a high current electron beam from a vacuum arc plasma. A preexisting vacuum arc ion source was used, with the extraction voltage reversed in polarity so as to form an electron beam rather than an ion beam; no other changes were required. The beam formed was of energy up to 33 keV, beam current up to 70 A, beam diameter about 10 cm, pulse width 500 /spl mu/s, and energy density up to 25 J/cm/sup 2/. This kind of source can be used for material surface modification.

Boron-rich plasma by high power impulse magnetron sputtering of lanthanum hexaboride

Boron-rich plasmas have been obtained using a LaB6 target in a high power impulse sputtering (HiPIMS) system. The presence of 10B+, 11B+, Ar2+, Ar+, La2+, and La+ and traces of La3+, 12C+, 14N+, and 16O+ have been detected using an integrated mass and energy spectrometer. Peak currents as low as 20 A were sufficient to obtain plasma dominated by 11B+ from a 5 cm planar magnetron. The ion energy distribution function for boron exhibits an energetic tail extending over several 10 eV, while argon shows a pronounced peak at low energy (some eV). This is in agreement with models that consider sputtering (B, La) and gas supply (from background and “recycling”). Strong voltage oscillations develop at high current, greatly affecting power dissipation and plasma properties.

Electrostatic Plasma Lens Focusing of an Intense Electron Beam in an Electron Source With a Vacuum Arc Plasma Cathode

In this paper, we present the research results on focusing and transport of an intense (up to 100 A) nonrelativistic (up to 20 kV) pulsed electron beam using an axially symmetric device with a high-current plasma lens configuration. The electron source is based on electron extraction from the plasma of a hollow-anode vacuum arc discharge. The arc is initiated by a dielectric surface flashover. The emission hole is covered with a fine mesh grid. The beam is extracted and accelerated in a diode-type electro-optical system formed between the grid surface and an open anode plasma boundary. The anode plasma is produced in an electron beam transport channel through residual gas ionization by beam electrons and a plasma lens discharge. The plasma lens configuration of crossed electric and magnetic fields provides an attractive means to obtain a stable low-pressure plasma discharge. This geometry allows the compression of the electron beam in diameter from 6 to 1 cm with more than 100-

Experimentally established correlation between ion charge state distributions and kinetic ion energy distributions in a direct current vacuum arc discharge

{\rm A}/{\rm cm}^{2}

Effect of N2 and Ar gas on DC arc plasma generation and film composition from Ti-Al compound cathodes

beam current density at the collector.

Surface modification of ferritic steels using MEVVA and duoplasmatron ion sources

DC arc plasmas from Al, Ti, Cu, Mo, and W cathodes have been characterized with respect to plasma chemistry and charge-state-resolved ion energy. The evaluated average ionization energies in the plasmas were found to be linearly correlated with the kinetic ion energies. This was further supported by evaluation of previously published data for 42 elements. A comparison of the total ion kinetic energy distribution and the corresponding ion charge state distribution, as defined by the ionization energies of the constituent ions, showed close to equivalent shapes and widths, for all cathodes analyzed. This suggests that the energy provided for ionization and acceleration varies simultaneously during plasma generation in the arc spot. The presented results provide a link between the ionization and acceleration processes, and may provide further insight into the fundamentals of cathode spot evolution and plasma generation.