A. Hershcovitch

High-pressure arcs as vacuum-atmosphere interface and plasma lens for nonvacuum electron beam welding machines, electron beam melting, and nonvacuum ion material modification

Atmospheric pressure plasmas can be used to provide a vacuum‐atmosphere interface as an alternative to differential pumping. Vacuum‐atmosphere interface utilizing a cascade arc discharge was successfully demonstrated and a 175 keV electron beam was successfully propagated from vacuum through such a plasma interface and out into atmospheric pressure. Included in the article are a theoretical framework, experimental results, and possible applications for this novel interface.

A plasma window for transmission of particle beams and radiation from vacuum to atmosphere for various applications

Many industrial and scientific processes like ion material modification, electron beam melting, and welding, as well as generation of synchrotron radiation are performed exclusively in vacuum nowadays, since electron guns, ion guns, their extractors, and accelerators must be kept at a reasonably high vacuum. Consequently, there are numerous limitations, among which are low production rates due to required pumping time, limits on the size of target objects, and degradation of particle beams and radiation through foils or differentially pumped sections. A novel apparatus, which utilized a short plasma arc, was successfully used to provide a vacuum-atmosphere interface as an alternative to differential pumping. Successful transmission of charged particle beams from a vacuum through the plasma to atmosphere was accomplished. Included in the article are a theoretical framework, experimental results, and possible applications for this novel interface.

Extraction of superthermal electrons in a high current, low emittance, steady state electron gun with a plasma cathode

Major limitations of plasma cathodes have been overcome in an electron gun based on extraction of superthermal electrons from a discharge characterized by a large component of high energy electrons with a low thermal spread. A grid is employed to select these electrons for extraction while retaining the bulk electrons in the discharge. Steady state extraction of electron beams corresponding to over 60% of the total arc discharge current has been observed. This extracted electron current far exceeds the thermal electron flux. A perveance of over 280 microperv was reached with the extraction of 9 A at 1 keV from a 6 mm aperture.

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.

Electron-beam enhancement of the metal vapor vacuum arc ion source

We report detailed investigations of the electron-beam metal vapor vacuum arc (E-MEVVA) ion source. The experiments were performed in Moscow and Tomsk with nearly the same design of ion sources. We recently reported the first conclusive demonstration of electron-beam enhancement of MEVVA performance using lead and bismuth cathodes, which yielded maximum ion charge states of Pb7+ and Bi8+ for E-MEVVA, as compared to Pb2+ and Bi2+ for conventional MEVVA operation. In this article we report extensive results for additional cathode materials, further details of the Moscow and Tomsk ion sources, and a discussion of electron beam effects on E-MEVVA performance. These results can be considered as a proof of the E-MEVVA principle.

High‐intensity H− ion source with steady‐state plasma injection

To satisfy the requirements of a negative‐ion‐based neutral beam line for future fusion applications, an H− ion source was developed using surface production of negative ions and a steady‐state plasma injection from hollow cathode discharges. Steady‐state and stable generation of H− ions on a negatively biased, cesiated converter has been achieved over periods of several hours; H− beam current pulses of 0.3‐A amplitude with 1‐s duration have been extracted over the same period, but steady‐state operation of the extractor has not been achieved yet due to a lack of extractor cooling. The source meets some of the neutral beam line requirements, i.e., the obtained linear current density, background gas pressure, and the proven capability for steady‐state H− ion generation.

Observation of a two‐component electron population in a hollow cathode discharge

A retarding potential technique has been employed to select electrons for extraction from a hollow cathode discharge. In ‘‘normal’’ operating pressures, the electron distribution function is Gaussian like with a superthermal tail. At low operating pressures, the electron distribution function has an additional distinct component of electrons with a very low thermal spread of 0.13 eV and an energy corresponding to the cathode potential.

High Current Energy Recovery Linac at BNL

We present the design and parameters of an energy recovery linac (ERL) facility, which is under construction in the Collider-Accelerator Department at BNL. This R&D facility has the goal of demonstrating CW operation of an ERL with an average beam current in the range of 0.1 - 1 ampere and with very high efficiency of energy recovery. The possibility of a future upgrade to a two-pass ERL is also being considered. The heart of the facility is a 5-cell 703.75 MHz super-conducting RF linac with strong Higher Order Mode (HOM) damping. The flexible lattice of the ERL provides a test-bed for exploring issues of transverse and longitudinal instabilities and diagnostics of intense CW electron beams. This ERL is also perfectly suited for a far-IR FEL. We present the status and plans for construction and commissioning of this facility.

Hollow cathode discharge as a plasma source for H— production

Experiments have been performed on a hollow cathode discharge to test the feasibility of an efficient H— production. When a negatively biased converter electrode was placed in the plasma column of the discharge, a current density of up to 2.7 A/cm2 was recorded. Strong evidence was found for an H— surface production with a lower limit of about 0.3 A/cm2 at the converter.

Results from energetic electron beam metal vapor vacuum arc and Z-discharge plasma metal vapor vacuum arc: Development of new sources of intense high charge state heavy-ion beams

We are exploring a new approach for heavy-ion beam injection (e.g., into the relativistic heavy-ion collider at BNL), as well as new sources of intense high charge state ions to be mounted on a relatively low voltage platform for high energy ion implantation. While conventional metal vapor vacuum arc (Mevva) ion sources can produce up to hundreds of milliamps or more of several-times-ionized metal ions (e.g., U3+), the recent results from Batalin et al. indicate that the addition of an energetic electron beam may lead to considerably higher charge states. An alternative way to produce the electron beam is where a Z-discharge plasma is used to enhance multiple ionization. As the vacuum arc plasma plume expands into a magnetized drift region, a Z-discharge is triggered in the drifting metal plasma. The ions are then extracted and analyzed using a time-of-flight system. We report initial results using these schemes with applied discharge and electron beam voltages from 1 to 2 kV.