A. Anders

Pulsed dye laser diagnostics of vacuum arc cathode spots

The ignition and arc phases of vacuum arcs were investigated using differential dye laser absorption photography with simultaneous high spatial (micrometer) and temporal (nanosecond) resolution. The discharge duration was 800 ns, the current 50-150 A, the electrode material copper, and the cathode-anode distance less than 50 mu m. A 0.4 ns laser pulse (tunable, gamma =480-530 nm) was used to obtain momentary absorption photographs of the cathode region. During ignition, an optically thick anode plasma expanded toward the cathode, decaying within 25 ns after bridging the electrode gap. In the arc phase, a fragmentary structure of the cathode spots was observed in situ for the first time. The microspots have a characteristic size of 5-10 mu m. They appear and disappear on a nanosecond time scale. The plasma density of the microspots was estimated to be greater than (3-6)*10/sup 26/ m/sup -3/. >

Streaming metal plasma generation by vacuum arc plasma guns

We have developed several different embodiments of repetitively pulsed vacuum arc metal plasma gun, including miniature versions, multicathode versions that can produce up to 18 different metal plasma species between which one can switch, and a compact high-duty cycle well-cooled version, as well as a larger dc gun. Plasma guns of this kind can be incorporated into a vacuum arc ion source for the production of high-energy metal ion beams, or used as a plasma source for thin film formation and for metal plasma immersion ion implantation and deposition. The source can also be viewed as a low-energy metal ion source with ion drift velocity in the range 20–200 eV depending on the metal species used. Here we describe the plasma sources that we have developed, the properties of the plasma generated, and summarize their performance and limitations.

Time dependence of vacuum arc parameters

Time-resolved investigations of the expanded plasma of vacuum arc cathode spots are described, including the study of the ion charge state distribution, the random cathode spot motion, and the crater formation. It was found that the ion charge state distribution changes over a timescale on the order of hundreds of microseconds. For the random spot motion two timescales were observed: a very short spot residence time of tens of nanoseconds which gives, combined with the step width, the diffusion parameter of the random motion, and a longer timescale on the order of 100 mu s during which the diffusion parameter changes. Crater formation studies by scanning electron microscopy indicate the occurrence of larger craters at the end of crater chains. The existence of a timescale much longer than the elementary times for crater formation and spot residence can be explained by local heat accumulation. >

Hollow-anode plasma source for molecular beam epitaxy of gallium nitride

GaN films have been grown by molecular beam epitaxy (MBE) using a hollow‐anode nitrogen plasma source. The source was developed to minimize defect formation as a result of contamination and ion damage. The hollow‐anode discharge is a special form of glow discharge with very small anode area. A positive anode voltage drop of 30–40 V and an increased anode sheath thickness leads to ignition of a relatively dense plasma in front of the anode hole. Driven by the pressure gradient, the ‘‘anode’’ plasma forms a bright plasma jet streaming with supersonic velocity towards the substrate. Films of GaN have been grown on (0001) SiC and (0001) Al2O3 at 600–800 °C. The films were investigated by photoluminescence, cathodoluminescence, x‐ray diffraction, Rutherford backscattering, and particle‐induced x‐ray emission. The film with the highest structural quality had a rocking curve width of 5 arcmin, the lowest reported value for MBE growth to date.

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.

Measurement of total ion flux in vacuum are discharges

A vacuum arc ion source was modified allowing us to collect ions from arc plasma streaming through an anode mesh. The mesh had a geometric transmittance of 60%, which was taken into account as a correction factor. The ion current from twenty-two cathode materials was measured at an arc current of 100 A. The ion current normalized by the arc current was found to depend on the cathode material, with values in the range from 5% to 11%. The normalized ion current is generally greater for light elements than for heavy elements. The ion erosion rates were determined from values of ion current and ion charge states, which were previously measured in the same experimental system. The ion erosion rates range from 12-94 μg/C.

Thermal instability of a super-high-pressure xenon discharge

Needlelike voltage pulses were detected when the burning voltage of super-high-pressure xenon discharge lamps was investigated. Above a certain current, the voltage pulses disappeared abruptly. The rise time of each pulse was determined to be shorter than 1 ns. The characteristic values of the phenomenon monotonically depend on the discharge current and cover the following parameter range: pulse duration, 3.4-3.8 ns; pulse amplitude, 2-5.1 V; and repetition frequency, 2.5-33 MHz. It is argued that the generation of voltage pulses is due to thermal instability of the plasma bulk, based on a higher rate of electron gas heating by the electric field rather than cooling by collisions of electrons with heavy particles. The results of an appropriate instability analysis are in agreement with the experimental observations. >

A periodic table of ion charge state distributions observed in the transition region between vacuum sparks and vacuum arcs

Ion charge state distributions have been measured with high time resolution for short and long arc pulses. Data for 3 /spl mu/s and quasi-steady state are given for most conductive elements in a Periodic Table. The mean ion charge states can be fitted by functions of the form Q~=Q~/sub t/spl rarr//spl infin// [1+A exp(-t//spl tau/)] where A is an enhancement function that depends on the power density. For the present conditions, A/spl sim/1 and /spl tau//spl sim/50 /spl mu/s.

Peristaltic ion source (invited)

Conventional ion sources generate energetic ion beams by accelerating the plasma‐produced ions through a voltage drop at the extractor, and since it is usual that the ion beam is to propagate in a space which is at ground potential, the plasma source is biased at extractor voltage. For high ion beam energy the plasma source and electrical systems need to be raised to high voltage, a task that adds considerable complexity and expense to the total ion source system. We have developed a system which though forming energetic ion beams at ground potential as usual, operates with the plasma source and electronics at ground potential also. Plasma produced by a nearby source streams into a gridded chamber that is repetitively pulsed from ground to high positive potential, sequentially accepting plasma into its interior region and ejecting it energetically. We call the device a peristaltic ion source. In preliminary tests we’ve produced nitrogen and titanium ion beams at energies from 1 to 40 keV. Here we describe the philosophy behind the approach, the test embodiment that we have made, and some preliminary results.Conventional ion sources generate energetic ion beams by accelerating the plasma‐produced ions through a voltage drop at the extractor, and since it is usual that the ion beam is to propagate in a space which is at ground potential, the plasma source is biased at extractor voltage. For high ion beam energy the plasma source and electrical systems need to be raised to high voltage, a task that adds considerable complexity and expense to the total ion source system. We have developed a system which though forming energetic ion beams at ground potential as usual, operates with the plasma source and electronics at ground potential also. Plasma produced by a nearby source streams into a gridded chamber that is repetitively pulsed from ground to high positive potential, sequentially accepting plasma into its interior region and ejecting it energetically. We call the device a peristaltic ion source. In preliminary tests we’ve produced nitrogen and titanium ion beams at energies from 1 to 40 keV. Here we describe...