V. P. Frolova

A vacuum spark ion source: High charge state metal ion beams

High ion charge state is often important in ion beam physics, among other reasons for the very practical purpose that it leads to proportionately higher ion beam energy for fixed accelerating voltage. The ion charge state of metal ion beams can be increased by replacing a vacuum arc ion source by a vacuum spark ion source. Since the voltage between anode and cathode remains high in a spark discharge compared to the vacuum arc, higher metal ion charge states are generated which can then be extracted as an ion beam. The use of a spark of pulse duration less than 10 μs and with current up to 10 kA allows the production of ion beams with current of several amperes at a pulse repetition rate of up to 5 pps. We have demonstrated the formation of high charge state heavy ions (bismuth) of up to 15 + and a mean ion charge state of more than 10 +. The physics and techniques of our vacuum spark ion source are described.

Boron ion beam generation utilizing lanthanum hexaboride cathodes: Comparison of vacuum arc and planar magnetron glow

Boron ion beams are widely used for semiconductor ion implantation and for surface modification for improving the operating parameters and increasing the lifetime of machine parts and tools. For the latter application, the purity requirements of boron ion beams are not as stringent as for semiconductor technology, and a composite cathode of lanthanum hexaboride may be suitable for the production of boron ions. We have explored the use of two different approaches to boron plasma production: vacuum arc and planar high power impulse magnetron in self-sputtering mode. For the arc discharge, the boron plasma is generated at cathode spots, whereas for the magnetron discharge, the main process is sputtering of cathode material. We present here the results of comparative test experiments for both kinds of discharge, aimed at determining the optimal discharge parameters for maximum yield of boron ions. For both discharges, the extracted ion beam current reaches hundreds of milliamps and the fraction of boron ions in the total extracted ion beam is as high as 80%.

Generation of boron plasma in vacuum arc with lanthanum hexaboride cathode

The mass–charge composition of vacuum-arc plasma in discharge with lanthanum hexaboride cathode has been experimentally studied. It is established that this cathode material ensures the generation of plasma with high (up to 90%) content of boron ions. Temporal variation of the plasma composition and boron ion fraction during discharge pulse and changes in the mass–charge composition of plasma with increasing pressure in the region of discharge operation have been studied.

Plasma of Vacuum Discharges: The Pursuit of Elevating Metal Ion Charge States, Including a Recent Record of Producing Bi 13+

Metal ions in the plasma of vacuum discharges are commonly multiply charged with ion charge states from 1+ to 3+, reaching 4+ and 5+ for some metals. The elevation of metal ion charge states in vacuum discharge plasma is an interesting challenge for plasma physics because it requires a deeper understanding of the processes leading to a more intense ionization of the electrode material. It also has practical implications, for example, for metal ion sources: elevation of ion charge state leads to a proportional increase in ion beam energy for a given accelerating voltage. During the last two decades, various techniques have been used to increase the ion charge states, including: 1) application of a strong magnetic field to the cathode region of the vacuum arc; 2) application of supplemental microwave power to the discharge plasma; 3) injection of an electron beam into the discharge area; and 4) application of a short current pulse to the discharge as to transiently increase the discharge voltage and power, emulating the conditions of a high-current vacuum spark. In this paper, we briefly survey the different techniques of metal ion charge state elevation and then present new experimental results by utilizing the spark regime and combining it with a strong pulsed magnetic field applied to the cathode region. Beams of ions with high charge state, up to a record Bi13+, were extracted from vacuum spark plasma. It is argued that the addition of a magnetic field to the spark plasma magnetizes the electrons and limits plasma expansion, which leads to an increase in the electron temperature relative to the free expansion case and to an increase in the likelihood of electrons to cause ionizing collisions.

Charge state, angular distribution, and kinetic energy of ions from multicomponent-cathodes in vacuum arc devices

We present research results on vacuum arc plasma produced with multicomponent cathode made of several different elements. The ion mass-to-charge-state spectra of the plasmas were studied by time-of-flight spectrometry. The angular distributions of different ion species were measured, and the kinetic energy of their directed (streaming) motion was determined. It is shown that the fractional composition of ions of different cathode components in the plasma flow from the cathode spot closely matches the fractional content of these components in the composite cathode. The charge states of ions of the various cathode components are determined by the average electron temperature in the cathode spot plasma. The angular distribution of lower mass ions in the plasma from a multicomponent cathode is less isotropic and broader than for the plasma from a single-component cathode of the same light element. The directed kinetic energies of the ions of the different components for plasma from a multicomponent cathode are lower for lighter elements and greater for heavier elements compared to the ion directed energy for plasmas from single-component cathodes made of the same materials. The physical processes responsible for these changes in the ion charge states in multicomponent-cathode vacuum arc plasma are discussed.

Generation of hydrogen isotope ions in a vacuum arc discharge with a composite zirconium deuteride cathode

The mass and charge composition of the plasma of a vacuum arc with thick and film-type zirconium cathodes containing deuterium and hydrogen is investigated experimentally and theoretically. For a thick cathode, it is shown that such a system ensures effective generation of deuterium ions with an integral fraction per arc current pulse of approximately 60%; the maximal concentration of deuterium is observed at the initial stage of the arc operation. In the case of the film cathode, such a concentration of hydrogen isotopes can be attained for currents exceeding 400 A and for an arc duration at a level of a few tens of microseconds. Occlusion of deuterium in the cathode leads to additional energy expenditures for its ionization and, as a consequence, a decrease in the average charge of ions of the cathode material in the arc plasma. Deuterium in the cathode spot is ionized completely, and the drift velocity of its ions almost coincides with the velocity of ions of the cathode material due to the high frequency of ion-ion collisions in the cathode region. The interaction of a dense (∼1020 cm−3) cathode-spot plasma with microinhomogeneities of the cathode surface leads to the development of thermal instability in them over time intervals that do not exceed tens of nanoseconds.

Plasma mass-charge composition of a vacuum arc with deuterium saturated zirconium cathode

An experimental study of the mass-charge composition of a vacuum arc plasma with zirconium cathode saturated with deuterium is carried out. It is shown that this system provides effective generation of deuterium ions with an integral fraction of about 60% per pulse of an arc current. The deuterium content in a vacuum arc plasma is maximum in the initial stage of discharge burning and considerably decreases during the first 150 μs of an arc current pulse. The cathode deuteration also leads to decreasing average ion charge of metallic substrate in the plasma of a vacuum arc discharge.

Effect of the axial magnetic field on a metallic gas-puff pinch implosion

The effect of an axial magnetic field Bz on an imploding metallic gas-puff Z-pinch was studied using 2D time-gated visible self-emission imaging. Experiments were performed on the IMRI-5 generator (450 kA, 450 ns). The ambient field Bz was varied from 0.15 to 1.35 T. It was found that the initial density profile of a metallic gas-puff Z-pinch can be approximated by a power law. Time-gated images showed that the magneto-Rayleigh–Taylor instabilities were suppressed during the run-in phase both without axial magnetic field and with axial magnetic field. Helical instability structures were detected during the stagnation phase for Bz < 1.1 T. For Bz = 1.35 T, the pinch plasma boundary was observed to be stable in both run-in and stagnation phases. When a magnetic field of 0.3 T was applied to the pinch, the soft x-ray energy was about twice that generated without axial magnetic field, mostly due to longer dwell time at stagnation.

Lifetime of hydrogenated composite cathodes in a vacuum arc ion source

The paper reports on a study of the mass-charge state of the plasma produced in a vacuum arc discharge with composite cathodes which were copper-disk coated with a hydrogenated Zr film of thicknesses 9, 22, and 35 μm. The cathodes allow the generation of multicomponent gas and metal ion beams with a hydrogen ion content from several to several tens of percent. Also investigated is the dependence of the H ion fraction in a beam on the Zr film thickness during erosion to the point of disappearance of Zr peaks in mass-charge spectra. The ability of the vacuum arc system to produce H ions is analyzed by analyzing the cathode lifetime as a function of the film thickness and pulse repetition frequency.

An experimental setup for studying the interaction of dense supersonic plasma flows with an arched magnetic field

We propose a new experimental approach to laboratory investigations of the interaction of supersonic (ionic Mach number up to 2.7) high-density (up to 1015 cm–3) plasma flows and inhomogeneous magnetic field (up to 3.3 T in magnetic mirrors of arched magnetic trap). This approach offers wide possibilities for modeling processes taking place in both near-Earth and space plasma.