Ion Beam Density Research Articles

Interferometric phase velocity measurements

Phase velocities of plasma waves near the lower hybrid frequency were measured with an interferometer composed of two spatially separated electron density probes. The plasma waves were produced in the F‐region ionosphere by an argon ion beam. By calculating the normalized cross spectrum of the plasma waves a coherency of .98 was estimated along with a maximum phase difference of π/3 radians between the two probes. This implies that the wavelength was 6 meters compared to an O+ gyroradius of 3.8 meters and that the phase velocity was 45 km/sec compared to an ion beam velocity of 12.4 km/sec. These numbers compare favorably with recent predictions of a nonresonant mode produced by a dense ion beam.

Solar wind deceleration and MHD turbulence in the Earth's foreshock region: ISEE 1 and 2 and IMP 8 observations

The interaction of the solar wind with ions backstreaming from the earth's bow shock is investigated using plasma and magnetic field measurements on ISEE 1 and 2 and IMP 8 at widely separated positions in the earth's foreshock. This technique separates temporal and spatial variations within the foreshock. It is found that the solar wind acceleration associated with backstreaming ions is correlated with the amplitude of the MHD turbulence and that the largest decelerations are seen close to the bow shock. The density of the backstreaming ion beam is strongly correlated with distance from the shock and decreases by about a factor of 3 in a distance of about 3 RE.

High Current Density Implantation and Ion Beam Annealing in Si

ABSTRACTAnnealing of amorphous layers in Si by high flux, selfion irradiation will be discussed. The mechanism for the lattice recovery is presented and related to the structure of the residual damage. It will be shown that highly supersaturated, alloyed regions, free from extended defects, result from the annealing process.

The crossfield current-driven ion-acoustic instability in a collisional plasma

Using the Bhatnagar–Gross–Krook collision model, the effects of charge-transfer and elastic ion-neutral collisions on the crossfield beam-driven ion-acoustic instability in a double plasma device are studied. Elastic collisions cause a slowing of the beam, while charge exchange gives rise to an exponential decay in ion beam density. The resultant damping of the instability is calculated and compared with experiment.

Flow enhancement in magnetically insulated ion injectors

Magnetically insulated gaps have proved useful for the production of high current density ion beams. Although ion flow well beyond conventional space charge limits has been observed, previous theoretical treatments have not predicted such flow enhancement at the magnetic fields typically used. One-dimensional numerical simulations are presented which demonstrate that trapped electron distributions are accessible that cancel much of the effect of ion space charge. This allows ion flow more than an order of magnitude higher than the Child limit. Two processes that result in modified electron distributions have been investigated; the use of a distributed initial electron source and electron diffusion across insulating field lines (specific instabilities were not studied). In both cases, high flow enhancement can be obtained with relatively small electron loss current.

Sputter-enhanced diffusion phenomena in Cu/Ni alloys at elevated temperatures

Compositional changes of the surface and near surface layers were studied in copper-nickel alloys during argon ion bombardment (3 keV) at temperatures up to 873 K, using Auger electron spectroscopy (AES) and x-ray microanalysis (XMA). A long range depletion of copper on the order of 100 nm in thickness was observed depending on the sputtering time, specimen temperature, and ion beam density. An enhanced diffusion of copper was noted in the damaged layer at low temperatures. The apparent activation energy calculated, assuming an Arrhenius type equation, was 5–6 kcal/mol. It is suggested that an interstitial type diffusion mechanism was predominant in the damaged layer.

Long-Pulse Ion Source Using Filament-Free LaB6 Multi-Cathodes

A prototype of a new ion source for neutral beam injection has been developed. To produce a uniform, high current density ion beam over a large area, the discharge electrode rows are arranged as two parallel channels in this source. A power supply system with two discharge current controllers is used to obtain arc discharges smoothly. In this system, the use of series resistors ana diodes was effective in eliminating mutual interaction between the two discharge regions. Preliminary experiments on high-density plasma production were carried out successfully in a quasi-steady mode. The ion current density estimated from the ion saturation current was as high as 0.25 A/cm2 12 cm from the extractor grid surface over an area of 10 cm × 15 cm with a transverse inhomogeneity of less than 10%.

A microprocessor-based beam sweep control unit

Using a microprocessor a beam sweep control module has been developed that facilitates ion beam experiments or ion implantation at a constant ion beam density. The density is regulated by varying the amplitude of an x- y-deflection system. The control unit supplies an analogue output voltage which is employed to adjust this amplitude. In our experiment a commercially available DANFYSIK Model 730 beam sweep system has been modified to enable external amplitude control. The control unit can be operated manually or by computer support. In any case the initial ion beam density and ion dose are to be set manually. Four functions (START, STOP, RESET, optional: SHUTTER) are controlled via push buttons or external signals. Coincident with the START-signal the shutter is opened. The ion charge on the target is now accumulated for 10 s. If deviations larger than 2% full scale from the initial beam density are measured the analogue output voltage of the control unit is changed. In case the initial value cannot be regulated within a limited voltage range, a warning is given (lamp and level). Under normal operation the shutter is closed when the preset ion dose has been accumulated. The control unit is manufactured in a standard NIM housing (four units) and has the following features: analogue instruments for (1) ion current and (2) deviation between initial and actual ion current, (3) three digits and one exponent to preset the ion charge, digital displays for (4) accumulated ion charge and (5) time, and (6) a potentiometer to set the initial control voltage, i.e. the ion beam density. The combination of these features together with the simple three - pushbutton - operation makes the instrument a very versatile tool for ion beam experiments.

Laser-induced fluorescence spectroscopy of an N +2-ion beam

The internal state distribution of an N + 2-ion beam which has been produced in a plasma ion source has been determined by laser-induced fluorescence. The results show a marked deviation from a thermal Boltzmann distribution. Ion-beam densities of less than 500 ions/cm in a particular vibration-rotation state have been measured.

Ion-beam generated ion acoustic solitons in beam plasma system with non-isothermal electrons

Reductive perturbation method has been employed to obtain a modified KdV equation and to show the excitation of ion acoustic solitons in an ion beam-plasma system consisting of bulk of cold ions, beam ions and non-isothermal and resonant electrons (both trapped and free electrons). The effect of ion beam velocity, beam ion density and plasma ion to beam ion mass ration on the amplitude and width of ion acoustic solitons has been investigated. The beam amplification rate of solitons amplitude is found to depend upon the ion beam velocity. There exists a critical ion beam velocity below which the ion beam is unable to generate solitons and this threshold ion beam velocity is an increasing function of the ion beam concentration and the mass ratio of background plasma ions and beam ions.

Evidence for a thermal spike mechanism in the erosion of frozen xenon

Abstract Recent measurements of the sputtering of frozen gases by MeV ions exhibit yields that are several orders of magnitude larger than predicted by collision cascade theory. Of the models considered, only a “thermal spike” model is consistent with our experimental observations. The various experimental parameters studied include the energy (0.3–2.0 MeV), atomic number (H+. He+. N+ and Ar+). angle of incidence and current density of the incident ion beam. the thermal conductivity of the substrate, and the temperature and thickness of the frozen film. Calculations for 1.0 MeV He + in Xe. based on a thermal spike mechanism, indicate that a localized cylindrical region around each incident ion track is raised to a temperature sufficiently high to evaporate a significant quantity of the frozen gas film.

An Ion Beam Pumped X-Ray Laser - A Progress Report

This work reports progress toward an ion-atom collision-pumped, traveling wave x-ray laser. The prototype system now under construction seeks to stimulate the emission of photons from L vacancies induced in intense 50 keV Al+ beams during their passage through thin aluminum films. An analytical study of the conditions required of the medium and the ion sources needed to produce it indicates that carefully focused and bunched ion pulses from an electrohydrodynamic (EHD) source may be pumped to inversion, provided their overall space charge is neutralized by electron pick-up. During recently completed testing of a developmental At EHD source unfocused, unbunched fluxes of about 25% of the desired ion current level were produced. An improved EHD source is now being built and will soon be used in a study of the soft x-ray outputs from foil excited dense ion beams under non-inverted and later, perhaps, inverted conditions.

Slow ions in plasma wind tunnels

One of the limitations of simulation experiments for the study of interaction between a satellite and its space environment is the background of slow ions in the plasma chamber. These ions appear to be created by charge exchange between the beam ions and residual neutral gas and may affect measurements of the current and potential in the wake. Results are presented for a plasma wind tunnel experiment to study the effect of slow ions on both the ion and electron current distribution and the electron temperature in the wake of a body in a streaming plasma. It is shown that the effect of slow ions for beam ion density not exceeding 3 is not significant for measurements of ion current variations in the wake zone. This is not the case when studies are aimed at the quantitative examination of electron current and temperature variations in the near wake zone. In these instances, the measurements of electron properties in the wake should be done at very low system pressures or over a range of system pressures in order to ascertain the influence of slow ions.

Low-energy ion gun (5-50 eV) for the study of the discharge-surface interactions

The current density of a low-energy ion beam (5-50 eV) impinging on a target is limited by the space-charge divergence. The ion gun proposed improves the ion bombardment rate in this energy range by one to two orders of magnitude, by reduction of the drift path of the non-neutral un-neutralized beam. A dense plasma, produced by a low-pressure gas discharge, diffuses in a toroidal magnetic field which separates the neutral component of the plasma from its charged component. This latter is transported along the field lines, the neutrality is conserved up to the vicinity of the target where the electrostatic separation and selection of the ions is achieved. The choice of the plasma source permits ions to be delivered from any element. The design is also able to select the projectiles issued from the plasma.

Plasma Turbulence Generated by Ion-Ion Two-Stream instability

An instability is generated by injecting a high density ion-beam with a large cross-section into an unmagnetized homogeneous plasma. The linear dispersion relation of the wave in the ion-ion two-stream system is experimentally confirmed. Three-dimensional investigations of the turbulence thus produced reveal some new features: The turbulent waves propagate at large angles to the beam with extremely short correlation lengths. At these angles a rapid relaxation of the velocity distribution of the ions is observed until the beam temperature reaches around 0.2 T e . the observed rapid heating of ions can be explained by a theory incorporating the effect of orbit modification.

A double beam ion source

The twin anode electrostatic ion source has been modified to produce two separate ion beams of equal intensity. Ion beam densities of up to 1.5 mA/cm 2 have been obtained for argon. The source can be used to ion etch two specimens at the same time under identical conditions of beam density, ion energy and pressure.

Comments on ``Extraction of Dense Ion Beams from Plasmas''

First Page

Extraction of Dense Ion Beams from Plasmas

An ion optical system suitable for extracting dense ion beams from gas discharges is described. A study of the beam forming characteristics of the system was performed by using an automatic trajectory tracer with space-charge simulation. Experimental measurements show that the extraction system operates satisfactorily over a wide range of current densities; total mercury-ion beam currents in excess of 1 A have been achieved. An empirical method of estimating perveance as a function of geometry, based on electrolytic tank measurements, is in agreement with experimental results.

Ion etching of dental tissues in a scanning electron microscope.

TOOTH sections have been bombarded with a 5-kV argon ion beam inside the specimen chamber of a scanning electron microscope. This apparatus was designed so that the ion beam could be focused into a 1.5-mm diameter spot on the surface of the specimen, covering the area viewed by the microscope. The ions were extracted from a radio-frequency ion source and were focused by two electrostatic lenses on to the specimen, through two apertures. The pressure in the specimen chamber when the ion source was running was better than 10−6 mm mercury. The ion beam density was not uniform over its cross-section, but varied from about 10 m. amp/cm2 at the centre to zero at the edge; this enabled us to check that, for the current densities we utilized, the nature of the changes in the specimen surface that was produced was not a function of the rate of etching. The bombarded area was kept to a minimum to reduce the amount of material deposited on the electron collection system, and reduce the temperature rise, which we have calculated to be less than 100° C, using the values of thermal conductivity given by Craig and Peyton1. The total ion current was kept to about 15µ amp. The micrographs were taken with a 16-kV scanning electron beam and only the high-energy reflected electrons were used to form the picture. This allowed micrographs to be taken with a resolution better than 500 A., even though the specimen tended to charge under the electron beam. This method of observation also allows pictures to be taken while the surface is under bombardment; but for this work it was found preferable alternately to bombard for, say, 5 min, and then observe the surface at leisure. The ion beam made an angle of about 40° to the specimen normal: the direction is indicated by the arrow on the micrographs. The surface was also viewed at an oblique angle, as is indicated by the magnification symbols on the micrographs (Figs. 1 and 2).