Positive Ion Beam Research Articles

Power Consumption in Positive Ion Beam Converter with Electrostatic Electron Suppressor

The power recovery characteristics of an in-line direct beam converter provided with electrostatic electron suppressor were studied numerically by tracing the orbits of fast primary ions and secondary charged particles generated along their beam path by collision with background gas molecules. It is shown that, in reference to the electrostatic field potential at the point of impact, the energy distribution of secondary ions impinging on the suppressor has two peaks—one corresponding to a zone of high positive potential surrounding the collector and the other to one of slightly negative potential around the electron suppressor. Secondary electron emission from the suppressor is ascribed mainly to the latter peak, associated with impingement of slower secondary ions. Far much power consumed in secondary particle acceleration is spent for emitting electrons from the suppressor than for secondary ions generated by beam-gas collision. The upper limit of background pressure is discussed on the basis of criteria prescribed for restricting the power consumed in this secondary particle acceleration, as for practical convenience of electrode cooling. Numerical examples are given of calculations based on particle trajectory analysis of both primary ions and secondary particles, for the case of a 100 keV-proton sheet beam 10 cm thick of 35mA/cm2 current density.

Power consumption in positive ion beam converter with electrostatic electron suppressor.

The power recovery characteristics of an in-line direct beam converter provided with electrostatic electron suppressor were studied numerically by tracing the orbits of fast primary ions and secondary charged particles generated along their beam path by collision with background gas molecules. It is shown that, in reference to the electrostatic field potential at the point of impact, the energy distribution of secondary ions impinging on the suppressor has two peaks—one corresponding to a zone of high positive potential surrounding the collector and the other to one of slightly negative potential around the electron suppressor. Secondary electron emission from the suppressor is ascribed mainly to the latter peak, associated with impingement of slower secondary ions. Far much power consumed in secondary particle acceleration is spent for emitting electrons from the suppressor than for secondary ions generated by beam-gas collision. The upper limit of background pressure is discussed on the basis of criteri...

Production of Atomic-Oxygen Negative-Ion Beam by Positive Ion-Induced Sputtering of Semiconductive BaTiO3 Ceramic

When a semiconductive BaTiO3 ceramic surface is bombarded by a keV positive-ion beam, atomic-oxygen negative ions are emitted from the surface in number several tens of times greater than the numbers of other negative ions and neutrals. The dependence of the number of oxygen negative ions on the angle of incidence, the energy and the mass of the bombarding positive ions was investigated, and the ion yield was estimated to be several times greater than 0.1 for the typical conditions. The significance of this finding on techniques for producting negative ions is discussed.

Apparatus for producing controlled positive or negative corona discharge for semiconductor materials processing

An apparatus capable of producing current or voltage controlled positive and negative ion beams in 1-atm oxidizing and nonoxidizing ambients between 25° and 950 °C is presented. The apparatus utilizes a point-to-plane corona discharge in the gas phase. Room-temperature experiments revealed that the small differences in ion beam shape between the two polarities is primarily due to the difference in applied voltage required to produce the same current for the two polarities. Due to increased ion mobility and decreased threshold voltage at higher temperatures, a slight broadening of the ion beam current-density profiles for both positive and negative ions is predicted.

Beam properties of a high-perveance single-aperture dc extraction system

In continuous operation a positive hydrogen ion beam up to 240 mA has been extracted at 50 kV out of a single aperture, corresponding to a perveance of 2.1×10−8 A/V3/2. The effective mass of the extracted ion beam was measured to be about 1.8 amu. The current density distribution of the extracted ion beam was measured with a small calorimetric probe. It was found that the beam distribution strongly depends on the matching between the plasma current density and the extraction voltage. At overmatched conditions the ion beam may even become hollow. The distribution of the extracted ion beam depends on the pressure along the beam path. At pressures above 3×10−4 Torr H2, the distribution was observed to be Gaussian. At a pressure 2×10−5 Torr H2, the intensity in the wings of the beam is relatively high with respect to the intensity in the beam center. The beam divergence decreases with the square root of the extraction voltage. The space-charge potential of the neutralized beam is estimated to be about 5.6 V at 2×10−5 Torr decreasing to 1.6 V at 10−3 Torr independent of the beam energy.

Production of a dense low energy positive hydrogen ion beam

We present measurements and computer calculations concerning the generation of a dense, quasi dc (10 s) positive hydrogen ion beam of energies between 1 and 5 keV. Maximum beam current densities ranging from 200 to 230 mA cm −2 have been transported through a three-electrode extraction-deceleration system with divergencies between 9.5 and 3.5°. The measurements are compared to trajectory calculations, carried out with a modified version of the SLAC code. The highest transmission for a low energy beam with a small divergence is obtained when the calculated beam trajectories exhibit a cross-over in the region between the extraction and the decel electrode. This cross-over can be achieved when the extraction slit has a shaping angle (45°) smaller than the Pierce angle (67.5°).

High power beams for neutral injection heating

High power neutral beams are commonly used for the heating of magnetically confined plasmas in the fusion programme throughout the world. These beams are generated by the acceleration of multi-ampere beams of positive ions extracted from large area ion sources. The paper describes the design of such systems, with special reference to the prototype of the 5 MW long-pulse accelerator being developed for use on the Joint European Torus (JET) tokamak.

Scanning MeV-ion microprobe for light and heavy ions

The University of Oregon scanning ion microprobe uses a 65 cm focal length plasma lens to form 8.65 × demagnified image of an object aperture. The plasma lens focuses a positive ion beam using the self-electric field of a trapped cylindrical column of electrons of density 3−9 × 10 9 cm −3 and length 13–18 cm. Since the focusing field is electric, the focal length depends only on ion accelerating voltage and not on ion mass or charge state. Our 5 MV Van de Graaff accelerator illuminates the object aperture with a current density of ∼ 0.5 pA/mu;m 2. The lens aperture is defined by a set of slits 3.35 m beyond the object aperture slits and 2.79 m from the lens. Four pairs of deflection plates are located between the intermediate aperture and the lens. Two pairs of plates are used for each scanning direction so the beam always passes through the lens center during rastering. The 1 kV operational amplifiers that drive these plates combine three sets of signals. Computer generated voltages raster the beam. Individual dc offset voltages align the beam with the lens axis. A small 60 Hz signal cancels the effects of background 60 Hz magnetic fields along the beam line. With 1 kV rastering voltage the rastered field at the focal plane is 3 mm square for 3 MeV ions. Focal spot size is now 10 μm with a 2 mm diameter lens aperture and 5 μm with a 0.5 mm lens aperture.

Neutral satellites in ion beams from liquid metal sources

A strong electric field acting on the tip of a needle anode covered with a thin layer of liquid metal draws out a cusp from which a beam of positive ions is emitted. However, the rate of loss of ionic mass was found to be only a fraction of the total mass loss while the larger part consisted of microparticles of size less than 1 mu m with a low specific positive surface charge. The presence of a neutral component was proven by examining the particles that are adjuncts to the decaying ion beam. It transpired that the main beam constituents are accompanied by relatively few fast neutral atomic 'satellites' with energies spread over a very wide range.

Application of thermal ionization mass spectrometry to diffusion studies in solids: Diffusion of halogens in transition metals

A study concerning the diffusion of halogen atoms implanted at low energy (E < 4 keV) in the vicinity of some transition metal surfaces is presented. Desorption kinetics of the negative halogen ions formed by negative surface ionization are recorded by sampling the negative ion signal during the off-time of the incident positive ion beam. The general features of the diffusion of halogens atoms are discussed from the kinetics observed at different temperatures for various energies of the incident beam. We show that the comparison of the experimental curves with those obtained from a computer simulation of the diffusion flux of implanted species using a one dimensional model allows the determination of the diffusion coefficients.

Boron isotope ratio measurement by negative thermal ionization mass spectrometry

Boron isotope ratio measurements are carried out by negative thermal ionization mass spectrometry. For that purpose, a boron compound together with a lanthanum nitrate addition is deposited on the filament of a single-filament thermal ion source. At a filament temperature of 1000°C negative boron dioxide ions are measured, producing an ion intensity which is a factor 100–1000 higher in comparison with the positive thermal ion beam. Without lanthanum addition, the attainable negative ion beam is lower by a factor of 0.001–0.0001. Using boracic acid, borax and sodium fluoborate for sample, relative external standard deviations of 0.08 % are available for the boron isotope ratio measurement.

Radiation Protection Considerations of Ion Implantation Systems

Ion implanters produce beams of positive ions that impinge on target materials located in an enclosed vacuum system and are not intended to produce external beams or any other external ionizing radiation. The deceleration of positive ions produces negligible bremsstrahlung external to the implanters. The deceleration of electrons when striking beam-tube components produces bremsstrahlung resulting in significant quantities of unintended external radiation. These electrons are created by the interaction of positive ions with component parts of the implanter and with the residual gas and are subsequently accelerated in the implanter beam tube) At implanter ion energies (< 500 keV), the bremsstrahlung is emitted in lobes essentially perpendicular to the electron stream (1800 from the ion beam) modified by self-shielding from implanter structures. Experimentally, the radiation exposure rate is generally proportional to the electron current and to the square of the electron energy in essential agreement with theory. Implanter x-ray spectra are also in general agreement with spectra taken under laboratory conditions. Principles for reducing external radiation include the judicious combination of implanter design, materials with low atomic number for electrons to strike, cleanliness in assembly, use of suppression fields to reduce the number of electrons, shielding, interlocks, and operation. The exposure rate limits can vary from the federal and state value of 0.5 milliroentgens per hour for electronic products to various lower values which may be established for other countries or by individual manufacturers.

Double bifurcation optimization for reduction of secondary loading in neutral beam injectors

From the solution to coupled Poisson-Vlasov equations that solves the plasma sheath explicitly, an electrode design for the minimization of neutralizer ion induced secondary loading is considered. The loading using such a design is reduced by a factor of 330, virtually eliminating it. This has application to steady-state neutral beam injectors based on intense positive ion beams.

The Frankfurt PIG ion source

A small sized, versatile PIG ion source with end extraction is described by which positive ion beams of most of the elements can be produced with intensities in the microampere range. The source operates either with gas or with a mixture of gas and metal vapor. In the latter case the supply of the metal atoms to the gas discharge is done by evaporation or sputtering depending on the kind of metal. A characteristic feature of the source is the small power consumption so that cooling of the source housing by air only is sufficient.

An inorganic resist for ion beam microfabrication

We report the properties of an inorganic material as a positive ion-beam resist. The ion beam (H+) exposure characteristics of g–GexSe1−x chalcogenide amorphous glass films were investigated. These films exhibit higher sensitivity as ion-beam resists than they do as positive photo- and electron-beam resists. Pattern replication and delineation with this resist material is demonstrated with a conformal gold mask.

Desorption of halogens from Nb, Mo, Ta and W polycrystalline surfaces

The desorption kinetics of halogens from Nb, Ta, Mo and W polycrystalline surfaces are studied, at low coverage (Θ ⩽ 10 −2 of a monolayer) and high temperature (1800–2400 K), using a pulsed ionic beam method. For a relatively high energy incident positive ion beam (ϵ = 2500 eV), the diffusion kinetics of implanted Br and I ions towards the surface are observed. At low energy (ϵ < 500 eV), a first order kinetic corresponding to the desorption of halogens from these surfaces is found. Preliminary results are given on the desorption energy and the preexponential factor of halogens adsorbed on Nb, Mo, Ta and W polycrystalline surfaces. The main result of this systematic study is that the mean adsorption lifetime for a given temperature, as the desorption energy decreases for F through I on all of the studied surfaces. A similarity between Nb and Ta, as between Mo and W surfaces for the desorption energies of halogens is also found.

Formation of hydrogen negative ions by surface and volume processes with application to negative ion sources

During the last few decades interest in negative-hydrogen ion sources has been directed mainly toward synchrotron and other particle accelerator applications, with emphasis on high current densities delivered for short pulses. But within the last several years there has been an awareness in the magnetic fusion program of the future need for negative ions as a means for generating high energy neutral beams, beams with energies above a few hundred keV. Negative ions seem to be the only effective intermediary for efficiently producing such beams. Although methods for generating negative ion beams have relied upon synchrotron concepts, the requirements for fusion are very different: here one is interested in more moderate current densities, up to 100 m A cm/sup -2/, but with continuous operation. Proposed source modules would accelerate of the order of 10 A of beam current and deliver several megawatts of beam power. Both H/sup -/ and D/sup -/ beams are being considered for application in different reactor systems. The conceptualization of negative ion sources is now in a very volatile stage. But of the great variety of proposals that have been offered to date, three general areas appear ready for development. These are: first, the double chargemore » exchange method for converting a positive ion beam into a negative ion beam; second, electron-volume processes wherein low energy electrons interacting with molecular species lead to negative ion products via dissociative attachment or recombination; and third, generation of negative ions in surface interactions, principally via desorption and backscattering. Both our qualitative and our quantitative understanding of these processes diminishes as one proceeds from the first through the third. The physics of these three methods is considered in detail.« less

Spacecraft charging due to positive ion emission: An experimental study

Results have been obtained from the flight of a sounding rocket payload instrumented to determine the cause and extent of charging of vehicles in the ionosphere during the ejection of energetic charged particles. Vehicle potential was measured using several independent probes while beams of positive ions or electrons were ejected. Positive and negative potentials were created by the ejection of electrons and positive ions respectively. For constant ion currents of 8 microamperes and energies of 1 keV, vehicle potentials ranged from volts to hundreds of volts, depending on ambient plasma density but independent of neutral density and vehicle pitch angle. A maximum vehicle potential greater than 1 kV was obtained during the emission of 400 microamperes of 2 keV positive xenon ions.

Analysis of particle species evolution in neutral-beam injection lines

Analytic solutions to the rate equations describing the species evolution of a multispecies positive ion beam of hydrogen due to charge exchange and molecular dissociation are derived as a function of the background gas (H2) line density in the neutralizing gas cell and in the drift tube. Using the solutions, calculations are presented for the relative abundance of each species as a function of the gas-cell thickness, the reionization loss in the drift tube, and the neutral-beam power as a function of the beam energy and the species composition of the original ion beam.

Space charge neutralization regions for mixed positive and negative ion beams

As in the previous studied case of space charge neutralization of positive ion beam by electron injection, there are necessary conditions on mean and thermal speeds at the mixing position, for the downstream mixed positive and negative ion beam to be stable and space charge neutral.