Atomic Ion Fraction Research Articles

Contribution of thermal dissociation during steel nitriding in electron beam plasma

We report the results of measurements of mass-to-charge ion composition of the beam plasma generated by a forevacuum plasma-cathode electron source in a nitrogen atmosphere during electron beam interaction with the surface of a steel sample. It has been found that as the sample surface temperature increases from 540 ℃ to 930 ℃, the fraction of atomic nitrogen ions in the spectrum increases by 4-10%. This increase in atomic nitrogen occurs at the backdrop of decreasing number of molecular nitrogen monoxide ions and is associated with thermal dissociation of its molecules.

Effects of gas pressure and bias potential on electron beam nitriding of titanium

We have studied the effects of gas pressure and bias potential on ion-plasma nitriding of titanium in an electron beam-produced plasma generated in the forevacuum pressure range. We present the results of scanning electron microscopy, x-ray analysis, and tribological studies of the modified layers of nitrided samples. The results of measurements are given, and the correlation between parameters (ion mass-to-charge composition, plasma density, electron temperature) of the beam-plasma and of the nitrided surface is found. Increased working gas pressure leads to an increase in the fraction of atomic nitrogen ions in the plasma, which results in increased surface layer hardness, depth of penetration of nitrogen atoms into the sample, and wear resistance. Higher absolute values of sample bias potential contribute to increased plasma density near the sample surface and a corresponding increase of hardness and wear resistance of the sample.

Effect of a magnetic cusp configuration on the ion species ratio in a NBI ion source

In arc discharge plasma, ion species ratio depends on plasma density and ion confinement time. To increase an atomic ion species ratio in an NBI (neutral beam injector) ion source, various ion sources have been designed with a higher ratio of plasma volume to effective ion loss area for increasing ion confinement time. In such designs, most NBI ion sources have a large arc chamber with strong cusp magnetic field. KSTAR NBI ion source also has a large arc chamber with a strong azimuthal line cusp configuration but its atomic ion (H+) beam fraction is small because its normal operation power levels of the arc discharges are comparatively lower than those of other NBI ion sources. In particular, the H2+ ion beam fraction is relatively large at about 40%. This means that there are many primary electrons in the beam extraction region. Through several Ele-orbit code simulations, it was found that some magnetic cusp configurations are effective in preventing primary electrons from nearing the plasma grid facing a plasma and having a high voltage potential. To reduce the molecular ion fraction in KSTAR NBI ion source and consequently increase its atomic ion fraction, experiments regarding the application of several new magnetic cusp configurations to the arc chamber were performed. Experimental results show that the new magnetic cusp configuration can increase the atomic ion fraction to approximately 80%, despite the low arc operation power at KSTAR NBI ion source.

Measurement of the atomic ion fraction of ion emitted from a miniature penning ion source

Penning-type ion source performance for neutron generator applications is characterized partly by the atomic ion fraction, providing one path by which source performance can be improved for increased neutron yields. A miniature penning ion source has been fabricated to investigate the atomic ion fraction of deuterium plasma by a mass-energy analyzer. The discharge current and atomic ion fraction increase with the increase of anode voltage and pressure. Effects of electrode materials on discharge characteristics have been studied. The atomic ion fraction could increase about 2% of the original atomic fraction by Au cathode instead of steel cathode, and it could increase about 1% of the original atomic fraction by Al cathode. The results can provide useful information for improving source performance by selecting more suitable anode voltage, pressure and electrode material.

Characterizing the dominant ions in low-temperature argon plasmas in the range of 1–800 Torr

The dominant ions in low-temperature rare gas plasmas can be either molecular ions or atomic ions depending on the discharge regime. In this paper, the dominant ions in low-temperature argon plasmas are characterized in a wide range of gas pressure (1–800 Torr). The channels for creation of molecular ions include atom assisted association, dissociative recombination, dissociation by atom impact (DAI), and dissociation by electron impact (DEI). The latter two were previously less often considered. It is found that the DEI reaction has a significant impact on the ion fractions, while the effect of the DAI reaction is much less important in the whole investigated gas pressure regime. As the gas pressure increases from 1 to 800 Torr, the atomic ion fraction drops rapidly in conjunction with an increase of the molecular ion fraction. This phenomenon confirms that in low-temperature argon plasmas the dominant ion will be the atomic ion in the low pressure regime but the molecular ion in the high pressure regime. The impact of power density is also investigated in combination with the DEI reaction. The results show that both the DEI reaction and the power density serve to delay the transition trend of the ion fraction, shifting the dominance of molecular ions to a higher pressure.

Corpuscular Diagnosis of the Plasma of Penning Ion Sources

The results of the studies of the energy distribution and atomic-molecular composition of the ions emitted from two Penning ion sources are presented. The transitions between different discharge modes, when the operating pressure and voltage on the anode are changed, are investigated. The energy spectra and mass-charge spectra are measured and the dependences of the fraction of atomic ions on the discharge parameters are determined.

Effect of water vapor on the ionic composition of the hydrogen beam-plasma discharge

The effect of optimization of the ionic composition of the hydrogen beam-plasma discharge (BPD) in the proton component when filling water vapor into the setup vacuum chamber is described. Experiments for studying the effect of controllable filling of water vapor on properties of highly nonequilibrium plasma show that under certain conditions the discharge ionic composition is redistributed toward increasing the atomic ion fraction. It is assumed that this effect is caused by the interaction of water vapor with the wall of the vacuum chamber of the plasma setup, which results in surface oxidation and a decrease in the recombination coefficient. Furthermore, the presence of active OH radical shifts chemical equilibrium toward increasing the atomic ion fraction.

Plasma studies of the permanent magnet electron cyclotron resonance ion source at Peking University

At Peking University (PKU) we have developed several 2.45 GHz Permanent Magnet Electron Cyclotron Resonance ion sources for PKUNIFTY, SFRFQ, Coupled RFQ&SFRFQ, and Dielectric-Wall Accelerator (DWA) projects (respectively, 50 mA of D(+), 10 mA of O(+), 10 mA of He(+), and 50 mA of H(+)). In order to improve performance of these ion sources, it is necessary to better understand the principal factors that influence the plasma density and the atomic ion fraction. Theoretical analysis about microwave transmission and cut-off inside the discharge chamber were carried out to study the influence of the discharge chamber diameters. As a consequence, experimental studies on plasma density and ion fraction with different discharge chamber sizes have been carried out. Due to the difficulties in measuring plasma density inside the discharge chamber, the output beam current was measured to reflect the plasma density. Experimental results show that the plasma density increases to the maximum and then decreases significantly as the diameter changed from 64 mm to 30 mm, and the atomic ion fraction has the same tendency. The maximum beam intensity was obtained with the diameter of 35 mm, but the maximum atomic ion fraction with a diameter of 40 mm. The experimental results are basically accordant with the theoretical calculation. Details are presented in this paper.

Novel methods for improvement of a Penning ion source for neutron generator applications

Penning ion source performance for neutron generator applications is characterized by the atomic ion fraction and beam current density, providing two paths by which source performance can be improved for increased neutron yields. We have fabricated a Penning ion source to investigate novel methods for improving source performance, including optimization of wall materials and electrode geometry, advanced magnetic confinement, and integration of field emitter arrays for electron injection. Effects of several electrode geometries on discharge characteristics and extracted ion current were studied. Additional magnetic confinement resulted in a factor of two increase in beam current density. First results indicate unchanged proton fraction and increased beam current density due to electron injection from carbon nanofiber arrays.

Development of a RF-Driven Neutron Generator for Associated Particle Imaging

We present recent work in the development of a compact, radio frequency (RF) D-T neutron generator with a maximum neutron yield of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> n/s for the application of associated particle imaging (API) used in explosive and contraband detection. API makes use of alpha particles produced in conjunction with 14 MeV neutrons in the D-T reaction to locate the neutron interaction and reduce background noise. To achieve high spatial resolution in API, a beam diameter of les1 mm on target is desired. For portable neutron generators used in API, the ion source and target cannot be water-cooled and the power deposited on the target must be low. By increasing the atomic ion fraction, the ion beam can be used more efficiently to generate neutrons, resulting in a lower beam power requirement and an increased lifetime of the alpha detector inside the acceleration column. The RF ion source produces very high atomic ion fractions (> 90%) compared to traditional Penning ion sources that produce less than 10% of atomic ions. Experimental measurements of the ion source plasma parameters including ion current density, atomic ion fraction, ignition and operating pressures, will be presented along with a discussion on the ion optics and engineering challenges.

Evolution of the plasma composition of a high power impulse magnetron sputtering system studied with a time-of-flight spectrometer

The plasma of a high power impulse magnetron sputtering system has been investigated using a time-of-flight spectrometer. The target materials included high sputter yield materials (Cu, Ag), transition metals (Nb, Cr, Ti), and carbon (graphite); the sputtering gases were argon, krypton, and nitrogen, and two different target thicknesses were selected to consider the role of the magnetic field strength. Measurements for selected combinations of those parameters give quantitative information on the transition from gas-dominated to metal-dominated (self-sputtering) plasma, on the fractions of ion charge states, and in the case of molecular gases, on the fraction of atomic and molecular ions.

Low-energy linear oxygen plasma source

A new version of a constricted plasma source is described, characterized by all metal-ceramic construction, a linear slit exit of 180 mm length, and cw operation (typically 50 kHz) at an average power of 1.5 kW. The plasma source is here operated with oxygen gas, producing streaming plasma that contains mainly positive molecular and atomic ions, and to a much lesser degree, negative ions. The maximum total ion current obtained was about 0.5 A. The fraction of atomic ions reached more than 10% of all ions when the flow rate was less then 10 SCCM O(2), corresponding to a chamber pressure of about 0.5 Pa for the selected pumping speed. The energy distribution functions of the different ion species were measured with a combined mass spectrometer and energy analyzer. The time-averaged distribution functions were broad and ranged from about 30 to 90 eV at 200 kHz and higher frequencies, while they were only several eV broad at 50 kHz and lower frequencies, with the maximum located at about 40 eV for the grounded anode case. This maximum was shifted down to about 7 eV when the anode was floating, indicating the important role of the plasma potential for the ion energy for a given substrate potential. The source could be scaled to greater length and may be useful for functionalization of surfaces and plasma-assisted deposition of compound films.

A compact filament-driven multicusp ion source

A compact filament-driven multicusp ion source has been studied using both hydrogen and helium. Three aspects of the source have been investigated: hydrogen ion species, axial energy spread and extractable current. An atomic ion fraction (H +) of approximately 30% could be obtained with a discharge power of 80 V and 3 A. A magnetic analyzer was used to determine the axial energy spread of the extracted (i.e. accelerated) ion beam species, and an electrostatic energy analyzer was used to determine the energy spread of the ions at the source exit. The energy spread of the extracted beam for the individual species of positive hydrogen ions (H +, H 2 +, H 3 +) and that for the negative hydrogen ions (H −) was measured as well. Energy spreads as low as 2.3 eV were obtained for H +, 2 eV for H 2 +, 1.7 eV for H 3 +, and 1 eV for H −. The axial energy spread in the source exit without extraction for hydrogen and helium was measured to be approximately 1 eV for both cases. The source can generate a hydrogen beam current density of approximately 12 mA/cm 2.

Analysis of the ion beam obtained from a small multicusp ion source

The small multicusp ion source developed at Lawrence Berkeley Laboratory (LBL) has been equipped with a low voltage ratio, single aperture extraction system. The influence of the potential of the plasma electrode and of a dipole filter field on the beam emittance are measured. A simple method to reduce hash is suggested. The aim of these investigations is to produce nitrogen ion beams with a high atomic ion fraction and a low emittance as required for a RFQ-accelerator, which will be built for ion implantation.

Ion composition in an IBC class II aurora: 2. Model

Relatively good agreement is obtained between the ion composition measurements for a steady class II aurora and a simple auroral model. Better agreement would require a more precise neutral model for N2, O2, O, and NO. Auroral ion composition model computations were performed by using the mean Cira (1965) and the Jacchia (1971) 800°K thermopause neutral models as extremes. The computations suggest that the true N2, O2, and O concentrations lie between these two models in contrast to a previous study of a class I aurora, which suggested a higher O/O2 ratio than either model. The required O/O2 ratio is found to increase in proportion to the fraction of atomic oxygen ions formed in metastable states. The peak NO concentration, about 4×108 cm−3; near 110 km, is seen to be much less sensitive to the major neutral gas model employed in the computations. The deduced nitric oxide profile agrees with the profile deduced earlier for the class I aurora within the estimated uncertainties for both profiles.