Positive Ion Current Research Articles

Langmuir Probe Diagnostics of a Low Temperature Non‐Isothermal Plasma in a Weak Magnetic Field

AbstractThis article presents measurements by a cylindrical Langmuir probe in the plasma of a DC cylindrical magnetron discharge át the pressure 1.5 Pa that aim at the experimental assessment of the influence of a weak magnetic field to the estimation of the electron density when using conventional methods of probe data interpretation. The probe data was obtained under the presence of a weak magnetic field in the range 1.10−2−5.10−2 T. The influence of the magnetic field on the electron probe current is experimentally assessed for two cylindrical probes with different radii, 50 μm and 21 μm. This assessment is based on comparison of the values of the electron density estimated from the electron current part with the values of the positive ion density estimated from the positive ion current part of the probe characteristic respectively by assuming that at the magnetic field strengths used in the present study the probe positive ion currents are possible to be assumed as uninfluenced by the magnetic field. For interpretation of the probe positive ion current two theories are used and compared to each other: the radial motion model by Allen, Boyd and Reynolds [10] and Chen [11] and the model that accounts for the collisions of positive ions with neutrals in the probe space charge sheath that we call Chen‐Talbot model [8]. At lower magnetic field 3 · 10−2 T the positive ion density values interpreted by using the Chen‐Talbot model [8] are in better agreement with the values of electron density compared to those obtained by using the theory [10,11]; therefore the model [8] is used for calculation of the positive ion density from the probe data at higher magnetic fields. The comparison of the positive ion and electron density values calculated from the same probe data at higher magnetic fields shows that up to the magnetic field strength 4 . 10−2 T with the probe 100 μm and up to 5 . 10−2 T with the probe 42 μm in diameter respectively the decrease of the magnitude of the electron current at the space potential due to the magnetic field does not exceed the error limits that are usual for Langmuir probe measurements (absolute error ±20%).

A multi-laminate wire mesh ionizer for a Cs sputter negative ion source

A new ventilated surface ionizer has been developed for a Cs sputter negative ion source after the cylindrical and spherical geometry ionizers, equipped with metal sheets, were successfully operated in a 860 A sputter source at BNU in 1990. The new ionizer is made up of multi-laminate nickel wiremesh and a spiral heater, and is shaped to a spherical-geometry with a 36 mm diameter. Low pressure cesium vapor will be diffused through the wiremesh to the extraction side of ionizer where, or before where, the neutral cesium can be ionized. The ionizer is capable of producing positive ion current in the millampere range and the negative ion currents are 250 μA of C and more than 150 μA of O and Si. Because of the reduction in erosion crater on the sample to approximately a diameter of 1 mm, the emittance is a lower compared to that of previous ionizers.

A microwave-driven negative ion source

A 2.45 GHz microwave-driven ion source, originally developed to generate a high-current 1H+ beam, has been adapted to serve as a universal negative-ion injector for the Tandem Accelerator Superconducting Cyclotron (TASCC) facility. Positive heavy ion currents from the source are converted into negative currents by means of a charge-exchange canal. So far, beams of 4He−, 16O−, and 18O− have been accelerated through the TASCC Tandem and 3He− and 209Bi− beams were accelerated through both the TASCC Tandem and cyclotron. A production version of the source has been designed, with emphasis on enhanced compactness and serviceability.

Study of influence of pulsed gas admission and material of insulating ring on extracting ion current

For heating the plasma of HT-6 Tokamak we have developed a multiampere ion source, which is able to provide positive ion current (pulsed 20 A). In this article we briefly introduce two significant studies we made while developing the big multicusp ion source. One is optimizing the pulse width of preadmission to obtain a uniform pulsed ion current; the other is making an insulating ring of two sorts of material to enhance the electrical character of the ring. Both of them made contributions to the extracted ion current.

Negative-ion source for implantation and surface interaction of negative-ion beams (invited)

An rf-plasma-sputter type heavy negative-ion source can deliver a few mA to 12 mA of dc negative-ion currents such as boron, carbon, silicon, and copper negative-ion beams. These negative-ion currents are comparable with the positive-ion currents for conventional high current positive-ion implantation, and thus could be newly used in place of positive ions. Surface interaction of negative-ion beams is extremely distinctive when negative ions are implanted into insulated materials or insulators. The surface charge-up voltage of insulated materials by negative-ion implantation saturates at only several volts, so that no breakdown of insulators takes place. As a whole system, a negative-ion implanter without charge neutralizer would be simpler and more economical than a positive-ion implanter.

Space charge effects in neutralized ion-compensated beams

Space charge effects in transport channels may be suppressed by using neutralized beams with equal currents of negative and positive ions. Variations of beam envelopes in strong focusing channels result in an increase of the incoherent oscillation frequency with the current. The frequency of the quardrupole coherent oscillations increases as well and it moves to the resonance appearing due to strong focusing. Moreover, the beam perveance is limited by non-linear coherent resonances of the same kind as in uniform beams. Space charge limitations with account of all these effects have been calculated.

CHARGE TRANSFER BETWEEN ALKALI ATOMS AND W,Pt SURFACES

The interaction of thermal alkali atoms on hot tungsten and platinum filaments has been studied experimentally. The positive ion current has been measured as a function of the surface temperature. Electron exchange between the adatom and metal surface and the change of work function of the adsorbed metal surface have been discussed.

Aerosol Charge Neutralization by a Corona Ionizer

A new aerosol neutralizer, which makes use of a corona ionizer to produce bipolar ions, has been designed and evaluated. The corona neutralizer consists of three components: a dual-needle ionizer for the generation of bipolar ions, an opposing-jet mixer for the uniform mixing of ions and particles, and a charging chamber for particle charge neutralization by ions. The bipolar ionizer consists of two unipolar ionizers for producing positive and negative ions, respectively. They are mixed to obtain the appropriate mixture of bipolar ions. Aerosol particals are then mixed with the bipolar ions in the mixer and are electrically discharged by collision with the bipolar ions in the charging chamber. The charge distribution of aerosol particles exiting the new corona neutralizer is found to depend on the ratio of electric current of positive ions and negative ions I − / I − and to attain the Boltzmann charge distribution at I + / I − = 1.0. The neutralizer has been shown to effectively reduce the particle charge...

Secondary electrons generated by fast neutral bombardment of the growing film during laser ablation deposition

Using an ion-probe method, time-of-flight measurements have been performed on the particles produced during excimer ablation of TiN. It is found that the number of ions produced in these ablation processes is quite small. The positive current due to secondary electrons departing from the detector and generated by fast neutral bombardment of the growing film vastly exceeds the current of positive ions arriving at the detector. This renders the ion-probe method unsuited for the determination of ion concentrations and ion velocity distributions.

Static operation of an AMS system using the beam monitor method

The AMS system with a beam monitor method using the tandem accelerator of Tokyo University has been improved. The negative ion beam currents of two isotopes (or molecular ions) can be measured by two Faraday cups in order to monitor the fluctuation of the ion currents during the injection of the ions of interest into the tandem accelerator. The Faraday cups can be moved semiautomatically using computer-controlled stepping motors which search for the current peaks. Using these negative ion beam monitors and another one for accelerated positive ion currents, static operation of the whole system has become possible. The precision of measurements obtained by this method has been improved to ± 0.6% (1σ) for 10Be and 26Al-AMS and ±0.8% (1σ) for 14C-AMS.

H− production from a barium converter with different discharge configurations

The production of H− ions at a barium converter immersed in a hydrogen plasma has been investigated experimentally for a pulsed discharge. It is found that the scaling of the negative ion output with incident positive ion current on the converter depends greatly on the actual geometry of the plasma generator. A less than linear scaling is observed in a multicusp system. A more than linear scaling is obtained in a configuration where the filament and the converter are separated by a permanent magnet filter.

The Application of Langmuir Probes to the Measurements in Flowing Afterglow Plasma

AbstractLangmuir probe measurements in argon flowing afterglow are presented together with probe data analyses. The charged particles density is evaluated from the electron probe current at the space potential and from the saturated positive ion current. The latter is analysed using the method by Zakrzewski and Kopiczynski [15, 16] and both results are compared against each other. A satisfactory agreement almost within the estimated experimental error range suggests that the method [15, 16] brings a reasonable description of a positive ion collection by a Langmuir probe.

生体内胆汁酸の高感度・高性能クロマトグラフィー

Separation and determination of bile acids in biological fluids by high-performance liquid chromatography are reviewed. The capacity ratios of bile acids on an ODS column were affected by the number, position and configuration of the hydroxyl group on the steroid nucleus, and the chromatographic behavior was markedly influenced by the pH of a mobile phase according to the conjugated form at C-24. A new pre-column derivatization reagent, 1-anthroyl nitrile, was developed and applied to the analysis of bile acids in biological fluids. Bile acids were derivatized through the 3 alpha-hydroxyl group into the corresponding esters, separated on an ODS column, and monitored by a fluorescence detector with detection limit of 20 fmol. The sensitive method for the determination of bile acids in biological materials by gas chromatography (GC) in combination with negative ion chemical ionization (NICI) mass spectrometry is also described. Of various derivatives for the carboxyl group, the pentafluorobenzyl (PFB) ester provided the highest value of the ratio of the negative to positive ion current. A characteristic carboxylate anion [M-181]- was produced as the most abundant ion by the loss of the PFB group in NICI. PFB esters of bile acids were further derivatized into the dimethylethylsilyl ethers and then separated by GC. The detection limit was 2 fg when the characteristic anion was monitored in the NICI mode. The preparation of 18O-labelled bile acids, as the internal standard for the trace analysis or the tracer for the metabolic study, was developed. Finally, the clean-up procedure for bile acids in biological fluids was investigated. The combined use of solid-phase extraction with a Sep-pak C18 or Bond Elut cartridge and group separation on a lipophilic ion-exchange gel, piperidinohydroxypropyl Sephadex LH-20, was found most effective for this purpose.

Review of ion-based coating processes derived from the cathodic arc

The cathodic vacuum arc provides a means of producing large currents of positive ions of a wide variety of materials. These ions can be utilized to produce coatings with improved properties such as higher density and adhesion. The processing is particularly useful in reactive deposition of ceramic coatings having excellent stoichiometry. In this review, emerging aspects of the technology are emphasized.

Auger-induced valence processes in alkali halides following alkali core-hole creation.

Ab initio calculated values are presented for the interatomic Auger and radiative decay rates for initial (2p${)}^{2}$ and 2p hole states of the Na ion in NaF. A comparison of the rates shows that these hole states decay overwhelmingly by the nonradiative Auger process. The knowledge of the rates makes possible a clarification of the mechanism of photon-stimulated positive-ion desorption presented by Parks et al. [Phys. Rev. B 28, 4793 (1983)]. Following the creation of a Na(1s) hole in NaF, the ensuing Auger cascade results in the creation of four to six valence holes, or one ${\mathrm{F}}^{+}$ ion and two to four valence holes, on the neighboring negative fluorine ions within an elapsed time of a few times ${10}^{\mathrm{\ensuremath{-}}14}$ s. The majority of the valence holes are produced as hot holes, and are likely to diffuse several tens to hundreds of angstroms in times of the order of picoseconds before self-trapping to form ${V}_{K}$ centers. The fast diffusion of these holes greatly reduces their chance of participating in ion desorption processes. The possibility that a fraction of the holes self-trap immediately and assist in the desorption of positive ions is considered. It is found that positive-ion desorption can occur if enough holes self-trap immediately. However, the required configurations are so special that they may occur with too small a probability to account for the observed positive-ion current. The fast hole diffusion also makes it unlikely that the ${V}_{K}$ centers from the cascade will exhibit the clustering effects which would have been expected had the holes self-trapped in place. An increase in the yield of electron-hole pairs of about 6% is predicted to occur at the Na K edge.

Hyperthermal Surface Ionization

AbstractSurface ionization of molecules with hyperthermal kinetic energy (1–20 eV) was found to be very efficient, demonstrating several unique features. We have used the technique of aerodynamic acceleration in supersonic seeded beams in order to obtain molecular kinetic energies in the range 1–20 eV. Three types of hyperthermal surface ionization (HSI) processes were observed: (a) surface‐molecule electron transfer was demonstrated in the I2/diamond system where negative molecular iodine (I−2) ions were produced; (b) molecule‐surface electron transfer was found for the anthracene molecule where positive molecular anthracene ions were generated; (c) hyperthermal surface‐induced dissociative ionization (HSIDI) was observed in 1‐iodopropane, 1‐chloro‐3‐iodopropane, benzyl bromide, and many other molecules. In these processes, we have observed a large current of negative halogen ions and positive molecular residue ions (ion‐pair formation). The mechanism of HSI is described in terms of electron transfer processes. It occurs due to a curve crossing between the neutral scattering interaction potential surface and the ionic interaction potential surface and nonunity reneutralization second curve crossing of the scattered molecules or fragment ions.

Efficient emitters for surface ionization organic mass spectrometry

In order to search for the emitter which surface-ionizes organic compounds most efficiently, surface ionization organic mass spectrometry (SIOMS) has been performed for eleven kinds of refractory metals in terms of relative ionization efficiency, time stability, and durability. On the understanding that the work function of the surface is greatly increased by the chemisorption of electronegative gases, the measurements of positive ion current from the eleven metals has been made for cyclohexene as a test sample in the presence of oxygen, chlorine, or fluorine. The conclusion is that oxidized Re seems to be the best emitter material for detecting organic substances for practical use in SIOMS. As far as the highest efficiency is concerned, the Re emitter, in the simultaneous presence of oxygen and fluorine, provides 5.5 times more (MH) + ions of cyclohexene (M) than the Re oxide emitter. However, the former is more complicated and is less easily operated than the latter.

Fine scale structure and turbulence parameters in the equatorial middle atmosphere

Two rockets carrying identical spherical probe payloads were launched from Thumba to measure positive ion density of the mesosphere and lower thermosphere over an equatorial location. Data obtained show the presence of strong irregularities in the ion density. From the measured positive ion current, the spectra of the spatial density fluctuation, turbulent velocity, energy dissipation rate and eddy diffusion coefficients have been derived in the altitude range of 70–100 km. The results are found to be different from those at middle and high latitudes.

Aspects of negative-ion generation and extraction from a refocus geometry cesium sputter negative-ion source. II

The basic parameters of the refocus geometry cesium sputter ion source for the production of negative ions from a low-sputtering material (carbon) and a high-sputtering material (copper) have been measured. The question of the influence of sample temperature on the negative-ion generation process has been addressed. Negative-ion yields from carbon samples are found to be weakly dependent on temperature over a range from 26° to 400 °C, while those from copper samples are found to be essentially independent of temperature over a range of 26° to 200 °C. The results of these investigations indicate that (1) sample temperature does not strongly influence negative formation in these sources, and (2) the maximum negative-ion yields for a given sample are dependent on both cesium-ion energy and positive-ion current. In addition to these data, the negative-ion extraction optics of the source are briefly discussed and cursory estimates of the probabilities for negative-ion formation presented.

Studies on steroids : CCXXVIII. Trace analysis of bile acids by gas chromatography-mass spectrometry with negative ion chemical ionization detection

A suitable derivatization method for the trace analysis of bile acids by gas chromatography (GC) in combination with negative ion chemical ionization (NICI) mass spectrometry is described. Of various derivatives for the carboxyl group, the pentafluorobenzyl (PFB) ester provided the highest value of the ratio of the negative top the positive ion current. A chracteristic carboxylate anion [M − 181] − was produced as the most abundant ion by the loss of the PFB group in NICI. The PFB esters were further derivatized to the dimethylethylsilyl (DMES) ethers, whereby lithocholic acid, deoxycholic acid, chenodeoxycholic acid, ursodeoxycholic acid and cholic acid were distinctly separated by GC on a cross-linked methyl silicon fused-silica capillary column. The detection limit for the PFB-DMES derivatives of dihydroxylated bile acids was 2 fg when the fragment ion was monitored at m/z 563 in the NICI mode using isobutane as a reagent gas.