Energy spectrum of a multicomponent flux of accelerated ions

We have experimentally studied the energy spectrum of an accelerated ion beam moving in a drift gap upon separation by a grid electrode system from a metal plasma jet of a pulsed high-current vacuum arc. It is established that the average energy per unit ion charge in the beam is below the value corresponding to the accelerating voltage. The width of the energy spectrum of accelerated ions is close to their average energy and significantly exceeds the width of the energy spectrum of the initial beam. The results of model calculations show that the observed phenomena can be explained by the action of a nonstationary electric field of the space charge of the ion beam transported via the drift gap.     

Singularity in the current density distribution at the boundary of emitting region on a flat cathode surface

The process of electron emission from the boundary of regions with infinite and zero emissivity on a flat cathode surface of a vacuum diode in the space-charge-limited current regime has been theoretically studied. The space charge density, electric field, and velocity field distributions in the interelectrode gap are determined using self-similar solutions of the equations of motion of charged particles. It is established that a singularity, where the emission current density tends to infinity according to a power law, appears at the boundary. From this special point, electrons are emitted at an angle of 103° relative to the electrode surface.     

Ion beam effects on dust grains: 2—Influence of charging history

Dust grains in space are charged by various processes. Impacts of energetic ions lead to deposition of positive charge on the grain, increasing the grain potential and, as a consequence, the electric field at its surface. The accumulated charge is spontaneously released as an emission current when the electric field reaches a threshold. This discharging current is usually attributed to field ionization of any gas surrounding the grain or to ion field emission and would thus be predominantly a function of the surface potential. However, preliminary studies [Velyhan A, Z?ilavý P, Pavl? J, S?afránková J, Něme?ek Z. Ion beam effects on dust grains. Vacuum 2004;76:447-55] using melamine formaldehyde spheres have shown that the discharging current depends strongly on the energy of primary ions. The present paper continues these investigations with the motivation to understand the whole charging/discharging process. The experiment is based on the capture of a single dust grain in an electrodynamic quadrupole. The trapped grain is exposed to an ion beam with different energies up to 5 keV and its charge and surface potential are estimated from the frequency of its oscillations in the quadrupole. The charging/discharging currents are determined from temporal changes of the grain charge. Our results suggest that the grain charge is accumulated in a thick surface layer of non-conducting samples. The thickness of this layer depends on the mass and energy of primary ions. On the other hand, the beam ions probably recombine on the metallic surfaces and create an adsorbed layer there. We believe that the main discharging process is field desorption complemented in this particular case with post-ionization.     

Multiple ionization of vacuum-arc-generated metal ions in a magnetic trap heated by high-power microwave radiation

We present experimental deta on the multiple ionization of refractory metal ions formed in the vacuum-arc discharge plasma, which is achieved by injecting plasma into a magnetic trap and additionally heating it by high-power microwave radiation in the millimeter range. An increase in the high-power microwave source (gyrotron) frequency from 37.5 to 75 GHz allowed plasma of greater density to be heated to a sufficiently high electron temperature ensuring the multiple ionization of trapped particles. In the case of platinum, the microwave heating led to an increase in the average ion charge from 2 to 7, while the maximum platinum ion charge reached 10 and the total ion beam current amounted to 300 mA.     

Generation of charged nanoparticles in point ion sources

The conditions of formation of a dispersed phase in a point source of indium and tin ions have been studied. Charged droplets are generated in a threshold manner at a certain value of the ion beam current. This is accompanied by the excitation of ion current oscillations at a frequency of 15–20 MHz as a result of the capillary instability of the conducting liquid surface in a strong electric field. The dimensions of emitted nanoparticles determined using an electron microscope are continuously distributed in a range of 2–20 nm, and their average specific charge amounts to 5 × 10⁴ C/kg. Such charged nanodroplets can be used to create various surface quantum structures. The possibility of obtaining nanoparticles of semiconductor materials using a modified source with a porous electrode is discussed.     

Obtaining a proton beam with 5-mA current in a tandem accelerator with vacuum insulation

Suppression of parasitic electron flows and positive ions formed in the beam tract of a tandem accelerator with vacuum insulation allowed a more than threefold increase (from 1.6 to 5 mA) in the current of accelerated 2-MeV protons. Details of the modification are described. Results of experimental investigation of the suppression of secondary charged particles and data on the characteristics of accelerated proton beam with increased current are presented.     

Structure of a picosecond electron beam inside a vacuum diode

The structure of a picosecond (∼150 ps) electron beam in the cathode-anode gap of a vacuum diode is determined. The electron beam is modeled in the form of flat quasiparticles with a definite charge density which follow one after another in equal time intervals. It is shown that the expansion of concentric layers of the beam under the action of the electric and magnetic self-fields strongly depends on the current strength. The experimental confirmation of the computed estimates is illustrated by recording the structure of the electron beam at the anode using a film which is sensitive to electron radiation. Pis’ma Zh. Tekh. Fiz. 25, 39–45 (September 26, 1999)     

Ion recombination on dust particles in a dense gas plasma excited by a hard ionizing factor

A theoretical model is proposed that describes the main kinetic mechanisms operating in a dust plasma formed upon the bulk ionization of a dense gas by a hard factor. An expression is derived for the ion density distribution around a charged dust particle. It is shown that ions in the plasma recombine with charged dust particles according to the Langevin mechanism. A condition is established under which the dust particles are bearing all the negative charge in the plasma, compensated by the positive charge of ion clouds. A new criterion for manifestation of the collective properties in the dust plasma is formulated.     

Photochemistry on a polarisable semi-conductor: what do we understand today?

The continued development of ferroelectric materials into more and more consumer led applications has been at the forefront of recent ferroelectric material research. It is, however, possible to view a ferroelectric as a wide band gap semi-conductor that can sustain a surface charge density. This charge density arises from the movement of ions in the crystal lattice and the need to compensate for this charge. When viewing ferroelectrics as polarisable semi-conductors a large number of new interactions are possible. One such is the use of super band gap illumination to generate electron–hole pairs. These photogenerated carriers can then perform local electrochemistry. What is most interesting for ferroelectric materials is that the REDOX chemistry can be chosen by selectively modifying the domain structure of the ferroelectric—we can perform oxidation and reduction on the surface of the same material at spatially separate locations, or use the material to drive photoexcited carriers apart. This means we can separate the REDOX products or produce patterns of photogenerated material in places we have predetermined. This review aims to introduce the background research that has led to the current understanding as well a highlight some of the current areas that require further development.     

Fast multiple electron capture dissociation in a linear radio frequency quadrupole ion trap

We developed a fast electron capture dissociation (ECD) device using a linear radio frequency-quadrupole (RFQ) ion trap. The device dissociated peptides and proteins using a focused electron beam with an intensity of 0.5 microA and a diameter of 1 mm. The electron capture rate was 13%/ms for doubly charged peptides, and the total amount of ECD products was identical to the theoretical limit, i.e., 50% of incident precursor ions were observed as maximum ECD products by electron irradiation of 7 ms in a pulse counting detection scheme. Coupling this ECD device to a time-of-flight mass spectrometer, we applied multiple ECD. Protonated ubiquitin precursor ions with a charge state of 10 were repeatedly cleaved by ECD, i.e., charge-reduced species and their highly charged fragments were cleaved again and again, creating lower charged products, leaving only singly to triply charged states among the final products. Meanwhile with the amount of electron irradiated, lower charged products increased. Applying an electron beam for 8 ms, we obtained 96% of the total sequence coverage using a 40 fmol sample except at three proline sites. This fast ECD device should be widely applicable to proteomics including post-translational modification analysis and top-down analysis.     

Peptide sequencing and characterization of post-translational modifications by enhanced ion-charging and liquid chromatography electron-transfer dissociation tandem mass spectrometry

We have tested the effect of m-nitrobenzyl alcohol (m-NBA) as a method to increase the average charge state of protonated gas-phase molecular ions generated by ESI from tryptic peptides and phosphopeptides. Various concentrations of m-NBA were added to the mobile phases of a liquid chromatography system coupled to an ESI tandem mass spectrometer. Addition of just 0.1% m-NBA changed the average charge state for the identified tryptic BSA peptides from 2.2+ to 2.6+. As a result, the predominant charge states for BSA peptides were changed from 2+ to > or =3+. To evaluate the benefits of peptide charge enhancement, the ETD fragmentation efficiency and Mascot peptide score were compared for BSA peptides in charge states 2+ and 3+. In all cases but one, triply charged peptides fragmented more efficiently than the analogues 2+ peptide ions. On average, triply charged peptides received a 68% higher Mascot score (24 units) than doubly charged peptides. m-NBA also increased the average charge state of phosphopeptides by up to 0.5 charge unit. The ease of implementation and the analytical benefits of charge enhancement of tryptic peptides by addition of m-NBA to the LC solvents suggest the general application of this reagent in proteomic studies that employ ETD-MS/MS and related techniques.     

Visualization of the spatial distribution of current densities in a photographic system with a semiconductor photodetector

Abstract

This paper studies the visualization of the spatial distribution pattern of current density in a semiconductor photographic system with a gallium arsenide semiconductor photodetector. The spatial distribution of the current in the filaments was determined by photometric analysis of the gas discharge light emission when a current was passed through a photographic cell. This method ensured spatial resolution of ~ 10/mm and made it possible to describe quantitatively the distributions involving a drop in current density of ? 10². Transformation of the profile and amplitude of the current density of the filaments in the different regions of the current-voltage characteristic (CVC) has been studied. The filamentation (i.e. an inhomogencous distribution of the current density) was primarily due to the formation of a space charge of positive ions in the discharge gap between the photodetector and a transparent anode plate that changed the discharge from the Townsend type to the glow type.     

Corona discharge in charge reduction electrospray mass spectrometry

Corona discharge is applied to charge reduction electrospray mass spectrometry for the analysis of complex mixtures of biological molecules. Recent work has described a method of charge reduction (reducing the charge states of analyte ions generated by the electrospray process) employing the radioactive isotope 210Po to produce neutralizing species. A variation to this approach is presented, in which charge neutralization is mediated by ions produced in a corona discharge. Varying the corona discharge voltage controls the current and the degree of charge reduction, providing predominantly singly charged ions that are detected by a commercial electrospray time-of-flight mass spectrometer. This technique provides charge reduction for the simplification of ESI spectra, without need for any radioactive material.     

Electron-induced dissociation of singly charged organic cations as a tool for structural characterization of pharmaceutical type molecules

Collision-induced dissociation (CID) and electron-induced dissociation (EID) have been investigated for a selection of small, singly charged organic molecules of pharmaceutical interest. Comparison of these techniques has shown that EID carried out on an FTICR MS and CID performed on a linear ion trap MS produce complementary data. In a study of 33 molecule-cations, EID generated over 300 product ions compared to 190 product ions by CID with an average of only 3 product ions per precursor ion common to both tandem MS techniques. Even multiple stages of CID failed to generate many of the product ions observed following EID. The charge carrying species is also shown to have a very significant effect on the degree of fragmentation and types of product ion resulting from EID. Protonated species behave much like the ammonium adduct with suggestion of a hydrogen atom from the charge carrying species strongly affecting the fragmentation mechanism. Sodium and potassium are retained by nearly every product ion formed from [M + Na](+) or [M + K](+) and provide information to complement the EID of [M + H](+) or [M + NH(4)](+). In summary, EID is proven to be a fitting partner to CID in the structural elucidation of small singly charged ions and by studying EID of a molecule-ion holding different charge carrying species, an even greater depth of detail can be obtained for functional groups commonly used in synthetic chemistry.     

Direct recovery of ion beam energy using magnetic electron suppression: I. Analysis of ORNL energy recovery experiments

The charge-exchange neutralization efficiency of positive ion-based neutral beams used in plasma heating applications decreases as the beam energy increases. Direct energy recovery from the remaining charged particles can be accomplished by electrostatically decelerating the positive ions; the space-charge neutralizing electrons are constrained from being accelerated by the application of a transverse magnetic field. A finite difference nonlinear sheath analysis is used to analyze the transverse magnetic field electron suppression experiments carried out at Oak Ridge National Laboratory in the early 1980s. A double plasma model, which assumes an equilibrium Boltzmann distribution of electrons at both the neutralizer potential and the ion collector potential, is used to study the experimental data obtained from operating 40 keV, 10 A ion beam energy recovery experiments. The effects of the magnetic field strength, ion “boost” energy, and ion beam current density are examined in detail.     

Dynamics of a charged gas suspension with an initial spatially nonuniform distribution of the average dispersed phase density during the transition to the equilibrium state

Results of calculations are presented for the transition of a charged gas suspension from the initial nonequilibrium state with a spatially nonuniform distribution of the average density to a state with a uniform distribution of the average density and charge in a bounded volume. A numerical solution is obtained for the system of equations describing the motion of a polydisperse multirate and multitemperature gas suspension, the particles of which carry an electric discharge and create a self-consistent electric field.     

Charge separation of plasma flow in curvilinear magnetic field

The energy spectra of ions occurring in various charged states in a flow of titanium plasma in a steady-state vacuum arc discharge have been studied. It is established that, during the motion of this flow through a plasma transport system based on a curvilinear magnetic field, ions with different charges are spatially separated. As a result, at the system output, ions with greater charges are concentrated in the inner part of the plasma flow, so that the average charge of ions in this region is higher than that in the outer part of the flow.     

Distribution of ion space charge in the volume of a multiwire proportional chamber at high counting rates

The accumulation of ion space charge in the drift gap of a MWPC, separated from the amplification gap by a cathode grid, is studied. A significant dependence of the cathode grid transparency to ions on the counting rate is observed. It is shown that the decrease of ion transparency with beam intensity is defined both by the parameters and the mode of operation of the MWPC as well as by the gas mixture chosen. On the basis of a suggested explanation of this effect a formula is obtained, that proved to be in good agreement with the experimental results.     

Charge ratio analysis method: approach for the deconvolution of electrospray mass spectra

A new method to interpret electrospray mass spectral data based on calculating the ratio of mass-to-charge (m/z) values of multiply charged ions is described. The mass-to-charge ratios of any two multiply charged ions corresponding to a single compound are unique numbers that enable the charge states for each ion to be unequivocally identified. The multiply charged ions in electrospray mass spectra originate from the addition or abstraction of protons, cations, or anions to and from a compound under analysis. In contrast to existing deconvolution processes, the charge ratio analysis method (CRAM), identifies the charge states of multiply charged ions without any prior knowledge of the nature of the charge-carrying species. In the case of high-resolution electrospray mass spectral data, in which multiply charged ions are resolved to their isotopic components, the CRAM is capable of correlating the isotope peaks of different multiply charged ions that share the same isotopic composition. This relative ratio method is illustrated here for electrospray mass spectral data of lysozyme and oxidized ubiquitin recorded at low- to high-mass resolution on quadrupole ion trap and Fourier transform ion cyclotron mass spectrometers, and theoretical data for the protein calmodulin based upon a reported spectrum recorded on the latter.