Ion Optical System Research Articles

Comprehensive modelling of secondary-ion energy spectra measured with a magnetic sector field instrument: I. Concept and basic features

In order to correlate measured secondary-ion energy spectra with theoretically predicted energy distributions of sputtered atoms, one needs to know the properties of the employed mass spectrometer. Here the resolution function and the transmission of a frequently employed magnetic sector-field instrument (IMS-nf series) are described using a simple concept. Modelling of energy spectra is greatly facilitated by the recently provided evidence that the ionisation probability of secondary ions is independent of their emission velocity. Hence the ejected ions may be described by Thompson-type energy distributions which are fully defined by the surface binding energy, Es, of sputtered atoms. On passage through the mass spectrometer the energy distributions are modified in response to the energy dependent transmission of the ion optical system, the aberrations of the lenses and the non-negligible size of the resolution defining apertures, notably the circular contrast aperture (CA) on the entrance side of the spherical energy analyser (aperture diameter d) and the energy slit on the exit side (slit width s). The diameter d determines a characteristic energy Ec up to which secondary ions with emission angles up to 90° to the surface normal are fed through the CA. At higher energies the transmission theoretically decreases as 1/E. Resolution functions are derived by convolving a rectangular box of unit height and width ω with a Gaussian featuring a standard deviation σ. The shape, width and height of the modelled energy spectra depend in a characteristic manner on ω, σ, Ec and Es. Owing to the very low characteristic energies (Ec≤0.1eV) associated with small CAs (d≤50μm), the resulting energy spectra contain very little useful information on the physics of the emission process. The peak position and the width of energy distributions are reproduced best using contrast apertures large enough to achieve Ec>2Es, in combination with a slit width corresponding to ω<Es/4. Inspecting calculated spectra on absolute scales, variations of ω are seen to shift the low-energy edge towards lower (‘negative’) energies by ω/2. With Es as an input parameter, the true origin of the energy scale may be assessed with an accuracy of about ±0.05eV. In measured spectra, the full widths at half maximum of the original distribution and the resolution function add up roughly in quadrature. Evidence in favour of the described concept is presented in the companion paper.

The 25 mA continuous-wave surface-plasma source of H− ions

The ion source with the Penning geometry of electrodes producing continuous-wave beam of H(-) ions with current up to 25 mA was developed. Several improvements were introduced to increase source intensity, reliability, and lifetime. The collar around the emission aperture increases the electrons filtering. The apertures' diameters of the ion-optical system electrodes were increased to generate the beam with higher intensity. An optimization of electrodes' temperature was performed.

Gabor lenses for capture and energy selection of laser driven ion beams in cancer treatment

AbstractThe application of laser accelerated ion beams in Hadron therapy requires ion beam optics with unique features. It has been shown that due to the spectral and spatial distribution of laser accelerated protons a lens based focusing system has advantages over aperture collimated beam formation. We present a compact ion optical system with therapy applications, based on Gabor space charge lenses for collecting, focusing and energy filtering the laser produced proton beam. For a full therapy solution, a scenario based on three space charge lenses is presented. In this very compact beam line an aperture is foreseen for energy selection.

Ion Source for 10 sec Diagnostic Neutral Beam

The ion source for along pulse operating diagnostic neutral beam injector has been developed. The ion source has to form an 8 A, 60 keV hydrogen ion beam. The ion source consists of an RF-plasma driver and a four grids ion-optical system. Both ion-optical system and plasma driver have an augmented water-cooling system. Geometry of the ion-optical system was optimized by the numerical simulations in order to achieve the minimal beam angular divergence. The performance of the developed water-cooling system of the ion source was numerically simulated. Design of the developed ion source and experimental and numerical simulations results are presented in the paper.

A versatile high-resolution magnetic spectrometer for energy compression, reaction studies and nuclear spectroscopy

The Super-FRS in-flight fragment separator at FAIR will be used to produce exotic nuclei at relativistic energies. High resolution experiments in ion traps with the separated fragments will be performed after they are slowed-down in matter and quickly extracted from a gas-filled stopping cell. Such experiments require an effective range bunching with a high-resolution magnetic spectrometer in combination with a wedge-shaped degrader at the dispersive focal plane. The ion-optical system of the spectrometer proposed here is rather versatile because it consists of several dipole and quadrupole magnets arranged such to serve as an energy-buncher and also as a large-acceptance spectrometer for the analysis of secondary reaction products. The performance and the experimental potential of such a multi-purpose spectrometer are described.

Use of the focusing multi-slit ion optical system at RUssian Diagnostic Injector (RUDI)

The upgrade of the diagnostic neutral beam injector RUDI in 2010 was performed to increase the beam density at the focal plane in accordance with the requirements of charge-exchange recombination spectroscopy diagnostics. A new focusing ion-optical system (IOS) with slit beamlets and an enlarged aperture was optimized for 50% higher nominal beam current and reduced angular divergence with respect to the previous multi-aperture IOS version. The upgraded injector provides the beam current up to 3 A, the measured beam divergence in the direction along the slits is 0.35°. Additionally, the plasma generator was modified to extend the beam pulse to 8 s.

Commissioning of heating neutral beams for COMPASS-D tokamak

Two neutral beam injectors have been developed for plasma heating on COMPASS-D tokamak (Institute of Plasma Physics, Prague). The 4-electrodes multihole ion-optical system with beam focusing was chosen to provide the low divergence 300 kW power in both deuterium and hydrogen atoms. The accelerating voltage is 40 kV at extracted ion current up to 15 A. The power supply system provides the continuous and modulated mode of the beam injection at a maximal pulse length 300 ms. The optimal arrangement of the cryopanels and the beam duct elements provides sufficiently short-length beamline which reduces the beam losses. The evolution of the impurities and molecular fraction content is studied in the process of the high voltage conditioning of the newly made ion sources. Two injectors of the same type have been successfully tested and are ready for operation at tokamak in IPP, Prague.

Ultra High Resolution Linear Ion Trap Orbitrap Mass Spectrometer (Orbitrap Elite) Facilitates Top Down LC MS/MS and Versatile Peptide Fragmentation Modes

Although only a few years old, the combination of a linear ion trap with an Orbitrap analyzer has become one of the standard mass spectrometers to characterize proteins and proteomes. Here we describe a novel version of this instrument family, the Orbitrap Elite, which is improved in three main areas. The ion transfer optics has an ion path that blocks the line of sight to achieve more robust operation. The tandem MS acquisition speed of the dual cell linear ion trap now exceeds 12 Hz. Most importantly, the resolving power of the Orbitrap analyzer has been increased twofold for the same transient length by employing a compact, high-field Orbitrap analyzer that almost doubles the observed frequencies. An enhanced Fourier Transform algorithm—incorporating phase information—further doubles the resolving power to 240,000 at m/z 400 for a 768 ms transient. For top-down experiments, we combine a survey scan with a selected ion monitoring scan of the charge state of the protein to be fragmented and with several HCD microscans. Despite the 120,000 resolving power for SIM and HCD scans, the total cycle time is within several seconds and therefore suitable for liquid chromatography tandem MS. For bottom-up proteomics, we combined survey scans at 240,000 resolving power with data-dependent collision-induced dissociation of the 20 most abundant precursors in a total cycle time of 2.5 s—increasing protein identifications in complex mixtures by about 30%. The speed of the Orbitrap Elite furthermore allows scan modes in which complementary dissociation mechanisms are routinely obtained of all fragmented peptides.

Tuner: A tool for designing and optimizing ion optical systems

Designing and optimizing ion optical systems is often a complex and difficult task, which requires the use of computational tools to iterate and converge towards the desired characteristics and performances of the system. Very often these tools are not well adapted for exploring the numerous degrees of freedom, rendering the process long and tedious, as well as somewhat random due to the very large number of local minima typically found when looking for a particular optical solution. This paper presents a novel approach to finding the desired solution of an optical system, by providing the user with an instant feedback of the effects of changing parameters. The process of finding an approximate solution by manually adjusting parameters is greatly facilitated, at which point the final tune can be calculated by minimization according to a number of constraints.

Xenon nano-beams for materials research

The advantage of focused Xe+ beams over other rare gas species have been investigated [Zhukov, V.; Kalbitzer, S. Russ. Microelectron. 2011, 40(1), 17–24]. In particular, the higher operation temperature of a super-tip gas field ion source for xenon ions is one outstanding technical feature. The properties of focused Xe+ beams are estimated with special reference to optimised ion-optical transport systems. Applications to both ion beam materials modification and ion beam materials analysis are outlined.

A new, ohmic-heating based thermal ionization cavity source for mass spectrometry

We present a new, ohmic-heating based, thermal ionization cavity source for mass spectrometry. We employ independent control of ionization and evaporation to keep sample in the evaporator while increasing the temperature of the ionizer. The ‘cold end’ phenomenon is eliminated by using optimized blade-shaped connection electrode. We simulated the processes of ionization and transition in the ion source using SIMION program based on Monte Carlo method and obtained spatial and kinetic energy distribution data for the atoms and ions at the outlet of the ionizer. We demonstrate considerable improvements in the extraction and transmission efficiency of the ion optics system. The total efficiencies achieved for uranium were 0.5–2% and the estimated ionization efficiencies - 3–7%. For plutonium, these values were 4–9% and 20–60%, respectively.

Optimisation and assessment of an innovative high sensitivity 90-degree reflecting ICP-MS ion optics system for the validation of simultaneous determination of arsenic and selenium in water matrices

A routine ICP-MS method for the determination of As and Se in fresh and sewage water has been developed. After a first phase of optimisation, where the torch alignment, flow of gases and ion optic adjustments were calibrated, the method was validated successfully. The parameters limits of detection, linearity, working range, sensitivity and the interferences, precision and accuracy were studied with three procedures for the method: measuring two SRMs (BCR-714 of influent wastewater and NIST 1643e of groundwater); spiking four different matrices at three levels of concentration; and doing a proficiency test. The limits of detection obtained were 0.2 and 0.8 µg L−1 for As and Se, respectively. The percentages of linearity obtained were 99.2% for As and 99.8% for Se. All recovery values were according to the AOAC intervals, from 95.3% for As in the wastewater matrix to 106.4% for Se in the seawater matrix. The accuracy was also studied determined via a proficiency testing resulting in acceptable Z-scores of 0.65 and 0.4 respectively for As and Se. The complete method allowed analysis of water samples according to European Directive 2000/60 EC and the different guides for method validation.

Tandem Time-of-Flight Mass Spectrometer with High Precursor Ion Selectivity Employing Spiral Ion Trajectory and Improved Offset Parabolic Reflectron

In this study, we have developed a tandem time-of-flight mass spectrometry (TOF/TOF) technique involving the use of a matrix-assisted laser desorption/ionization ion source that exhibits high precursor ion selectivity. An ion optical system with a 17 m spiral ion trajectory was used in the first time-of-flight mass spectrometer. High precursor ion selectivity was achieved by realizing a 15 m flight path, which is considerably longer than that of the conventional MALDI-TOF/TOF before the precursor ion selection by an ion gate; monoisotopic ions could be selected properly up to m/z 2500. Furthermore, the first time-of-flight mass spectrometer was composed of electrostatic sectors and could eliminate post-source decay (PSD) ions. Precursor ions with 20 keV kinetic energy were selected and injected into a collision cell, leading to the generation of fragment ions by high-energy collision-induced dissociation (HE-CID). The optimized second time-of-flight mass spectrometer included a post-acceleration region and an offset parabolic reflectron to record product ion spectra in the entire mass range. Our system could generate a simple HE-CID product ion spectrum because each fragment pathway could be observed as a single peak by the selection of monoisotopic ions of all precursor ions and HE-CID fragment pathways could be predominantly observed by the PSD ion elimination.

New features of the Monte-Carlo code MOCADI

MOCADI, the Monte-Carlo code for tracking of ions in ion-optical systems with non-Liouvillian elements, has been extended. Accurate atomic and nuclear interactions are taken into account when ions penetrate gaseous and solid matter placed within the ion-optical system. The new features of MOCADI are described in this article with practical examples which demonstrate the new possibilities, such as new event-generators for targets and spontaneous nuclear decay, the option of atomic-charge state fluctuation in matter, loops for multi-turn ion-optical systems and a graphical user interface for easier operating and control of the program. Experiments for investigation of nuclear structure and reactions with ions circulating in a storage ring can now be ideally studied with MOCADI.

Development of Focused Neutral Beams with Small Angular Divergence for Plasma Heating and Diagnostics

A series of neutral beam injectors for plasma heating and diagnostics in modern magnetic fusion devices has been developed in the Budker Institute of Nuclear Physics. In ion sources of these injectors arc discharge or RF plasma boxes are used. Ion optical systems are optimized to produce ion beams with a low enough angular divergence. In order to provide beam focusing, the grids are formed as spherical segments. Such ballistically focused beams are further neutralized in a gas target and subsequently are used to heat or diagnose plasma. Obtained diagnostic neutral beams with precise focusing are widely used to measure plasma parameters by beam emission spectroscopy methods in tokamaks, stellarators, reversed field pinches and open traps. High power focused beams with small divergence are also necessary for heating of localized regions of plasma and in the devices with narrow access ports through which only small size, high power density beams can be transported. Transition to steady state operation regime of the injectors is discussed.

Majority Ion Heating by Neutral Beam Injection and Confinement of Fast Ions in the Madison Symmetric Torus Reversed Field Pinch

A new 1 MW neutral beam injector (START-20F) is in operation on the Madison Symmetric Torus (MST) reversed field pinch. The beam, consisting of two arc discharge plasma generators, an optimized ion optical system and an integrated neutralizer/injector tank, operates at 25kV and up to 40A of neutrals for a 20 msec pulse (compared to a typical MST pulse length of 60 msec). The injected 1 MW of hydrogen neutrals (with approximately 85% in the full energy component) is significant compared to the 3-4 MW of ohmic input power in a typical target discharge. At this beam energy and a background electron density of about 1x1019 m–3 and temperature ≤1keV, roughly 90% of the injected power is deposited within the plasma. Initial experiments with the high power NBI show a large heating of the bulk ions: the fit of the width of energy spectrum as measured by Rutherford scattering (which is generally related to core ion temperature) quickly increases from 180eV to 230eV. This apparent significant and rapid heating of bulk ions is difficult to explain by classical collisions only, as modeling predicts 75% of the injected power is deposited on electrons and 15% on ions. The confinement of the fast ions (measured by the persistence in time of fusion neutrons due to a small fraction of deuterium in the beam fuel) is much greater than the canonical 1 msec confinement of particles and energy in the MST. The fast particle confinement is measured to increase with magnetic field strength. Further recent experiments document fast particle confinement time versus direction of injection (parallel or antiparallel to central magnetic field), beam energy, and background plasma properties.

Advantages of the use of supertip Xe+ sources with field gas ionization in ion microscopes

Advantages of the use of beams of focused Xe+ ions obtained from supertip sources with field ionization (GFIS*) in ion microscopy compared with conventional sources of different ions are considered. Specifically, the higher working temperature of supertip Xe+ sources with field gas ionization as well as the higher values of the interaction sections of the Xe+ ions with different solid targets are a considerable technical advantage. The properties of focused Xe+ ion beams with a special accent on optimization of ion-optical systems of microscopes are evaluated. The possibilities of nanomodification and elemental analysis of materials with the use of beams of Xe+ ions are considered.

A Continuously Operated 70mA, 50keV Proton Source for Plasma Diagnostic and Other Applications

A radio-frequency ion source has been developed in the Budker Institute of Nuclear Physics, Novosibirsk. Ion source is designed to operate in cw mode with beam modulation. The ion source works with hydrogen at 50kV producing an ion beam current of up to 70mA. The beam is extracted and accelerated by using a four electrode ion-optical system with single aperture. Initial beam diameter is 8 mm. The measured beam divergence is less than 0.5 deg. half angle.This paper describes the experimental results obtained during the ion source testing. Ion source can be applied in many areas including plasma diagnostic, accelerator technique, ion implantation, etc.

Time-of-flight energy compensation to improve energy resolution in low-energy radioactive beam experiments at the TwinSol facility

Abstract Improved time-of-flight (ToF) capability has been added to the University of Michigan (UM)–University of Notre Dame (UND) TwinSol low-energy dual 6T solenoid-based radioactive nuclear-beam (RNB) facility at the UND FN tandem Van de Graaff accelerator. An extended low-background beam line has been combined with a newly-constructed ca.1 meter diameter ISO-250 based scattering chamber capable of sub-ns ToF of RNBs. These together with the intrinsic, low ToF spread in the TwinSol ion-optical system yields improved ToF information for low-energy RNB nuclear reaction studies. Using either the intrinsic accelerator bunched and pulsed-selected beams with RF timing, or with the addition of an MCP+foil timing system (or similar) detector at the dual-solenoid cross-over region allows for reduction of the energy spread inherent in these low-energy RNB reactions. As an example, using the ToF information reduces the FWHM energy spread in a high-intensity 28 MeV 8Li beam from about 1 MeV to

The determination of the atomic mass constant with ion accumulation: status and perspectives

An experiment is described for determining the atomic mass constant by accumulating ions from an ion beam up to a weighable mass. It establishes a link between the international prototype of the kilogram, the realization of the associated SI unit and an atomic mass. The items necessary for such an experiment are a high-current ion source, an ion optical system with high transmission, a suitable ion collector and a vacuum balance. With the most recent measurement, a mass of more than 320 mg of bismuth was accumulated and its atomic mass was determined with a relative standard uncertainty of 9.4 × 10−5. Special emphasis is placed on determining the mass loss of the accumulated ions caused through sputter effects in the ion collector.Although work on this experiment at the PTB has now been stopped, we conclude with a number of suggestions that could lead to much smaller relative uncertainty in a future experiment.