Ion Optics Research Articles

Effects of Electrode Beam Radius on Properties of four-poles Electrostatic Double Lens for Point-to-Point Focus

This study investigates the effect of selection the electric pole radius of the four poles electrostatic doublets lens on the optical property. Also, a comparison between the optical property and performance of two lenses with different electrode radius was made. The results showed that the electric pole diameter of the lens plays main function in the imaging characteristics of the four poles electrostatic doublets lens. The results show that the aberration figures can be enhanced when the electrode diameter of the quadrupole lens is equal to the lens aperture, while the chromatic aberration can be improved by choosing the electrode diameter according to the equation.

Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens.

The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e-e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from -20 to -1100 V/mm for Ekin = 100eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 μm above the sample surface for Ekin = 100eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at Ekin = 107eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm2 (retarding field -21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm2, it increases by 0.5eV (pump wavelength 1030nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at Ekin = 3830eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments.

Development of novel projection-type imaging mass spectrometer.

We developed a novel imaging mass spectrometer based on our accumulating technology for projection-type imaging mass spectrometry, the simulation of an accurate ion trajectory, and the theory for ion optics. The newly developed apparatus yields high spatial resolution with a substantially shorter image-acquisition time compared with conventional scanning-type imaging mass spectrometers. In order to maintain a high mass resolution, a multi-turn time-of-flight mass spectrometer is combined with post-extraction differential acceleration methods. Consequently, a mass resolution of m/Δm ∼ 10 000 and a spatial resolution of 1μm were achieved simultaneously in this study. Application of our newly established apparatus to biological samples accomplished successful imaging mass spectrometry by exhibiting an organ-specific distribution of endogenous ions as well as a localized distribution of exogenously applied ions with an ultra-high spatial resolution image in the size of 18.5 megapixels.

The impact of neutralizer-free ignition of a radio frequency ion thruster on the lifetime of the ion optics system

In the initial stage of a radio frequency ion thruster (RIT) ignition, an influx of electrons is required from an external source into the discharge chamber and ionization of the neutral gas propellant. A neutralizer-free method for Townsend breakdown discharge ignition based on Paschen’s law was developed in this study. The feasibility of the ignition method was confirmed by performing thousands of ignition experiments. Metallic Molybdenum (Mo), pyrolytic graphite (PG) and Zr[Formula: see text]Ti[Formula: see text]Cu[Formula: see text]Ni[Formula: see text]Be[Formula: see text]alloy acceleration grids were prepared, and ignition-induced damage on the grids was investigated. A field-emission scanning electron microscope was used to inspect surface damage on the grids after multiple ignitions and to analyze the influence of the ignition method on the lifetime of the ion optical system. Grid materials for space missions that require multiple RIT ignitions (10[Formula: see text] should be high-strength blocks that are resistant to sputtering corrosion and high temperature.

Maximal kinetic energy and angular distribution analysis of spatial map imaging: Application to photoelectrons from a single quantum state of H2O.

Dynamical or spatial properties of charged species can be obtained using electrostatic lenses by velocity map imaging (VMI) or spatial map imaging (SMI), respectively. Here, we report an approach for extracting dynamical and spatial information from patterns in SMI images that map the initial coordinates, velocity vectors, and angular distributions of charged particles onto the detector, using the same apparatus as in VMI. Deciphering these patterns required analysis and modeling, involving both their predictions from convolved spatial and velocity distributions and fitting observed images to kinetic energies (KEs) and anisotropy parameters (βs). As the first demonstration of this capability of SMI, the ensuing photoelectrons resulting from (2 + 1) resonant ionization of water in a selected rotational state were chosen to provide a rigorous basis for comparison to VMI. Operation with low acceleration voltages led to a measured SMI pattern with a unique vertical intensity profile that could be least-squares fitted to yield KE and β, in good agreement with VMI measurement. Due to the potential for improved resolution and the extended KE range achievable by this new technique, we expect that it might augment VMI in applications that require the analysis of charged particles and particularly in processes with high KE release.

A compact electrostatic lens for velocity map imaging experiments

The traditional three-electrode Velocity Map Imaging (VMI) electrostatic lens developed by Eppink and Parker has proven to be a useful tool in a wide range of applications in molecular dynamics, including photodissociation experiments taking place in a molecular beam. However, in some other cases, such as crossed molecular beam experiments, the large diameter (typically ∼70 mm) of this conventional VMI lens can be inconvenient. We describe here a VMI lens with a much smaller diameter (down to ∼20 mm) using tubular electrodes. The performance is evaluated by imaging O+ fragments produced by the photodissociation of O2 at around 225 nm, and by inelastic scattering of NO with Ar. Our results suggest that the recorded images are of acceptable quality compared to those from a standard lens while achieving a fourfold reduction of the VMI lens diameter.

Three-dimensional nanoprinting via charged aerosol jets.

Three-dimensional (3D) printing1-9 has revolutionized manufacturing processes for electronics10-12, optics13-15, energy16,17, robotics18, bioengineering19-21 and sensing22. Downscaling 3D printing23 will enable applications that take advantage of the properties of micro- and nanostructures24,25. However, existing techniques for 3D nanoprinting of metals require a polymer-metal mixture, metallic salts or rheological inks, limiting the choice of material and the purity of the resulting structures. Aerosol lithography has previously been used to assemble arrays of high-purity 3D metal nanostructures on a prepatterned substrate26,27, but in limited geometries26-30. Here we introduce a technique for direct 3D printing of arrays of metal nanostructures with flexible geometry and feature sizes down to hundreds of nanometres, using various materials. The printing process occurs in a dry atmosphere, without the need for polymers or inks. Instead, ions and charged aerosol particles are directed onto a dielectric mask containing an array of holes that floats over a biased silicon substrate. The ions accumulate around each hole, generating electrostatic lenses that focus the charged aerosol particles into nanoscale jets. These jets are guided by converged electric-field lines that form under the hole-containing mask, which acts similarly to the nozzle of a conventional 3D printer, enabling 3D printing of aerosol particles onto the silicon substrate. By moving the substrate during printing, we successfully print various 3D structures, including helices, overhanging nanopillars, rings and letters. In addition, to demonstrate the potential applications of our technique, we printed an array of vertical split-ring resonator structures. In combination with other 3D-printing methods, we expect our 3D-nanoprinting technique to enable substantial advances in nanofabrication.

Featuring ultimate sensitivity of high-resolution LC-MS analysis of phenolics in rat plasma.

Sensitive analysis of very low-molecular weight metabolites using liquid chromatography with quadrupole-time-of-flight mass spectrometry is challenging due to the high losses of ions in a time-of-flight analyzer. Improvement in sensitivity for these analytes via the optimization of advanced parameters, including quadrupole profile, ion guide parameters, and duty cycle, has been achieved. The optimization of the method was carried out using a large spectrum of structurally different compounds including (iso)flavonoids and their known metabolites. These compounds can be categorized into two major groups, that is, compounds with (iso)flavonoid core and low-molecular weight phenolics. The optimization of the duty cycle enabled up to a 15-fold increase in analyte responses while the contribution of tuning ion optics and quadrupole profile was negligible. The limits of quantifications of our new method were assessed using both standard solutions and rat plasma. They were decreased at least 10 times for several low-molecular weight phenolics enabling measurement of their concentrations in a range of 1-50ng/mL in rat plasma after protein precipitation. Concurrently, the limits of quantifications for compounds with (iso)flavonoid core did not increase distinctly allowing their detection in a range of 0.5-10ng/mL. The new method was used for the targeting of phenolics in biological samples from pharmacokinetics experiments.

A versatile microscope

Quantum Gases Quantum microscopes have been used for more than a decade now to image the many-body states of quantum gases in optical lattices. However, their resolution is limited to optical wavelengths, and they can typically image only a single layer of atoms. As a complementary technique for imaging quantum gases, Veit et al. developed an ion microscope. Atoms were first ionized by laser light, and the resulting ions were controlled using electrostatic lenses and imaged. Although the technique was demonstrated on rubidium atoms in their ground state, it is expected that it will also work on highly excited Rydberg atoms and will enable three-dimensional and time-dependent imaging. Phys. Rev. X 11 , 011036 (2021).

A three-dimensional velocity-map imaging setup designed for crossed ion-molecule scattering studies

In this study, we report the design and simulation of an electrostatic ion lens system consisting of 22 round metal plates. The opening of the extractor plate is covered with metal mesh, which is for shielding the interaction region of the lens system from the high DC voltages applied to all other plates than the repeller and extractor plates. The Simion simulation shows that both velocity-mapping and time focusing can be achieved simultaneously when appropriate voltages are applied to each of the plates. This makes the ion lens system be able to focus large ionic volumes in all three dimensions, which is an essential requirement for crossed ion-molecule scattering studies. A three-dimensional ion velocity measurement system with multi-hit and potential multi-mass capability is built, which consists of a microchannel plate (MCP), a P47 phosphor screen, a CMOS camera, a fast photomultiplier tube (PMT), and a high-speed digitizer. The two velocity components perpendicular to the flight axis are measured by the CMOS camera, and the time-of-flight, from which the velocity component along the flight axis can be deduced, is measured by the PMT. A Labview program is written to combine the two measurements for building the full three-dimensional ion velocity in real time on a frame-by-frame basis. The multi-hit capability comes from the fact that multiple ions from the camera and PMT in the same frame can be correlated with each other based on their various intensities. We demonstrate this by using the photodissociation of CH3I at 304 nm.

Confinement characteristic of primary electrons with the variation of channel width in the discharge chamber of annular ion thruster

In this paper, the confinement characteristic of primary electrons in the non-axisymmetric discharge chamber of annular ion thruster is investigated by a three-dimensional (3D) non-self-consistent particle tracking model with Monte Carlo Collision (MCC) method. The results show that: The density of primary electrons upstream of ion optics on cathode axis is about 5–50 times higher than that of the other side, which means that the density of primary electrons is obviously non-axisymmetric. The channel width has a significant effect on the average density and uniformity of primary electrons. The average density can be increased by nearly 1.5 times under the appropriate channel width, meanwhile, the variance of density distribution can be reduced by more than 2 times. This is because that increasing the channel width can improve the average confinement length of primary electrons greatly. Furthermore, there is an infection point in the increase of primary electron average confinement length with the channel width, which caused by the significant change of magnetic field structure. Under the case after the inflection point, primary electron mainly (more than 50%) moves to the channel center, which makes the average confinement length of primary electrons and their number in the discharge chamber increase largely.

Improvement in the spatial resolution of heavy ion beam probe measurements through application of ion optics.

A technique for more accurately modeling and improving the spatial resolution of heavy ion beam probe measurements is described. We use a set of particle trajectories to numerically determine the focusing properties of a complicated three-dimensional magnetic field and characterize these properties with a transfer matrix. We then modify the transfer matrix approach of traditional ion optics to include a parameter that describes the ionization location of the detected ions. The ion optics model calculated using this technique enables a more accurate description of the particle trajectories than previously feasible. The model also allows one to easily determine an initial beam focus that could be used during experimental operation to optimize the spatial resolution of measurements. The technique has been applied to the design of a heavy ion beam probe diagnostic for the Wendelstein 7-X stellarator, and improvements in the modeled spatial resolution by a factor of about 2 over previous estimates are possible. The improved spatial resolution will enable measurements of plasma fluctuations with smaller wavelengths than would otherwise be possible.

A superconducting Fabry–Perot cavity for trapping cold molecules

A superconducting Fabry–Perot microwave cavity for a molecular trap was designed, constructed and characterized. The cavity was designed to create intense microwave fields that are sufficient to trap cold polar molecules by the AC Stark force. By coating the mirror surfaces with a superconducting material, an unloaded quality factor of up to 1.1 × 106 at 24.087 GHz was achieved at a temperature of 2.24 K. The field strength of 1.06 MV m−1 obtained for a TEM02 transverse mode with an input power of 10 W is sufficiently intense to trap ammonia molecules at a temperature of 85 mK, which is achievable by a conventional Stark molecular decelerator. We have also implemented an ion optics assembly for sensitive detection of molecules inside the microwave cavity by resonance enhanced multi-photon ionization.

Flat electron mirror

Electron beams can be reflected by an electrode that is at a more negative potential than the cathode from which the beam is emitted. We want to design a mirror with a flat mirror electrode where the electrons are reflected at a plane very close to the electrode. The wave front of an electron can then be shaped when the mirror contains a surface topography or modulated potential. However, electron beams reflected by flat electron mirrors are usually characterized by high coefficients of chromatic and spherical aberration. When the mirror is combined with an electrostatic lens to form a tetrode mirror system, the situation deteriorates even further. This places a restrictive limit on the maximum aperture angle of the beam, and consequently also limits the attainable resolution at the image plane. We have numerically studied the dependence of these aberrations as a function of design parameters of the tetrode mirror consisting of a ground, lens, cap, and mirror electrode, and limited ourselves to only using flat electrodes with round apertures, at fixed electrode spacing. It turns out that the third order spherical aberration can be made negative. The negative third order aberration is then used to partially compensate the positive fifth order aberration. This way, a system configuration is obtained that, at 2 keV beam energy, provides a diffraction limited resolution of 7.6 nm at an image plane 25 mm from the mirror at beam semi-angles of 2.3 mrad, enabling an illumination radius of 40 μm at the mirror. The presented tetrode mirror design could spark innovative use of patterned electron mirrors as phase plates for electron microscopy in general, and for use as coherent beam splitters in Quantum Electron Microscopy in particular. An appendix presents a method to calculate the spot size of a focused beam in the presence of both third and fifth order spherical aberration coefficients, which is also applicable to Scanning (Transmission) Electron Microscopes with aberration correctors.

Issues in the numerical modelling of positive ion extraction

The wide application of ion sources requires the optimization of the ion optics to achieve a balance between the performance, life cycle and size. A 2D hybrid model is represented to simulate the ion optics for positive ion extraction where adaptive particle management and parallel computing are considered. Then, misunderstandings and inconsistencies in the previous numerical models of positive ion extraction are discussed, and new methods for determining the key simulation parameters are proposed. The local model of positive ion extraction is found to be unable to form a presheath and simulate a Bohm sheath self-consistently, which indicates that the upstream surface of the numerical domain should be the sheath edge and the simulation parameters should satisfy the sheath characteristics. Accordingly, the methods for determining the length of the upstream domain, the potential of the upstream surface and the injection of ions are discussed, as well as the effects of key parameters on the simulation results. Further simulations demonstrate that the thermal velocities of ions can affect the perveance condition of the ion beamlet. By comparing the simulation results with earlier experimental results, it is shown that our 2D model can describe the beam divergence characteristics correctly and predict the perveance limit well with high computational efficiency.

Focusing properties of a very thin electrostatic quadrupole lens

The rainbow effect in transmission of a proton beam of energy of 10 keV through a very thin electrostatic quadrupole lens is considered theoretically. The incident ion velocity vectors are taken to be parallel to the lens axis. The distance between the positively or negatively charged lens electrodes is 10 mm and the electrode voltage modulus is 1 kV. The approach is based on the momentum approximation. The rainbow effect is studied via a detailed analysis of the ion differential transmission cross-section, which is determined by the Jacobian of the ion transmission angle components. This variable contains a term describing the ion deflection from the individual electrodes and a term describing the coupling of the electrodes that is seen by the ion. It has been found that the latter term is the source of the rainbow effect, which is, thus, a complex effect in ion beam dynamics. The final conclusion of the calculations is that the lens acts on the ion beam in a way to reshape it qualitatively into a beam containing four narrow sub-beams directed between the lens electrodes.

Simulation Study for Beam Extraction with an Electrostatic Einzel Lens for Low Energy Accelerator Mass Spectrometry

Radiocarbon dating using a compact or low energy accelerator mass spectrometer has been rapidly developing in recent times. A gaseous ion source was designed and simulated to reduce sample preparation times. Then, a simulation study for low-energy accelerator mass spectrometry using the designed ion source for low energy beam transport was conducted to analyze beam separation optimization. Based on our simulation results, the use of an electrostatic Einzel lens led to an enhancement in beam focusing and separation. The ion beam trajectories were calculated using a SIMION program. These simulation results for ion beam extraction are presented in this paper.

Ion beam diagnostic for the assessment of miniaturized electric propulsion systems.

An automated plume diagnostic system is presented for two-dimensional mapping of the ion beam produced by using miniaturized electric propulsion devices. The instrument is designed to operate with noble and reactive gases and consists of a semi-circular probe array with a unique angular distribution following a Gaussian shape for improved beam profile resolution. To demonstrate the operation of the diagnostic system, a series of experiments are conducted with a low-power gridded ion thruster using xenon and iodine propellants. The two-dimensional current density maps generated are used to calculate important beam properties for performance estimates, including the total ion current and the effective divergence angle, and we show how the measurements can be used to assess and optimize the design of ion optics for gridded ion thrusters. All current density measurements with the new instrument are corrected for secondary electron emission (SEE) due to ion bombardment, and using a separate, specially designed, electrostatic probe, we present the first measurements of the SEE yield for iodine ions impacting molybdenum and SAE 304 stainless steel surfaces.

The Seven S Criteria for Evaluating the Performance of a MALDI Mass Spectrometer for MSI

Our goal in this work is to evaluate a new combination linear/reflector MALDI-TOF instrument toward satisfying all "7S criteria" for the ideal MSI mass spectrometer. The linear analyzer satisfies all of the 7 criteria except for Specificity. The new instrument described here adds a reflector to provide up to 50,000 mass resolving power with ppm mass accuracy and with no sacrifice in speed, spatial resolution, and sensitivity demonstrated earlier for the linear MALDI-TOF. This instrument employs new laser optics that produces a 5 kHz laser beam with 2.5-25 μm diameter under computer control. The most important advance is the patented combination of laser and ion optics that provides very high efficiency for production and detection of ions generated by laser desorption using small diameter laser beams. This provides spectra with a wide dynamic range summing a relatively small number of laser shots/pixels. Rat and mouse brain tissues have been used for these initial studies. Examples of negative ion images of lipids and positive ion images from tryptic digestion of proteins are presented. These results demonstrate a very high speed for MSI. This speed is derived from a combination of high laser rate (5 kHz), fast motion of sample relative to the laser beam (20 mm/s), very high ionization efficiency (up to 50%), and the ability to acquire, process, and save spectra at a very high rate (1000/s). As a result, the speed that is possible is imposed by other limits, including the mass range, concentration of samples on the surface, and the spatial resolution required.

A simulation study of time-of-flight mass spectrometry with an angular chirp enhanced delay cavity

Recent developments of the time-stretch in optics have been closely related to the concept of time-of-flight in mass spectrometry. While the resolution of optical wavelength in time-stretch techniques relies on the differentiation of optical path lengths induced by either free space or dispersive media, the mass resolution in time-of-flight mass spectrometry depends on the overall length of the field-free region during a fly of ions group. In this study, we introduce the latest concept of free-space angular-chirp-enhanced delay (FACED) cavity from the field of optical time-stretch into the time-of-flight mass spectrometry. The FACED cavity in time-of-flight ion optics will be realized using a pair of angularly misaligned electrostatic ion mirrors. We study the properties of the FACED cavity in ion optics using SIMION and demonstrate how the multiple reflections in a time-of-flight mass analysis can be significantly increased.