Ion Optics Research Articles

Fast trajectory reconstruction techniques for the large acceptance magnetic spectrometer VAMOS++

The large angular and momentum acceptance magnetic spectrometer VAMOS++, at GANIL, France, is frequently used for nuclear structure and reaction dynamics studies. It provides an event-by-event identification of heavy ions produced in nuclear reactions at beam energies around the Coulomb barrier. The highly non-linear ion optics of VAMOS++ requires the use of the heavy ion trajectory reconstruction methods in the spectrometer to obtain the high-resolution definition of the measured atomic mass number. Three different trajectory reconstruction methods, developed and used for VAMOS++, are presented in this work. The performances obtained, in terms of resolution of reconstructed atomic mass number, are demonstrated and discussed using a single data-set of fission fragments detected in the spectrometer.

Investigation of the Acceleration and Deceleration Performance of the New Geometrical Shapes of the Three Electrode

Electrostatic lenses are established using billed electrodes with cylinder bores which are exactly organized on a line of symmetry. In this paper, four designs of three-element cylindrical electrostatic lenses in the acceleration and deceleration mode with different geometrical shapes were created to prove how the operational characteristics of the lenses are affected by the electrode geometry according to potentials, the electron beam diameter in the image plane, kinetic energy, velocity, and acceleration before and after the electron beam passes through each of the four lenses. The best lens is chosen from the four lenses based on its optical performance. It has been found that the lens with the concave outer electrode surface has the best optical performance as an accelerating lens, and the lens with the concave inner electrode surface has the best optical performance as a decelerating lens. It was discovered that the geometry of the electrodes plays a major role in minimizing the beam electron diameter at the image plane and in accelerating and decelerating the electron beam. The calculations are carried out by using the SIMION8.0 and SL Tools Programs package.

Design of an apertureless two-stage acceleration lens for a single-ion implantation system

A single-ion-implantation (SII) system with an implantation accuracy of several tens of nanometers and energy of several tens of kilo-electron volts (keV) is developed to produce an array of nitrogen-vacancy centers (NV centres). The device with the array is expected to be used for quantum information technologies. The system consists of a Paul-trap type laser cooling device (PTLCD) and an electrostatic focusing lens. The PTLCD generates a few single-nitrogen-ions via an ion-cooling procedure. The SII system needs the implantation of single-nitrogen ions with 100% probability, called “deterministic implantation,” to build the array. A focusing lens without an aperture and with a magnification of ≪ 1 must be developed to realize the deterministic implantation. In this study, an electrostatic focusing lens, namely a two-stage acceleration lens, was designed by mainly investigating a second acceleration lens that is suitable for the SII system based on the previously developed two-stage acceleration lens. A new two-stage acceleration lens with a magnification of 0.006 was designed and a numerical simulation based on the lens demonstrated that the beam width for a single-nitrogen ion with an energy of 14 keV was 4.5 nm including intrinsic aberrations at full width at half maximum.

Determination of the ion beam energy width in tandem Van de Graaff accelerators via Auger projectile spectroscopy

Fast ion beams delivered by tandem Van de Graaff accelerators intrinsically have an energy width, ΔEP/EP, due to the voltage ripple of the accelerator, the production mechanism of the highly charged ions inside the tank of the accelerator, as well as geometrical considerations related to the propagation of the ion beam. The ion beam energy width directly affects the experimental resolution and thus its value should be considered in experiments and the corresponding data analysis. Here, we present a method for determining the energy width of ion beams delivered by tandem Van de Graaff accelerators, based on high resolution Auger projectile electron spectroscopy combined with data analysis via Monte Carlo simulations within the SIMION ion optics package.

Voltage scanning and technical upgrades at the Collinear Resonance Ionization Spectroscopy experiment

To optimize the performance of the Collinear Resonance Ionization Spectroscopy (CRIS) experiment at CERN-ISOLDE, technical upgrades are continuously introduced, aiming to enhance its sensitivity, precision, stability, and efficiency. Recently, a voltage-scanning setup was developed and commissioned at CRIS, which improved the scanning speed by a factor of three as compared to the current laser-frequency scanning approach. This leads to faster measurements of the hyperfine structure for systems with high yields (more than a few thousand ions per second). Additionally, several beamline sections have been redesigned and manufactured, including a new field-ionization unit, an electrostatic bend with a larger deflection angle, and improved ion optics. The beamline upgrades are expected to yield an improvement of at least a factor of 5 in the signal-to-noise ratio by avoiding the use of high-power lasers (which yield non-resonantly produced ions) and providing time-of-flight separation between the resonant ions and the collisional background.

Recent progress in the construction of KoBRA for low-energy nuclear physics experiments

A multi-purpose experimental instrument, called as KoBRA (Korea Broad acceptance Recoil spectrometer and Apparatus), has been constructed for low-energy nuclear physics experiments at RAON (Rare Isotope Accelerator complex for ON-line experiments), and is being prepared in order to produce rare isotope beams at an energy of about 20 MeV/nucleon during the first beam commissioning phase. A test was performed to measure the positions of 4He ions at the dispersive and achromatic foci of KoBRA using an 241Am α-source placed at the production target position, so as to examine the momentum dispersion and ion beam transport. We report on the results of the test and the detailed design of KoBRA including ion optics, together with the status of detectors for particle identification of rare isotopes.

KoBRA Wien filter for low-energy RI beam production and recoil separation

The Wien filter is one of the key components in ion optics to improve the mass separation performance. The KoBRA Wien filter will be installed at the low-energy beamline KoBRA of RAON in Korea. The specifications of the KoBRA Wien filter were determined based on the ion beams expected in the KoBRA beamline, especially, beam energies less than about 5 MeV/nucleon suitable for nuclear astrophysics experiments. The Wien filter is designed to have the maximum field intensities of 0.2 T for the magnetic field and 2.0 kV/mm for the electric field in the ±75(H)×±50(V)×2500(L)mm3 good-field region. Performance of the Wien filter was estimated by the ion optics calculations of the KoBRA beamline for 40Ar beams at 18.5 MeV/nucleon and 14O beams at 2.5 MeV/nucleon. The mass resolving powers are 42.65 and 517, respectively. Currently, the KoBRA Wien filter is being manufactured, and will perform a factory acceptance test.

2D and 3D Electrospinning of Nanofibrous Structures by Far-Field Jet Writing

Electrospinning offers remarkable versatility in producing superfine fibrous materials and is hence widely used in many applications such as tissue scaffolds, filters, electrolyte fuel cells, biosensors, battery electrodes, and separators. Nevertheless, it is a challenge to print pre-designed 2D/3D nanofibrous structures using electrospinning due to its inherent jet instability. Here, we report on a novel far-field jet writing technique for precisely controlling the polymer jet in nanofiber deposition, which was achieved through a combination of reducing the nozzle voltage, adjusting the electric field, and applying a set of passively focusing electrostatic lenses. By optimizing the applied voltage, the circular aperture of lenses, and the distance between the adjacent lenses, the best precision achieved using this technique was approximately 200 μm, similar to that of a conventional polymer-based 3D printer. This development makes it possible for printing 2D/3D nanofibrous structures by far-field jet writing for different applications with enhanced performance.

TEMGYM Advanced: Software for electron lens aberrations and parallelised electron ray tracing

Characterisation of the electron beams trajectory in an electron microscope is possible in a few select commercial software packages, but these tools and their source code are not available in a free and accessible manner. This paper introduces the free and open-source software TEMGYM Advanced, which implements ray tracing methods that calculate the path of electrons through a magnetic or electrostatic lens and allow evaluation of the first-order properties and third-order geometric aberrations. Validation of the aberration coefficient calculations is performed by implementing two independent methods – the aberration integral and differential algebra (DA) methods and by comparing the results of each. This paper also demonstrates parallelised electron ray tracing through a series of magnetic components, which enables near real-time generation of a physically accurate beam-spot including aberrations and brings closer the realisation of a digital twin of an electron microscope. TEMGYM Advanced represents a valuable resource for the electron microscopy community, providing an accessible and open source means of characterising electron lenses. This software utilises the Python programming language to complement the growing ecosystem of free and open-source software within the electron microscopy community, and to facilitate the application of machine learning to an electron microscope digital twin for instrument automation. The software is available under GNU Public License number Three (GPL 3).

Simulation of ion optics in a gas-filled solenoid GASSOL

The results of simulation of ion optics in a gas-filled solenoid GASSOL are presented. The principle of operation of the gas-filled solenoid is described and a method for implementing a separation system based on a static turbine is proposed. On the basis of the simulation results, the potential capabilities of the setup for studying the chemical properties of superheavy elements from Fl to Ts are demonstrated.

On the space-charge effects in the beam extraction process of ion thrusters: the roles of compensating electrons and changing beam radius

Space-charge effects limit the beam-extraction capability of the ion optics and thus hinder the miniaturization and other performance improvements of ion thrusters. This paper presents numerical studies of the space-charge effects in ion optics using hybrid and full particle-in-cell (PIC) simulations, and proposes a modified Child–Langmuir (CL) law. As the injected current increases, the parallel-plane electrode system which corresponds to the classical CL law will reach an unstable and oscillatory state, while the ion optics system remains stable because the electrons from the bulk plasma compensate for the space-charge effects. Furthermore, the radial expansion of the ion beam and the loss of ions on the grids can counteract the space-charge effects when the injected current increases. In general, the space-charge effects in ion optics are self-consistently adjusted by the compensating electrons and the variation of the beam radius. Accordingly, we identify a region in ion optics where, generally, no electrons exist to exclude the influence of electron compensation, and then we modify the CL law of this region by taking into account the effect of the change in the beam radius. We validate the modified CL law and demonstrate its effectiveness in predicting the operating points of the ion optics, such as the perveance-limit point.

Three-dimensional analysis of RF-biased ion optics with misalignments of apertures

Accelerator grid hole shift is a critical reason for erosion failure of optic systems in ion thrusters, which may also cause an unexpected roll torque about the ion beam axis. A three-dimensional (3D) model of ion optics is developed based on a particle-in-cell Monte Carlo collision method to investigate the plasma dynamics and performance of radio frequency (RF) grid systems with misalignments of apertures, which are compared with those in the direct current (DC) grid system. For the benchmark case, the 3D model gives a better agreement with the experiments in the ion energy distribution function (IEDF) compared with the two-dimensional model from a previous publication, with 36.4% and 47.9% relative error reduction of the peak position and full width at half maximum (FWHM), respectively, indicating the effectiveness of the developed 3D model. Simulations show that in the RF grid system the ion beamlet is deflected in the direction opposite to the shift of the accelerator grid hole, while electrons move first in the hole shift direction, and then deflect in the opposite direction. The average ion beamlet deflection angle calculated is consistent with the predictions from linear optical theory in both the RF and DC grid system. The amplitude of beamlet deflection angle fluctuation with time decreases with the increase of RF frequency. When the grid holes shift, the ion beamlet will deflect with the divergence angle almost unchanged in the DC grid system, while the beamlet divergence angle increases in the RF grid system. When RF frequency is low, the big vortex-like structure in the electron velocity phase diagram breaks into small vortices, showing a reduced oscillation intensity. The hole shift also causes high-frequency oscillation in the shift direction. In terms of performance, the RF grid system is more sensitive to grid hole shift than the DC grid system.

TEMGYM Advanced – NanoMi lens characterisation

A complete analysis including finite element method (FEM) calculation, focal length properties, and thirdorder geometric aberrations of the open-source electrostatic lens from the NanoMi project is presented. The analysis is carried out by the software TEMGYM Advanced, a free package developed to carry out ray-tracing and lens characterisation in Python. Previously TEMGYM Advanced has shown how to analyse the aberrations of analytical lens fields; this paper expands upon this work to demonstrate how to apply a suitable fitting method to discrete lens fields obtained via FEM methods so that the aberrations of real lens designs can be calculated. Each software platform used in this paper is freely available in the community and creates a free and viable alternative to commercial lens design packages.

Compact 2D electron analyzer CoDELMA: Simultaneous wide reciprocal and real space analysis using wide-angle deceleration lens, CMA and projection lens

A compact wide-acceptance angle (around ± 50°) high-energy-resolution 2D electron analyzer CoDELMA (Compact DELMA) is proposed, constructed and tested. CoDELMA is composed of a recently-proposed variable-deceleration-ratio wide-acceptance-angle electrostatic lens (VD-WAAEL), a cylindrical mirror analyzer (CMA) and a projection lens. After a brief review of VD-WAAEL, the use of a plane grid in the lens is presented as a new design option, which allows keeping the focusing angle almost constant irrespective of the deceleration ratio. A detailed consideration is given to the combination of CMA and a projection lens, which allows simultaneous analysis of wide reciprocal ( ± 8°) and real space ( ± 5 mm) at the entrance of CMA. CoDELMA is the successor to the previously developed DELMA instrument including a concentric hemispherical analyzer. The whole system of CoDELMA becomes much more compact than that of DELMA owing to the use of VD-WAAEL and CMA, and this results in considerable reduction of the production cost. Results of test measurement of energy spectrum and angular distribution using an electron gun as an excitation source confirmed that the energy resolution and the acceptance angle are almost the same as the designed values.

Ion thruster accelerator grid erosion mechanism under extreme conditions of cylindrical erosion and chamfer erosion

In this paper, the abnormal experimental phenomenon on barrel erosion under extreme working conditions in the ultra-long life experiment (>10000 h) of ion thruster ion optics is studied by the Immersed-Finite-Element Particle-In-Cell Monte-Carlo-Collision (IFE-PIC-MCC) method and the grid erosion evaluation model. The transport process of beam ions and Charge Exchange (CEX) ions in the grid system, and the characteristics and mechanisms of the aperture barrel erosion under extreme erosion conditions (i.e. the cylindrical erosion and chamfer erosion) were systematically studied. Thanks to the advantage of the IFE method for dealing with complex boundaries in structured mesh, the aperture barrel erosion morphology of the accelerator grid is reconstructed accurately based on the experimental results. The results show that, with the evolution of working conditions, the mechanism of the aperture barrel erosion changes significantly, which relies heavily on the accelerator grid morphology. The change of the accelerator grid aperture barrel morphology has a significant effect on the behavior of CEX ions, and only affects the local electric field distribution, but has no effect on the upstream plasma sheath. As the erosion progresses, the erosion position moves downstream along the grid aperture axis direction, and the erosion range becomes narrower. Regardless of the erosion phase, the erosion rate of the CEX ions located downstream of the decelerator grid is the largest. The erosion rate is related to the mean incident energy and angle, and their variation is closely related to the position and trajectory of CEX ions.

Optimized design of the ion optics based on the over-perveance characteristics of the beam current

To illustrate the erosion mechanism of the edge aperture by high-energy ions, a three-dimension simulation model was established, and a particle-in-cell method was applied in tracking the hitting processes as well as a Monte Carlo collision method was used to deal with the particles’ collision. Numerical results showed that the potentials interacted among the sheaths of adjacent holes and the beam ions were over-focused in the accelerator gap, causing some ions to be intercepted on the downstream wall of the decelerator grid. This was a typical under-perveance characteristic of the beam current, operating at significantly less than the optimal perveance and corresponding to lower beam currents than optimal combination, which pushed the sheath to the left farther into the plasma. The farther the grid was, the more serious erosion patterns developed. In addition, the charge exchange ions were attracted and bombarded on the decelerator and the accelerator grids, respectively. The ion optics of the 30 cm ion thruster was optimized from four-zone to three-zone, where the plasma density upstream of the screen grid was increased by 1.5% and the perveance characteristics of the beam ions were improved such that many ions struck the edge hole. The background pressure was reduced from 3.0 × 10−3 to 8.0 × 10−4 Pa to decrease the erosion depths of the grids caused by the charge exchange ions, about 9.09% and two orders of magnitude, respectively, for the accelerator and decelerator grids.

A simple in situ method for optimizing settings for the Einzel lens elements in a focused ion beam.

Ion beams have had an incredible impact on research in the past couple of decades. One major reason for this is the continued development of systems having optimal beam currents that allows one to image more clearly at different spot sizes to include higher currents that allow for faster milling. The advancements for Focused ion beam (FIB) columns have developed rapidly due to the computational optimization of lens designs. However, once a system has been produced, the optimal column settings for these lenses may change or simply become obscure. Our work involves regaining this optimization with the newly applied values through a new algorithm, requiring hours, rather than the days or weeks that existing methods require. FIB columns frequently utilize electrostatic lens elements (normally two, condenser and objective). This work presents a method to quickly determine the optimal lens 1 (L1) values for large beam currents (∼1 nA or greater), from a carefully acquired set of images without any detailed knowledge of the column geometry. Each set of images, acquired through a voltage sweep of the objective lens (L2) for a preset L1, is partitioned for its spectral content. The sharpest position at each spectral level is used to assess how close the preset L1 is to the optimal. This procedure is conducted for a range of L1 values, the optimal being the one having the smallest range in spectral sharpness. For a system that has suitable automation in place, the time to optimize L1 for a given beam energy and aperture diameter is ∼1.5 h or less. In addition to the technique for finding optimal condenser and objective lens parameters, an alternative peak determination method is presented.

Start-up process of ion optics in ion thrusters

In this study, a particle-in-cell model of ion optics was established to simulate the changing trend of an ion beam shape over time during an ion optic start-up. This model was established to study the effect of ion beam shape change on the accel grid during ion optical start-up and to discuss the start-up strategy of ion optics. The results show that the ions at the edge of the beam have a radial velocity component toward the centerline of the ion optics owing to the electric field; this causes the ion beam to shrink gradually toward the centerline over time. Furthermore, the accel grid can be prevented from being bombarded by ions by design at the operating voltage. However, the grid may still be bombarded by ions during the low-voltage start-up process, thereby affecting its life. Thus, the ion dynamics must be considered during the start-up process.

A new high parallel-field spectrometer at TRIUMF's β-NMR facility.

A new high field spectrometer has been built to extend the capabilities of the β-detected nuclear magnetic resonance (β-NMR) facility at TRIUMF. This new beamline extension allows β-NMR spectroscopy to be performed with fields up to 200 mT parallel to a sample's surface (perpendicular to the ion beam), allowing depth-resolved studies of local electromagnetic fields with spin polarized probes at a much higher applied magnetic field than previously available in this configuration. The primary motivation and application is to allow studies of superconducting radio frequency (SRF) materials close to the critical fields of Nb metal, which is extensively used to fabricate SRF cavities. The details of the design considerations and implementation of the ultra-high vacuum (UHV) system, ion optics, and beam diagnostics are presented here. Commissioning of the beamline and spectrometer with radioactive ions are also reported here. Future capabilities and applications in other areas are also described.

Design of Low Aberration Electrostatic Lens Operated under Zero Magnification Condition

تعد الطريقة العكسية من الطرق المهمة في تصميم العدسات الكهروستاتيكية والتي اعتمدت في هذا البحث بأستخدام تقنية جديدة تعتمد على تحديد معادلة مناسبة للجهد المحوري على شكل متعدد حدود ومن ثم حل معادلة الشعاع المحوري لابجاد مسار الجسيمات والذي يحقق معادلة الجهد المفروضة. في هذا البحث تم تصميم عدسة كهروستاتيكية بٍأستخدام حلول معادلة لابلاس بأستخدام طريقة العناصر المحددة. وقد بينت النتائج قيم قليلة لمعاملات الزيغين الكروي واللوني حيث كانت بحدود 3.9للزيغ الكروي و o.24 للزيغ اللوني عند نسبة الجهد 10 وهذا يعطي مؤشرآ جيدآ لتصميم العدسة .