Acceleration Electrode Research Articles

Do collisions inside the collision cell play a relevant role in CID‐LIFT experiments?

Collision experiments are a valid approach to characterize the ionic species generated by matrix assisted laser desorption ionization (MALDI). When a time-of-flight analyzer is employed, three different approaches are available for these experiments: the postsource decay (PSD), the LIFT and the MALDI-TOF/TOF. The last two are of particular interest because of the overcoming of the PSD problems related to mass calibration of the product ion spectra. Experiments performed by LIFT on linear or cyclic peptides, in presence or in absence of collision gas in the collision cell, gave evidence of an unexpected behavior: the two spectra were practically superimposable, and in the former case only a few new fragmentation channels were activated with low yield. These results mean that the selected ion exhibits a large amount of internal energy, capable of promoting fragmentation processes in the time window corresponding to the flight time between ion source and the acceleration electrode placed after the collision cell. Experiments performed by varying the plume density show that this internal energy uptake occurs in the expanding plume, through multiple collisions. The LIFT data have been compared with those achieved by collisions of ESI-generated [MH](+) ions of angotensin II performed under 'in-source' conditions and by triple-quadrupole experiments. The obtained results show a strong similarity among the spectra, indicating that the internal energy uptake in a MALDI source is comparable with that of 40-eV ions colliding with Ar in a triple-quadrupole instrument.

Spectroscopic study of plasma flow created by a magnetoplasma compressor

Magnetoplasma compressor with a semi-transparent electrode system that operates in the ion current transfer regime is studied. The thermodynamic parameters of the discharge and the compression plasma flow generated in N2+5 % H2 mixture at 500 Pa pressure with input energy 4.9 kJ have been measured. Special construction of the accelerator electrode system enable the electrode shielding by the self-magnetic field resulting in protection from the erosion. Fully predominant N II spectrum is observed in the compression plasma flow. The plasma flow velocity, electron temperature and electron density maximum values are measured close to 35 km/s, 4 eV and 2 × 1016cm−3, respectively.

Distortion of ion trajectories in a time-of-flight mass spectrometer: simulations and experiments with a position sensitive detector

We used a position sensitive detector (PSD) to show the image of an ion packet at the end of a time-of-flight mass-spectrometer. The spread of the ion packet due to wires in the accelerator electrodes is directly observed on the image. A huge effect on the size and shape of the ion packet is observed. Experimental results are compared to SIMION simulations performed using a procedure developed to compute the trajectories of ions close to wires.

Electric and thermodynamic properties of plasma flows created by a magnetoplasma compressor

A magnetoplasma compressor of compact geometry (MPC-CG) with a semi-transparent electrode system that operates in the ion current transfer regime was constructed and studied. The electric and thermodynamic parameters of the discharge and the plasma flow created in different gases and their mixtures (hydrogen, nitrogen, argon and Ar + 3% H2) have been measured to optimize the working conditions within the 100–3000 Pa pressure range for input energy up to 6.4 kJ. A special construction of the accelerator electrode system shielded by the self-magnetic field results in protection from erosion, which is the main cause of the high current cut-off in conventional plasma accelerators. It was found that the compression plasma flow velocity, electron density and temperature predominantly depend on the energy conversion rate from the energy supply to the plasma, since the current cut-off is avoided. The maximum energy conversion rate for MPC-CG was found when operating in hydrogen. The plasma flow velocity and electron density maximum values are measured close to 100 km s−1 and 1017 cm−3, respectively, for input energy of 6.4 kJ at 1000 Pa pressure in hydrogen. Our results appear in good agreement with existing theoretical and experimental data.

Nano-beam production by RFQ with ion-trap injector

Nano-beam production by RFQ with ion-trap injector

Theoretical study of the field distributions in a linear electrohydrodynamic charged particle source for space applications

Electrohydrodynamic sources have been proposed as thrusters in electric space propulsion. Compared with chemical propulsion, electric propulsion is characterized by a relatively low thrust and high exhaust velocity of the propellant. These inherent characteristics of electric propulsion offer direct applications in space missions. A study of a field emission electric propulsion system has centered around an emitter modular unit. This module has evolved from a simple single-pin or point source emitter, through linear arrays of stacked needles to the solid slit type emitter module used by Bartoli et al. [J. Phys. D: Appl. Phys. L7 (1984) 2473; J. IEEE Trans. Plasma Sci. P515 (1987) 593]. To model the device, Mitterauer assumed the liquid metal emitter to be an infinitely long cylinder. In order to preserve the symmetry and make the problem tractable, he assumed the accelerator electrode to be a concentric cylinder, which was then used to study the condition for the onset of instability within a surface capillary wave model. Although this model can be used to estimate the field, it does not exhibit the actual geometry associated with the finite size in the device. In this paper, we use a more accurate model to simulate the experimental configuration. The electric field distributions for a finite slit-type liquid metal charged particle source emitter with a parallel planar anode are calculated using an analytic truncated series solution of Laplace's equation. This approach obviates the difficulties of the finite element method which becomes computationally less efficient when there are large gradients in the electric field. The form of the charge density on the emitter is approximated by a truncated series whose coefficients are determined by the boundary conditions on the emitter and the anode. The electric potential and field distribution are determined in both the plane perpendicular to the slit and in the plane of the slit. The calculated field distribution, as a function of scale of the device, suggests the significance of geometry for controlling the spatial characteristics of the emitted current density.

Pulsed laser ablation of glassy carbon targets for the coating of ion accelerator electrodes

The synthesis of hard carbon films grown by laser ablation of glassy-carbon targets is reported. The carbon films were deposited under various conditions using XeCl pulsed laser deposition at 10 −4 torr on Mo substrates kept at temperatures ranging from 200 to 500°C. The structural features of the deposits were investigated by means of reflection high energy electron diffraction (RHEED) and Raman spectroscopy. Ion sputtering experiments have been performed using 1-keV Ar + beams in order to verify the suitability of such coatings to protect spatial components against erosion by accelerated ions.

Three-Dimensional Numerical Simulation of Field-Emission-Electric-Propulsion Backflow Contamination

A three-dimensional full particle particle-in-cell with Monte Carlo collision simulation model is developed to study charge-exchange ion backe ow from a e eld-emission-electric-propulsion thruster using liquid cesium as a propellant. Contamination from backe ow of low-energy charge-exchange ions generated in the plume is an important spacecraft integration issue. Simulations show that the backe ow ion current is on the order of 0.01% of the total emitting current under typical thruster operating conditions. Nearly all charge-exchange ions are collected by the accelerator electrode close to the emitter. Operating a e eld emission thruster at lower neutral e ux, lower emitter current, or higher emitter potentials helps to reduce the backe ow current and possible spacecraft contamination. The operation of a neutralizer does not have a signie cant effect on the backe ow current under typical space plasma environments. A semi-analytical expression is also derived for quick estimations of the e eld emission thruster backe ow current. Nomenclature Aacc = accelerator area, m 2 Ae; Ai; An = exit area of the electron, ion, and neutral beam, m 2 a = emitter‐accelerator distance, m b = half of accelerator aperature, m C

Measurement of Residual Radioactivity of Machine Elements and Concrete on the Cyclotron Decommissioning

The AVF cyclotron was decommissioned at the CYRIC (Cyclotron and Radioisotope Center) of Tohoku University for installing a new cyclotron. With this decommissioning, we sampled the main part of machine elements such as accelerator box, acceleration electrodes, magnet yoke, and the concrete of the cyclotron vault which had been strongly activated for about 20 years operation. We measured the residual radioactivities of these samples with a high pure Ge detector and identified the nuclides of 22Na, 46Sc, 51Cr, 54Mn,59Fe, 56Co, 58Co, 60Co, 65Zn, 134Cs, 152Eu from the gamma-ray spectra and half-lives. The peak counts per unit time (cps) were converted into radioactivities per unit mass (Bq/g) with the Ge peak efficiency In addition, we measured the spatial dose distribution in the cyclotron room and the surface dose of electromagnet with a Nal (Tl) survey meter. These results will be useful for the future accelerator decommissioning.

Identifying charge-exchange collision products within the ion-energy distribution of electrostatically accelerated plasmas

When used in flowing electrostatically accelerated plasmas, electrostatic energy analyzers, such as Retarding Potential Analyzers (RPA’s) or Cylindrical Mirror Analyzers (CMA’s), occasionally yield data which seem implausible: for a known applied plasma acceleration voltage, electrostatic analyzers may indicate populations of ions having voltages much greater than that available through the electrostatic discharge. The process responsible for this phenomenon is resonant charge-exchange (CEX) collisions between ions of different charge species. Through the transfer of electrons, an ion of charge n+ can appear in an electrostatic analyzer with equivalent voltage of n-times the available acceleration potential. This paper discusses the phenomena responsible for the appearance of such high-voltage ions in the measured voltage distribution function and presents evidence for such reactions in the flowing xenon plasma produced by a Hall-effect current accelerator designed for spacecraft propulsion. For a 300 V applied potential on the Hall accelerator electrodes, CEX collisions are shown to produce ions having voltages as high as 800 V when measured using an electrostatic analyzer.

Numerical Model of Cesium Ion Beams in Field Emission Electric Propulsion Thrusters

A numerical model of the ion beam in field emission electric propulsion thrusters is presented. The objective of the study is to develop a computational tool capable of predicting the effects of thruster configuration parameters (geometry, voltage distribution between the electrodes) on thruster performance (beam divergence, ion exhaust velocity). A two-dimensional, steady model has been adopted. A simple emission model has been assumed, neglecting the detailed, microscopic description of the physical emission mechanism. The electric field is evaluated by a finite element scheme; boundary conditions are provided by an auxiliary technique based on the placement of fictitious charges in proximity to the electrode surfaces. The beam is simulated through a discrete number of charged particles; the effects of the associated space charge density distribution on the electric field is considered. The results of the simulations of typical thruster configurations are presented and discussed. The need to take space charge effects into account to attain a sufficiently accurate solution is indicated: the beam charge density modifies the electric potential distribution generated by the electrodes voltage difference, causing a decrease in the local electric field at emitter tip and an increase in beam divergence. Neutralization at a finite distance must be taken into account to fully appreciate the effects of different voltage distributions between the electrodes: a higher accelerator electrode voltage determines a lower ion exhaust velocity and a larger beam divergence. The capability of an additional, decelerating neutral electrode to focus the ion beam is also investigated, showing that only little improvement in beam containment is attained.

Effects of CT injector acceleration electrode configuration on tokamak penetration

Through compact toroid (CT) injection experiments on the TEXT-U tokamak (with BT ≃ 10 kG and IP ≃ 100 kA), it has been shown that theacceleration electrode configuration, particularly in the vicinity of the toroidal field (TF) coils of the tokamak, has a strong effect on penetrationperformance. In initial experiments, premature stopping of CTs within theinjector was seen at anomalously low TF strengths. Two modifications werefound to greatly improve performance: (a) removal of a section of the innerelectrode and (b) increased diameter of the `drift tube' (which guides theCT into the tokamak after acceleration). It is proposed that the primary drag mechanism slowing CTs is toroidal flux trapping, which occurs when a CT displaces transverse TF trapped within the flux conserving walls of the acceleration electrodes (ordrift tube). For a simple two dimensional (2-D) geometry, a magnetostatic analysis produces a CT kinetic energy requirement of 1/2ρv2 ⩾ α(B02/2μ0), with α = 2/(1-a2/R2) adimensionless number that is dependent on the CT radius a normalized by the drift tube radius R. For a typical CT, this can greatly increase the required energies. A numerical analysis in 3-D confirms the analytical result for long CTs (with length L such that L/a ≳ 10). In addition to flux trapping, the CT shape is also shown to affect theenergy criterion. These findings indicate that a realistic assessment of the kinetic energyrequired for a CT to penetrate a particular tokamak TF must take intoaccount the interaction of the magnetic field with the electrode walls of the injector.

Separation of beam and electrons in the spallation neutron source H− ion source

The spallation neutron source requires an ion source producing an H− beam with a peak current of 35 mA at a 6.2% duty factor. For the design of this ion source, extracted electrons must be transported and dumped without adversely affecting the H− beam optics. Two issues are considered: (1) electron containment transport and controlled removal; and (2) first-order H− beam steering. For electron containment, various magnetic, geometric, and electrode biasing configurations are analyzed. A kinetic description for the negative ions and electrons is employed with self-consistent fields obtained from a steady-state solution to Poisson’s equation. Guiding center electron trajectories are used when the gyroradius is sufficiently small. The magnetic fields used to control the transport of the electrons and the asymmetric sheath produced by the gyrating electrons steer the ion beam. Scenarios for correcting this steering by split acceleration and focusing electrodes will be considered in some detail.

Experimental approaches to crystalline ion beams

Facing the difficulties associated with the preparation of crystalline ion beams in heavy ion storage rings we propose two complementary table top schemes to elucidate fundamental issues like stability criteria for moving ion crystals at low velocities. One is based on a small radio-frequency quadrupole (RFQ) storage ring, where by laser cooling Mg+ ion crystals at rest were observed, but conceptionally modified with additional acceleration electrodes and manufactured with highest mechanical precision. The other approach suggested here implies the use of an RFQ ion guide for the lateral confinement of the ions combined with buffer gas viscous damping, which could lead to an ideal starting condition for first time direct three-dimensional laser cooling in a single pass arrangement.

Miniature electron beam column with a silicon micro field emitter

Silicon micro field emitters (Si-MFEs) are expected to be promising as electron sources for their high brightness and ease in making arrayed microcolumns due to small physical size. We have developed a Si-MFE electron gun assembly that produces an electron beam of 1 keV. All components including the Si acceleration electrode are mounted on an integrated circuit (IC) package stem (TO-8) by anodic and eutectic bonding. A high brightness (75 μA/sr) and a long lifetime (>1000 h) have been observed. To overcome the intrinsic emission instability of Si-MFEs, we developed a simple feedback circuit which controls an extraction voltage. The source position shift and the aberration coefficient change caused by stabilization were evaluated analytically and found to be negligible due to the scaling law as applied to micron size. We confirmed that the total emission fluctuation could be stabilized to less than 1% by detecting the absorption current but also found that this detection should be done in the electron beam column to stabilize the probe current due to the instability of the emission angle. Using the Si-MFE electron gun along with a miniature electron beam column 5 cm in length, we evaluated the electron optical properties and succeeded in demonstrating a scanning electron microscope operation with a resolution less than 0.5 μm.

Magnification and mass resolution in local-electrode atom probes

The mass resolution of a local-electrode atom probe can be improved by accelerating the evaporated ions to a higher energy in order to reduce the energy-deficit-related dispersion. A simple model of the instrument is developed and used to estimate the effects of this secondary acceleration on image magnification and mass resolution. Effects of non-instantaneous secondary acceleration, variations in the secondary acceleration field distribution and electrode length are evaluated using the model. The addition of an acceleration electrode after the extraction electrode is shown to improve the performance of local-electrode atom probes.

Energetic negative ion source for fusion at NIFS (invited)

Large high current hydrogen negative ion sources have been developed for the negative ion based neutral beam injector of the Large Helical Device (LHD) at NIFS. The prototype of the negative ion source is required to deliver a negative ion beam of 45 A at the beam energy of 125 keV. The optimization of 1/3 scale ion sources which are multicusp ion source with a rod and an external magnetic filter, respectively, has been investigated for the operation parameter of the plasma source. A total H− current of 16 A is extracted at an operating pressure of 0.9–0.45 Pa with Cs seeding operation. Negative hydrogen ion current is proportional to the input arc power and a beam current density of 45 mA/cm2 is attained. The beam extraction and acceleration characteristics are studied for a single-stage and a two-stage acceleration electrode. A beam divergence angle of 5 mrad is obtained. The results of research and development of a hydrogen negative ion source at NIFS will be reviewed.

Minimum feature sizes and ion beam profile for a focused ion beam system with post-objective lens retarding and acceleration mode

The beam characteristics of a focused Ga+-ion beam (FIB) at ion energies between 10 and 150 keV have been investigated by using a 100 keV focused ion beam system with an additional post-objective lens retarding and acceleration electrode. Line structures with minimum feature sizes down to 20 nm almost independent of the landing energy of the ions could be realized by sputtering line patterns in a gold-coated GaAs substrate. The current density profile was determined over six orders of magnitude by measuring the radii of FIB-exposed dot structures as a function of the ion dose. The beam diameters have been found to be in the range between 30 and 45 nm, nearly independent of the available ion energies. The photoluminescence signal of locally intermixed AlGaAs/GaAs quantum well structures was used to study the spatial resolution of FIB-implanted line patterns. The results were compared with the lateral resolution, which can be calculated from the current density distribution of the beam profile.

High resolution focussed ion beam implantation with post objective lens retarding and acceleration

Abstract The focussing properties of a focussed ion beam system with a post objective lens retarding and acceleration electrode was investigated. Ga+ ions exposed PMMA samples show linewidths of 30 nm at 70 keV implantation energy. Using post objective lens retarding a minimum beam diameter of 20 nm (width of sputtered lines on a Au coated sample) was obtained for 10 keV Ga+ ions. In PMMA linewidths of 70 nm have been realized by using 10 keV Ga+ ions. Only a slight reduction of the resolution was observed with post objective lens acceleration up to 150 keVtotal ion energy.

Erosion rate diagnostics in ion thrusters using laser-induced fluorescence

We have used laser-induced fluorescence (LIF) to monitor the charge-exchange ion erosion of the molybdenum accelerator electrode in ion thrusters. This real-time, nonintrusive method was implemented by operating a 30cm-diam ring-cusp thruster using xenon propellant. With the thruster operating at a total power of 5 kW, laser radiation at a wavelength of 390 nm (corresponding to a ground state atomic transition of molybdenum) was directed through the extracted ion beam adjacent to the downstream surface of the molybdenum accelerator electrode. Molybdenum atoms, sputtered from this surface as a result of charge-exchange ion erosion, were excited by the laser radiation. The intensity of the laser-induced fluorescence radiation, which is proportional to the sputter rate of the molybdenum atoms, was measured and correlated with variations in thruster operating conditions such as accelerator electrode voltage, accelerator electrode current, and test facility background pressure. We also demonstrated that the LIF technique has sufficient sensitivity and spatial resolution to evaluate accelerator electrode lifetime in ground-based test facilities.