Wehnelt Electrode Research Articles

Wehnelt photoemission in an ultrafast electron microscope: Stability and usability

We tested and compared the stability and usability of three different cathode materials and configurations in a thermionic-based ultrafast electron microscope: (1) on-axis thermionic and photoemission from a custom 100 μm diameter LaB6 source with a graphite guard ring, (2) off-axis photoemission from the Ni aperture surface of the Wehnelt electrode, and (3) on-axis thermionic and photoemission from a custom 200 μm diameter polycrystalline Ta source. For each cathode type and configuration, including the Ni Wehnelt aperture, we illustrate how the photoelectron beam-current stability is deleteriously impacted by simultaneous cooling of the source following thermionic heating. Furthermore, we demonstrate usability via collection of parallel- and convergent-beam electron diffraction patterns and by formation of the optimum probe size. We find that usability of the off-axis Ni Wehnelt-aperture photoemission is at least comparable to on-axis LaB6 thermionic emission, as well as to on-axis photoemission [the heretofore conventional approach to ultrafast electron microscopy (UEM) in thermionic-based instruments]. However, the stability and achievable beam currents for off-axis photoemission from the Wehnelt aperture were superior to that of the other cathode types and configurations, regardless of the electron-emission mechanism. Beam-current stability for this configuration was found to be ±1% (one standard deviation from the mean) for 70 min (longest duration tested), and steady-state beam current was reached within the sampling-time resolution used here (∼1 s) for 15 pA beam currents (i.e., 460 electrons per packet for a 200 kHz repetition rate). Repeatability and robustness of the steady-state condition were also found to be within ±1% of the mean. We discuss the implications of these findings for UEM imaging and diffraction experiments, for pulsed-beam damage measurements, and for practical switching between optimum conventional TEM and UEM operation within the same instrument.

Photo-emission electron gun and electron optical simulation for ultrafast scanning electron microscope

Ultrafast scanning electron microscope (USEM) integrates pump-probe technique with microscopic imaging, enabling the visualizing of photon-induced surface charge dynamics with high spatial and temporal resolution. This capability is crucial for high-resolution detection of semiconductor surface states and optoelectronic devices. This work discusses the parametric design of a thermionic emission electron gun that has been modified into a photoemission electron gun, based on a home-built ultrafast scanning electron microscope. Given that the dose of the photoemitting electron beam is usually much lower than that of thermionic emission, the transition to photoemission requires the removal of the self-bias voltage function of the original electron microscope power supply to ensure the normal operation of the Wehnelt electrode. We quantitatively analyze the dependence of bias voltage, cathode, Wehnelt electrode, and anode on the position, size and divergence angle of crossover, which helps to improve the parameter adjustment of the modified electron gun. The analysis results indicate that if the distance between the Wehnelt electrode and the anode is adjusted from 8 to 23 mm, the distance between the filament and wehnelt can be changes from 0.65 to 0.45 mm to cooperate with the bias adjustment, so that the normal use of high-resolution thermionic emission mode, low voltage mode and photoemission mode can be realized. Subsequently, the effect of the mirror’s position on the electron optical path is analyzed. It is found that when the anode is raised 1.4 mm above the mirror, the influence on the electron optical path can be ignored. Additionally, the zero-of-time and temporal broadening of the photo-electron pulse are further simulated. The results indicate that with the increase of bias voltage, the time zero of photoemission will be delayed and the temporal broadening will become larger. This study lays a foundation for the future development of ultrafast electron microscope and the design of photoemission electron sources.

Design and beam dynamics of a Transmission Electron Microscope electron gun assembly

We present the design and beam dynamics analysis of a Transmission Electron Microscope illumination system. A LaB6 emitter is used as a cathode/source with an emissive surface area of diameter 50 μm generating fixed emission current of 5 mA which is biased by −300 V with Wehnelt electrode. The gun is comprised of five electrostatic lenses in-between the focusing and anode electrodes to accelerate and focus the beam with integrated energy of 160 keV in the post anode region with diameter around 228 μm. Finally, the beam is focused using the condensed electro-magnetic lenses of field strengths 183 mT and 331 mT, respectively to spot size 1.8 μm in the post-lens position. The obtained focal spots are largely symmetric along the two transverse directions. The illumination system geometry offers full beam collimation control. The study provides a theoretical basis for the structure and optics design of the Transmission Electron Microscope.

Numerical simulation of beam current control mechanism in the thermionic electron gun

The thermionic electron gun is one of essential parts of electron accelerators and vacuum tubes. For electron beam generation it is basically configured by cathode, anode and it can have control electrode. In common electron guns, beam current is typically controlled by the bias voltage on the Wehnelt electrode. To deeply study the control mechanism of electron beam current, a three-dimensional model of a 60 kV/6 kW thermionic electron gun is established and the simulation of electric field distribution with CST software is shown in this paper. The particle tracking model is used to track the movement of emitted electrons and the quantitative relationship between beam current and bias voltage is established. Results show that electron beam current is inversely proportional to the bias voltage within a certain range. As the bias voltage increases, the electric field distribution in electron gun changes and the equipotential line of accelerating voltage moves towards the emitting surface of cathode, therefore the Effective Emitting Area (EEA) of cathode decreases and, accordingly, the beam current also decreases. Besides, when the bias voltage is fixed, fluctuations of the accelerating voltage cause fluctuations of the beam current, which is especially severe when the beam current is small.

Imaging and electron energy-loss spectroscopy using single nanosecond electron pulses

We implement a parametric study with single electron pulses having a 7 ns duration to find the optimal conditions for imaging, diffraction, and electron energy-loss spectroscopy (EELS) in the single-shot approach. Photoelectron pulses are generated by illuminating a flat tantalum cathode with 213 nm nanosecond laser pulses in a 200 kV transmission electron microscope (TEM) with thermionic gun and Wehnelt electrode. For the first time, an EEL spectrometer is used to measure the energy distribution of single nanosecond electron pulses which is crucial for understanding the ideal imaging conditions of the single-shot approach. By varying the laser power, the Wehnelt bias, and the condenser lens settings, the optimum TEM operation conditions for the single-shot approach are revealed. Due to space charge and the Boersch effect, the energy width of the pulses under maximized emission conditions is far too high for imaging or spectroscopy. However, by using the Wehnelt electrode as an energy filter, the energy width of the pulses can be reduced to 2 eV, though at the expense of intensity. The first EEL spectra taken with nanosecond electron pulses are shown in this study. With 7 ns pulses, an image resolution of 25 nm is attained. It is shown how the spherical and chromatic aberrations of the objective lens as well as shot noise limit the resolution. We summarize by giving perspectives for improving the single-shot time-resolved approach by using aberration correction.

Electron beam dynamics in an ultrafast transmission electron microscope with Wehnelt electrode

High temporal resolution transmission electron microscopy techniques have shown significant progress in recent years. Using photoelectron pulses induced by ultrashort laser pulses on the cathode, these methods can probe ultrafast materials processes and have revealed numerous dynamic phenomena at the nanoscale. Most recently, the technique has been implemented in standard thermionic electron microscopes that provide a flexible platform for studying material's dynamics over a wide range of spatial and temporal scales. In this study, the electron pulses in such an ultrafast transmission electron microscope are characterized in detail. The microscope is based on a thermionic gun with a Wehnelt electrode and is operated in a stroboscopic photoelectron mode. It is shown that the Wehnelt bias has a decisive influence on the temporal and energy spread of the picosecond electron pulses. Depending on the shape of the cathode and the cathode-Wehnelt distance, different emission patterns with different pulse parameters are obtained. The energy spread of the pulses is determined by space charge and Boersch effects, given by the number of electrons in a pulse. However, filtering effects due to the chromatic aberrations of the Wehnelt electrode allow the extraction of pulses with narrow energy spreads. The temporal spread is governed by electron trajectories of different length and in different electrostatic potentials. High temporal resolution is obtained by excluding shank emission from the cathode and aberration-induced halos in the emission pattern. By varying the cathode-Wehnelt gap, the Wehnelt bias, and the number of photoelectrons in a pulse, tradeoffs between energy and temporal resolution as well as beam intensity can be made as needed for experiments. Based on the characterization of the electron pulses, the optimal conditions for the operation of ultrafast TEMs with thermionic gun assembly are elaborated.

Communication: Effects of thermionic-gun parameters on operating modes in ultrafast electron microscopy.

Ultrafast electron microscopes with thermionic guns and LaB6 sources can be operated in both the nanosecond, single-shot and femtosecond, single-electron modes. This has been demonstrated with conventional Wehnelt electrodes and absent any applied bias. Here, by conducting simulations using the General Particle Tracer code, we define the electron-gun parameter space within which various modes may be optimized. The properties of interest include electron collection efficiency, temporal and energy spreads, and effects of laser-pulse duration incident on the LaB6 source. We find that collection efficiencies can reach 100% for all modes, despite there being no bias applied to the electrode.

Analytical Method for Electron Gun Calculation

AbstractThe model of thermionic electron gun was developed. The Dirichlet problem for the cylinder (the Wehnelt electrode) restricted by two bottoms, one of them imitates a plane cathode and another imitates the equipotential surface, was solved analytically. It allows to study electron optical properties of the gun and its behaviour in dependence on Wehnelt potential for different cylinder depths. When the focal distance and the crossover size have the minimal value, this mode is called a work one. The crossover size and the beam half-angle values in this mode were approximated and the analytical method for electron gun calculation was developed.

Numerical Study of Space Charge Effects in a Point Cathode Electron Gun: Distribution of Space Charge Density

The space charge effects in a thermionic emission gun using a tungsten point cathode have been studied with numerical method. The method is based on the integral form of Poisson equation, where the space charges are treated with a large number of coaxial charged rings surrounding the cathode. Each ring charge is estimated from many electron trajectories traced in 3D. The distribution of the space charge density around the cathode of the tip radius 0.4 μm was calculated for different bias conditions and different cathode temperatures. The space charge effects were examined by comparing these distributions as well as the axial potential and field distributions near the tip. The numerical results show that even at the temperature as high as 3100 K the cathode can provide the emission without any noticeable space charge effects when the negative bias on the Wehnelt electrode is shifted by several volts from the cutoff. [DOI: 10.1380/ejssnt.2008.91]

Characteristic X-ray Generator Utilizing Angle Dependence of Bremsstrahlung X-ray Distribution

This generator consists of the following components: a constant high-voltage power supply, a filament power supply, a turbomolecular pump, and an X-ray tube. The X-ray tube is a demountable diode which is connected to the turbomolecular pump and consists of the following major devices: a molybdenum rod target, a tungsten hairpin cathode (filament), a focusing (Wehnelt) electrode, a polyethylene terephthalate X-ray window 0.25 mm in thickness, and a stainless-steel tube body. In the X-ray tube, the positive high voltage is applied to the anode (target) electrode, and the cathode is connected to the tube body (ground potential). In this experiment, the tube voltage applied was from 22 to 36 kV, and the tube current was regulated to within 100 µA by the filament temperature. The exposure time is controlled in order to obtain optimum X-ray intensity. The electron beams from the cathode are converged to the target by the focusing electrode, and clean K-series characteristic X-rays are produced through the focusing electrode without using a filter. The X-ray intensity was 26.6 µGy/s at 1.0 m from the X-ray source with a tube voltage of 30 kV and a tube current of 100 µA, and quasi-monochromatic radiography was performed using a computed radiography system.

Low brightness and high emittance electron gun for a reducing image projection system

An electron gun for an accelerating voltage of 100 kV to be operated under space charge limited condition is designed, and its emittance and brightness are calculated. To obtain low brightness, a Pierce-type electron gun with a Wehnelt electrode and a control anode is adopted. Choosing a low value of the ratio between anode and cathode curvatures reduces the electric field strength at the cathode. A maximum emittance of 1.8 mm mrad and a minimum brightness of 350 A/(cm2 sr) are obtained under the above conditions with a cathode diameter of 1.66 mm. The radius of the Wehnelt aperture and its axial position are optimized. The effects of deviations of the shapes of the Wehnelt and control electrodes from concentric sphere are studied. It is found that in the space between the anode and the control anode the electrodes must be spherical up to an angle of at least 55°, whereas in the space between the Wehnelt electrode and the control anode the electrodes must be spherical up to an angle of at least 21°.

High-Emittance and Low-Brightness Electron Gun for Reducing-Image Projection System: Computer Simulation

The required electron gun characteristics for a reducing-image projection system so that the emittance is much larger than 187 µ m· mrad and the brightness is variable around 7.3×104 A/cm2 sr are estimated. For an electron gun with flat cathode, Wehnelt, control-anode and anode electrodes, an emittance of more than 1000 µ m· mrad is obtained by optimization of the cathode to control anode distance, the control-anode radius and the Wehnelt electrode shape. The brightness is variable from 3.9×104 to 9×104 A/cm2 sr by control of the control-anode voltage. The optimum control-anode radius increases and the brightness tends to decrease with increasing cathode to control-anode distance. The angle between the beam edge and Wehnelt electrode is optimum at about 62°, which is smaller than the well-known 67.5° in a Pierce-type electron gun.

A Magnetically Guided Slow Positron Beam for Defect Studies

The design and construction of a compact, magnetically guided slow positron beam is discussed. The system uses a 30 mCi 22 Νa source. It consists of three main parts: (i) the source chamber with a tungsten foil transmission moderator and the extraction optics, (ii) the beam line with magnetic beam guidance, a bent tube for the separation of the fast positrons and the accelerator stage, (iii) the target chamber with the sample holder and the detector electronics. The energy of the incident positrons can be varied from 30 eV up to 50 keV. Furthermore source geometries, pre-acceleration, main acceleration sections and various magnetic induction profiles have been considered, such as (i) rectangular, conical and bent Wehnelt electrodes, (ű) pre-accelerator voltage shared over several electrodes, (iii) weak, strong, constant and z-dependent B-profiles, (iv) geometric options in the main accelerator region, (v) purely electrostatic and combined electric/magnetic fields. The beam is mainly designed for defect profile studies in ultra high vacuum conditions. PACS numbers: 06.90.+v, 07.90.+c

Design of a LaB6 gun using EGN2 and INTMAG

Abstract In order to launch a high-density electron beam to be focused in the 5 T superconducting solenoid of the Frankfurt EBIS [R. Becker et al., Nucl. Instr. and Meth. B24 (1987) 838], an electron gun has been designed, with a 0.5 mm diameter LaB 6 cathode (FEI Comp., Beaverton, USA) in a 70 mm diameter electrode geometry. The emitting surface is placed in the axial fringing field of the solenoid, modified by an axial shielding disk and a bucking coil, to provide either immersed flow or Brillouin flow conditions for the focused beam. Since the cathode diameter is small as compared to the electrodes, a new feature of EGN2 [W.B. Herrmannsfeldt, SLAC-331 (1988)] had to be used in order to have a sufficient number of meshes along the emitting surface. By starting a field line in the large geometry, a curved Neumann boundary is found for a subdivided part of the gun, which represents the influence of the larger part. EGN2 writes the coordinates of this field line on a file, which can be used by POLYGON [R. Becker, Nucl. Instr. and Meth. B42 (1989) 162] (a boundary setup program for EGN2) to define a curved Neumann boundary. By this procedure, it becomes possible to get a reliable simulation of the emission properties of a small cathode in large gun electrodes. The magnetostatic field calculations have been performed with INTMAG [R. Becker, Nucl. Instr. and Meth. B42 (1989) 303], which is a new program of the boundary element method type. Due to the integration calculus, the results do not need smoothing or “Maxwellisation” for the use in EGN2, where the off-axis fields are evaluated by radial expansion. INTMAG provides an output file, which is suitably formatted to be read in by EGN2. The gun design is based on space-charge-limited emission, but no Pierce-type electrode has been provided in the vicinity of the cathode; instead a Wehnelt electrode on negative bias with respect to the cathode is used to create the correct Pierce-type equipotential in free space, ending on the cathode edge with the correct angle. This gives an additional adjustment tool, if the axial position of the gun is not perfect and it relaxes the radial tolerance requirements considerably.

Point-Cathode Electron Gun Using Electron-Beam Bombardment for Cathode Tip Heating

Point cathode electron gun with high brightness and long cathode life has been developed. In this gun, a straightened tungsten wire is used as the point cathode, and the tip is locally heated to higher temperatures by electron beam bombardment. The high brightness operation and some findings on the local heating are presented.Gun construction is shown in Fig.l. Small heater assembly (annular electron gun: 5 keV, 1 mA) is set inside the Wehnelt electrode. The heater provides a disk-shaped bombarding electron beam focusing onto the cathode tip. The cathode is the tungsten wire of 0.1 mm in diameter. The tip temperature is raised to the melting point (3,650 K) at the beam power of 5 W, without any serious problem of secondary electrons for the gun operation. Figure 2 shows the cathode after a long time operation at high temperatures, or high brightnesses. Evaporation occurs at the tip, and the tip part retains a conical shape. The cathode can be used for a long period of time. The tip apex keeps the radius of curvature of 0.4 μm at 3,000 K and 0.3 μm at 3,200 K. The gun provides the stable beam up to the brightness of 6.4×106 A/cm2sr (3,150 K) at the accelerating voltage of 50 kV. At 3.4×l06 A/cm2sr (3,040 K), the tip recedes at a slow rate (26 μm/h), so that the effect can be offset by adjusting the Wehnelt bias voltage. The tip temperature is decreased as the tip moves out from the original position, but it can be kept at constant by increasing the bombarding beam power. This way of operation is possible for 10 h. A stepwise movement of the cathode is enough for the subsequent operation. Higher brightness operations with the rapid receding rates of the tip may be improved by a continuous movement of the wire cathode during the operations. Figure 3 shows the relation between the beam brightness, the tip receding rate by evaporation (αis the half-angle of the tip cone), and the cathode life per unit length, as a function of the cathode temperature. The working life of the point cathode is greatly improved by the local heating.

Rate control for electron gun evaporation

Principles for obtaining high-quality rate control for electron gun evaporation are discussed. The design criteria for rate controllers are derived from this analysis. Results are presented which have been obtained with e-guns whose evaporation rate is controlled by a Wehnelt electrode or by sweeping of the electron beam. Further improvements of rate stability can be obtained by improved design of e-guns and power supplies.

Novel Monte Carlo simulation of space-charge-induced energy broadening in laser irradiated cathodes

Laser irradiation is one alternative to the use of a Wehnelt for confining the area of emission of an electron gun. The absence of the retarding Wehnelt electrode allows the direct acceleration of the electrons to the anode, reducing space‐charge‐induced energy broadening in several ways. Fewer crossovers are necessary to achieve a given spot size, and the space‐charge density and total current near the source are reduced. A novel Monte Carlo algorithm has been developed to simulate for the first time the space‐charge interactions in this type of gun for thermionic and photonic emission from various source materials. A novel data structure incorporating a linear ordering of the particles in each of three dimensions allows an efficient determination of particle neighbors. The central processing unit time is proportional to n1.4 instead of n2, where n is the number of particles. This provides a savings in the cost of a simulation or allows more particles to be simulated. The simulation results indicate that...

Two electron guns with adjustable beam compression and perveance

Abstract Two electron guns have been designed by computer simulation, in order to produce high density electron beams focused by a superconducting solenoid. A spherical impregnated tungsten cathode is used in a high perveance Pierce type gun, whereas a Rogowski type gun of low perveance may be operated with a directly heated tantalum strip. By variation of the Wehnelt electrode bias both guns are adjustable in the compression—perveance relation with tolerable deterioration of the beam quality.

Entwicklung einer verbesserten sondenstromstabilisierung für rasterelektronenmikroskope development of an improved probe current stabilizing unit for SEMs

AbstractA probe current stabilizing unit is important in quantitative XRMA with long counting times. The uncorrected long‐term drift in a Cambridge Mk II A SEM is of the order of 40% during the first 3 h after switch‐on and is caused by an increase in the emission current by more than 20%, probably due to thermal expansion of the long metal tube containing the filament mounting which reduces the distance to the Wehnelt electrode, and a decrease in probe current due to a horizontal shift of the filament tip and a misalignment of the electron beam. Summarizing the literature the following stabilizing methods can be determined: A motor‐driven readjustment of the filament (Balk et al 1975), control of the Wehnelt bias (Cleaver and Smith 1972, 1973), regulation of the filament current (Chapman 1972, Close and Yarwood 1972, Wellenstein and Ensman 1973, Reed 1968, Nakagawa and Yanaka 1975, Baines et al 1975). Our first method of stabilizing the total emission current (Arndt et al 1978) is working well, but does not solve the problem of constant probe current. In the present experiments, we insulated the three spray apertures (Fig. 1) and recorded the emission current and the specimen current during the first 3 h (Fig. 2). The increasing current at aperture 1 confirms the effect (a) mentioned above. The approximately equal decrease in the current at aperture 3 and at the specimen, initiated experiments aimed at stabilizing the probe current using the current from spray aperture 3. However, this was not an appropriate method because overcompensation effects were observed. Therefore, we finally insulated the whole final aperture holder containing the three limiting apertures below the final lens. The voltage drop on a resistor of a few MΩ is amplified by an FET, and the difference voltage U compared to the compensation voltage is used to make a feed‐back loop to the power supply unit of condenser lens 1. Fig. 3 shows the dependence of the probe current on the current IC1 of this lens. The normal operating point is at IC1 = 0.55 A. We limit the current of this lens to between values of IC1 = 0.4–0.7A to avoid the cut‐off region below 0.4A. The compensation voltage can be changed by a potentiometer “zero” (Fig. 12) to calibrate IC1 to 0.55A with a current meter on the panel, because the electron emission of the filament does not only change with time but also from filament to filament. Fig. 4 shows the variations in the feedback voltage and IC1 with a low and high sensitivity of the feed‐back loop. Fig. 5 shows the dependence of the probe current on IC2‐ Increasing values of IC2 result in an equivalent decrease in IC1. In order to change the probe current using condenser lens 2 a compensation voltage is generated by a logarithmic amplification of a voltage drop at a resistor in series with the lens current meter. Fig. 6 shows the difference between the actual voltage obtained by this method and the voltage required for ideal compensation. This difference causes a small shift of IC1 if IC2 is changed (Fig. 7). The time constant was of the order of 4s to avoid oscillations, because the magnetic lenses are slow to react to fast current changes.The result obtained with the stabilizing unit is presented in Fig. 8. The preexisting decrease of 47% in 3 h was reduced to approximately 1.6% (for comparison see Fig. 2). Figs. 9 and 10 demonstrate how the stabilizing unit regulates the current using high and low sensitivity when the filament current is decreased from 3.0 to 2.1 A. The decrease in emission current is actually compensated for over a broad range. The overall efficiency of the stabilizing unit can be seen from Fig. 11.No defocusing could be seen at low and intermediate magnifications but a small shift of 1 μm was observed at M = 20000 × when the full range of stabilizing was used. It normally takes several hours to reach the limits of the stabilizing range and this unwanted shift can therefore be ignored.

Computer analysis of electron gun characteristics by surface charge method

Computer analysis of an electromagnetic field is indispensable when we come into design problems of electron optical devices. The difference method has been commonly applied where the potential of meshed space is solved using the successive over-relaxation (SOR), but it is fairly unfavorable in the electron optical application, since the potential derivatives necessary for the ray tracing must be evaluated numerically, open boundaries involve ambiguity in specifying their prerequisite potential values, and a large size of computer memories must be provided for mesh points, most of which are to cover the off-axis region where no electron paths take place. These problems become more serious for the case of an electron gun where the cathode has a very sharp curvature at the tip compared with the other configuration. It is for this reason that theoretical investigations could hardly present a quantitatively consistent interpretation over fundamental characteristics of the gun except for some papers which utilize analytical potential functions contrived under several approximations for simplified geometry such as a sphere cathode and an infinitely thin Wehnelt electrode.