Large Beam Currents Research Articles

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.

Elimination of the charge effect on zirconia coatings using a focused ion beam for characterization of the three-dimensional microstructure

How to improve the acquisition efficiency of electron back scatter diffraction (EBSD) and meanwhile avoid the drifting of images caused by the charge effect is still an issue that needs to be solved urgently for three-dimensional (3D) EBSD characterization of non-conductive ceramic materials. The purpose of this study is to solve the problem based on an experimental and theoretical basis using the yttria-stabilized zirconia (YSZ) coating as the research subject. To eliminate the charge effect, the charge equilibrium point of the YSZ coating was determined using the Duane–Hunt limit method. When the incident energy is equal to the E2 value (3.06 keV), the charge effect on the scanning electron microscopy images of the YSZ coating disappears. However, the incident energy of 3.06 keV exhibits lower EBSD collection efficiency for a 3D EBSD result because of the long exposure time; therefore, increasing the incident energy to 7 keV under the large beam current is a practical way to improve EBSD collection efficiency, which inevitably results in the charge problem again. Furthermore, Ga ion implantation caused the charge effect on the YSZ coating at 7 keV under the large beam current to disappear, which can be attributed to the decrease in bandgap widths and the improvement of electrical conductivity of the t-YSZ phase after Ga ion implantation based on first principles. Calculations of the cohesive energies reveal that Ga ions are more likely to occupy the lattice positions of the YSZ coating after Ga ion implantation. Finally, a high-resolution 3D EBSD image of the YSZ coating was obtained.

Filling pattern dependence of regenerative beam breakup instability in energy recovery linacs

Beam breakup instability is a potential issue for all particle accelerators and is often the limiting factor for the maximum beam current that can be achieved. This is particularly relevant for Energy Recovery Linacs with multiple passes where a relatively small amount of charge can result in a large beam current. Recent studies have shown that the choice of filling pattern and recirculation scheme for a multi-pass energy recovery linac can drastically affect the interactions between the beam and RF system. In this paper we further explore this topic to study how filling patterns affect the beam breakup instability and how this can allow us to optimise the design in order to minimise this effect. We present a theoretical model of the beam-RF interaction as well as numerical modeling and show that the threshold current can vary by factors of 2-4, and potentially even more depending on the machine design parameters. Therefore a judicious choice of filling pattern can greatly increase the onset of BBU, expanding the utility of future ERLs.

G-band Rectangular Beam Extended Interaction Klystron Based on Bi-Periodic Structure

A rectangular beam G-band extended interaction klystron (EIK) based on planar bi-periodic slow wave structure (SWS) is proposed. Such structure has the advantages of uniform and strong electric field, as well as the large frequency spacing, which implies that EIK adopting such structure will have strong beam–wave interaction and will be easy to avoid the mode competition. Furthermore, the planar structure is compatible to the microfabrication technology, thus reducing the fabrication difficulty of THz vacuum electron devices with small sizes. Compared with the sheet beam and large beam current, the rectangular beam with small aspect ratio and the decreased current is easy to realize the beam focusing and high power in engineering. Under the voltage of 16.5 kV, current of 0.3 A, and beam size of 0.30 mm × 0.13 mm, particle in cell simulations show a considerable output power of 390 W, with gain of 38.9 dB, efficiency of 7.9%, and relatively wide bandwidth of 800 MHz. The high-frequency structure has been fabricated by computer numerical control method, and the measurement results are close to the simulations. The proposed planar bi-periodic SWS shows certain superiorities in the performance improvement of millimeter-wave and terahertz EIK.

RAON 중이온 가속기용 EBIS 전하증식기

EBIS charge breeder will be installed at the ISOL beam line to boost the charge state of the ions and accommodate the input beam requirement of 10keV/u for the superconducting linear accelerator of RAON. Developing EBIS charge breeder requires special techniques related to high voltage, ultra high vacuum, large electron beam current, very strong magnetic field, and precise beam alignment etc. In this paper, we describe the operation principle of EBIS charge breeder and the current development status of the RAON EBIS.

真空エレクトロスプレーを用いるイオン銃の開発

Atmospheric pressure electrospray had been used previously in our laboratory to generate massive cluster ion beam for charged droplet ionization; a method we called electrospray droplet impact (EDI). Previous ion gun that employed atmospheric pressure electrospray lacks adequate beam current and density for surface and interface analysis. To improve the EDI beam performance, we have proposed a technique for producing a stable electrospray of aqueous solution under vacuum. It is well known that vacuum electrospray of volatile liquids is difficult because of the freezing of the liquids by evaporative cooling under vacuum. Vacuum electrospray of aqueous solution was achieved by irradiating the tip of the electrospray emitter with infrared or near-infrared laser to prevent the freezing under high vacuum (<0.05 Pa). A stable and large beam current was also obtained by optimizing the vacuum electrospray conditions. On the basis of these results, the vacuum electrospray can be expected to be a high-performance massive cluster ion gun.

Design and fabrication of a sub-millimeter multi-beam folded waveguide structure

A novel multi-beam folded waveguide (MBFW) circuit, which can enhance the output power and interaction efficiency of sub-terahertz (THz) traveling wave tube (TWT), is presented in the paper. Operating with fundamental mode and multiple electron beams means that a larger beam current can be used for a higher output power. The characteristics of the MBFW structure are analyzed and optimized. Compared with the single-beam folded waveguide (SBFW) TWT, the output power of the MBFW TWT increases from 3.64 W to 25.45 W at 140 GHz and its electronic efficiency increases from 1.06% to 7.4% under the conditions of an input peak power of 10 mW, a beam voltage of 9.55 kV and a current of 12 mA. The optimized MBFW structure can be successfully fabricated by micro milling, with dimension errors below expectation, and the measured transmission characteristics are in good agreement with the design.

Multipacting in a coaxial coupler with bias voltage for SRF operation with a large beam current

A superconducting radio-frequency (SRF) module is commonly used for a high-energy accelerator; its purpose is to provide energy to the particle beam. Because of the low power dissipation and smaller impedance of a higher-order mode for this module, it can provide more power to the particle beam with better stability through decreasing the couple bunch instability. A RF coupler is necessary to transfer the high power from a RF generator to the cavity. A coupler of coaxial type is a common choice. With high-power operation, it might suffer from multipacting, which is a resonance phenomenon due to re-emission of secondary electrons. Applying a bias voltage between inner and outer conductors of the coaxial coupler might increase or decrease the strength of the multipacting effect. We studied the effect of a bias voltage on multipacting using numerical simulation to track the motion of the electrons. The simulation results and an application for SRF operation with a large beam current are presented in this paper.

Theoretical study on mode competition between fundamental and second harmonic modes in a 0.42 THz gyrotron with gradually tapered complex cavity

In this paper, the nonlinear dynamics of mode competition in the complex cavity gyrotron are studied by using multi-frequency, time-dependent theory with the cold-cavity longitudinal profile approximation. Based on the theory, a code is written to simulate the mode competition in the gradually tapered complex cavity gyrotron operating at second harmonic oscillation. The simulations tracking seven competition modes show that single mode oscillation of the desired mode TE17.4 at 150 kW level can be expected with proper choice of operating parameters. Through studying on mode competition, it is proved that the complex cavity has a good capability for suppressing the mode competition. Meanwhile, it is found that TE17.3 could be excited in the first cavity as a competition mode when the gyrotron operating at large beam current, which leads to that TE17.3 and TE17.4 with different frequencies can coexist stably in the complex cavity gyrotron with very close amplitudes. Thus, the complex cavity might be used for multi-frequency output gyrotron.

Effect of beam current on dispersion relation of plasma-filled dielectric Cherenkov maser

Using plasma fluid theory, from linear perturbation equations, the effect of the beam current size on the dispersion relation of plasma-filled dielectric Cherenkov maser is examined when the beam current is complete, or not effectively neutralized by the return current in the background plasma. The dispersion relation and the wave growth of the TM0n are solved under both cases. Based on this, a discussion on the effect of the beam current on the dispersion relation and wave growth is given. Both of the cases of plasma return current can or not effectively neutralized electron beams current has different dispersion relation and wave growth. When the parameters of waveguide and plasma are fixed, the electron beam current is not the bigger the better, too large electron beam current is harmful for improving the efficiency of the device, and there has an optimal matching relationship between them.

Effect of normalized plasma frequency on electron phase-space orbits in a free-electron laser

Irregular phase-space orbits of the electrons are harmful to the electron-beam transport quality and hence deteriorate the performance of a free-electron laser (FEL). In previous literature, it was demonstrated that the irregularity of the electron phase-space orbits could be caused in several ways, such as varying the wiggler amplitude and inducing sidebands. Based on a Hamiltonian model with a set of self-consistent differential equations, it is shown in this paper that the electron-beam normalized plasma frequency functions not only couple the electron motion with the FEL wave, which results in the evolution of the FEL wave field and a possible power saturation at a large beam current, but also cause the irregularity of the electron phase-space orbits when the normalized plasma frequency has a sufficiently large value, even if the initial energy of the electron is equal to the synchronous energy or the FEL wave does not reach power saturation.

Performance and Operation Results of the RF Systems for the KEK B-Factory

KEK B-Factory (KEKB) was terminated in 2010 after achieving the world record luminosity of 2.11 × 1034 cm−2s−1. This paper describes the design features, performance, and operation results of the KEKB RF systems, which include a lot of original RF technologies and methods to store an electron/positron beam over 1 A stably. To cope with beam instabilities caused by large beam currents, two new types of normal and superconducting (SC) cavities with large stored energy were developed and successfully operated. For a new beam colliding scheme called crab crossing, two SC crab cavities were introduced and boosted the peak luminosity. Performance of these cavities is reported, along with the lowlevel and high-power RF systems, and the cryogenic system for the SC cavities. The status of the RF system upgrade towards SuperKEKB, which aims at a luminosity of 8.0 × 1035 cm−2s−1, is briefly described. © The Author(s) 2013. Published by Oxford University Press on behalf of the Physical Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Disclaimer: Please note that abstracts for content published before 1996 were created through digital scanning and may therefore not exactly replicate the text of the original print issues. All efforts have been made to ensure accuracy, but the Publisher will not be held responsible for any remaining inaccuracies. If you require any further clarification, please contact our Customer Services Department. Copyright © 2013 The Physical Society of Japan Other Oxford University Press sites: Oxford University Press

Electron beam direct write of chalcogenide glass integrated optics

The authors present experimental results of electron beam driven structuring of GexSe1-x glass films with composition (x ∼ 0.2) that lies very close to the floppy to rigid stiffness transition for the purpose of achieving planar ridge optical waveguides. Chalcogenide thin films were deposited by pulsed laser deposition and spin-coating on thermally oxidized silicon substrates. The height and width of the induced deformations tend to increase with slower beam scan rates, greater number of exposures, larger beam currents, and thicker GexSe1-x films. Deformations greater than three times the original film thickness were obtained. Numerical analysis shows that guided wave optical modes are supported. Fabricated directional coupler, ring resonator, and grating structures are promising advances toward realizing planar lightwave devices by direct writing with an electron beam.

Using a controlled pressure field emission gun scanning electron microscope to acquire EBSD data on non-conductive samples

The present work is dealing with electron backscattered diffraction. It is well known that EBSD requires most of the time and up to now, both an important high voltage and a large beam current. However, in such conditions, acquiring EBSD maps on non-conductive samples require to either coat the sample with a conductive thin layer or to use a controlled pressure SEM and then introduce some gas in the sample chamber. The idea here is to present some data comparisons between both solutions. Patterns, orientation images and simulations were carried out to try to access meaningful quantitative results.

A 140-GHz Two-Beam Overmoded Folded-Waveguide Traveling-Wave Tube

A folded-waveguide (FW) traveling-wave tube (TWT) with two electron beams, which operates at the higher order mode, is simulated. Operating with two electron beams means that a larger beam current can be used for a higher output power. Meanwhile, the electric field density of the fundamental mode is not the strongest at the center of the beam channel, so the competition of the fundamental mode is significantly suppressed. A four-section two-beam FWTWT at 140 GHz is designed to get 100 W of average power. From the simulation results, we can see that this kind of tube is unstable if the gain per section is over 11 dB, because both fundamental and band-edge modes are strongly excited. Then, another way for a stable FWTWT operation with two electron beams by loading with dielectric is also presented in this paper. The results presented here can provide a new way to obtain high power radiation at the terahertz frequency.

The beam divergence of an indium LMIS at a distance of 50μm as determined by plasma diagnostic measurements

The current-dependent beam divergence at a distance of 50 μm from an indium-liquid metal ion source is derived from experimental data obtained by measuring the beam spread with a 3D Plasma diagnostic system at a distance of 10 cm from the needle tip. The observed relationship between emission current and beam divergence in vicinity of the emitting needle is used to design a focusing electrode for a field-emission electric propulsion thruster operating at currents up to 150 μA. Another application involves focused ion beam columns which may choose to forego a beam-limiting aperture, such as LMIS-based rapid machining tools with large beam currents.

Water-cooled, in-cavity apertures for high power operation of FEL oscillators

In an oscillator FEL, higher-order harmonic radiation from wigglers can cause serious damage to the downstream FEL resonator mirror and limit the maximum electron beam current for FEL operation due to thermal overload. These problems can be effectively dealt with for FELs driven by helical wigglers using a system to block off-axis wiggler harmonic radiation. In this paper, we report a new scheme to block the off-axis radiation from helical wigglers using a set of motorized, water-cooled, in-cavity apertures. These apertures can reduce the wiggler harmonic radiation power load on the downstream FEL resonator mirror by two orders of magnitude or more. With these apertures, we were able to operate the Duke FEL with record high intracavity power in infrared and visible wavelengths and extend FEL operation into ultraviolet wavelengths with a large electron beam current. The technique for limiting wiggler harmonic radiation using in-cavity apertures is expected to be useful for other types of FEL oscillators including high average power FEL oscillators driven by superconducting linacs.

Performance of the far-IR beamline of the 6 MeV tabletop synchrotron light source

The performance of the far-infrared (FIR) beamline of the 6 MeV tabletop synchrotron light source MIRRORCLE-6FIR dedicated to far-infrared spectroscopy is presented. MIRRORCLE-6FIR is equipped with a perfectly circular optical system (PhSR) placed around the 1 m-long circumference electron orbit. To illustrate the facility of this light source, the FIR output as well as its spectra were measured. The optimum optical system was designed by using the ray-tracing simulation code ZEMAX. The measured FIR intensity with the PhSR in place is about five times higher than that without the PhSR, which is in good agreement with the simulation results. The MIRRORCLE-6FIR spectral flux is compared with a standard thermal source and is found to be 1000 times greater than that from a typical thermal source at approximately 15 cm(-1). It is also observed that the MIRRORCLE-6FIR radiation has a highly coherent nature. The broadband infrared allows the facility to reach the spectral range from 10 cm(-1) to 100 cm(-1). MIRRORCLE-6FIR, owing to a large beam current, the PhSR mirror system, a large dynamic aperture and small ring energy, can deliver a bright flux of photons in the FIR/THz region useful for broadband spectroscopy.

Thermal effects during focused electron beam induced deposition of nanocomposite magnetic-cobalt-containing tips

Focused electron beam induced deposition of tips with cobalt carbonyl is studied with respect to deposit composition and nanostructure. Increasing the beam current from 20 pA to 3 μA at 25 keV, increases the average Co content from 12 to 80 at.% and the initial vertical deposition rate from 2 to 45 nm/s. At the initial growth stage, tip deposits show a smooth surface. This is related to a carbon-rich nano-composite structure with dispersed Co nanocrystals of 1–2 nm in size as shown by transmission electron microscopy (TEM) investigations. With increasing tip length or larger beam currents, transitions to corrugated and polycrystalline surfaces are observed. The observed phenomena fit into heat dissipation estimations where the formation of deposits with high Co-content is attributed to local thermal precursor decomposition at the tip apex.

A GaMn liquid metal ion source for spintronic device fabrication

A new alloy liquid metal ion source (LMIS) has been developed which may be used for the focused ion beam patterning of magnetic elements into GaAs wafers. The GaMn alloy source had a composition of 90% Ga and 10% Mn by atomic weight. The source displayed good operating characteristics with stable and low emission currents. Typical turn-on currents were about 10 μA at an extraction voltage of −6 kV. The maximum current attained for the Mn 2+ beam was 0.8 nA (current density ∼0.1 A/cm 2), which would allow the real-time patterning of approximately (100 μm) 2 of Ga 0.95Mn 0.05As during the wafer growth. Beam resolution was 5 μm, and with the excellent mass separation between the different ion species, and reasonably large beam currents, it demonstrates the suitability of the source for high-resolution device patterning.