DC Electron Guns Research Articles

Performance of laser patterned copper plasmonic photocathodes

We study ultrafast laser surface nanopatterning as an alternative to improve the photo-emissive properties of metallic photocathodes. By tailoring the physical dimensions of these surface nanostructures, one can localize the optical field intensity and exploit plasmonic effects occurring in such nanostructures. As a result, this surface nanopatterning technique can become a great tool for improving metallic photocathodes photoemission behavior enabling their use for next generation high brightness electron sources. Our goal is to investigate such surface-plasmon assisted photoemission processes with a view on simplifying the photocathode production at CERN while extending the lifetime of existing photoinjectors. The performance of two different femtosecond laser nanopatterned plasmonic photocathodes was analyzed by measuring the quantum yield with a 65kV DC electron gun utilizing 266nm laser excitation generated by a nanosecond laser with 5ns pulse duration and 10Hz repetition rate. By comparing the electron emission of the copper surface nanostructured areas with that of a flat area, our results suggest quantum yield enhancements of up to a factor of 5.

A cryogenically cooled 200kV DC photoemission electron gun for ultralow emittance photocathodes.

Novel photocathode materials like ordered surfaces of single crystal metals, epitaxially grown high quantum efficiency thin films, and topologically non-trivial materials with dirac cones show great promise for generating brighter electron beams for various accelerator and ultrafast electron scattering applications. Despite several materials being identified as brighter photocathodes, none of them have been tested in electron guns to extract electron beams due to technical and logistical challenges. In this paper, we present the design and commissioning of a cryocooled 200kV DC electron gun that is capable of testing a wide variety of novel photocathode materials over a broad range of temperatures from 298 to 35K for bright electron beam generation. This gun is designed to enable easy transfer of the photocathode to various standard ultra-high-vacuum surface diagnostics and preparation techniques, allowing a full characterization of the dependence of beam brightness on the photocathode material and surface properties. We demonstrate the development of such a high-voltage, high-gradient gun using materials and equipment that are easily available in any standard university lab, making the development of such 200kV electron guns more accessible.

Operation of Cs–Sb–O activated GaAs in a high voltage DC electron gun at high average current

Negative Electron Affinity (NEA) activated GaAs photocathodes are the most popular option for generating a high current (>1 mA) spin-polarized electron beam. Despite its popularity, a short operational lifetime is the main drawback of this material. Recent works have shown that the lifetime can be improved by using a robust Cs–Sb–O NEA layer with minimal adverse effects. In this work, we operate GaAs photocathodes with this new activation method in a high voltage environment to extract a high current. We demonstrate that improved chemical resistance of Cs–Sb–O activated GaAs photocathodes allowed them to survive a day-long transport process from a separate vacuum system using a vacuum suitcase. During beam running, we observed spectral dependence on lifetime improvement. In particular, we saw a 45% increase in the lifetime at 780 nm on average for Cs–Sb–O activated GaAs compared to Cs–O activated GaAs.

An alternative technique for verification of terahertz Smith–Purcell radiation

An alternative technique to verify terahertz (THz) Smith–Purcell radiation (SPR) was proposed by using different grating periods. A compact DC electron gun, serving as a THz source, was modified to generate continuous 35 keV electron beam that passes by a metal-coated silicon grating in order to induce SPR. A design on beam optics to generate high-brightness beam was carried out by using Computer Simulation Technology (CST) Studio Suite® software package. The optimization of beam parameters was obtained and experimentally performed. Results show that the root-mean-square (rms) beam size (diameter) is about 1 mm when supplying beam current of 200 µA, and rms beam emittance is about 3π mm mrad. The CST simulation software was, furthermore, employed to observe radiation spectra when using different grating structures. A liquid–helium cooled silicon bolometer together with a low-pass filter was used to detect radiation power. The results reveal that incoherent THz signal with power of 1.5 µW cm−2 is observed when the continuous-beam current is about 200 µA. The dependence of THz frequency and power on the grating period, that is associated to the SPR, was also investigated in this work.

Design of 325 MHz RF grid-controlled high voltage thermionic cathode electron gun

The electron gun with high repetition rate and high average current has a very wide range of applications. This paper presents the design of a microwave grid-controlled high-voltage thermionic electron gun working in CW mode with a bunch repetition rate of 325 MHz and elaborates the experimental principles of this kind of electron guns. Firstly, simulation software EGUN, POISSON (Poisson Superfish) and GPT (General Particle Tracer) are used to accomplish the structure design of a300kV high-voltage DC electron gun and beam dynamics verification.Secondly, to feed the microwave into the gap between the cathode and the grid of the electron gunefficiently, design of a power supply with a scheme of impedance matching from the radio frequency power source to the cathode is completed.Accordingly, a 325 MHz dual-mode coaxial power supply device is designed, and its feasibility is verified and analyzed..

Synthesis, surface chemical analysis, lifetime studies and degradation mechanisms of Cs-K-Sb photocathodes

We report synthesis and characterization of a batch of three cesium potassium antimonide photocathodes that have been grown on pure copper substrates via a ternary co-deposition method whose procedure is described herein. A deposition system that was designed for synthesis of two-element photocathodes has been utilized for synthesis of the aforementioned three-element photocathodes with slope of the in situ photocurrent as the driver for the growth process. A variation of substrate temperature and deposition parameters among the three photocathodes during synthesis has yielded a maximum quantum efficiency of 6% for 140°C substrate temperature. Lifetime studies performed in a 65-kV DC electron gun on two of the photocathodes, but under oxidized states, at tens of μA average currents (ampere-level peak currents) extracted utilizing a 532-nm wavelength, 1-kHz repetition rate laser, have resulted in charge-lifetimes of 6.13 C and 13.78 C, respectively. X-ray photoelectron spectroscopy analysis of the photocathode with the highest quantum efficiency has revealed a nearly impurity-free surface with stoichiometry Cs1.3K1.8Sb1.0 when pristine. Furthermore, it has been found that oxidation of the alkali surface atoms as well as carbon adsorption from hydrocarbons and minor fluorine uptake are the causes of quantum efficiency reduction during laser illumination in the utilized experimental set-up.

Design of ultrabright 270 keV DC photoelectron gun for ultrafast electron diffraction

Compact DC photoelectron guns of high-voltage are highly desired to output ultrabright and ultrashort electron pulses for accessing irreversible processes by using the ultrafast electron diffraction (UED) technique. The high-voltage breakdown, however, is a major technical barrier to providing an intense electric field strength in a condensed space between the photocathode and the anode when the voltage is over 120 kV. In this work, by adopting the concept of voltage division, we propose a novel design of ultrabright near-relativistic DC photoelectron guns for UED. The electric field breakdown mainly caused by micro-particle collision could be avoided such that an optimized three-level acceleration DC gun can work with an electron energy of up to 270 keV and an electric field strength of up to 15 MV/m. N-particle simulations of the electron pulse propagation show that, with such a DC electron gun, it is possible to have ultrabright and ultrashort electron probe pulses with no jitter issue.

Transport of intense beams with current-dependent initial conditions in linearly tapered solenoid channels

AbstractIn electron beams where space charge plays an important role in the beam transport, the beams’ transverse and longitudinal properties will become coupled. One example of this is the transverse–longitudinal correlation produced in a current-modulated beam generated in a DC electron gun, formed through the competition between the time-dependent radial space charge force and the time-independent radial focusing force. This correlation will cause both the slice radius and divergence of the beam extracted from the gun to depend on the slice current. Here we consider the transport of such a beam in a linearly tapered solenoid focusing channel. Transport performance was generally improved with longer taper lengths, minimal initial correlation between slice divergence and slice current, and moderate degrees of initial correlation between initial slice radius and slice current. Performance was also generally improved with lower slice emittances, although surprisingly transport was improved by slightly increasing the assumed slice emittance in certain limited circumstances.

Femtosecond phase control in high-field terahertz-driven ultrafast electron sources

Terahertz-based electron acceleration has recently emerged as a promising candidate for driving next-generation high-brightness electron sources. Although initial demonstrations have proven the feasibility of this technology for accelerating and manipulating the phase space of electrons, further demonstrations of exquisite timing control are required to make use of terahertz acceleration for demanding applications such as light sources and ultrafast electron diffraction. In this paper, we use a two-stage segmented-terahertz-electron-accelerator-and-manipulator (STEAM) setup to demonstrate control over the electron beam energy, energy spread, and emittance. The first rebunching stage is used to tune the duration of 55 keV electron bunches from a DC electron gun that enables femtosecond phase control at the second accelerating stage. For optimized parameters, energy spread and emittance are reduced by 4× and 6×, respectively, relative to operation with the first stage off. A record energy gain of ∼70 keV was achieved at a peak accelerating field of 200 MV/m, resulting in a >100% energy boost in a terahertz-powered accelerator for the first time. These results represent a critical step forward for the practical implementation of terahertz-powered devices in ultrafast electron sources.

A cryogenically cooled high voltage DC photoemission electron source.

Linear electron accelerators and their applications such as ultrafast electron diffraction require compact high-brightness electron sources with high voltage and electric field at the photocathode to maximize the electron density and minimize space-charge induced emittance growth. Achieving high brightness from a compact source is a challenging task because it involves an often-conflicting interplay between various requirements imposed by photoemission, acceleration, and beam dynamics. Here we present a new design for a compact high voltage DC electron gun with a novel cryogenic photocathode system and report on its construction and commissioning process. This photoemission gun can operate at ∼200 kV at both room temperature and cryogenic temperature with a corresponding electric field of 10 MV/m, necessary for achieving high quality electron beams without requiring the complexity of guns, e.g., based on RF superconductivity. It hosts a compact photocathode plug compatible with that used in several other laboratories opening the possibility of generating and characterizing electron beam from photocathodes developed at other institutions.

Solenoid transport of beams with current-dependent initial conditions

Intense charged particle beams will generally be formed with an initial correlation between their longitudinal properties, including longitudinal variations in current, and their transverse properties, including their radius and divergence. This is due to the competition between the transverse focusing fields in the beam source and the time-varying space charge forces in the beam. In DC electron guns where the current modulation is slow compared to the electron transit time, the nature of these correlations was previously shown to depend on the gun geometry, exhibiting a linear dependence of the beam radius and divergence on the beam current at the gun exit. Here, we extend the previous work to study the transport of beams with such correlation in uniform and periodic solenoid channels. For each transport channel configuration studied, the transverse envelope equation is used to calculate the envelope of 101 beam slices differing in their slice currents, as well as initial radius and divergence (due to their dependence on slice current). For each channel configuration, these calculations are performed 546 times, with each of these iterations considering a different degree of correlation between the radius and divergence, and the slice current. It is found that some degree of correlation between the initial radius and slice current actually aids in beam transport, and the required strength of correlation can be estimated with simple models. Increasing the degree of correlation between the initial divergence and slice current is generally counterproductive, and the degree of sensitivity to such correlations depends on the design of the transport channel.

Constructions and Preliminary HV Conditioning of a Photocathode Direct-Current Electron Gun at IHEP**Supported by the Innovation and Technology Fund of Institute of High Energy Physics.

As one of the most important key technologies for future advanced light source based on the energy recovery linac, a photocathode dc electron gun is supported by Institute of High Energy Physics (IHEP) to address the technical challenges of producing very low emittance beams at high average current. Construction of the dc gun is completed and a preliminary high voltage conditioning is carried out up to 440 kV. The design, construction and preliminary HV conditioning results for the dc gun are described.

Design of Turkish Accelerator and Radiation Laboratory in Ankara (TARLA) facility injector magnets

The installation of a high current thermionic gun-based injector capable to accelerate a maximum 1.5 mA electron beam at 250 keV and continuous wave (CW) for various bunch repetition rates has been continuing at the Turkish Accelerator and Radiation Laboratory in Ankara (TARLA) since 2014. The injector, which is based on totally normal conducting technology, will mainly consist of a thermionic DC electron gun, two stage bunch compressor cavities operating at 260 MHz and 1.3 GHz, one dipole magnet, five solenoid lenses and an additional one dipole magnet that is going to be used as spectrometer, and several steerer magnets. In this paper we discuss the design criteria of all magnets of the TARLA injector and present design results.

Extreme regimes of femtosecond photoemission from a copper cathode in a dc electron gun

The femtosecond photoemission yield from a copper cathode and the emittance of the created electron beams has been studied in a $12\text{ }\text{ }\mathrm{MeV}/\mathrm{m}$, 100 keV dc electron gun over a wide range of laser fluence, from the linear photoemission regime until the onset of image charge limitations and cathode damaging. The measured photoemission curves can be described well with available theory which includes the Schottky effect, second-order photoemission, and image charge limitation. The second-order photoemission can be explained by thermally assisted one-photon photoemission (1PPE) and by above-threshold two-photon photoemission (2PPE). Measurements with a fresh cathode suggest that the 2PPE process is dominant. The beam emittance has been measured for the entire range of initial surface charge densities as well. The emittance measurements of space-charge dominated beams can be described well by an envelope equation with generalized perveance. The dc gun produces 0.1 pC bunches with 25 nm rms normalized emittance, corresponding to a normalized brightness usually associated with rf photoguns. In this experimental study the limits of femtosecond photoemission from a copper cathode have been explored and analyzed in great detail, resulting in improved understanding of the underlying mechanisms.

High voltage threshold for stable operation in a dc electron gun

We report clear observation of a high voltage (HV) threshold for stable operation in a dc electron gun. The HV hold-off time without any discharge is longer than many hours for operation below the threshold, while it is roughly 10 min above the threshold. The HV threshold corresponds to the minimum voltage where discharge ceases. The threshold increases with the number of discharges during HV conditioning of the gun. Above the threshold, the amount of gas desorption per discharge increases linearly with the voltage difference from the threshold. The present experimental observations can be explained by an avalanche discharge model based on the interplay between electron stimulated desorption (ESD) from the anode surface and subsequent secondary electron emission from the cathode by the impact of ionic components of the ESD molecules or atoms.

Electron gun for diffraction experiments off controlled molecules

A dc electron gun, generating picosecond pulses with up to 8 × 106 electrons per pulse has been developed. Its applicability for future time-resolved diffraction experiments on state- and conformer-selected laser-aligned or oriented gaseous samples was characterized. The focusing electrodes were arranged in a velocity-map imaging spectrometer configuration. This allowed us to directly measure the spatial and velocity distributions of the electron pulses emitted from the cathode. The coherence length and pulse duration of the electron beam were characterized by these measurements combined with electron trajectory simulations. Electron diffraction data from a thin aluminum foil illustrated the coherence and resolution of the electron-gun setup.

Comparative study of evaporation using DC and AC filament electron guns

Electron beam assisted physical vapour deposition (EB-PVD) and purification of metal by repeated melting using electron guns is a well-established technique in industrial metallurgy. Strip electron gun is considered a cost effective alternative to multiple pencil guns for handling of large size substrates. In the electron guns, the thermionic emission of the electrons from a filament is achieved by using AC or DC filament heating. A study of their relative merits and demerits was conducted for the both types of electron guns. Due to finite length of the filament, the magnetic field generated around the filament by heating current drops down towards ends. The DC filament heating results in electron beam with a comet shape having high power density hot spot at one end with low power density tails. With AC filament heating, electron beam oscillates with the frequency as that of heating current. The study of vapour flux distribution using DC gun revealed that highly directional vapour evolution takes place from a smaller hot spot whereas with AC gun vapour evolution occurs from an oscillatory 2D-evaporating source. The vapour deposit on substrate indicated that evaporation using DC gun caused splashing and granular deposit due to volumetric melting and evaporation from the ingot. This is contrary to the AC filament heating wherein quiet evaporation was observed due to surface melting and evaporation. The experimental results are critically reviewed to decide the configuration of electron guns for large-scale evaporation.

Transverse-longitudinal correlations in electron guns

When a modulated beam is produced in a DC electron gun, the time-dependent space charge force and the time-independent focusing force produce a correlation between current, position along the beam, and the beam’s radius and divergence. This has implications for beam extraction and downstream beam transport. Here, we simulate an electrostatically-focused DC electron gun to study this correlation. Beam radius and divergence at the anode are found to be approximately linear functions of beam current, and the consequences for beam loss and extraction are considered.

High-voltage isolation transformer for sub-nanosecond rise time pulses constructed with annular parallel-strip transmission lines

A novel annular parallel-strip transmission line was devised to construct high-voltage high-speed pulse isolation transformers. The transmission lines can easily realize stable high-voltage operation and good impedance matching between primary and secondary circuits. The time constant for the step response of the transformer was calculated by introducing a simple low-frequency equivalent circuit model. Results show that the relation between the time constant and low-cut-off frequency of the transformer conforms to the theory of the general first-order linear time-invariant system. Results also show that the test transformer composed of the new transmission lines can transmit about 600 ps rise time pulses across the dc potential difference of more than 150 kV with insertion loss of -2.5 dB. The measured effective time constant of 12 ns agreed exactly with the theoretically predicted value. For practical applications involving the delivery of synchronized trigger signals to a dc high-voltage electron gun station, the transformer described in this paper exhibited advantages over methods using fiber optic cables for the signal transfer system. This transformer has no jitter or breakdown problems that invariably occur in active circuit components.

Physical design of a 10 MeV LINAC for polymer radiation processing

In China, polymer radiation processing has become one of the most important processing industries. The radiation processing source may be an electron beam accelerator or a radioactive source. Physical design of an electron beam facility applied for radiation crosslinking is introduced in this paper because of it's much higher dose rate and efficiency. Main part of this facility is a 10 MeV travelling wave electron linac with constant impedance accelerating structure. A start to end simulation concerning the linac is reported in this paper. The codes Opera-3d, Poisson-superfish and Parmela are used to describe electromagnetic elements of the accelerator and track particle distribution from the cathode to the end of the linac. After beam dynamic optimization, wave phase velocities in the structure have been chosen to be 0.56, 0.9 and 0.999 respectively. Physical parameters about the main elements such as DC electron gun, iris-loaded periodic structure, solenoids, etc, are presented. Simulation results proves that it can satisfy the industrial requirement. The linac is under construction. Some components have been finished. Measurements proved that they are in a good agreement with the design values.