Photoemission Gun Research Articles

High voltage dc gun for high intensity polarized electron source

The high intensity polarized electron source is a critical component for future nuclear physics facilities. The Electron Ion Collider (EIC) requires a polarized electron gun with higher voltage and higher bunch charge compared to any existing polarized electron source. At Brookhaven National Laboratory, we have built an inverted high voltage direct current (HVDC) photoemission gun with a large cathode size. We report on the performances of GaAs photocathodes in a high gradient with up to a 16 nC bunch charge. The measurements were performed at a stable operating gap voltage of 300 kV - demonstrating outstanding lifetime, and robustness. We observed obvious lifetime enhancement by biasing the anode. The gun also integrated a cathode cooling system for potential application on high current electron sources. The various novel features implemented and demonstrated in this polarized HVDC gun open the door towards future high intensity-high average current electron accelerator facilities.

Study on characteristics and beam emission theory of UTEM electron gun

In the ultrafast transmission electron microscopy (UTEM), the speed of the electron will be close to the light, when the electron pulse passes through the acceleration field. So the radiation effect will be produced when the electron beam moves at high speed, which will cause energy loss. In this paper, the performance of femtosecond mode photoemission gun is analyzed and studied by simplifying the electron gun model. According to the knowledge of electrodynamics, the radiation power of pulsed electron beam is deduced, and the corresponding radiation energy is obtained, which can provide a reference for the ultrafast dynamics research and design of the photoemission gun.

High-Brightness Continuous-Wave Electron Beams from Superconducting Radio-Frequency Photoemission Gun.

Continuous-wave photoinjectors operating at high accelerating gradients promise to revolutionize many areas of science and applications. They can establish the basis for a new generation of monochromatic x-ray free electron lasers, high-brightness hadron beams, or a new generation of microchip production. In this Letter we report on the record-performing superconducting rf electron gun with CsK_{2}Sb photocathode. The gun is generating high charge electron bunches (up to 10 nC/bunch) and low transverse emittances, while operating for months with a single photocathode. This achievement opens a new era in generating high-power beams with a very high average brightness.

High-brightness electron beams for linac-based bunched beam electron cooling

A high-current high-brightness electron accelerator for low-energy RHIC electron cooling (LEReC) was successfully commissioned at Brookhaven National Laboratory. The LEReC accelerator includes a dc photoemission gun, a laser system, a photocathode delivery system, magnets, beam diagnostics, a superconducting rf booster cavity, and a set of normal conducting rf cavities to provide enough flexibility to tune the beam in the longitudinal phase space. Cooling with nonmagnetized rf accelerated electron beams requires longitudinal corrections to obtain a small momentum spread while preserving the transverse emittances. Electron beams with kinetic energies of 1.6 and 2.0 MeV with a beam quality suitable for cooling were successfully propagated through 100 m of beam lines, including dispersion sections, maintained through both cooling sections in RHIC and used for cooling ions in both RHIC rings simultaneously. The beam quality suitable for cooling RHIC beams was achieved in 2018, which led to the first experimental demonstration of bunched beam electron cooling of hadron beams in 2019.7 MoreReceived 22 November 2019Accepted 23 January 2020DOI:https://doi.org/10.1103/PhysRevAccelBeams.23.021003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeam coolingBeam opticsHigh intensity beam dynamicsAccelerators & Beams

Stable operation of a high-voltage high-current dc photoemission gun for the bunched beam electron cooler in RHIC

Stable operation of a high-voltage high-current dc photoemission gun for the bunched beam electron cooler in RHIC

Operational experience of a 500 kV photoemission gun

Operational experience of a 500 kV photoemission gun at the compact energy recovery linac (cERL) of the High Energy Accelerator Research Organization is presented. The gun, developed at the Japan Atomic Energy Agency, was found to have failures in two out of the ten-segment ceramic insulator just after installation at cERL. The gun had been operated at 390 kV with eight segments until April 2015 and provided a 0.9 mA beam. An additional two-segment insulator was installed on the top of the existing insulators to recover the high-voltage performance. The gun was then conditioned up to 539 kV and has been operated stably at 500 kV. No discharge caused by the gun itself was observed at 500 kV once the high threshold voltage for stable operation exceeded 500 kV. A dark current of a few picoamperes was generated at 500 kV from a photocathode puck with a semiconducting wafer, while no dark current was observed without a semiconducting wafer. Stable generation of a 500 keV beam with current greater than 0.8 mA was demonstrated for more than two hours.

Relativistic Ultrafast Electron Microscopy: Single-Shot Diffraction Imaging with Femtosecond Electron Pulses

We report on a single-shot diffraction imaging methodology using relativistic femtosecond electron pulses generated by a radio-frequency acceleration-based photoemission gun. The electron pulses exhibit excellent characteristics, including a root-mean-square (rms) illumination convergence of 31 ± 2 μrad, a spatial coherence length of 5.6 ± 0.4 nm, and a pulse duration of approximately 100 fs with (6.3 ± 0.6) × 106 electrons per pulse at 3.1 MeV energy. These pulses facilitate high-quality diffraction images of gold single crystals with a single shot. The rms spot width of the diffracted beams was obtained as 0.018 ± 0.001 Å−1, indicating excellent spatial resolution.

Design and test of 704 MHz and 2.1 GHz normal conducting cavities for low energy RHIC electron cooler

The Low Energy RHIC electron Cooler (LEReC) is currently under commissioning at BNL to improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon. The linac of LEReC consists of a DC photoemission gun, one 704 MHz superconducting radio frequency (SRF) booster cavity, and three normal conducting cavities. It is designed to deliver a 1.6 MeV to 2.6 MeV electron beam, with peak-to-peak momentum spread dp/p of less than 7e4. Two of the three normal conducting cavities will be used in LEReC for energy spread correction. A single-cell 704 MHz cavity for energy de-chirping and a three-cell 2.1 GHz third harmonic cavity for RF curvature correction. In this paper, we present the designs and RF test results of these two cavities.

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.

Ultrafast electron microscopy with relativistic femtosecond electron pulses.

Ultrafast electron microscopy (UEM) with femtosecond temporal resolution is a 'dream machine' that has been long envisioned for the study of ultrafast structural dynamics in materials. For this purpose, we developed a prototype UEM with relativistic femtosecond electron pulses generated by a radio-frequency acceleration-based photoemission gun. TEM images of polystyrene latex particles and gold nanoparticles were observed using approximately 100-fs-long electron pulses with energies of 3.1 MeV. The effect of emittance and the number of pulses on the images were investigated. We demonstrated single-shot imaging with the femtosecond electron pulse at low magnification of approximately 500×.

High voltage studies of inverted-geometry ceramic insulators for a 350 kV DC polarized electron gun

Jefferson Lab is constructing a 350 kV direct current high voltage photoemission gun employing a compact inverted-geometry insulator. This photogun will produce polarized electron beams at an injector test facility intended for low energy nuclear physics experiments, and to assist the development of new technology for the Continuous Electron Beam Accelerator Facility. A photogun operating at 350kV bias voltage reduces the complexity of the injector design, by eliminating the need for a graded-beta radio frequency �capture� section employed to boost lower voltage beams to relativistic speed. However, reliable photogun operation at 350 kV necessitates solving serious high voltage problems related to breakdown and field emission. This study focuses on developing effective methods to avoid breakdown at the interface between the insulator and the commercial high voltage cable that connects the photogun to the high voltage power supply. Three types of inverted insulators were tested, in combination with two electrode configurations. Our results indicate that tailoring the conductivity of the insulator material, and/or adding a cathode triple-junction screening electrode, effectively serves to increase the hold-off voltage from 300kV to more than 375kV. Electrostatic field maps suggest these configurations serve to produce a more uniform potential gradient across the insulator.

Design, conditioning, and performance of a high voltage, high brightness dc photoelectron gun with variable gap.

A new high voltage photoemission gun has been constructed at Cornell University which features a segmented insulator and a movable anode, allowing the cathode-anode gap to be adjusted. In this work, we describe the gun's overall mechanical and high voltage design, the surface preparation of components, as well as the clean construction methods. We present high voltage conditioning data using a 50 mm cathode-anode gap, in which the conditioning voltage exceeds 500 kV, as well as at smaller gaps. Finally, we present simulated emittance results obtained from a genetic optimization scheme using voltage values based on the conditioning data. These results indicate that for charges up to 100 pC, a 30 mm gap at 400 kV has equal or smaller 100% emittance than a 50 mm gap at 450 kV, and also a smaller core emittance, when placed as the source for the Cornell energy recovery linac photoinjector with bunch length constrained to be <3 ps rms. For 100 pC up to 0.5 nC charges, the 50 mm gap has larger core emittance than the 30 mm gap, but conversely smaller 100% emittance.

Experimental investigation of an optimum configuration for a high-voltage photoemission gun for operation at≥500 kV

We demonstrated the generation of a 500-keV electron beam from a high dc voltage photoemission gun for an energy recovery linac light source [N. Nishimori et al., Appl. Phys. Lett. 102, 234103 (2013)]. This demonstration was achieved by addressing two discharge problems that lead to vacuum breakdown in the dc gun. One is field emission generated from a central stem electrode. We employed a segmented insulator to protect the ceramic insulator surface from the field emission. The other is microdischarge at an anode electrode or a vacuum chamber, which is triggered by microparticle transfer or field emission from a cathode electrode. An experimental investigation revealed that a larger acceleration gap, optimized mainly to reduce the surface electric field of the anode electrode, suppresses the microdischarge events that accompany gas desorption. It was also found that nonevaporable getter pumps placed around the acceleration gap greatly help to suppress those microdischarge events. The applied voltage as a function of the total gas desorption is shown to be a good measure for finding the optimum dc gun configuration.

Generation of a 500-keV electron beam from a high voltage photoemission gun

High-brightness, high-current electron guns for energy recovery linac light sources and high repetition rate X-ray free-electron lasers require an exit beam energy of ≥500 keV to reduce space-charge induced emittance growth in the drift space from the gun exit to the following superconducting accelerator entrance. At the Japan Atomic Energy Agency, we have developed a DC photoemission gun employing a segmented insulator to mitigate the field emission problem, which is a major obstacle for operation of DC guns at ≥500 kV. The first demonstration of generating a 500-keV electron beam with currents up to 1.8 mA is presented.

Record high-average current from a high-brightness photoinjector

High-power, high-brightness electron beams are of interest for many applications, especially as drivers for free electron lasers and energy recovery linac light sources. For these particular applications, photoemission injectors are used in most cases, and the initial beam brightness from the injector sets a limit on the quality of the light generated at the end of the accelerator. At Cornell University, we have built such a high-power injector using a DC photoemission gun followed by a superconducting accelerating module. Recent results will be presented demonstrating record setting performance up to 65 mA average current with beam energies of 4–5 MeV.

Surface science analysis of GaAs photocathodes following sustained electron beam delivery

Degradation of the photocathode materials employed in photoinjectors represents a challenge for sustained operation of nuclear physics accelerators and high power free electron lasers (FEL). Photocathode quantum efficiency degradation is due to residual gases in the electron source vacuum system being ionized and accelerated back to the photocathode. These investigations are a first attempt to characterize the nature of the photocathode degradation, and employ multiple surface and bulk analysis techniques to investigate damage mechanisms including sputtering of the Cs-oxidant surface monolayer, other surface chemistry effects, and ion implantation. Surface and bulk analysis studies were conducted on two GaAs photocathodes, which were removed from the JLab FEL DC photoemission gun after delivering electron beam, and two control samples. The analysis techniques include helium ion microscopy, Rutherford backscattering spectrometry (RBS), atomic force microscopy, and secondary ion mass spectrometry (SIMS). In addition, two high-polarization strained superlattice GaAs photocathode samples, one removed from the continuous electron beam accelerator facility (CEBAF) photoinjector and one unused, were also analyzed using transmission electron microscopy (TEM) and SIMS. It was found that heat cleaning the FEL GaAs wafer introduces surface roughness, which seems to be reduced by prolonged use. The bulk GaAs samples retained a fairly well organized crystalline structure after delivering beam but show evidence of Cs depletion on the surface. Within the precision of the SIMS and RBS measurements, the data showed no indication of hydrogen implantation or lattice damage from ion back bombardment in the bulk GaAs wafers. In contrast, SIMS and TEM measurements of the strained superlattice photocathode show clear crystal damage in the wafer from ion back bombardment.

Recent Developments in High Voltage Photocathode DC Gun for Accelerator

A photoinjector which can generate a high brightness, low emittance and short duration electron bunches is required for future light source accelerators and applications such as electron microscopes. A DC high voltage gun which has a semiconductor photocathode in a high DC electric field is one of the technology choices for the generation of high quality electron beam. Many techniques to the development of semiconductor based photoemission gun are introduced in briefly.

Thermal emittance and response time of a cesium antimonide photocathode

Measurements of the intrinsic emittance and response time of a Cs3Sb photocathode are presented. The emittance is obtained with a solenoid scan technique using a high voltage dc photoemission gun. Photoemission response time is evaluated using a RF deflecting cavity synchronized to a picosecond laser pulse train. We find that Cs3Sb has both small mean transverse energy, 160 ± 10 meV at 532 nm laser wavelength, and a prompt response time (below the resolution of our measurement) making it a suitable material for high brightness electron photoinjectors.

Thermal emittance measurements of a cesium potassium antimonide photocathode

Thermal emittance measurements of a CsK2Sb photocathode at several laser wavelengths are presented. The emittance is obtained with a solenoid scan technique using a high voltage dc photoemission gun. The thermal emittance is 0.56±0.03 mm mrad/mm(rms) at 532 nm wavelength. The results are compared with a simple photoemission model and found to be in a good agreement.

Benchmarking of 3D space charge codes using direct phase space measurements from photoemission high voltage dc gun

We present a comparison between space charge calculations and direct measurements of the transverse phase space of space charge dominated electron bunches from a high voltage dc photoemission gun followed by an emittance compensation solenoid magnet. The measurements were performed using a double-slit emittance measurement system over a range of bunch charge and solenoid current values. The data are compared with detailed simulations using the 3D space charge codes GPT and Parmela3D. The initial particle distributions were generated from measured transverse and temporal laser beam profiles at the photocathode. The beam brightness as a function of beam fraction is calculated for the measured phase space maps and found to approach within a factor of 2 the theoretical maximum set by the thermal energy and the accelerating field at the photocathode.