Photocathode Gun Research Articles

Photoinjector optimization using a derivative-free, model-based trust-region algorithm for the Argonne Wakefield Accelerator

Model-based, derivative-free, trust-region algorithms are increasingly popular for optimizing computationally expensive numerical simulations. A strength of such methods is their efficient use of function evaluations. In this paper, we use one such algorithm to optimize the beam dynamics in two cases of interest at the Argonne Wakefield Accelerator (AWA) facility. First, we minimize the emittance of a 1 nC electron bunch produced by the AWA rf photocathode gun by adjusting three parameters: rf gun phase, solenoid strength, and laser radius. The algorithm converges to a set of parameters that yield an emittance of 1.08 μm. Second, we expand the number of optimization parameters to model the complete AWA rf photoinjector (the gun and six accelerating cavities) at 40 nC. The optimization algorithm is used in a Pareto study that compares the trade-off between emittance and bunch length for the AWA 70MeV photoinjector.

InSitu Observation of Dark Current Emission in a High Gradient rf Photocathode Gun.

Undesirable electron field emission (also known as dark current) in high gradient rf photocathode guns deteriorates the quality of the photoemission current and limits the operational gradient. To improve the understanding of dark current emission, a high-resolution (∼100 μm) dark current imaging experiment has been performed in an L-band photocathode gun operating at ∼100 MV/m of surface gradient. Scattered strong emission areas with high current have been observed on the cathode. The field enhancement factor β of selected regions on the cathode has been measured. The postexaminations with scanning electron microscopy and white light interferometry reveal the origins of ∼75% strong emission areas overlap with the spots where rf breakdown has occurred.

Hybrid high-energy high-power pulsewidth-tunable picosecond source.

A hybrid ytterbium-doped fiber-bulk laser source allowing the generation of 3 ps, 350 μJ, 116 MW peak power Fourier transform-limited pulses at 50 kHz repetition rate and 1030 nm wavelength is described. Pulse duration tunability is provided by an adjustable spectral compression-based seeder system. Energy scaling capabilities of the architecture by use of the divided-pulse amplification method are investigated. This source provides a robust, compact, and versatile solution for applications such as RF photocathode guns, x- and γ-ray generation by inverse Compton scattering, and optical parametric chirped-pulse amplification pumping.

Temporal profile measurement of an electron bunch with the two-cell rf deflecting cavity at Waseda University

Our group at Waseda University developed an rf deflector system for measuring longitudinal. In this study we evaluated the temporal resolution of an S-band two-cell rf deflector cavity built into the system and performed a series of longitudinal profile measurements of electron bunch from an rf photocathode gun. The rf deflector system achieved a temporal resolution of 147.6 fs (rms), the value gave in close agreement with the design specification. We present our experimental results and discussion how the temporal profile was influenced by the rf electric field and space charge effect in the electron bunch from the rf photo cathode gun. Our results agreed with the results of a beam tracking simulation. The successful results of longitudinal profile measurements in this study confirm that our deflector system will be quite useful for improving an rf electron gun cavities.

用于光阴极的光纤激光研制实验

用于光阴极的光纤激光研制实验

Development of fiber laser system for a photocathode gun

It is difficult to find commercial laser products meeting the requirements of ERL photocathode gun. Therefore, a drive laser system is being developed at Institute of High Energy Physics (IHEP), which comprises two kinds of laser seeds, 1.3 GHz and 100 MHz at repetition rate. Fiber amplifiers were being constructed and a test system was set up. The output power of the main amplifier can reach up to 27 W. With a 5-mm-long LBO crystal, the green second harmonic laser of 6 W was achieved in the preliminary validation experiment. ©, 2015, Editorial Office of High Power Laser and Particle Beams. All right reserved.

光阴极直流高压电子枪束流偏轴发射研究

光阴极直流高压电子枪束流偏轴发射研究

Observation of temporal evolution following laser triggered rf breakdown in vacuum

Radio frequency breakdown is one of the fundamental phenomena that limits the operational performance of most of high power and high gradient vacuum rf devices. We report on experimental results of rf breakdown in an $S$-band photocathode gun triggered by an intensity controlled laser. Through measurement and analysis of the time dependence of the collected current at the gun exit and the stored rf energy in the cavity, one can gain insight into the time evolution of the rf breakdown process. Multiple breakdowns were observed within one rf pulse due to power flow between cells after the initial emission. Similarities of the laser-triggered breakdowns to those occurring in the course of cavity conditioning and normal operation are found by comparing the postbreakdown signals in both cases. It is shown that an intense laser can offer a more controllable and flexible method for rf breakdown studies.

Start-to-end simulation of CAEP FEL-THz beamline

A THz free electron laser (FEL-THz) is being constructed in China Academy of Engineering Physics (CAEP) with the aim of proving high-power, monochromatic and tunable radiation for research in a variety of experimental fields. This facility consists of an injector equipped with a photo-cathode direct current gun, an achromatic transfer line, and an undulator for THz radiation. The physical design of the beamline is accomplished with the PARMELA and TRANSPORT programs. Start-to-end simulation is performed and the result is crosschecked with the codes of IMPACT-T and ELEGANT. The results show that it is feasible to achieve an electron beam with energy of 7~8 MeV, beam current of about 5 mA, and normalized transverse emittance typically below 10 mmmrad at the entrance of the undulator.

Development of a high average current rf linac thermionic injector

Thermionic electron guns are capable of operating at high average currents in a variety of vacuum electronic applications, including conventional microwave tubes, but have been replaced by laser photocathode injectors for most applications requiring high-brightness electron beams. However, while laser photocathode guns are capable of providing the very high-brightness beams, they provide an increased level of system complexity and do not extrapolate well to injectors for high average current applications requiring high beam quality. We are developing a 714 MHz injector based on a gridded thermionic electron gun for these applications. This paper presents an experimental study, computer simulations, and analysis of the performance of an existing gridded thermionic electron gun as an injector prototype, and a design concept for an improved injector configuration based on these results.

Temporal Response Measurements of GaAs-Based Photocathodes

It is well known that a negative electron affinity GaAs photocathode shows a moderate temporal response when excited by a laser pulse of wavelength close to its band gap energy. We show here that the temporal response can be estimated using a diffusion model that describes the internal transport of the conduction electrons. Using a transverse deflection cavity system, we measured the temporal profile of the electron bunch generated by a DC photocathode gun illuminated by a ps pulsed laser. A systematic set of measurements of GaAs cathodes with various active layer thicknesses and boundary conditions confirmed that the observed temporal response is well understood by the diffusion model calculation.

Design of a focused electron beam column for ring-cathode sources

This paper describes focused electron beam columns designed for ring-cathode sources. Each cathode is intended to have a nano-size ring-edge thickness (typically around 50 nm), while its radius can measure hundreds of microns or larger, providing an area of emission that is several orders of magnitude greater than that of conventional field emission cathodes. Using methods of first-order and second-order geometric aberration correction, direct ray tracing simulations predict that these columns will be capable of combining high probe current (>1 μA) with high spatial resolution (<20 nm). Several simple column designs consisting of a ring-cathode gun unit combined with either a single magnetic or electric objective lens are presented. The ring-cathode concept is applicable to both field emission and photo-cathode guns.

Impact of the spatial laser distribution on photocathode gun operation

It is widely believed that a drive laser with uniform temporal and spatial laser profiles is required to generate the lowest emittance beam at the photoinjector. However, for a given 3 ps smooth-Gaussian laser temporal profile, our recent simulations indicate that a truncated-Gaussian laser spatial profile produces an electron beam with smaller emittance. The simulation results are qualitatively confirmed by later analytical calculation, and also confirmed by measurements: emittance reduction of $\ensuremath{\sim}25%$ was observed at the linac coherent light source (LCLS) injector with a truncated-Gaussian laser spatial profile at the nominal operating bunch charge of 150 pC. There was a significant secondary benefit---laser transmission through the iris for the truncated-Gaussian profile was about twice that compared to the nearly uniform distribution, which significantly loosens the laser power and quantum efficiency requirements for drive laser system and photocathode. Since February 9, 2012, the drive laser with the truncated-Gaussian spatial distribution has been used for LCLS routine user operations and the corresponding free electron laser power is at least the same as the one when using the nearly uniform spatial profile.

Advanced Laser Technologies for High-brightness Photocathode Electron Gun

A laser-excited photocathode RF gun is one of the most reliable high-brightness electron beam sources for XFELs. Several 3D laser shaping methods have been developed as ideal photocathode illumination sources at SPring-8 since 2001. To suppress the emittance growth caused by nonlinear space-charge forces, the 3D cylindrical UV-pulse was optimized spatially as a flattop and temporally as squarely stacked chirped pulses. This shaping system is a serial combination of a deformable mirror that adaptively shapes the spatial profile with a genetic algorithm and a UV-pulse stacker that consists of four birefringent α-BBO crystal rods for temporal shaping. Using this 3D-shaped pulse, a normalized emittance of 1.4 π mm mrad was obtained in 2006. Utilizing laser's Z-polarization, Schottky-effect-gated photocathode gun was proposed in 2006. The cathode work functions are reduced by a laser-induced Schottky effect. As a result of focusing a radially polarized laser pulse with a hollow lens in vacuum, the Z-field (Z-polarization) is generated at the cathode.

Recent Developments of Low-emittance Electron Gun for Accelerator

Recent developments of low-emittance electron guns for accelerator are reviewed. In the accelerator field, DC biased triode thermionic gun (Pierce type gun) has been widely used and is still conventional. On the other hand, because of strong demands on the high brightness electron beam by FEL and other advanced accelerator concepts based on linear accelerator, the low emittance beam generation becomes one of the most important issue in the accelerator science. The R&D effort is “accelerated” by two technological innovations, photo-cathode and RF gun. They made a large improvement on the beam emittance. After the explanations on the technical and physical aspects of the low emittance electron beam generation, advanced electron sources for accelerators are reviewed.

A novel scaling law relating the geometrical dimensions of a photocathode radio frequency gun to its radio frequency properties

Developing a photocathode RF gun with the desired RF properties of the π-mode, such as field balance (e(b)) ~1, resonant frequency f(π) = 2856 MHz, and waveguide-to-cavity coupling coefficient β(π) ~1, requires precise tuning of the resonant frequencies of the independent full- and half-cells (f(f) and f(h)), and of the waveguide-to-full-cell coupling coefficient (β(f)). While contemporary electromagnetic codes and precision machining capability have made it possible to design and tune independent cells of a photocathode RF gun for desired RF properties, thereby eliminating the need for tuning, access to such computational resources and quality of machining is not very widespread. Therefore, many such structures require tuning after machining by employing conventional tuning techniques that are iterative in nature. Any procedure that improves understanding of the tuning process and consequently reduces the number of iterations and the associated risks in tuning a photocathode gun would, therefore, be useful. In this paper, we discuss a method devised by us to tune a photocathode RF gun for desired RF properties under operating conditions. We develop and employ a simple scaling law that accounts for inter-dependence between frequency of independent cells and waveguide-to-cavity coupling coefficient, and the effect of brazing clearance for joining of the two cells. The method has been employed to successfully develop multiple 1.6 cell BNL∕SLAC/UCLA type S-band photocathode RF guns with the desired RF properties, without the need to tune them by a tiresome cut-and-measure process. Our analysis also provides a physical insight into how the geometrical dimensions affect the RF properties of the photo-cathode RF gun.

High‐brightness electron sources for a next‐generation light source based on an energy recovery linac

AbstractA high‐brightness electron source of ultrasmall emittance and high average current is one of the most important components for next‐generation light sources based on an energy‐recovery linac (ERL). Such a high‐brightness electron source can be realized by a DC photocathode gun driven by laser pulses tailored in the temporal and spatial dimensions. We propose a novel photocathode based on a quantum cascade laser (QCL). Since the ultrafast response of photoelectron emission from QCL is compatible with the tailored laser pulses, it is a candidate electron source for ERLs. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 177(3): 46–53, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21154

Design of a high repetition rate S-band photocathode gun

Photocathode RF guns have been developed in many laboratories for generating high quality electron beams for free-electron lasers based on linear accelerators. Such guns can generate electron beams with an exceptionally high peak current as well as a small transverse emittance. Their applications have been recently expanded for ultrafast electron diffraction, coherent terahertz radiation, and X-ray or γ ‐ ray radiation by Compton scattering. In this paper, we design an S-band normal-conducting gun with capabilities of high quality beam generation and high repetition rate operation. The RF design and thermal analysis of the gun cavity and coupler are introduced. Optimal position of the gun focusing solenoid for low emittance beam generation is found by performing particle tracking simulations. Then, the gun system is designed to be able to afford the optimal solenoid position. The cooling-water channel surrounding the gun cavity and coupler is designed and analyzed numerically. The pressure in the gun is simulated with a vacuum model containing the detailed inner structure of the gun. An injector for a free-electron laser application is designed by using this gun and the beam dynamics simulation is shown. A cold test with a prototype gun for confirmation of the RF design is reported.

Production of a sub-10 fs electron beam with107electrons

We study the possibility to produce a 1.6 pC electron beam (${10}^{7}$ electrons) with a bunch length of less than 10 fs and a beam energy of a few MeV. Such a short, relativistic beam will be useful for an electron diffraction experiment with a 10 fs time resolution. An electron beam with ${10}^{7}$ electrons will allow a single-shot experiment with a laser pulse pump and an electron beam probe. In this design, an S-band photocathode gun is used for generating and accelerating a beam and a buncher consisting of two S-band four-cell cavities is used for temporally compressing the beam. Focusing solenoids control the beam transverse divergence and size at the sample. Numerical optimization is carried out to achieve a beam with a 4 fs full-width-at-half-maximum length, a 26 microradian root-mean-square divergence, and a 2 nm transverse coherence length at a 3.24 MeV beam energy. When state-of-the-art rf stability is considered, beam arrival time jitter at the sample is calculated to be about 10 fs.

Development of high brightness, high repetition rate photoelectron injectors at STFC Daresbury Laboratory

Accelerator drivers for Energy Recovery Linac (ERL) and Free-Electron Laser (FEL) based light sources demand electron injectors which deliver high brightness bunches on the several hundreds of picocoulomb scale, at repetition rates between 1 MHz and 1 GHz (or higher), corresponding to an average current between 0.1 and 100 mA. Simultaneous satisfaction of these injector requirements is considerably beyond the current state-of-the-art. Daresbury Laboratory is concentrating efforts on the development of high average current III-V and XnY3−nSb photocathode-based DC and SRF photocathode guns for ERL applications. The ultimate goal of this research is their integration with the ALICE ERL.