Photocathode RF Gun Research Articles

Experimental demonstration of high quality MeV ultrafast electron diffraction

The simulation optimization and an experimental demonstration of improved performances of mega-electron-volt ultrafast electron diffraction (MeV UED) are reported in this paper. Using ultrashort high quality electron pulses from an S-band photocathode rf gun and a polycrystalline aluminum foil as the sample, we experimentally demonstrated an improved spatial resolution of MeV UED, in which the Debye-Scherrer rings of the (111) and (200) planes were clearly resolved. This result showed that MeV UED is capable to achieve an atomic level spatial resolution and a approximately 100 fs temporal resolution simultaneously, and will be a unique tool for ultrafast structural dynamics studies.

Quantum efficiency and thermal emittance of metal photocathodes

Modern electron beams have demonstrated the brilliance needed to drive free electron lasers at x-ray wavelengths with major advances occurring since the invention of the photocathode gun and the realization of emittance compensation. These state-of-the-art electron beams are now becoming limited by the intrinsic thermal emittance of the cathode. In both dc and rf photocathode guns details of the cathode emission physics strongly influence the quantum efficiency and the thermal emittance. Therefore improving cathode performance is essential to increasing the brightness of beams. It is especially important to understand the fundamentals of cathode quantum efficiency and thermal emittance. This paper investigates the relationship between the quantum efficiency and the thermal emittance for metal cathodes using the Fermi-Dirac model for the electron distribution. We use a consistent theory to derive the quantum efficiency and thermal emittance, and compare our results to those of others.

Non-invasive single bunch monitoring for ps pulse radiolysis

A single-shot electro-optic (EO) diagnostic has been installed on the ELYSE photocathode RF gun accelerator to monitor the electron bunch at the place and under the conditions of the ps pulse radiolysis experiments. The EO signal is due to the coulombic field of the electron bunch and to a contribution of a free-space THz radiation generated by the same electron pulse. This signal is recorded shot-to-shot at the repetition rate of the accelerator. The jitter of the arrival time of the electron bunch is characterized for the first time with a non-invasive method and is confirmed to be around 1 ps.

Design of an MeV ultra-fast electron diffraction experiment at Tsinghua university

Time-resolved MeV ultra-fast electron diffraction (UED) is a powerful tool for structure dynamics studies. In this paper, we present a design of a MeV UED facility based on a photocathode RF gun at Tsinghua University. Electron beam qualities are optimized with numerical simulations, indicating that resolutions of 250 fs and 0.01 A, and bunch charge exceeding 105 electrons are expected with technically achievable machine parameters. Status of experiment preparation is also presented.

Ultra-low emittance X-band photocathode RF gun

In this paper, we present the simulation results of a 1.6 cell X-band photocathode RF gun for ultra-low emittance electron beams. It will work at 9.3 GHz. The emittance, bunch length, electron energy and energy spread at the gun exit are optimized at bunch charge of 1pC using PARMELA. Electron bunches with emittance about 0.1 mm · mrad and bunch length less than 100 fs can be obtained from this gun. A PITZ type coupler is adopted in this gun and an initial simulation by MAFIA is also given in this paper.

Tsinghua Thomson scattering X-ray source

We proposed the Tsinghua Thomson scattering X-ray (TTX) source as an ultra-fast, high flux source for advanced X-ray imaging studies and applications. A linac system, which consists of an S-band photocathode RF gun, a SLAC type 3 m traveling wave tube and two X-band structures, generates ultra-short, high brightness electron pulses to scatter with tera-watt femto-second laser pulses. A compact low energy electron storage ring is also designed to dramatically enhance the average X-ray flux. In this paper, we present the simulation studies and optimized parameters of the electron and X-ray pulses. The construction and commissioning status of TTX is also reported.

100-femtosecond MeV electron source for ultrafast electron diffraction

A near-relativistic 100-fs MeV electron beam is developed by using a photocathode rf gun for revealing the hidden ultrafast dynamics of intricate molecular and atomic processes in materials through experimentation of ultrafast time-resolved electron diffraction (UED). The transverse and longitudinal dynamics of femtosecond electron beam in the rf gun were studied theoretically by particle simulation. The growths of the emittance, bunch length and energy spread due to the rf and space charge effects were investigated by changing the laser parameters, field gradient and electron charge. The theoretical studies indicate that a 100-fs MeV electron beam with the transverse emittance of 0.1 mm mrad and the relative energy spread of 10 −3–10 −4 at bunch charge of 0.1–2 pC (10 6–10 7 electrons per pulse) is achievable for UED, in which the intensity is three orders of magnitude higher than that produced by the conventional dc or pulsed guns.

First observation of multi-pulse X-ray train via multi-collision laser Compton scattering

A compact hard X-ray source via laser Compton scattering (LCS) has been developed for biological and medical applications at the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. The multi-collision LCS has been investigated in order to enhance the X-ray yields. The first observation of multi-pulse X-ray train with 6 pulses via the multi-collision LCS has been successfully demonstrated between the multi-bunch electron train with 6 bunches and the multi-pulse Ti:Sa laser train with 6 pulses. The 32 MeV electron train was generated from a Cs 2Te photocathode rf gun with a multi-pulse UV laser and the S-band linac. The Ti:Sa laser train was obtained with the chirp pulse amplification (CPA) including the modified regenerative amplifier. The X-ray train with 6 pulses with 12.6 ns spacing was observed with the micro-channel plate (MCP). The maximum energy of the X-ray is analytically estimated to be about 24 keV and the total number of generated photons was calculated to be about 1.8×10 6 photons/train.

Temporal resolution of MeV ultrafast electron diffraction based on a photocathode RF gun

Photocathode RF guns serve as electron sources in various ultrafast pump–probe experiments, such as in X-ray free-electron lasers, Thomson scattering X-ray sources, and MeV ultrafast electron diffractions (UED), for studying structural dynamics on the fundamental scales of atomic motions. The temporal resolutions of these experiments are of great concern. In this paper, we study each contributing term to the temporal resolution, including the RF compression effect, the dependence of time-of-flight of the electron pulses on the RF-to-laser timing jitter, and the effect of fluctuation of the RF field amplitude. We employ a simple single-particle model, and demonstrate inherent connections of all these effects to the single-particle dynamics. Particularly, the correlation between the RF compression effect and the fluctuation of time-of-flight induced by the RF-to-laser timing jitter, makes the temporal resolution considerably smaller than simply adding each term quadratically. We apply these results to a MeV UED and evaluate its temporal resolution, showing that a ∼ 100 fs resolution is already achievable with state-of-the-art hardware performances. Since these results are inherent properties of photocathode RF guns, they are also applicable to other facilities.

On the theory of photocathode rf guns

In this paper we give a set of analytical formulae to describe the characteristics of photocathode rf guns at any rf frequencies, such as energy, energy spread, bunch length, out going current, and emittance etc. as functions of the laser injection phase, which are useful in the design and practical operation of rf guns.

Design of a mode separated RF photo cathode gun

Our group at the Laser Undulator Compact X-ray source facility (LUCX), KEK (High Energy Accelerator Research Organization) is performing experiments to generate X-rays by inverse Compton scattering. The system, to generate the X-rays requires a high intensity beam with low emittance. In this research we propose modifications to the existing system to achieve an acceleration of 200 nC in 100 bunches. Specifically, we seek to demonstrate that a new RF gun with high mode separation and high Q value will achieve a beam of good quality. A new RF power system with a modified power delivery scheme provides a good beam loading compensation with a high-intensity multi-bunch electron beam. The acceleration of 200 nC in 100 bunches with low emittance will help increase the intensity of X-rays by inverse Compton scattering.

Analysis and experiments of a waveguide post's influence on photocathode RF gun

Several BNL/KEK/SHI type photocathode RF guns have been fabricated for high-quality electron beams in Accelerator Laboratory of Tsinghua University. This paper describes how the characteristics of a waveguide post can be chosen to correct for a mismatch in power coupling without affecting the pi-mode resonant frequency and the balance of fields between the two cells. Microwave circuit theories are used to analyze how to select the proper location and depth of the waveguide post. The tolerance on the post positioning is evaluated based on gun field quality requirements. MAFIA simulations and RF experiments have been done to confirm the theoretical analysis.

RobustCsBr/Cuphotocathodes for the linac coherent light source

The linac coherent light source (LCLS), an x-ray free-electron laser project presently under construction at SLAC, uses a 2.856 GHz rf photocathode gun with a copper cathode for its electron source. While the copper cathode is performing well for the LCLS project, a cathode material with higher quantum efficiency would reduce the drive laser requirements and allow a greater range of operating conditions. Therefore a robust CsBr/Cu photocathode with greater than 50 times the quantum yield at 257 nm relative to the present LCLS copper cathode has been investigated. Preliminary experiments using a dedicated electron source development test stand at SLAC/SSRL are encouraging and are presented in this paper.

Dynamic optical modulation of an electron beam on a photocathode RF gun: Toward intensity-modulated radiation therapy (IMRT)

In intensity-modulated radiation therapy (IMRT), the aim is to deliver reduced doses of radiation to normal tissue. As a step toward IMRT, we examined dynamic optical modulation of an electron beam produced by a photocathode RF gun. Images on photomasks were transferred onto a photocathode by relay imaging. The resulting beam was controlled by a remote mirror. The modulated electron beam maintained its shape on acceleration, had a fine spatial resolution, and could be moved dynamically by optical methods.

Recent progress of a soft X-ray generation system based on inverse Compton scattering at Waseda University

At Waseda University, we are developing a table-top size soft X-ray source based on inverse Compton scattering between a high quality electron beam and a high power laser. Using 1047 nm laser beam (Nd:YLF) and 4.6 MeV electron beam generated from a photo-cathode rf-gun, we had already succeeded in generating inverse Compton X-rays. The energies are within the “water window” region (250–500 eV) which can be applied to biological studies. For good signal to noise ratio ( S/ N) and a larger number of photons, we remodeled our collision chamber and laser amplifier system. S/ N is defined by the X-ray photon signal over all other noises and backgrounds such as a bremsstrahlung X-rays from a lost electron beam and an electric noises caused by a radiation and rf power source. With these modifications, the X-ray photons detected by a micro channel plate (MCP) have increased 10-fold to reach 312 ph/pulse. Total generated photons were estimated to be 3.3×10 4. Further, we succeeded in generating stable soft X-rays for more than 12 h. Good S/ N ratio, stable X-rays have made it possible to observe the beam–laser interaction very precisely. By using this technique, we have measured the electron beam size at the collision point as 251 μm ( σ x )×56 μm ( σ y ).

Ultra-fast pulse radiolysis: A review of the recent system progress and its application to study on initial yields and solvation processes of solvated electrons in various kinds of alcohols

In order to study radiation-induced fast phenomena, a new pulse radiolysis system with higher time resolution based on pulse-and-probe method was developed and utilized for practical work. A few picosecond electron beam generated from a linear accelerator, in which a laser photocathode RF-gun is introduced, was synchronized with a femtosecond laser pulse which is employed as the analyzing light. The synchronization precision between them was suppressed within 1.6 ps (rms). Converting the fundamental laser into white light continuum or optical parametric amplification allows to measure in the wide wavelength from visible to infrared region.

Analysis of slice transverse emittance evolution in a photocathode RF gun

The slice transverse emittance of an electron beam is of critical significance for an X-ray FEL. In a photocathode RF gun, the slice transverse emittance is not only determined by the emission process, but also influenced strongly by the nonlinear space charge effect. In this paper, we study the slice transverse emittance evolution in a photocathode RF gun using a simple model that includes effects of RF acceleration, focusing, and space charge force. The results are compared with IMPACT-T space charge simulations and may be used to understand the development of the slice emittance in an RF gun.

Development of Cs 2Te photocathode rf gun system for compact THz SASE-FEL

A compact terahertz (THz) SASE-FEL source has been developed with a compact S-band electron linac at AIST. The S-band linac has been improved using a Cs 2Te photocathode rf gun with a compact load-lock system. The 40 MeV electron beam which has a bunch charge of more than 2 nC/bunch was stably generated using our system with the Cs 2Te photocathode, and the quantum efficiency (QE) typically achieved was about 0.3%. The surface observation of the Cs 2Te photocathode to obtain the surface micrographs and QE mapping images was successfully performed with a photoelectron emission microscopy (PEEM).

Multislit-Based Emittance Measurement of Electron Beam from a Photocathode Radio-Frequency Gun

Measurement of emittance for a space-charge dominated electron beam from a photocathode rf gun is performed by employing the multislit-based method at Accelerator Laboratory of Tsinghua University. We present the design considerations on the multislit system and the experimental results, with special attention to the study of space charge induced emittance growth. The experimental results are in reasonable agreement with the PARMELA simulations.

Genetic-algorithm-based method to optimize spatial profile utilizing characteristics of electrostatic actuator deformable mirror

Arbitrary spatial beam shaping was demonstrated with a membrane electrostatic actuator type deformable mirror (DM). An automatic closed loop system must optimize such beam shapes as flattop. Well-characterized short pulse laser beam is widely required for a photocathode RF gun or for microscopic processing, etc. We propose a new sophisticated optimizing method based on a genetic algorithm (GA) for spatial shaping. A membrane type DM is driven by electrostatic attraction power, and applied electrode voltages vs displacement of membrane surface have a square function relationship. We prepare discrete electrode voltages to linearly change displacement as a utilized gene of the initial population in GA. Using uniform crossover without mutation in this method, we can make an arbitrary spatial beam shape quasi-flattop.