Radio-frequency Gun Research Articles

Investigation of Electron Beam Parameter in Seeded THz-FEL Amplifier using Photocathode RF Gun

Simple, compact, and powerful seeded terahertz-free electron laser (THz-FEL) which consists of a photocathode radio-frequency gun, a focusing solenoid, an undulator and a THz wave parametric generator has been designed. A start-to-end calculation has been performed by using the particle tracking code PARMELA and the FEL gain simulation code GENESIS1.3. To obtain a higher FEL gain, we searched a better condition of the electron beam parameter and the seed light. In the present design scheme, the FEL peak power was calculated to be 3kW at the FEL resonant wavelength of 179 m. Since the power of the seed light was 0.20W, the light power was amplified almost 104 times.

Development of double-decker pulse radiolysis

Double-decker pulse radiolysis (DDPR), which utilizes double-decker electron beams, was investigated to develop a new pulse radiolysis with a high time resolution. The double-decker electron beams were generated by injecting two UV pulses into a photocathode radio-frequency gun. In the pulse radiolysis, one electron beam was used as a pump beam, and the other was converted to a probe pulse. Finally, as its first application, the DDPR was successfully used for observing solvated electrons in water, with a 10%-90% rise time of 8.6 ps.

Experimental investigation of thermal emittance components of copper photocathode

With progress of photoinjector technology, thermal emittance has become the primary limitation of electron beam brightness. Extensive efforts have been devoted to study thermal emittance, but experiment results differ between research groups and few can be well interpreted. Besides the ambiguity of photoemission mechanism, variations of cathode surface conditions during cathode preparation, such as work function, field enhancement factor, and surface roughness, will cause thermal emittance differences. In this paper, we report an experimental study of electric field dependence of copper cathode quantum efficiency (QE) and thermal emittance in a radio frequency (rf) gun, through which in situ cathode surface parameters and thermal emittance contributions from photon energy, Schottky effect, and surface roughness are extracted. It is found the QE of a copper cathode illuminated by a 266 nm UV laser increased substantially to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ after cathode cleaning during rf conditioning, and a copper work function of 4.16 eV, which is much lower than nominal value (4.65 eV), was measured. Experimental results also show a thermal emittance growth as much as $0.92\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}/\mathrm{mm}$ at $50\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ due to the cathode surface roughness effect, which is consistent with cathode surface morphology measurements.

UV pulse trains by α-BBO crystal stacking for the production of THz-rap-rate electron bunches

AbstractUltrashort electron bunch trains can be used for plasma wake field acceleration (PWFA) to overcome the limit of transformer ratio of a single electron bunch, or high-power terahertz (Thz) radiation production by various radiation mechanisms. Basic facility for high-power THz radiation development based on ultrashort electron beam has been set up at accelerator lab of TUB. Using birefringent crystal serials, ultraviolet (UV) pulse shaping for photocathode radio frequency gun to produce THz-repetition-rate pulse train was realized. Driven by such pulses, ultrashort electron bunch train with picosecond (ps) spacing was obtained for THz production. Measurement of the stacked UV pulse trains was done by difference frequency generation (DFG), and the measured group velocity mismatch of α-BBO crystal at 266.7-nm wavelength was 0.8 ps/mm. This method may also be applied to form ramped electron bunch trains for PWFA.

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.

Status and perspectives of superconducting radio-frequency gun development for BERLinPro

As part of the BERLinPro study, HZB is developing an SRF photoelectron injector. The R&D will be carried out in three stages, the first of which is currently being installed at HZB's HoBiCaT facility. It consists of an SRF cavity with SC solenoid, electron beam diagnosics and drivelaser systems.

Surface photoemission in a high-brightness electron beam radio frequency gun

We report the experimental characterization of high-brightness electron beam generation from a magnesium (Mg) photocathode. Both the quantum efficiency (QE) and the thermal emittance of an Mg cathode are experimentally investigated. The measured QE∼0.2% is the highest reported for a metal cathode. We observed no change in the Mg cathode thermal emittance as the QE varies from 0.015% to 0.15%. The upper-limit of the thermal emittance, 0.5 mm mrad, is about 50% lower than the theoretical prediction. Our results demonstrated the feasibility of having a high QE and a low thermal emittance simultaneously for a robust metal photocathode.

Characteristics of terahertz coherent transition radiation generated from picosecond ultrashort electron bunches

This paper presents a method of generating terahertz (THz) coherent transition radiation (CTR) from picosecond ultrashort electron bunches including single and train bunches, which are produced by a photocathode radio frequency gun. The radiation characteristics of THz CTR including formation factor and energy spectrum are analysed in detail. With the help of a 2-dimensional particle-in-cell simulation, the radiation characteristics including power, energy and magnetic field are analysed. The results show that the radiation frequency can be adjusted by tuning the repetition frequency of the train bunch and the energy can be enhanced with the train bunches.

On electron beam motion near the SPARC photoinjector cathode

Description of the electron beam behavior at the moment of its generation inside a radio-frequency gun of the SPARC1 photo-injector is presented by this research work aiming to show the influence of laser radiation presence on the beam accelerated by the electric field. The electron motion inside the photo-injector RF-gun has been analytically solved within the reasonable approximations. The partial qualitative analysis of both electron beam distribution in a space as well as pulse photoelectron distribution at the moment of beam generation are given. The electron redistribution inside the beam was analyzed and visualized by means of the developed MATLAB code. The obtained estimations have been performed using the parameters of electron beam defined during the SPARC commissioning.

Generation of pre-bunched electron beams in photocathode RF gun for THz-FEL superradiation

A simple laser technology is proposed to generate a laser pulse sequence with a pulse rate of 2 THz frequency. The laser pulse sequence is used as the driving laser for a photocathode radio frequency (RF) gun. The RF gun generates the pre-bunched electron beams into an undulator for immediate superradiation. Terahertz (THz) radiation, based on a free electron laser (FEL) at a megawatt (MW) level, could be achieved from a compact FEL facility. We describe the laser handling technology for a 16-pulse laser. The detailed calculation of the bunching factor due to the injection phase, beam charge, and bunch number, as well as the differences in the time spacing and energy spread among the laser pulses, are also presented.

Short bunch effect on tabletop THz FEL amplification

A tabletop THz free-electron laser (FEL) amplifier, which consists of a photocathode radio-frequency gun and an undulator, has been studied at the Institute of Advanced Energy, Kyoto University. The concepts of the THz amplifier are (1) a high peak power of about 1 MW, (2) a whole length of less than 4 m, and (3) a spectral range of 150–300 μm. For the feasibility study for the conceptual design, a start-to-end simulation was conducted by using Parmela and GENESIS 1.3. The slippage effect and the diffraction loss were taken into account in the FEL simulation. An output power of about 350 kW is expected from the present design scheme. The calculation shows that the short bunch electron beam has an advantage toward obtaining a higher FEL gain because of its small energy spread. Since the slippage effect becomes dominant, however, the FEL gain drops with the excessive short bunch beam.

Construction and performance of the magnetic bunch compressor for the THz facility at Chiang Mai University

The Plasma and Beam Physics Research Facility at Chiang Mai University has established a THz facility to focus on the study of ultra-short electron pulses. Short electron bunches can be generated from a system that consists of a radio-frequency (RF) gun with a thermionic cathode, an alpha magnet as a magnetic bunch compressor, and a linear accelerator as a post-acceleration section. The alpha magnet is a conventional and simple instrument for low-energy electron bunch compression. With the alpha magnet constructed in-house, several hundred femtosecond electron bunches for THz radiation production can be generated from the thermionic RF gun. The construction and performance of the alpha magnet, as well as some experimental results, are presented in this paper.

Soft X-ray generation experiment at the Tsinghua Thomson scattering X-ray source

The development, design, and construction of the Tsinghua Thomson scattering X-ray (TTX) source have been ongoing since 2001. The TTX source is based on the Thomson scattering of a femtosecond laser pulse by a relativistic electron beam, which aims at an ultra-fast, high-flux, monochromatic, and tunable X-ray source for scientific, medical, and industrial applications. A recent experiment sought to generate a soft X-ray pulse through Thomson scattering. In this experiment, a 3-eV electron beam generated from a photocathode radio-frequency gun is focused by quadrupole magnets to several hundred microns and collided with a 120-mJ, 60-fs laser beam. The generated X-ray is detected by a micro-channel plate. The energy, pulse duration, and number of X-ray photons is estimated to be 290.4 eV, 1 ps, and 6.4×10 3, respectively.

Particle-in-cell/Monte Carlo simulation of ion back bombardment in a high average current RF photo-gun

In this paper, we report on study of ion back bombardment in a high average current radio-frequency (RF) photo-gun using a particle-in-cell/Monte Carlo simulation method. Using this method, we systematically studied effects of gas pressure, RF frequency, RF initial phase, electric field profile, magnetic field, laser repetition rate, different ion species on ion particle line density distribution, kinetic energy spectrum, and ion power line density distribution back bombardment onto the photocathode. These simulation results suggest that effects of ion back bombardment could increase linearly with the background gas pressure and laser repetition rate. The RF frequency significantly affects the ion motion inside the gun so that the ion power deposition on the photocathode in an RF gun can be several orders of magnitude lower than that in a DC gun. The ion back bombardment can be minimized by appropriately choosing the electric field profile and the initial phase.

TW Laser system for Thomson scattering X-ray light source at Tsinghua University

A TW (Tera Watt) laser system based on Ti:sapphire mainly for the Tsinghua Thomson scattering X-ray light source (TTX) is being built. Both UV (ultraviolet) laser pulse for driving the photocathode radio-frequency (RF) gun and the IR (infrared) laser pulse as the electron-beam-scattered-light are provided by the system. Efforts have also been made in laser pulse shaping and laser beam transport to optimize the high-brightness electron beam production by the photocathode RF gun.

Electron emission characterization of Mg photocathode grown by pulsed laser deposition within anS-band rf gun

Pulsed laser deposition (PLD) has been proposed several years ago as a suitable technique to deposit a pure Mg film over a radio frequency (rf) gun Cu backflange in order to obtain a high efficiency photocathode surface for the generation of high brightness electron beams. In this paper we report preliminary experimental results on the emission properties of a PLD grown Mg film within the high electric field gradients of a rf gun showing the effects of the rf conditioning process on the cathode surface. Even though a laser cleaning process should be performed on the sample surface in order to remove contaminated layers, the results presented here are very promising for the realization of a final Mg-based photocathode.

Transverse phase space tomography using a solenoid applied to a thermal emittance measurement

Recently, Stratakis and collaborators performed the tomographic phase space mapping of intense particle beams in the energy range of a few keV using a pair of solenoids [D. Stratakis et al., Phys. Plasmas 14, 120703 (2007)]. In this paper, we report the tomographic phase space mapping for the relativistic electron beam from a photocathode radio-frequency gun using a single solenoid at an energy of a few MeV. The experiment is conducted at a very low charge level aiming at mapping the phase space distributions of thermal emittance dominated beam. The phase space distribution is reconstructed with the algebraic reconstruction technique using the 14 beam profiles measured under various solenoid currents. Results from the computerized tomography (CT) technique are in good agreement with that from the solenoid scan method. The reconstructed phase space from the CT technique is further analyzed and shows a stochastic distribution which is a strong confirmation for the measurement of thermal emittance. Details on measurement principles and experimental implementation of the CT technique and solenoid scan method are presented.

A plasma cathode for a radio-frequency gun

A plasma ferroelectric cathode is used to form electron beams with a high pulse charge and a high charge in an electron bunch in an rf electron gun of a 10-cm wavelength range. The design of the cathode is described, and the results of calculations of the densities of the cathode-emitted and the gun-outputted currents are presented. The operation of the cathode in the rf gun was studied experimentally: the electron energy, the pulse current, and the transverse emittance of the beam were measured. The electron beam obtained at the output of the single-resonator gun had a pulse current of up to 10 A, a pulse duration of 60 ns, and an electron energy of ≅500 keV. The normalized beam emittance was 40 mm mrad.

Mg based photocathodes for high brightness RF photoinjectors

Advanced high brightness radio frequency (RF) gun injectors require photocathodes with a fast response, high quantum efficiency (QE) and good surface uniformity. Metal films deposited by various techniques on the gun back wall could satisfy these requirements. A new deposition technique has been recently proposed, i.e. pulsed laser ablation. Several Mg samples have been deposited by this technique: the emission performance and morphological changes induced on the cathode surface during laser activation are compared and discussed.

Double-decker femtosecond electron beam accelerator for pulse radiolysis

A new concept of double-decker electron beam accelerator based on a photocathode radio-frequency (rf) gun was proposed for studying chemical kinetics and primary processes or reactions of radiation chemistry. The synchronized double-decker electron beams with time interval of 1.4ns were generated in the rf gun by injecting two laser beams on the photocathode. The double-decker electron beams were accelerated by a booster linear accelerator (linac) up to 31.8MeV with energy-phase correlation and compressed into femtosecond by rotating the bunch in the energy-phase distribution in magnetic fields. The normalized transverse emittance of both beams downstream of the linac was obtained to be 2.5±0.6mmmrad for the up beam with bunch charge of 0.47nC and 3.6±0.7mmmrad for the down beam with bunch charge of 0.65nC. The minimum relative energy spread was (0.14±0.03)% for the two beams. The compressed bunch length was obtained to be 430±25fs for the up bunch and 510±20fs for the down bunch.