Photocathode Radio Frequency Gun Research Articles

Commissioning the photocathode radio frequency gun: a candidate electron source for Hefei Advanced Light Facility

Commissioning the photocathode radio frequency gun: a candidate electron source for Hefei Advanced Light Facility

Relativistic Attosecond Electron Pulses from a Photocathode Radio-Frequency Gun

The interaction of light and electrons has been one of the important frontiers for generating ultrashort electron pulses in free-electron lasers, ultrafast science, and dynamical analysis of matter. However, the generation of a relativistic attosecond electron beam remains a challenge. In a photocathode radio-frequency (rf) gun, this work identifies a regime for obtaining an isolated or a train of relativistic attosecond electron pulses. A photoelectron beam is generated from the cathode under the illumination of a driving laser pulse in the electron gun, which is subsequently microbunched to optical wavelength scale by the field of a synchronized radially polarized laser (RPL) pulse that is focused near the cathode surface. The rf field in the gun cavity simultaneously accelerates the electron pulse to several MeV. The rf field causes a velocity differential inside the electron beam as it passes into the region near the focal point of the RPL (microbunching is processed), and the microbunch at the head is substantially faster than that at the tail. Using this knowledge, the spacing between consecutive microbunches, referred to as bunch spacing, may be controlled across a wide range by regulating the velocity difference, which can be accomplished by tuning the phase and amplitude of the rf field. Numerical simulations show that a train of attosecond bunches with tunable bunch spacing can be generated using realistic laser parameters corresponding to current GW-power-level laser systems, and an isolated attosecond pulse can be obtained when the driving laser pulse is about 50 fs. This regime may result in opportunities in ultrafast electron diffraction and microscopy, free-electron lasers, and other applications that require high-energy electrons with the temporal structure of single-cycle light.

Beam Dynamics Simulation of Photocathode RF Electron Gun at the PBP-CMU Linac Laboratory

Photocathode radio-frequency (RF) electron guns are widely used at many particle accelerator laboratories due to high quality of produced electron beams. By using a short-pulse laser to induce the photoemission process, the electrons are emitted with low energy spread. Moreover, the photocathode RF guns are not suffered from the electron back bombardment effect, which can cause the limited electron current and accelerated energy. In this research, we aim to develop the photocathode RF gun for the linac-based THz radiation source. Its design is based on the existing gun at the PBP-CMU Linac Laboratory. The gun consists of a one and a half cell S-band standing-wave RF cavities with a maximum electric field of about 60 MV/m at the centre of the full cell. We study the beam dynamics of electrons traveling through the electromagnetic field inside the RF gun by using the particle tracking program ASTRA. The laser properties i.e. transverse size and injecting phase are optimized to obtain low transverse emittance. In addition, the solenoid magnet is applied for beam focusing and emittance compensation. The proper solenoid magnetic field is then investigated to find the optimum value for proper emittance conservation condition.

High quality single shot ultrafast MeV electron diffraction from a photocathode radio-frequency gun.

A compact ultrafast electron diffractometer, consisting of an s-band 1.6 cell photocathode radio-frequency gun, a multi-function changeable sample chamber, and a sensitive relativistic electron detector, was built at Shanghai Jiao Tong University. High-quality single-shot transmission electron diffraction patterns have been recorded by scattering 2.5 MeV electrons off single crystalline gold and polycrystalline aluminum samples. The high quality diffraction pattern indicates an excellent spatial resolution, with the ratio of the diffraction ring radius over the ring rms width beyond 10. The electron pulse width is estimated to be about 300 fs. The high temporal and spatial resolution may open new opportunities in various areas of sciences.

1 ms Pulse Beam Generation and Acceleration by Photocathode Radio Frequency Gun and Superconducting Accelerator

We report the successful generation and acceleration of a 1-ms-long pulse and multibunch electron beam by a normal conducting photocathode RF gun and a super conducting accelerator at the KEK Superconducting Test Facility (STF). A 1.3 GHz normal conducting RF gun generates a 1 ms and 10 mA macropulse that fully satisfies the designed parameters. This is the longest macropulse generated by a normal conducting RF gun with a frequency of more than 1 GHz. A beam acceleration of up to 40 MeV was demonstrated with more than 60% of the designed average current. The accelerated beam properties were evaluated: the intensity and energy fluctuations were 3.8% and less than 0.08%, respectively. These beam properties are sufficient for the requirement of the STF operation including that in the quantum beam project, which is high-brightness-X-ray generation by inverse laser Compton scattering. The intensity fluctuation should be improved toward the International Linear Collider (ILC) for uniform acceleration.

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.

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.

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.

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.

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.

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.

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 Measurement Method of Time Jitter of a Laser Pulse withRespect to the Radio-Frequency Wave Phase in a PhotocathodeRadio-Frequency Gun

In a photo-cathode radio-frequency (rf) gun, the micro-bunched charge output from the gunis dependent linearly on the laser injection phase, due to the Scottayeffect in the process of photoemission and the procedure of the electronlongitudinal acceleration. Based on this principle, a new method isproposed, which should be utilized to measure the time jitter betweenthe driving laser pulse and the rf phase with a very high resolution ofa few tens of femtoseconds.

Low-emittance electron-beam generation with laser pulse shaping in photocathode radio-frequency gun

A technique of laser-pulse shaping was developed for low-emittance electron-beam generation in a photocathode radio-frequency (rf) gun. The emittance growth due to space charge and rf effects in the rf gun was experimentally investigated with square and gaussian temporal pulse shapes. It was found that the square pulse shaping was a useful tool for both the reduction of nonlinear space-charge force and the correction of linear space charge. The normalized transverse rms emittance at 1 nC was obtained to be 1.20 πmm-mrad for the square pulse shape with pulse length of 9 ps full width at half maximum. The emittance was measured as a function of the electron bunch charge and the laser-pulse length.

Experimental studies of emittance growth and energy spread in a photocathode RF gun

In this paper we report on a low emittance electron source, based on a photocathode RF gun, a solenoid magnet and a subsequent linac. The dependencies of the beam transverse emittance and relative energy spread with respect to the laser injection phase of the radio-frequency (RF) gun, the RF phase of the linac and the bunch charge were investigated experimentally. It was found that a lower beam emittance is observed when the laser injection phase in the RF gun is low. The emittance increases almost linearly with the bunch charge under a constant solenoid magnetic field. The corrected relative energy spread of the beam is not strongly dependent on the bunch charge. Finally, an optimal normalized rms transverse emittance of 1.91±0.28πmmmrad at a bunch charge of 0.6nC was obtained when the RF gun was driven by a picosecond Nd:YAG laser. A corrected relative rms energy spread of 0.2–0.25% at a bunch charge of 0.3–2nC was obtained after the beam was accelerated to 14MeV by the subsequent linac.

First operation of a photocathode radio frequency gun injector at high duty factor

Tests of the electron beam injector for the Boeing/Los Alamos Average Power Laser Experiment have demonstrated first time operation of a photocathode radio frequency gun accelerator at 25% duty factor, exceeding previous photocathode operating parameters by three orders of magnitude. The macropulse format was 30 Hz and 8.3 ms with a micropulse frequency of 27 MHz. Average beam currents of up to 32 mA have been accelerated to 5 MeV for an average beam power of 160 kW. The macropulse peak current was 128 mA. The 32 mA average beam current exceeds previous cathode performance by a factor of 1000. Emittance measurements demonstrate excellent electron beam quality.