Photocathode RF Gun Research Articles

Theory and design consideration of a THz superradiant waveguide FEL

We present theoretical analysis and design considerations of a THz superradiant FEL. We derive analytical expressions for the spectral parameter of THz radiation, emitted superradiantly in a rectangular waveguide using a Longitudinal Section Magnetic mode expansion. The results compare well with numerical simulations using UCLA GPTFEL code. GPT simulations of the accelerator e-beam transport show that the chirp provided by a hybrid photocathode RF gun, can produce tight bunching at the undulator site below σ = 100fs. This enables intense superradiant emission up to 3THz, limited by the beam bunching factor. Phase-space analysis of the beam transport indicates that keeping the beam bunching parameter small enough for higher THz frequency operation is limited by the energy spread of the beam in the gun.

Development of a solenoid magnet for emittance compensation

This paper explores the implementation of a solenoid field to achieve electron beam focusing and compensation of transverse emittance, which is generated from photocathode RF gun. An extensive investigation was conducted to determine the optimal solenoid magnetic field required for focusing an electron bunch with a charge of 0.5 nC. A magnetic field strength of 0.25 T was chosen for emittance compensation, enabling the reduction of emittance to below 1.20 mm-mrad at the end of linac. The engineering design of the solenoid magnet for this system is presented in detail, ensuring precise dimensional tolerances within the range of ±30 µm. To ensure its effectiveness and reliability, extensive thermal and mechanical analyses were conducted. Currently, the prototype solenoid magnet is in the fabrication process.

Design and Beam Dynamic Studies of an Injector for a Compact THz Coherent Radiation Source

An intense narrow-band terahertz (THz) radiation source has been designed to generate a broad tuning range of radiation frequencies between 0.5 THz and 5.0 THz. The THz radiation is produced when a short-bunch electron beam propagates through an undulator. To achieve high-power peak radiation, the source requires high-brightness electron beams with low beam emittance and short bunch length. A proposed design for the photocathode RF gun used as the electron source is presented. The gun with high mode separation and high Q-factor can be achieved for producing a good beam quality. The beam dynamics of the injector have been preliminarily optimized using the software ASTRA and Elegant, investigating the impact of laser pulse shape on electron beam quality. The results of the beam dynamics studies are comprehensively discussed in this paper.

The Particle-Tracking Simulation of a New Photocathode RF Gun in the Free-Electron Laser Facility, KU-FEL

A project is underway that aims to generate attosecond pulses via high-harmonic generation in rare gases, driven by extremely short and highly intense pulses from free-electron-laser oscillators. For this purpose, it has been planned that a new photocathode RF gun, dedicated to high-bunch-charge operation, will be installed at the KU-FEL (Kyoto University Free Electron Laser) oscillator facility. In this study, RF guns with two different structures (1.6-cell and 1.4-cell) were compared, from the perspective of exploring the possibility of introducing bunch-interval modulation, which is important for achieving high extraction efficiency in the FEL oscillator. As a result, it was confirmed that the introduction of bunch-phase modulation would be possible only in the case of the 1.6-cell RF gun. After the structure of the RF gun was decided on, particle-tracking simulations were performed, to study the electron-beam parameters using the 1.6-cell RF gun and 1 nC bunch charge. The results showed that we could obtain the peak current of 1 kA without a large degradation of the other parameters.

Considered design and radiation calculation for a compact THz coherent radiation source

Since the light obtained from the Siam Photon Source (SPS) in Thailand is the high-intensity radiation in only the region of the infrared (IR) to high-energy X-rays, it has great attention in many areas of scientific research and industrial development. In order to increase in the number of users and applications, the terahertz (THz) region covering the frequency range from 0.1 to 10 THz needs to be fulfilled. Therefore, a compact accelerator based free-electron laser or coherent radiation is particularly interesting for providing an intense THz radiation. A 1.6-cell photocathode RF gun is used as an electron source to generate a high-brightness electron beam. The energy of electron beam can be raised up to meet the target of 30 MeV in a linear accelerator (Linac) before moving to a chicane bunch compressor. The THz radiation is produced when a short bunch electron beam propagates through an undulator. In order to achieve the intense narrow-band THz coherent radiation at tunable frequencies between 0.5 THz and 5.0 THz with MW peak power, a THz radiation source needs to be studied and developed. Design of high-power THz radiation source is described in this paper.

Femtosecond diagnostics of ultrashort electron beam using a pinhole-based THz-driven deflector

THz-driven resonant structures such as rectangular slits has been widely used for femtosecond temporal characterization of ultrashort electron beams produced in photocathode rf guns. In this work, we show that a simple non-resonant subwavelength pinhole perforated on a metal foil can also provide femtosecond temporal diagnostics to ultrashort electron beams. The streaking effect of the pinhole structure is dominated by the reflected THz field near the surface. A time resolution of 1.5 fs is achieved for a 3 MeV electron beam using a pinhole with diameter of 100 μm. A secondary beam, common in photocathode rf gun but hard to observe with conventional beam diagnostic methods, is also observed with the THz pinhole deflector. In addition to measuring the temporal information of the electron beam, we show that the pinhole may also be used to measure the properties of the THz pulse using the electron beam as the probe. We believe such simple and commercially available pinhole structure is a robust and viable method for femtosecond temporal diagnostics of ultrashort electron beams.

Development of a 1.4-cell RF photocathode gun for single-shot MeV ultrafast electron diffraction devices with femtosecond resolution

The MeV ultrafast electron diffraction (UED) has proven to be a powerful tool for exploring the ultrafast process at the atomic level with femtosecond resolution. However, MeV UED facilities are struggling to achieve sub-100 fs temporal resolution at pC level charges due to the rapid and severe bunch lengthening near the cathode region. Both analytical work and simulation show that the bunch lengthening near the cathode originates from both the strong space charge effect (SCE) within the sub-relativistic electron beam and the bunch tail trailing due to the mirror charge effect (MCE). In this paper, we report the development of a new 1.4-cell RF gun that significantly increases the extraction field at the cathode to suppress both the SCE and the MCE. Compared to the conventional 1.6-cell RF gun, the new 1.4-cell RF gun is more suitable to produce an electron beam with a shorter bunch length and less distorted longitudinal phase space. The shorter bunch length leads directly to a temporal resolution improvement in UEDs. Meanwhile, the beam longitudinal phase space with better linearity ensures a smaller compressed bunch length when the bunch compression schemes are applied.

A precision LLRF control system for UED

An ultrafast electron diffraction (UED) facility is located at Shanghai Jiao Tong University. The layout of the UED is divided into two modules. The accelerating module has one S-band photocathode RF gun, one C-band harmonic buncher, and one C-band deflector. The power-generating module has one S-band klystron and one C-band klystron. To meet the high-precision requirements, a MicroTCA.4-based low level radio frequency (LLRF) system is used to provide a stable RF field in the cavities. In this paper, we introduce the RF distributions, LLRF system, firmware and software of the feedback controller, and field stability of the RF gun; and present an analysis of the measurement results for UED.

Design, fabrication and cold-test results of a 3.6 cell C-band photocathode RF gun for SXFEL

The increasing demands for electron beams with short bunch length and high brightness for Soft X-ray Free Electron Laser (SXFEL) have stimulated active efforts to develop suitable photoinjectors for these accelerators. One promising direction in this regard is increasing the gradient of rf gun. The rf design, beam dynamics and cold-test results of a novel 3.6 cell C-band 150 MV/m rf photocathode gun are presented in this paper. The rf gun and its beam dynamics modeling have been done using a combination of codes including POSSION, CST, and ASTRA. The impact of first half-cell length, cell number, and solenoid on beam dynamic are analyzed and described. The gun has state-of-the-art rf features including: elliptical irises profile; a non-brazed cathode plate; and a rectangular mode launcher. The fabrication and cold-test of the rf gun prototype are successfully implemented, and the measured solenoid field agrees well with simulations.

Longitudinal phase space manipulation with planar corrugated wakefield structures

Wakefield structures have found a broad range of applications in beam longitudinal phase space manipulation in recent years. In this paper, we report on several electron beam manipulation experiments with planar corrugated wakefield structures following a photocathode RF gun, in which the bunch length of the electron beams are much longer, comparable, and much shorter than the characteristic wavelength of the corrugated structures, respectively. (1) A 15.6-ps beam forms a 0.17-THz bunch train via ballistic bunching, after gaining periodic energy modulation through these structures via self-induced wakefield. (2) For a 6-ps beam, the combination of wake structures and a drift converts its flat-top longitudinal profile into a ramped shape, remarkably improving its capability of efficiently exciting electromagnetic fields in plasma. (3) A 0.2-pC, 250-fs electron pulse subject to a drive beam’s wake is manipulated for different purposes, such as acceleration or deceleration with a ±200 keV gain in energy; or compression to ∼70 fs via ballistic compression. The results illustrate the unique and versatile capability of wakefield structures in beam manipulation for various applications. The simplicity, robustness, and inherent synchronization of such devices may open up many new possibilities in accelerator facilities.

Development of an L-band photocathode RF gun at Tsinghua University

This study reports on the development of an L-band photocathode RF gun for delivering high brightness electron beams with high average power. We designed and fabricated an L-band normal conducting photocathode RF gun at Tsinghua University. RF performance is improved after optimization of the cavity geometry referring to the PITZ gun. A newly designed solenoid that encloses the entire cavity body has been designed to suppress the emittance growth, and dynamic simulation shows a lower transverse emittance. A coupled 3D RF-thermal-structure analysis is carried out to investigate the thermal-induced effects under the operating condition. Cold test results are also presented in this paper. This gun is now under commissioning and beam tests will be discussed in future works.

Simulations of the millimeter wavelength accelerating structure excitation

We present the work on development of the millimeter wavelength accelerating structure (also called the W-band structure). It consists of cylindrical cavities with the operating frequency of 96 GHz. The structure will be excited by the picosecond electron beam from the photocathode RF gun. In order to define exactly both the structure and the exciting beam parameters, analytical estimations and simulations of the structure excitation were performed. For the successful propagation of the exciting beam through it, focusing system is needed. It is proposed to use the permanent magnets producing solenoidal field, their preliminary design being discussed. Prototype of the millimeter wavelength structure has been manufactured, its first measurements are presented.

Submicropulse electron-beam dynamics correlated with short-range wakefields in Tesla-type superconducting rf cavities

We report direct observations of submicropulse beam centroid shifts (head-tail kicks) correlated with short-range wakefields generated by off-axis electron-beam steering in Tesla-type superconducting rf cavities. The experiments were performed at the Fermilab Accelerator Science and Technology (FAST) Facility using its unique configuration of a photocathode rf gun injecting beam into two separated nine-cell cavities. The cavities are in series with corrector magnets and beam position monitors (BPMs) located before, between, and after them. The off-axis steering in the cavity was guided by the rf BPM data and higher-order mode circuitry targeting the first and second dipole passbands. The centroid shifts of up to 300 μm from head to tail of the ∼10-ps-long micropulses at 500 pC/b in a 3-MHz pulse train were measured via optical transition radiation at a downstream screen with a Hamamatsu C5680 synchroscan streak camera. We also showed that we could compensate such kicks from the first cavity with the short-range wakefields (SRWs) in the second cavity, and we observed the dilution of the beam size in the tail of the pulses. A simple numerical model of the SRW effect in a single Tesla cavity is compared to the experiment successfully. In principle, these fundamental results may be scaled to cryomodule configurations of major free-electron laser (FEL) facilities such as the European XFEL, Linac Coherent Light Source or LCLS-II XFEL, and the conceptual international linear collider.

Durability improvement of Cesium Telluride photocathode for an rf-gun

At Waseda University, we have been studying for high quality electron beam generation using 1.6 cell Cesium Telluride (Cs-Te) photocathode rf-gun. We use photocathode as the electron source, which can generate high-quality electron beam such as low emittance, and short bunch. The performance of photocathode is evaluated mainly in terms of quantum efficiency (Q.E.) and lifetime. Cs-Te photocathode is known for high Q.E. about 10% with UV light and relatively longer lifetime among semiconductor photocathodes. Since it is a hard environment for photocathode inside the gun, it is necessary to replace the photocathode every several months. In other words, in order to achieve long-term operation of rf-gun, it is necessary to establish highly durable photocathode recipe. In this report, we report the evaluation results of Cs-Te photocathode by co-evaporation, CsBr coated photocathode performance in an rf-gun and future prospects.

Single shot cathode transverse momentum imaging in high brightness photoinjectors

In state of the art photoinjector electron sources, thermal emittance from photoemission dominates the final injector emittance. Therefore, low thermal emittance cathode developments and diagnostics are very important. Conventional thermal emittance measurements for the high gradient gun are time-consuming and thus thermal emittance is not measured as frequently as quantum efficiency during the lifetime of photocathodes, although both are important properties for the photoinjector optimizations. In this paper, a single shot measurement of photoemission transverse momentum, i.e., thermal emittance per rms laser spot size, is proposed for photocathode RF guns. By tuning the gun solenoid focusing, the electrons transverse momenta at the cathode are imaged to a downstream screen, which enables a single shot measurement of both the rms value and the detailed spectra of the photoelectrons transverse momenta. Both simulations and proof of principle experiments are reported.

Demonstration of variable polarized coherent terahertz source

We are developing an accelerator-based terahertz source that can produce arbitrary polarization states from a linearly polarized coherent undulator radiation (CUR). The polarization manipulation of the CUR can be realized using the Martin–Puplett interferometer employed as an optical phase shifter. This study also demonstrates a variable polarization manipulator by using the terahertz CUR (THz-CUR) source driven by a compact photocathode RF-gun at the Institute of Advanced Energy, Kyoto University. The vertically polarized CUR with a frequency of 0.16 THz was manipulated into a horizontal linear polarization or circular polarization, and Stokes parameters were measured to derive the degree of polarization.

Commissioning of the electron injector for the AWAKE experiment

The advanced wakefield experiment (AWAKE) at CERN is the first proton beam-driven plasma wakefield acceleration experiment. The main goal of AWAKE RUN 1 was to demonstrate seeded self-modulation (SSM) of the proton beam and electron witness beam acceleration in the plasma wakefield. For the AWAKE experiment, a 10-meter-long Rubidium-vapor cell together with a high-power laser for ionization was used to generate the plasma. The plasma wakefield is driven by a 400 GeV/c proton beam extracted from the super proton synchrotron (SPS), which undergoes a seeded self-modulation process in the plasma. The electron witness beam used to probe the wakefields is generated from an S-band RF photo-cathode gun and then accelerated by a booster structure up to energies between 16 and 20 MeV. The first run of the AWAKE experiment revealed that the maximum energy gain after the plasma cell is 2 GeV, and the SSM mechanism of the proton beam was verified. In this paper, we will present the details of the AWAKE electron injector. A comparison of the measured electron beam parameters, such as beam size, energy, and normalized emittance, with the simulation results was performed.

Improvement of 6D brightness by a 1.4-cell photocathode RF gun for MeV Ultrafast electron diffraction

Recent research indicates that ultrafast electron diffraction and microscopy (UED/M) have unprecedented potential in probing ultrafast dynamic processes, especially in organic and biological materials. However, reaching the required brightness while maintaining high spatiotemporal resolution requires new design of electron source. In order to produce ultrashort electron beam with extreme high brightness, a 1.4-cell RF gun is being developed to reach higher acceleration gradient near the photocathode and thus suppress the space charge effect in the low energy region. Simulation of the 1.4-cell RF photocathode gun shows considerable improvement in bunch length, emittance and energy spread, which all lead to better temporal and spatial resolution comparing to traditional 1.6-cell RF photocathode gun. The results demonstrate the feasibility of sub-ps temporal resolution with normalized emittance less than 0.1 πmm·mrad while maintaining 1 pC electron pulse.

Using an alpha magnet to compress the electron bunches from a photocathode RF gun

We report here the design, simulation, and characterization of a compact electron injector for the formation of high-current, femtosecond electron bunches from a photocathode RF gun without mandatory use of ultra-short laser pulses and a high power RF supply. The proposed injector consists of a 1.6-cell S-band photocathode RF gun to produce the electron beam and a small bunch compressor to increase the peak current. The RF gun is designed to operate at a 65 MV/m accelerating field gradient, and the bunch compressor is a small α-magnet with a maximum magnetic field gradient of 340 G/cm. The output beam from the gun has 100 pC charge per-bunch that is equivalent to a beam current of ∼15 A, and output beam from the injector has an RMS bunch length of ∼ 439 fs, a peak current of 0∼ 12 A and an RMS normalized horizontal emittance of 1.21 mm-mrad.

Properties of THz coherent undulator radiation generated from a compact accelerator source at Kyoto University

A compact accelerator-base source of THz Coherent Undulator Radiation (THz-CUR) at the Kyoto University has been developed with the purpose of providing intense quasimonochromatic and tunable THz-CUR at frequencies below 1 THz. The source is driven by a compact accelerator using a photocathode RF gun, which emits an electron beam with a fixed energy of 4.6 MeV and high bunch charge. The THz-CUR from our source can be generated when a compressed electron bunch passes through a planar undulator. In this study, we investigate the properties of this radiation, including the total radiation energy, spatial distribution, and power spectrum. With an electron beam of 160 pC bunch charge, the total radiation energy of THz-CUR at 0.16 THz was 1.2 µJ in the micropulse. The THz-CUR covering the frequency range from 0.16 THz to 0.65 THz could be produced by changing the magnetic field of the undulator at a 60 pC bunch charge. Due to the influence of the space charge forces causing the degradation of electron beam qualities, saturation of the radiation energy occurred, and the radiation power spectrum at a frequency of 0.65 THz could not be observed in the high charge condition (160 pC). The effects of bunch lengthening, energy spread, and emittance growth and the influence of the phase error on the generation of THz radiation are also discussed in this paper. This opens up the possibility of understanding the generation of THz-CUR and sheds further light on the enhancement of the radiation power.