RF Gun Research Articles

Dark current studies of an L-band normal conducting RF gun

L-band normal conducting RF guns operating at long pulse mode and high gradient are installed at the European X-ray Free-Electron Laser (European XFEL) and the Free-electron LASer in Hamburg (FLASH) as high brightness electron sources. The dark current generated from the gun may cause severe issues, such as component activation, superconducting radio frequency (SRF) cavity quench and undulator degradation. The dark current is one of the parameters limiting the gun accelerating gradient and performance. In this paper, we report the dark current tracking simulations, including the detailed RF field distribution in the gap between cathode plug and copper hole in the backplane. The transmission ratios of the field emission from cathode regions were simulated for different copper hole geometries and different plug insertions. The dark current imaging measurements were conducted in an L-band normal conducting RF gun (gun 4.2) at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The dark current imaging comparison before and after a 180 degree cathode plug rotation shows the main dark current emissions come from the copper hole corner and Cs2Te film edge, which is consistent with the dark current tracking simulations.

Design and low-power RF test of a C-band cryogenic RF gun

Next-generation photoinjectors with high brightness are under development for advanced accelerator facilities, such as Free-Electron Laser, Ultrafast Electron Diffraction, and Ultrafast Electron Microscopy. A feasible and prospective method to produce electron beams with higher quality is based on high gradient acceleration technology. The gradient could be as high as 200 MV/m at the photocathode in a C-band RF gun in a 20 K cryostat. In this paper, a new cryogenic C-band photocathode electron gun is systematically designed and optimized in both beam dynamics and RF design. Moreover, a test platform has been set up for some preliminary experiments with a 3-cell prototype. The results on the RF characteristics at cryogenic temperatures are in good agreement with the design and simulation, and will pave the way for the practical operation of the cryogenic electron gun and the design of other cryogenic structures.

Experimental benchmark study of multiphysics simulations of an L-band high average power RF gun

Multiphysics simulations are widely used in designing high average power RF cavities, which require iterations of RF simulation, thermal simulation and mechanical simulation in a closed loop. The reliability and accuracy of the multiphysics simulations are crucial, otherwise extra design margins are needed for simulation uncertainties. We present an experimental benchmark for the multiphysics simulations of a 40 kW L-band RF gun at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The gun temperature distribution and frequency shift due to average RF heating power are measured and compared with multiphysics simulations.

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.

Simulation study of first and second harmonic photocathode bimodal gun

The increasing demands for electron beams with high quality and low timing jitter for accelerator-based applications such as ultrafast electron microscopy (UEM) and ultrafast electron diffraction (UED) have stimulated active efforts to develop suitable injectors for these accelerators. One promising direction in this regard is using of a bimodal rf gun operating in fundamental mode and a phase-locked harmonically-related higher mode. Work reported in this paper is on analysis and design of a bimodal rf gun operating in S-band as the fundamental and C-band as the second-harmonic modes. The rf design for such an rf gun includes the new design like directional coupler and rectangular mode launcher, which presented good transmission, low reflective cross-talk, and allowed multiple iterations of the modification. Compared to the fundamental mode, the new design achieved reduction in rf emittance and energy spread for MeV UEM initial distribution, and reduction in slope of flight time as function of injection phase for MeV UED applications.

Experimental validation of collective effects modeling at injector section of X-ray free-electron laser

We consider the collective beam dynamics at the injector section of the European XFEL. The results of the measurements for the longitudinal phase space (LPS) of the electron beam are compared with those obtained from numerical modeling. A new approach is proposed for the analysis of the self-field effects in the LPS measurements. It allows to determine accurately the synchronous RF phase in an accelerating module and to subtract properly the RF curvature imprinted in the LPS. A further incorporation with the simulation made it possible to separate the collective effects originating from different sources and thus to quantify, individually, the impact of these effects on the beam dynamics. This includes the space-charge dominated beam dynamics in the RF gun as well as the collective effects dominated by the wake fields after the gun up to the end of the injector section. A new analytical model is also proposed for the short-range wake function of a finite chain of RF cavities and verified by both numerical analysis and experimental results. It is shown that the physical models used in the simulation of the beam dynamics after the gun allow to reduce the absolute error in the modeling of the correlated energy chirp by order of magnitude in comparison with the case when the collective effects after the gun are neglected.

Conceptual design for an hard X-ray Free Electron Laser based on CLIC X-band structure

In this paper, a conceptual design of a hard X-ray Free-Electron Laser (FEL) facility based on CLIC-like X-band accelerating structure has been presented. The injector, based on an X-band rf gun, the main accelerating sections, using high-gradient CLIC-like structures including rf power distribution, and the two bunch compressors have been discussed. Simulations of the rf gun, of the injector, and of the linac have been performed, successfully meeting the stringent requirements in terms of minimum projected emittance, sliced emittance, and minimum bunch length. Full 6-D start-to-end linac simulations have been performed taking into account wakefield effects, misalignment of accelerator components, Coherent Synchrotron Radiation (CSR), and small dynamic variations of phase, voltage, and injection time. The performance of FEL at λ=1 Angstrom (Å) has also been evaluated for self-amplified spontaneous emission (SASE) FEL mode. A radiation power of about 10 GW has been achieved with a photon pulse length of 30 fs. The main advantage of the proposed design is that it achieved the required beam parameters in very short length, rendering the facility quite compact.

Dogleg design for an MeV Ultra-fast electron Diffraction beamline for the hybrid Photo-emitted RF GUN at Ariel University

A secondary parallel beamline is proposed for the construction of a Mega electron Volt Ultrafast electron Diffraction (Mev UED) facility, based on the novel hybrid 6 MeV RF-GUN in the Schlesinger center for compact accelerators in Ariel University. The Addition of a second beamline requires kicking the beam sideways and back, using a dogleg section. In order to change the trajectory of the beam, while preserving as much as possible the quality of the relativistic electron beam, requires a careful design and simulation process. Start-to-end comprehensive simulations of the dogleg design were performed using GPT (General Particle Tracer) software followed by analysis and optimization for the simulations in order to achieve optimal beam parameters in the second beamline. This paper presents the results of the dogleg design and optimization.

Photoemission studies of yttrium photocathodes by using the visible radiation

The present work reports on Yttrium based photocathodes. A Yttrium (Y) thin film is deposited via pulsed laser deposition (PLD) on the copper (Cu) back flange of a radio frequency (rf) gun for photocathode application. Because of a lower work function with respect to Cu, Y photocathodes are particularly appealing for the possibility to illuminate them with visible laser pulses, with the advantage of a higher energy per pulse, paving the way to high repetition rate photoinjectors, driven by conventional laser sources. In addition, working at $\ensuremath{\lambda}\ensuremath{\sim}400\text{ }\text{ }\mathrm{nm}$ the small energy difference between the Y work function (about 3 eV) and the laser photon energy reduces the contribution of the intrinsic emittance of the material. Photoelectrons, emitted by the thin film Y photocathode driven by the second harmonic of a Ti:Sapphire laser, have been characterized in terms of quantum efficiency and transverse emittance. Results have been compared with the theoretical ones obtained by the three-step model of Spicer for metallic photocathodes.

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.

Demonstration of nitrogen-incorporated ultrananocrystalline diamond photocathodes in a RF gun environment

Quantum efficiency (QE), intrinsic emittance, and robustness are the three most important figures of merit for photocathodes, the first two determine the ultimate achievable brightness of an electron beam, and the third one directly correlates with the complications of a beamline design. Nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] materials are promising candidates for photocathode applications due to their remarkable electron emission performance as well as the moderate vacuum requirement. Two (N)UNCD photocathodes have been characterized in a realistic RF gun environment with the nicely balanced performance of all three figures of merit. The QE of the first (N)UNCD cathode (stored in air for two years before the test) was found to be 3.8 × 10−4 using a 262 nm UV laser and a cathode surface field of 30 MV/m. It was found that the QE of the second (N)UNCD sample (grown days before the test) was nearly the same and, therefore, demonstrates the exceptional environmental tolerance of the material. The intrinsic emittance of (N)UNCD was measured to be 1.00 μm/mm.

Simulation studies on the RF gun saturated emission

The emission of a short high charged electron bunch in a radiofrequency gun was simulated. The numerical model is supplemented by a positive charge arising and dynamically changing during emission in the cathode region The numerical results are compared with experimental data.

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.

Low-emittance radio-frequency electron gun using a gridded thermionic cathode

We developed an electron gun system capable of delivering high brightness beams to drive soft x-ray free-electron lasers (SXFEL). We modified the conventional radio-frequency (rf) gun concept by using a 50-kV gun with a gridded thermionic cathode connected with a 238-MHz acceleration cavity. We achieved a low target emittance by applying the following two methods: (1) to obtain a parallel beam after the control grid, we optimized the grid voltage to compensate for the electric potential distortion near the grid (i.e., the lens effect); and (2) to avoid emittance growth due to the space charge effect at low energy regions, the 50-kV gun was connected to a 238-MHz rf cavity with a minimum distance, which immediately accelerated the beam energy to 500 keV. We built a gun test stand to verify the high beam performance, i.e., a normalized emittance of less than 4 mm mrad with a bunch charge of 1 nC and a pulse length shorter than 0.7 ns, as predicted by cst and parmela codes. Our proof-of-performance experiments demonstrated that a projection emittance of 1.7 mm mrad was obtained as a core part, representing 60% of all extracted electrons and the measured emittance agreed well with the simulation value of 2.0 mm mrad. This paper presents an overview of the electron gun system and our proof-of-performance experimental results.

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.

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.

Beam dynamics of realistic bunches at the injector section of the European X-ray Free-Electron Laser

We consider a 3D electron bunch at the cathode, modelled via photoemission, with realistic (asymmetric) spatial and temporal distributions for accelerator beam dynamics studies at the European X-ray Free-Electron Laser (European XFEL). A series of measurements are performed for low energy beams in the European XFEL injector. Using modeled 3D electron bunches, beam dynamics simulations, enabled by a three-dimensional (3D) space-charge solver with valid image-charge calculation on the photocathode plane, have shown improved agreements on measured beam properties (e.g., charge, bunch length, and shape) over a large range of variable machine parameters (e.g., cathode drive laser pulse energy, rf gun phase) for injector operation. In addition, the beam dynamics close to the cathode in a strongly space-charge dominated regime is studied in the rf gun. In light of the choice of a suitable machine working point along a so-called emission curve, the impact of the space-charge effect on the longitudinal length and shape of the produced electron bunch is analyzed in comparison to the measurements downstream the injector exit.

Quasi-monochromatic THz pulse generation using Cherenkov radiation from a spatially modulated electron beam

Cherenkov radiation forms a wave front in a direction that depends on the density of the medium and the velocity of charged particles; therefore, ordinary electron bunches do not generate coherent radiation unless they are sufficiently compressed. In this work, we present the generation of a quasi-monochromatic THz pulse using an electron bunch with a periodic structure generated from a small rf gun. A coherent THz pulse was generated by matching the tilt angle of the electron bunch and Cherenkov radiation angle with a rf deflector. Furthermore, we attempted to render the THz pulse quasi-monochromatic by adding a periodic structure to the electron bunch using a multi-slit. Finally, using several band-pass filters, we successfully confirmed that the quasi-monochromatic operation was performed at frequencies of 0.2 and 0.3 THz.

Breaking 50 Femtosecond Resolution Barrier in MeV Ultrafast Electron Diffraction with a Double Bend Achromat Compressor.

We propose and demonstrate a novel scheme to produce ultrashort and ultrastable MeV electron beam. In this scheme, the electron beam produced in a photocathode radio frequency (rf) gun first expands under its own Coulomb force with which a positive energy chirp is imprinted in the beam longitudinal phase space. The beam is then sent through a double bend achromat with positive longitudinal dispersion where electrons at the bunch tail with lower energies follow shorter paths and thus catch up with the bunch head, leading to longitudinal bunch compression. We show that with optimized parameter sets, the whole beam path from the electron source to the compression point can be made isochronous such that the time of flight for the electron beam is immune to the fluctuations of rf amplitude. With a laser-driven THz deflector, the bunch length and arrival time jitter for a 20fC beam after bunch compression are measured to be about 29fs (FWHM) and 22fs (FWHM), respectively. Such an ultrashort and ultrastable electron beam allows us to achieve 50 femtosecond (FWHM) resolution in MeV ultrafast electron diffraction where lattice oscillation at 2.6THz corresponding to Bismuth A_{1g} mode is clearly observed without correcting both the short-term timing jitter and long-term timing drift. Furthermore, oscillating weak diffuse scattering signal related to phonon coupling and decay is also clearly resolved thanks to the improved temporal resolution and increased electron flux. We expect that this technique will have a strong impact in emerging ultrashort electron beam based facilities and applications.