Photocathode RF Gun Research Articles

Commissioning the Linac Coherent Light Source injector

The Linac Coherent Light Source is a SASE x-ray free-electron laser (FEL) project presently under construction at SLAC [J. Arthur et al., SLAC-R-593, 2002.]. The injector section, from drive laser and rf photocathode gun through first bunch compressor chicane, was installed in the fall of 2006. The initial system commissioning with an electron beam was completed in August of 2007, with the goal of a 1.2-micron emittance in a 1-nC bunch demonstrated. The second phase of commissioning, including second bunch compressor and full linac, is planned for 2008, with FEL commissioning in 2009. We report experimental results and experience gained in the first phase of commissioning, including the photocathode drive laser, rf gun, photocathode, S-band and X-band rf systems, first bunch compressor, and the various beam diagnostics.

Single-side electron multipacting at the photocathode in rf guns

Multiple electron impacting (multipacting) can take place in rf fields when the rf components are composed of materials with a secondary electron yield greater than one. In rf gun cavities, multipacting may change the properties of the vacuum components or even damage them. First systematic measurements of the multipacting occurring in a photocathode rf gun were made at the Fermilab/NICADD Photoinjector Laboratory in 2000. The multipacting properties were found to depend on the cathode material and the solenoid field configuration. In this study, we measure the multipacting properties in more detail and model the secondary electron generation for numerical simulation. Measurements and simulations for the photoinjectors at Fermilab and DESY are compared. The multipacting takes place at the photocathode in rf guns and is categorized as single-side multipacting. In a low rf field, the electrons emitted from the cathode area do not leave the gun cavity within one rf cycle and have an opportunity to travel back and hit the cathode. The solenoid field distribution in the vicinity of the cathode changes the probability of electron bombardment of the cathode and makes a major contribution to the multipacting behavior.

SYNCHRONIZATION BETWEEN THE LASER AND ELECTRON BEAM IN A PHOTOCATHODE RF GUN

The chemical reactions of hot or room temperature and/or critical water in a time-range of picosecond and sub-picoseconds have been carried out by the 18 MeV S -band linac and a Mg photocathode RF gun with the irradiation of third harmonic Ti : Sapphire laser, at Nuclear Engineering Research Laboratory (NERL), the University of Tokyo. Although this short bunch and 100 fs laser light are enough to perform the experiment of radiation chemistry in the time-range of sub-picoseconds, the total time-resolution become worse by the instability of synchronization between laser and radio frequency of linac. We found that the fluctuation of room temperature causes the instability, particularly the cycle of turning on/off of the air-conditioner. When we decrease the fraction of the room temperature within 0.1 degrees, the timing drift in an hour reaches 600 fs, closed to the timing jitter of 340 fs than previous results.

RF GUN PHOTO-EMISSION MODEL FOR METAL CATHODES INCLUDING TIME DEPENDENT EMISSION

The quantum efficiency from a metal cathode is strongly dependent on the field at the cathode due to the Schottky effect. Since the field is time dependent the quantum efficiency is also time dependent. Thus the laser pulse shape used to generate electrons in a photocathode rf gun is not the same as the electron bunch shape. In addition since the thermal emittance and quantum efficiency are related, the thermal emittance is also time dependent.

Improvements in time resolution and signal-to-noise ratio in a compact pico-second pulse radiolysis system

A compact pico-second pulse radiolysis system has been developing at Waseda University for studying primary processes in radiation chemistry. The system is composed of a photo-injector system and a pico-second all-solid-state laser system. An infrared (IR) and an ultraviolet (UV) laser pulses are obtained from mode-locked Nd:YLF laser system and used for generation of the white light continuum as a probe light and the irradiation to the Cu cathode of a photo-cathode RF-gun, respectively. To improve signal-to-noise (S/N) ratio and time resolution of this pulse radiolysis system, we optimized both probe light and pump electron beam. As a result, our pico-second pulse radiolysis system has been enough to study the primary processes of radiation chemistry. The experimental results and the improvements of our system are described in this paper.

First Beam Measurements of the S-Band photocathodeRadio-Frequency Gun at Tsinghua University

During the last decades, photocathode rf gun has been proven to besuccessful in generating the high brightness electron beam(~1nC,~1πmmmrad,~1 ps) which is required by the ILC,XFEL, Thomson scattering x-ray source, etc. A photocathode rf gun systemis built to develop electron source for the Thomson scattering x-raysource at Accelerator Laboratory of Tsinghua University. The systemconsists of a BNL/ATF-type 1.6 cell S-band rf cavity, a solenoid foremittance compensation, a laser system and some simple equipments forbeam diagnosis. The first beam measurements of the photocathode rf gun,including the dark current, transverse beam profile, charge and quantumefficiency, are reported.

Laser System with a Photocathode RF Gun at the Pohang Accelerator Laboratory

At the Pohang Accelerator Laboratory, a photocathode RF gun has been studied for a photoinjector of the 4th generation light source. This photocathode RF gun provides electron bunches with an energy of 5 MeV, a charge of 1 nC, and a bunch length of 10 ps. For this purpose, a Ti:Sapphire laser system that can deliver laser pulses of 2 mJ in energy and 110 fs in pulse width at a 800-nm wavelength was installed. The laser pulse passes through a third harmonic generator and pulse stretcher to generate a 266-nm laser pulse with a controllable pulse width (1 ps ∼ 15 ps). In this article, we introduce our laser system and report on recent progress in electron beam generation.

RF Measurement and High-Power Test of a 1.6-Cell Photocathode RF Gun at Pohang Accelerator Laboratory

RF Measurement and High-Power Test of a 1.6-Cell Photocathode RF Gun at Pohang Accelerator Laboratory

High Power Beam Test and Measurement of Emittance Evolution of a 1.6-Cell Photocathode RF Gun at Pohang Accelerator Laboratory

A Brookhaven National Laboratory (BNL) GUN-IV type photocathode rf gun has been fabricated to use in femtosecond electron diffraction (FED), femtosecond far infrared radiation (fs-FIR) facility, and X-ray free electron laser (XFEL) facilities at the Pohang Accelerator Laboratory (PAL). The gun consists of a 1.6-cell cavity with a copper cathode, a solenoid magnet, beam diagnostic components and auxiliary systems. We report here the measurement of the basic beam parameters which confirm a successful fabrication of the photocathode RF gun system. The emittance evolution is measured by an emittance meter and compared with the PARMELA simulation, which shows a good agreement.

INJECTOR STUDY FOR COMPACT HARD X-RAY SOURCE VIA LASER COMPTON SCATTERING

Compact hard X-ray source via laser Compton sattering has been developed in SHI and AIST. Our system has the injector and the linac and the high power laser system. The injector has a photo-cathode rf gun with a solenoid magnet. To enhance the X-ray yeild, we are planning to increase electron beam charge up to 5 nC/bunch and to make multi-bunch beam. The beam tracking simulation in the injector have been performed by changing laser spot size, laser pulse width, rf phase and solenoid field to optimize the distance between the injector and the linac for 5 nC/bunch high charge beam. In addition, high charge multi-bunch beam simulation in rf-gun cavity have been carried out to investigate the influence by the beam loading and the wake field.

DEVELOPMENT OF RADIO FREQUENCY GUNS AT NSRRC

Technologies for rf gun-based high brightness injector are being developed at NSRRC. An X-band photo-cathode rf gun has been constructed and operated successfully for coherent electromagnetic radiation research at U.C. Davis. Recent measurement of a 1.5-2 MeV beam from this gun with bunch charge at 25 pC showed that it has a normalized emittance at 1.65 mm-mrad. As a successive project, a 100 MeV electron beam injector system with a thermionic rf gun and a photo-cathode rf gun that share the same 2998 MHz rf linac is being designed as the pre-injector of the TPS (Taiwan Photon Source) booster as well as beam injector for novel light source research. For evaluation purposes, a prototype thermionic rf gun system is now under construction at NSRRC with the existing 2856 MHz high power microwave system as the driver source.

Transverse-to-longitudinal emittance exchange to improve performance of high-gain free-electron lasers

The ability to generate small transverse emittance is perhaps the main limiting factor for the performance of high-gain x-ray free-electron lasers (FELs). Noting that beams from an rf photocathode gun can have energy spread much smaller than required for efficient FEL interaction, we present a method to produce normalized transverse emittance at or below about $0.1\text{ }\ensuremath{\mu}\mathrm{m}$, which will lead to a significantly shorter length undulator as well as a lower electron beam energy for an x-ray FEL project. The beam manipulation consists of producing an unequal partition of the initially equal emittances into two dissimilar emittances by a flat-beam technique and exchanging the larger transverse emittance with a smaller longitudinal emittance. We study various issues involved in the manipulation. In particular, a new emittance exchange optics we found enables an exact emittance exchange necessary for this scheme.

Femto-second electron pulses from an rf linear accelerator as a far-infrared radiation source

Femto-second radiation pulses are becoming an important tool for studying the dynamics of materials at an atomic level. Various methods of generating femto-second electron pulses have been developed, including a thermionic rf gun combined with an alpha magnet. In this paper, we focus on the utilization of a photo-cathode rf gun and an rf linear accelerator to generate far-infrared (FIR) radiation. We demonstrate that with a magnetic chicane the electron beam pulse can be compressed to less than rms 20 fs at an energy between 30 and 50 MeV. Geometrical wake fields due to the rf linear accelerator and the coherent synchrotron radiation effects in the magnetic chicane are included in the study. We suggest a possible improvement of the existing 80 MeV rf linear accelerator at the Pohang Accelerator Laboratory to employ it as a coherent FIR radiation source. We focus on extending the radiation wave number to greater than 1500 cm−1 with a micro-pulse energy of more than 100 µJ, which are the conditions required for femto-dynamics studies.

Beam diagnostics for high quality electron beam emitted from photocathode rf-gun

High quality electron beam generation using photocathode rf-gun system and beam diagnostic techniques have been developed at Waseda University. This system can generate up to 4.6 MeV low emittance and short bunch electron beam. For bunch length monitor, rf-kicker technique is able to streak the electron bunch directly, using transverse magnetic field on the beam orbit. This technique is able to observe the longitudinal profile of the bunch and to achieve higher resolution than streak camera method.

High-intensity multi-bunch beam generation by a photo-cathode RF gun

A multi-bunch photo-cathode RF gun system has been developed as an electron source for the production of intense quasi-monochromatic X-rays based on inverse Compton scattering. The desired multi-bunch beam is 100 bunches/pulse with a total charge of 500 nC and a bunch spacing of 2.8 ns. We modified the gun cavity of a ‘BNL-type IV’ RF gun to allow a CsTe cathode plug in the end plate. The system uses a four-dipole chicane beam line to allow the injection of laser light normal to the cathode surface. We compensate the gun cavity beam loading caused by the high-intensity multi-bunch electron beam by injecting the laser pulse before RF power has filled the cavity. We have achieved a total intensity of 220 nC in 100 bunches with a bunch-to-bunch energy spread under 1.3% (peak-to-peak). This paper concentrates on experiments to generate the high-intensity multi-bunch beam with compensation of the bunch-to-bunch energy spread due to heavy beam loading.

Laser Photo-Cathode RF-Gun

High quality beam generation project based on High-Tech Research Center Project, which has been approved by Ministry of Education, Culture, Sports, Science and Technology in 1999, has been conducted by advance research institute for science and engineering, Waseda University. In the project, laser photo-cathode RF-gun has been selected for the high quality electron beam source. RF cavities with low dark current, which were made by diamond turning technique, have been successfully manufactured. The low emittance electron beam was realized by choosing the modified laser injection technique. The obtained normalized emittance was about 3 mm mrad at 100 pC of electron charge. The soft X-ray beam generation with the energy of 370 eV, which is in the energy region of so-called “water window” , by inverse Compton scattering has been performed by the collision between IR laser and the low emittance electron beams.

Compact soft x-ray source using Thomson scattering

A compact soft x-ray source using Thomson scattering, enabled by the combination of a picosecond laser and an electron rf gun, was developed aiming at biological studies such as those using an x-ray microscope. The x-ray source included both a photoinjector system and a picosecond laser system with a tabletop size of 2×2m2. An infrared laser beam (λ0=1047nm) was obtained from an all-solid-state mode-locked Nd:YLF laser system and injected into the photocathode of an accelerator system. A 4.2MeV electron beam was generated from a laser-driven photocathode rf gun system. The residual laser beam was amplified up to about 4.2mJ/pulse using a flash-lamp-pumped laser amplifier. Upon collision of the electron beam with the amplified laser beam, 300eV soft x rays were generated by Thomson backscattering. The stable interaction between the two beams was achieved using the same seed laser pulse for irradiating the photocathode and the scattering process with laser photons.

Adaptive shaping system for both spatial and temporal profiles of a highly stabilized UV laser light source for a photocathode RF gun

We have been developing a stable and highly qualified ultraviolet (UV) laser pulse as a light source of an RF gun for an injector candidate of future light sources. Our gun cavity is a single-cell pillbox, and the copper inner wall is used as a photocathode. The chirped pulse amplification (CPA) Ti:sapphire laser system is operated at a repetition rate of 10 Hz. At the third harmonic generation (central wavelength—263 nm), the laser pulse energy after a 45 cm silica rod is up to 850 μJ/pulse. In its present status, the laser's pulse energy stability has been improved down to 0.2∼0.3% at the fundamental, and 0.7–1.4% (rms; 10 pps; 33,818 shots) at the third harmonic generation, respectively. This stability has been held for 1 month continuously, 24 h a day. The improvements we had passively implemented were to stabilize the laser system as well as the environmental conditions. We introduced a humidity-control system kept at 50–60% in a clean room to reduce damage to the optics. In addition, we prepared a deformable mirror for spatial shaping and a spatial light modulator based on fused-silica plates for temporal shaping. We are applying both the adaptive optics to automatic optimization of the electron beam bunch to produce lower emittance with the feedback routine. Before the improvements, the electron beam produced from a cathode suffered inhomogeneous distribution caused by the quantum efficiency effect, and some pulse distortions caused by its response time. However, we can now freely form any arbitrary electron beam distribution on the surface of the cathode.

フォトカソードRFガンの安定化・低エミッタンス化・高QE化

フォトカソードRFガンの安定化・低エミッタンス化・高QE化

Transverse beam envelope measurements and the limitations of the 3-screen emittance method for space-charge dominated beams

In its normal mode of operation, the Argonne Wakefield Accelerator Facility uses a high charge (10–100 nC), short pulse (3–5 ps) drive bunch to excite high-gradient accelerating fields in various slow-wave structures. To generate this bunch, we designed a 1.5 cell, L-band, rf photocathode gun with an emittance compensating solenoid to give optimal performance at high charge; it has recently completed commissioning. More recently, we have begun to investigate the possibility of using this gun in a high-brightness, low-charge operating mode, with charge equal to approximately 1 nC, for high-precision measurements of wakefields. Two related measurements are reported on in this paper: (1) measurements of the transverse beam envelope are compared to predictions from the beam dynamics code PARMELA; and (2) investigations into the use of a modified 3-screen emittance measurement method that uses a beam envelope model that includes both space-charge and emittance effects. Both measurements were made for the 1 nC, 8 MeV beam in the drift region directly following the rf photocathode gun.