High brightness, high repetition rate electron beams are key components for optimizing the performance of next generation scientific instruments, such as MHz-class X-ray Free Electron Laser (XFEL) and Ultra-fast Electron Diffraction/Microscopy (UED/UEM). In the Advanced Photo-injector EXperiment (APEX) at Berkeley Lab, a photoelectron gun based on a 185.7 MHz normal conducting re-entrant RF cavity, has been proven to be a feasible solution to provide high brightness, high repetition rate electron beam for both XFEL and UED/UEM. Based on the success of APEX, a new electron gun system, named APEX2, has been under development to further improve the electron beam brightness. For APEX2, we have designed a new 162.5 MHz two-cell photoelectron gun and achieved a significant increase on the cathode launching field and the beam exit energy. For a fixed charge per bunch, these improvements will allow for the emittance reduction and hence to an increased beam brightness. The design of APEX2 gun cavity is a complex problem with multiple design goals and restrictions, some even competing each other. For a systematic and comprehensive search for the optimized cavity geometry, we have developed and implemented a novel optimization method based on the Multi-Objective Genetic Algorithm (MOGA).

We report recent progress on the design of a normal conducting CW electron gun, APEX-II (Advanced Photo-injector EXperiment-II) at Lawrence Berkeley National Laboratory. APEX-II is an upgrade of the successful APEX gun and the LCLS-II (Linac Coherent Light Source-II) injector, aiming at applications for Free electron laser (FEL) such as LCLS-II High Energy upgrade, UED (Ultrafast Electron Diffraction) and UEM (Ultrafast Electron Microscopy). The APEX-II adopted a two-cell cavity design with resonant frequency of 162.5 MHz. The APEX-II gun is targeting to achieve exceeding 30 MV/m of launch gradient at the cathode and output energy above 1.5 MeV with transverse emittance of 0.1 um at 100 pC. Advanced MOGA optimization technique has been used for both the RF cavity design and extensive beam dynamics studies using APEX-like and LCLS-II like injector layout. Detailed RF designs, beam dynamics studies, preliminary engineering design and FEA analysis will be presented, with cavity features that were demonstrated to be crucial in the operation of the APEX gun.

The past decade was characterized by an increasing scientific demand for extending towards higher repetition rates (MHz class and beyond) the performance of already operating lower repetition rate accelerator-based instruments such as x-ray free electron lasers (FELs) and ultrafast electron diffraction (UED) and microscopy (UEM) instruments. Such a need stimulated a worldwide spread of a vibrant R&D activity targeting the development of high-brightness electron sources capable of operating at these challenging rates. Among the different technologies pursued, rf guns based on room-temperature structures resonating in the very high frequency (VHF) range (30-300 MHz) and operating in continuous wave successfully demonstrated in the past few years the targeted brightness and reliability. Nevertheless, recently proposed upgrades for x-ray FELs and the always brightness-frontier applications such as UED and UEM are now requiring a further step forward in terms of beam brightness in electron sources. In this paper, we present a few possible upgrade paths that would allow one to extend, in a relatively simple and cost-effective way, the performance of the present VHF technology to the required new goals.

The need to operate an rf-gun-based electron photoinjector with a beam emitted away from the cathode center can occur under various circumstances. First, in some cases the cathode can be affected by ion back-bombardment that progressively reduces the quantum efficiency (QE) in its center, making off-axis operation mandatory; second, in some cases the drive laser intensity can be sufficiently high to generate QE depletion in the cathode area illuminated by the laser, forcing off-axis operation; last, in cathodes with nonuniform QE distribution it could be convenient to operate off axis to exploit a better QE. However, operation in this mode may lead to growth of the projected transverse beam emittances due to correlations between the transverse and longitudinal degrees of freedom that are introduced within the gun and downstream rf cavities. A strategy is described to mitigate this emittance growth by allowing the beam to propagate along a carefully tuned off-axis trajectory in downstream rf cavities to remove the time-dependent rf kicks introduced in the gun. Along this trajectory, short range wakefields do not degrade the emittance, and long range wakefields degrade the emittance for very high repetition rate only.

A high repetition rate, MHz-class, high-brightness electron source is a key element in future high-repetition-rate x-ray free electron laser-based light sources. The VHF-gun, a novel low frequency radio-frequency gun, is the Lawrence Berkeley National Laboratory (LBNL) response to that need. The gun design is based on a normal conducting, single cell cavity resonating at 186 MHz in the VHF band and capable of continuous wave operation while still delivering the high accelerating fields at the cathode required for the high brightness performance. The VHF-gun was fabricated and successfully commissioned in the framework of the Advanced Photo-injector EXperiment, an injector built at LBNL to demonstrate the capability of the gun to deliver the required beam quality. The basis for the selection of the VHF-gun technology, novel design features, and fabrication techniques are described.

As one of the main components of the injector II of China ADS LINAC project, an RFQ working at 162.5MHz is used to accelerate proton beams of 15mA from 30 keV to 2.1 MeV. The four vane RFQ has been designed in collaboration with Lawrence Berkeley National Laboratory and built at the workshop of the Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS). Low power test of the cavity has been completed, and it shows the field flatness is within ±1% and the unloaded Q is 12600. RF conditioning has been completed, results of preliminary beam test show the output beam energy is 2.16 MeV with energy spread of 3.5% and the transmission efficiency is 97.9%. Continuous wave (CW) beam of 2.3 mA has been accelerated for more than one hour.

The design of a 1.3 GHz buncher cavity for the APEX project, a MHz repetition rate high-brightness photoinjector, is presented. The buncher cavity operates at 240 kV in CW mode, and it compresses the 750 keV beam from APEX gun through ballistic compression. Compared with a single cell design, a two-cell cavity doubles the shunt impedance to 7.8 M� , which greatly relaxes the requirements for both RF amplifier and cavity cooling. Coupler design, multipacting analysis, HOM analysis and thermal analysis will be presented in this paper.

A high repetition rate, MHz, electron source is a key element in future FEL based light sources. The Advanced Photo-injector Experiment (APEX) at Lawrence Berkeley National Laboratory (LBNL) consists of a high repetition rate 186 MHz (VHF-band) CW electron gun, 1 MHz UV laser source, a pulsed 30 MeV linac, and the diagnostic components necessary to quantify the gun’s performance. The gun design is based on well-established, conventional RF cavity design, with a couple notable exceptions. The basis for the selection of this technology, novel design features, fabrication techniques and measured cavity performance are presented.

The Advanced Photoinjector Experiment (APEX) at the Lawrence Berkeley National Laboratory is dedicated to the development of a high-brightness high-repetition rate (MHz-class) electron injector for x-ray free-electron laser (FEL) and other applications where high repetition rates and high brightness are simultaneously required. The injector is based on a new concept rf gun utilizing a normal-conducting (NC) cavity resonating in the VHF band at 186 MHz, and operating in continuous wave (cw) mode in conjunction with high quantum efficiency photocathodes capable of delivering the required charge at MHz repetition rates with available laser technology. The APEX activities are staged in three phases. In phase 0, the NC cw gun is built and tested to demonstrate the major milestones to validate the gun design and performance. Also, starting in phase 0 and continuing in phase I, different photocathodes are tested at the gun energy and at full repetition rate for validating candidate materials to operate in a high-repetition rate FEL. In phase II, a room-temperature pulsed linac is added for accelerating the beam at several tens of MeV to reduce space charge effects and allow the measurement of the brightness of the beam from the gun when integrated in an injector scheme. The installation of the phase 0 beam line and the commissioning of the VHF gun are completed, phase I components are under fabrication, and initial design and specification of components and layout for phase II are under way. This paper presents the phase 0 commissioning results with emphasis on the experimental milestones that have successfully demonstrated the APEX gun capability of operating at the required performance.

The APEX project at the Lawrence Berkeley National Laboratory is devoted to the development of a high repetition rate (MHz-class) electron injector for X-ray FEL applications. The injector is based on a new concept photo-gun, utilizing a normal conducting 186 MHz (VHF) RF cavity operating in CW mode in conjunction with high quantum efficiency photo-cathodes able to deliver the required repetition rates with available laser technology. The APEX activities are staged in phases. In Phases 0 and I, the electron photo-gun is constructed, tested and several different photo-cathodes, such as multialkali antimonides, cesium telluride [1], and diamond amplifiers [2], are tested at full repetition rate. In Phase II, a pulsed linac is added for accelerating the beam at several tens of MeV to prove the high brightness performance of the gun when integrated in an injector scheme. APEX is located in an existing area with a radiation shielding configuration limiting the repetition rate at Phase II energies down to several Hz. Based on funding availability, after Phase II the program could also include testing of new undulator technologies and FEL studies. The status of Phases 0 and I, in the initial experimental phase, is described together with plans and activities for Phase II and beyond.

We report on on-going studies of a superconducting CW linac driver intended to support a high repetition rate FEL operating in the soft x-rays spectrum. We present a pointdesign for a 1.8 GeV machine tuned for 300 pC bunches and delivering low-emittance, low-energy spread beams as needed for the SASE and seeded beamlines.

In this paper, we will present the status of the beam dynamics simulations for a Next Generation Light Source (NGLS) injector, based on a high repetition rate (1 MHz), high brightness design. A multi-stage beam compression scheme is proposed, based on the concepts of velocity bunching and emittance compensation. For the optimization of the design parameters we use a genetic algorithm approach, and we focus on a mode providing charges of 300 pC, with normalized transverse emittance less than 0.6 microns, suitable to operate a next generation light source based on an X-ray FEL. In addition, we discuss the effects of bunch compression and linearity of the transverse and longitudinal phase space of the beam.

A baseline H{sup -} ion source and low energy beam transport system (LEBT) have been identified for Project X. The filament-discharge H{sup -} ion source has been fabricated by D-Pace, Inc. and is now in operation at LBNL. The source is capable of delivering over 10mA of H{sup -} beam in cw operation with normalized 4rms emittances less than 0.7 {pi} mm mrad. A two-solenoid magnetic lens LEBT system has been design. The design has been validated with simulations of beam transport for 5 mA 30 keV H- beams using various simulation codes.

The Next Generation Light Source (NGLS) is a design concept, under development at LBNL, for a 10� beamline xray FEL array powered by a superconducting linear accelerator of ~2 GeV, operating with a 1 MHz bunch repetition rate. The CW superconducting linear accelerator is supplied by a high-brightness, high- repetition-rate photocathode electron gun. Beam is distributed from the linac to the array of independently configurable FEL beamlines with nominal bunch rates up to 100 kHz, with even pulse spacing. Individual FELs may be configured for EEHG, HGHG, SASE, or oscillator mode of operation, and will produce high peak and average brightness x-rays with a flexible pulse format ranging from sub-femtoseconds to hundreds of femtoseconds.

The fabrication and installation at the Lawrence Berkeley National Laboratory of a high-brightness highrepetition rate photo-gun, based on a normal conducting 187 MHz (VHF) RF cavity operating in CW mode, is in an advanced phase. The cavity will generate an electric field at the cathode plane of ~ 20 MV/m to accelerate the electron bunches up to ~ 750 keV, with peak current, energy spread and transverse emittance suitable for FEL and ERL applications. The gun vacuum system has been designed for achieving pressures compatible with the use of "delicate" high quantum efficiency semiconductor cathodes to generate up to a nC bunches at MHz repetition rate with present laser technology. Several photo-cathode/laser systems are under consideration, and in particular photo-cathodes based on K2CsSb are being developed for the gun and have already achieved a QE of 8% at 532 nm wavelength, or close to 20% including the Schottky barrier lowering. The cathode will be operated by a μJ fiber laser in conjunction with refractive transverse beam shaping to create a flat top transverse profile, as well as a birefringent pulse stacker to create a flat top temporal profile. The present status and the plan for future activities are presented.

The scientific potential of femtosecond x-ray pulses at linac-driven free-electron lasers such as the Linac Coherent Light Source is tremendous. Time-resolved pump-probe experiments require a measure of the relative arrival time of each x-ray pulse with respect to the experimental pump laser. An optical timing system based on stabilized fiber links has been developed for the LCLS to provide this synchronization. Preliminary results show synchronization of the installed stabilized links at the sub-20-femtosecond level. We present details of the implementation at LCLS and potential for future development.

PRELIMINARY CONCEPT FOR THE PROJECT X CW RADIO- FREQUENCY QUADRUPOLE (RFQ)* S. P. Virostek, M . D. Hoff, D. L i , and J. W. Staples Lawrence Berkeley National Laboratory, Berkeley, C A , USA Abstract Project X is a proposed multi-MW proton facility at Fermi National Accelerator Laboratory [1]. It is the key element for future accelerator complex development intended to support world-leading High Energy Physics (HEP) programs. The Project X front-end would consist of an H - ion source, a low-energy beam transport ( L E B T ) , a radio-frequency quadrupole (RFQ) accelerator, and a medium-energy beam transport (MEBT). To support current and future H E P experiments at Fermilab, a C W RFQ is required. One of the chosen R F Q designs has a resonant frequency of 325 M H z . A 162.5 M H z option is also being considered but is not presented here. The R F Q provides bunching of the 10 mA H - beam with acceleration from 30 keV to 2.5 M e V and wall power losses of less than 250 kW. Lawrence Berkeley National Laboratory ( L B N L ) is currently developing the early designs for various components in the Project X front-end [2]. The R F Q design concept and the preliminary thermal analyses are presented here. INTRODUCTION The Project X baseline R F Q design is 2.66 m long and will accelerate a 10 mA H - beam to 2.5 M e V , with a 64 kV vane-to-vane voltage (corresponding to a 1.55 Kilpatrick peak field). Most of the RF input power is dissipated on the cavity walls to establish the needed RF field with only about 17% of the total power transferred to the beam. Each of the two 1.33 m long R F Q modules will consist of four solid O F H C copper vanes that are modulated prior to being brazed together. A brazed copper structure has been chosen due to the high power, C W operation. A 304 stainless steel outer shell is to be bolted to the cavity by means of thread inserts in the copper. A series of 32 water-cooled pi-mode rods provides quadrupole mode stabilization, and a set of 48 evenly spaced fixed slug tuners is used for final frequency adjustment and local field perturbation correction. The Project X R F Q design incorporates technology validated by recent RFQ's developed at L B N L , including for the Spallation Neutron Source (SNS) Front End [3] as well as a recent design completed for the Accelerator Driven Neutron Source (ADNS) [4]. The use of proven and reliable fabrication and assembly methods permits construction using readily available machinery incorporating previously proven techniques. The bolt-on, stainless steel outer stiffening plates provide the necessary structural rigidity as well as a means for reliably applying vacuum and RF sealing forces for the tuners, couplers, * This work was supported by the Office of Science, U. S. Department of Energy, under Contract No. DE-AC02-05CH11231. sensing loops and vacuum pumping manifolds. The outer shell also provides for a relatively simple method to interconnect the modules. A preliminary 3-D C A D model of the RFQ conceptual design has been developed and is used here to present a description of the design characteristics. A n overall view of a single R F Q module is shown in Fig. 1. Figure 1: C A D model of a single RFQ module. RFQ DESIGN DETAILS Cavity Body Each of the four vanes in a module are to be machined from a single piece of copper and will include simple cooling channels produced using an established gun boring technique. The R F Q vane tips are to be modulated by means of a fly cutter technique previously developed at L B N L using a commonly available programmable mill. Fiducial surfaces that also act as mating surfaces will be machined directly onto the vanes to provide high precision during both machining and assembly. Two vane geometries will be used (major and minor) with the opposing vanes being identical. Other features such as tuner ports, RF coupling ports, vane cut back cooling passages, cooling taps, vacuum pumping ports, pi-mode rod penetrations, sensing loop ports and tapped holes for the stainless steel backing plates are to be machined prior to finish machining of the cavity surfaces and vane tips. Note that all vacuum seals to the cavity for penetrations are recessed beyond the outer layer of stainless steel and are to be applied directly to the O F H C . The finished vanes are to be brazed together along axially running joints. A zero-thickness brazing process will be used in order to maintain the tight vane tip-to-vane tip tolerance, which is dictated by the high dependence of cavity frequency on vane tip spacing. Wire braze alloy will be loaded into grooves in the joint surfaces such that the alloy spreads throughout the joint during the braze

Several recent reports have identified the scientific requirements for a future soft X-ray light source [Citation1, Citation2, Citation3, Citation4, Citation5], and a high-repetition-rate free-electron laser (FEL) facility responsive to them is being studied at Lawrence Berkeley National Laboratory (LBNL) [Citation6]. The facility is based on a continuous-wave (CW) superconducting linear accelerator with beam supplied by a high-brightness, high-repetition-rate photocathode electron gun operating in CW mode, and on an array of FELs to which the accelerated beam is distributed, each operating at high repetition rate and with even pulse spacing. Dependent on the experimental requirements, the individual FELs may be configured for either self-amplified spontaneous emission (SASE), seeded high-gain harmonic generation (HGHG), echo-enabled harmonic generation (EEHG), or oscillator mode of operation, and will produce high peak and average brightness X-rays with a flexible pulse format ranging from sub-femtoseconds to hundreds of femtoseconds. This new light source would serve a broad community of scientists in many areas of research, similar to existing utilization of storage ring based light sources. Acknowledgments The authors gratefully acknowledge helpful contributions from G. Stupakov and D. Xiang at SLAC for their assistance with FEL design studies.