C. Hernandez-Garcia

Electron sources for accelerators

Photoemission shines as a source of the bright electron beams required for free-electron lasers and particle-physics accelerators.

A High Average Current DC GAAS Photocathode Gun for ERLS and FELS

The Jefferson Lab (JLab) 10 kW IR Upgrade FEL DC GaAs photocathode gun is presently the highest average current electron source operational in the U.S., delivering a record 9.1 mA CW, 350 kV electron beam with 122 pC/bunch at 75 MHz rep rate. Pulsed operation has also been demonstrated with 8 mA per pulse (110 pC/bunch) in 16 ms-long pulses at 2 Hz rep rate. Routinely the gun delivers 5 mA CW and pulse current at 135 pC/bunch for FEL operations. The Upgrade DC photocathode gun is a direct evolution of the DC photocathode gun used in the previous JLab 1 kW IR Demo FEL. Improvements in the vacuum conditions, incorporation of two UHV motion mechanisms (a retractable cathode and a photocathode shield door) and a new way to add cesium to the GaAs photocathode surface have extended its lifetime to over 500 Coulombs delivered between re-cesiations (quantum efficiency replenishment). With each photocathode activation quantum efficiencies above 6% are routinely achieved. The photocathode activation and performance will be described in detail.

High power operation of the JLab IR FEL driver accelerator

Operation of the JLab IR Upgrade FEL at CW powers in excess of 10 kW requires sustained production of high electron beam powers by the driver ERL. This in turn demands attention to numerous issues and effects, including: cathode lifetime; control of beamline and RF system vacuum during high current operation; longitudinal space charge; longitudinal and transverse matching of irregular/large volume phase space distributions; halo management; management of remnant dispersive effects; resistive wall, wake-field, and RF heating of beam vacuum chambers; the beam break up instability; the impact of coherent synchrotron radiation (both on beam quality and the performance of laser optics); magnetic component stability and reproducibility; and RF stability and reproducibility. We discuss our experience with these issues and describe the modus vivendi that has evolved during prolonged high current, high power beam and laser operation.

DC High Voltage Conditioning of Photoemission Guns at Jefferson Lab FEL

DC high voltage photoemission electron guns with GaAs photocathodes have been used to produce polarized electron beams for nuclear physics experiments for about 3 decades with great success. In the late 1990s, Jefferson Lab adopted this gun technology for a free electron laser (FEL), but to assist with high bunch charge operation, considerably higher bias voltage is required compared to the photoguns used at the Jefferson Lab Continuous Electron Beam Accelerator Facility. The FEL gun has been conditioned above 400 kV several times, albeit encountering non‐trivial challenges with ceramic insulators and field emission from electrodes. Recently, high voltage processing with krypton gas was employed to process very stubborn field emitters. This work presents a summary of the high voltage techniques used to high voltage condition the Jefferson Lab FEL photoemission gun.

Correlation of CsK2Sb photocathode lifetime with antimony thickness

CsK2Sb photocathodes with quantum efficiency on the order of 10% at 532 nm, and lifetime greater than 90 days at low voltage, were successfully manufactured via co-deposition of alkali species emanating from an effusion source. Photocathodes were characterized as a function of antimony layer thickness and alkali consumption, inside a vacuum chamber that was initially baked, but frequently vented without re-baking. Photocathode lifetime measured at low voltage is correlated with the antimony layer thickness. Photocathodes manufactured with comparatively thick antimony layers exhibited the best lifetime. We speculate that the antimony layer serves as a reservoir, or sponge, for the alkali.

Status of the Jefferson Lab IR/UV High Average Power Light Source

Jefferson Lab is in the process of building an upgrade to our Free-Electron Laser Facility with broad wavelength range and timing flexibility. The facility will have two cw free-electron lasers, one in the infrared operating from 1 to 14 microns and one in the infrared operating from 0.25 to 1 micron [1]. In addition, there will be beamlines for Thompson-backscattered femtosecond X-rays, and broadband THz radiation. The average power levels for each of these devices will exceed any other available sources by at least 2 orders of magnitude. Timing of the available laser pulses can be continuously mode-locked at least 4 different (MHz) repetition rates or in macropulse mode with pulses of a few microseconds in duration with a repetition rate of many kHz. The status of the construction of this facility and a review of its capabilities will be presented.

Jlamp: an amplifier-based fel in the jlab srf erl driver

Notional designs for energy-recovering linac (ERL)-driven high average power free electron lasers (FELs) often invoke amplifier-based architectures. To date, however, amplifier FELs have been limited in average power output to values several orders of magnitude lower than those demonstrated in optical-resonator based systems; this is due at least in part to the limited electron beam powers available from their driver accelerators. In order to directly contrast the performance available from amplifiers to that provided by high-power cavity-based resonators, we have developed a scheme to test an amplifier FEL in the JLab SRF ERL driver. We describe an accelerator system design that can seamlessly and non-invasively integrate a 10 m wiggler into the existing system and which provides, at least in principle, performance that would support high-efficiency lasing in an amplifier configuration. Details of the design and an accelerator performance analysis will be presented.

Electrostatic design and conditioning of a triple point junction shield for a −200 kV DC high voltage photogun

Nuclear physics experiments performed at the Continuous Electron Beam Accelerator Facility (CEBAF) at the Jefferson Lab require a DC high voltage photogun to generate polarized electron beams from GaAs photocathodes. The photogun uses a tapered ceramic insulator that extends into the vacuum chamber and mechanically holds the cathode electrode. Increasing the operating voltage from nominal -130 kV to -200 kV will provide lower beam emittance, better transmission through injector apertures, and improved photocathode lifetime. This desire to increase the photogun operating voltage led to the design of a triple-point-junction shield electrode which minimizes the electric field at the delicate insulator-metal-vacuum interface and linearizes the potential across the insulator, thus reducing the risk of arcing along the ceramic insulator. This work describes the results obtained using COMSOL® electrostatic-field simulation software and presents the high voltage conditioning results of the upgraded -200 kV CEBAF photogun.

High voltage performance of a dc photoemission electron gun with centrifugal barrel-polished electrodes

The design and fabrication of electrodes for direct current (dc) high voltage photoemission electron guns can significantly influence their performance, most notably in terms of maximum achievable bias voltage. Proper electrostatic design of the triple-point junction shield electrode minimizes the risk of electrical breakdown (arcing) along the insulator-cable plug interface, while the electrode shape is designed to maintain <10 MV/m at the desired operating voltage aiming at little or no field emission once conditioned. Typical electrode surface preparation involves diamond-paste polishing by skilled personnel, requiring several weeks of effort per electrode. In this work, we describe a centrifugal barrel-polishing technique commonly used for polishing the interior surface of superconducting radio frequency cavities but implemented here for the first time to polish electrodes for dc high voltage photoguns. The technique reduced polishing time from weeks to hours while providing surface roughness comparable to that obtained with diamond-paste polishing and with unprecedented consistency between different electrode samples. We present electrode design considerations and high voltage conditioning results to 360 kV (∼11 MV/m), comparing barrel-polished electrode performance to that of diamond-paste polished electrodes. Tests were performed using a dc high voltage photogun with an inverted-geometry ceramic insulator design.

Prospects for an accelerator program in Mexico focused on photon science

Recent interest in developing Mexican expertise in Accelerator Science and Technology has resulted in several actions by the Division of Particles and Fields in Mexico, and by the electron accelerator community in the United States. We report on the very encouraging activities over the past two years which were aimed at developing a light source as the most effective starting point. We present a number of possibilities to initiate and grow an accelerator science program and present a path that would lead to building, commissioning and operating a third or fourth generation light source in Mexico.