Novel Ultrabright and Air-Stable Photocathodes Discovered from Machine Learning and Density Functional Theory Driven Screening.

The high brightness, low emittance electron beams achieved in modern X-ray free-electron lasers (XFELs) have enabled powerful X-ray imaging tools, allowing molecular systems to be imaged at picosecond time scales and sub-nanometer length scales. One of the most promising directions for increasing the brightness of XFELs is through the development of novel photocathode materials. Whereas past efforts aimed at discovering photocathode materials have typically employed trial-and-error-based iterative approaches, this work represents the first data-driven screening for high brightness photocathode materials.

A cryogenically cooled high voltage DC photoemission electron source.

Linear electron accelerators and their applications such as ultrafast electron diffraction require compact high-brightness electron sources with high voltage and electric field at the photocathode to maximize the electron density and minimize space-charge induced emittance growth. Achieving high brightness from a compact source is a challenging task because it involves an often-conflicting interplay between various requirements imposed by photoemission, acceleration, and beam dynamics. Here we present a new design for a compact high voltage DC electron gun with a novel cryogenic photocathode system and report on its construction and commissioning process.

Generation of 167 W infrared and 124 W green power from a 1.3-GHz, 1-ps rod fiber amplifier.

We report on the direct amplification of 1-ps pulses at 1.3 GHz repetition rate by using a large mode area rod fiber amplifier. An average power of 167 W at 1040 nm with nearly transform-limited duration is generated and converted into 124 W average green power through second harmonic generation.

Design, conditioning, and performance of a high voltage, high brightness dc photoelectron gun with variable gap.

A new high voltage photoemission gun has been constructed at Cornell University which features a segmented insulator and a movable anode, allowing the cathode-anode gap to be adjusted. In this work, we describe the gun's overall mechanical and high voltage design, the surface preparation of components, as well as the clean construction methods. We present high voltage conditioning data using a 50 mm cathode-anode gap, in which the conditioning voltage exceeds 500 kV, as well as at smaller gaps.

Ultrabright and ultrafast III-V semiconductor photocathodes.

Crucial photoemission properties of layered III-V semiconductor cathodes are predicted using Monte Carlo simulations. Using this modeling, a layered GaAs structure is designed to reduce simultaneously the transverse energy and response time of the emitted electrons. This structure, grown by molecular beam epitaxy and activated to negative electron affinity, is characterized.

Generation of 110 W infrared and 65 W green power from a 1.3-GHz sub-picosecond fiber amplifier.

A fiber amplifier that generates nearly transform-limited sub-picosecond pulses and greater than 100 W average power at 1.3-GHz repetition rate is described. Modest stretching of the seed pulses allows the amplifier to be operated in the linear regime.

Maximum achievable beam brightness from photoinjectors.

Electron injectors delivering relativistic electron beams with very high brightness are essential for a number of current and proposed electron accelerator applications. These high brightness beams are generally produced from photoemission cathodes. We formulate a limit on the electron beam brightness from such cathodes set by the transverse thermal energy of the electrons leaving the photocathode and the accelerating field at the cathode.

Efficient temporal shaping of ultrashort pulses with birefringent crystals.

We report on a simple and robust technique to temporally shape ultrashort pulses. A number of birefringent crystals with appropriate crystal length and orientation form a crystal set. When a short pulse propagates through the crystal set, the pulse is divided into numerous pulses, producing a desired temporal shape.