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. Through screening over 74 000 semiconducting materials, a vast photocathode dataset is generated, resulting in statistically meaningful insights into the nature of high brightness photocathode materials. This screening results in a diverse list of photocathode materials that exhibit intrinsic emittances that are up to 4x lower than currently used photocathodes. In a second effort, multiobjective screening is employed to identify the family of M2 O (M = Na, K, Rb) that exhibits photoemission properties that are comparable to the current state-of-the-art photocathode materials, but with superior air stability. This family represents perhaps the first intrinsically bright, visible light photocathode materials that are resistant to reactions with oxygen, allowing for their transport and storage in dry air environments.

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. This photoemission gun can operate at ∼200 kV at both room temperature and cryogenic temperature with a corresponding electric field of 10 MV/m, necessary for achieving high quality electron beams without requiring the complexity of guns, e.g., based on RF superconductivity. It hosts a compact photocathode plug compatible with that used in several other laboratories opening the possibility of generating and characterizing electron beam from photocathodes developed at other institutions.

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. Both infrared and green pulses exhibit diffraction-limited beam quality. A frequency doubling efficiency of 75% and a pump-to-green efficiency of 45% have been achieved, which are to our knowledge the highest reported for fiber laser systems.

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. Finally, we present simulated emittance results obtained from a genetic optimization scheme using voltage values based on the conditioning data. These results indicate that for charges up to 100 pC, a 30 mm gap at 400 kV has equal or smaller 100% emittance than a 50 mm gap at 450 kV, and also a smaller core emittance, when placed as the source for the Cornell energy recovery linac photoinjector with bunch length constrained to be <3 ps rms. For 100 pC up to 0.5 nC charges, the 50 mm gap has larger core emittance than the 30 mm gap, but conversely smaller 100% emittance.

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. The measured values of quantum efficiency and transverse energy are found to agree well with the simulations. Such advanced layered structures will allow generation of short electron bunches from photoinjectors with superior beam brightness.

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. The amplified and dechirped pulses exhibit excellent beam quality, and can be frequency-doubled to produce green pulses at 65 W average power. Detailed characterization of the performance is presented.

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. Two specific examples--direct measurement of the transverse phase space of a space charge dominated beam from a high-voltage photoemission electron gun and a numerical optimization of the same at a higher gun voltage--illustrate the importance of this limit.

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. Flexibility in the final pulse shape is achieved through varying initial pulse duration, divided-pulse number, the polarization-mode delay, and energy distribution of the divided pulses. The energy efficiency of the technique is near 100% for a pulse train of alternating polarizations, and 50% for a linearly polarized pulse train.