A cryogenically cooled 200 kV DC photoemission electron gun for ultralow emittance photocathodes.

Novel photocathode materials like ordered surfaces of single crystal metals, epitaxially grown high quantum efficiency thin films, and topologically non-trivial materials with dirac cones show great promise for generating brighter electron beams for various accelerator and ultrafast electron scattering applications. Despite several materials being identified as brighter photocathodes, none of them have been tested in electron guns to extract electron beams due to technical and logistical challenges. In this paper, we present the design and commissioning of a cryocooled 200 kV DC electron gun that is capable of testing a wide variety of novel photocathode materials over a broad range of temperatures from 298 to 35 K for bright electron beam generation. This gun is designed to enable easy transfer of the photocathode to various standard ultra-high-vacuum surface diagnostics and preparation techniques, allowing a full characterization of the dependence of beam brightness on the photocathode material and surface properties. We demonstrate the development of such a high-voltage, high-gradient gun using materials and equipment that are easily available in any standard university lab, making the development of such 200 kV electron guns more accessible.

The effects of oxygen-induced phase segregation on the interfacial electronic structure and quantum efficiency of Cs3Sb photocathodes.

High-performance photocathodes for many prominent particle accelerator applications, such as x-ray free-electron lasers, cannot be grown in situ. These highly reactive materials must be grown and then transported to the electron gun in an ultrahigh-vacuum (UHV) suitcase, during which time monolayer-level oxidation is unavoidable. Thin film Cs3Sb photocathodes were grown on a variety of substrates. Their performance and chemical state were measured by x-ray photoelectron spectroscopy after transport in a UHV suitcase as well as after O2-induced oxidation. The unusual chemistry of cesium oxides enabled trace amounts of oxygen to drive structural reorganization at the photocathode surface. This reorganization pulled cesium from the bulk photocathode, leading to the development of a structurally complex and O2-exposure-dependent cesium oxide layer. This oxidation-induced phase segregation led to downward band bending of at least 0.36 eV as measured from shifts in the Cs 3d5/2 binding energy. At low O2 exposures, the surface developed a low work function cesium suboxide overlayer that had little effect on quantum efficiency (QE). At somewhat higher O2 exposures, the overlayer transformed to Cs2O; no antimony or antimony oxides were observed in the near-surface region. The development of this overlayer was accompanied by a 1000-fold decrease in QE, which effectively destroyed the photocathode via the formation of a tunnel barrier. The O2 exposures necessary for degradation were quantified. As little as 100 L of O2 irreversibly damaged the photocathode. These observations are discussed in the context of the rich chemistry of alkali oxides, along with potential material strategies for photocathode improvement.

Ultracold Electrons via Near-Threshold Photoemission from Single-Crystal Cu(100).

Achieving a low mean transverse energy or temperature of electrons emitted from the photocathode-based electron sources is critical to the development of next-generation and compact x-ray free electron lasers and ultrafast electron diffraction, spectroscopy, and microscopy experiments. In this Letter, we demonstrate a record low mean transverse energy of 5 meV from the cryo-cooled (100) surface of copper using near-threshold photoemission. Further, we also show that the electron energy spread obtained from such a surface is less than 11.5 meV, making it the smallest energy spread electron source known to date: more than an order of magnitude smaller than any existing photoemission, field emission, or thermionic emission based electron source. Our measurements also shed light on the physics of electron emission and show how the energy spread at few meV scale energies is limited by both the temperature and the vacuum density of states.

Direct Measurement of Sub-10 fs Relativistic Electron Beams with Ultralow Emittance.

Ultralow emittance (≤20 nm, normalized) electron beams with 10^{5} electrons per bunch are obtained by tightly focusing an ultrafast (∼100 fs) laser pulse on the cathode of a 1.6 cell radio frequency photoinjector. Taking advantage of the small initial longitudinal emittance, a downstream velocity bunching cavity is used to compress the beam to <10 fs rms bunch length. The measurement is performed using a thick high-voltage deflecting cavity which is shown to be well suited to measure ultrashort durations of bunching beams, provided that the beam reaches a ballistic longitudinal focus at the cavity center.

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.

Classification and analysis of excitations in a coupled finite nanoparticle cylinder-shell lattice.

We present a general model for computing an optical response function of a finite shell lattice of semiconducting or metallic nanoparticles. Within a second quantization formalism, a cylindrical shell of induced, coupled dipoles is considered in the presence of an external electric field. Numerical analysis of the eigenmodes and quantum mechanical response function allow us to identify resonator effects due to constructive interferometric interaction of the light to the dipole lattice. Adjusting the wavelength of the external electric field, a coherent resonance excitation is possible for a fixed parameter of the cylinder radius.