Comparison of DC and SRF Photoemission Guns For High Brightness High Average Current Beam Production

A comparison of the two most prominent electron sources of high average current high brightness electron beams, DC and superconducting RF photoemission guns, is carried out using a large-scale multivariate genetic optimizer interfaced with space charge simulation codes. The gun geometry for each case is varied concurrently with laser pulse shape and parameters of the downstream beamline elements of the photoinjector to obtain minimum emittance as a function of bunch charge. Realistic constraints are imposed on maximum field values for the two gun types. The SRF and DC gun emittances and beam envelopes are compared for various values of photocathode thermal emittance. The performance of the two systems is found to be largely comparable provided low intrinsic emittance photocathodes can be employed

Experimental characterization of photoemission from plasmonic nanogroove arrays

Metal photocathodes are an important source of high-brightness electron beams, ubiquitous in the operation of both large-scale accelerators and table-top microscopes. When the surface of a metal is nano-engineered with patterns on the order of the optical wavelength, it can lead to the excitation and confinement of surface plasmon polariton waves which drive nonlinear photoemission. In this work, we aim to evaluate gold plasmonic nanogrooves as a concept for producing bright electron beams for accelerators via nonlinear photoemission. We do this by first comparing their optical properties to numerical calculations from first principles to confirm our ability to fabricate these nanoscale structures. Their nonlinear photoemission yield is found by measuring emitted photocurrent as the intensity of their driving laser is varied. Finally, the mean transverse energy of this electron source is found using the solenoid scan technique. Our data demonstrate the ability of these cathodes to provide a tenfold enhancement in the efficiency of photoemission over flat metals driven with a linear process. We find that these cathodes are robust and capable of reaching sustained average currents over 100 nA at optical intensities larger than 2 GW/cm$^2$ with no degradation of performance. The emittance of the generated beam is found to be highly asymmetric, a fact we can explain with calculations involving the also asymmetric roughness of the patterned surface. These results demonstrate the use of nano-engineered surfaces as enhanced photocathodes, providing a robust, air-stable source of high average current electron beams with great potential for industrial and scientific applications.Comment: 9 pages, 9 figure

Low intrinsic emittance in modern photoinjector brightness

Reducing the intrinsic emittance of photocathodes is one of the most promising routes to improving the brightness of electron sources. However, when emittance growth occurs during beam transport (for example, due to space charge), it is possible that this emittance growth overwhelms the contribution of the photocathode, and, thus, in this case source emittance improvements are not beneficial. Using multiobjective genetic optimization, we investigate the role intrinsic emittance plays in determining the final emittance of several space-charge-dominated photoinjectors, including those for high-repetition-rate free electron lasers and ultrafast electron diffraction. We introduce a new metric to predict the scale of photocathode emittance improvements that remain beneficial and explain how additional tuning is required to take full advantage of new photocathode technologies. Additionally, we determine the scale of emittance growth due to point-to-point Coulomb interactions with a fast tree-based space-charge solver. Our results show that, in the realistic high-brightness photoinjector applications under study, the reduction of thermal emittance to values as low as $50\text{ }\text{ }\mathrm{pm}/{\mu}\mathrm{m}$ (1 meV mean transverse energy) remains a viable option for the improvement of beam brightness

Low energy photoemission from (100) Ba

Recent research on photocathodes for photoinjectors has focused on the understanding of the photoemission process at low energy (i.e. at photon energy close to the material’s work function) as well as on the study of ordered and innovative photocathode materials, with the aim of minimizing the emittance at the cathode. We here present a preliminary study on low energy photoemission from (100) oriented Ba1−xLaxSnO3 thin films, characterizing their quantum efficiency and the mean transverse energy of the photoelectrons. The aim of the study is to pave the way for future experiments on innovative photocathodes based on perovkite oxides

Nearly tight bounds for testing function isomorphism

We study the problem of testing isomorphism (equivalence up to relabelling of the variables) of two Boolean functions f, g: {0, 1} n → {0, 1}. Our main focus is on the most studied case, where one of the functions is given (explicitly) and the other function may be queried. We prove that for every k ≤ n, the worst-case query complexity of testing isomorphism to a given k-junta is Ω(k) and O(k log k). Consequently, the query complexity of testing function isomorphism is e Θ(n). Prior to this work, only lower bounds of Ω(log k) queries were known, for limited ranges of k, proved by Fischer et al. (FOCS 2002), Blais and O’Donnell (CCC 2010), and recently by Alon and Blais (RANDOM 2010). The nearly tight O(k log k) upper bound improves on the e O(k 4) upper bound from Fischer et al. (FOCS 2002). Extending the lower bound proof, we also show polynomial query-complexity lower bounds for the problems of testing whether a function can be computed by a circuit of size ≤ s, and testing whether the Fourier degree of a function is ≤ d. This answers questions posed by Diakonikolas et al. (FOCS 2007). We also address two closely related problems – 1. Testing isomorphism to a k-junta with one-sided error: we prove that for any 1 < k < n − 1, the query complexity is Ω(log ` ´ n), which is almost optimal. Thi