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Development of photoinjectors is a critical research area for modern accelerators. Photoinjectors produce the high brightness electron beams that have enabled the realization among others of Ultrafast Electron Diffraction and Microscopy setups, X-ray Free Electron Lasers, Energy Recovery Linacs, high average current sources for bunched electron beam cooling and spin polarized electron beams used in colliders. All these accelerator machines rely on photocathodes to produce electron beams with precisely controlled temporal and spatial shapes, often with stringent requirements on emittance, temporal response, and spin polarization. This 3-day workshop hosted by Brookhaven National Laboratory (October 3–5, 2023) will explore the current state of the art in accelerator photocathodes, from operational, theoretical and a materials science perspective, aiming also at identifying directions for future research and new opportunities for collaboration.
Topics Include:
The organizers envision a workshop format that will allow plenty of time for discussion.
Event ID: 44380
The interplay between a photocathode electron source, the laser driving the photoemission and the electric field into which those electrons emerge define the electron source emittance. The need to characterise photocathode performance and specifically to measure the transverse energy spread of photoemitted electrons is therefore crucial to minimise emittance and maximise beam brightness.
The TESS concept has been developed over the last decade, and has proved to be a simple and powerful method to characterise photoemissive performance. TESS offers insights into the performance of these materials at any illumination wavelength, at room temperature or under cryogenic conditions. TESS also allows photocathode materials to be studied while subjected to progressive degradation, allowing us to assess their robustness. Furthermore, the high gain available with the TESS detector facilitates photocathode studies at the emission threshold where the energy spread approaches the thermal limit and the quantum efficiency is vanishingly small.
I will present an overview of the TESS system, highlighting the substantial developments which have been made in recent years, and will present data on some of our more recent work using the TESS in our development of a CsTe deposition capability at Daresbury, and in the use of ultra-thin metal oxide films to enhance photoemission.
One of the simplest models of tunneling makes use of a δ-function barrier V (x) = ¯h2γδ(x)/2m which depends on a single parameter γ, making it a useful tool to examine a variety of technologically important applications (i) theoretical predictions of photoemission from materials such as cesiated GaAs surfaces often include a high and thin barrier at the surface attributed to a submonolayer coating of an alkali metal such as cesium to bring theoretically predicted yields in line with photoemission experiments; (ii) time evolution simulations of quantum mechanical (QM) tunneling effects using the Wigner distribution function (WDF) and Schrodinger equation (SE) use the δ-function barrier differently to gauge how abruptness and symmetry affect numerical time evolution and because the δ-function barrier demonstrates that the passage of a wave packet incident through it is delayed, even though the barrier is of zero thickness; (iii) the δ-function describes IV characteristics of normal-superconducting (NS) point contacts where the transition from the metallic to the superconducting limit is governed by a strength γ of the barrier. Left and right states are coupled, affect the crossover from metallic to small-area tunnel-junction behavior and impact current calculations and charge imbalance processes at the normal-superconducting (NS) interface; and (iv) the interest in conduction, transport, and/or emission when either the barriers or wells have a time dependent behavior, for which using steady state emission equations may be insufficient demand the creation of new exact methods that we consider. In such cases, an examination of the physics of tractable models gives a means to examine methods useful for more complex configurations.
The analytic model of a δ-function barrier inside a confining well is extended to exactly evaluate the eigenstates and show their dependencies. The time evolution of a superposition of the lowest eigenstates is considered for barriers having comparable Gamow tunneling factors so as to quantify the impact of barrier height and shape on time evolution in a simple and exact system, and thereby serve as a proxy for tunneling time. Lastly, density profiles and associated quantum potentials are examined for coupled wells to show changes induced by weaker and wider barriers. The single parameter δ-function model is useful for the rapid evaluation of current density and is demonstrated. The simplicity of the barrier is further useful in drawing a connection between quantum well bound state problems to tunneling and transport problems, but it is a blunt tool that forces the eigen-energies in a closed system to take on simple limiting values. In the present work, we examine the eigenstates associated with a δ-function barrier in a confining well with infinite walls to identify how the δ-function barrier relates to the more complicated barrier representations. The methods are to be generalized to examine barriers of other shapes that differ from the rectangular barrier by a shape factor to simulate how barrier parameters affect either the dynamic behavior associated with density migration from one side of the well to the other through the barrier or the parameterization of the NS interface.
References
[1] K. L. Jensen et al., “A delta barrier in a well and the exact time evolution of its eigenstates,”
J. Appl. Phys., vol. 133, no. 17, pp. 174402, 2023
The adjoint method promises to greatly improve the calculation of the effect of fabrication tolerances and may also lead to highly efficient optimization algorithms for general charged-particle beamline design. The adjoint method applied to an electron beam source design is discussed. We note that such methods vastly improve computational efficiency by requiring only two or three runs of the simulation code to calculate the sensitivities (derivatives) of a figure of merit to variations of a number of physical parameters, as compared with (N+1) runs required to compute sensitivities of a figure of merit to N physical parameters. The computational savings can be enormous for large values of N. Additionally, in codes that employ computational grids matched to their geometries or volumetric features of interest, remeshing is not required for the subsequent runs, often-times allowing the second run to be restarted from the first, reducing overall run time. The other benefit is a higher-accuracy sensitivity reductions by using the same mesh.
The excitation laser polarization and coupling geometry metastructures define the spin-orbit interaction of surface plasmon polaritons at metal-vacuum interfaces. We image the topological plasmonic fields by imaging nonlinear electron emission by photoemission electron microscopy. We record with nanometer spatial and femtosecond temporal resolution the generated space-time spin textures of plasmonic vortex fields and how they can dress the space-time invariance of matter. The angular momentum of light is transferred to photoemitted electrons in a coherent two-photon emission. We conclude that the angular momentum must be carried away by the generated electron beam. The plasmonic vortex cores act as sources of magnetoelectric interaction on 10 nm scale.
Abstract:
Photocathodes based on GaAs and other III–V semiconductors are capable of producing highly spin-polarized electron beams. GaAs/GaAsP superlattice photocathodes exhibit high spin polarization; however, the quantum efficiency (QE) is limited to less than 1%. To increase the QE, GaAs/GaAsP superlattice photocathodes with a Distributed Bragg Reflector (DBR) are usually used underneath. This configuration creates a Fabry–Pérot cavity between the DBR and the GaAs surface, enhancing the absorption of incident light and, consequently, the QE. However, the peak QE often deviates from the design requirements of around 780 nm. To investigate this, we have used transmission electron microscopy and improved heat cleaning techniques to mitigate temperature-induced diffusion of atoms from one layer to another. Secondary Ion Mass Spectroscopy is also performed to evaluate the effect of heat treatment on these photocathodes.