arXiv subjects

S. Karkare

Publications and source records attributed to S. Karkare.

7 recordsLinked to original sources

arXiv2026

Report on the Advanced Linear Collider Study Group (ALEGRO) Workshop 2026

The 7th ALEGRO workshop, hosted by INFN Frascati National Laboratories from 3rd to 5th March 2026, brought together the international Advanced Novel Accelerators (ANA) community to discuss the role of advanced wakefield accelerators (AWA) in particle and high-energy physics. Organized under the ICFA-ANA panel, the workshop highlighted recent progress in plasma- and structure-based wakefield acceleration and strengthened international collaboration toward future energy-frontier colliders. A major focus was the ongoing 10 TeV linear collider design study, launched in 2025 following the US P5 recommendations. A dedicated session covered accelerator concepts, enabling technologies, and the associated physics case, while recognizing that future priorities will depend on the outcome of the European Strategy for Particle Physics Update. The workshop also reviewed nearer-term applications of advanced accelerators, including fixed-target experiments, injectors for future colliders and light sources, plasma-based Higgs factory concepts, and proton-driven plasma wakefield acceleration. Beyond high-energy physics, sessions covered free-electron lasers, synchrotron light sources, and strong-field QED. Recent demonstrations of FEL lasing with plasma-accelerated electron beams highlighted significant progress in beam quality and accelerator performance. Operational challenges for reliable user facilities, including high-availability laser and electron-beam systems, were discussed alongside the growing role of artificial intelligence and machine learning for accelerator optimization and control. This report summarizes the workshop discussions and conclusions from the chairs, together with short contributions from the presenters, providing an overview of the current status and future prospects of advanced wakefield accelerators.

physics.acc-ph↗

arXiv2024

A structural analysis of ordered Cs$_{3}$Sb films grown on single crystal graphene and silicon carbide substrates

Alkali antimonides are well established as high efficiency, low intrinsic emittance photocathodes for accelerators and photon detectors. However, conventionally grown alkali antimonide films are polycrystalline with surface disorder and roughness that can limit achievable beam brightness. Ordering the crystalline structure of alkali antimonides has the potential to deliver higher brightness electron beams by reducing surface disorder and enabling the engineering of material properties at the level of atomic layers. In this report, we demonstrate the growth of ordered Cs$_{3}$Sb films on single crystal substrates 3C-SiC and graphene-coated 4H-SiC using pulsed laser deposition and conventional thermal evaporation growth techniques. The crystalline structures of the Cs$_{3}$Sb films were examined using reflection high energy electron diffraction (RHEED) and X-ray diffraction (XRD) diagnostics, while film thickness and roughness estimates were made using x-ray reflectivity (XRR). With these tools, we observed ordered domains in less than 10 nm thick films with quantum efficiencies greater than one percent at 530 nm. Moreover, we identify structural features such as Laue oscillations indicative of highly ordered films. We found that Cs$_{3}$Sb films grew with flat, fiber-textured surfaces on 3C-SiC and with multiple ordered domains and sub-nanometer surface roughness on graphene-coated 4H-SiC under our growth conditions. We identify the crystallographic orientations of Cs$_{3}$Sb grown on graphene-coated 4H-SiC substrates and discuss the significance of examining the crystal structure of these films for growing epitaxial heterostructures in future experiments.

cond-mat.mtrl-sci↗

arXiv2022

Ab initio study of the crystal and electronic structure of mono- and bi-alkali antimonides: Stability, Goldschmidt-like tolerance factors, and optical properties

Mono- and bi-alkali antimonides, X$_2$YSb (X and Y from Group I), are promising for next-generation electron emitters due to their capability of producing high-quality electron beams. However, these materials are not yet well understood, in part due to the technical challenges in growing pure, ordered alkali antimonides. For example, in the current literature there is a lack of complete understanding of the mechanically stable crystal structures of these materials. As a first step towards understanding this issue, this paper presents an ab initio study of stability of single-crystal mono- and bi-alkali antimonides in the $D0_3$ structure, the structure generally assumed in the literature for these materials. Finding that many of these materials actually are unstable in the $D0_3$ structure, we formulate a new set of Goldschmidt-like tolerance factors that accurately predict $D0_3$ stability using a procedure analogous to machine-learning perceptron-based analysis. Next, we consider possible stable structures for materials that we predict to be unstable in the $D0_3$ structure. Taking as examples the mono- and bi-alkali antimonides Cs$_3$Sb and Cs$_2$KSb, which also are technologically interesting for photoemission and photoabsorption applications, respectively, we note that the most unstable phonon displacements are consistent with the cubic structure, and we therefore perform extensive ab initio searches to identify potential ground-state structures in a cubic lattice. Our X-ray diffraction experiments confirm that indeed these two materials are not stable in the $D0_3$ structure and show scattering that is consistent with our new, proposed stable structures. Finally, we explore ab initio the implications of the breaking of the $D0_3$ symmetry on the electronic structure, showing significant impact on the location of the optical absorption edge.

cond-mat.mtrl-sci↗

arXiv2021

Photoemission characterization of N-polar III-Nitride photocathodes as bright electron beam source for accelerator applications

We report on the growth and characterization of a new class of photocathode structures for application as electron sources to produce high brightness electron beams for accelerator applications. The sources are realized using III-Nitride materials and are designed to leverage the strong polarization field characteristic of this class material while grown in their wurtzite crystal structure to produce a negative electron affinity condition without the use of Cs, possibly allowing these materials to be operated in RF gun. A Quantum Efficiency (QE) of about 1x10$^{-3}$ and a Mean Transverse Energy (MTE) of electron of about 100 meV are measured at the operating wavelength of 265 nm. In a vacuum level of 3x10$^{-10}$ Torr the QE does not decrease after more than 24 hours of continuous operation. The lowest MTE, about 50 meV, is measured at 300 nm where the measured QE is 1.5x10$^{-5}$. Surface characterizations reveal possible contribution to the MTE due to the surface morphology calling for more detailed studies.

physics.acc-ph↗

arXiv2020

An Ultra-Compact X-Ray Free-Electron Laser

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV/m. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 meters. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by ~50 nm-rad normalized emittances. These beams, when injected into innovative, short-period (1-10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine which may profit from this new model of performing XFEL science.

physics.acc-ph↗

arXiv2019

Development of a 3-D energy-momentum analyzer for meV-scale energy electrons

In this article, we report on the development of a time-of-flight based electron energy analyzer capable of measuring the 3-D momentum and energy distributions of very low energy (meV-scale) photoemitted electrons. This analyzer is capable of measuring energy and 3-D momentum distributions of electrons with energies down to 1 meV with a sub-meV energy resolution. This analyzer is an ideal tool for studying photoemission processes very close to the photoemission threshold and also for studying the physics of photoemission based electron sources.

physics.ins-det↗

arXiv2016

Ultrafast laser pulse heating of metallic photocathodes and its contribution to intrinsic emittance

The heating of the electronic distribution of a copper photocathode due to an intense drive laser pulse is calculated under the two-temperature model using fluences and pulse lengths typical in RF photoinjector operation. Using the finite temperature-extended relations for the photocathode intrinsic emittance and quantum efficiency, the time-dependent emittance growth due to the same photoemission laser pulse is calculated. This laser heating is seen to limit the intrinsic emittance achievable for photoinjectors using short laser pulses and low quantum efficiency metal photocathodes. A pump-probe photocathode experiment in a standard 1.6 cell S-band gun is proposed, in which simulations show the time dependent thermal emittance modulation within the bunch from laser heating can persist for meters downstream and, in principle, be measured using a slice emittance diagnostic.

physics.acc-ph↗