arXiv subjects

Adham Naji

Publications and source records attributed to Adham Naji.

7 recordsLinked to original sources

arXiv2025

Full-scatter vector field analysis of an overmoded and periodically-loaded cylindrical structure for the transportation of THz radiation

Highly overmoded and periodically loaded structures, such as the iris-line waveguide, offer an attractive solution for the efficient transportation of diffraction-prone THz pulses over long distances (hundreds of meters). This paper presents the full-scatter field theory that allows us to analytically derive all the spectral (modal) coefficients on the discontuities of the iris line. The spectral analysis uses vector fields, superseding scalar field descriptions, to account for diffraction loss as well as polarization effects and ohmic loss on practical conductive surfaces. An advanced application of Lorentz's reciprocity theory, using a generalized guided-field configuration, is developed to reduce complexity of the mode-matching problem over nonuniform sections. The used technique is quite general and applies to a wide class of structures, as it only assumes a paraxial incidence (i.e. a parabolic wave equation) along the axis of the structure. It removes the traditional assumption of very thin screens, allowing for the study of thicker screens in the high-frequency limit, while formulating the problem efficiently by scattering matrices whose coefficients are found analytically. The theory agrees with and expands previously established techniques, including Vainstein's asymptotic limit and the forward-scatter approximation. The used formulation also facilitates accurate visualization of the transient regime at the entrance of the structure and how it evolves to reach steady state.

physics.acc-ph↗

arXiv2025

Characteristic transition of the dominant power loss from diffractive to ohmic in overmoded and periodically loaded waveguides at high frequency

The analysis of electromagnetic fields in cylindrical waveguiding structures that contain periodic ring loading, whether for applications in charged-particle accelerators or radiation transportation, has been traditionally conducted under simplifying limits, where the structure is either single-moded at the lower-frequency limit or overmoded at high-frequency limit. These limits have often allowed us to find spectral (modal) expansions for the fields under simpler analytical and computational conditions, with ohmic effects typically being the dominant power loss mechanism in the lower limit, while diffraction effects dominate the loss in the higher limit. In this Letter, we report the observation of a transition point in the character of the main loss mechanism, where ohmic loss becomes dominant in a structure typically presumed to be dominated by diffraction loss. The results follow a formal analysis for the scattered vector fields in a highly overmoded THz waveguide. The findings bridge between the traditional theoretical descriptions for the two limits and reveal key tradeoffs that inform experiments for the transportation of THz radiation over long distances.

physics.app-ph↗

arXiv2024

Chisel-edged screens to reduce loss in highly overmoded THz iris lines with finite screen thickness

In this note we report observations on the growth trends of diffraction power loss and ohmic power loss, as functions of screen thickness, in highly overmoded THz iris-line waveguides that are constructed from thin screens. Recent theoretical developments have given a detailed field description, including eigen and transient analyses, that characterizes such waveguides under paraxial dipole-mode excitation. Informed by these analyses, we can better estimate -- and minimize -- power loss effects due to finite screen thickness in practical realizations. A geometric variation is proposed whereby we limit the growth trend in ohmic loss, even when using relatively thicker screens in practice.

physics.acc-ph↗

arXiv2022

Paraxial forward-scatter field analysis for a THz pulse traveling down a highly overmoded iris-line waveguide

Highly overmoded iris-line structures carry desirable features for the efficient transportation of THz radiation over long distances. Previous studies have analyzed the iris line, modeled approximately as a long open-resonator structure with thin screens, using methods such as Vainstein's impedance boundary condition or perturbative mode matching. The aim in those methods was to seek the eigensolution that represents the dominant (least lossy) propagation mode in a long iris-line structure at steady state. In this paper, a forward-traveling wave analysis is presented, wherein a short THz pulse paraxially traverses the oversized structure cell by cell, including the transient regime. The iris line's periodic discontinuities are analyzed in terms of forward-wave orthogonal mode decompositions, which are then used to build a 'forward-scatter' model (matrix) for each cell. The presented approach predicts a diffraction loss behavior that agrees with that of the eigensolution found by the previous analytical methods at the steady state for a long waveguide, but adds the ability of analyzing waveguides of finite lengths, showing transients as they evolve and settle to the dominant mode down the line, and offers numerical implementation speeds that are typically an order-of-magnitude faster than those previously reported for the mode-matching method. This approach may also be easily extended to analyze iris lines with mechanical imperfections (e.g. misalignments or aperiodic defects), allowing for investigations of performance sensitivity against manufacturing tolerances.

physics.acc-ph↗

arXiv2022

XCC: An X-ray FEL-based $γγ$ Collider Higgs Factory

This report describes the design of a $γγ$ Higgs factory in which 62.8 GeV electron beams collide with 1 keV X-ray free electron laser (XFEL) beams to produce colliding beams of 62.5 GeV photons. The Higgs boson production rate is 34,000 Higgs bosons per $10^7$ second year, roughly the same as the ILC Higgs rate. The electron accelerator is based on cold copper distributed coupling (C$^3$) accelerator technology. The 0.7 J pulse energy of the XFEL represents a 300-fold increase over the pulse energy of current soft x-ray FEL's. Design challenges are discussed, along with the R\&D to address them, including demonstrators.

hep-ex↗

arXiv2021

Field Analysis for a Highly-Overmoded Iris Line, with Application to THz Radiation Transport at LCLS

Vector field analysis is presented for a highly overmoded iris-line structure that can act as a medium of transportation for THz radiation. The axisymmetric structure is capable of supporting hybrid modes with desirable features such as low propagation loss, uniformly linear polarization and approximately-Gaussian intensity profile across the iris. A specific application that can benefit from these features is the transportation of THz undulator radiation over hundreds of meters to reach the experimental halls at the LCLS facility, SLAC, Stanford. Such a structure has been modelled before as a boundary-value problem using Vainstein's impedance boundary condition and assuming infinitely-thin screens. Given that physical realizations of such screens must have finite thickness and that the THz wavelength in the 3-15 THz range is expected to be smaller than convenient screen thicknesses in practice, the question of the impact of finite screen thickness on propagation performance becomes rather pressing. To address this question, we present a mode-matching analysis of the structure, with finite screen thickness and perturbatively clustered (localized) field expansions, for computational feasibility. The effect of screen thickness is seen to lower the attenuation constant on the iris line, which is dominated by diffraction loss. The propagation loss predictions based on the Vainstein-model are compared with the numerical results from mode-matching for infinitely-thin screens, where the former method is observed to agree with the numerical results better at higher Fresnel numbers (highly-overmoded structures). The properties of the dominant mode fields are formally derived from first principles and a recommended approach is discussed for the inclusion of screen-thickness effects into propagation loss estimations.

physics.acc-ph↗

arXiv2021

Variational Self-Consistent Theory for Beam-Loaded Cavities

A new variational theory is presented for beam loading in microwave cavities. The beam--field interaction is formulated as a dynamical interaction whose stationarity according to Hamilton's principle will naturally lead to steady-state solutions that indicate how a cavity's resonant frequency, $Q$ and optimal coupling coefficient will detune as a result of the beam loading. A driven cavity Lagrangian is derived from first principles, including the effects of cavity wall losses, input power and beam interaction. The general formulation is applied to a typical klystron input cavity to predict the appropriate detuning parameters required to maximize the gain (or modulation depth) in the average Lorentz factor boost, $\langle Δγ\rangle$. Numerical examples are presented, showing agreement with the general detuning trends previously observed in the literature. The developed formulation carries several advantages for beam-loaded cavity structures. It provides a self-consistent model for the dynamical (nonlinear) beam--field interaction, a procedure for maximizing gain under beam-loading conditions, and a useful set of parameters to guide cavity-shape optimization during the design of beam-loaded systems. Enhanced clarity of the physical picture underlying the problem seems to be gained using this approach, allowing straightforward inclusion or exclusion of different field configurations in the calculation and expressing the final results in terms of measurable quantities. Two field configurations are discussed for the klystron input cavity, using finite magnetic confinement or no confinement at all. Formulating the problem in a language that is directly accessible to the powerful techniques found in Hamiltonian dynamics and canonical transformations may potentially carry an additional advantage in terms of analytical computational gains, under suitable conditions.

physics.acc-ph↗