arXiv subjects

Levi K. C. Fisher

Publications and source records attributed to Levi K. C. Fisher.

2 recordsLinked to original sources

arXiv2026

Three-Dimensional Simulation of the University of Hawai`i FEL Oscillator with Cavity Desynchronization

In this paper, we present three-dimensional, time-dependent simulations of the University of Hawai`i (UH) at Mānoa free-electron laser (FEL) oscillator. Using beam parameters from the UH facility, we study the pass-by-pass evolution of the radiation field, including its temporal, spectral, and transverse properties. At nominal bunch length, the radiation pulse develops temporal spiking near saturation, together with sideband formation and increased sensitivity to machine timing jitter. Our results show that modest cavity desynchronization can enhance the radiation energy by 63%. Large cavity desynchronization, on the other hand, can effectively suppress the spiking instabilities and improve robustness to timing fluctuations. Finally, we simulate a short-bunch operational mode with a bunch length comparable to the slippage length, which accelerates saturation and further amplifies the FEL power. Overall, these results provide a quantitative foundation for pulse control studies in the UH FEL oscillator and a critical benchmark for future experimental validation and machine optimization.

physics.acc-ph↗

arXiv2025

Scaling Relations for Dark Matter Halos Hosting Ultra-Faint Dwarf Galaxies

We consider the extraction of parameters of dark matter halos hosting ultra-faint dwarf galaxies, in the case where there are only ${\cal O}(10)$ identified member stars with measured line-of-sight velocities. This scenario is likely to be increasingly common, as upcoming newly discovered dwarf galaxies in the Milky Way, by e.g. the Rubin Observatory, will likely (at least initially) have only a few identified members. Assuming an NFW dark matter profile, equilibrium modeling likely can only robustly extract one halo parameter ($ρ_s r_s$), but the scale radius itself will typically be unconstrained. In these cases, the results obtainable from Jeans modeling can be well replicated by a simple scaling relation motivated by the half-light mass estimator. As a application, we examine the recently discovered stellar system Ursa Major III, which has been optimistically assessed to have the largest $J$-factor of any known object. We suggest that, because of the presence of outlier stars, the $J$-factor obtained from modeling of Ursa Major III is likely inflated, as it is inconsistent with the half-light mass estimator, while removal of the outliers will leave the $J$-factor unconstrained from below.

astro-ph.CO↗