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Webb, Stephen

Publications and source records attributed to Webb, Stephen.

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Averaged invariants in storage rings with synchrotron motion

In an ideal accelerator, the single-particle dynamics can be decoupled into transverse motion—the betatron oscillations— and longitudinal motion—the synchrotron oscillations. Chromatic and dispersive effects introduce a coupling between these dynamics, the so-called synchro-betatron coupling. We present an analysis of the fully coupled dynamics over a single synchrotron oscillation that leads to an averaged invariant with synchro-betatron coupling in a generic lattice. Here, we apply this analysis to two problems: first, a toy lattice where the computations are analytically tractable, then a design for a rapid cycling synchrotron built using the integrable optics described by Danilov and Nagaitsev, showing that although there is fairly complex behavior over the course of a synchrotron oscillation, the Danilov-Nagaitsev invariants are nevertheless periodic with the synchrotron motion.

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

Theoretical and Computational Modeling of a Plasma Wakefield BBU Instability

Plasma wakefield accelerators achieve accelerating gradients on the order of the wave-breaking limit, m c² k_{p}/e, so that higher accelerating gradients correspond to shorter plasma wavelengths. Small-scale accelerating structures, such as plasma and dielectric wakefields, are susceptible to the beam break-up instability (BBU), which can be understood from the Panofsky-Wenzel theorem: if the fundamental accelerating mode scales as b⁻¹ for a structure radius b, then the dipole mode must scale as b⁻³, meaning that high accelerating gradients necessarily come with strong dipole wake fields. Because of this relationship, any plasma-accelerator-based future collider will require detailed study of the trade-offs between extracting the maximum energy from the driver and mitigating the beam break-up instability. Recent theoretical work* predicts the tradeoff between the witness bunch stability and the amount of energy that can be extracted from the drive bunch, a so-called efficiency-instability relation . We will discuss the beam break-up instability and the efficiency-instability relation and the theoretical assumptions made in reaching this conclusion. We will also present preliminary particle-in-cell simulations of a beam-driven plasma wakefield accelerator used to test the domain of validity for the assumptions made in this model.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Effects of Synchrotron Motion on Nonlinear Integrable Optics

An integrable Rapid-Cycling Synchrotron (iRCS) has been proposed as a replacement for the Fermilab Booster to achieve multi-MW beam power for the Fermilab high-energy neutrino program.* The successful application of nonlinear integrable optics to proton synchrotrons requires careful examination of single-particle longitudinal effects, especially synchrotron motion. For example, synchrobetatron coupling may excite transverse resonances in the ring. We will use the Synergia code to simulate the effects of this synchrobetatron coupling on the iRCS design with nonlinear inserts. Our goal will be to identify new invariants in the presence of this coupling. Assuming the synchrotron tune is sufficiently small, we have identified one or more adiabatic invariants of the motion. These invariants suggest that integrable optics with synchrobetatron coupling retains integrability when averaged over a synchrotron period.

43 PARTICLE ACCELERATORS↗
DOE OSTI2013

Suppressing Transverse Beam Halo with Nonlinear Magnetic Fields

Traditional space charge driven resonances, such as beam halo, arise due to the underlying linear nature of accelerator lattices. In this talk, we present initial results on a new class of intrinsically nonlinear lattices, which introduce a large tune spread naturally. The resulting nonlinear decoherence suppresses the onset of beam halo.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

EOS7C-ECBM Version 1.0

EOS7C is an equation of state module for the TOUGH2 program for CO2 or N2 in Methane (CH4) Reservoirs. In the present work, additions have been made to the EOS7C Version 1.0 module to include the Enhanced Coal Bed Methane (ECBM) modifications developed by Webb (2003). In addition, the Dusty Gas Model for gas-phase diffusion (Webb 2001) has been included. The ECBM modification to the EOS7C equation of state incorporate the extended Langmuir isothem for sorbing gases, including the change in porosity associated with the sorbed gas mass. Comparison to hand calculations for pure gas and binary mixture shows very good agreement. Application to a CO2 well injection problem by Law et al. (2002). The Dusty Model modification add options to calculate gas diffusion using the Dusty-Gas Model including separate and coupled approaches. Comparison to low-permeability pure gas diffusion data shows excellent agreement. The results from the DGM are compared to the Fick's Law behavior for diffusion across a capillary fringe. The differences between the models are small due to the relatively high permeability (10-11 m2) of the problem.

Oldenburg, CurtisM↗