Search Results (6 total)

We describe a tunable slow light device based on a photonic-crystal with a mechanically adjustable coupled-resonator optical waveguide structure. The lateral energy confinement is implemented along a lattice shear defect with the group velocity actively controlled by shifting the shear along the defect interface over a distance of one crystal period. The group velocity tuning range can be anywhere from arbitrarily small (determined by the waveguide structure) to near the value expected in bulk media. We present the theory and a demonstration (via simulation) of a device configuration that is realistic to fabricate and achieves a tunable range of group velocity spanning at least three orders of magnitude. The conditions for stopping the light are also discussed for different configurations.

Knowledge of the three-dimensional structure of a charged particle beam bunch is essential for understanding its evolution and for initializing computer simulations, especially when space charge is involved. This paper presents a novel experimental method for time-sliced mapping of the transverse phase space of a space-charge dominated beam based on tomographic principles. The combination of a high precision tomographic diagnostic with fast imaging screens and a gated camera are used to produce phase-space maps of two beams: one with a parabolic current profile and another with a short perturbation atop a rectangular pulse. The correlations between longitudinal and transverse phase spaces are apparent and their impact on the dynamics is discussed.

This is the sixth paper in the series developing the formalism to manage the effective scattering theory of strong interactions. Relying on the theoretical scheme suggested in our previous publications we concentrate here on the practical aspect and apply our technique to the elastic pion-nucleon scattering amplitude. We test numerically the πN spectrum sum rules that follow from the tree-level bootstrap constraints. We show how these constraints can be used to estimate the tensor and vector NNρ coupling constants. Last, we demonstrate that the tree-level low-energy expansion coefficients computed in the framework of our approach show a nice agreement with known experimental data. These results allow us to claim that the extended perturbation scheme is quite reasonable from the computational point of view.

Density modulation of charged particle beams may occur as a consequence of deliberate action, or may occur inadvertently because of imperfections in the particle source or acceleration method. In the case of intense beams, where space charge and external focusing govern the beam dynamics, density modulation may, under some circumstances, be converted to velocity modulation, with a corresponding conversion of potential energy to kinetic energy. Whether this will occur depends on the properties of the beam and the initial modulation. This paper describes the evolution of discrete and continuous density modulations on intense beams and discusses three recent experiments related to the dynamics of density-modulated electron beams.

This is the fifth paper in the series devoted to explicit formulation of the rules needed to manage an effective field theory of strong interactions in S-matrix sector. We discuss the principles of constructing the meaningful perturbation series and formulate two basic ones: uniformity and summability. Relying on these principles, one obtains the bootstrap conditions which restrict the allowed values of the physical (observable) parameters appearing in the extended perturbation scheme built for a given localizable effective theory. The renormalization prescriptions needed to fix the finite parts of counterterms in such a scheme can be divided into two subsets: minimal, needed to fix the S-matrix, and nonminimal, for eventual calculation of Green functions; in this paper we consider only the minimal one. In particular, it is shown that, in theories with the asymptotic behavior governed by known Regge intercepts, the system of independent renormalization conditions only contains those fixing the counterterm vertices with n≤3 lines, while other prescriptions are determined by self-consistency requirements. Moreover, the prescriptions for n≤3 cannot be taken arbitrarily: an infinite number of bootstrap conditions should be respected. The concept of localizability, introduced and explained in this article, is closely connected with the notion of resonance in the framework of perturbative quantum field theory. We discuss this point and, finally, compare the cornerstones of our approach with the philosophy known as “analytic S-matrix.”

Studies of the dynamics of longitudinal space-charge waves in space-charge dominated beams propagating through a transport channel with a long solenoid are performed at the University of Maryland. In this paper, we report some experimental results on the energy modulations converted from density modulations. By changing the working conditions of the electron gun, pure initial density modulations are generated. Energy perturbation waveforms are measured with a high-resolution energy analyzer. The experimental results are compared with both the linear theory and the simulation results. Good agreements are achieved for the relationship between the energy and current perturbation strengths.