Search Results (15 total)

In a recent Comment, Decca [Phys. Rev. A 79, 026101 (2009)] discussed the origin of the anomalies recently reported by us in Phys. Rev. A 78, 036102(R) (2008). Here we restate our view corroborated by their considerations that quantitative geometrical and electrostatic characterizations of the conducting surfaces (a topic not discussed explicitly in the literature until very recently) are critical for the assessment of precision and accuracy of the demonstration of the Casimir force and for deriving meaningful limits on the existence of Yukawian components possibly superimposed to the Newtonian gravitational interaction.

We have performed precision electrostatic calibrations in the sphere-plane geometry and observed anomalous behavior. Namely, the scaling exponent of the electrostatic signal with distance was found to be smaller than expected on the basis of the pure Coulombian contribution and the residual potential found to be distance dependent. We argue that these findings affect the accuracy of the electrostatic calibrations and invite reanalysis of previous determinations of the Casimir force.

We propose an experiment for generating and detecting vacuum-induced dissipative motion. A high frequency mechanical resonator driven in resonance is expected to dissipate mechanical energy in quantum vacuum via photon emission. The photons are stored in a high quality electromagnetic cavity and detected through their interaction with ultracold alkali-metal atoms prepared in an inverted population of hyperfine states. Superradiant amplification of the generated photons results in a detectable radio-frequency signal temporally distinguishable from the expected background.

We report the use of coherent Smith-Purcell radiation to measure the bunch length of femtosecond-scale, 15 MeV electron bunches produced by a 17 GHz rf accelerator. The Smith-Purcell radiation was produced by passing a train of electron bunches above a metal grating. The radiation was verified as Smith-Purcell radiation by measuring the resonance condition, dependence on beam current, and dependence on beam height above the grating. Measurements of the intensity of the radiation vs emission angle were analyzed to obtain the bunch length. The accelerator was operated in two different modes, producing bunches that were determined to have bunch lengths of 600 and 1000±200  fs. These nondestructive bunch length measurements were found to agree well with an independent, but destructive, measurement using a microwave deflecting cavity.

The passage of a finely focused electron beam near the surface of a periodic metallic grating produces radiation known as Smith-Purcell radiation. This paper presents an analysis of the role of the grating profile in the case of echelle-type gratings whose period consists of two facets only. Particular emphasis is placed on the ultrarelativistic regime and a comparison is made with recent experimental results in this region. It is shown that the details of the profile of the grating play an important role in the optimization of the radiated energy. The behavior of higher order modes and the limitations of the surface current description of the radiative process are also discussed briefly.

Čerenkov free-electron lasers have primarily operated on the fundamental guided mode of the dielectric waveguide. Higher-order generation would allow short wavelength emission in a relatively large scale resonator. In comparison to the fundamental mode, we find that gain on higher-order modes can be significant in a planar geometry. This analysis is presented with a discussion of practical limits.

Coherent enhancement of the Smith-Purcell radiation produced from the interaction of a 1.8 MeV electron beam with a grating has been observed. The emitted radiation has been measured at angles in the 40° to 120° range, which correspond to wavelengths from 0.65 to 4 mm, approximately. The radiated power was 320 mW at 90°. Its angular distribution agrees well with the description of the process in terms of induced surface currents and has been used to infer the longitudinal profile of the electron bunch. It is concluded that the bunch has an approximately triangular profile, with 85% of the bunch particles contained within 14 ps. The possibilities of the technique as a bunch-shape diagnostic tool are also discussed.

We present in this paper compelling evidence supporting the three-wave traveling-wave theory developed by Pierce fifty years ago. The transition in a Smith-Purcell free-electron laser from low, through moderate amplified spontaneous emission, to strong gain conditions was carefully controlled. Below threshold, the emitted far-infrared power exhibits oscillations with a cubic dependence on the electron beam current. Both characteristics are expected in a three-wave interaction yet, to date, have not been observed.

A theoretical analysis of Smith-Purcell radiation at very high energies is presented. The energy per unit frequency and solid angle is expressed in closed form as a function of the grating geometry, beam energy, and viewing angles. A certain choice of grating geometry is shown to optimize the output energy for a particular order of radiation. Scaling laws are derived for the energy emitted into all orders of radiation in the relativistic limit. It is shown that the total energy emitted into each order scales as the three-halves power of the beam voltage.

An analytic solution for the radiated intensity distribution produced by an electron beam passing over a metallic diffraction grating (the Smith-Purcell effect) is derived. The approach is based upon an expression for the current traveling over the grating surface and the method can deal with arbitrary grating profiles. Although collective behavior in the electron beam is neglected, very high power density is predicted if high energy, short electron bunches are employed. The electron beam characteristics of various accelerators are used to illustrate the potential of high energy, accelerator based Smith-Purcell radiation sources.

Second harmonic generation in Cs vapor is produced by the combined action of a polarized laser pulse and a magnetic field. It is delayed from the 10 ps excitation pulse by several hundred ps. The laser is two-photon resonant with the 6S_{1 / 2}-6D_{3 / 2} transition and initially induces a macroscopic quadrupole moment whose radiation pattern has a null in the forward direction. The magnetic field reshapes this pattern to produce second harmonic light, which is simply a free polarization decay signal evolving in a manner akin to the free induction decay following an adiabatic demagnetization.

A laser pulse two-photon resonant with the 6S_1/2-6D_3/2 442 nm transition in Cs vapor generates delayed superfluorescence (SF) on the upper 6D_3/2-6P_1/2 876 nm and lower 6P_1/2-6S_1/2 894 nm transitions. Yoked SF is observed in the forward (along the laser) direction as evidenced by the simultaneous appearance of SF on both transitions. Cascade SF is observed in the backward direction, where SF on the lower transition only evolves after it has terminated on the upper. The four SF signals exhibit quantum beats associated with the 6P_1/2 hyperfine splitting.

The classical electron-ion collision rate, ν_ei, has been deduced in the absence of Ohmic heating from the inverse bremsstrahlung absorption rate measured on a laboratory plasma subjected to an intense microwave electric field E whose frequency ω is near the electron plasma frequency ω_p. The observations, carried out in the transition range, where eE / (mω)≈(kT_e / m)^{1 / 2}, yield a simple empirical relation that describes the decrease of ν_ei with increasing E.