OSA grants to the Author(s) (or their employers, in the case of works made for hire) the following rights:(a) The right, after publication by OSA, to use all or part of the Work without revision or modification, including the OSAformatted version, in personal compilations or other publications consisting solely of the Author(s') own works, including the Author(s') personal web home page, and to make copies of all or part of the Work for the Author(s') use for lecture or classroom purposes; (b) The right to post and update his or her Work on any internet site (other than the Author(s') personal web home page) provided that the following conditions are met: (i) access to the server does not depend on payment for access, subscription or membership fees; and (ii) any such posting made or updated after acceptance of the Work for publication includes and prominently displays the correct bibliographic data and an OSA copyright notice (e.g. "© 2009 The Optical Society").. Abstract: We demonstrate long-distance (100-km) synchronization of the phase of a radio-frequency reference over an optical-fiber network without needing to actively stabilize the optical path length. Frequency mixing is used to achieve passive phase-conjugate cancellation of fiber-length fluctuations, ensuring that the phase difference between the reference and synchronized oscillators is independent of the link length. The fractional radio-frequency-transfer stability through a 100-km "real-world" urban optical-fiber network is 6 × 10 17 with an averaging time of 10 4 s. Our compensation technique is robust, providing long-term stability superior to that of a hydrogen maser. By combining our technique with the short-term stability provided by a remote, high-quality quartz oscillator, this system is potentially applicable to transcontinental optical-fiber time and frequency dissemination where the optical round-trip propagation time is significant. 723-727 (2010). 14. G. Marra, H. S. Margolis, S. N. Lea, and P. Gill, "High-stability microwave frequency transfer by propagation of an optical frequency comb over 50 km of optical fiber," Opt. Lett. 35(7), 1025-1027 (2010). 15. G. Marra, R. Slavík, H. S. Margolis, S. N. Lea, P. Petropoulos, D. J. Richardson, and P. Gill, "High-resolution microwave frequency transfer over an 86-km-long optical fiber network using a mode-locked laser," Opt. Lett. ©2013 Optical Society of America 36(4),

Very-long-baseline interferometry (VLBI) for high-resolution astronomical imaging requires phase-stable frequency references at widely separated radio-telescope antennas. For the first time to our knowledge, we have disseminated a suitable radio-frequency (RF) reference for VLBI over a "real-world" telecom optical-fiber link between radio telescopes that are >100 km apart, by means of an innovative phase-conjugation technique. Bidirectional optical amplification is used in parallel with live traffic, and phase perturbations in the effective optical-fiber path length are compensated. This RF-over-fiber approach obviates the need for separate hydrogen masers at each antenna, offering significant advantages for radio-astronomy facilities such as the Square Kilometer Array.

Study of Langmuir collapse thresholds shows that they have little polarization dependence and that moving packets have the lowest thresholds in the undamped case. However, incorporation of damping into the density response inhibits collapse of packets moving at more than a small fraction of the sound speed. Investigation of energy transfer to packets localized in density wells—the nucleation process—shows that at most a few trapped states can exist and that energy transfer is most effective when there is a single barely-trapped state. Coupled with an argument that closely packed wave packets have lower collapse thresholds, this argument yields an estimate of the number density of localized nucleating states in a turbulent plasma. It also leads to a simple and direct semiquantitative estimate of the collapse threshold. All these results are in accord with previous numerical simulations incorporating ion-sound damping, which show a preponderance of slow-moving or stationary packets with little or no intrinsic polarization dependence of thresholds. Likewise, the number densities obtained are in good agreement with simulation values, and the simple estimate of the threshold is semiquantitatively correct. The extent of the agreement supports the nucleation scenario with close-packed nucleation sites in the turbulent state.

Two-photon laser-induced fluorescence (LIF) is used to study the production and loss of H atoms in a pulsed microwave discharge in over the pressure range 1-50 Torr. Absolute measurements of the H atom density are made at the end of the pulse. These measurements were calibrated using a new technique based on the decay rate of the LIF signal. The temporal variation of emission during pulsing of the discharge is used to estimate the rate of dissociation of , which compares well with the predictions of a one-dimensional model for the electron energy distribution function. This measurement also gives the wall recombination probability for H atoms, which is compared with that obtained by LIF measurement of the decay of the H atom density in the pulse afterglow.

Abstract-Cryogenic