A Resonant-Cavity ICRF Coupler for Large Tokamaks

A new resonant cavity ICRF coupler is proposed for large tokamaks. The design features a novel resonant cavity, an RF magneticfield orientation that effectively radiates fast Alfvén waves, matching to 40-? transmission lines, and an electric-field orientation so that the strongest RF electric fields are orthogonal to the main toroidal magnetic field, thereby benefiting from magnetic insulation. As a result, the power handling capability is excellent. For the case of the "Big-Dee" Doublet III tokamak, a single 35 × 50-cm coupler can launch 20 mW of fast Alfvén waves. Extrapolation to fusion reactor parameters is straightforward.     

Extension of measurement range in Brillouin optical correlation domain analysis by pump-probe switching

We propose and experimentally demonstrate a novel method for the extension of the measurement range in Brillouin optical correlation domain analysis by switching the pump and probe waves for selectively inducing the stimulated Brillouin scattering (SBS) in the sensing fiber. In this scheme, two correlation peaks are located in the sensing fiber and SBS is alternatively induced at each position by pump/probe selective switching and optical attenuation. In the experiment, twice enlargement of measurement range is successfully achieved with 146-m fiber with a 10-cm spatial resolution.     

Bragg Effects in Microwave Transmission through Stationary Plasma Structures

We describe the first of a series of experiments designed to study, in a double-plasma (DP) device (3-cm) microwave Bragg scattering from stationary plasma structures, and from systems of plasma waves and "irregularities." This paper deals with Bragg effects in microwave transmission through stationary plasma density structures, consisting of a) a cylindrical plasma column, b) a single plasma slab, and c) a rotatable system of multiple equally spaced plasma slabs.     

Search for a stochastic background of 100-MHz gravitational waves with laser interferometers

This Letter reports the results of a search for a stochastic background of gravitational waves (GW) at 100 MHz by laser interferometry. We have developed a GW detector, which is a pair of 75-cm baseline synchronous recycling (resonant recycling) interferometers. Each interferometer has a strain sensitivity of approximately 10;{-16} Hz;{-1/2} at 100 MHz. By cross-correlating the outputs of the two interferometers within 1000 seconds, we found h{100};{2}Omega_{gw}<6 x 10;{25} to be an upper limit on the energy density spectrum of the GW background in a 2-kHz bandwidth around 100 MHz, where a flat spectrum is assumed.     

Phase correction in double-pass quasi-phase-matched second-harmonic generation with a wedged crystal

Compensation for dispersive elements is necessary for efficient multiple-pass and intracavity nonlinear frequency-conversion devices. We describe the use of a wedged quasi-phase-matched crystal to compensate for the phase shifts introduced by mirrors in such devices, taking advantage of the periodic variation in the relative phases of the interacting waves in a quasi-phase-matching grating. A representative double-pass second-harmonic generation experiment with a 5-cm -long periodically poled lithium niobate crystal showed the expected conversion efficiency enhancement.     

Study of main-lobe direction variations in backscattering of 3-cm radio waves by sea surface due to wind-direction variations

We study the correlation between minute variations of the wind direction near the sea surface and those of the main-lobe direction in the backscattering pattern of 3-cm radio waves. The correlation coefficient between wind-direction variations, which were 10°–20°, and main-lobe direction variations varied from 0.52 for nonfiltered signals to 0.83 after filtering frequencies above 1/60 Hz. The response of the scattering pattern was delayed by no more than 20 sec with respect to wind-direction variations. This result shows that short surface waves have a considerable effect on the anisotropy of radio-wave scattering by the sea surface. It also follows that, along with variations of the magnitude of the wind velocity, temporal variations of the wind direction determine the noise background in cases where not the wind itself but, for example, slicks, internal waves, streams, etc. are the object of observation. Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 6, pp. 535–542, June 1999.     

Reducing the effect of thermal noise in optical cavities

Thermal noise in optical cavities imposes a severe limitation in the stability of the most advanced frequency standards at a level of a few $10^{-16}\sqrt{\hbox{s}/\tau}$ for long averaging times τ. In this paper, we describe two schemes for reducing the effect of thermal noise in a reference cavity. In the first approach, we investigate the potential and limitations of operating the cavity close to instability, where the beam diameter on the mirrors becomes large. Our analysis shows that even a 10-cm short cavity can achieve a thermal-noise-limited fractional frequency instability in the low 10^?16 regime. In the second approach, we increase the length of the optical cavity. We show that a 39.5-cm long cavity has the potential for a fractional frequency instability even below 10^?16, while it seems feasible to achieve a reduced sensitivity of <10^?10/g for vibration-induced fractional length changes in all three directions.     

Gravitational wave production at the end of inflation

We consider gravitational wave production due to parametric resonance at the end of inflation, or "preheating." This leads to large inhomogeneities that source a stochastic background of gravitational waves at scales inside the comoving Hubble horizon at the end of inflation. We confirm that the present amplitude of these gravitational waves need not depend on the inflationary energy scale. We analyze an explicit model where the inflationary energy scale is approximately 10{9} GeV, yielding a signal close to the sensitivity of Advanced Laser Interferometer Gravitational Wave Observatory and Big Bang Observer. This signal highlights the possibility of a new observational "window" into inflationary physics and provides significant motivation for searches for stochastic backgrounds of gravitational waves in the Hz to GHz range, with an amplitude on the order of Omega_{gw}(k)h{2} approximately 10{-11}.     

Acoustic properties of proton exchanged LiNbO3 investigated by Brillouin scattering

The propagation of surface acoustic waves at microwave frequencies (10¹⁰ Hz) was studied on proton exchanged LiNbO₃ crystals by means of Brillouin scattering. The proton exchange causes a large velocity reduction for surface acoustic waves propagating in the x–y plane of ay-cut crystal as well as for longitudinal bulk acoustic waves travelling in the proton exchanged sub-surface region. The velocity reduction amounts to about 20% for both types of waves. The corresponding elastic constants are reduced even by about 40% since the density remains almost constant. This softening seems to involve both the shear and compressional elastic constants, but in an anisotropic way.Thus by proton exchange it is possible to build acoustic waveguides adjacent to the surface, similar to the construction of optical waveguides. By a lateral control of the proton exchange rate optical elements for ultrasonic waves, for example, acoustic lenses can be produced without deformation of the flat surface.The absorption of surface acoustic waves on proton exchanged surfaces is stronger than on pure LiNbO₃ indicating a novel absorption mechanism becoming active in the proton exchanged material.     

Optical bulk and surface waves with negative refraction

In materials with negative refraction, the direction of wave propagation is opposite to the direction of the wave vector. Using an approach that characterizes the optical response of a medium totally by a generalized dielectric permittivity, , we discuss the possibility of seeing negative refraction for optical waves in a number of nonmagnetic media. These include bulk waves in organic materials and in gyrotropic materials where additional exciton–polariton waves can have a negative group velocity. It is known that dispersion of surface waves can be engineered by tailoring a surface transition layer. We show how this effect can be used to obtain surface waves with negative refraction.     

Comparison of optical detection system ,PVDF detection system, and PVDF needle hydrophone for optoacoustic tomography

Optoacoustic tomography (PAT) is a two-dimensional medical imaging method that has the advantage of optical contrast and resolution of ultrasonic waves. The detection systems with a high sensitivity can be used for detecting small tumors, located deeply in human tissues, such as the breast. In this study, the sensitivity of existing ultrasonic detection systems has been compared experimentally with that by using thermoelastic waves as a broadband ultrasonic source. For the comparison, an optical stress transducer(OST), a polyvinylidene difluoride (PVDF) sheet and a calibrated PVDF needle hydrophone were used.To ensure all of the detection systems interrogated by the same ultrasonic field, a small optical instrument that fixed the generating laser head was constructed. The sensitivity was evaluated by measuring signalto-noise ratios (SNRs) and noise equivalent pressures (NEPs). The PVDF system, with a 4-kPa NEP has a 22 dB better performance than the OST. The OST showed nearly the same sensitivity as the hydrophone for detecting ultrasound waves at a 1-cm distance in water. PVDF detection system provides a useful tool for imaging of soft tissues because of its high sensitivity and broad detection range.     

Multiscale cosmological dynamics

The recently developed mean field theory of relativistic gravitation predicts the emergence of an “apparent matter” field at large scales describing the net effect of small-scale fluctuations on the large-scale dynamics of the universe. It is found that this so-called back reaction effect is much stronger for gravitational waves than for matter density fluctuations. At large scales, gravitational waves behave like radiation and, for them, the perturbative effect scales as the squared relative amplitude times squared frequency. In particular, a bath of gravitational waves of relative amplitude 10⁻⁵ and frequency would not be directly detectable by today’s technology but would generate an effective large-scale radiation of amplitude comparable to the unperturbed matter density of the universe.     

Bispectral Analysis of Gravity Capillary Waves

We perform a laboratory study of the nonlinear phase relation between short gravity waves with wavelengths of the order of 10 cm and their higher harmonics including spurious capillary ripple. The presence of higher harmonics of the basic low-frequency component in the spectrum of gravity capillary waves (GCWs) is confirmed on the basis of the methods of bispectral analysis. To determine the quantitative relations between the contributions of free and forced components in the GCW spectrum, we use the normalized bicoherence function. We show that it is correctly defined for both broadband random processes and polyharmonic random signals. The quantitative relation between the contributions from free and forced waves in the GCW spectrum at the frequency of the second harmonic is obtained.     

Results of radius scaling experiments and analysis of neon K-shellradiation data from an inductively driven Z-pinch

The K-shell radiated energy (yield) from neon Z-pinch implosions with annular, gas-puff nozzle radii of 1, 1.75, and 2.5 cm was measured for implosion times from 50 to 300 ns while systematically keeping the implosion kinetic energy nearly constant. The implosions were driven by the Hawk inductive-storage generator at the 0.65-MA level. Initial neutral-neon density distributions from the nozzles were determined with laser interferometry. Measured yields are compared with predictions from zero-dimensional (0-D) scaling models of ideal. One-dimensional (1-D) pinch behavior to both benchmark the scaling models, and to determine their utility for predicting K-shell yields for argon implosions of 200 to >300 ns driven by corresponding currents of 4 to 9 MA, such as envisioned for the DECADE QUAD. For all three nozzles, the 0-D models correctly predict the Z-pinch mass for maximum yield. For the 1and 1.75-cm radius nozzles, the scaling models accurately match the measured yields if the ratio of initial to final radius (compression ratio) is assumed to be 8:1. For the 2.5-cm radius nozzle, the measured yields are only one-third of the predictions. Analysis of K-shell spectral measurements suggest that as much as 70% (50%) of the imploded mass is radiating in the K-shell for the 1-cm (1.75-cm) radius nozzle. That fraction is only 10% for the 2.5-cm radius nozzle. The 0-D scaling models are useful for predicting 1-D-like K-shell radiation yields (better than a factor-of-two accuracy) when a nominal (≈10:1) compression ratio is assumed. However, the compression ratio assumed in the models is only an “effective” quantity, so that further interpretations based on the 0-D analysis require additional justification. The lower-than-predicted yield for the 2.5-cm radius nozzle is associated with larger radius and not with longer implosion time, and is probably a result of two-dimensional effects     

Limit on the detectability of the energy scale of inflation

We show that the polarization of the cosmic microwave background can be used to detect gravity waves from inflation if the energy scale of inflation is above 2x10(15) GeV. These gravity waves generate polarization patterns with a curl, whereas (to first order in perturbation theory) density perturbations do not. The limiting "noise" arises from the second-order generation of curl from density perturbations, or rather residuals from its subtraction. We calculate optimal sky coverage and detectability limits as a function of detector sensitivity and observing time.     

Broadband femtosecond transient infrared spectroscopy using a 256 x 256 element indium antimonide focal-plane detector

Application and characterization of large-format IR focal-plane arrays as detectors for ultrafast, high-resolution IR spectroscopy are discussed. We also present generation of broadband IR probe-reference pulses by use of collinear non-phase-matched geometry and shot-to-shot dual-track normalization to obtain transient spectra from broadly absorbing hydrogen-bonded systems. As much as 400-cm(-1)-wide coverage with 15-cm(-1) FWHM spectral resolution and +/-6.4 x 10(-4)(DOD = 3 x 10(-4)) baseline standard deviation (+/-1sigmas) is demonstrated near 2.9 microm.     

Recovery properties of vacuum spark gaps

The authors investigated the recovery properties of a 90-kV, 15-kA multiple site, triggered vacuum spark gap. Triggering was accomplished with a multisite surface flashover plasma source with approximately 60 sites distributed over a 10 cm² area. Gap dimensions were 1-cm spacing by 7.5-cm diameter. Recovery measurements were performed utilizing an inductively recharged double pulse circuit. Under a sinusoidal recharge, the time to a 40-kV recovery occurred in 330 μs for an average current fall rate approaching 10¹⁰ A/s. These vacuum spark gap switches may provide an alternative method of high-average-power switching     

暗怪波的相互作用

以耦合非线性薛定谔方程为理论模型,数值研究了两个一阶暗怪波在正常色散单模光纤中的相互作用.基于一阶暗怪波精确解,采用分步傅里叶数值模拟法,从间距、相位差和振幅系数比方面讨论相邻两个一阶暗怪波之间的相互作用.基于二阶暗怪波精确解,讨论了两个一阶暗怪波的非线性相互作用.研究结果表明:同相位情况下,间距参数T1为0、5、20时,相邻两个一阶暗怪波相互作用激发产生“扭结型”暗怪波.相比较于单个暗怪波发生能量的弥散,“扭结型”暗怪波分裂形成多个次暗怪波.反相位情况下,间距参数T1为2、7、12时,相邻两个一阶暗怪波相互作用也可以激发产生“扭结型”暗怪波.并且“扭结型”暗怪波初始激发的空间位置偏离原始单个暗怪波的位置5.振幅系数比越大,该空间位置越接近5.二阶暗怪波可以看作是两个一阶暗怪波的非线性叠加,复合型和三组分型二阶暗怪波与相邻两个一阶暗怪波的相互作用略有相似.     

The Green’s Function of the Navier-Stokes Equations for Gas Dynamics in ℝ<Superscript>3</Superscript>

In this paper we derive pointwise estimates for the Greens function of the Navier-Stokes equations for the compressible fluid. Our analysis shows that the short time behavior of the Greens function is dominated by the high frequency waves but the large time behavior is dictated by low frequency waves. Furthermore, the low frequency waves consist of entropy and acoustic waves that demonstrate a weaker form of Huygens Principle.     

A Laboratory Study of Strong Modulation of Radar Signals due to Long Waves on the Water Surface Covered with a Surfactant Film

Modulation of radar backscatter due to long wind waves in the presence of surfactant films is studied for the first time under wave-tank conditions. It is found that the modulation coefficient can increase significantly with increasing surfactant concentration. We show that the existing theoretical models of modulation of free surface waves underestimate the observed modulation levels. It is demonstrated that the experimental radar Doppler shifts cannot be explained by only the scattering due to free gravity-capillary waves (GCW) and are also determined by coupled waves, which are nonlinear harmonics of the longer wind wave components. Estimates of the relative intensity of coupled waves in the wind-wave spectrum at the Bragg wavelength are presented. It is shown that the contribution of coupled waves is considerable and increases with increasing surfactant concentration, thus confirming the hypothesis for a cascade mechanism of strong backscatter modulation in the long-wave field.