Cw Source Research Articles

Matched-field inversion using the down hill simplex algorithm

In recent years inversion techniques have been proposed to effectively and efficiently determine geophysical parameters. The focus of this matched-field inversion (MFI) study is to determine the geoacoustic and geometric parameters for a North-East Pacific shallow water data set. MFI correlates modeled data, called replicas, with the measured data and uses a search algorithm to find model values which maximize the correlation function. In our MFI the parabolic equation (PE) is used to compute the replicas to account for mode coupling in the range-dependent environment. The first stage of the optimization algorithm employs a random search to determine N+1 parameter sets with the best correlations, where N is the number of parameters being determined. Next the N+1 sets are the input to the downhill simplex algorithm. The algorithm is shown to perform well for simulated vertical line array data for search parameter ranges representative of the environment in the shallow North-East Pacific Ocean. The inversion technique was then applied to measured data obtained in the North-East Pacific on the continental shelf using towed CW and broadband sources and a vertical array of hydrophones. The geometric parameters of the experiment and dominant geoacoustic parameters were successfully recovered.

Quartz antenna for radio frequency ion source operation

Radio-frequency (rf) driven multicusp ion sources developed at the Lawrence Berkeley National Laboratory use an internal induction coil (antenna) for plasma generation. The copper rf-antenna with a thin layer of porcelain coating, which is presently used, cannot fully satisfy the increasing demands on source cleanliness and antenna lifetime under high power cw or pulsed operation in applications where water cooling is not possible. A quartz antenna has been designed and operated in the multicusp ion source. It has been demonstrated that the overall performance of the new antenna exceeds that of the regular porcelain-coated antenna. It can be operated with a long lifetime in different discharge plasmas. The quartz antenna has also been tested at the Paul Scherrer Institute for cw source operation at rf power higher than 5 kW. Results demonstrated that the antenna can survive under dense plasma discharge operations.

Superradiant Smith-Purcell Emission

A diffraction grating mounted in the electron beam focal region of a scanning electron microscope has been used to produce superradiant emission over the 300--900 $\ensuremath{\mu}\mathrm{m}$ wavelength range. Feedback is provided by the grating itself and the electron beam is focused and positioned over the grating by the microscope's electron optical system. Extensions of this technique promise a new tunable, coherent, cw source for the difficult to access far infrared (30--1000 $\ensuremath{\mu}\mathrm{m}$) range of the spectrum.

Ultra-broad-bandwidth spectral continuum generation in fibre Raman amplifier

Picosecond seeding pulses derived from a CW source and electroabsorption modulator operating at a 10 GHz repetition rate have been used with gain saturation and nonlinear conversion in a high power fibre Raman amplifier to provide a broad-band, 100 nm, spectral continuum with an average power > 1W.

Next generation neutron source project in Japan Atomic Energy Research Institute (JAERI)

JAERI is carrying out a project to construct an intense-pulsed spallation source to promote a wide range of basic science and a CW source for nuclear power technologies such as nuclear transmutation of rare actinides from nuclear wastes. The most important basic science of the project is neutron scattering. The major facility is a 1.5 GeV superconducting H +/H − linac, with a peak current of 30 mA and a time-averaged beam current of 5.33 mA, which can provide 8 MW in total. For neutron-scattering experiments, 5 MW is available. A general view of the project and neutron-scattering facility are reported. Neutron-scattering experiments with a short-pulse spallation source (SPSS) are our final goal, but with a long-pulse spallation source (LPSS) might be optionally possible, if we have financial and/or technical difficulties.

Evaluation of different techniques in matched-field processing using measured data

The data from May 1996 Shallow Water evaluation cell Experiment (SWellEx-96) are used to evaluate the performance of commonly used processors in matched-field processing (MFP). The objective of SWellEx-96, which was conducted west of Point Loma in shallow water, was to investigate source tracking using matched-field processing and low-level signal detection. Data were recorded on a 64-element 118-m vertical line array and on an identical 45-deg tilted line array anchored in 200 m of water. A cw source emitting tonals from 49–388 Hz was towed over tracks ranging up to 6 km from the array. Signals were transmitted in group of frequencies. The source level in each frequency group decreased by 3 dB every 3 Hz, thus making it possible to study MFP as a function of the signal-to-noise ratio (SNR). Various figures of merits have been used to evaluate the performance of the Bartlett, the minimum variance distortionless response (MVDR) as well as MVDR with white-noise gain-constraint (MVDRWNC) processors.

Range-dependent multitone matched-field processing in SWellEX-3

SWellEX-3 was an experimental study of shallow-water propagation and matched-field processing conducted 10 km off San Diego in July, 1994. Data were recorded on a 64-element, 118-m aperture vertical line array anchored in 200 m of water. A cw source emitting tonals from 53–197 Hz was towed over tracks where the water depth varied from 200–75 m. Examples of range-dependent matched-field processing of this multitone signal using the finite-element parabolic equation program for propagation modeling will be presented.

Experimental measurement of the array bearing error caused by bathymetric refraction

Experimental measurements have been carried out to identify the effects of a realistic bottom bathymetry on detecting a source with a horizontal line array. The measurements have been conducted over a 1:10000 scale model of the Santa Lucia Escarpment. The experiments measured the bearing error obtained in locating a CW source with a horizontal line array using plane-wave beamforming. The error in the detected bearing is caused by the bending of sound rays in the horizontal plane, commonly referred to as horizontal or bathymetric refraction. The results of the experiment demonstrate large bearing errors which vary rapidly and do not increase monotonically with range. The rapid variation of the bearing errors was unexpected and was a result which has not been previously identified. The magnitude of the errors was also a function of the across slope look direction and frequency, with bearing errors as large as 26/spl deg/ for low frequencies. At higher frequencies, the bearing error is reduced. An analytical solution for the acoustic field over a shear supporting sloping bottom has been used to approximate the results of the experiment. The simulation confirmed the trends in the experimental results by showing bearing errors of the same order of magnitude with the same dependence on across slope direction and frequency. Most importantly, the theoretical results also showed a rapidly varying bearing error as a function of across slope range.

Ion charge-state distribution in a high-power pulsed electron cyclotron resonance discharge sustained by millimeter-wavelength radiation

Measurements have been made of the ion charge-state distribution in a high-power, pulsed, electron-cyclotron discharge in argon sustained by millimeter radiation. It has been observed that the maximum of the distribution is shifted toward higher charge states compared with the distributions in conventional cw sources of multiply charged ions pumped by centimeter radiation. This shift of the distribution maximum evidently occurs because a so-called quasigasdynamic plasma confinement regime can be established in a magnetic trap when higher-power, shorter-wavelength gyrotron radiation is used, and in this regime, an increase in plasma density is accompanied by an increase in the confinement parameter Nτ, which determines the ion charge distribution.

Theoretical investigation of the array bearing error caused by bathymetric refraction

Experimental measurements over a laboratory scale model with a real three-dimensional bathymetry have identified the effects of bathymetric refraction on detecting a cw (continuous wave) source with a horizontal line array. The results of these experiments showed large bearing errors which vary rapidly and do not increase monotonically with range. The rapid variation of the bearing errors was unexpected, and a result which has not been previously identified. In order to verify the oscillation in bearing error seen in the experimental results, a simulation of bearing detection using a horizontal line array in a wedge shaped environment was evaluated. This approach uses the method of images to approximate the three-dimensional acoustic field. This theory uses a series of image sources which are centered around the apex of the wedge. The rapid variation in bearing error is shown to be caused by the interference of these image sources which represent eigenrays with different propagation paths. Numerical simulations also investigate the bearing ambiguity caused by bathymetric refraction at full ocean scale. These results identify the magnitude of the bearing error as a function of the acoustic similarity parameter kh (wave number multiplied by water depth). [Work supported by ONR.]

Coherent localization of an unknown cw source

Let D j =D j (f) be the data spectrum from thej th element of a hydrophone array of any shape, all of whose phones have the same unknown transfer function with unknown gains. ThenD j =B dG j ( m ¯ d ), where B d is the product of the unknown source spectrum and the unknown hydrophone response, G j is the Green’s function for thej th hydrophone location, and m ¯ d is the unknown true value of the vector m ¯ ̄ containing source location and environmental parameters. Similarly, let S j =B sG j ( m ¯ s ) be a candidate synthetic spectrum for comparison with D j ; here B s is a guess at B d and m ¯ s is a candidate parameter vector. Conventional coherent localization searches for an m ¯ s that maximizes the agreement of D j and S j , an agreement clearly dependent on the guess B s . To remove the problem of the unknown source define F ij =D iG j ( m ¯ s ) and search for the m ¯ s that maximizes agreement of F ij with F ji . To see why this works note that F ij =B dG i ( m ¯ d )G j ( m ¯ s ), whereasF ji =B dG j ( m ¯ d )G i ( m ¯ s ). Thus F ij and F ji have the same source function B d , and F ij equals F ji only if m̄ s =m̄ d . Note that knowledge of the source is not required; in particular it need not be compact in time, hence cw data can be used to localize with ≥ two hydrophones. The localizer for hydrophones with equal gains is φ1=|∑ i≠j ξ i ξ j ∑ f ζ fF ij F ji *|2/(∑ i≠j ξ i ξ j ∑ f ζ f F ij F ij *)2, in which optional real ξ i give array shading, and optional real ζ f give spectral shading. A localizer allowing hydrophones to have different unknown gains is φ2=|∑ i≠j ξ i ξ j ∑ f ζ f F ̂ ij F ̂ F̂ ji *|2/(∑ i≠j ξ i ξ j )2, where F̂ ij =F ij / ∑ f ζ f F i j F i j * .

A shallow-water experiment for estimating bottom geoacoustic properties at a site off Numazu

An experiment to estimate bottom geoacoustic properties was conducted in the coastal area of Gulf Suruga off Numazu in August 1995. The measurements were made at a 70-m-depth site using a vertical line array of 12 receivers spaced at 5-m intervals. A cw source output at 55 Hz with 167 dB re: 1 μPa source level was towed at a constant depth over 3 km in horizontal range along the coast. Acoustic fields measured by a synthetic array at each channel were processed with a Hankel transform, whose output wave-number spectrum provided several peaks corresponding to mode eigenvalues. The position of each peak, however, varies slightly depending on the channel; this variation is considered to be caused by the range dependence of the waveguide and/or by mutual interaction of adjacent mode spectrum due to the finite aperture. Applying an inversion method to the observed mode eigenvalues, bottom geoacoustic parameters (sound speed, attenuation, and density) were estimated under several assumptions. Propagation loss calculated with use of the estimated parameters was found to be in good agreement with the observed one.

Characterize reverberation spatial distribution using an L-shape array

Matched-field processing (MFP) is a generalized beamforming. Instead of using plane-wave replica vectors in the plane-wave beamforming, in MFP, an acoustic model is used to calculate replica vectors in a search space. In addition to azimuthal and elevation information, which can be obtained by plane-wave beamforming, MFP also provides range and depth information. In active systems, scattered signals are usually modeled by two-way, source-to-scatterer and scatterer-to-receiver, propagation. Assuming that scatterers are illuminated and reradiate energy as point sources, the scattering field can be characterized by using just one-way, scatterer-to-receiver propagation. Passive MFP can then be used to map the spatial distribution of the scatterers. A shallow-water experiment took place off the Gulf of Mexico in November 1995; reverberation was measured on an L-shaped array from a bottomed cw source. Passive MFP was used to characterize the spatial distribution of the measured reverberation. [Work supported by U.S. Navy.]

Some general reflexions on "long pulse" neutron sources

A long pulse spallation neutron source (LPSS) having about 20 times more time average thermal flux than its short pulse counterpart (SPSS) at the same proton beam power and featuring a pronounced time structure not available on cw sources (CWNS) of e

Double security level in a telecommunication system based on phase coding and false data transmission

A new communication system exhibiting a double security level is demonstrated. Information coding is based on relative phase changes between consecutive time intervals of the light emitted from a highly coherent CW source. An imbalanced Mach-Zehnder interferometer whose time delay matches the intervals duration is used to decode the data. The first security level is related to the constant optical power in the transmission link. To further enhance the security, a low coherent source is used to send false data down the transmission link. Common mode rejection is used to separate the real signal from the false signal. This system provides a high signal-to-noise ratio as well as a low crosstalk in a wavelength division multiplexing network.

Photodynamic therapy with pulsed light sources: a theoretical analysis

The effectiveness of photodynamic therapy using pulsed sources was evaluated using a mathematical model describing the time-dependent excitation and de-excitation of the photosensitizer molecule. Using the various numerical data available in the literature on haematoporphyrin we calculated that the effectiveness of pulsed excitation in PDT is identical to that of CW excitation for peak fluence rates below . Above this threshold the effectiveness drops significantly. In practice this effect will occur with sources with high pulse energy and low repetition frequency. The commonly used dye lasers pumped by either a Cu vapour laser or a frequency doubled Nd:YAG laser have a PDT effectiveness identical to that of a CW source of the same wavelength and the same average fluence rate.

Absolute frequency stabilization of an injection-seeded optical parametric oscillator

A method is described that provides absolute frequency stabilization and calibration of the signal and idler waves generated by an injection-seeded optical parametric oscillator (OPO). The method makes use of a He-Ne stabilized transfer cavity (TC) to control the frequencies of the cw sources used to seed both the pump laser and OPO cavity. The TC serves as a stable calibration source for the signal and idler waves by providing marker fringes as the seed laser is scanned. Additionally, an acoustic-optic modulator (AOM) is used to shift the OPO seed laser's frequency before locking it onto the TC. The sidebands of the AOM are tunable over more than one free spectral range of the TC, thereby permitting stabilization of the signal and idler waves at any frequency. A ±25-MHz residual error in the absolute frequency stabilities of the pump, signal, and idler waves is experimentally demonstrated, which is roughly 30% of the 160-MHz near-transform-limited linewidths of the signal and idler pulses.

Study of H− beams for ion-projection lithography

The desired properties of ion sources used in ion-projection lithography are high brightness and low energy spread. Due to a very stringent demand in many other applications, the state of the art of negative hydrogen ion (H−) beams has advanced significantly in recent years. H− ion beams with the transverse ion temperature of about 0.3 eV have been obtained from surface plasma sources in pulsed mode operation [V. G. Dudnikov and G. E. Derevyankin, AIP Conf. Proc. 287, 239 (1994)]; a design for a cw source to achieve a comparable, or even lower energy spread and high brightness has been reported [S. K. Guharay, M. Reiser, and V. G. Dudnikov, Rev. Sci. Instrum. 65, 1745 (1994)]. The energy spread of H+ ion beams now in use is in the range of about 6–10 eV. This renders the H− ion beams an attractive alternative to the H+ ion beams for ion-projection lithography. No data of H− ion exposure of resists are available, and we have carried out experiments illuminating PMMA resists with various doses of H− ions and H+ ions at 24 keV. Within experimental accuracy the exposure characteristics (e.g., sensitivity and contrast) are identical for both H− and H+ ions.

Modal sensitivity of acoustic interactions with internal waves in shallow water

Analyses of individual acoustic mode interactions with internal waves should permit both qualitative and quantitative understanding of the influences of internal waves on multimode signals in shallow water. In this report, some experimental results taken in the Yellow Sea during the summer with strong thermoclines, will be reported. Numerical simulations will be used to analyze the potential internal wave interactions suggested by these data. (a) A multimode signal obtained at a single hydrophone and the first acoustic mode obtained by a mode filter array, both from a fixed 442-Hz cw source, exhibit very different fluctuation characteristics. (b) The transmission loss (TL) measured with explosive sources indicates a strong source/receiver depth dependence. (c) TL is, generally, anisotropic and can be abnormally large over some frequency range. It has been shown that the acoustic mode-coupling induced by internal waves can be an important loss mechanism for sound propagation during the summer [Zhou etal., J. Acoust. Soc. Am. 90, 2042–2054 (1991)]. The observed fluctuations can be explained in terms of the propagation characteristics of individual modes to the presence of internal waves. In so far as the mode-coupling and both the intensity and arrival time fluctuations are concerned, certain acoustic modes are more sensitive to internal waves than others. The characteristics of acoustic-internal wave interaction strongly depend on signal frequency, thermocline profile, seabottom properties, source/receiver depths and distance. [Work supported by ONR.]

Shallow-water acoustic measurements in the southern Adriatic Sea

Shallow-water acoustic and geoacoustic measurements were made in the southern Adriatic Sea in May–June 1995 in combination with detailed oceanographic, geological, and geophysical surveys. Seismic subbotom profiling, narrow-beam echosounding, and bottom coring results were used to select two shallow water sites with downward refracting conditions for acoustic experiments: (1) a flat seafloor at 90-m depth with about 1 m of rigid, nongaseous clay over a sequence of laminated clays and (2) a seafloor sloping from 49- to 37-m depth and consisting of soft gaseous mud over laminated clay. Broadband sparker and 180-g TNT explosive sources were used for propagation and reverberation to a vertical array receiver in addition to cw sources for propagation. Initial results for the deeper water site show frequency-independent propagation that is consistent with modeled results using geoacoustic parameters extrapolated from coring results. A modification of the Dix method to estimate sediment sound speed versus depth from wide-angle reflectivity experiments made at the deeper water site is described. [Work supported by ONR.]