Backscattering experiments at microwave frequencies were conducted off the west coast of Scotland in the summer of 1991. Using a dual-polarization, eightfrequency, X band, coherent scatterometer mounted on the bow of a boat, we measured time-resolved backscattering from ocean waves at a range of grazing angles from 10 ø to 70 ø. From the grazing-angle-dependent signals and their Doppler spectra, we are able to differentiate Bragg scattering from non-Bragg scattering and resolve "peak separation" between the vertical and horizontal polarizations. We observe instances of "super" events, i.e., instances when the horizontal polarization return power equals or exceeds the vertical polarization power at particular frequencies. We find that "super" events occur not only at low grazing angles but at any grazing angle for upwind viewing directions and obtain statistics for such occurrences as a function of grazing angle. We study the coherence properties of scatterers and find strong evidence that at low grazing angles, lifetime-dominated, non-Bragg scattering contributes noticeably to returns of both polarizations, but is dominant in providing returns for the horizontal polarization. We examine "spiking" events and find that they can be related to, but need not be limited to, breaking wave events. By comparing the data of upwind runs with cross-wind and circle runs, we obtain wind direction dependence of Doppler spectra, which further assists in the identification of scattering mechanisms. 2591 2592 LEE ET AL.' X BAND MICROWAVE BACKSCATTERING FROM OCEAN WAVESTable 1. Frequency Pairs for Microwave Scatterometer Polarization, GHz Pair Vertical Horizontal 1 9.020 9.021 2 9.170 9.171 3 9.320 9.321 4 9.470 9.471 four quadrature mixers, one for each transmitted frequency. Each mixer generates an in-phase (I) and quadrature (Q) signal with a frequency response of 0 to 1000 Hz. With complex amplitude thus generated for each frequency, there are 16 channels of output. During an experiment the 16 signals are recorded digitally on a multichannel cassette recorder.An absolute calibration of the scatterometer system [Barter et al., 1993] was conducted in a large (10 m x 10 m x 30 m) anechoic chamber using spheres and cylinders of various sizes as well as corner reflector targets. Measurements consisted of establishing the output power and receiver gain of each of the eight frequency channels, the system radiation patterns in two perpendicular planes in the forward half sphere, the range dependence of the signal power, and the cross-polarization isolation of the scatterometer system. The essential results of the calibration are as follows: total power (eight channels), -1 W; nominal receiver gain each channel, -60 dB; system radiation pattern, approximately Gaussian main lobe; azimuthal plane -3 dB beam width, 8.7 ø for VV and 10.3 ø for HH; vertical plane -3 dB beam width, 11 ø for VV and 9.6 ø for HH; average -3 dB beam width, 9.5ø; antenna gain, 26.6 dB; cross-polarization isolation, >35 dB; image rejection ratio, --45 dB. As an ex...
Results of X-band microwave backscatter from sea surfaces at a small-grazing angle for a range of wind speeds (3.8-10.3 d s ) are presented. The data indicate that the speed of faster-than-Bragg scatterers and their time-averaged polarization ratio both increase with wind speed. At a friction velocity above -21 c d s , super events (i.e., backscattering events where the horizontal-polarization power exceeds the vertical-polarization power) begin to appear. The fraction of fast scatterers which produce super events also increases with wind speed. The presence of super events indicates that in addition to scattering mechanisms, such as Bragg-resonant scattering, composite surface theory, wedge scattering, and specular reflection, other mechanisms which favor the horizontal-polarization returns must be considered.
In thermal-barrier experiments in the tandem mirror experiment upgrade, axial confinement times of 50 to 100 ms have been achieved. During enhanced confinement we measured the thermal-barrier potential profile using a neutral-particle-beam probe. The experimental data agree qualitatively and quantitatively with the theory of thermal-barrier formation in a tandem mirror.
Ion cyclotron heating (ICH) is applied to TMX-U to improve the thermal barrier performance by reducing the passing ion collisionality. During its development, measurements of the antenna loading resistance, R , and the absorption efficiency, rj, were compared with calculations with the antenna design code ANTENA over a wide range of densities and frequencies. Good agreement in R was obtained in the short wavelength slow wave regime but not for long wavelength fast waves because the experimental magnetic field gradients are not modelled in ANTENA. Similarly, r) is much larger experimentally (40%) than in ANTENA (10%) due to the magnetic beach in TMX-U. In its application, ICH successfully decreased the passing ion collisionality tenfold but did not extend thermal barrier plugging to higher density, indicating that collisional barrier filling is not currently limiting TMX-U performance.
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