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991.
Finite pulse durations in diverse pulse schemes lead to the reduction of the magnitude of the magnetization vector due to T1 and T2 effects during the radio-frequency pulses. This paper presents an analysis of the steady state signal in the presence of relaxation effects during radio-frequency pulses in MRI spoiled gradient echo sequences. It is shown that minor attenuations of the magnetization vector can have dramatic consequences on the measured signal, and may thus entail a loss in SNR benefits at high static magnetic fields if a careful analysis is not performed. It is emphasized that it is the time-integrated magnetization vector trajectory that matters for these effects and not only the pulse duration. Some experimental results obtained on a phantom at 3 T verify this analysis. 相似文献
992.
H. Nowakowska M. Jasiński J. Mizeraczyk 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2009,54(2):511-518
We present results from simulations of 2D distributions of the electromagnetic field inside a waveguide-based axial-type microwave
plasma source (MPS) used for hydrogen production via methane reforming. The studies are aimed at optimization of discharge
processes and hydrogen production. We derive equations for determining electromagnetic field distributions and next determine
the electromagnetic field distributions for two cases – without and with plasma inside the MPS. For the first case, we examine
the influence of the length of the inner conductor of the coaxial line on electromagnetic field distributions. We have obtained
standing wave patterns along the coaxial line and found resonances for certain positions of the coaxial line inner conductor.
For the case with plasma inside the MPS, we perform calculations assuming that distributions of plasma parameters are known.
Simulations are done for several values of maximum electron density. We have found that for values of electron density greater
than strong skin effect in the plasma is observed. Consequently, plasma may be treated as an extension of the inner conductor
of the coaxial line. We have used FlexPDE software for the calculations. 相似文献
993.
994.
RF system design and measurement of HIRF-CSRe 总被引:1,自引:0,他引:1
An RF system for the CSRe (cooling storage experimental ring) is designed and manufactured domestically. The present paper mainly describes the RF system design in five main sections: ferrite ring, RF cavity, RF generator, low level system and cavity cooling. The cavity is based on a type of coaxial resonator which is shorted at the end with one gap and loaded with domestic ferrite rings. The RF generator is designed in the push-pull mode and the low level control system is based on a DSP+FGPA+DDS+USB interface and has three feedback loops. Finally we give the results of the measurement on our system. 相似文献
995.
Thermal dependence on the small signal gain of Nd/Cr:YAG (yttrium aluminum garnet) ceramics was observed experimentally. Usually, Nd:YAG crystal and ceramics have remarkable gain reduction when optical pumping is performed and the temperature of the laser media is upped to 373 K. A CW laser light generated in a 1064 nm Nd:YAG laser oscillator was amplified by Nd/Cr:YAG ceramic amplifier, and the output power was measured at non-saturation level. Laser small signal gain of the ceramic disk was kept to 470 K. This property was remarkably different from one of Nd:YAG crystals or ceramics. The peak shift of the fluorescence was observed experimentally when the temperature is high. 相似文献
996.
Modal properties of all-active InGaAsP/InP microring lasers 总被引:1,自引:0,他引:1
I. Stamataki A. Kapsalis S. Mikroulis D. Syvridis M. Hamacher U. Troppenz H. Heidrich 《Optics Communications》2009,282(12):2388-2393
An experimental investigation of the multimode dynamics of 1.55 μm-InGaAsP/InP microring lasers is demonstrated. Different operation regimes are observed, bidirectional multimode, unidirectional single mode, bidirectional single mode and mode-hopping. The extent of each of the above operation regimes is examined with respect to the ring current level, bus waveguide reflectivity and ring radius. The back-reflections generated from the bus waveguide facets are indirectly controlled via the bus waveguide current. In order to clarify the physical mechanisms beyond experimental results, a multimode model based on the rate equation approximation is utilized. The experimental results are in good agreement to those predicted by the multimode model. 相似文献
997.
T. Saiki S. Motokoshi K. Fujioka H. Fujita C. Yamanaka 《Optics Communications》2009,282(13):2556-2984
Laser pulses with high repetitive rate generated from a Q-switch microchip Nd:YAG oscillator were amplified by Active mirror composed of Nd/Cr:YAG ceramic pumped by CW arc-lamp. The laser pulses were amplified, and saturation of averaged output laser power was observed. Repetitive ratio of injected laser pulses was changed from 20 to 100 kHz. Averaged output laser power and gain were measured, and gain coefficient and power-extraction efficiency were evaluated. Output laser power was calculated and compared to experimentally measured one, and the calculated results are consistent with the experimental ones. For amplification of laser pulses with high repetitive ratio, this laser material can realize very high laser gain with low pumping power density and high optical-optical conversion efficiency under CW-lamp-light pumping. 相似文献
998.
C.H. Kim H.B. Moon S.S. Min Y.H. Jang J.H. Cho 《Solid State Communications》2009,149(39-40):1611-1615
We have studied the nanoscale electrical properties of NiO thin films by using conducting atomic force microscopy (CAFM) to understand the mechanism of resistance change of the NiO thin films as we changed the applied voltage. We observed that inhomogeneous conducting filaments were generated by external voltage bias; in addition, some of the inhomogeneous conducting filaments were durable while some of them were not, and they disappeared. We deduced that the resistance change of the NiO thin films was related to inhomogeneous filamentary conducting paths generated by both Ni ions in thermodynamically unstable NiO and the existence of conducting filament segments generated by high voltage bias. 相似文献
999.
1000.