The results of an analytical treatment of the time-dependent 2D full-wave equation are presented here for the case of ultra-short pulse (USP) reflectometry. We consider several models of the plasma geometry, namely linear and nonlinear slab models, as well as a 2D plasma density profile with cylindrical symmetry. The latter model is more realistic when compared to the 1D stratified plasma models previously employed in all the analytical, and most numerical, treatments, since the plasma in fusion toroidal devices, mirror machines and plasma processing chambers can often be considered axially symmetric on the scale relevant to microwave reflectometry.Based on the results of analytical modelling, a signal record analysis method of profile reconstruction is proposed. The method has the advantage of using raw signal records instead of poorly localized frequency modes, which makes it robust for the profile measurements using USP reflectometry.
Millimeter-wave imaging systems in the frequency range of 70–140 GHz have been developed for diagnostics of magnetically confined plasmas. The 70 GHz imaging system successfully measures time evolutions of both radial and axial profiles of line density and electron cyclotron emission (ECE) in a tandem mirror. The imaging system is being installed in Large Helical Device (LHD) at the National Institute for Fusion Science. In order to cover the frequency range of the second harmonic ECE on LHD, a new detector using monolithic microwave integrated circuit technology has been designed and fabricated. The detector consists of the integration of a receiving antenna, a down-converting mixer diode, and an intermediate frequency amplifier on a GaAs substrate chip. The heterodyne response up to 10 GHz was confirmed at 70–140 GHz. The optical system consisting of an ellipsoidal mirror and a flat mirror was designed by using a ray-tracing code and evaluated experimentally at 140 GHz.
We present an analysis of the time delay methods of plasma profile reconstruction applied to ultra-short pulse (USP) reflectometry. As the instantaneous frequencies become poorly localized in the time domain, even the advanced time-frequency analysis fails to produce reliable values of the time delay for corresponding modes. Based on the results of analytical modeling of USP propagation in plasma the signal record analysis method of profile reconstruction is proposed. The method has an advantage of relying on a row signal record rather than on the delay time of each frequency mode, which makes it more robust and reliable for the problem of density profile measurements using USP reflectometry.
Significant advances in microwave and millimeter wave technology have enabled the development of a new generation of imaging diagnostics in this frequency region. Millimeter wave imaging radar is expected to be one of the most promising diagnostic methods for this purpose. It consists of a frequencymodulated continuous wave or pulsed wave as a probe beam and quasi-optical focusing optics followed by a planar-type detector array. We have started to develop a diagnostic system for the achievement of imaging radar. Representative experimental results obtained with related diagnostic systems are presented.
Design of an X-mode fast-scanning reflectometry for edge density profile measurement on HT-7 tokamak Rev. Sci. Instrum. 74, 1522 (2003; 10.1063/1.1527253 Millimeter-wave reflectometry for electron density profile and fluctuation measurements on NSTX Rev. Sci. Instrum. 72, 348 (2001); 10.1063/1.1329657 Ultrashort pulse reflectometry for electron density profile measurements on SSPX Rev. Sci. Instrum. 72, 332 (2001); 10.1063/1.1308999Preliminary electron density profile and fluctuation measurements on GAMMA 10 using ultrashort-pulse reflectometry Rev.An ultrashort-pulse reflectometry ͑USRM͒ has been applied to the inductively coupled steady-state plasma ͑ICP͒ for density profile measurement. A reflected wave from ICP is directly recorded into a digitizing sampling scope and is analyzed by the signal record analysis ͑SRA͒ method. From repetition of the measurement, we have obtained the time evolution of the density profile. In order to apply the USRM and SRA method to high density plasmas such as core region of the Large Helical Device plasma, generation of the pulse with higher frequency up to 90 GHz is essential. We have utilized a frequency upconverter and an active doubler for this purpose. We confirmed the generation of the upconverted pulse in the range of 60-90 GHz and the second-harmonic frequency in the range of 26 -40 GHz. Time of flight measurements have been preliminarily performed with a metal target.
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