K. Kawahata

T. Kaneba

T. Tokuzawa

National Institute for Fusion Science (NIFS-Repository)

with 20dS attenuattion(b)6lS--64--6..'t?__65 65.5 66 66.5 67--------...=;o 62.6 62.8 63 63.2 63.4time(ns)Fig 2.(a) 40 - 26 GHz chirped waveform propagating through a50 cm R-band waveguide and (b) Time evolution of the frequencycomponent of the chirped waveform from 62.5 ns to 63.5 ns.Fig 3. Schematic of ultrashort-pulse reflectometer system.-2500 20 40 60 80 100 120 140 160 180. .. 1 fr (ps) 1 .Fig 1. -2.2 V, 23 ps FWHM Impulse SIgna om lmpu se lormmgnetwork:>E'i 0&8 -5front of the mixer the bandpass filter (26 - 40 GHz) is set to avoidthe unexpected frequency component.Positions of cut-off layers are estimated by the time-of-flightmeasurement. The reference pulse from CFD is used to the startsignal and the reflected pulse is used to the stop signal for atime-to-amplitude converter (TAC). The TAC outputs the analogvoltage proportional to the time differences between the start signaland the stop signal. The TAC outputs are sent to CAMAC and thedata is acquired by a personal computer.0F==F=='F::::r::::;,..,----,-......,-----r-----,,----.---,-r-r:::-----:;l:::::::::!:l-50:>&100"'5-200§ 21. Development of Ultrashort-PulseReflectometerAn plane wave of an electromagnetic wave which is launchedperpendicularly in the direction to the external magnetic field into aplasma is distinguished between the ordinary mode (O-mode) andthe extraordinary mode (X-mode) for the direction of the electricfield component. When these waves arrive at the critical regions,they can not propagate and then reflect. In the case of O-modeoperation, the critical layer is determined by only the electrondensity profile. In order to measure the electron density profile andfluctuation in LHD plasmas, we have been developing a newreflectometer system using an ultrashort-pulse. A reflectometer hashigher resolution for time and space than other conventionalmethods. The ultrashort-pulse has broad band frequencycomponents. When they are launched into the plasma, they reflectfrom the cOITesponding each cut-off layers. When the time-of-flightmeasurement of each frequency component pulse is done, theelectron density profile can be determined.To extract the desired range of the millimeter wave from theimpulse, we use a rectangular waveguide. When an ultrashort-pulseis launched into the waveguide, it is transformed intoelectromagnetic waves including a broad frequency spectrum. Thistransformation is owing to the dispersion effect of the waveguide.The waveform of the ultrashort-pulse is shown in Fig. 1. It is -2.2 V,23 ps FWHM impulse. A 50 cm R-band rectangular waveguide isused to disperse the impulse. Output from the waveguide is shownin Fig. 2(a). The resultant frequency component calculated byusing the zero-cross method is shown in Fig. 2(b). In consequencethe frequency changes from high to low gradually and we obtain 26- 40 GHz millimeter wave for the measurement. Our reflectometersystem is shown in Fig. 3. The chirped wave is amplified by apower amplifier and divided from the incident wave to get thereference wave by the directional coupler. The reference wave isdetected by a Schottky barrier diode and then sent to a constantfraction discriminator (CFD).As a method to receive the reflected waves a heterodyne system isadopted. The reflected wave is mixed with a 42 GHz continuouswave of the local oscillator. The output is amplified by theintermediate frequency (IF) amplifier (2 - 18 GHz) and thendivided to six. Each IF signal is filtered by bandpass filters whichthe center frequencies are 3, 5, 7, 9, 11, and 13 GHz and each bandwidth is ±0.5 GHz. The detected pulses at the Schottky barrierdiode are amplified by the pulse amplifier and sent to CFDs. InKaneba, T. (Grad. Univ. Advanced Studies),Tokuzawa, T., Kawahata, K.170

Development of Ultrashort-Pulse Reflectometer

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