Application of Ultrashort-Pulse Reflectometry to Large Helical Device

The ultrashort-pulse refrectometer (USRM) system is being prepared to apply to Large Helical Device (LHD). At the present time, we preliminary applied this USRM system to a metal plate as a target, and confirmed that detection time of the reflected pulse is delayed proportionally as increasing in double path length. Recently, we are constructing the USRM system at National Institute for Fusion Science (NIFS). The experiment using this USRM system will be performed since February this year. The USRM system for LHD is as follows. The system utilize an impulse generator as a source, which can transmit an impulse with a pulse width of 22 ps and a voltage amplitude of 3 V. The impulse is fed to an active frequencydoubler via a WD-750 waveguide and a low loss coaxicial cable to produce higher frequency component suitable for high density plasma such as LHD plasma. The charped millimeter wave is amplified by a wide band power amplifier and transmitted by a conical horn in ordinary (O) mode. We utilize optical lenses in front of the transmitter and the receiver. The reflected wave is received by the receiver horn and amplified by low-noise amplifiers. The signal is recorded directly by a high-speed sampling scope (50 GSample/s). The repetition rate of impulse transmission is fixed at 1GHz, and data acquisition is completed for 10 ms. In the analysis for reconstructing density profile from the signal, we utilize signal record analysis (SRA) method, which can introduce less receiver channel requirements and reliable profile reconstruction. The SRA is the method that utilizes a phase difference at each frequency between presence and absence of plasma rather than a time delay at each frequency. In the conference, we will report the result of application of the USRM system to LHD.