title: Reconstruction of edge density profiles on Large Helical Device using ultrashort-pulse reflectometry.
abstract: Reflectometry has been expected to be one of the key diagnostics to measure density profiles. We have applied an ultrashort-pulse reflectometry (USPR) system to Large Helical Device in the National Institute for Fusion Science. Wide frequency band system is required to obtain wide density profile since an incident wave is reflected at the density layer corresponding to its cutoff frequency. The reflectometry utilizes an impulse with less than 30 ps pulse width as a source. Since the bandwidth of an impulse has an inverse relation to the pulse width, we can cover the frequency range of micro- to millimeter waves (18-40 GHz) with a single source. The density profiles can be reconstructed by collecting time-of-flight (TOF) signals for each frequency component of an impulse reflected from the corresponding cutoff layer. We utilize the signal record analysis (SRA) method to reconstruct the density profiles from the TOF signal. The effectiveness of the SRA method for the profile reconstruction is confirmed by a simulation study of the USPR using a finite-difference time domain method.
authors.affiliation: Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga 816-8580, Japan.
authors.forename: Yuya
authors.initials: Y
authors.lastname: Yokota
authors.forename: Atsushi
authors.initials: A
authors.lastname: Mase
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Kogi
authors.forename: Tokihiko
authors.initials: T
authors.lastname: Tokuzawa
authors.forename: Kazuo
authors.initials: K
authors.lastname: Kawahata
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Nagayama
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Hojo