Computer Simulation of Ultra short-Pulse Reflectometry in Helical Plasmas

A. Fukuchi1), H. Hojo1), A. Itakura1) and A. Mase2)

1)Plasma Research Center, University of Tsukuba, Tsukuba 305-8577, Japan
2)Advanced Science and Technology Center for Cooperative Research,Kyushu University,
Kasuga 816-8580, Japan

Ultrashort-pulse reflectometry has been recently developed as a nonperturbing diagnostic for probing density profile and its fluctuation in tandem mirror, tokamak and also helical system. In this paper, we study the computer simulation on ultrashort-pulse reflectometry in helical plasmas such as the LHD device. In the simulation, we solve the Maxwell equations for electromagnetic wave fields and an equation of motion for plasma current with the use of the finite difference time domain (FDTD) method in two dimensions. We here present the propagation of an ultrashort -pulse electromagnetic wave, the cross-polarization scattering of the ordinary mode to the extraordinary mode due to the effect of strong magnetic shear in a helical plasma. It is discussed the relationship between this mode conversion and the magnetic shear strength. We also present computer simulations on ultrashort-pulse reflectometry for helical plasmas. An incident ultrashort pulse is launched into plasma, and the delayed time of the reflected wave from plasma is measured numerically as a function of frequency. Then, the density profile reconstruction is done with the use of the Abel inversion method. It is shown that the reconstructed density profile is in good agreement with the density profile used in the simulation. The possibility of multi-dimensional measurements in ultrashort-pulse reflectometry is also discussed for imaging on the density profile of plasma. .