Electron density profile measurement using Ka-band microwave impulse radar reflectometer on LHD

We have installed a ten channel Ka-band ultrashort pulsed radar reflectometer system in the Large Helical Device. The main objective of this diagnostic is to provide reliable information about the edge electron density profile. The time-of-flight measurement technique is used for the delay time measurement of the reflected pulses from each cut-off layer in the plasma, in order to avoid a mixture of spurious radiation such as electron cyclotron emission and undesired reflection such as another polarization wave mode. As for the signal detection section, a filter bank system is applied; the reflected pulse shape is affected by the bandwidth of band pass filter; thus, the optimum bandwidth must be found experimentally. The reconstructed electron density profile is obtained using an Abel inversion method from the profile of the delay time as a function of the probing frequency. In density modulation experiments, the static island width can be estimated from the temporal change of the density profile.