###### Abstract
The advantages of this method are that it can utilize the intermittent but broad frequency components of a single frequency comb output, and the repetitive oscillation frequency can be increased to several hundred MHz or more, allowing it to be handled like a pseudo-continuous wave. In other words, a simple measurement system using a single source can extract a broad frequency component and achieve high spatial and temporal resolution at the same time. A frequency comb is a signal with periodic peak frequency components and utilizes oscillation from a single source such as a non-linear transmission line (NLTL) or step recovery diode (SRD), as shown in Figure 2, or comb-like frequency component radiation using a multiplier [13; 21; 22; 23; 24; 25]. Recently, we have been applying frequency comb Doppler reflectometers in the LHD [6; 8], and the precise radial profiles of the perpendicular velocity are obtained up to 20-40 radial points, simultaneously, by using a high sampling rate (80 GS/s) data acquisition system. This is a very useful technique, but the observation time is limited due to the limitation of the stored memory. To solve this problem by slowing down the sampling rate, we try to reduce the intermediate frequency (IF) components in the mixer output of the heterodyne detection circuit. In addition, if the IF frequency can be lowered, not only can the sampling rate be reduced, but the frequency bandwidth of the filter bank can also be lowered. This makes it easier to reduce the associated component costs, add more frequency channels, and improve spatial resolution. Figure 1: Schematic diagram of various methods of multi-frequency reflectometer technology. Images of temporal behavior of output signal wave forms (green) and frequencies (red). (**a**) Different fixed frequency sources method utilizes independent continuous wave with different frequency components. (**b**) Frequency hopping or frequency sweeping method utilizes time varying frequencies. (**c**) Ultrashort pulse method utilizes a train of impulse which has broadband frequency component. (**d**) Frequency comb method utilizes simultaneous discrete frequency components. Figure 2: Examples of frequency spectrum of different frequency comb sources: (**a**) Marki Microwave, Inc. model: NLTL-60265 (**b**) Herotek, Inc. model: GCA2026B. Both comb sources are controlled to output frequency components every 2 GHz. Here, the spectral power degradation in the region above 20 GHz is due to the frequency bandwidth limitation, since the data was collected by an oscilloscope with a bandwidth of 20 GHz and a sampling rate of 80 GS/s. In this paper we report on the development of a new Doppler reflectometer system based on a frequency comb source and the results of a demonstration experiment in which it was applied to an LHD plasma. Section 2 introduces the principle of the Doppler reflectometer, and Section 3 describes the dual-comb system using two frequency comb sources, which contributes to various cost reductions, and explains the operating principle. Section 4 describes the dual-comb Doppler reflectometer system for LHD plasma measurement and describes the results of its operational tests and initial plasma demonstration observation.