###### Abstract  
A new Doppler radar using millimeter-waves in the Ka-band, named the \"dual-comb Doppler reflectometer\", has been developed to measure the turbulence intensity and its velocity in high-temperature plasmas. For the realization of a fusion power generation, it is very important to know the spatial structure of turbulence, which is the cause of plasma confinement degradation. As a non-invasive and high spatial resolution measurement method for this purpose, we apply a multi-frequency Doppler radar especially with simultaneous multi-point measurement using a frequency comb. The newly developed method of synchronizing two frequency combs allows a lower intermediate frequency (IF) than the previously developed frequency comb radar, lowering the bandwidth of the data acquisition system and enabling low-cost, long-duration plasma measurements. In the current dual-comb radar system, IF bandwidth is less than 0.5 GHz; it used to be 8 GHz for the entire Ka-band probing. We applied this system to the high-temperature plasma experimental device, the Large Helical Device (LHD), and, to demonstrate this system, verified that it shows time variation similar to that of the existing Doppler radar measurements. millieter-wave; frequency comb; Doppler reflectometer; DBS; LHD +
Footnote †: journal: Applied Sciences  
machines (C-2 FRC [26], GAMMA-10 [27]). In particular, in recent years, systems capable of simultaneous multi-frequency observation have been developed with the aim of understanding the instantaneous spatial structure of turbulence [6; 7; 8; 13; 17; 21; 22; 23; 24; 25]. When observing turbulence in torus plasmas with this Doppler reflectometer, an ordinary or extraordinary wave is injected into the magnetic confined plasma, and the back-scattered wave from the vicinity of the cut-off position corresponding to the injecting frequency is observed, so the measurement position can be changed by changing the frequency. Therefore, it is necessary to inject electromagnetic waves of different frequencies to obtain information at various radial positions in the plasma. This multi-frequency measurement technique has been studied and developed in various ways, as shown in Figure 1. The first is to prepare multiple millimeter-wave sources with fixed frequencies and inject them together [28; 29; 30]. This is the most robust method, but it is difficult to increase the number of frequencies significantly, due to the need to prepare many millimeter-wave sources and to mix signals. The second method is to vary the frequency [3; 4; 5; 10; 11; 12; 14; 15; 16; 18; 19; 20]. This method has the advantage of requiring only one set of millimeter-wave sources as hardware and is relatively easy to calibrate. The method of varying frequencies in a staircase fashion is called frequency hopping, and is also used for zonal flow measurement with high spatial resolution [20]. The disadvantage of this method is that it does not allow instantaneous measurements. If fast plasma changes, such as transition phenomena, occur within the time required for the frequency sweep, it becomes difficult to understand the spatial structure. Another method is to sweep the frequency continuously to obtain the density profile with high temporal resolution, and then extract the fluctuation component by calculation [31]. The third method is to use pulsed electromagnetic waves as a source [32; 33; 34]. This method, called ultrashort pulse radar, uses the broad frequency component of the pulse to ensure a wide bandwidth. The disadvantages of this method are the limited time resolution and the relative complexity of the system. For example, the repetition rate of conventional pulse output is up to about 1 MHz. The time-of-flight instrument used in this method requires a large number of filter banks for received signal processing in order to improve spatial resolution. The fourth method is to use a frequency comb [6; 7; 8; 17].