# Review of Advanced Implementation of Doppler Backscattering Method in Globus-M
## 2 Doppler Backscattering on Globus-M
### DBS Systems
\begin{table}
\begin{tabular}{c c c c} \hline
**DBS Systems** & **\#1** & **\#2** & **\#3** \\ \hline Number of systems & 1 & 2 & 1 \\ \hline Number of frequencies & 1 & 1 & 4 \\ \hline Frequency values, GHz & 18–26 & 27–38 & 20, 29, 39, 48 \\ \hline Waveguide & WR-42 & WR-28 & WR-42, WR-28 \\ \hline Wave propagation & O-mode & O-mode & O-mode \\ \hline Poloidal tilt angles, \({}^{\circ}\) & 6–9 & 6–9 & 6–9 \\ \hline Toroidal tilt angles, \({}^{\circ}\) & 2–5 & 2–5 & 2–5 \\ \hline Probing beam angle, \({}^{\circ}\) & \(-\)30, \(-\)45 & 45, \(-\)30, \(-\)45 & \(-\)30 \\ \hline Position & port \#2,3 & port \#1,2,3 & port \#2 \\ \hline Cut-off radii \(R\), m & \(\sim\)0.55–0.58 & \(\sim\)0.53–0.58 & \(\sim\)0.51–0.59 \\ \hline Field of study & geodesic acoustic modes, filaments & geodesic acoustic modes, filaments, Alfven eigenmodes & geodesic acoustic modes, filaments, Alfven eigenmodes, turbulence \\ \hline \end{tabular}
\end{table} 
Table 1: DBS systems on Globus-M.  
DBS system was developed so as to probe and record the scattered signal at larger radii (system #2 in Table 1). The microwave circuit of system #2 was similar to the one in system #1 with the exception of a higher frequency probing range of 27-38 GHz. The same antenna that was used for system #1 to probe the plasma was employed on system #2. The antenna was connected to this DBS system using the WR-28 to WR-42 waveguide transition. Because only one port (port #2) located in the equatorial plane was initially allocated for the DBS diagnostic, only one of the two available systems could be applied during a discharge. Subsequently, two additional ports (ports #1 and #3) were made available for the development of Globus-M DBS diagnostic that were located at the same toroidal angle, but several centimeters, respectively, above and below the equatorial plane. Additional antenna systems were installed in these ports, and one more DBS system similar to system #2 was acquired. The simultaneous use of several DBS systems, spread out in a poloidal direction, made it possible to develop poloidal correlation Doppler reflectometry and to determine the poloidal scale of the developing plasma instabilities. However, the use of systems #1 and #2 made it difficult to construct radial profiles of the plasma processes under investigation by means of changing the probing frequency from discharge to discharge. This method required precise repetition of plasma discharge parameters. The fluctuation of these very parameters made it difficult to accurately interpret the acquired results. Therefore, a DBS scheme with four fixed probing frequencies was developed. The 4-channel DBS system in Globus-M comprised two \(\times\) two-channel DBS microwave schemes with different probing frequencies [33]. Two pairs of fixed frequencies were chosen--20, 29 GHz and 39, 48 GHz. Each frequency channel included a microwave circuit with dual homodyne detection. Two steerable antennas were used to probe the plasma by O-mode microwaves. Each of the antennas was used both as an incident and a receiving one, and were connected to a waveguide that allows only a certain range of wavelengths to pass through. One antenna was used for frequencies of 20 and 29 GHz, whereas the second one was used for the 39 and 48 GHz frequencies. The detection region covered a considerable radial interval of normalized small radii \(\rho\) = 0.6-1 for a typical Globus-M discharge.