# Review of Advanced Implementation of Doppler Backscattering Method in Globus-M
## 3 Application Results
### Alfven Mode  
For the first time, the novel idea to use DBS for the detection of Alfven eigenmodes (AEs) was proposed on Globus-M. This is possible due to the nature of the Alfven oscillations being that of electromagnetic waves. These oscillations of the magnetic field lead to the fluctuations of the E \(\times\) B velocity that is measured using DBS, which means that \begin{table}
\begin{tabular}{c c c c} \hline
**Investigated Property** & **ELM Filament** & **Inter-ELM Filament** & **MHD Induced Filament** \\ \hline Radial size, cm & 2–6 & \textless{}2 & \textless{}1 \\ Poloidal size, cm & \textless{}1–3 & \textless{}1–3 & \textless{}1–3 \\ Distance between filaments, cm & \textless{}14 & \textless{}7 & \textless{}10 \\ Velocity, km s\({}^{-1}\) & \textless{}11–14 & \textless{}6–12 & \textless{}12 \\ Quasi-toroidal mode numbers & \textless{}12 & \textless{}25 & \textless{}15 \\ Critical density \(\mathrm{n_{e}}\), 10\({}^{19}\) m\({}^{-3}\) & \textless{}1.2 & 1.5 & \textless{}2–3.5 \\ Location R, cm & 55 & 54.8 & 57 \\ D\({}_{\alpha}\) activity & present & not present & not present \\ Langmuir probe activity & present & not present & not present \\ Magnetic probe activity & present & not present & present \\ \hline \end{tabular}
\end{table} 
Table 6: Properties of filaments in Globus-M.  
the amplitude of the Alfven perpendicular velocity, radial electric field can be extracted from the measurements. Studies were conducted in the Globus-M tokamak with early neutral beam injection at the stage of growth of the plasma current [66; 67]. Initially, the detection of toroidal Alfven eigenmodes (TAE) using DBS was confirmed by comparing the experimentally obtained frequency of the fluctuations presented in Table 7 to the Alfven continuum frequency gaps \(f_{TAE}=v_{A}/4\pi qR\), where \(v_{A}=B/\mu_{0}\rho_{D}\) is the Alfven velocity (B-magnetic field, \(\rho_{D}\)-mass density of plasma ions), \(q\) is the safety factor, and \(R\) is the major radius [68]. The results of the calculations were similar to those acquired in experiments. Apart from that, the calculated spectrograms of rotation velocity determined using the DBS method highlighted the fact that the frequency of the TAE decreases with time, which corresponds to the concept of the slowing down of the Alfven wave. These properties were also observed in the spectrograms of the magnetic probe signals that confirmed the observation of Alfven eigenmodes. The nature of the TAE varied depending on the characteristics of the injected beam (it was shown that the isotope of the injected particles had an effect on what type of Alfven mode could develop) as well as the plasma density. The instability was observed either in the form of a short burst lasting from 0.1 up to 0.5 ms or as continuous oscillations of several ms. The length of the TAE burst apparently can be defined through a predator-prey model [69]. The DBS measurements were used to calculate various properties of the Alfven eigenmodes such as the amplitude of the radial electric field and accordingly their magnetic field. The obtained values are presented in Table 7 as well as works [70; 71; 72]. It is worthy of note that the magnetic field amplitude determined using DBS was compared with the measurements of the magnetic probes \(B^{MP}\) located on the wall of the vacuum chamber. It was observed that the \(B^{MP}\) values were generally significantly smaller than those calculated from the DBS measurements, which seems to indicate the spatial damping of the instability [71]. While several DBS systems were used to study the location of the Alfven eigenmodes, the four-frequency system had yielded the best results. The toriodal number \(n=1\) TAE was detected in the area with major radii values of 0.50-0.56 m. It was noted that a clear maximum of the amplitude for this n was not determined.