# Review of Advanced Implementation of Doppler Backscattering Method in Globus-M
## 3 Application Results  
### Geodesic Acoustic Mode  
DBS was implemented to study geodesic acoustic modes (GAMs) on the Globus-M tokamak in the Ohmic phase of discharges at relatively low plasma density (\(n\approx\) (2-3) \(\times\) 10\({}^{19}\) m\({}^{-3}\)) when the toroidal ion drift was directed away from the X-point [35; 36; 37]. All DBS systems (see Table 1) and in some experiments even their combinations were used to obtain information about various properties of GAMs presented in Table 3 (see the end of Section 3.1). GAMs were identified through spectral analysis of the perpendicular velocity V\({}_{\perp}\)(t) = V\({}_{\text{E}\times\text{B}}\)(t) + V\({}_{\text{phase}}\)(t). Since GAMs lead to oscillations of V\({}_{\text{E}\times\text{B}}\)(t), they can be detected by DBS. The measured Doppler frequency shift was obtained either from the spectrum shift of the IQ detector signal or as the phase derivative of the complex signal. GAMs manifest themselves as coherent oscillations in the frequency domain of 23-28 kHz in the case of deuterium plasma and around 36 kHz in hydrogen plasma [38; 39]. These frequencies are within the large orbit drift width limit when compared with theoretical predictions [40]. The study of the temporal evolution of GAMs showed that they do not exist continually; rather, an intermittent increase and decrease of GAM amplitude was observed. There were two characteristic times for the GAM level changes [39]. The first one is the duration of a quasi-coherent GAM burst which was about 0.2 ms with an interval of 0.4 ms between bursts. This behavior corresponds to the well-known predator-prey model [41]. Another timescale is a very slow evolution of the GAM level with a period of about 5 ms. This is also the characteristic time of the evolution of the ion pressure gradient on Globus-M. The GAMs velocity amplitude V\({}_{\perp}\)(t) was recalculated to the radial electric field E\({}_{\rm r}\) assuming that the phase velocity V\({}_{\rm phase}\)(t) induced by GAM oscillations is relatively small. The amplitude of the electric field oscillations reached up to 3 kVm\({}^{-1}\), which sometimes exceeded the mean value of the radial electric field causing a velocity reversal at a certain phase of the GAMs oscillations [39]. Initially the GAM location was studied by varying the probing frequencies from shot to shot while all plasma parameters were kept identical. These experiments showed that GAMs exist in a very narrow layer of a few centimeters inside the separatrix [39]. The obtained radial profile of the GAM velocity amplitude is presented in Figure 2. A clear maximum and decrease in amplitude is observed in this several centimeter layer. The GAM frequency was also investigated in different locations and it was determined that no significant change of its values was present. Experiments with a four frequency DBS scheme confirmed the narrow localization of GAM [42]. Only the peripheral DBS channel could register GAM oscillations near the normalized minor radius 0.95, whereas the other channels did not detect oscillations exceeding the background level at the GAM frequency. This narrow area of GAM existence might be caused by the prevention of GAMs development by Landau damping induced by the steep q-value decrease in the edge region of Globus-M. The width of the GAMs existence layer was about 1 cm with the GAM frequency not changing significantly within this area [39]. Such observations correspond to the global GAM eigenmode nature which was predicted in work [43]. Poloidal correlation DBS using two microwave schemes with the cut-offs positioned at different poloidal angles (\(-30^{\circ}\) and \(45^{\circ}\)) of the same minor cross-section was employed in Globus-M to study the poloidal structure of GAMs [32; 44].