# Review of Advanced Implementation of Doppler Backscattering Method in Globus-M
## 4 Conclusions  
In summary, the DBS diagnostics was successfully used to study peripheral plasma processes in the Globus-M tokamak. The application of this method was implemented on a spherical tokamak for the first time. Subsequently, DBS was implemented on the spherical tokamak MAST [8]. The simultaneous application of several DBS systems located at different poloidal angles and with different probing frequencies made it possible to determine the spatial structure of different plasma processes. As a result, geodesic acoustic modes, limit cycle oscillations, quasi-coherent fluctuations, filaments, Alfven eigenmodes, and plasma turbulence were investigated. Two types of systems were used to study the radial distribution of GAMs: (1) two single-frequency systems with the possibility to change the probing frequency in the ranges of 18-26 GHz and 27-38 GHz, (2) a four-frequency system with fixed probing frequencies of 20, 29, 39, and 48 GHz. It turned out that the very narrow localization of GAMs near the periphery could only be studied in detail with the first type of system. It is worth noting, however, that discharge repetition in this case required much effort and additional diagnostics of various plasma parameters. It was not possible to construct a radial profile of the GAM oscillation amplitude using a four-frequency system, as the cut-offs for the selected frequencies were spaced much farther apart than the size of the GAM localization layer. Therefore, GAMs were only detected on the peripheral channel of the four-frequency system, whereas on other channels they were not observed. It was also useful for the purpose of GAM research to use poloidally spread single-frequency DBS systems positioned in a manner so that their cut-offs were located on the same magnetic surface. Such an application of DBS diagnostics in this configuration allowed for the determination of the poloidal mode number of the GAM velocity oscillations to be \(m=0\). A four-frequency DBS system was used with the intent to study LCO. In contrast to GAMs, the localization of the LCO turned out to be much wider, and all four DBS channels were able to detect oscillations at this frequency, albeit with different amplitudes. The study of fluctuations in the velocity and amplitude of turbulence at different radii had made it possible to investigate the characteristic properties of the development of LCO. The phase relationship between oscillations of velocity and turbulence amplitude near the top of the pedestal region corresponded to the interaction of zonal fluxes and turbulence observed on other devices (see [78], for example). Yet near the separatrix where both velocity and turbulence fluctuations were observed, their phase coupling changed and the velocity started to increase slightly earlier than the level of turbulence. These results contradict  
Figure 7: Evolution of DBS amplitude for different cut-off radii. (**a**) low-frequency ELMs, (**b**) high-frequency ELMs. The turbulence amplitude of each DBS-channel is calculated in different arbitrary units [76]. the known zero-dimensional model of the interaction of zonal flows and turbulence [48] and indicate that the description of LCO should be at least one-dimensional. A study of the spectral characteristics of the LCO oscillations on Globus-M showed an increase in their frequency when compared to larger devices. The dependence of the frequency of oscillations on the size of the devices cannot be predicted by a predator-prey model of coupling of zonal flows with turbulence. DBS was first utilized for filament research. Subsequently, the detection of filaments using DBS on the ASDEX Upgrade tokamak was reported [79]. The use of various probing frequencies on Globus-M made it possible to detect them both near the separatrix and in the more inner plasma regions. The simultaneous use of several probing frequencies allowed one to study their radial size. It had been found that the filaments that develop during ELMs are radially longer than the filaments that develop during inter-ELM periods.