# Review of Advanced Implementation of Doppler Backscattering Method in Globus-M
## 1 Introduction  
Doppler backscattering (DBS) is a microwave diagnostics method applied on many magnetic confinement fusion devices typically with the aim to study the plasma rotation velocity [1; 2; 3; 4; 5; 6; 7; 8; 9]. Since the L-H transition was shown to be caused by the suppression of turbulent plasma perturbations by the shear of the plasma drift velocity in the radial electric field (E \(\times\) B velocity) [10], DBS was successfully applied to study this transition. In the tokamak ASDEX Upgrade, data for the analysis of L-H transitions with low density were obtained using this method [11]. A comparison of neoclassical calculations for the radial electric field and DBS data was made, which showed a good agreement between the two methods for different modes of operation of the tokamak. The TCV tokamak used a heterodyne V-band Doppler backscattering diagnostic system [12]. The first results show that the perpendicular rotational velocities obtained with DBS are consistent with the estimates of the poloidal rotation obtained from the charge exchange recombination spectroscopy diagnostics. A radial profile of perpendicular velocity was successfully obtained on the LHD tokamak using a Doppler reflectometer system [13]. The radial electric field was extracted from measurements of perpendicular velocity. In the work [14] the rotation characteristics of the plasma in the tokamak HL-2A before and after supersonic molecular beam injection were also analyzed. It was shown that an SMBI pulse can reduce the Doppler shift frequency, which corresponds to the flattening of the electric field. In a series of experiments on the EAST tokamak using two separate DBS systems, one for the Q-band (33-50 GHz) and the other for the V-band (50-75 GHz), Doppler shifted signals were obtained and radial profiles of perpendicular velocities were computed for the L- and H-mode [15]. In the Globus-M2 tokamak, the first results of the Doppler backscatteringdiagnostics during a discharge with the L-H transition show that the transition process is linked to the deformation of the poloidal rotation velocity profile [16]. This diagnostic made it possible to conduct studies in the hotter and more central plasma regions, which is a useful addition to the velocity and field data obtained by other methods. Apart from these conventional techniques more advanced forms of DBS implementation were suggested. More specifically the study of a variety of oscillating processes was performed using DBS. For example, for the investigation of the high-frequency branch of zonal flows--the geodesic acoustic mode--the DBS diagnostics were used on JET [17; 18], ASDEX Upgrade [19], HL-2A [20], EAST [21], DIII-D [22], TUMAN-3M [23], and other devices [24]. Doppler backscattering has also proved useful for the study of limit cycle oscillations whose typical frequency is significantly lower than that of the geodesic acoustic mode frequency. Such research was carried out in the ASDEX Upgrade tokamak [25], DIII-D [26; 27], and HL-2A [28; 29]. In this review we aim to present a detailed description of all the methods and techniques employed in Globus-M in regard to DBS diagnostics used to study all the various phenomena that occur in plasma during a discharge. A multitude of results that had been obtained using DBS in the last 10 years up until the moments of the tokamak's ultimate shutdown in 2017 are revisited and described. These include the investigation of such phenomena like geodesic acoustic modes or limit cycle oscillations using similar research practices that had been previously adopted on other devices. Apart from that, innovative works are reexamined regarding Alfven eigenmodes and filaments that were the first of their kind and had provided important and novel results for the plasma physics community. Furthermore, the specific aspects of DBS application on a spherical tokamak are discussed and highlighted.