First data and preliminary experimental results from a new Doppler Backscattering system on the MAST-U spherical tokamak

TL;DR

This paper introduces a new Doppler backscattering system on MAST-U, demonstrating its capability to measure plasma turbulence and velocity profiles across a wide radial range with initial experimental validation.

Contribution

The paper presents the design, installation, and initial experimental results of a novel dual-band DBS system on MAST-U, enabling comprehensive turbulence and velocity measurements.

Findings

Successful first data acquisition from the new DBS system.

Validation of data through comparison with existing DBS system.

Measurement of velocity profiles from plasma edge to core.

Abstract

A new Doppler backscattering (DBS) system, consisting of Q-band and V-band, has been installed and achieved its first data on the MAST-U spherical tokamak. The Q-band and V-band have separate microwave source systems, but share the same optical front-end components. The Q-band and V-band sources simultaneously generate eight (34, 36, 38, 40, 42, 44, 46 and 48 GHz) and seven (52.5, 55, 57.5, 60, 62.5, 65 and 67.5 GHz) fixed frequency probe beams, respectively. These frequencies provide a large range of radial positions from the low-field-side edge plasma to the core, and possibly to the high-field-side edge, depending on the plasma conditions. The quasi-optical system consists of a remotely-tunable polarizer, a focusing lens and a remotely-steerable mirror. By steering the mirror, the system provides remote control of the probed density fluctuation wavenumber, and allow the launch angle to match the magnetic field. The range of accessible turbulence wavenumbers (k_\theta) is reasonably large with normalized wavenumber k_\theta\rho_s ranging from <0.5 to 9. The first data acquired by this DBS system is validated by comparing with the data from the other DBS system on MAST-U (introduced in Ref. [21]). An example of measuring the velocity profile spanning from the edge to the center in a high-density plasma is presented, indicating the robust capabilities of the integrated Q-band and V-band DBS systems.