Design elements and first data from a new Doppler backscattering system on the MAST-U spherical tokamak

Abstract

A new Doppler backscattering (DBS) system has been installed and tested on the MAST-U spherical tokamak. It utilizes eight simultaneous fixed frequency probe beams (32.5, 35, 37.5, 40, 42.5, 45, 47.5, and 50 GHz). These frequencies provide a range of radial positions from the edge plasma to the core depending on plasma conditions. The system utilizes a combination of novel features to provide remote control of the probed density wavenumber, the launched polarization (X vs O-mode), and the angle of the launched DBS to match the magnetic field pitch angle. The range of accessible density turbulence wavenumbers ( k θ) is reasonably large with normalized wavenumbers k θ ρ s ranging from ≤0.5 to 9 (ion sound gyroradius ρ s = 1 cm). This wavenumber range is relevant to a variety of instabilities believed to be important in establishing plasma transport (e.g., ion temperature gradient, trapped electron, electron temperature gradient, micro-tearing, kinetic ballooning modes). The system is specifically designed to address the requirement of density fluctuation wavevector alignment which can significantly reduce the SNR if not accounted for.

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

“…As mentioned above, a DBS system developed by UCLA has been installed on MAST-U in 2021 [21]. Following an extensive scan in both toroidal and poloidal launch angles, we have gained confidence in the data from the UCLA DBS.…”

“…It is worth noting that this DBS system is distinct from the other DBS system on MAST-U in ref. [21], which is developed by the University of California (UCLA) team. This new DBS system is developed by the Southwestern Institute of Physics (SWIP).…”

“…For clarity's sake, the DBS system from ref. [21] will be referred to as the UCLA DBS, while the new DBS system presented in this paper will be referred to as the SWIP DBS. This SWIP DBS is comprised of a Q-band and a V-band system.…”

The investigation of edge-localized modes on the Globus-M2 tokamak using Doppler backscattering

“…DBS, sometimes referred to as Doppler reflectometry, is a method which involves the detection of the backscattered radiation by means of introducing a microwave beam at oblique incidence into the plasma. DBS is currently installed on most modern tokamaks [14][15][16][17][18][19][20][21][22][23][24][25] and stellarators [26][27][28][29][30], and is also being developed for new devices, such as JT60-SA [31] and ITER [32].…”

Overview of fast particle experiments in the first MAST Upgrade experimental campaigns

“…The fast-ion diagnostics in MAST-U include a fast-ion deuterium-α (FIDA) spectrometer [10], measuring Doppler-shifted Balmer-α emission due to charge exchange between fast ions and neutrals; a solid-state neutral particle analyser (SSNPA) [11] detecting fast neutrals from charge exchange reactions; a scintillatorbased fast ion loss detector (FILD) [12], providing the velocity space distribution of lost fast ions and the frequencies of modes correlated with these losses; an upgraded neutron camera (NCU) [13] measuring neutron flux mainly resulting from beam-thermal fusion reactions along 6 lines of sight on the equatorial plane; and a fission chamber (FC) [14], measuring the total neutron flux. For measurements of fast iondriven instabilities, the MAST-U is equipped with the outboard Mirnov coil array for high frequency acquisition (OMAHA) [15], providing d(δB)/dt measurements that makes it possible to detect magnetic perturbations in the plasma; beam emission spectroscopy (BES) [16] and Doppler backscattering (DBS) [17], which provide in situ measurements of fluctuations and also spatial information on mode structure. A motional Stark effect (MSE) diagnostic [18] measures the equilibrium magnetic field pitch as function of radius, which is also an important tool for studying the co-evolution of fast-ions and thermal plasma.…”