Design of the cross-polarization scattering diagnostic on the HL-2A tokamak

To measure plasma density and magnetic fluctuations on the HL-2M tokamak simultaneously, a new diagnostic system combining Doppler backscattering (DBS) and cross-polarization scattering (CPS) is under development. It is essential to understand the propagation of injected and scattered rays to support the electronic/quasi-optical design and subsequent interpretation of the detected signals of the multi-channel DBS/CPS measurements. Thus, ray-tracing analysis with the axisymmetric ray-tracing code BORAY has been performed to estimate the scattering location and wavenumbers of the density and magnetic fluctuations. In addition, the influence of accordance between toroidal and poloidal launch angles is investigated. The received DBS/CPS signal quality can be improved by matching the parallel wavenumber in the direction of magnetic field lines.

Ray-tracing analysis for combined Doppler backscattering and cross-polarization scattering diagnostic on the HL-2M tokamak

Zhou

Tong

Zhong

et al. 2023

Quasi-optical design for the cross-polarization scattering diagnostic on the HL-3 tokamak

Zhou,

Tong,

Zhong

et al. 2024

As the plasma beta (β) increases in high-performance tokamaks, electromagnetic turbulence becomes more significant, potentially constraining their operational range. To investigate this turbulence, a cross-polarization scattering (CPS) diagnostic system is being developed on the HL-3 tokamak for simultaneous measurements of density and magnetic fluctuations. In this work, a quasi-optical system has been designed and analyzed for the Q-band CPS diagnostic. The system includes a lens group for beam waist size optimization, a rotatable wire-grid polarizer for polarization adjustment, and a reflector group for measurement range regulation and system response enhancement. Laboratory tests demonstrated a beam radius of order 4 cm at the target measurement location (near the plasma pedestal), cross-polarization isolation exceeding 30 dB, and poloidal and toroidal angle adjustment ranges of ±40° and ±15°, respectively. These results verify the system’s feasibility through laboratory evaluations. The quasi-optical system has been installed on the HL-3 tokamak during the 2023 experimental campaign to support the development of CPS diagnostics.

New millimeter-wave diagnostics to locally probe internal density and magnetic field fluctuations in National Spherical Torus Experiment-Upgrade (invited)

Macwan,

Barada,

Kubota

et al. 2024

A set of new millimeter-wave diagnostics will deliver unique measurement capabilities for National Spherical Torus Experiment-Upgrade to address a variety of plasma instabilities believed to be important in determining thermal and particle transport, such as micro-tearing, global Alfvén eigenmodes, kinetic ballooning, trapped electron, and electron temperature gradient modes. These diagnostics include a new integrated intermediate-k Doppler backscattering (DBS) and cross-polarization scattering (CPS) system (four channels, 82.5–87 GHz) to measure density and magnetic fluctuations, respectively. The system can access reasonably large normalized wavenumbers kθρs ranging from ≤0.5 to 15 (where ion sound gyroradius ρs = 1 cm and kθ is the binormal density turbulence wavenumber). The system addresses the challenges for making useful DBS/CPS measurements with a remote control of launch polarization (X- or O-mode), probed wavenumber, polarization match of the launch beam with the edge magnetic field pitch angle, and beam steering of the launched beam for wave-vector alignment. In addition, a low-k DBS system consisting of eight fixed frequencies (34–52 GHz) and four tunable frequencies (55–75 GHz) for low-k density turbulence and fast ion physics will be located at a nearby port location. The combined systems cover the near LCFS and pedestal regions (34–52 GHz), the pedestal or mid-radius (50–75 GHz), and core plasmas (82.5–87 GHz).