Design of a Doppler backscattering diagnostic for WHAM: The Wisconsin HTS Axisymmetric Mirror

The Wisconsin HTS Axisymmetric Mirror (WHAM) is a compact high-field magnetic mirror. In such magnetic mirrors, cross-field transport is dominated by the flute instability [M. Rosenbluth and C. Longmire, Ann. Phys. 1, 120–140 (1957)]. To investigate density fluctuations associated with the flute instability, we designed a Doppler backscattering (DBS) diagnostic for WHAM, to be installed at the midplane port window. The diagnostic uses a two-channel tunable Ka-band (26.5–40 GHz) source and X-mode polarization. The azimuthal launch angle is set mechanically by rotating the external quasi-optical assembly. As such, the system is reconfigurable during dedicated setup periods. Using the Scotty beam-tracing code [V. H. Hall-Chen et al., Plasma Phys. Controlled Fusion 64, 095002 (2022)], we show that the proposed DBS system can measure density fluctuations with perpendicular wavenumbers |k⊥| ≤ 3 cm−1 over radial locations 0.7 ≤ ρ ≤ 0.9, where ρ is the normalized radial coordinate. This is achieved with probe frequencies between 28 and 38.5 GHz, an elevation launch angle of 0°, and azimuthal launch angles in the range of 1°–3°. The selected configurations have low mismatch angles at cutoff, |θm,c| < 1°. The quasi-optical system uses a Ka-band horn and a biconvex ultra-high molecular weight polyethylene lens and satisfies the port-access constraints in WHAM. The planned microwave system has a monostatic, homodyne architecture based on two phase-coupled Ka-band microwave channels. These two channels will be for the transmitted signal and coherent local oscillator for IQ downconversion, respectively. As the two phase-coupled channels can be independently tuned or swept with a controlled frequency offset, the same microwave chain is intended to retain compatibility with future profile-reflectometry operation.