Combination Doppler backscattering/cross-polarization scattering diagnostic for the C-2W field-reversed configuration

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10H116 (2018) Combination Doppler backscattering/cross-polarization scattering diagnostic for the C-2W field-reversed configuration L. Schmitz,1,2,a) B. Deng,2 M. Thompson,2 H. Gota,2 C. Lau,2 D. P. Fulton,2 Z. Lin,3 T. Tajima,2,3 M. Binderbauer,2 and TAE Team2,b) 1Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA 2TAE Technologies, Inc., Foothill Ranch, California 92610, USA 3Department of Physics and Astronomy, University of California Irvine, Irvine, California 92697, USA (Presented 18 April 2018; received 7 May 2018; accepted 20 June 2018; published online 5 October 2018) A versatile combination Doppler backscattering and Cross-Polarization Scattering (CPS) diagnos- tic for the C-2W beam-driven field-reversed configuration is described. This system is capable of measuring density fluctuations and perpendicular magnetic field fluctuations across a wide wavenum- ber range (2.5 ≤ kθρs ≤ 50), with typical resolution ∆kθ/kθ ≤ 0.4-0.8. Four tunable frequencies (26 GHz ≤ f ≤ 60 GHz corresponding to plasma cut-off densities 0.8 × 1019 ≤ ne ≤ 4.4 × 1019 m−3) are launched via quasi-optical beam combiners/polarizers and an adjustable parabolic focusing mir- ror selecting the beam incidence angle. GENRAY ray tracing shows that the incident O-mode and backscattered CPS X-mode beam trajectories for C-2W plasma parameters nearly overlap, allowing ˜˜ simultaneous detection of n˜ and Br or Bθ from essentially the same scattering volume. Published by AIP Publishing. https://doi.org/10.1063/1.5038914 I. INTRODUCTION Field-reversed configurations (FRCs) are axisymmetric, compact plasmas characterized by high β (the ratio of kinetic to magnetic pressure). Typically, in fusion plasmas, radial particle and heat transport in excess of classical or neoclas- sical transport is observed, which is caused by plasma tur- bulence. In the C-2U FRC device at TAE Technologies,1,2 Doppler Backscattering (DBS) measurements3,4 with a diag- nostic described earlier5 clearly show that fluctuations at low 6 Gyrokinetic stability analysis has attributed core stability to the combined effect of large ion Larmor radius, short field-line connection length restricting the parallel wavenumber spec- 7,8 toroidal wavenumber are absent/stable in the FRC core. tant as β is near unity and well in excess of (me/mi) . In this paper, we describe an extension of the existing DBS diagnostic for density fluctuation, capable of measuring in addition spec- trally resolved radial or poloidal magnetic field fluctuations via Cross-Polarization Scattering (CPS),11–17 simultaneously with DBS density fluctuation measurements. trum, and favorable magnetic field gradient. Only low-level electron-scale fluctuations are observed in the FRC core. How- ever, substantial density fluctuation amplitudes with an expo- nentially decreasing wavenumber spectrum were measured in the scrape-off layer (SOL) and near the excluded-flux radius (near the magnetic separatrix). Anomalous electron radial heat loss is also observed, and the diagnosis of radial magnetic field fluctuations is therefore clearly important. In FRC plasmas, the radial thermal electron heat flux Qet is typically anoma- 􏰂ωi􏰃2􏰄 c σ 􏰅 iε0ωi ∂J(i) ∂t lously high compared to the classical value. Qe can formally be linked to the radial fluxes produced by correlated density, electron temperature, and toroidal electric field fluctuations, and fluctuations of the parallel heat flux and radial magnetic field,9 Note: Paper published as part of the Proceedings of the 22nd Topical Confer- ence on High-Temperature Plasma Diagnostics, San Diego, California, April 2018. a) Author to whom correspondence should be addressed: lschmitz@ucla.edu b)TAE Team members are listed in Nucl. Fusion 57, 116021 (2017). 1− (i) iε0ωpen˜ ωi 􏰌 ˜ 􏰍 , (2) (3) −∇×(∇×Es)+ t2 3 e2eeeθθ∥r 􏰀􏰆􏰀˜ 􏰁˜􏰇􏰆 ˜􏰇􏰁􏰆˜􏰇 Q = (nT )/B T /T E + (n˜/n)E + q˜ B . (1) t Here, the first two terms on the right-hand side correspond to the electrostatically driven electron heat flux, and the third term describes the electromagnetic contribution. The diagnostic principle of CPS is based on scattering of an incident microwave beam into the opposite polarization. Using the wave equation15 for the scattered field Es and the per- turbed nonlinear equation of motion for the plasma electrons, the current J(i) induced by the incident wave (with incident electric field E i ) is described by In an FRC, electromagnetic contributions could be impor- 1/2 10 Es=−iμ0 J = ωi nEi+ε0ω2σσEi×B/B. pe Here, σ is the unperturbed plasma conductivity. The sec- ond term in Eq. (3) describes the electron current induced via cross-polarization scattering. If a Gaussian microwave beam, for example, in O-mode polarization, is launched toward the plasma at an oblique angle to the magnetic flux sur- faces, the beam is refracted in the FRC plasma and its tra- jectory/wavenumber will be approximately toroidal near the 0034-6748/2018/89(10)/10H116/5/$30.00 89, 10H116-1 Published by AIP Publishing.