A sensitivity assessment of millimeter-wave polarimetry for measurement of magnetic fluctuations associated with microtearing modes in NSTX-U

Recent nonlinear gyrokinetic calculations have indicated that microtearing modes are driven unstable in NSTX (National Spherical Torus experiment) and may account for the observed anomalous electron thermal transport (Guttenfelder et al 2011 Phys. Rev. Lett. 106 155004). In order to study magnetic fluctuations of both coherent and incoherent modes, a 288 GHz (λ ≈ 1 mm) polarimeter is under development (Zhang et al 2012 Rev. Sci. Instrum. 83 10E321) for NSTX-U (NSTX-Upgrade) (Menard et al 2012 Nucl. Fusion 52 083015). The system will utilize a retro-reflective geometry and view the plasma along the major radius close to the midplane. In order to assess whether the system will have sufficient sensitivity to observe microtearing modes in NSTX-U, a synthetic diagnostic code is developed and utilized to determine the expected phase fluctuation level. The fluctuating profiles for density and magnetic field generated by the non-linear gyrokinetic simulation are used as input to the code. Results indicate that the polarimeter phase fluctuation level due to the modeled microtearing modes is ≳2°. Utilizing the same model, it was also established that the calculated phase fluctuations are dominated by magnetic, not density fluctuations. This was especially true when the horizontal viewing chord was close (within ±5 cm) to the plasma midplane. These results indicate that the polarimeter planned for NSTX-U should have sufficient sensitivity to observe magnetic fluctuations associated with microtearing modes. (paper)