Effect of magnetic-field orientation on electron-density measurement using microwave cutoff probes in weakly magnetized plasma

In this study, the wave transmission characteristics and cutoff-frequency shift of the microwave cutoff probes were investigated in weakly magnetized plasma using electromagnetic simulations and a magnetized equivalent circuit model. The external magnetic field was varied from 0 to 200 G, and the relative orientation of the electric field between the probe antennas was changed to examine its effect on cutoff-frequency. For the square cutoff probe, the cutoff frequency coincided with the input plasma frequency under parallel configuration, whereas it shifted toward the upper-hybrid resonance frequency under the perpendicular configuration due to electron cyclotron motion. This field-orientation-dependent shift was well described by the magnetized circuit model using the anisotropic dielectric tensor of magnetized plasma, with the electron-density discrepancy remaining within 2% under the parallel and perpendicular configurations and within 3% under oblique-field orientations. The cylindrical cutoff probe showed the same trend and demonstrated valid electron-density measurement capability, although a slightly increased discrepancy was observed under the perpendicular configuration, reaching up to 6%. Electric-field distribution analysis revealed that this difference originates from the more limited effective antenna-to-antenna field region in which the electric field is well aligned with the nominal configuration for the cylindrical probe. Misaligned field components near the antenna tip introduce additional deviation, which becomes more pronounced with increasing magnetic-field strength. Nevertheless, the discrepancy remained within acceptable levels across all investigated conditions, demonstrating that accurate electron-density measurement using microwave cutoff probes remains feasible in weakly magnetized plasma when the relative field orientation is properly considered.