The geometrical circuit dimensions designed for the first spatial harmonic operation (n = 1) provide a broad bandwidth and large inherent volume to preserve a sufficient amount of RF energy. As shown in Fig. 1(b), in the pass band of the optimally designed circuit, the beam line is very close to the upper cutoff and crosses over the n = 1 and n = 2 space harmonic backward wave modes, which can cause instability during device operation due to parasitic cutoff oscillations or backward wave oscillations.