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Figure 2: Simulation results of Ez field distributions of the spiral field Smith-Purcell radiation with the angular momentum l = 1. (a) and (c) Radiated Ez field distribution at z = 0 (a) and the corresponding phase distribution for l = -1 (c). (b) and (d) Radiated Ez field distribution at z = 0 (b) and the corresponding phase distribution for l = +1 (d). The working frequency is 1.5 THz.