Performance characteristics of a cylindrical Hall thruster depending on the depth of the annular region (L_a) in front of the anode were investigated. The effect of the annular region was examined by operating thrusters corresponding to four different values of L_a (0, 4, 6, and 10 mm) and a fixed length of the cylindrical region (25 mm). Various measurements such as electron and ion currents, thrust, anode efficiency, current and propellant utilizations, and ion energy distribution functions were performed. Such measurements lead to an interpretation that (1) a considerable potential difference may exist between the anode and the ionization region, which is presumably located near the end of the annular region where magnetic field lines converge; (2) this potential difference increases with respect to increasing L_a; and (3) the presence of the annular region near the anode reduces the specific impulse and anode efficiency for the examined thrusters.
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