Increased thrust-to-power ratio of a stepped-diameter helicon plasma thruster with krypton propellant is investigated in a study. Krypton gas propellant is introduced near the open exit of the stepped-diameter radio frequency (rf) plasma thruster, which was previously operated with argon propellant. The experiment is performed in a 1-m-diam and 2-m-long vacuum chamber evacuated by three 3000 ls-1 turbomolecular pumps to a base pressure less than 10-4 Pa. The chamber pressure is proportional to the gas flow rate and about 40 mPa for the krypton gas flow rate of 100 sccm. A pendulum thrust balance, same as the previous experiment, is installed inside the chamber. The balance consists of an aluminum plate suspended by four flexible thin plates from a top plate. This structure allows the axial displacement by an axial force exerted to the aluminum plate. The thruster is attached to the aluminum plate and a displacement of the thruster induced by plasma production is measured by a laser sensor, where the force can be obtained by multiplying a calibration coefficient to the measured displacement.
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