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The paper deals with an experimental activity carried out on an applied field MPD thruster with a preionisation chamber, called HPT (hybrid plasma thruster). The HPT has been jointly developed by RIAME-MAI and Centrospazio, starting from a design proposed by Prof. Tikhonov. The HPT consists of two stages (chambers) divided by a half-transparent anode. The first stage serves for the preliminary ionisation of part of the propellant by means of a secondary discharge between peripheral cathodes and the anode itself and the second one is used for ionisation and acceleration of the total plasma flow. The results illustrated was obtained using argon as propellant at a mass flow rate of 220 mg/s, injected in different proportion from the central and the peripheral cathodes, with an applied field up to 100 mT on the axis and an arc power from 100 to 400 kW. Results indicate the injection mode can significantly affect the performance of the HPT, also without the activation of the secondary discharge. The total mass flow rate the same, the performance increase by increasing the percentage of mass flow injected by the peripheral injectors, especially at low power. The activation of the pre-ionisation chamber has given contradicting results. The improvements observed during a campaign indicate the substantial effectiveness of the concept, but the current HPT configuration has proven to be not reliable enough for a thorough assessment of the thruster operation, since the same improvements were not observed in a subsequent campaign. An improved HPT with independent peripheral cathodes will be tested in the following months.

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