Measurements of magnetic and electrostatic fluctuations in an applied field magneto-plasma-dynamic thruster have shown that a m/n=1/1 kink mode becomes unstable whenever the Kruskal-Shafranov limit is violated. A positive correlation is established between the kink and performance degradation at high current, which has until now prevented the use of this kind of thruster in space missions.

An experimental investigation of a pulsed, quasi-steady, 100-kW-class applied-field magnetoplasmadynamic thruster is discussed. Measurements were obtained with argon propellant for a variety of currents, mass flow rates, and magnetic field strengths in a power range between 20 and 250 kW. Tests were carried out in Alta's IV-10 vacuum facility. With a volume of about 200 m(3), IV-10 allowed for a current-pulse duration up to 1 s as well as for minimization of the environmental interaction with the plume. Although the shot duration was still too short to achieve steady-state thermal conditions, it allows for direct, time-resolved thrust measurements. To this purpose, a new single-axis thrust stand was designed to improve the frequency response of the existing thrust stands commonly employed in high-power devices. A maximum thrust efficiency of 28% was obtained at about 200 kW for an applied magnetic field of 120 mT and a mass flow rate of 60 mg/s. At 100 kW, for the same mass flow rate and magnetic field, a thrust efficiency of 22% and a specific impulse of about 2500 s were achieved. Moreover, during the arc ignition phase, the cathode current attachment was found to be distributed mainly at the external surface of the electrode while a transition from diffuse to hollow cathode behavior was observed after approximately 400 ms from the breakdown

An extensive experimental investigation of magnetic and electrostatic fluctuations in a magneto-plasma-dynamic (MPD) thruster, with and without the application of an external magnetic field, has shown that gross magnetohydrodynamic instabilities develop whenever the current rises beyond a threshold value. These instabilities are helical kink modes with azimuthal m and axial n periodicity m/n=1/1 and their occurrence can be described by the Kruskal–Shafranov stability criterion. The presence of these modes is found to be the cause of the loss of efficiency observed in MPD thrusters at high current.

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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.