Study of the stationary plasma thruster ground-based test conditions on its parameters and discharge current oscillation characteristics

Aeronautical and Space-Rocket Engineering


DOI: 10.34759/vst-2022-2-118-134

Аuthors

Baranov S. V.1*, Ermoshkin Y. M.2**, Kim V. P.1**, Merkur'ev D. V.1**, Svotina V. V.1***

1. Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia
2. Satellite Information Systems, SIS, 62, Lenin str., Zheleznogorsk, Krasnoyarsk Region, 862972, Russia

*e-mail: sergy@strizhi.ru
**e-mail: riame4@sokol.ru
***e-mail: riame@sokol.ru

Abstract

This article presents the results of preliminary study of influence of the Stationary Plasma Thruster (SPT) ground-based test conditions in the typical vacuum chambers on the SPT output parameters and discharge current oscillation characteristics. The SPT’s are operating already many years in space as a parts of the spacecraft (S/C) motion control systems and ensuring the S/C operation during 5-15 year service time. To reach their reliable SPT operation they undergo complex of the ground tests including that ones in the vacuum chambers imitating thruster operation conditions in space. And that it is impossible to reproduce these conditions fully. Then it s important to understand how the difference in the test conditions and those in space can influence on the thruster operation and performance. Particularly this difference could be responsible for the increase of the discharge current oscillation amplitudes after thruster switching on and further operation of the two SPT-100B in pair during their tests in the vacuum chamber in comparison with the case of switching on and operation of one such thruster in the same vacuum chamber. This event was obtained at the Russian JSC "Information Satellite Systems"(ISS) and initiated this study. Preliminary analysis had shown that the possible reason of the mentioned event is an increased release of the gases and sputtered material of the vacuum chamber wall due to their bombardment by the increased accelerated ion flow from the two thrusters. These products are able to penetrate into working volumes of the thruster parts and change the properties of surfaces of the mentioned parts such as a cathode emitter or discharge chamber walls. As a result they can change the thruster operation and its characteristics. The rate of the mentioned gas release of the adsorbed or absorbed gases and sputtered products depend on time and state of the internal vacuum chamber wall surfaces being in contact with atmosphere. Then, it is to be dependent on the history of the earlier electric thruster test before the given one because the accelerated ion flow are cleaning the mentioned wall surfaces. Taking all the mentioned into account the given investigation consisted of the study of the SPT-100 type thruster output parameters variation and discharge current oscillation characteristics in time during at least 100 hours of operation in the two different vacuum chambers of 2 m in diameter and 3m (chamber 1) and 5 m (chmber2) in length, respectively. The internal walls of these chambers had different state of their internal surfaces because the walls the chamber 1 was staying in contact with atmosphere around 6 months after test of the SPT with powers not exceeding 1kW. And chamber 2 stayed in contact with atmosphere around 3 months after test of the ion thruster model operated with power 10-13 kW and ion energies 5 keV around 50 hours. Thus, the vacuum chamber wall internal surfaces of these chambers were cleaned to different state due to different intensity of their bombardment by the ion flows during previous tests.

To estimate the rate of the sputtered products condensation on the discharge chamber wall internal surfaces there were installed the removable ring-shape internal reference samples (IRS) into the external and internal discharge chamber wall parts in between anode and eroding their parts in such a manner that they were not changing geometries of the mentioned walls. The IRS were made of the same ceramics as that of the discharge chamber. Then, to estimate the condensation rate of the sputtered from the vacuum chamber wall products on the exit side surfaces of the external magnetic poles there were installed the external reference samples (ERS) made of the same ceramics as that of the discharge chamber or made from the stainless steel. There was mounted also one Langmuir probe near one of ERS to estimate the plasma parameters near the surface of the external magnetic pole. The experimental study was made during 150 hours in the chamber 1 (cycle 1) and during 100 hours in the vacuum chamber 2 (cycle 2). During each cycle thruster model was switched on and thruster operated during 3-5 hours with the discharge voltage 300V and mass flow rate ensuring the discharge current 4,5A optimized by magnetization currents. And there was realized registration of the pressure in the vacuum chamber, thrust, discharge parameters and discharge current oscillations. There were made also periodic measurements of the plasma parameters by probe. After every 25-30 hours of thruster operation the vacuum chamber were opened and IRS and ERS were weighed.

Obtained results had shown the following:

— during 1st ~50 hours of thruster operation in the vacuum 1 there were obtained jumps of the vacuum chamber pressure after thruster switching on and there was obtained increased level of the discharge current amplitude which was regularly reduced in time. Such jumps was not observed during the cycle 2 tests;

— at the internal part of the acceleration channel walls the flows of the sputtered from the exit parts of the discharge chamber wall are drastically dominating in comparison with the vacuum chamber wall sputtering product flows;

— performance level was a little bit higher during cycle 2;

— the ceramic and metal ERS samples are slowly sputtered.

Finally, it was concluded that the most probable reason of the oscillation amplitudes increase during 1st period of the thruster pair operation in the vacuum chamber after its internal wall contact with atmosphere is the increased release of the active gases from the vacuum chamber walls being long time in contact with atmosphere under their bombardment by the increased and widened ion flow.

Keywords:

stationary plasma thruster, output parameters, discharge current, oscillations characteristics, ground-based tests, vacuum chamber, ion bombardment, chamber walls cleaning, effect of gaseous release

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