Development and research of the stationary plasma thruster with a hollow magnet anode
Aerospace propulsion engineering
Lomonosov Moscow State University, 1, Leninskie Gory, Moscow, 119991, Russia
The problem of wide range F ion sources (IS) constructing is very relevant nowadays. These sources are used in technological processes such as sputtering, cleaning, ion assistance, surface modification and as an electric propulsion system. The gridded ion sources are used in the applications where the ions with energies 200-1000eV are necessary. However such sources have limited service life. Possible alternative is an RF gridless ion sources developed by R.Boswell and C.Charles during the last decade. Its operational principle is based on the dense plasma production under conditions of the helicon waves excitation and ion acceleration in the double layer at the outlet of the IS. The maximal achieved ion energy is about 120eV.
The goal of the present work is to develop the low-energy IS with long service life and wide energy range (10-300eV). To do so three modifications of the RF gridless IS have been constructed and studied experimentally. The RF inductive discharge located in the external magnetic field was used as the operational process of the IS. The magnetic field value corresponded to the resonance conditions of helicons and oblique Langmuir waves excitation. The acceleration of ions took place due to the potential drop formed near the outlet of the IS. Besides the double layer formation due to the magnetic plasma contraction similar to R.W.Boswell experiments, potential drop was enhanced using an additional geometrical plasma contraction. For this purpose two IS configurations where provided with constrictions of different diameters near the outlet. The third IS configuration was constructed for checking the possibility of ions acceleration due to the potential drop appearing near electrodes in the capacitive RF discharge. To do so a coaxial electrode was mounted near constriction and connected to the inductor lowest coil. Between electrode and inductor a variable separating capacity was placed. This provided the existence of RF voltage between the lower part of the inductor and coaxial electrode. The RF voltage in the capacitive channel of the discharge could be controlled by the separating capacity value. In order to organize the acceleration of ions parallel to the IS axis the diameter of constriction was made smaller than the characteristic dimension of the near electrode sheath.
Experiments showed that the existence of the constriction at the outlet of the IS leads to the mean ion beam energy increase. The smaller the constriction and longer, the energy of the accelerated ions is the bigger. The idea to increase the potential drop near the outlet of IS due to electronegative self-bias effect using IS with capacitive channel was experimentally confirmed with average improvement of 20-60eV. As the result of the work an effective ion source with independent flexible control options has been developed. Such device can vary ion beam energy from 10 to 300eV and ion current value — from 0 to 250µA/cm2
Keywords:electric propulsion systems, ion thrusters, radio frequency, inductive discharge, helicons, oblique Langmuir waves
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