Study of the influence of azimuthal magnetic field nonuniformity inside the discharge chamber on the ion thruster performance

Аuthors

Lovtsov A. S.^*, Selivanov M. Y.^**, Belikov M. B.^***

Keldysh Research Centre, 8, Onezhskaya str., Moscow, 125438, Russia

*e-mail: alovtsov@mail.ru
**e-mail: michael.selivanov@gmail.com
***e-mail: mich-bor@mail.ru

Abstract

Configuration of magnetic field in the discharge chamber of ion thruster is traditionally considered in the axisymmetric approximation. This approach is reasonable for the thrusters with permanent magnet rings (ring cusp thruster). However, as it is shown hereinafter, the Kaufman thrusters using magnetic systems with electromagnets placed around the discharge chamber periphery have significant azimuthal nonuniformity of magnetic field. This nonuniformity can lead to the reduction of magnetic field capability to confine electrons from direct loss to the anode. There are two reasons of such reduction. First, magnetic field has too low strength in the regions between magnetic coils to confine electrons efficiently. Second, new mechanism of electron drift appears in the nonuniform magnetic field.
In this paper, two magnetic system configurations of the ion thruster IT-300 are investigated. The first one has 12 coils with the current I = 3 A, the second one has 6 coils with the current I = 6 A. According to the magnetic field measurements, such configurations have the same average magnetic field strength (~3,7 mT) and different degrees of azimuthal nonuniformity.
Laboratory tests were conducted for two investigated configurations for the purpose of determination of radial and azimuthal distribution of the beam current density. This distribution is characterized by the nonuniformity coefficient being a very important parameter in terms of the thruster lifetime. Measurements were carried out using five Faraday probes. The probes were placed on a mounting bracket so that the distances between the accelerating grid and collector of each of the probes were the same. Collector was made of molybdenum and had the collecting surface diameter of 10 mm.
Tests to determine relationship between the discharge loss and mass utilization efficiency were conducted for the estimation of discharge chamber operation efficiency. Tests were carried out with the accelerating voltage of 2500 V and beam current of 200 1400 mA.

Keywords:

ion thruster, magnetic field, laboratory tests

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