Technology of closed-drift thrusters

Abstract

Introduction A CLOSED-drift thruster is defined herein as a thruster in which ions are electrostatically accelerated in essentially the thrust direction, with the accelerating electric field established by an electron current interacting with a transverse magnetic field. One component of the electron motion is counter to the ion flow. Another component is normal to that direction. The current associated with this normal component is called the Hall current. In a closed-drift accelerator there is a complete, or closed, path for the Hall current. In addition, for the ions to be accelerated in essentially a single thrust direction, the ion cyclotron radius must be much larger than the total acceleration length. Closed-drift thrusters usually employ axially symmetric electrodes and pole pieces, with the magnetic field in the radial direction and the electric field in the axial direction. The Hall current flows in a circular closed path in such a configuration. A few closed-drift thrusters without axial symmetry have also been investigated. The closed-drift thruster is particularly well suited for operation in the 1000-2000 s range of specific impulse (approximately 10,000-20,000 m/s exhaust velocity). It is difficult to operate above about 1000 s with an electrothermal thruster due to excessive excitation and ionization losses. On the other hand, the space-charge-flow limitations of gridded electrostatic thrusters will not permit practical ion current densities below about 2000 s. Within the 1000-2000 s range, the electron backflow required to establish ion acceleration can, for the most part, be used to generate ions. The generation of ions constitutes the major closed-drift thruster loss in this range of specific impulse, and this loss can be under 100 eV per beam ion. The power processing requirements are also moderate. In a properly designed closed-drift thruster only one power circuit is required for steady-state operation, with the voltage of this circuit typically in the 50-500 V range.