Hall Thruster Clusters

The development of high-power Hall thrusters falls into two categories: one case involves investigating single, monolithic thrusters, while the second case involves clustering several small thrusters. Generally, clustering is favorable because of several merits including a cheaper manufacturing cost, less demanding requirement from test facilities, more robustness and an ability to tolerate failure of single thrusters.

The right figure shows a cluster of 4 BHT200 Hall thruster in operation(Courtesy of PEPL, the University of Michigan).

There are several major interests in numerical simulation of plasma flows from a cluster of Hall thrusters. One interest is to investigate the plume interactions, especially in the complex and important near field locations. The performance of a thruster in a cluster may be different from a stand alone situation. Another interest is to estimate plume impingement, which involves high-energy ions and Charge Exchange (CEX) ions, onto sensitive spacecraft surfaces such as solar arrays. When a fast ion collides with a slow neutral, one or two electrons may transfer from the neutral to the ion, resulting in a slow ion and a fast neutral. Under the electric field, this ion may drift behind the thruster. Severe impingement of ions onto spacecraft surfaces may result eventually in failure of devices or even a final failure of the whole mission. If severe impingement is predicted, then a change of design philosophy must be considered to reduce the impingement.

In this study, several three-dimensional particle simulations are performed to study the plume flows from a cluster of Hall thrusters. The direction simulation Monte Carlo(DSMC) method is used to model the collisions and movements of neutrals while the Particle-In-Cell(PIC)method is used to model charged ion movement in the flowfield. The electron is modeled with a detailed fluid model proposed by Boyd and Yim, which is much superior than the most widely used, and the simplest Boltzmann relation. These simulations adopt three-dimensional unstructured meshes.

These simulations yield some quite interesting results. For example, the right figure shows the ion number density taken at the station X=2cm downstream with 4 BHT200 in operation.

Note that the contour increments are not uniform in this result. With a cluster of four thrusters in operation, a special three-dimensional structure result in the ion number densities. With two thrusters in operation, the middle point between the two thrusters is an unstable saddle point which has a lower potential than the plume core, but a higher potential value than the far field. Hence, slow ions diffuse along all directions, there exists two spots close to the cathodes with relatively high density of slow ions due to the CEX effects. However with four thrusters in operation, the topographic pattern of ion density in the near field is quite different. The four thruster centers represent high potential values because of large amount of ions in the plume cores. When CEX happens, slow ion particles may freely diffuse away along the electric field direction opposite to the cluster center, hence low ion densities exist along four major diffusion directions of 45 degree, 135 degree, 225 degree and 315 degree, as indicated by this figure. Meanwhile, a portion of slow ions may travel along the electric field direction to the cluster center, which has a relatively lower potential value than the plume core and a zero value of electric field. With more slow ions accumulated at the cluster center, a special cusp shape is formed at the cluster center. There are slow ions coming from the four thruster plume beams, and at the same time, there are slow ions escaping through the four low potential gaps between the thrusters, i.e. 0 degree, 90 degree, 180 degree and 270 degree. These escaping slow ions form a special pattern of a four-leaved clover. There are four small secondary leaves in the contours as well, which are generated by the cathode. A relatively large amount of slow ions are created around the cathodes, without electric field effect, they should diffuse upwards or downwards. However, with the effects of the electric field, these slow ions diffuse outside along the four directions with the strongest electric field strength: 45 degree, 135 degree, 225 degree and 315 degree. In the near field, a special topography is formed: four strong plume cores with a low potential at the cluster center, 4 major leaves and 4 secondary leaves represent diffusion directions for the slow ions. Further downstream, the strengths of these four beams decrease quickly and eventually merge into one plume. This figure clearly illustrates the CEX effects, clustering effects and cathode effects.

Acknowledgments

The work is partially supported by the Air Force Office of Scientific Research through Grant F49620-03-1-0123 and FA9550-05-1-0042.

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