Development of the DRACO Code for Modeling Electric Propulsion Plume Interactions

COLISEUM is a plasma simulation package for modeling electric plume interactions currently being developed by Air Force Research Lab, MIT, and Virginia Tech. One of the major components of COLISEUM is the DRACO code. DRACO, designed to perform firstprinciple based, high fidelity simulations of plume spacecraft interactions, include several simulation modules. As the purpose of DRACO is to simulate real engineering problems, DRACO reads in spacecraft configurations defined by commercial CAD tools. A unique feature of DRACO is that it incorporates the recently developed immersed finite element particle-in-cell (IFE-PIC) algorithm. This method allows one to use a Cartesian mesh to handle realistic spacecraft geometry without scarifying the accuracy in electric field solutions. The computational speed of an IFE-PIC simulation is about the same as that of standard PIC simulation. DRACO is cross platform and runs on Windows, Linux, and Unix. This paper presents an overview of the DRACO code and presents simulation results for simulation of CEX backflow in a vacuum tank as well as plume/spacecraft interaction in the presence of a charged plume shield. The Air Force Research Laboratory (AFRL) is sponsoring the development of a flexible, user-friendly, plasma computational package called COLISEUM. The core library of COLISEUM provides the rudimentary input/output support to several plasma simulation packages. The complexity of the simulation packages ranges from a simple ray-tracing algorith, through a prescribed plume model to several ES-PIC simulation modules. The DRACO module, being developed at Virginia Tech, is described in this paper. Additional information about COLISEUM and its simulation packages is available in Ref. 1-2. DRACO is a multi-purpose electro-static, particle-in-cell (ES-PIC) plasma simulation package. As shown in Fig. I, DRACO allows the user to choose from several Poisson solvers. The quasi-neutral solver obtains the potential by assuming the Boltzmann distribution for the electron density and a constant electron temperature. The quasi-neutral solver is intended for quick calculations and cannot be used to resolve the plasma sheath. The DADI solver uses a standard finite-dierence formulation to solve the electric field. It is designed for problems with relatively simple geometries. The Immersed Finite Element (IFE) is DRACO’s most sophisticated solver. IFE is based on a finite element formulation, and is designed to perform simulations accurately for problems involving complex geometric and material boundary conditions. Instead of using a complex body-fitted mesh, the IFE method uses a structured mesh without consideration of the object surface location. Thus the standard, Cartesian mesh based method for particle-mesh interpolations and pushing particles can be used even in simulations involving complex geometric boundaries. This allows DRACO to retain the computation speed of a standard PIC code. Many of DRACO s subroutines are based on 3-D plasma simulation codes previously developed by J. Wang to simulate ion thruster plume interactions 4 and ion optics plasma flow. 5 The legacy code produced results that were in excellent agreement with data from Deep Space 1 in-flight measurements and the long duration test of the NSTAR thruster.