1. Department of Aerospace (GALCIT), California Institute of Technology, Pasadena, California 91001, USA
2. UCLA Samueli School Of Engineering, University of California, Los Angeles, California 90095, USA
3. Electric propulsion group, Jet Propulsion Laboratory, Pasadena, California 91109, USA
4. Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid and IFIMAC, Cantoblanco, Madrid E28049, Spain