For krypton to become a realistic option for Hall thruster operation, it is necessary to understand the performance gap between xenon and krypton and what can be done to reduce it. A floating emissive probe is used with the Plasmadynamics and Electric Propulsion Laboratory's High-speed Axial Reciprocating Probe system to map the internal plasma potential structure of the NASA-173Mv1 Hall thruster ͓R. R. Hofer, R. S. Jankovsky, and A. D. Gallimore, J. Propulsion Power 22, 721 ͑2006͒; 22, 732 ͑2006͔͒ using xenon and krypton propellant. Measurements are taken for both propellants at discharge voltages of 500 and 600 V. Electron temperatures and electric fields are also reported. The acceleration zone and equipotential lines are found to be strongly linked to the magnetic-field lines. The electrostatic plasma lens of the NASA-173Mv1 Hall thruster strongly focuses the xenon ions toward the center of the discharge channel, whereas the krypton ions are defocused. Krypton is also found to have a longer acceleration zone than the xenon cases. These results explain the large beam divergence observed with krypton operation. Krypton and xenon have similar maximum electron temperatures and similar lengths of the high electron temperature zone, although the high electron temperature zone is located farther downstream in the krypton case.

Magnetic field topology has been found to be a central design concern for high-efficiency Hall thrusters. For future improvements in Hall thruster design, it is necessary to better understand the effects that magnetic field topology has on the internal plasma structure. The Plasmadynamics and Electric Propulsion Laboratory's High-speed Axial Reciprocating Probe system is used in conjunction with a floating emissive probe to map the internal plasma potential structure of the NASA-173Mv1 Hall thruster ͓R. R. Hofer, R. S. Jankovsky, and A. D. Gallimore, J. Propul. Power 22, 721 ͑2006͒; 22, 732 ͑2006͔͒. Measurements are taken at 300 and 500 V with a xenon propellant. Electron temperature and electric field are also measured and reported. The acceleration zone and equipotential lines are found to be strongly linked to the magnetic field lines. Moreover, in some cases the ions are accelerated strongly toward the center of the discharge channel. The agreement between magnetic field lines and equipotential lines is best for high-voltage operation. These results have strong implications on the performance and lifetime optimization of Hall thrusters.

Krypton has recently become the focus of attention in the Hall thruster community because of its relatively large specific impulse compared with xenon and its potential to operate with comparable efficiencies. However, before krypton can be considered a viable propellant choice for missions, the performance gap between xenon and krypton must be reduced. A series of diagnostic measurements are taken for xenon and krypton propellant using the NASA-173Mv1 Hall thruster and the results are analyzed using a phenomenological performance model. The combined use of experiments and modeling enables a direct comparison of several efficiency components for each propellant to be made. With this method, it is possible to pinpoint the exact causes for the efficiency gap between xenon and krypton. It is also possible to see the effect of the magnetic field topology on Hall thruster performance and where gains are being made due to the magnetic field. Although there is a large series of competing components that differentiate krypton and xenon performance, the largest factors that dictate the efficiency difference between krypton and xenon are krypton's inferior propellant utilization and beam divergence. For xenon, the propellant utilization is 5-10% higher and the beam divergence efficiency is approximately 8% higher.