Emissive Probes in Plasma Propulsion: Noah Hershkowitz's Legacy

For plasma propulsion devices such as Hall thrusters, knowledge of the ion accelerating electric field is essential for understanding of how thrust is generated. Emissive probes have become a widely used tool for diagnosing these devices with flowing plasmas. This is in great part thanks to Noah Hershkowitz, who developed and refined emissive probe techniques for measuring plasma space potentials. By emitting electrons below the plasma potential, but not above it, emissive probes can be used to make measurements in a wide variety of conditions, making them well suited for electric propulsion plasmas. Noah taught the plasma community how to account for space-charge effects, which make interpreting emissive probe data more complex than it first appears [1]. He also pioneered a careful technique that uses emissive probes to provide insights into electric propulsion devices with the accuracy that cannot be obtained in any other way [2]. In this talk, we will highlight results from the Hall thruster research which we conducted together with Noah [2,3]. Noah’s remarkable technique is standing the test of time. Despite the development of recent theories of inverse sheath for the floating emissive walls [4] which clears the way for the use of a simple floating probe measurements in some quiescent plasmas [5], it remains that Noah’s method of emissive probe measurements using the inflection point in the limit of zero electron emission [1] is critically needed to minimize space charge effects particularly in flowing plasmas. Among many other areas of flowing plasmas, these conditions pertain to our own area of research, plasma thrusters [2,3,5]. 

[1] J. R. Smith, N. Hershkowitz, P. Coakley, Rev. Sci. Instrum. 50, 210 (1979)

[2] J. P. Sheehan et al., Phys. Plasmas 18, 073501 (2011)

[3] J. P. Sheehan et al., J. Prop. Power 33, B35697 (2017)

[4] M. D. Campanell and M. V. Umansky, Phys. Rev. Lett. 116, 085003 (2016)

[5] B. F. Kraus and Y. Raitses, Phys. Plasmas 25, 030701 (2018)