1. Ion Emission Energetics from a Charged Hollow Cathode Plasma Contactor System
Grant Miars1, Omar Leon 2 , Brian Gilchrist1, Gian Luca Delzanno3
1Department of Electrical Engineering, The University of Michigan, Ann Arbor, MI
2Applied Physics Program, The University of Michigan, Ann Arbor, MI
3Applied Mathematics and Plasma Physics, Los Alamos National Laboratory, Sante Fe, NM
Results and Discussion
Introduction
Simulations suggest that a hollow cathode is able to balance the current of active electron beam emission in the absence of a background plasma.[1] In this system, ions are emitted from the quasi-neutral contactor plume surface to balance the electron beam current. Maximum ion emission from the plume surface is space-charge limited and determined by the ion drift velocity, ion temperature, spacecraft potential, and plasma sheath thickness according to the Child-Langmuir Law. Based on these parameters and the surface area of the quasi-neutral plume, one can determine if ion emission is capable of balancing the electron beam current. Below we present experimental results from a ground-based hollow cathode system which investigates the relationship between spacecraft potential and the ion energetics which define maximum ion emission.
Table 1. Net current emission densities as measured by planar probes at various locations on the chamber wall. Biasing the S/C positive leads to net ion emission at all planar probe locations.
Materials and Methods
Figure 5. Radial IEDFs at varying biases. Radial ion drift energies are very low and unaffected by S/C bias. This suggests the sample population is largely charge exchange (CEX) ions.
Conclusions
Figure 3. Sample planar probe I-V sweeps at various S/C biases. Shifting the sweeps by the local plasma potential causes the transition regions to overlap. This signifies that the emitted ion energies are increased by the plasma potential. Electron depletion is also evident at large biases in the electron saturation regime.
Ion Emission Current is seen to increase with S/C bias
Electron depletion is also observed with increasing S/C bias
Emitted particles acquire the energy of the sheath drop by the time they reach the wall
Ions acquire the keeper potential energy as drift energy in the axial direction
Ion temperature appears insensitive to S/C charging
Ions do not have significant drift in the radial direction and it is likely this population is dominated by charge exchange ions
Ion emission occurs primarily downstream of the hollow cathode for positive S/C charging
Figure 1. Planar probe mounted on the chamber wall with copper tape used to limit sheath expansion (left). RPA and emissive probe positioned to determine directional IEDFs (right).
Acknowledgements
I wish to thank Brian Gilchrist for his guidance and support throughout my research. I would also like to acknowledge the PEPL team for their help during the setup and implementation of these experiments. Finally, Gian Luca Delzanno, Joe Borovsky, and Federico Castello of Los Alamos were instrumental in guiding our experiments and increasing our understanding of the problem.
Work supported by the Center for Space and Earth Science & Los Alamos National Laboratory
References
Figure 2. Probe locations relative to the hollow cathode plasma contactor. The RPA locations within LVTF (left) and planar probes within CTF (right) are shown.
Figure 4. Axial IEDFs at varying biases. Ion temperature is fairly constant and a high energy tail is evident. The ion drift energy is set by the potential drop from the hollow cathode keeper.
[1] G.L. Delzanno, J.E. Borovsky, M.F. Thomsen, and J.D. Moulton, J. Geophys. Res. Space Physics. 120, (2015).