# Why might krypton have a lower utilization fraction than xenon for ion propulsion, and what can be done to improve it?

A comment below Why will Starlink satellites use krypton instead of xenon for electric propulsion? links to the 2011 preprint A Performance Comparison of Xenon and KryptonPropellant on an SPT-100 Hall Thruster; IEPC-2011-003 which explains that in this study using a SPT-100 Hall Thruster krypton had a lower utilization fraction than xenon.

The methodology section says:

Studies of krypton propellant have documented that increased flow rate will improve the propellant utilization fraction.13 A lower propellant utilization fraction is one of the major reasons krypton performance has been known to be inferior to that of xenon. A promising feature of krypton is that it can potentially have a higher specific impulse than xenon due to its lower atomic mass. Therefore, exploration of operating conditions at higher than nominal discharge potential was of interest (specific impulse is proportional to the square root of discharge voltage) to see if advantageous specific impulse could be realized in spite of krypton’s inferior propellant utilization.

Basically a larger fraction of krypton propellant remained unionized, so while it contributed to mass loss ($$\dot{m}$$) it was not electrostatically accelerated and so did not exit with the roughly ~30 km/s exhaust velocity.

Questions:

1. Why might krypton have a lower utilization fraction than xenon for ion propulsion?
2. What can be done to improve it?

Answers to What performance specification would be lower for Krypton than for Xenon in Hall effect thrusters? may serve as useful starting points for an answer here.

• From a cursory glance, the SPT-100 was optimized for Xe, not Kr. From experience, getting the best current out of an ion source requires tweaking the source parameters (gas flow, magnetic field, aperature, etc.) which does not seem to have occurred here - they took the SPT-100 and ran Kr in it. – Jon Custer Jun 1 '20 at 18:55