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

Question

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.

Answer 1

As stated in High-Specific Impulse Hall Thrusters, Part 2: Efficiency Analysis (section "Krypton Operation"), the lower propellant utilization efficiency of Krypton is due to its higher ionization energy when compared to xenon (13.99 eV instead of 12.12 eV), which drains more power from the discharge to sustain a plasma with the same density. On the other hand, as the authors say, the fact that the second ionization energy of krypton is also higher (24.35 eV instead of 20.97 eV), decreases the presence of doubly-charged ions, potentially increasing the lifetime of the Hall thruster.

Besides offering higher specific impulse, krypton may prove to be a better propellant than xenon at high-specific impulse because the thruster lifetime should be higher with krypton. Whereas ionizing krypton requires more energy than xenon (which decreases the mass utilization efficiency), this attribute also decreases the production of multiply charged ions. The lower mass of krypton with respect to xenon also decreases the sputtering yield approximately by the square root of the mass ratio. If Hall thruster efficiency on krypton can be improved to approach that of xenon, a krypton thruster could benefit several interplanetary missions due to its higher specific impulse and longer lifetime. Recent investigations of several Hall thrusters operating on krypton have shown promise, where total efficiencies greater than 50% have been measured.