For a DAWN-like solar-electric deep space mission, would using propellant particles much heavier than xenon atoms be beneficial?

Question

For deep space missions using ion propulsion that are power-limited, this answer explains that a large mass/charge ratio ($m/q$) species is desirable. The solar-electric propulsion system used by the DAWN mission to Ceres at ~3 AU) and its immediate progenitor NSTAR (used by Deep Space 1) used xenon atoms as Xe⁺¹.

Do Xe⁺¹ have the ideal $m/q$ ratio or would even heavier particles be better for power-limited missions from a propulsion point of view? It's certainly possible to produce singly ionized molecules that are heavier than xenon atoms, but one can even imagine nanoparticles, liquid droplets or even ball-bearings with a single charge.

Question: For a DAWN-like solar-electric deep space mission, should propellant be particles much heavier than xenon atoms? Ignoring practicalities of producing plasmas with larger molecules, does the benefit of using larger and larger $m/q$ ratio particles continue without limit?

Answer 1

Ion thruster

Ion thrusters use beams of ions (electrically charged atoms or molecules) to create thrust in accordance with momentum conservation. The method of accelerating the ions varies, but all designs take advantage of the charge/mass ratio of the ions. This ratio means that relatively small potential differences can create high exhaust velocities. This reduces the amount of reaction mass or propellant required, but increases the amount of specific power required compared to chemical rockets. Ion thrusters are therefore able to achieve high specific impulses.

The point of ion thrustsers is to increase exhaust velocity to reduce reaction mass, at the cost of energy, taking advantage of infinite energy of the sun, to improve specific impulse over chemical engines. Here m/q is irrelevant. The key is low reaction mass flow and high exhaust velocity. For the same thrust, specific impulse requirement and power budget, you can certainly use heavier particles as long as you eject them fast: you simply eject heavier particles less frequently.

In other words, if you would use canon balls you might as well electro-mechanically eject them, e.g. instead of a particle accelerator that ejects ions continuously, you have a rail gun that shoots one canon ball a day.

But there are limitations: even the best rail gun now only have a muzzle velocity of 3km/s, which is below the typical exhaust velocity of chemical rockets. In other words, at this scale, you have lowered specific impulse so much that, for the same thrust you have so much reaction mass that it can sufficiently store energy chemically, i.e. instead of inert projectile plus electric power, you can do better with chemicals, and there you have an chemical rocket.

Finally, and maybe more importantly, ionizing is not free and contributes to the efficiency loss of the thruster. Higher m/q usually means cheaper ionizing per unit mass, which is why heavy particle is preferred over extremely light ones like hydrogen. I'm not an expert in this area but I think you are asking a very valid question: even though m/q is irrelevant for thrust and specific impluse, there should be more economical particles for ionizing that can and should be (and probably is being) explored.