One problem with HETs is that their plumes diverge quickly and thus can impinge on other spacecraft parts leading to thermal and contamination problems.
Example: Hall thrusters typically operate at over 50% thrust efficiency, provide specific impulse from 1200-1800 seconds, and thrust to power ratios of 50-70 mN/kW.
The plasma gas is typically xenon because of its high molecular weight and low ionization potential.
See also: spacecraft propulsion