If we assembled an array of today's best ion thrusters in orbit, (the x3?) how many would be needed to accelerate a 1000 tonne spacecraft at 1g?
I'm thinking Space Tug, for repeated Mars missions. Nuclear powered of course.
Each thruster provides thrust, but each thruster has mass, as do the power sources needed to power them and the tanks to store their fuel.
No currently existing ion thruster is able to produce anywhere near that much thrust for its mass, and more significantly, even the best power sources (even speculative ones or those at low TRL's) don't provide enough power for the mass to power such an assembly, no matter how large it is. You're looking at milliG's or microG's of acceleration.
More generally, a thruster that was realistically even vaguely practical for accelerating at even fractional G's would almost certainly stretch the definition of "ion thruster" which is originally normally used to refer to gridded ion thrusters which have pretty low limits in thrust to weight ratio.
If you want to accelerate a multi-ton spacecraft at 1 G, then you want chemical rockets, Or if you absolutely insist, possibly an ultra high performance nuclear thermal rocket can be developed (current designs aren't really designed to accelerate at 1 G when mounted on a spacecraft, though they do have greater than one thrust weight ratio).