Probably such things have been hashed out before, but allow me to
solicit the opinion of the readership on this question that has
come up in some off-line discussions:

Supposing you wanted a reactive propulsion system -- say a
nuclear rocket, electrical thruster, mass driver or other
throw-mass-out-the-back widget that had continuous thrust =
1 kN, ISP = 10,000 sec (= 100 km/sec Ve) and a total run
time = 1 megasecond or greater, preferably getting toward
10 Ms.

If it needs a reactor or other power supply, plumbing,
radiators to get rid of excess heat, etc., ignore that and
concentrate on the thrust-producing part. It's ok to have a
system composed of shorter lifetime parts if that helps
(e.g., banks of 100 N thrusters with individual lifetimes
of 100,000 seconds that get shed as they burn out).

Does current technology or anything that can be reasonably
foreseen in the next 20 years support anything like that? If
so, what might it be?