Cyclotron engines as a means of thrust for interplanetary travel?

Eject the particles at relativistic speeds, into a magnetic decelerator to generating a vector force in the opposite direction. Matter at that speed will increase in mass, to infinity actually, so a small particle could have substantial force through the deceleration cycle, relative to the energy input. Seal the entire unit, then move the particles back to the cyclotron at lower mass/velocity for recycling into the process.

The net effect would be particles at very high speed and mass moving in one direction for some period of time, then moving back the same distance, at low speed, in the opposite direction into the cyclotron for reacceleration with a 0 net force until they enter the decelerating field again. Eventually the particles would be exhausted then reused in a pulsating manner, over and over again forever. A self contained force generating device that could accelerate great mass to relativistic speeds with no matter input or loss, just energy, leveraging the ability of matter to increase in mass as it accelerates.

To over come the torque effects, just use to counter rotating cyclotrons, vary the particles to each in the beginning after each pulse cycle to correct for any imbalances. One could even use four cyclotrons, two counter rotating in the y axis and two in the x axis to control pitch, yaw and roll. Six in three axis would give you the ability to move in any direction, pulse cycles permitting.

A cyclotron is a type of Ion engine. Cyclotrons (in the classical sense of the word) haven't been used because they have a very low power-to-weight ratio (tons of magnet to accelerate tiny amounts of matter).

There are ion engines that use the Electron cyclotron Resonance principle; these use the cyclotron princple at a tiny scale to create ions that are then accelerated in a linear accelerator.