"A new concept for in-space propulsion is proposed in which propellant is not ejected from the engine, but instead is captured to create a nearly infinite specific impulse. The engine accelerates ions confined in a loop to moderate relativistic speeds, and then varies their velocity to make slight changes to their mass. The engine then moves ions back and forth along the direction of travel to produce thrust. This in-space engine could be used for long-term satellite station-keeping without refueling. It could also propel spacecraft across interstellar distances, reaching close to the speed of light. The engine has no moving parts other than ions traveling in a vacuum line, trapped inside electric and magnetic fields."
https://www.nextbigfuture.com/2019/10/nasa-inertial-drive-with-a-helical-engine-using-a-particle-accelerator.html
"In a paper posted in NASA’s Technical Reports Server – a public database containing aerospace documents created or funded by NASA – Burns explained that the helical engine consists of a helix-shaped particle accelerator, an apparatus that produces a beam of fast-moving, electrically charged particles.
Burns, an engineer for NASA’s Marshall Space Flight Center, said that this particle accelerator can propel a spacecraft even without propellant."
https://physics.news/2020-11-04-helical-engine-paves-way-fuel-less-engines.html#
"But the helical engine, according to Burns, could make space travel faster and fuel-free. The apparatus works by accelerating ions confined in a loop. The ions are driven at near the speed of light to produce thrust and then decelerated to keep the spacecraft moving forward. The engine, according to Burns, has no moving parts other than the ions moving in a vacuum line.
“This in-space engine could be used for long-term satellite station-keeping without refueling,” said Burns. “It could also propel spacecraft across interstellar distances, reaching close to the speed of light.”"
https://futuretech.news/2020-11-04-helical-engine-paves-way-fuel-less-engines.html#
A NASA Engineer Wants to Use a Particle Accelerator to Power Rockets
https://www.popularmechanics.com/science/energy/a29443247/particle-accelerator-power-rockets/