It's a particle accelerator, which was a well-known technology back when Star Trek was produced, and bears no resemblance to the antimatter reactor depicted on the left, which is not a real technology and likely never will be. Partly because of the problems of safely containing antimatter (indeed, it seems every other episode the Enterprise is in danger of spontaneous combustion for this exact reason), but mostly because antimatter doesn't even exist in nature (a fact which is puzzling to physicists, but damned inconvenient for starship captains looking for a place to refuel).