The Ion Drive Linkable is similar to the Ion Drive in that it uses electricity to generate thrust, but is capable of firing multiple units that are attached to the same power grid. This can be used to compensate for their low individual thrust.
It can be unlocked after completing Mission 36.
In real life
Instead of conventional engines, which combust a fuel and an oxidiser, ion engines only require a different type of fuel. Said fuel - often xenon - is ionised and accelerated it to very high speeds using an electric field, exerting a force on the spacecraft. The thrust generated is very low (25-250 milinewtons), but due to their incredibly high efficiency they can be powered for a long time to cause very large changes in velocity.