# When did ion propulsion first see some serious development efforts?

In the video below at about 01:35 Dr. Marc D. Rayman, Chief Propulsion Engineer, NASA JPL mentions that the first time he had heard about ion propulsion was on the Star Trek TOS third season premiere episode Spock's Brain which was first aired on 20-Dec-1968.

In reality, when did ion propulsion first see some serious development efforts? Where and when were the first experimental and first sound theoretical investigations of propulsion based ion electrostatic accelerations of ions as a reaction mass for propulsion in space?