Is a magnetic assisted takeoff feasible?
Catapults are used to launch airplanes on aircraft carriers, most are currently steam powered but the US Navy are developing an electronic catapult system called EMALS to replace steam ones in the future. Other countries are developing the same technology. The US Navy system has been shipboard tested, apparently there are still some problems but they will likely get worked out, so the technology is viable - it is possible.
Shipboard catapults are used to accelerate airplanes quickly in the very short distance you have on an aircraft carrier deck, not to save energy or fuel. It doesn't make practical or economic sense to use the technology to accelerate airplanes in a normal airfield for the following reasons:

It's very expensive technology to buy and maintain. It would cost enormous amounts of energy to deploy this technology
It will limit airplane operations to those airfields which have the technology. I can fly a Cessna 172 into and out of remote strips, but if I require a catapult of some kind my options become very limited
It doesn't actually help much. It's not the take-off roll which is energy intensive, it's the climb to altitude, and a catapult wouldn't help with that. When you think about it the take-off roll is a minute, climbing to altitude is 20 minutes in a commercial jet, you'd be reducing the energy needed by at most 5%, which isn't worth the investment