This engine was making news few years ago, but I have not heard much about it lately. Were there any updates regarding its tests, further development or anything?
There's a timely article at Ars Technica today on this.
The conclusion (unsurprisingly to me at least):
And the winner is… Physics, without much doubt. Even with a power of just a couple of Watts, the EM-drive generates thrust in the expected direction (e.g., the torsion bar twists in the right direction). If you reverse the direction of the thruster, the balance swings back the other way: the thrust is reversed. Unfortunately, the EM drive also generates the thrust when the thruster is directed so that it cannot produce a torque on the balance (e.g., the null test also produces thrust). And likewise, that “thrust” reverses when you reverse the direction of the thruster.
The best part is that the results are the same when the attenuator is put into the circuit. In this case, there is basically no radiation in the microwave cavity, yet the WTF-thruster thrusts on.
So, where does the force come from? The Earth’s magnetic field, most likely. The cables that carry the current to the microwave amplifier run along the arm of the torsion bar. Although the cable is shielded, it is not perfect (because the researchers did not have enough mu metal). The current in the cable experiences a force due to the Earth’s magnetic field that is precisely perpendicular to the torsion bar. And, depending on the orientation of the thruster, the direction of the current will reverse and the force will reverse. The researchers made some calculations, based on the location of the experiment and the amplifier current, and got a torque that agreed quite well with the measured torque.
This is, of course, not the final word. But it is an excellent cautionary tale. The thrust that the researchers measured with just a couple of Watts of power was the same as that measured previously with 50W of power. And that was all due to a shielding problem. When the amplifiers are properly mounted and the shielding is in place, it will be even more difficult to detect the thrust, because the effects of noise will grow as well. I expect a flood of null results in the next year.
Space unicorns are not your friend