Most effecient ion thruster

I am providing the following link in order to help answer this question. It is a link to the website of an agency called Qinetiq, who is one half of what used to be D.E.R.A (Defence Evaluation and Research Agency) They build Ion thrusters among other things. This particular thruster (the T5) may or may not be the most efficient available, I leave it to you to decide. www.qinetiq.com/home/defence/defence_solutions/space/ep/t5_ion_thruster.html