Since you mentioned an inaccessible room full of "plasma generators", I think I know where you are talking about.
Is that keypad controlling access to the elevator to the next level and in the same wall with 2 shielded laser wall guns and one
wall camera?
Go to the west end( to left end of the player screen), there is a small cubic with a guard guarding some valves. Kill him, and near him there is a small computer screen in the wall containing the passcode. If you fail to prevent the wall camera from raising an alarm, you have to shut that alarm first then the activated force field. Anyway, your answer lies in that very small cubic.