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The MSD is a device used to cancel the natural telepathic abilities of certain species, such as Betazoids. It functions by creating random static bursts on the frequency at which telepathic powers function. At lower levels it can be a distraction or completely block telepathic powers, at higher levels it can cause severe pain or even unconsciousness.
The MSD field requires no medium except the aether, and the size of the field is dependant on the size of the generator. To fill a small building, a generator only roughly the size of a cat is needed. To cover a large ship (such as a Galaxy-class) requires a generator about 5x5x5 meters, and to affect other ships in the vacinity it is often necessary to use an entire cargo bay for the generator. To protect a single individual would only require a generator about the size of a ring.
It is suggested that this be incorporated into a 7th Fleet scenario in which the commanding officer of the ship which encounters it is telepathic (for instance, Admiral Llerroc aboard the USS Exploration is a Betazoid).
Not long after that encounter, Starfleet could develope a primitive version of it which is not as compact and declare all MSDs capable of being set to a level high enough to cause pain to be illegal (ie just completely block powers is maximum legal setting), except for authorized Federation research teams.
~Jeremy Lennert, Simon Dark of the 7th Fleet