Shields and Deflectors
Since the leading edge of the warp field pushes other objects aside anyway, a navigational
deflector is unnecessary: tidal forces in the leading edge would either propel any obstacle away from the ship at warp speeds or simply crush it to pieces. In other cases, though, the deflector screens
may be necessary to provide 360 degree protection even when the ship is not in motion.
The ship's deflector screens are housed in the front ends of the warp nacelles: the red domes in the TOS Enterprise and the E-D, or the complicated grillework in the Ent-A. Deflector screens work on the same principle as warp drive, so the deflector coils in the front of the nacelle are basically modified warp coils. The deflector field is different from other subspace fields in that it is mainly a flat region of space surrounded by a raised "wall" of space (take the "coffee cup" from the above example and sit it on top
of the grid instead of sunken into it). This means any matter or energy that enters the field will experience intense acceleration away from the ship until it either crosses it or is deflected away from it. At full intensity the deflectors can produce accelerations of up to 3kps^2; this is equivalent to the gravitational distortion produced by a star around ten solar masses, but with the entire effect of gravitation taking effect in a spheroid region ten meters thick.
The deflector field requires immense quantities of energy in order to reach its full output, similar to the amount of power used by the warp engines or the main deflector dish. Even he deflector screen's longevity largely depends on the survival of the gravitomagnetic field coils in the nacelle ends: as they handle more load the coils themselves experience mechanical stress and vibration (action-reaction) and tend to overheat. The hotter the deflector coils get, the less the effect of the deflector field, so the practical limit to how much load the deflectors can handle is dependent on the capacity of the ship's cooling systems. Overheating of either the deflectors or the warp coils themselves is handled by pumping coolant over the coils and then venting it through the emergency flush vents in the nacelle pylons.
Shields--different from "deflectors"--are an energy forcefield conformal to the hull of the ship. Shields are basically layers of additional armor composed of a "pseudo-material," that is, a material that has the property of being solid only in the presence of high electric potentials. The material itself is conductive and is secreted along the outer hull by the shield grid, the application of high voltage current holds the material in place. This pseudomaterial has, potentially, extremely high tensile strength as well as elasticity, but it is only as strong as the current that passes through it. Shield material is usually replaced just as quickly as it is ablated; on the other hand, loss of shield material may result in damage to the electrode terminals or gas dispensers in the hull, in which case any shield-layer damage becomes permanent until the damaged part of the defense grid is repaired. Personal and security forcefields work on the exact same operating principle, but at much lower voltages since their protective layers do not have to be as thick.