Electrical – Electric field in ionization area of a corona discharge

Russell's answer is good. To clarify the way corona works:

When the electric field at a point in air is strong enough, some electrons are stripped from the atoms and the air becomes a conductive plasma instead of an insulator. When an object is at a high voltage, and has a small curvature (wire, sharp points), the field in the region around it can have a high enough field strength for this to happen, forming a conductive layer of air, which is then surrounded by normal insulating air. If the strength around this conductive layer is also high enough (if the conductive region is pointy), the conductive layer can increase and increase in size (usually in one direction) until it forms a complete conductive path to another object, and then a huge current flows in the form of an arc.

Even if the shape of the field is such that it can't form an arc, it can still form a corona discharge. This is when a conductive layer of plasma stops expanding and just sits there around the wire, and ions in the plasma layer are repelled into neutral air, where they drift slowly until they discharge on another object. This is still a flow of current, just much smaller than an arc.

The corona ring changes the shape of the electric field, spreading it out in a sense, so that it is no longer strong enough at any point to ionize the air.

As the slab is brought to the plates, it will experience an attractive force that will tend to pull it into the gap. If you do not restrict it, the slab will accelerate.

In an ideal environment, the energy loss in the capacitor will be balanced by the increase in kinetic energy of the slab. Over time, this energy will be lost to friction or drag.

Once the slab is at rest inside the plates, work must be done to remove it against the electric field. This work will raise the energy stored in the capacitor.