Upper limit to electric charge confinement? The following questions are not intended for conventional capacitors (which stores excess charge on the outside surface), rather a type of hypothetical capacitor which is designed to confine charge by trapping it inside the cavity of a closed dielectric.  Lets suppose it is spherical.
1.) Does there exist a well known upper limit to the charge density which can be achieved in the lab?  
2.) What are the mechanical limitations or physical constraints commonly encountered in practice which forbids the experimentalist from constructing an unbelievably strong static electric field?
3.) What are well known methods for constructing devices which can confine charge indefinitely?
4.) How does one avoid voltage breakdown?
5.) The graphic below is just an example of an extremely basic design of a device I am envisioning which may store charge.  The outside is some dielectric and its sole purpose would be to keep the charges confined. Are these sort of devices practical?

 A: The first two parts of the question seem similar to this previous question
i.e. the limiting charge density on a conductor would be determined by when the energy of the electric field outside the conductor  became comparable to the work function of the conducting material.
Capacitors are devices that store charge. They require the presence of two conductors at different electrical potentials. I'm not aware of any capacitor design that can store charge indefinitely (i.e. forever), all capacitors will have some amount of shunt resistance and thus self-discharge over time. 
The limiting capacitance of a device is determined by its geometry and the dielectric material it contains. Dielectric breakdown is one of the limiting factors in how large a capacitance is possible -there isn't a way to avoid it.
The device in the drawing looks like just a conducting shell immersed in an effectively infinite dielectric. It wouldn't function as an effective capacitor, compared to a capacitor with two conducting surfaces at different electrical potentials.
Such a configuration does have some self-capacitance, but unless the radius of the sphere were very large, it would not be significant compared to a ordinary capacitor.
To see this, suppose we made a capacitor by placing a smaller sphere inside the larger sphere. Let the larger sphere have radius $b$ and the smaller radius $a$.
It can be shown (from Gauss' Law) that the capacitance  of the concentric spheres would be
C= $\dfrac{4\pi\epsilon} {{1\over a } - {1\over b}}$
So the self capacitance of a single conducting sphere would be given by taking the limit as $b$ goes to infinity, which gives
C= $4\pi\epsilon a$
For any practical configuration this would be much less than the two concentric spheres configuration.
