AC purturbation on top of a DC voltage

Please see the attached MPH file for drawing.

1. How to excite a slab with an AC perturbation voltage on top of a DC bias voltage in COMSOL? Let's say I have a slab, I want to apply voltage on both ends as,

2. How do I properly apply the fixed constraint? If I apply, let's say, a dc voltage to the slab on both sides, it will be squeezed from both ends.

3. How to incorporate an infinite artificial domain around the slab so that the electric field is zero at infinity?