A plasma lamp for an electrodeless plasma lamp having a shaped dielectric waveguide body. The shaped body may have a relatively thin region containing a bulb, and a second region thicker than the first region. Microwave probes may be positioned in the second region to provide power to the waveguide body. The body may be shaped to intensify the electric field in the first region adjacent to the bulb to allow operation at a lower frequency than a solid cylindrical or rectangular waveguide body having the same volume and dielectric constant.