Extruded round edges don't appear smooth

Hi all,

I'm a complete COMSOL novice, so I'm probably missing something extremely obvious, however:

I am drawing essentially a circle with a rectangle cut out of it to provide a flat edge, and extrude this in the z direction so I have a flat-sided cylinder.
In the 2D workplane, the edge of the shape appears very smooth, however, after extrusion the circular top appears jagged, and not a smooth curve like it was pre-extrusion.

Does anyone have any tips/fixes for resolving this issue?
I've attached a screenshot of the image- you can see it's not very smooth.
Many thanks in advance,

Sam