I think you are running into a “list of lists” issue. There are 2 options that you could do, (as your code exists).
1 : use an additional python component by simply setting a = x
2: use an additional import statement to access a “list to data tree” function (Rhino 6 only)
import rhinoscriptsyntax as rs
import ghpythonlib as gh
start = S
step = N
count = C
radii = [start + (i * step) for i in range(int(count))]
a = []
for i in radii:
circles = rs.AddCircle([0, 0, 0], i)
a.append(circles)
b = []
segs = E
for i in a:
pts = rs.DivideCurve(i, segs)
b.append(pts)
a = gh.treehelpers.list_to_tree(a, True, [0])
b = gh.treehelpers.list_to_tree(b, True, [0])
