A plasma source is provided including a core element extending from a first end to a second end along a first axis. The plasma source further includes one or more coils disposed around respective one or more first portions of the core element. The plasma source further includes a plasma block having one or more interior walls at least partially enclosing an annular plasma-generating volume that is disposed around a second portion of the core element. The annular plasma-generating volume includes a first region that is symmetrical about a plurality of perpendicular axes that are perpendicular to a first point positioned on the first axis, the first region having a width in a direction parallel to the first axis and a depth in a direction perpendicular from the first axis. The first region has a width that is at least three times greater than the depth of the first region.