A coil assembly for controlling a magnetic field in a plasma chamber is provided herein. In some embodiments, the coil assembly may include a mandrel including an annular body that includes at least one upper body coolant channel and at least one lower body coolant channel, and a plurality of cooling fins disposed circumferentially about an outer diameter of the body and radially outward from the outer diameter, an inner electromagnetic cosine-theta (cos θ) coil ring including a first set of inner coils wrapped around the plurality of cooling fins in the body and configured to generate a magnetic field in a first direction, an outer electromagnetic cosine-theta (cos θ) coil ring including a second set of outer coils wrapped around the plurality of cooling fins and configured to generate a magnetic field in a second direction orthogonal to the first direction.
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