In one embodiment, an apparatus comprises a coil structure comprising a first coil and a second coil, each of the first coil and the second coil having a first side portion, a center portion, and a second side portion, wherein the first side portion forms a first angle with the center portion and the second side portion forms a second angle with the center portion, and a layer of magnetic material adjacent to the center portion of the first coil and the center portion of the second coil, the first coil and the second coil being configured such that when a current flows in a first spatial direction in the first coil a current flows in a second spatial direction different from the first spatial direction in the second coil. In one embodiment, the apparatus further comprises a power circuit configured to provide a time-varying current to the coil structure and a battery configured to provide a direct current to the power circuit.