Design and numerical analysis of a Birdcage resonator without lumped capacitors

To handle the high, continuously transmitted power, a birdcage resonator without lumped capacitors is created. The discussed design consists of eight partially overlapping sheets of copper on polyimide. The resulting structure (polyimide substrate between two copper layers) creates the capacitive part of the resonator, without the need for any added capacity. The fine-tuning of the resonance frequency by the change in the area of the overlapping part is described. Finally, the frequency of the fundamental resonance mode is adjusted to 300 MHz.

1. Introduction

The birdcage resonators were created for use in magnetic resonance imaging (MRI) [1]. The resonator consists of two endrings and an even number of legs, which connect the endrings together. There are two basic designs, based on the position of the capacitors: high-pass and low-pass [2]. Recently, birdcage resonators were also proposed for utilization in plasma sources [3]. However, for plasma processing, one might expect long-term operation, with hundreds of watts of transmitted power. While the power was usually delivered in the pulse regime in the field of MRI, the continuous regime is expected within the plasma source. This condition will create thermal stress on all parts of the birdcage resonator. Following the standard design, the inserted capacitors will be the weak points. The dimensions of the resonator are in the range of units or tens of centimeters, but the capacitors are usually delivered in SMD housing. Heat can be better dissipated in designs that rely on distributed capacity.