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An ICRF strap antenna solution exploiting the high impedance technique

Ion Cyclotron Range of Frequencies (ICRF) strap antennas are routinely adopted in most of the existing nuclear fusion experiments, even though their main goal, i.e. to couple high power to the plasma (MW), is often limited by rather severe drawbacks due to high fields on the antenna itself and on unmatched part of the feeding lines directly connected to the antenna. In this work, we propose, describe, prototype and measure an ICRF strap antenna based on the high impedance surfaces concept that is matched at a specific tunable frequency. The adopted high-impedance structure, positioned between the strap and the backwall, is a metallic patch displaced on top of a dielectric block and grounded by means of a vertical post, in a mushroom-like shape. This structure presents a high impedance, within a given very narrow frequency band, such that the image currents are in-phase with the currents of the strap itself, thus determining a significant efficiency increase. After a general description on the properties of high impedance surfaces applied to ICRF antennas, we describe the optimization steps, carried on by means of numerical codes, to define an antenna configuration suitable for a nuclear fusion experiment. The antenna has been then manufactured and measured; strengths and weaknesses of the proposed solution are outlined.