ICRF Heating in Alcator C-Mod:

Alcator C-Mod, the high field, high density, diverted, compact tokamak in the world's portfolio of high performance plasma fusion devices, is heated exclusively with ICRF auxiliary power. In this paper an overview of recent results is summarized, with particular attention given to the importance of RF operation and the flexibility afforded by different heating scenarios. Besides the routine minority heating operation, results in the mode conversion heating regime are also presented (mainly direct electron heating through mode converted ion Bernstein waves). Recent attempts at improving plasma performance by establishing internal transport barriers (ITBs) by various transient profile control techniques (the so-called Advanced Tokamak mode of operation) are also presented. Future improvements in performance afforded by the recent addition of a new 4-strap antenna and 4 MW of tunable (40-80 MHz) ICRF power are also discussed. Mode-conversion current drive (MCCD) and fast wave current drive (FWCD) will be among the many new options that will be tested with the goal of improving plasma performance. INTRODUCTION The primary auxiliary heating power in Alcator C-Mod has been provided by the 4.0 MW ICRF heating system operating at 80 MHz in D(H), D(3 He) or 3He(H) plasmas [1-5]. This frequency corresponds to on-axis minority fundamental cyclotron resonance of protons at 5.3 T, and 3He at 7.9 T in a dominantly deuterium plasma. In addition, mode-conversion heating in various combinations of the above listed ion species has also been tested [6]. To date up to 3.5 MW of RF power at 80 MHz has been injected by two pairs of poloidal strap antennas, driven 180 degrees out of phase between adjacent straps (dipole phasing) [7]. The two antennas are installed in adjacent ports. The current straps and the antenna housing boxes are plated with copper, and are protected by slanted Faraday shield rods which are coated with TiCN and B 4N. Each of these antennas is capable of withstanding 50 kV, and has been operated in both vacuum and plasmas at 40 kV at the full power capability (10 MW/M 2 ). The antennas are