Initial operation of a polarization-controlled beamed ECH launcher

A system for launching 53.2-GHz electron cyclotron heating (ECH) power is being installed on the Advanced Toroidal Facility (ATF) fusion experiment at Oak Ridge National Laboratory. The launcher provides a narrow linearly polarized beam. The plane of polarization is easily rotated. Measurements indicate that the launcher will have a -20 dB beam width of 15 cm at the plasma center and polarization purity of >97%. The ATF (1) requires 53.2-GHz microwave power for ECH plasma ionization and heating; this power is generated by a 200-kW cw gyrotron. Theoretical ray tracing studies have shown almost complete first-pass power absorption is possible if the power is within the saddle point in the proper polarization (2). Therefore, a narrow-beam-width launcher with adjustable polarization is being developed. In ATF the confining magnetic field can operate continuously at 1 T or in long pulses at 2 T. Either second harmonic or fundamental ECH absorption occurs, depending on the field strength, and it is of interest to operate experiments using both values. With perpendicular propagation and low-field-side launch, the ECH power must be in the extraordinary mode (X-mode, Erf perpendicular to Bo) for second harmonic (ω = 2 ωce) or in the ordinary mode (O-mode, Enr parallel to Bo) for fundamental (ω = ωce) operation. The magnetic field in ATF is formed primarily by helical windings, which produce a shear in the plane of the field along a radial chord. This shear may cause polarization rotation of the ECH wave as it travels through the outer plasma regions because of the unequal O- and X-mode phase velocities at higher plasma density (similar to Faraday rotation). The ECH resonance zone is a 10-cm-wide saddle point near the plasma center.