Development of capacitively-coupled combline antennas for current drive in tokamaks

摘要

The capacitively-coupled combline (CCC) antenna has been developed for current drive by the lower hybrid wave (LHW) on the TST-2 spherical tokamak. The combline antenna was developed to satisfy the requirements of high directionality, low reflectivity, and simple feeding. Since the combline antenna makes use of mutual coupling between neighboring elements, only the first and the last elements are connected to external feedlines. RF powers and power densities of the order of 100 kW and 1 MW m(-2) can be achieved easily in small antennas of the order of 0.1 m(2). The two CCC antennas installed in TST-2 (outboard-launch and top-launch) excite toroidal refractive index (n(phi)) spectra peaked around five with the frequency of 200 MHz. Wave excitation calculation using a finite element code shows that the excited power of the n(phi) = 5 LHW component increases rapidly when the plasma cutoff density layer (where n(e) = 5 x 10(14) m(-3)) becomes closer than 27 mm from the antenna surface, in agreement with experiment. Experimentally, the density profile in front of the antenna can be controlled by adjusting the side limiter location or antenna-plasma distance, and should be optimized for antenna-plasma coupling, since too high coupling results in a broadened and less directional n(phi) spectrum, and too low coupling results in a less efficient power coupling. Using these antennas, successful ST plasma start-up and I-p ramp-up to over 25 kA (about 1/4 of the nominal I-p for OH operation) have been achieved with RF power of less than 100 kW in about 40 ms.