Development of a plasma current ramp-up technique for spherical tokamaks by the lower hybrid wave

Spherical tokamaks (STs) have the advantage of high beta capability, but the realization of a compact reactor requires the elimination of the central solenoid (CS). The possibility of using the lower hybrid wave (LHW) to ramp up the plasma current (I-p) from zero to a high enough level required for fusion burn in ST is examined theoretically and experimentally. Excitation of a travelling fast wave (FW) by the combline antenna installed on TST-2 was confirmed by a finite element analysis, but efficient current drive requires excitation of the LHW, either directly by the antenna or by mode conversion from the FW. The analysis using the TORLH full-wave solver indicates that core current drive by LHW is possible in the low-density, low I-p plasma formed by electron cyclotron heating (ECH). It is important to keep the density low during I-p ramp-up, and the wavenumber must be reduced as I-p increases. Initial results from TST-2 demonstrate that RF power in the LH frequency range (200 MHz) can achieve initial I-p formation, and is more effective than ECH for further ramp-up of I-p. I-p ramp-up to over 12 kA was achieved by combining ramp-up of the externally applied vertical magnetic field and ramp-up of the RF power. The significant asymmetry observed between co-current drive and counter-current drive is attributed to the presence of RF driven current. An optimized LHW antenna with appropriate polarization and wavenumber spectrum controllability is being designed. The success of the TST-2 experiment would provide a scientific basis for quantitatively evaluating the required CS capability for a low-aspect-ratio reactor.