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

Y. Takase, A. Ejiri, H. Kakuda, Y. Nagashima, T. Wakatsuki, O. Watanabe, P. Bonoli, O. Meneghini, S. Shiraiwa, J. Wright, C. Moeller, H. Kasahara, R. Kumazawa, T. Mutoh, K. Saito

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26 Citations (Scopus)


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 (Ip) 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 Ip plasma formed by electron cyclotron heating (ECH). It is important to keep the density low during Ip 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 Ip formation, and is more effective than ECH for further ramp-up of Ip. 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.

Original languageEnglish
Article number063017
JournalNuclear Fusion
Issue number6
Publication statusPublished - Jun 2011
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Nuclear and High Energy Physics
  • Condensed Matter Physics


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