Abstract
Removal of the central solenoid is essential to realize an economical spherical tokamak fusion reactor, but non-inductive plasma start-up is a challenge. On the TST-2 spherical tokamak, non-inductive plasma start-up using lower-hybrid (LH) waves has been investigated. Using the capacitively-coupled combline (CCC) antenna installed at the outboard midplane, fully non-inductive plasma current ramp-up up to a quarter of that of the typical Ohmic discharges has been achieved. Although it was desirable to keep the density low during the plasma current ramp-up to avoid the LH density limit, it was recognized that there was a maximum current density that could be carried by a given electron density. Since the density needed to increase as the plasma current was ramped-up, the achievable plasma current was limited by the maximum operational toroidal field of TST-2. The top-launch CCC antenna was installed to access higher density with up-shift of the parallel index of refraction. Numerical analysis of LH current drive with the outboard-launch and top-launch antennas was performed and the results were qualitatively consistent with the experimental observations.
Highlights
Spherical tokamaks (STs) are able to operate at high beta [1] and have the potential to realize an economical fusion reactor with low magnetic field
electron cyclotron heating (ECH) is turned off during the plasma current ramp-up for the results shown in this paper
For demonstration of the improvement of the wave accessibility, we show the comparison of the discharges with and without the top-launch antenna power during the ramp-down of the toroidal field since it was difficult to perform accurate density control with the present system implemented on TST-2
Summary
Spherical tokamaks (STs) are able to operate at high beta [1] and have the potential to realize an economical fusion reactor with low magnetic field. There is no fundamental reason to prohibit application of LH waves to STs, operation at high density is more challenging than conventional tokamaks due to the lower toroidal field. For parameters of present day STs, the parallel index of refraction of the LH waves needs to be relatively large and high current drive efficiency observed in conventional tokamaks [13,14,15] may be hard to realize. Future ST reactors [2,3] will have a toroidal field of a few tesla, and efficient current drive can be possible using LH waves especially for plasma start-up when the density is low. We present our recent numerical analysis of the LH driven fully non-inductive discharges on TST-2
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