Lower Hybrid Current Drive for DEMO: Physics Assessment and Technology Maturity

Recent experiments on lower hybrid (LH) penetration at reactor-relevant densities, together with the recent demonstration of the technological viability of the passive-active multijunction launcher on long pulses, have removed major concerns about the employment of LH waves on next-generation tokamaks, where LH could profitably drive far-off-axis plasma current, allowing current profile control and helping in sustaining burning performance. In this frame and with the aim of being prepared for the design phase of the next experimental reactors, preliminary investigations on the possibility of using LH on DEMO have been started under the supervision of the European Fusion Development Agreement. This paper reports the outcomes of these studies, addressing three main questions: Is LH useful for DEMO? If so, which setting of physics parameters makes it as effective as possible? Last, can available technology fulfill such demands?

From the physics viewpoint, deposition sensitivity to launcher poloidal position, scrape-off layer parameters, and peak n|n+ have been analyzed, indicating the equatorial injection of 5-GHz waves with n|n+_peak = 1.8 as the most favorable option. On the engineering side, specific research and development needs have been investigated on the basis of available information and sensible assumptions, showing that most of the components of the transmission line and, of highest priority, radio-frequency vacuum windows demand intense development.