Quantification of lower hybrid wave absorption in the edge of the Alcator C-Mod Tokamak

Abstract

A 1 MW Lower Hybrid Current drive (LHCD) radiofrequency system is used to replace inductive drive on the Alcator C-Mod tokamak. It was designed to test Advanced Tokamak (AT) scenarios for future steady-state diverted, high field tokamaks. However, at reactor-relevant densities (n̄e > 1 . 10 20 m-3), an anomalous current drive loss is observed. This loss, known as the LHCD density limit, occurs in diverted plasmas and is correlated with the plasma current and plasma density. Several mechanisms have been implicated in the loss of current drive, with both experimental and theoretical results suggesting edge power loss. Power modulation is a standard technique used for characterizing power sources and plasma power balance. In this case, the Lower Hybrid radiofrequency (LHRF) power is modulated in time in a set of plasmas across the density range from efficient to negligible current drive. This data is used to characterize the absorption of LHRF power through the calculation of the LHRF power balance within 15%, typical of power balance studies. This power balance is used to derive characteristics of the cause behind the LHCD density limit. The immediate nature of LHRF-induced conducted and radiated power losses confirm that LHRF power is absorbed in the edge plasma, even at the lowest densities. The edge losses increase to balance the reduced current drive, indicating that the observed power in the scrape-off-layer (SOL) limits the available power for current drive and the edge losses represent a parasitic mechanism. Unlike edge losses of other radiofrequency systems, this absorption occurs with a high degree of toroidal symmetry near the plasma separatrix. This indicates absorption occurs just inside the separatrix, or just outside the separatrix over multiple SOL traversals. Measurements of the poloidal distribution of ionization and recombination in the edge were made using a specially designed Ly[alpha] pinhole camera. It utilizes a MgF2 filter and AXUV diode array to measure Ly[alpha] emission from the lower to upper divertor. Edge deposited LHRF power was found to promptly ionize the active divertor plasma in all diverted topologies. This result highlights the power flow and importance of the divertor plasma in the LHCD density limit. Three independent characteristics indicate the thermal absorption of LHRF power. First, in- /out balance of radiated and conducted LHRF power change with the reversal of the tokamak magnetic fields. Second, comparisons of the conducted heat via Langmuir probes and IR thermography are similar with and without LHRF power. Lastly, the Langmuir probe ratio of Vf l/Te does not significantly modulate with modulated LHRF. A second experiment utilized a high strike-point diverted discharge to determine the edge loss of fast electrons. The high strike point could be observed using the hard X-ray camera, which can compare core and edge X-ray emission. The measured count rates from thick-target bremsstrahlung were interpreted into fast electron fluxes using the Win X-ray code. Theoretical treatments of the fast-electron confinement time were also calculated for Alcator C-Mod. In all cases the fast-electron edge losses are minimal and will be unimportant for future tokamaks due to the small fast electron diffusivity and their large size. The loss of current drive in high density diverted plasmas correlates with high edge plasma collisionality. The newly derived characteristics set stringent requirements in nk for electron Landau damping to cause the edge absorption of LHRF power. Several observed attributes, namely high frequency modulation and low density absorption do not correlate with Landau damping characteristics. However, parasitic collisional absorption in the divertor plasma yields the necessary plasma current, topology, symmetry, thermal, and ionization characteristics. High divertor plasma collisionality is expected if not required for future tokamaks. LHRF systems of future tokamaks must must avoid propagation through collisional regions, even on the first traversal through the SOL.