High field side launch of lower hybrid waves: A scoping study for ADX

Launching lower hybrid (LH) waves from the high field side (HFS) of a tokamak offers significant advantages over low field side (LFS) launch with respect to both wave physics and plasma material interactions (PM!). The higher magnetic field opens the window between wave accessibility and the condition for strong electron Landau damping, allowing LH waves from the HFS to penetrate into the core of a burning plasma, while waves launched from the LFS are restricted to the periphery of the plasma. The lower parallel refractive index (n||) of waves launched from the HFS yields a higher current drive efficiency as well. The absence of turbulent heat and particle fluxes on the HFS, particularly in double null configuration, makes it the ideal location to minimize PM! damage to the antenna structure. The quiescent SOL also eliminates the need to couple LH waves across a long distance to the separatrix, as the antenna can be located close to plasma without risking damage to the structure. The Advanced Divertor eXperiment (ADX) will include an LH launcher located on the HFS. Scoping studies with the GENRAY/CQL3D ray tracing/Fokker-Planck simulation package show good absorption for rays launched from the HFS into target discharges with C-Mod-like plasma parameters. These studies identify optimum wave launch parameters (n||, vertical position, number of rows, net power) for non-inductive operation of ADX. The LH system for ADX will make use of existing infrastructure from Alcator C-Mod, including sixteen 250 kW klystrons at 4.6 GHz (total source power of 4 MW), high voltage power supply, and controls. The ADX vacuum vessel design includes dedicated space for waveguide runs, pressure windows, and vacuum feedthrus for accessing the HFS wall. Compact antenna designs based on proven technologies (e.g. multijunction and `4-way splitter' antennas) fit within the available space on the HFS of ADX. Wave coupling simulations of these launchers with HFS SOL density profiles show good coupling can be obtained by adjusting the distance between the separatrix and the HFS wall. Guard limiters on each side of the LH antenna protect the structure during ramp-up, ramp-down, and off-normal events.