Long distance coupling of lower hybrid waves in JET plasmas with edge and core transport barriers

Efficient coupling of lower hybrid (LH) waves in conditions close to those foreseen in ITER has been obtained in advanced scenario plasmas in the JET tokamak. Up to 3 MW of lower hybrid current drive (LHCD) power has been coupled at a distance between the separatrix and the launcher of 11 cm, in the presence of edge localized mode activity. The key to the improved LH wave coupling is local control of the Scrape-Off-Layer (SOL) density through gas injection in the region magnetically connected to the launcher. This increases the electron density in front of the launcher so as to improve the coupling of the LH waves, i.e. reduce the reflected power in the launcher. The average power reflection coefficient was 5.7% with gas injection, at 11 cm distance between the separatrix and the launcher. A change in the gas injection design has made the gas puffing more efficient, making the use of D₂ injection possible as an alternative to CD₄. At similar injected electrons/s rates, D₂ gives higher electron density in the SOL than CD₄, resulting in better LH coupling with D₂. The possibility of using D₂ instead of the earlier used CD₄ is an encouraging result in view of ITER operation, as CD₄ may not be compatible in ITER due to the problem of tritium retention in deposited carbon layers.