Long distance coupling of lower hybrid
waves in JET plasmas with edge and core
transport barriers
A. Ekedahl, G. Granucci, J. Mailloux, Y. Baranov,
S.K. Erents, E. Joffrin, X. Litaudon, A. Loarte,
P.J. Lomas, D.C. McDonald, V. Petrzilka, K. Rantamäki,
F.G. Rimini, C. Silva, M. Stamp, A.A. Tuccillo and JET EFDA
Contributors
advanced scenario plasmas in the JET tokamak. Up to 3MW 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.