Isotopic effect in experiments on lower hybrid current drive in the FT-2 tokamak

S I Lashkul,S V Shatalin,E Z Gusakov,D V Kouprienko,L A Esipov,A Yu Stepanov,A D Gurchenko,A N Saveliev,V V D’Yachenko,A B Altukhov,M Yu Kantor,M A Irzak

doi:10.1134/s1063780x15120089

Abstract

To analyze factors influencing the limiting value of the plasma density at which lower hybrid (LH) current drive terminates, the isotopic factor (the difference in the LH resonance densities in hydrogen and deuterium plasmas) was used for the first time in experiments carried out at the FT-2 tokamak. It is experimentally found that the efficiency of LH current drive in deuterium plasma is appreciably higher than that in hydrogen plasma. The significant role of the parametric decay of the LH pumping wave, which hampers the use of the LH range of RF waves for current drive at high plasma densities, is confirmed. It is demonstrated that the parameters characterizing LH current drive agree well with the earlier results obtained at large tokamaks.

Highlights

The results of experiments carried out at the FT-2 tokamak allow us to conclude that the most probable reason for LH current drive (LHCD) termination in both hydrogen and deuterium plasmas at a relatively low plasma current of I = 22 kA and, a low electron temperature is an additional reduction in Te at the periphery of the plasma column during the lower hybrid (LH) pulse [2]
This results in a lower threshold for the parametric decay (~Te/ne) of the pumping wave
Parametric process on the periphery of the plasma column are considered as the main mechanism of growth N‫ ׀׀‬resulting in a significant reduction of LHCD efficiency and depletion of the pump wave power due to the absorption of ions in the peripheral plasma [3, 4]

Summary

Result of the LHCD efficiency study

The most efficient method for sustaining the quasisteady plasma current by means of lower hybrid (LH) waves can be implemented only at relatively low plasma densities not exceeding a certain density limit, nDL. The results of experiments carried out at the FT-2 tokamak allow us to conclude that the most probable reason for LHCD termination in both hydrogen and deuterium plasmas at a relatively low plasma current of I = 22 kA and, a low electron temperature is an additional reduction in Te at the periphery of the plasma column during the LH pulse [2]. This results in a lower threshold for the parametric decay (~Te/ne) of the pumping wave.

Parametric processes intensify with increasing plasma density

RF ms