Study and optimization of lower hybrid wave coupling in advanced scenario plasmas in JET

Abstract

A systematic study of the coupling of the Lower Hybrid (LH) power to JET plasmas has allowed us to overcome the problems of active current drive in the H-mode and ITB plasmas. As a result, LH power up to 4MW has been injected for longer than 8s in an ITB plasma. The LH coupling has been improved by fitting thepoloidal shapes of the plasma to that of the launcher, up to the degree allowed by the septum in the divertor, and by injecting deuterated methane (CD 4) from a pipe close to the LH antenna. Average values of the reflection coefficient as low as 4%, as in the previous Lmode operation are obtained. The reduced shape mismatch has strongly increased the poloidal uniformity of the plasma in front of the LH launcher and has permitted a balanced supply of the 3 grills. The major breakthrough, however, has been to puff CD 4 , which increases the density in front of the LH launcher at least by 1.5 times locally, raising it above the cutoff value. This technique does not affects the plasma performances nor contaminates the main plasma up to the maximum flow rate so far used, F CD4 >10 22 el./s. Now it is routinely applied in JET and has allowed us to modify the plasma current radial profile evolution during the ITB, producing very encouraging results. Even though CD 4 is not suitable for ITER for the large tritium retention, the possibility to control locally and safely the scrape off plasma density has been demonstrated.

Effect of gas puffing from different side on lower hybrid wave-plasma coupling in experimental advanced superconductive tokamak

“…As we know, the effect of LHW on ionization of neutral gas depends on the LH electric field, which is proportional to [P LHW (MW)] 0.5 . 11 With the development of LHCD system in ITER, the LHW source power in ITER is about 20 MW, 17 which is much larger than that in EAST at present. Considering the coupled power, the electric field which is the driving force for ionization will be larger by about a factor of 2-3, and the local density increase should be enhanced moderately with respect of these experiments in EAST (P LHW $ 0.8 MW), hence resulting in larger difference between GIM_e and GIM_i puffing due to the contrary movement direction of electrons.…”

“…It is seen that the LHW coupling indicated by the coupled power and RC is much improved by the local gas puffing, suggesting it is effective to improve LHW-plasma coupling and being in agreement with the results obtained in other devices. [8][9][10][11][12][13][14] Just due to the increased LHW power, plasma loop voltage decreases even if with a high density, implying more current is driven by LHW.…”

“…Such coupling experiments have been done and analyzed in devices JT-60U, ASDEX, JET, Tore-supra, and HT-7. [8][9][10][11][12][13][14] It is expected that edge density is improved with a little gas amount so as not to affect core plasma too much. As for the gas puffing, it is can be puffed from different side of LHW antenna (electron-side or ion-side).…”

“…In order to minimize the effect of local gas puffing on core plasma as much as possible, it is necessary to compare the coupling with electron-side and ion-side gas puffing. Although the effect of gas puffing on LHW-plasma coupling has been already addressed on ASDEX, JT60, Tore-supra, and more recently on JET, [8][9][10][11][12][13] including different kinds of gas, the gas is puffed only from one side. There is no report yet on the comparison between ion-and electron-side gas puffing.…”

Investigation of lower hybrid wave coupling and current drive experiments at different configurations in experimental advanced superconducting tokamak

“…These results confirm earlier observation concerning the effect of the scrape-off layer and connection length on the LHW antenna coupling. 36,37 The CD efficiency in limiter plasmas is lower than that in divertor configurations. This is mainly because the evolution of N == along the ray path and the power spectrum upshift factor depend on the plasma configuration, hence affecting the current drive efficiency in qualitative agreement with the ray tracing=Fokker-Planck code simulations.…”