Improved confinement at high density, high field operation in FTU.

Use of pellet injection in conditions where injected particles can be ionised close to the plasma centre can lead to plasmas with improved confinement as observed in several experiments (Alcator-C, JET, Tore Supra, etc.). Improved confinement is attributed to steep pressure profiles and is called the PEP mode (Pellet Enhanced Phase) but this improved confinement phase does not last very long due to the rapid decay of the pressure gradients. In other experiments, such as ASDEX-U, multiple pellets have been used to increase and to maintain high-density plasmas but typically without deep penetration. Multiple deep pellet injection has been used in FTU in order to benefit from the high field capability of the device, hence high density operation, and the availability of a multiple fast pellet injector (up to 8 pellets at 1.3 km/s).

In previous FTU experiments, improved confinement was achieved in the second pellet phase with the following target plasma parameters: n = 1.5 1020 m-3, Teo = 1.5 keV, B=7.1T and I= 0.8MA. Following the second pellet launch, central density was up to 7 1020 m-3 and neutron yield up to 4 1012 n/sec. Transport analysis has indicated that ion diffusivity was neo-classical. These high performance plasmas are very sensitive to m=1 internal kink modes, which can grow to a large amplitude and couple, in some case, to more external modes, leading to mode-locking and disruptions. In the present campaign, technical availability of FTU has been extended up to 8T, 1.6 MA plasmas with a significant length of the current plateau (0.6 s at 1.6 MA).

Main results:

- Previous enhanced confinement regimes have been extended up to B=8T, I=1.25MA with multiple pellet injection, only limited by the availability of pellets and the time duration of the plateau. Disruptions were avoided by careful conditioning of the first wall (using titanisation) and adjusting the target plasma density so as to allow good pellet penetration. Time interval between pellets was selected to be 100 ms, about one energy confinement time.

- As shown in Fig1, the best confinement was achieved during the last sequence of pellets. Quasi steady conditions were achieved with a thermal neutron rate up to 1.8 x1013n/s, a line averaged density of 4 1020m-3 (peaked density is estimated to be 7-8 1020m-3) and central temperature of 2 keV. The resulting values of the fusion figure of merit, ntT, are in the range of 1020m-3 keVs and are achieved in conditions were electron and ion temperatures are equal and Zeff close to unity.

-  In an attempt to heat and/or to stabilise m=1 modes, effective coupling of LHCD to high density, high field plasmas; 1.2 1020 m-3, 7.9T, 1.2 MA, 0.8 MW has been achieved. A 40% increase in neutron yield together with a 20 % drop in loop voltage and good confinement was observed with about 1 MW of LHCD power. In some cases, m=1 modes were stabilised. At the very high density achieved during the multiple pellet phase, LHCD is not effective enough with the presently available power to really affect the plasma behaviour.

- Several 1.6 MA discharges were obtained during the last week of FTU high performance operation. Pellet injection, up to three pellets, was studied. Some of the discharges were terminated by a disruption for reasons under investigation, likely linked to the lack of tuning of the operation with pellet injection. MHD activity which could be associated with high plasma current density operation (qedge= 2.5) has not posed significant problems. Thanks to titanisation, disruption recovery was achieved with two or three standard discharges.

-  Steady neutron yield of 1013n/s was achieved in the post pellet phase, one of the highest achieved so far in FTU discharges. It confirms the trend indicating that performances of multiple pellet operation increase with increasing plasma current.Fig.

Fig. 1 Time traces of line-average density, neutron yield and central electron temperature for FTU pulse #18598, with five pellets injected during the 1.2 MA current plateau at B=8 T (edge safety factor q=3.3).

Experimental Reports