Transport Analysis of RI mode.

During the 2002 campaign the experiments on RI (Radiative Improved) mode in FTU have been devoted mainly to the optimisation of the ohmic discharge with Ne puffing and to the analysis of energy transport in this plasma regime.The main aim was to find the most important characteristics of the plasma in this mode and to compare the results with those achieved in other Tokamaks.
The plasma target was chosen with the following parameters: BT = 6 T; plasma current Ip = 0.9 MA, to avoid the onset of marfee; electron density (D2) larger than 1020 m-3, in order to be in the saturated ohmic confinement regime (SOC). The deuterium flow, injected by a fast valve to fuel the plasma, was stopped at the beginning of the current plateau (~ 0.45s), just before a short Ne puff (10-30 ms duration) was programmed (at 0.5-0.6 s). These experiments were done shortly after a fresh boronization, in order to have a target plasma with a low fraction of radiated power (< 50%).


After Ne injection, the line average density increases, much more than expected from the full Ne ionisation, while the electron density profile becomes more peaked. Central electron temperature increases slightly and the radiated power reaches ~ 85% of total input power. Neutron yield increases by a factor 4 with respect to a reference discharge.
Metallic impurities (Mo and Fe) are observed to decrease after Ne injection, as expected. Indeed the conducted power through the last closed magnetic surface decreases, and thus the plasma temperature in the scrape off layer would also decrease, corresponding to a smaller sputtering yield from the metallic surfaces in contact with the plasma (Mo toroidal limiter and stainless steel vacuum walls).
Fig. 1 shows the density, electron and ion temperature profiles for the RI mode discharge, compared to a reference one. The profiles refers to a time of 1.2 s into the discharge, when a quasi steady state condition is reached in the RI mode shot.


Fig. 1 a) density profile; b) electron temp. profile from Michels. interfer.; c) ion temp. profile from transp. code simulation: (red) w/o Ne; (violet) with Ne.

 

Density and electron temperature profiles are measured by a multichannel FIR laser interferometer and a ECE Michelson interferometer respectively. The ion temperature profile is deduced by using the 1-D transport code EVITA in the interpretative mode. The transport ion conductivity i is taken to be a ineocl. In order to reproduce the enhancement of the neutron yield for the discharge with Ne puff, the anomaly factor a must be decreased by at least a factor 2. Typically, in ohmic discharges in the SOC regime a ~ 3. While the total thermal energy increases substantially as soon as Ne is injected, the input power remains practically the same (Fig.2).

Fig.2 a) Total thermal energy; b) Ohmic power: (red) w/o Ne and (violet) with Ne puff.

As a consequence the energy confinement time E (shown in Fig. 3) is larger in the RI mode discharge. The improvement factor is 1.4.

Fig.3 Global energy confinement time (red) w/o Ne and (violet) with Ne puff.

The typical signatures of an RI mode have been thus observed in FTU ohmic plasmas. In the next experimental campaign devoted to RI mode experiments, the plan is to further optimise this regime, extending it to higher densities, where larger E improvements are expected , and finally to plasmas with strong additional heating.

 

Experimental Reports