RI mode in ohmic plasmas.

Controlled injection of gaseous impurities (mainly noble gases such as Ne, Ar and Kr) into the plasma has been found to lead, in certain conditions, to an improved confinement regime, the so called Radiative Improved (RI) mode on many Tokamaks. The interest in this regime for FTU lies on the fact that it can be obtained with different magnetic configurations (circular or elongated plasmas, limiter or divertor) and different heating systems (NBI, ICRH and Ohmic). Moreover it couples a good energy confinement with a large fraction of radiation losses ( up to 90% of total input power), thus alleviating the problems of plasma-wall interactions. Of course the price to be paid is a larger Zeff and a greater plasma diluition.
The strategy to look for RI mode in FTU is based on the interpretation given by TEXTOR]: impurities injection attenuates the growth rate of the Ion Temperature Gradient (ITG) instability. This leads to a smaller particle outflow and therefore to a peaking of the density profile. As a consequence ITG turbulence is further attenuated or even quenched. In cases where the ITG turbulence is the dominant heat loss mechanism, an increase of the energy confinement is achieved. In addition it has been found on TEXTOR that the energy confinement time increases with density.
An experimental campaign on FTU has begun at the end of 2001 to explore the possibility of an RI mode in ohmically heated plasmas, to reproduce the Improved Ohmic Confinement (IOC) regime, found on ASDEX .
The plasma target was choosen in such a way to see clearly this regime, if ever existed on FTU. At a magnetic field BT = 6 T, the plasma current was programmed to be at 0.8-0.9 MA to avoid the insurgence of MARFEs. Working density has been set up at 1020 m-3, to be well into the Saturated Ohmic Confinement (SOC) regime, where the energy confinement is independent of density. In FTU the critical density to access SOC is ~0.8 1020 m-3. Deuterium gas puffing is interrupted at 0.45 sec, just at the beginning of current flat-top, according to the experience on ASDEX. A Neon puff (10-30 ms duration) was injected at the beginning of current flat-top, 0.6 sec.
The usual set of standard diagnostics for FTU was employed to obtain the experimental results.
Fig.1.a) shows the central line average density for a discharge with a Ne puff of 20 ms, compared with that of a reference discharge with no Ne. It can be seen that there are two different phases: first a slowly increase of density after the Ne injection. This density increase cannot be accounted fully by electrons contributed by Ne ionization. Ne concentration can be roughly derived by the variation of Zeff (Fig.1.b). Radiation power losses increases and reaches 90% of total power, at the end of pulse (Fig. 1.c) Then apruptly the density increases at a stronger rate up to a disruption.
Neutron yield (Fig. 1.d) increases by a factor ~2, after the Ne puff.

Fig.1 Line average density (a), Zeff from bremmstrahlung (b), Radiated power (c) and neutron yield (d) for two discharges : one without Ne (red) ; and the other with Ne puff of 20 ms at 0.6 sec (violet).

In Fig.2 the total plasma energy (a), calculated assuming Ti=Te and the Ohmic power (b) are shown for the two discharges. Since at equivalent ohmic power the thermal energy is larger for the Ne puffed shot, an increase of the confinement time is deduced (c).

Fig.2 Total thermal energy (Ti=Te) (a), Ohmic power (b), and energy confinement time (c) for the two discahrges: w/o Ne (red) and with Ne (violet).

Density profiles are more peaked after Ne puff, while electron temperature remains the same or even increases somewhat. More accurate values of these parameters require the simulation with a transport code. The sudden increase at later time in the discharge with Ne is still unexplained and it is being analysed. As a first conclusion, we can state that this regime has all the signatures of a typical RI mode, but still much work remains to be done to compare this improved regime with those observed in other Tokamaks. This will the work of a dedicated experimental campaign during the next year.


Experimental Reports