Experimental Activity of 2002
During the year 2002 the main FTU experimental activity focussed
to the exploration of enhanced confinement regimes and Internal Transport Barrier
(ITB) formation. Long duration ITBs have been produced with combined injection
of Electron Cyclotron and Lower Hybrid waves both on the plasma current flat-top
and during the current ramp. Higher performance than before were achieved thanks
to boronisation techniques, which allowed operation at low Zeff. Stationary
ITBs for a duration of about 40 energy confinement times (corresponding to about
1.5 times the current diffusion time) have been obtained at ITER relevant density
and magnetic field values and negligible impurity accumulation. Up to 15 keV
were obtained in transient conditions. The ITB onset closely follows the same
criterion observed on JET, namely an increase above a threshold value of the
ratio between the ion Larmor radius (evaluated with the electron temperature)
and the local electron temperature scale length. Interestingly, the threshold
value is the same on JET and FTU. The global confinement of these discharges
is up to 60% higher than the ITER97 L-mode energy confinement scaling.
Improved confinement regimes have been obtained also with Ion Bernstein Wave injection, a result which has been extended during the 2002 campaign also to Deuterium plasmas and higher densities and plasma current values (up to 0.8MA) than before, thanks to an increase of the available power due to the insertion of a second launcher. A reduction up to 40% of the local electron thermal diffusivity is observed inside the absorption radius, where a sheared flow formation is theoretically expected, both in Hydrogen and Deuterium discharges.
On the basis of the results obtained in the last few years in various confinement regimes, a comprehensive analysis of the global energy confinement time has been made. In conventional ohmic discharges the energy confinement time is marginally lower than the ITER97 L-mode scaling. ITBs discharges achieve up to a factor 1.6 improvement. Pellet discharges, that are particularly interesting since they allow to extend the L-mode scaling validation up to very high density values, are in agreement within 10% with the L-mode scaling, with an increase in the energy confinement during transient phases up to 40% above the L-mode scaling.
Local transport studies have also continued using modulated ECRH within a multi machine collaboration involving FTU, Tore Supra, ASDEX-U and JET to elucidate the mechanism of electron temperature profile stiffness.
During the year 2002 the Passive Active Multijunction, prototype of the lower hybrid wave launcher envisaged for ITER, has been successfully characterised at low power and will be inserted in FTU during the year 2003.
The Frascati participation to JET has continued with a substantial effort devoted to the S2 (Advanced Regimes) and H (Heating and Current Drive) Task Forces. Particular emphasis has been given to the development of real time control of ITB plasmas, ITB at high density, FW-IBW mode conversion and active control of Lower Hybrid wave coupling.
Theoretical activity has continued on the understanding of the nonlinear Alfven eigenmode dynamics in ITB discharges. Specific attention has been given to the simulation of JET discharges with non monotonic q profiles in which Alfven cascades have been observed. The investigation of the origin of the Bohm-gyrobohm scaling of energy transport has continued in collaboration with the University of California at Irvine.
A collaboration has started with CEA Cadarache for a common exploitation of FTU and Tore Supra in order to elucidate the mechanisms underlying the electron transport physics, the plasma-radiofrequency wave interaction physics, the development of Lower Hybrid launchers for ITER and the theory of turbulence in plasmas.