Experimental activity and highlights of 1999

In the first six months of 1999 the experimental runs on FTU have been interrupted by two shutdowns of one month each, needed to install a new launching structure for 8 GHz electromagnetic waves (LH) and a set of rings, inside the vacuum chamber, for MHD measurements. In the second part of the year, the experiments only started at mid November, because of a problem to one of the bearing of the flywheel generator feeding the toroidal coils. Nevertheless, a total of 1110 shots were successfully performed. On request of the IBW experimental program, discharges were produced with a value of the toroidal magnetic field of 8.2 T on axis, greater than the design value of 8 T. The current flat top was limited to 0.5 s and the mechanical stresses measured were within the engineering tolerances.
In September, a titanisation system was installed on FTU to cover the vacuum chamber and the toroidal limiter with a thin layer of titanium, to control the light impurities, mainly Oxygen, during the plasma shots. Two filaments, located on different ports, permit to cover more than 60% of the chamber walls. The titanisation procedure is fully automatic and can be performed after every shot. The main result, as expected, was the decrease of Zeff down to 1.6 at ne 0.6 1020 m-3, as requested by some experimental programs, such as LH current drive and Ion Bernstein Wave Heating. In the future, a boronisation system is planned to be implemented to further reduce radiation losses.In order to optimise the current density profiles for obtaining best plasma performances in advanced Tokamak configurations, a reliable diagnostic of the magnetic field profile is needed. The best technique for measuring the local field direction in a laboratory plasma is based on the polarimetry analysis of the motional Stark components of the spectrum emitted by fast hydrogen atoms injected into the plasma. The realisation of this diagnostic depends on the installation of a neutral beam injector of relatively low intensity, not to perturb the plasma significantly. A neutral beam injector, developed at TdeV Laboratories (Canada) has been acquired: it has a 40 keV, 1 A neutral beam, with a minimumdivergence of 1°.

As for the results of the experimental 1999 campaign, here are the main highlights:

• A transport barrier was observed in the absorption region of 433 MHz wave, corresponding to the 4th ion cyclotron harmonics resonance layer. The peculiar feature of the IBW experiment on FTU is the waveguide antenna, used instead of the conventional loop antenna. In this way, good coupling of up to 350 kW of power has been achieved, with low impurity influx and reduced non linear wave interaction in the scrape-off layer (SOL). The plasma current was Ip ~ 0.3 MA, the density ne ~ 0.4 1020 m-3 and the magnetic field 7.9 T. An increase in central electron temperature larger than 2 keV was observed, together with a strong peaking of the plasma density profile. A transport code was used to estimate the value of the electron conductivity coefficient Chie in the central region. A substantial drop of Chie was deduced in the region beyond the barrier, located at about 1/3 of the minor radius.

• To study core energy transport in sawtooth-free plasma, up to 800 kW of Electron Cyclotron Resonance Heating (ECRH) at 140 GHz was injected from the low field side in ordinary polarisation during the current ramp-up. This frequency corresponds to fundamental cyclotron frequency at BT = 5 T. High electron temperatures (12-13 keV) were obtained with central deposition. Since the experiment was made at a relatively low density, electron-ion coupling is very weak, and the electron transport can be studied accurately. The analysis of MHD-free phases of the discharge has shown that the central electron thermal diffusivity is comparable with the ohmic one (0.2-0.4 m2/s), even at high
temperature and high temperature gradient.

Electron temperature profile from ECE (open circles) and Thomson scattering (closed circles) during current ramp-up and ECR heating


• Two 140 GHz gyrotrons have been used (PEC ~ 0.8 MW) with a typical pulse length of 0.1-0.3 s to suppress and/or control the sawtooth instability which, when present leads to an effective increase in energy transport in the plasma core. Complete stabilisation was achieved only when the wave absorption layer rabs was very close to the sawtoothinversion radius rinv. Transient stabilisation or significant elongation of the sawtooth period on the other hand, is achieved when rabs/rinv =0.85-0.9. Comparison between ECRH and ECCD (Electron Cyclotron Current drive) configuration has shown that local distortion of the current density profile, rather than changes of the global current inside rq=1, could control the occurrence of the sawtooth crash.

Sawteeth are stabilised when the absorption radius is very near to inversion radius. a) ECE
electron temperature; b) electron temperature profiles before and after stabilisation

• MHD fluctuations of the tearing type, affecting both plasma energy transport and global stability, stimulate great interest. The response of tearing modes to the effects of localised heating by ECRH was studied in much detail in FTU. The interpretation of these experimental results has led to identifying the mechanisms governing the rotation of toroidally coupled and independent magnetic islands. A non linear dynamic model has been developed which is able to reproduce the essential features of the observations. These results have confirmed the role of inertia and wall interaction in determining the phase relations between different modes of oscillations and the conditions for their coupling.

• High Z transport studies have been performed on FTU at high electron temperatures (Te ~ 10 keV). This activity was carried out in collaboration with the John Hopkins University (JHU) and Lawrence Livermore National Laboratory (LLNL).
LLNL and JHU have developed a collisional radiative model of the L-shell transitions of Mo30+ to Mo39+ and determined the relative charge state distribution at high temperature (Te = 5-15 keV). Mo line emission was measured by injecting ECRH during current rampup in FTU, and the resulting spectrum compared with that computed according to the model. Good agreement was found under the assumptions of negligible diffusion and central impurity peaking. Both these assumptions are consistent with a neo-classical transport regime in these conditions.

An extensive discussion has been ongoing in the last few years on how to significantly pursue the line of research on magnetic confinement, aftercompleting the planned experimental program on FTU. A consensus was reached on two proposals: a modification of FTU to achieve elongated plasma configurations (FTU-D) and a spherical Tokamak device (PROTO-SPHERA). Both proposals will have to be approved by ENEA and EURATOM.

Experimental Activity