G. Gatti

EXPERIMENTS IN FTU WITH DIFFERENT LIMITER MATERIALS

Over the last few years, a great deal of effort has been devoted to solving the problem of power and particle handling in divertors, which has been recognized as a critical issue for the operation of a magnetic fusion reactor. In particular, the choice of materials for plasma facing components has been examined with a view to developing heat and erosion resistant materials for divertor target plates. A large database on the behaviour of low-Z (carbon or beryllium) materials in tokamaks is available, while for high-Z materials there is little experience in the present generation of magnetic fusion devices. Frascati Tokamak Upgrade (FTU), a high field compact tokamak, has devoted part of its experimental campaign to studying the plasma characteristics when its limiter material is changed from the usual Inconel (nickel) to molybdenum and tungsten. Siliconization of the machine has also allowed the comparison of plasma performance when a relatively low-Z (silicon) ion is the dominant impurity. In this article, results are reported concerning the plasma operation, the differences in plasma characteristics and radiation losses, the impurity generation mechanisms and the relative impurity concentrations in the core plasma. A simulation of the experimental results, made with a self-consistent edge-core coupled model is presented, in order to provide evidence of the main physics mechanisms responsible for the observed behaviour

Reduction of the electron thermal conductivity produced by ion Bernstein waves on the Frascati Tokamak Upgrade tokamak

Operating with a high frequency and a wave guide antenna, the ion Bernstein wave (IBW) experiment on the Frascati Tokamak Upgrade is not dominated, as expected, by nonlinear plasma edge phenomena. By coupling IBW power, a simultaneous increase of plasma density and central electron temperature (⩾2 keV) is produced when the confinement magnetic field is adjusted to set an ion cyclotron resonant layer in the plasma bulk. Transport analysis indicates a reduction of the electron thermal transport inside the internal resonant layer larger than a factor of 2.

Steady improved confinement in FTU high field plasmas sustained by deep pellet injection

High density plasmas (n0 ≈ 8 × 1020m-3) featuring steady improved core confinement have been obtained in FTU up to the maximum nominal toroidal field (8 T) by deep multiple pellet injection. These plasmas also feature high purity efficient electron-ion coupling and peaked density profiles sustained for several confinement times. Neutron yields in excess of 1 × 1013 n/s are measured, consistent with the reduction of the ion transport to neoclassical levels.

High power heating and current drive experiments with EC waves in FTU tokamak

The EC system at 140 Ghz, 2 MW, being implemented on FTU tokamak for performing electron heating, profile control and current drive, is composed by four gyrotrons with 0.5 s pulse length capability, and four hybrid mirror/waveguide transmission lines. The launching system is capable of poloidal/toroidal beam steering through a set of tiltable in-vacuum mirrors, coupling to the O-mode at the fundamental resonance, with full control of the e.m. field polarization. Experiments have been made with two gyrotrons, by launching 0.8 MW to the plasma. In the high density regime (local ne from ≈0.8 up to 2 1020 m−3) the e-i energy transfer is significant, and ion heating is observed through the enhancement of the neutron emission in Deuterium plasmas. The highest electron heating is observed when on-axis ECRH is performed in the current ramp-up, sawtooth-free phase. In steady-state conditions, saw-tooth period is increased up to stabilization by off-axis ECRH. Localized ECCD is also performed to assist in the shapi...

Recent results of the ion Bernstein wave heating experiment on FTU

About 400 kW of rf power at the frequency of 433 MHz were coupled to the plasma of the FTU tokamak by a waveguide antenna. During the experiment, a peaking of the Te and ne profiles and a Hα drop have been observed during the IBW pulse, only when operating with toroidal magnetic field value which allows the wave penetration to in the plasma bulk. Moreover, the electron temperature increased typically from about 2.5 keV to about 4.5 keV with a coupled power of about 350 kW; the line-averaged central plasma density increased from 3×1019 m−3 to about 5×1019 m−3.

Lower hybrid current drive in FTU high density shear reversed discharges

Results are reported of the 8 GHz Lower Hybrid experiments on FTU after the installation of the new toroidal limiter. A figure of merit of the Current Drive efficiency ηCD≈0.11⋅1020 A/Wm2 is estimated for plasma density ne=1020 m−3 and no appreciable broadening of the launched frequency is detected. In low density experiments sawteeth are stabilised and m=1 activity is present in the plasma. Shear reversed discharges with large reversal radius, rs/a≈0.5, are obtained at higher density, lower temperature, BT=4 T, qa≈5.5, by off-axis LH CD. The reversed configurations exhibit high central temperature coexisting with regular m=2, n=1 relaxations of large amplitude and are maintained up to LH switch off. At higher magnetic field, B=5.2 T, qa≈7, irregular DTM crashes are present during the whole LH pulse. Confinement time of radiofrequency heated discharges (PLH=0.5÷2⋅POH) exhibits the same behaviour of FTU ohmic discharges following the ITER89-P scaling. Preliminary results of central 140 GHz Electron Cyclotron Resonant Heating (ECRH) during the plasma current ramp-up, aimed at obtaining shear reversed configurations are also reported.Results are reported of the 8 GHz Lower Hybrid experiments on FTU after the installation of the new toroidal limiter. A figure of merit of the Current Drive efficiency ηCD≈0.11⋅1020 A/Wm2 is estimated for plasma density ne=1020 m−3 and no appreciable broadening of the launched frequency is detected. In low density experiments sawteeth are stabilised and m=1 activity is present in the plasma. Shear reversed discharges with large reversal radius, rs/a≈0.5, are obtained at higher density, lower temperature, BT=4 T, qa≈5.5, by off-axis LH CD. The reversed configurations exhibit high central temperature coexisting with regular m=2, n=1 relaxations of large amplitude and are maintained up to LH switch off. At higher magnetic field, B=5.2 T, qa≈7, irregular DTM crashes are present during the whole LH pulse. Confinement time of radiofrequency heated discharges (PLH=0.5÷2⋅POH) exhibits the same behaviour of FTU ohmic discharges following the ITER89-P scaling. Preliminary results of central 140 GHz Electron Cyclot...

Local Improvement Confinement in the Ion Bernstein (IBW) Experiment on FTU (Frascati Tokamak Upgrade)

New experiments with the ion Bernstein waves (IBW) has been performed on FTU both in Hydrogen (H) and Deuterium (D) majority plasmas, at higher radio‐frequency power level, plasma density, current, lower effective ion charge than in previous campaign of 1999. Also in these conditions, no role is played by non‐linear edge physics, which prevented, instead, the RF penetration in the plasma bulk of DIII‐D. With resonant toroidal magnetic field (≈8T), improved confinement occurs inside a radial region of 1/3 of the minor radius operating in H‐plasma, and 2/3 of the minor radius operating in D‐majority plasma. Such behavior is consistent with the model of turbulence suppression by locally‐induced‐IBW‐sheared flow expected to occur close the resonant layer. The FTU results provide support for active control of the pressure profile by IBW which is of relevance for advanced tokamaks .