S. Ciattaglia

High Plasma Density Lower-Hybrid Current Drive in the FTU Tokamak

s Vol. 19C (European Physical Society, Geneva, 1995), Part III, p. 361. [15] G. Tonon et al., Plasma Phys. Controlled Fusion 35, A105

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.

MHD activity in FTU plasmas with reversed magnetic shear

The MHD activity of plasma configurations with reversed magnetic shear has been investigated on the FTU tokamak. In the presence of pairs of surfaces with the same rational value q = m/n of the safety factor, double-tearing modes are excited which give rise in most cases to bursts of sawtooth-like profile rearrangements. More stable regimes have also been found, in which the activity is dominated by rotating saturated modes. In a particular case with and a discharge without any detectable MHD activity during the current flat-top has been obtained. In high-temperature regimes ( at ), an irregular activity has been detected near the plasma centre which could be due to the excitation of resistive interchange modes.

Transport and MHD studies at high Te in FTU tokamak

Magnetohydrodynamic (MHD) activity and energy transport at rational-q surfaces is analysed on the basis of experimental results on current density profile control obtained with localized electron cyclotron resonance heating (ECRH) on FTU tokamak. The MHD response, in particular 2/1 and 1/1 modes, to ECRH is in agreement with expectations from a theoretical model including resistive wall braking and toroidal mode coupling. It is also shown that the magnetic shear atrqD1 could controlmD 1 mode saturation and magnetic reconnection. Heating results with ECRH at steady state indicate that transport enhancement is the dominant effect on confinement at theqD 2 surface, and suggest that conduction and convection inside the asymmetricmD 1 island should both be taken into account for a proper description of the thermal response to localized ECRH.

Enhanced confinement regimes with strong electron heating in the presence of flat or inverted safety factor profiles

The role of magnetic shear in affecting electron transport is discussed on the basis of the Frascati tokamak upgrade (FTU) data with central electron cyclotron resonance heating (ECRH) on the current ramp phase and with pellet injection. The results point out that strongly negative magnetic shear is not a necessary condition in order to have good electron transport and that magnetohydrodynamic (MHD) activity plays a crucial role in affecting the electron transport in the region with low/negative magnetic shear. The theoretical arguments for the dependence of transport on magnetic shear are reviewed and compared with the experimental evidence.

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 1020u200am−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 3×1019u200am−3 to about 5×1019u200am−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⋅1020u2009A/Wm2 is estimated for plasma density ne=1020u2009m−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=4u2009T, 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=5.2u2009T, 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⋅1020u2009A/Wm2 is estimated for plasma density ne=1020u2009m−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=4u2009T, 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=5.2u2009T, 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...

The Frascati Tokamak Upgrade machine: availability analysis, main results and future programme

The paper presents a general overview of the Frascati Tokamak Upgrade (FTU) machine operation in the last two years. About 3500 pulses were performed (mostly in D/sub 2/) with a toroidal field between 2.5 and 8 Tesla and a plasma current between 0.35-1.3 MA, according to the experimental programmes. An availability analysis of the FTU plant was performed, which pointed out the number of pulses per day and the causes of delay. The main research activities concentrated on plasma MHD studies with different heating scenarios: lower hybrid waves current drive (LHCD) and electron cyclotron resonance heating (ECRH) were used both alone and together to investigate synergetic effects. Strong effort was devoted to high density (1/spl times/10/sup 20/ m/sup -3/) current drive experiments and investigating the possibility of obtaining and sustaining shear reversal configurations. Near term scientific and technological programmes are also reported.