A. Bertocchi

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.

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.

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.

Improved confinement produced by ion Bernstein waves in hydrogen and deuterium plasmas of FTU

New experiments with the ion Bernstein waves (IBWs) have been performed in the Frascati Tokamak Upgrade (FTU) both in hydrogen and deuterium plasmas at higher power level, operation at higher density, higher plasma current and lower effective ion charge values than in the previous campaign. Improved confinement in a region with larger radius is obtained when operating in deuterium. Transport analysis shows a uniform decrease of the electron thermal conductivity by 40% over the radial region bounded by the absorption layer.

ITB formation with counter ECCD and LHCD and Suprathermal ECCD experiments on FTU in ITER relevant conditions

In this paper the results of the experiments of combined injection of LH waves and EC waves performed in the FTU tokamak are reported. Such experiments were mainly devoted to study and to control the access to the advanced tokamak scenarios in plasma conditions close to ITER parameters (Bt≈5T or higher, ne,line≈10 20 m -3 ). Two different absorption mechanisms were used for the EC waves. In the first one the cold resonance absorption of EC waves launched with a toroidal angle was used to induce small modification of the current profile mainly maintained by LH waves. In the second one the absorption through Doppler shift due to the fast electron tails generated by LHCD was used.

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...

High density internal transport barriers for burning plasma operation

A tokamak plasma with internal transport barriers (ITBs) is the best candidate for a steady ITER operation, since the high energy confinement allows working at plasma currents (Ip) lower than the reference scenario. To build and sustain an ITB at the ITER high density (10 20 m −3 ) and largely dominant electron (e − ) heating is not trivial in most existing tokamaks. FTU can instead meet both requests, thanks to its radiofrequency heating systems, lower hybrid (LH, up to 1.9 MW) and electron cyclotron (EC up to 1.2 MW). By the combined use of them, ITBs are obtained up to peak densities ne0 > 1.3 × 10 20 m −3 , with central e − temperatures Te0 ≈ 5.5 keV, and are sustained for as long as the heating pulse is applied (>35 confinement times, τE) .A tne0 ≈ 0.8 × 10 20 m −3 Te0 can be larger than 11 keV. Almost full current drive (CD) and an overall good steadiness is attained within about one τE, 20 times faster than the ohmic current relaxation time. The ITB extends over a central region with an almost flat or slightly reversed q profile and qmin ≈ 1.3 that is fully sustained by off-axis lower hybrid current drive. Consequent to this is the beneficial good alignment of the bootstrap current, generated by the ITB large pressure gradients, with the LH driven current. Reflectometry shows a clear change in the turbulence

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...