M. Zerbini

Towards fully non-inductive current drive operation in JET

Quasi-steady operation has been achieved at JET in the high-confinement regime with internal transport barriers (ITBs). The ITB has been maintained up to 11 s. This duration, much larger than the energy confinement time, is already approaching a current resistive time. The high-performance phase is limited only by plant constraints. The radial profiles of the thermal electron and ion pressures have steep gradients typically at mid-plasma radius. A large fraction of non-inductive current (above 80%) is sustained throughout the high-performance phase with a poloidal beta exceeding unity. The safety factor profile plays an important role in sustaining the ITB characteristics. In this regime where the self-generated bootstrap current (up to 1.0 MA) represents 50% of the total current, the resistive evolution of the non-monotonic q-profile is slowed down by using off-axis lower-hybrid current drive.

Integrated scenario in JET using real-time profile control

The recent development of real-time measurements and control tools in JET has enhanced the reliability and reproducibility of the relevant ITER scenarios. Diagnostics such as charge exchange, interfero-polarimetry, electron cyclotron emission have been upgraded for real-time measurements. In addition, real-time processes like magnetic equilibrium and q profile reconstruction have been developed and applied successfully in real-time q profile control experiments using model based control techniques. Plasma operation and control against magnetohydrodynamic instabilities are also benefiting from these new systems. The experience gained at JET in the field of real-time measurement and control experiments operation constitutes a very useful basis for the future operation of ITER scenarios.

A Fourier transform spectrometer with fast scanning capability for tokamak plasma diagnostic

Electron cyclotron emission spectra are routinely measured in the FTU tokamak by means of a fast scanning Fourier transform spectrometer. The key element of this instrument is a two‐beam interferometer in which the path difference is varied linearly in time by rotation of a helicoidal reflector. The shortest scan time obtained for a path difference excursion of 4 cm was 1.2 ms. The spectrometer has been absolutely calibrated and the measured temperature profiles are in good agreement with those derived from other diagnostics.

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.

ECRH at high heating power density in FTU tokamak

Plasma absorption at the fundamental EC resonance of e.m. waves at 140 GHz, 0.8 MW, Ordinary mode, provides in the FTU tokamak a localized (minimum δr ≃ 2.5 cm) and intense (up to ≃ 100 MW/m 3 ) heat source for the electrons. It is shown that at high density (≃ 10 20 mγ- -3 ) ions are heated (ΔT i /T i = 0.25) by collisional e - i energy transfer and assuming ion thermal diffusivity about three times larger (at most) than predictions in order to account for the observed T,. Off-center localization (r abs /a ≃ 0.2) of the ECRH heat source changes the electron temperature (and current density) profile in a way to fully stabilize sawteeth. On-axis absorption accelerates the sawtooth crash repetition rate if P ecrh ≤ P oh , but with P ecrh > P oh the sawtooth period is increased. A critical shear model for the sawtooth crash repetition rate is shown to be in agreement with its experimental dependence on resonance position and heating power. In discharges characterized by a flat temperature profile in the ohmic phase, ECRH may de-stabilize tearing modes of different order, with a strong degradation of core energy confinement.

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.

AMPLITUDE MODULATION REFLECTOMETRY SYSTEM FOR THE FTU TOKAMAK

Amplitude modulation (AM) reflectometry is a modification of the classical frequency sweep technique that allows to achieve unambiguous phase delay measurements. A broadband AM reflectometer has been realized for the measurement of density profiles on the FTU tokamak in the 0.34–1×1020 m−3 range. The characteristics of the instrument have been determined in extensive laboratory tests; particular attention has been devoted to the effect of interference with parasitic reflections. Possible ways to overcome this effect are indicated. An example of the first experimental data collected on FTU is discussed.

Experimental results of amplitude modulation reflectometry on the FTU tokamak

The amplitude modulation (AM) reflectometer realized for the FTU tokamak has been successfully operated in a wide range of plasma conditions. The reflectometer has been calibrated using the signal reflected by the inner part of the vacuum chamber (back wall) in absence of the plasma. Density profiles have been obtained for steady-state discharges by inverting the AM phase measured as a function of the carrier frequency. The results are in good agreement with density profiles obtained from multichannel far infrared interferometry.

Electron cyclotron emission diagnostic of high temperature electron cyclotron resonance heated plasmas on Frascati tokamak upgrade

The electron cyclotron emission (ECE) diagnostic on FTU tokamak is routinely performed with a Michelson interferometer with spectral range extending up to 1300 GHz. The diagnostic allowed accurate electron temperature measurements during the recent 140 GHz electron cyclotron resonance heating (ECRH) experiments on FTU. Very accurate measurements have been performed on a wide range of electron temperatures and profile peaking. The ECE measurements have been compared with Thomson scattering and with observations of x-ray spectra from highly stripped molybdenum ions. The suprathermal emission in these conditions has been studied.