FTU Electron Cyclotron Heating experiment
(joint experiment with CNR IFP Milan)
Electron cyclotron waves have the features to deposit their energy in very localized region inside the plasma, thus allowing a good control of the power deposition.
The capability of localized deposition is particularly interesting in view of the possibility of producing regions inside the plasma column with reduced energy transport (Internal Transport Barriers). If the deposition region of electron cyclotron waves is chosen in such a way to overlap with the region of reduced transport, a large increase in the local temperature is expected. This scenario might be of interest for ITER, with the alpha-particle heating playing the role of the electron cyclotron power. On FTU injecting ECRH power on the current ramp in B=5.3 T discharges has led to observe very high values of the central electron temperature (up to 15 keV). The electron thermal conductivity has been found to remain at the value of the ohmic phase in the deposition zone, in spite of much larger temperature and temperature gradients. In addition ECRH injection during the current ramp phase can delay the current density evolution and allow the formation of broad current density profiles, which can be subsequentky maintained by LH current drive during the flat-top phase. In this way, on FTU long lasting (many energy confinement times) electron ITB can be produced with high electron temperature (~ 11 keV) and a central density reaching 1e20 m-3.
Due to the variety of the available microwave systems, at present FTU is the only experiment capable of studying the synergy between lower hybrid and electron cyclotron heating. Synergistic effext between LH and EC radio-frequency waves open the possibility of combining the most interesting features of the two heating schemes, namely a high current drive efficiency for LH waves and a very localised tuneable and effective heating for EC waves. The synergy LHCD-EC is characterised by a substantial damping of the EC waves on the energetic LHCD-produced electrons at a magnetic field at which thermal electrons are not in resonance with the EC waves. Two distinct regimes can be investigated. In the so-called down-shifted regime the operating magnetic field is above the resonant value for EC absorption everywhere in the plasma. The EC waves cannot interact with the bulk electrons, whereas they can be absorbed by the supra-thermal electron tail induced by LHCD. In the up-shifted regime, the operating magnetic field is below the resonant value. The EC wave is launched with an angle wiith respect to the magnetic field. The wave can be absorbed in the high field sise of the discharge, but before thermal absorption take place, the wave is damped on the fast electron population.
The electron temperature profile response to strong ECRH can be used to study electron transport in Tokamak. For instance tests of the so-called stiffness of the electron temperature profile can be made. Modulated ECH can also be used to investigate temperature stiffness using transient transport techniques. The analysis of the amplitude and phase of the induced temperature modulation can lead to transport model that can be compared with theoretical ones based on electron temperature gradient driven turbulent transport.
Fig. 2 Amplitude of the temperature modulation during modulated ECH vs. radius. The deposition profile and the electron thermal conductivity are also shown.
From the technological point of view, the frequency chosen for FTU is in the range considered for ITER, and the experience gained on FTU is expected to be beneficial to the design of the ITER ECRH system.