G. Apruzzese

HIGH DENSITY OPERATION ON FRASCATI TOKAMAK UPGRADE

Plasma behaviour in the high density regime has been investigated on the Frascati Tokamak Upgrade (FTU). The items particularly addressed are density limit, MARFE characteristics and fuelling efficiency. With gas puffing, a maximum line average density of 3.2*1020 m-3 has been obtained in the ohmic regime at q=5.1, corresponding to 1.7 times the Greenwald limit; while with pellet injection, a value of 3.7*1020 m-3 has been reached at q=3.5. The density limit appears to be connected with the impurity content and edge parameters, so the best results are obtained with very clean plasmas and peaked electron density profiles. The MARFE phenomenon always appears beyond a critical density that depends on the total input power and the effective charge: emissivities in the range of 2 to 12 MW/m3 have been measured in this highly radiating region. The fuelling efficiency, starting from a value of about 50% at low density, progressively deteriorates, falling to 10% near the density limit: this behaviour is interpreted as being due to a decrease of the scrape-off layer transparency to incoming neutrals

Runaway electron generation and control

We present an overview of FTU experiments on runaway electron (RE) generation and control carried out through a comprehensive set of real-time (RT) diagnostics/control systems and newly installed RE diagnostics. An RE imaging spectrometer system detects visible and infrared synchrotron radiation. A Cherenkov probe measures RE escaping the plasma. A gamma camera provides hard x-ray radial profiles from RE bremsstrahlung interactions in the plasma. Experiments on the onset and suppression of RE show that the threshold electric field for RE generation is larger than that expected according to a purely collisional theory, but consistent with an increase due to synchrotron radiation losses. This might imply a lower density to be targeted with massive gas injection for RE suppression in ITER. Experiments on active control of disruption-generated RE have been performed through feedback on poloidal coils by implementing an RT boundary-reconstruction algorithm evaluated on magnetic moments. The results indicate that the slow plasma current ramp-down and the simultaneous reduction of the reference plasma external radius are beneficial in dissipating the RE beam energy and population, leading to reduced RE interactions with plasma facing components. RE active control is therefore suggested as a possible alternative or complementary technique to massive gas injection.

Particle transport in the Frascati Tokamak upgrade

Three methods of density modulation have been used in FTU discharges (gas puffing, pellet injection and current rampup). The analysis of particle balance confirms that the neoclassical theory is not adequate and the experimental results obtained by the three methods are used to verify the validity of an empirical expression for the radial particle flux. Such an expression is relevant for the design of future experiments and constitutes a constraint for any satisfactory theory.

Peaked density profiles due to neon injection on FTU

Neon injection in FTU can cause a spontaneous increase of the line-average density by a factor 2. The recent experiments were devoted to characterize the plasma response to the neon injection at different densities and plasma currents.A qualitative estimate from UV spectroscopy measurements indicates that the density behaviour cannot be attributed simply to the stripped electrons from the puffed impurity, but a modification of particle transport should be invoked in order to explain the spontaneous rise and the higher peaking.JETTO transport and GENE gyrokinetic codes analyses, as well as a calculation of the electron diffusion coefficients D and pinch velocity U, contribute to feature the peaking effect.

Radial particle flow, electrical conductivity and thermal energy content in the Frascati Tokamak Upgrade

Experimental determinations of the radial plasma flow velocity, of Zeff and of the thermal energy content, made on the Frascati Tokamak Upgrade (FTU), have been compared with the predictions of the conventional neoclassical theory, of the extended neoclassical theory and of the pseudoneoclassical theory. In the comparison only the momentum balance and continuity equations were used. The best overall agreement is found for the second of these theories, thus confirming the result of previous analyses

Experiments on magneto-hydrodynamics instabilities with ECH/ECCD in FTU using a minimal real-time control system

Experiments on real time control of magneto-hydrodynamic (MHD) instabilities using injection of electron cyclotron waves (ECW) are being performed with a control system based on only three real time key items: an equilibrium estimator based on a statistical regression, a MHD instability marker (SVDH) using a three-dimensional array of pick-up coils and a fast ECW launcher able to poloidally steer the EC absorption volume with dρ/dt = 0.1/30 ms maximum radial speed. The MHD instability, usually a tearing mode with poloidal mode number m and toroidal mode number n such that m/n = 2/1 or 3/2 is deliberately induced either by neon gas injection or by a density ramp hitting the density limit. No diagnostics providing the radial localization of the instabilities have been used. The sensitivity of the used MHD marker allows to close the control loop solely on the effect of the actuators action with little elaboration. The nature of the instability triggering mechanism in these plasma prevents that the stabilization lasts longer than the ECW pulse. However when the ECW power is switched on, the instability amplitude shows a marked sensitivity to the position of the absorption volume with an increase or decrease of its growth rate. Moreover the suppression of the dominant mode by ECRH performed at high plasma density even at relatively low power level facilitates the development of a secondary mode. This minimized set of control tools aim to explore some of the difficulties which can be expected in a fusion reactor where reduced diagnostic capabilities and reduced actuator flexibility can be expected.

First Spectroscopic Results with Tin Limiter on FTU Plasma

Since the end of 2016 experiments were performed on FTU with a tin limiter, for testing liquid metals under reactor relevant thermal load up to 17 MW/m2 in nearly stationary conditions. FTU is the first tokamak in the world operating with a liquid tin limiter and one of the pioneers in liquid metal application. The preliminary analysis of the experimental data has been focused in detecting the presence of tin in the discharge: suitable monitors are the spectroscopic diagnostics in the visible and UV ranges. The experimental observation of the tin spectral lines represents a new goal for extending the database of atomic nuclear data in the plasma tokamak research. In particular, 607.8 and 645.3 nm spectral lines of SnII have been observed. In addition, all the expected spectral lines in VUV range have been detected, 20.4 nm of SnXXI and 21.9 nm and 27.6 nm of SnXXII.