We report on the results achieved by installing two Chemical Vapor Deposition (CVD) single crystal diamond detectors in one of the equatorial ports of the Frascati Tokamak Upgrade (FTU) tokamak, ...during the last six months of operation of the machine. The devices were fabricated at the University of Rome “Tor Vergata” in a metal/instrinsic/p-type diamond layered structure, allowing them to be used as Schottky photodiodes for VUV and Soft-X ray detection. Both detectors were placed inside the machine and operated in current mode under high vacuum conditions.
The fast response capabilities of diamond detectors allowed to observe several plasma events, like the so-called Anomalous Doppler instabilities, the pellet injection and ablation, and Multifaceted Asymmetric Radiation From the Edge (MARFE). Diamond detectors often but not always followed the Magnetohydrodynamics (MHD) activity, depending on their localization relative to the emitting region. Core temperature oscillations following Electron Cyclotron Heating system (ECH) modulation were also observed. In addition, the diamond signals were compared to selected channels of the FTU bolometry system with similar line-of-sight: the encouraging results have launched an R&D program for the development of diamond-based bolometers.
Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confined in toroidal tokamak devices requires the development of efficient current drive methods. Previous ...experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing significant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.
In order to prevent the overheating of the liquid Li surface and the consequent Li evaporation for T>500°C, an advanced version of the liquid lithium limiter has been realized and installed on FTU. ...This new system, named Cooled Lithium Limiter (CLL), has been optimized to demonstrate the lithium limiter capability to sustain thermal loads as high as 10MW/m2 with up to 5s of plasma pulse duration. The CLL operates with an actively cooled system with water circulation at the temperature of about 200°C, for heating lithium up to the melting point and for the heat removal during the plasma discharges. To characterize CLL during discharges, a fast infrared camera and the spectroscopic signals from Li and D atom emission have been used. The experiments analyzed so far and simulated by ANSYS code, point out that heat loads as high as 2MW/m2 for 1.5s have been withstood without problems.
Since 2006, experiments using a liquid lithium limiter (LLL) were successfully performed on FTU, pointing out the problem of the quantity of lithium in the plasma, especially in conditions of strong ...evaporation due to the high temperature of limiter surface. In order to avoid the strong evaporation it is necessary to control the temperature by removing the heat from the limiter during the plasma exposure. To explore this issue a new actively cooled lithium limiter (CLL) has been installed and tested in FTU.
Suitable monitors to detect the presence of lithium in the plasma are the spectroscopic diagnostics in the visible range that permit to measure the flux of lithium, coming from the limiter surface, through the brightness of the LiI spectral lines.
For this aim an Optical Multichannel Analyser (OMA) spectrometer and a single wavelength impurities monitor have been used.
The analysis of the Li influx signals has permitted to monitor the effects of interaction between the plasma and the limiter connected to the thermal load. Particular attention has been paid on the possible occurrence of sudden rise of the signals, which is an index of a strong interaction that could lead to a disruption.
On the other hand, the appearance of significant signals gives useful indication if the interaction with the plasma has taken place.
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/m
2
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.
•The dependence of the principal frequency of the MARFE instability versus basic plasma parameters is investigated.•A study of the ion drift on the MARFE instability has been performed.•The MARFE ...localization with respect to the ion drift direction, which can influence the stable and unstable positions, is discussed.
The Frascati Tokamak Upgrade (FTU) device can operate at high electron density regimes of the order of 1020m−3, where the MARFE instability is present at various plasma current and magnetic field values. When the MARFE is well developed and oscillating, its movement causes continuous density fluctuation, contaminating the integral density measurements. The amplitude and frequency of these density fluctuations are well revealed by the high resolution interferometer available on FTU, the dependence of the frequency versus basic plasma parameters is investigated in this paper.
A specific experimental session on FTU, including some discharges with reversed toroidal magnetic field, and pushing the plasma column towards the internal or external side of the vacuum chamber, respectively, has shown that, when the plasma column is distant from the toroidal limiter, the MARFE is stable and does not oscillate around the mid plane. For these last cases the MARFE localization with respect to the ion drift direction, which can influence the stable and unstable positions, is also discussed.
Since the end of 2005 a liquid lithium limiter has been installed on FTU. The liquid lithium is confined by capillarity in a mat of stainless steel or other refractory material and the capillary ...force counteracts the electromagnetic force. In this paper we review some of the most important results obtained in the experimental campaigns led on FTU. Peaked electron density profiles are spontaneously formed when the density exceeds 1.0
×
10
20
m
−3. Despite to the fact that FTU is a fully metallic machine with a TZM toroidal limiter, the only impurity that is present in the plasma is lithium so that very clean plasma are obtained and the beneficial effects are discussed. Heat loads in excess of 5
MW/m
2 are withstood by the limiter without any damage also because the radiative losses from the evaporated lithium are able to dissipate most of the incoming heat flux.
Overview of the FTU results Pucella, G.; Amicucci, L.; Angelini, B. ...
Nuclear fusion,
10/2015, Letnik:
55, Številka:
10
Journal Article
Recenzirano
Odprti dostop
Since the 2012 IAEA-FEC Conference, FTU operations have been largely devoted to runaway electrons generation and control, to the exploitation of the 140 GHz electron cyclotron (EC) system and to ...liquid metal limiter elements. Experiments on runaway electrons have shown that the measured threshold electric field for their generation is larger than predicted by collisional theory and can be justified considering synchrotron radiation losses. A new runaway electrons control algorithm was developed and tested in presence of a runaway current plateau, allowing to minimize the interactions with plasma facing components and safely shut down the discharges. The experimental sessions with 140 GHz EC system have been mainly devoted to experiments on real-time control of magnetohydrodynamic (MHD) instabilities using the new EC launcher with fast steering capability. Experiments with central EC injection have shown the onset of 3/2 and 2/1 tearing modes, while EC assisted breakdown experiments have been focused on ITER start-up issues, exploring the polarization conversion at reflection from inner wall and the capability to assure plasma start-up even in presence of a large stray magnetic field. A new actively cooled lithium limiter has been installed and tested. The lithium limiter was inserted in the scrape-off layer, without any damage to the limiter surface. First elongated FTU plasmas with EC additional heating were obtained with the new cooled limiter. Density peaking and controlled MHD activity driven by neon injection were investigated at different plasma parameters. A full real-time algorithm for disruption prediction, based on MHD activity signals from Mirnov coils, was developed exploiting a large database of disruptions. Reciprocating Langmuir probes were used to measure the heat flux e-folding length in the scrape-off layer, with the plasma kept to lay on thea internal limiter to resemble the ITER start-up phase. New diagnostics were successfully installed and tested, as a diamond probe to detect Cherenkov radiation produced by fast electrons and a gamma camera for runaway electrons studies. Laser induced breakdown spectroscopy measurements were performed under vacuum and with toroidal magnetic field, so demonstrating their capability to provide useful information on the surface elemental composition and fuel retention in present and future tokamaks, such as ITER.
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