[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Experimental results on the position and current control of disruption
generated runaway electrons (RE) in FTU are presented. A scanning
interferometer diagnostic has been used to analyze the time evolution of the RE
beam radial position and its instabilities. Correspondence of the
interferometer time traces, radial profile reconstructed via magnetic
measurements and fission chamber signals are discussed. New RE control
algorithms, which define in real-time updated plasma current and position
references, have been tested in two experimental scenarios featuring disruption
generated RE plateaus: the experimental data confirm the effectiveness of the
control strategies as the RE beam interaction with the plasma facing components
is reduced while the current is ramped-down.
[Show abstract][Hide abstract] ABSTRACT: A digital upgrade of the analogue electronics of the Frascati Tokamak Upgrade (FTU) neutron camera was carried out in order to enable studies on runaway electrons by measuring the hard X-rays produced in the bremsstrahlung interactions between runaway electrons and plasma ions. This “gamma camera” system is based on six radial lines of sight equipped with liquid organic scintillators capable of neutron/gamma discrimination. The digital acquisition system is composed by three digitizers (14-bit, 400 Msamples/s) enabling the separation between neutron and hard X-ray events also in conditions of very high count rate (MHz range). The first measurement results indicate the capability of the diagnostic to provide HXR profiles for energies >0.1 MeV and information on the runaway population during the Ip ramp-up, flattop and ramp-down phases. Disruption events can also be analyzed with sub-millisecond time resolution, although in such phases the data are affected (mainly in the inner three detectors) by the presence of a strong HXR background not originating in the plasma.
[Show abstract][Hide abstract] ABSTRACT: The Lower Hybrid Current Drive (LHCD) has been analysed in DEMO tokamak plasma in the "pulsed and steady state regime" considering two plasma scenarios characterized, respectively, by flat density profile and peaked density profiles. We have obtained LH deposition profiles in cases of neglecting the effect of spectral broadening produced by PI at the edge. By comparing the Power Deposition Profiles for both DEMO scenarios ("flat" and "peaked"), the SOL of DEMO does not play any role in the absorption of the LH wave. In all cases the deposition is localized inside the separatrix layer r/a≤1. By lowering the parallel wave-number peak of the power spectrum from 1.8 to 1.5, the accessibility condition in both case prevents the power from reaching the deposition layer apart from a small fraction which pertains to the higher n∥ of the power spectrum. The spectrum centred at 1.8 is suggested to be useful in DEMO. More realistically, as supported by available data of LHCD in a wide range of operating densities, the effect of parametric decay instability (PDI) can produce a spectral broadening which should be included in the simulations. Further studies would be necessary for assessing the temperature profiles in the SOL at reactor-graded conditions. This is because, if the SOL temperature is at least of the order of 50 to 100 eV, the effect of PDI broads the spectrum up to n∥≤10, and the deposition profile is slightly wider but not much shifted outwards.
[Show abstract][Hide abstract] ABSTRACT: Three candidates of waveguide bends for the transmission lines of Lower Hybrid systems in large-size tokamaks are assessed in terms of RF performance and integration issues. The three options are respectively characterized by profiled curvature, corrugated waveguide and elliptical cross-section; the unsatisfactory behavior of a standard, smooth wall, circular bend is also shown for comparison. The design of ideal curves achieving no mode conversion at their output is rather attended in literature because it has always been a critical issue for long-distance high-power transmissions at microwave frequencies. Here the problem is addressed taking advantage of modern numerical advancements and, unlike usual, focusing on small curvature radii. Analysis outcomes indicate the bend based on corrugated waveguide as the most suitable option from the viewpoint of both RF performance and integration issues. Details about adopted method and results are discussed.
[Show abstract][Hide abstract] ABSTRACT: Recent experiments on lower hybrid (LH) penetra- tion at reactor-relevant densities, together with the re- cent demonstration of the technological viability of the passive-active multijunction launcher on long pulses, have removed major concerns about the employment of LH waves on next-generation tokamaks, where LH could prof- itably drive far-off-axis plasma current, allowing current profile control and helping in sustaining burning perfor- mance. In this frame and with the aim of being prepared for the design phase of the next experimental reactors, preliminary investigations on the possibility of using LH on DEMO have been started under the supervision of the European Fusion Development Agreement. This paper reports the outcomes of these studies, addressing three main questions: Is LH useful for DEMO? If so, which setting of physics parameters makes it as effective as possible? Last, can available technology fulfill such demands?
From the physics viewpoint, deposition sensitivity to launcher poloidal position, scrape-off layer parameters, and peak n7 have been analyzed, indicating the equato- rial injection of 5-GHz waves with n7peak 1.8 as the most favorable option. On the engineering side, specific research and development needs have been investigated on the basis of available information and sensible as- sumptions, showing that most of the components of the transmission line and, of highest priority, radio-frequency vacuum windows demand intense development.
Fusion Science and Technology 11/2013; 64(4):748-761. · 0.49 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Since the 2010 IAEA-FEC Conference, FTU has exploited improvements in cleaning procedures and in the density
control system to complete a systematic exploration of access to high-density conditions in a wide range of plasma
currents and magnetic fields. The line-averaged densities at the disruptive limit increased more than linearly with the
toroidal field, while no dependence on plasma current was found; in fact, the maximum density of 4.3 × 1020 m−3
was reached at B = 8 T even at the minimum current of 0.5 MA, corresponding to twice the Greenwald limit. The
lack of plasma current dependence was due to the increase in density peaking with the safety factor. Experiments
with the 140 GHz electron cyclotron resonance heating (ECRH) system were focused on the sawtooth (ST) period
control and on the commissioning of a new launcher with real-time steering capability that will act as the front-end
actuator of a real-time system for ST period control and tearing mode stabilization. Various ECRH and electron
cyclotron current-drive modulation schemes were used; with the fastest one, the ST period synchronized with an
8ms modulation period. The observed period variations were simulated using the JETTO code with a critical shear
model for the crash trigger. The new launcher was of the plug-in type, allowing quick insertion and connection to the
transmission line. Both beam characteristics and steering speed were in line with design expectation. Experimental
results on the connection between improved coupling of lower hybrid waves in high-density plasmas and reduced
wave spectral broadening were interpreted by fully kinetic, non-linear model calculations. A dual-frequency, timeof-
flight diagnostic for the measurement of density profiles was developed and successfully tested. Fishbone-like
instabilities driven by energetic electrons were simulated by the hybrid MHD-gyrokinetic XHMGC code.
[Show abstract][Hide abstract] ABSTRACT: The turbulence in the scrape-off layer (SOL) plasma of FTU is characterized in order to assess its effect on the current drive efficiency of the lower hybrid (LH) waves. Amplitude, frequency and perpendicular wave vector of the fluctuations are measured for a variety of the main plasma conditions in front of the LH antenna together with the temperature and density in the SOL and used as inputs for the linear scattering theory of the LH waves developed many years ago. This theoretical model can account for both the frequency spectral broadening of the LH pump and the variations of the driven current, inferred by the perpendicular fast electron bremsstrahlung signals. The fraction of the LH power that is then deduced to be effective for current drive appears to be well related to the calculated optical thickness τ of the SOL. It drops as low as 40% as τ increases, consistent with the model prediction. Possible ways to control the SOL optical depth are investigated and a clear relation of the fluctuation level with the collisionality is found.
[Show abstract][Hide abstract] ABSTRACT: New FTU ohmic discharges with a liquid lithium limiter at IP = 0.7–0.75 MA, BT = 7 T and ne0 ≥ 5 × 1020 m−3 confirm the spontaneous transition to an enhanced confinement regime, 1.3–1.4 times ITER-97-L, when the density peaking factor is above a threshold value of 1.7–1.8. The improved confinement derives from a reduction of electron thermal conductivity (χe) as density increases, while ion thermal conductivity (χi) remains close to neoclassical values. Linear microstability reveals the importance of lithium in triggering a turbulent inward flux for electrons and deuterium by changing the growth rates and phase of the ion-driven turbulence, while lithium flux is always directed outwards. A particle diffusion coefficient, D ~ 0.07 m2 s−1, and an inward pinch velocity, V ~ 0.27 m s−1, in qualitative agreement with Bohm–gyro-Bohm predictions are inferred in pellet fuelled lithized discharges. Radio frequency heated plasmas benefit from cleaner plasmas with edge optimized conditions. Lower hybrid waves penetration and current drive effects are clearly demonstrated at and above ITER densities thanks to a good control of edge parameters obtained by plasma operations with the external poloidal limiter, lithized walls and pellet fuelling. The electron cyclotron (EC) heating system is extensively exploited in FTU for contributing to ITER-relevant issues such as MHD control: sawtooth crash is actively controlled and density limit disruptions are avoided by central and off-axis deposition of 0.3 MW of EC power at 140 GHz. Fourier analysis shows that the density drop and the temperature rise, stimulated by modulated EC power in low collisionality plasmas are synchronous, implying that the heating method is the common cause of both the electron heating and the density drop. Perpendicularly injected electron cyclotron resonance heating is demonstrated to be more efficient than the obliquely injected one, reducing the minimum electric field required at breakdown by a factor of 3. Theoretical activity further develops the model to interpret high-frequency fishbones on FTU and other experiments as well as to characterize beta-induced Alfvén eigenmodes induced by magnetic islands in ohmic discharges. The theoretical framework of the general fishbone-like dispersion relation is used for implementing an extended version of the HMGC hybrid MHD gyrokinetic code. The upgraded version of HMGC will be able to handle fully compressible non-linear gyrokinetic equations and 3D MHD.
[Show abstract][Hide abstract] ABSTRACT: The present work compares and experimentally validates the results coming out from the following three Lower Hybrid (LH) coupling codes: Brambilla code (M. Brambilla), GRILL3D‐U (Mikhail Irzak, A. F. Ioffe Physico‐Technical Institute, Russia) and TOPLHA (Politecnico di Torino, Italy). The conventional grill antenna, operating in FTU in different scenarios, is used as benchmark.
The validation with experimental data is carried out with respect to the average reflection coefficients at the input of a row of the grill, considering two different phasings between adjacent waveguides: −90 ° and −75 °. A comparison between calculated power spectra is also presented.
Good agreement can be observed for all the simulated plasma profiles and waveguide phasings between experimental data and codes, in particular for the most recent numerical tools, namely GRILL3D‐U and TOPLHA.
19th Topical Conference of Radio Frequency Power in Plasmas; 06/2011
[Show abstract][Hide abstract] ABSTRACT: In the frame of the EFDA task HCD-08-03-01, a 5 GHz Lower Hybrid system which should be able to deliver 20 MW CW on ITER and sustain the expected high heat fluxes has been reviewed. The design and overall dimensions of the key RF elements of the launcher and its subsystem has been updated from the 2001 design in collaboration with ITER organization. Modeling of the LH wave propagation and absorption into the plasma shows that the optimal parallel index must be chosen between 1.9 and 2.0 for the ITER steady-state scenario. The present study has been made with n|| = 2.0 but can be adapted for n|| = 1.9. Individual components have been studied separately giving confidence on the global RF design of the whole antenna.
[Show abstract][Hide abstract] ABSTRACT: Steady-state long pulse operation of tokamaks requires fully non-inductive current drive, thus an external current drive method. Lower Hybrid Current Drive is recognized as one of the most efficient technique used in the present day tokamaks. Progress of the conceptual design of the LHCD ITER relevant system is reported, as well as critical technology issues.
[Show abstract][Hide abstract] ABSTRACT: In the framework of the EFDA task HCD-08-03-01, the ITER lower hybrid current drive (LHCD) system design has been reviewed. The system aims to generate 24 MW of RF power at 5 GHz, of which 20 MW would be coupled to the plasmas. The present state of the art does not allow envisaging a unitary output of the klystrons exceeding 500 kW, so the project is based on 48 klystron units, leaving some margin when the transmission lines losses are taken into account. A high voltage power supply (HVPS), required to operate the klystrons, is proposed. A single HVPS would be used to feed and operate four klystrons in parallel configuration. Based on the above considerations, it is proposed to design and develop twelve HVPS, based on pulse step modulator (PSM) technology, each rated for 90 kV/90 A. This paper describes in details, the typical electrical requirements and the conceptual design of the proposed HVPS for the ITER LHCD system.
[Show abstract][Hide abstract] ABSTRACT: The use of rectangular oversized waveguides in the Main Transmission Lines of the Lower Hybrid Current Drive (LHCD) system of ITER, requires to investigate the problem of bends. The principal specifications that characterize the oversized bend design concern the minimization of the reflection of the fundamental mode and the maximization of its transmission, limiting at the same time its coupling to spurious modes that could propagate at the operational frequency. In this paper, the performances of bends with different geometries are compared. They are simulated using the commercial finite element software Ansoft HFSS. An innovative modified mitre-bend solution with trapezoidal-elements is proposed and analyzed.
[Show abstract][Hide abstract] ABSTRACT: In order to assist the design of the future ITER Lower Hybrid launcher, coupling codes ALOHA, from CEA/IRFM, TOPLHA, from Politecnico di Torino, and GRILL3D, developed by Dr. Mikhail Irzak (A.F. Ioffe Physico-Technical Institute, St. Petersburg, Russia) and operated by ENEA Frascati, have been compared with the updated (six modules with four active waveguides per module) Passive-Active Multi-junction (PAM) Lower Hybrid antennas. Both ALOHA and GRILL3D formulate the problem in terms of rectangular waveguides modes, while TOPLHA is based on boundary-value problem with the adoption of a triangular cell-mesh to represent the relevant waveguides surfaces. Several plasma profiles, with varying edge density and density increase, have been adopted to provide a complete description of the simulated launcher in terms of reflection coefficient, computed at the beginning of each LH module, and of power spectra. Good agreement can be observed among codes for all the simulated profiles.
[Show abstract][Hide abstract] ABSTRACT: An overview of the FTU results during the period 1998-2000 is presented. FTU has operated up to the nominal parameters (B=8T, I=1.6MA) with good reliability. Using the high-speed, multiple pellet injection system in 8T/1.25MA discharges a phase lasting a few energy confinement time has been achieved with improved confinement properties with peaked density profiles (n=4×10 20 m -3), T e ≈T i and Z eff ≈ 1.3. Up to 14 keV of electron temperature have been obtained at high density using electron cyclotron resonance heating (ECRH) on the current ramp. The transport analysis shows a very low electron heat transport in the region with flat/hollow safety factor profile. Synergy studies have been performed with simultaneous injection of lower hybrid and electron cyclotron waves in 7.2T discharges, well above the ECRH resonance. Clear evidence has been obtained of electron cyclotron wave absorption by the lower hybrid produced electron tails. Stabilisation of m=2 tearing modes has been obtained using ECRH, with subsequent improvement of the energy confinement. Ion Bernstein wave injection in high magnetic field (B=8T) plasmas has shown the reduction of the electron thermal conductivity in the region inside the absorption radius possibly due to the formation of an internal transport barrier.
[Show abstract][Hide abstract] ABSTRACT: Oversized waveguides allow to transmit high electromagnetic power, but require components able to filter the unwanted modal content unavoidably excited along their path. In this paper mode filters based on corrugations partially filled with absorbing materials are described; they have been studied aiming at optimizing their performance and overcoming their intrinsic limitations. More precisely the dependence of the filtering performance on various geometrical parameters is derived using analytical and numerical approaches. The problems related with the power handling capability of mode filters are shortly considered too. Finally the role of degenerate modes in deteriorating the filtering performance is explained and a solution to this problem is presented.
[Show abstract][Hide abstract] ABSTRACT: 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.