[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.
Fusion Engineering and Design. 03/2011; 86(6-8):823-826.
[show abstract][hide abstract] ABSTRACT: In the frame of the EFDA Task HCD-08-03-01 (EU contribution to the ITER LHCD Development Plan) the preliminary design of the LHCD system, described in the Detailed Design Description DDD 5.4 LH (2001), has been revised to take into account progresses on technology and on knowledge. In particular a revised layout of the ITER machine and the successful development of a prototype klystron at 5 GHz with a target RF power of 500 kW CW have determined significant changes in the design of the main transmission lines and on the modularity of the launcher.
Fusion Engineering and Design 03/2011; 86(6-8):759-762. · 0.84 Impact Factor
[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.
Fusion Engineering and Design. 02/2011; 86(6-8):490-496.
[show abstract][hide abstract] ABSTRACT: A 20 MW Lower Hybrid Current Drive system using an antenna based on the Passive-Active Multijunction (PAM) concept is envisaged on ITER. This paper gives an overview of the mechanical analysis, modeling and design carried out on two major elements of the antenna: the grill front face, and the RF feed-through or windows. The front face will have to withstand high heat and fast neutrons fluxes directly from the plasma. It will be actively cooled and present a beryllium coating upon ITER requirement. The RF window being a critical safety importance class component (SIC) because of its tritium confinement function, two of them will be put in series on each line to achieve a double barrier. A design of a water cooled 5 GHz CW RF “pillbox” window capable of sustaining 500 kW of transmitted power is proposed. Both studies allow to move forward, and focus on critical issues, such as manufacturing processes and R&D associated programs including tests of mock-ups.
Fusion Engineering and Design 02/2011; 86(6-8):810-814. · 0.84 Impact Factor
[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.
Fusion Engineering and Design. 01/2011; 86(6-8):819-822.
[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.
Fusion Engineering and Design. 01/2011; 86(6-8):746-749.
[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.
Fusion Engineering and Design. 01/2011; 86(6-8):827-830.
[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.
Fusion Engineering and Design 12/2010; 86(6-8):909-912. · 0.84 Impact Factor
[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.
[show abstract][hide abstract] ABSTRACT: FAST is a new machine proposed to support ITER experimental exploitation as well as to anticipate DEMO relevant physics and technology. FAST is aimed at studying, under burning plasma relevant conditions, fast particle (FP) physics, plasma operations and plasma wall interaction in an integrated way. FAST has the capability to approach all the ITER scenarios significantly closer than the present day experiments using deuterium plasmas. The necessity of achieving ITER relevant performance with a moderate cost has led to conceiving a compact tokamak (R = 1.82 m, a = 0.64 m) with high toroidal field (BT up to 8.5 T) and plasma current (Ip up to 8 MA). In order to study FP behaviours under conditions similar to those of ITER, the project has been provided with a dominant ion cyclotron resonance heating system (ICRH; 30 MW on the plasma). Moreover, the experiment foresees the use of 6 MW of lower hybrid (LHCD), essentially for plasma control and for non-inductive current drive, and of electron cyclotron resonance heating (ECRH, 4 MW) for localized electron heating and plasma control. The ports have been designed to accommodate up to 10 MW of negative neutral beams (NNBI) in the energy range 0.5–1 MeV. The total power input will be in the 30–40 MW range under different plasma scenarios with a wall power load comparable to that of ITER (P/R ~ 22 MW m−1). All the ITER scenarios will be studied: from the reference H mode, with plasma edge and ELMs characteristics similar to the ITER ones (Q up to ≈1.5), to a full current drive scenario, lasting around 170 s. The first wall (FW) as well as the divertor plates will be of tungsten in order to ensure reactor relevant operation regimes. The divertor itself is designed to be completely removable by remote handling. This will allow us to study (in view of DEMO) the behaviour of innovative divertor concepts, such as those based on liquid lithium.FAST is capable of operating with very long pulses, up to 170 s, despite being a copper machine. The magnets initial operation temperature is 30 K, with cooling provided by helium gas. The in vessel components, namely FW and divertor, are actively cooled by pressurized water above 80 °C. The same water is also used to bake the vacuum vessel. FAST is equipped with ferromagnetic inserts to keep the toroidal field magnet ripple down to 0.3%.
[show abstract][hide abstract] ABSTRACT: FAST (Fusion Advanced Studies Torus) is a new machine that the Italian Fusion Association is proposing to the European scientific community as a Satellite Tokamak to support ITER exploitation and to anticipate DEMO relevant physics and technology. As recommended by the European Fusion Panel, FAST will not use Tritium (unnecessary to address burning plasma peculiar physics) to avoid the problems associated to this element. Therefore to understand the alpha-particles behaviors in burning plasmas, fast ions accelerated by powerful heating and current drive systems (ICRH, ECRH, LHCD) will be used. This paper describes the recent advancements on the design of FAST and its LHCD system.
[show abstract][hide abstract] ABSTRACT: The fishbone-like internal kink instability driven by supra-thermal electrons generated by lower hybrid current drive is an important issue of burning plasma research. Indeed, the trapped particle averaged bounce characterizing the interaction of trapped alpha particles with low-frequency MHD modes in burning plasmas depends on energy, not on mass, hence the charged fusion product effects can be usefully modelled by the analogous effect induced by the fast electrons on the low-frequency MHD modes. Fishbone-like internal kink instabilities driven by electrons were observed during experiments on Frascati Tokamak Upgrade (FTU) and interpreted in terms of an oscillating 'fixed point' activity followed by one of 'limit cycle', produced by suprathermal electrons in the presence of a q-profile with q min ≈ 1. As an interesting behaviour of the fast electron population produced by the LH power when the q-profile meets the condition q min ≈ 1, the fast electron bremsstrahlung data of FTU show a marked redistribution in space across layers centred at r/a ≈ 0.20–0.33. This redistribution occurs during and in phase with the fishbone oscillation observed by the MHD diagnostic, with the same time scale and at the same radial position of the peak emission seen by x-ray tomography.
[show abstract][hide abstract] ABSTRACT: FAST (Fusion Advanced Studies Torus) is a new tokamak machine proposed by the Italian Fusion Association as a Satellite Tokamak for the ITER programme. FAST will operate with deuterium plasmas to avoid the complexity deriving from the use of tritium. Therefore burning plasma conditions, where energy density of fast ions and of charged fusion products is a significant fraction of the total plasma energy density, will be achieved by accelerating plasma ions above the half-MeV range through an Ion Cyclotron Resonance Heating (ICRH) system (P = 30 MW, f = 60–90 MHz). For long pulse Advanced Tokamak (AT) scenarios, a Lower Hybrid Current Drive (LHCD) system (P = 6 MW, f = 3.7 GHz) has been envisaged to actively control the current profile, whereas an Electron Cyclotron Resonant Heating (ECRH) system (P = 4 MW, f = 170 GHz) will provide enough RF power for MHD control.