Silvio Ceccuzzi

Current drive at plasma densities required for thermonuclear reactors

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.

Directive EBG Antennas: A Comparison Between Two Different Radiating Mechanisms

This communication compares two different methods for the directivity enhancement of antennas by means of electromagnetic band-gap (EBG) structures. The first approach employs an EBG material working inside the band-gap to create a resonator, where a radiator is placed in between a groundplane and the EBG cover. The second mechanism adopts an EBG structure, working at the edge of the band-gap and embedding a radiating source to excite a proper Bloch wave. The methods are benchmarked in terms of directivity with reference to several optimized two-dimensional configurations, based on either square or triangular lattices of dielectric rods. For a set of selected configurations, the comparison is carried out in terms of electrical and geometrical parameters, as well as with respect to their frequency behaviour and efficiency. The resonator antenna achieves the highest directivity, but it turns out to be less efficient and is outranked by the other method when few rods per layer are employed. Strengths and weaknesses of the two approaches are fully discussed together with their generalization beyond the particular cases.

LOWER HYBRID CURRENT DRIVE FOR DEMO: PHYSICS ASSESSMENT AND TECHNOLOGY MATURITY

Abstract Recent experiments on lower hybrid (LH) penetration at reactor-relevant densities, together with the recent 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 profitably drive far-off-axis plasma current, allowing current profile control and helping in sustaining burning performance. 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 n‖ have been analyzed, indicating the equatorial injection of 5-GHz waves with n‖peak = 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 assumptions, showing that most of the components of the transmission line and, of highest priority, radio-frequency vacuum windows demand intense development.

Mode Filters for Oversized Rectangular Waveguides: A Modal Approach

A semi-analytical approach, relying on the mode-matching method and the resonator technique, is proposed. It is conceived to evaluate the performance of mode filters, based on corrugations partially filled with an absorbing material, in the case of oversized rectangular waveguides. The mathematical formulation allows an accurate and fast computation of the scattering parameters, through closed-form expression of the surface integrals and some matrix algebra. The theoretical model is implemented in a code that is benchmarked against a finite-element method to elucidate its advantages with respect to volumetric solvers. After giving an insight on the physical mechanisms ruling the performance of these devices, the modal method is used to run the particle swarm optimization algorithm for different types of devices, taking advantage of the reduced computation time of the developed tool.

Directive propagation in two EBG structures: A Comparison

Two methods for the enhancement of directive propagation in two-dimensional EBG structures, consisting of either square or triangular lattices of dielectric rods, are compared. The benchmark is carried out in terms of main radiation and geometrical parameters for a set of configurations, chosen among several arrangements having different number of layers and cylinders per layer. In one mechanism a resonator antenna is attained by inserting a line source in between an EBG cover and a ground plane, while, in the other, the same source is embedded within an EBG material, working near its band-gap edge. The favourable configurations of the two methods are identified together with their strengths and weaknesses.

On beam shaping of the field radiated by a line source coupled to finite or infinite photonic crystals

Comparison of the beam-shaping effect of a field radiated by a line source, when an ideal infinite structure constituted by two photonic crystals and an actual finite one are considered, has been carried out by means of two different methods. The lattice sums technique combined with the generalized reflection matrix method is used to rigorously investigate the radiation from the infinite photonic crystals, whereas radiation from crystals composed of a finite number of rods along the layers is analyzed using the cylindrical-wave approach. A directive radiation is observed with the line source embedded in the structure. With an increased separation distance between the crystals, a significant edge diffraction appears that provides the main radiation mechanism in the finite layout. Suitable absorbers are implemented to reduce the above-mentioned diffraction and the reflections at the boundaries, thus obtaining good agreement between radiation patterns of a localized line source coupled to finite and infinite photonic crystals, when the number of periods of the finite structure is properly chosen.

Directive EBG Antennas Based on Lattice Modes

The emission of radiators can be shaped by means of electromagnetic bandgap (EBG) materials exploiting proper Bloch waves supported by the lattice rather than its bandgaps. So far, such a method has relied only on the dispersion diagram of the periodic structure, i.e., on the eigenvalues of the lattice, neglecting the particular configuration of the associated eigenfunction. This paper explores the radiation mechanism under a novel viewpoint, which is mostly focused on the electric field pattern of lattice modes, allowing a deeper understanding of the underlying physics. Such an approach is profitably used here to improve the performance of antennas based on different Bloch waves by reducing spurious lattice modes. Geometrical configurations coupled to a line source are provided with little adjustments, getting EBGs to work at a frequency where they had no bandgap originally. As a proof-of-concept of the proposed perspectives, a compact antenna using a cheap low-permittivity dielectric is conceived, fabricated, and successfully tested.

Tapered EBG superstrates for low-permittivity resonator antennas

High-gain antennas can be implemented applying an EBG structure as superstrate to planar low-gain radiators. A cavity is formed between the ground plane backing the primary source and the EBG superstrate, that is realized with a low-permittivity dielectric, in order to be fabricated with a 3D printer. Due to the finite size of the EBG layers, diffracted fields occur at the edges of the cavity. Furthermore, notwithstanding the considerable gain enhancement, most times side-lobe level is not optimized. EBG layouts with tapered periodicity are proposed to control the reflectivity of the EBG superstrates from the center of the structure towards the edges, in order to reduce diffraction effects and lower the side-lobe level.

On the use of corrugations in mode filters for oversized rectangular waveguides

The filtering of unwanted modes that can propagate in oversized rectangular waveguides is addressed, focusing on longitudinal corrugations partially filled with an absorber. Lengthwise slots, in the middle of top and bottom waveguide walls, can indeed extract the power of some modes from the main waveguide with negligible insertion losses for the working dominant mode. Once some power is coupled through the slot into the junction, it travels toward the absorbing material, where it is damped. This behavior is studied comparing the performance of a single continuous slot, where various modes can propagate, with the one achieved by several shorter single-mode apertures. The excitation and absorption of modes in the corrugations is estimated by means of analytical expressions under a small coupling approximation. Mono-modal corrugations achieve better performances with respect to the overmoded junction.

Conditions for Lower Hybrid Current Drive in ITER

To control the plasma current profile represents one of the most important problems of the research of nuclear fusion energy based on the tokamak concept, as in the plasma column the necessary conditions of stability and confinement should be satisfied. This problem can be solved by using the lower hybrid current drive (LHCD) effect, which was demonstrated to occur also at reactor grade high plasma densities provided that a proper method should be utilised, as assessed on FTU (Frascati Tokamak Upgrade). This method, based on theoretical predictions confirmed by experiment, produces relatively high electron temperature at the plasma periphery and scrape-off layer (SOL), consequently reducing the broadening of the spectrum launched by the antenna produced by parasitic wave physics of the edge, namely parametric instability (PI). The new results presented here show that, for kinetic profiles now foreseen for the SOL of ITER, PI is expected to hugely broaden the antenna spectrum and prevent any penetration in the core of the coupled LH power. However, considering the FTU method and assuming higher electron temperature at the edge (which would be however reasonable for ITER) the PI-produced spectral broadening would be mitigated, and enable the penetration of the coupled LH power in the main plasma. By successful LHCD effect, the control of the plasma current profile at normalised minor radius of about 0.8 would be possible, with much higher efficiency than that obtainable by other tools. A very useful reinforce of bootstrap current effects would be thus possible by LHCD in ITER.