Davide Melazzi

ADAMANT : a surface and volume integral-equation solver for the analysis and design of helicon plasma sources

We present a full-wave numerical tool, dubbed ADAMANT (Advanced coDe for Anisotropic Media and ANTennas), devised for the analysis and design of radiofrequency antennas which drive the discharge in helicon plasma sources. ADAMANT relies on a set of coupled surface and volume integral equations in which the unknowns are the surface electric current density on the antenna conductors and the volume polarization current within the plasma. The latter can be inhomogeneous and anisotropic whereas the antenna can have arbitrary shape. The set of integral equations is solved numerically through the Method of Moments with sub-sectional surface and volume vector basis functions. This approach allows the accurate evaluation of the current distribution on the antenna and in the plasma as well as the antenna input impedance, a parameter crucial for the design of the feeding and matching network. We report several numerical examples which serve to validate ADAMANT against other well-established numerical approaches as well as experimental data. The numerical accuracy of the computed solution versus the number of basis functions in the plasma is also assessed. Finally, we employ ADAMANT to characterize the antenna of a real-life helicon plasma source.

SPIREs: A finite-difference frequency-domain electromagnetic solver for inhomogeneous magnetized plasma cylinders

Abstract We present SPIREs (plaSma Padova Inhomogeneous Radial Electromagnetic solver), a Finite-Difference Frequency-Domain (FDFD) electromagnetic solver in one dimension for the rapid calculation of the electromagnetic fields and the deposited power of a large variety of cylindrical plasma problems. The two Maxwell wave equations have been discretized using a staggered Yee mesh along the radial direction of the cylinder, and Fourier transformed along the other two dimensions and in time. By means of this kind of discretization, we have found that mode-coupling of fast and slow branches can be fully resolved without singularity issues that flawed other well-established methods in the past. Fields are forced by an antenna placed at a given distance from the plasma. The plasma can be inhomogeneous, finite-temperature, collisional, magnetized and multi-species. Finite-temperature Maxwellian effects, comprising Landau and cyclotron damping, have been included by means of the plasma Z dispersion function. Finite Larmor radius effects have been neglected. Radial variations of the plasma parameters are taken into account, thus extending the range of applications to a large variety of inhomogeneous plasma systems. The method proved to be fast and reliable, with accuracy depending on the spatial grid size. Two physical examples are reported: fields in a forced vacuum waveguide with the antenna inside, and forced plasma oscillations in the helicon radiofrequency range.

A comparative study of radiofrequency antennas for Helicon plasma sources

Since Helicon plasma sources can efficiently couple power and generate high-density plasma, they have received interest also as spacecraft propulsive devices, among other applications. In order to maximize the power deposited into the plasma, it is necessary to assess the performance of the radiofrequency (RF) antenna that drives the discharge, as typical plasma parameters (e.g. the density) are varied. For this reason, we have conducted a comparative analysis of three Helicon sources which feature different RF antennas, namely, the single-loop, the Nagoya type-III and the fractional helix. These antennas are compared in terms of input impedance and induced current density; in particular, the real part of the impedance constitutes a measure of the antenna ability to couple power into the plasma. The results presented in this work have been obtained through a full-wave approach which (being hinged on the numerical solution of a system of integral equations) allows computing the antenna current and impedance self-consistently. Our findings indicate that certain combinations of plasma parameters can indeed maximize the real part of the input impedance and, thus, the deposited power, and that one of the three antennas analyzed performs best for a given plasma. Furthermore, unlike other strategies which rely on approximate antenna models, our approach enables us to reveal that the antenna current density is not spatially uniform, and that a correlation exists between the plasma parameters and the spatial distribution of the current density.

Beam-Forming and Beam-Steering Capabilities of a Reconfigurable Plasma Antenna Array

We present the numerical parametric study of a reconfigurable plasma antenna array (PAA) composed of a metallic half-wavelength dipole and a set of cylindrical plasma discharges arranged in a planar square lattice. Our results, obtained with the linear embedding via Green’s operators (LEGO) method, indicate that beam-forming and beam-steering functionality can be achieved and controlled by appropriately choosing the number and position of the active plasma discharges around the dipole. Furthermore, we show that an external static magnetic field and the plasma density have a noticeable effect on the radiation pattern of the antenna.

Ray-tracing WKB analysis of Whistler waves in non-uniform magnetic fields applied to space thrusters

Radiofrequency magnetized cylindrical plasma sources are proposed for the development of space thrusters, whose thrust efficiency and specific impulse depend on the power coupled into the plasma. At this stage of research, emphasis has been on the absorption of Whistler wave energy by non-uniform plasmas but not much on the role played by the magneto-static confinement field, considered uniform, constant and aligned with the axis of the source. We present RAYWh (RAY-tracing Whistler), a three-dimensional (3D) ray-tracing solver for electromagnetic propagation and power deposition in cylindrical plasma sources for space plasma thrusters, where actual magnetic confinement configurations along with plasma density profiles are included. The propagation and absorption of Whistler waves are investigated by solving the 3D Maxwell–Vlasov model equations by a Wentzel–Kramers–Brillouin (WKB) asymptotic expansion. The reduced set of equations for the wave phase and for the square amplitude of the electric field is solved numerically by means of a modified Runge–Kutta algorithm. Unexpected cut-offs, resonances, radial reflections, mode conversions and power deposition profile of the excited waves are found, when realistic confinement magnetic fields are considered. An analysis of the influence of axial wavenumbers and the axial length of the system on the power deposition is presented.

Modelling and Optimization of Electrode-less Helicon Plasma Thruster with Different Propellants

In the framework of the ”Helicon Plasma Thruster for Space Missions AO7048” research program, a zero-dimensional numerical model is developed in order to assess the propulsive performances of the plasma thruster when fed with different molecular gases as propellants. The zero-dimensional model developed consists of detailed kinetic schemes for bulk plasma reactions and surface processes; particle in cell simulations are performed to properly describe the behaviour of the charged particles at the exhaust plume so to obtain exhaust parameters for the global model. Given a set of input parameters, such as geometry and mass flow rate, the model is able to follow the time evolution of plasma main physical quantities, such as neutral and charge densities and the electron temperature, and to calculate thruster performances. The model is then linked to the genetic algorithm optimization toolbox in Matlab environment in order to obtain high performance configurations. Propellants considered are H2, O2, N2 and N2O.

Numerical results on the performance of gaseous plasma antennas

A plasma antenna is a radiating device partially composed of plasma instead of metals or dielectrics, and as a result, it exhibits reconfigurable properties unlike conventional metallic antennas. By tuning the plasma discharge parameters, e.g., the plasma density, the antenna properties can be changed dynamically. In this work we report on recent numerical investigations into the characteristics of plasma antennas as a function of the plasma discharge parameters. We have used ADAMANT (Advanced coDe for Anisotropic Media and ANTennas) - a full-wave numerical tool based on integral equations - to assess the role played by plasma discharge parameters in shaping the gain function, and the antenna input impedance.

Plasma source optimization for multispecies helicon plasma thruster

Radiofrequency (RF) magnetized Helicon plasma sources have been proposed for the development of space thrusters, whose thrust efficiency and specific impulse depend on the power coupled into the plasma by the RF antenna that drives the discharge. In this work we report on a set of numerical experiments specifically conceived to optimize Helicon sources in terms of plasma parameters (e.g., gas species, plasma density, external magnetostatic field, neutral pressure), and the RF antenna shape. Results concerning the power coupled into the plasma and the antenna impedance for different antenna configurations and plasma parameters are presented and discussed.

An integral-equation approach to the analysis and design of plasma antennas

Plasma antennas constitute a promising alternative to conventional metallic antennas for applications in which reconfigurability with respect to some property is desired. The latter feature can be achieved by tuning the plasma discharge parameters. However, simplified models have been employed so far for the analysis of such devices, and the influence of an external magnetizing field on plasma antenna behavior have not been fully understood. In this communication we present an integral-equation approach for the analysis of a magnetized plasma in the presence of metal parts which in turn provide the plasma excitation. The radiation pattern is mainly determined by the plasma current distribution, whereas the input impedance of the overall antenna system ensues from the knowledge of the surface current density at the excitation port. Preliminary numerical results are in good agreement with data available in existing literature, and confirm that the radiation efficiency can be controlled by adjusting plasma density and magnetizing field.

Analytical and Numerical Study of a Gaseous Plasma Dipole in the UHF Frequency Band

Gaseous plasma antennas are appealing in applications in which reconfigurability is desired, because the radiation properties can be changed by tuning the plasma parameters. In this paper, an analytical and numerical analysis of a gaseous plasma dipole that works in the 0.3–3 GHz frequency range is presented; the plasma dipole is comprised of two discharges that are arranged similar to the arms of a center-fed metallic dipole. The analytical approach can provide the short- and open-circuit resonance frequencies of the plasma dipole, which have been proven reconfigurable by adjusting the discharge length and by adjusting the density of the plasma therein. Thanks to the numerical approach, the role of the type and pressure of the neutral gas species, and the magneto-static field on the reflection coefficient, and the gain function has been investigated. Finally, a feasibility study of an actual plasma dipole is presented; specifically, the choice of coupling the signal to be radiated by means of electrodes immersed in the plasma is a key to achieve radiation efficiency close to 90%.