Marco Manente

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 microwave interferometer for small and tenuous plasma density measurements

The non-intrusive density measurement of the thin plasma produced by a mini-helicon space thruster (HPH.com project) is a challenge, due to the broad density range (between 10(16) m(-3) and 10(19) m(-3)) and the small size of the plasma source (2 cm of diameter). A microwave interferometer has been developed for this purpose. Due to the small size of plasma, the probing beam wavelength must be small (λ = 4 mm), thus a very high sensitivity interferometer is required in order to observe the lower density values. A low noise digital phase detector with a phase noise of 0.02° has been used, corresponding to a density of 0.5 × 10(16) m(-3).

Off-line ionization tests using the surface and the plasma ion sources of the SPES project.

The development of new target ion source systems for the selective production of exotic species (SPES) facility is currently in progress at Legnaro National Laboratories. In this context, the study of ion sources and their performance in terms of ionization efficiency and transversal emittance is a crucial point in order to maximize the available yields, particularly for short-lived isotopes. In this work, preliminary off-line ionization efficiency and emittance measurements for the SPES surface and plasma ion sources are presented. The plasma source emittance measurements are supported by dedicated numerical calculations.

Ion Velocimetry Measurements and Particle-In-Cell Simulation of a Cylindrical Cusped Plasma Accelerator

The Stanford Cylindrical Cusped Field Thruster (CCFT) has been experimentally and numerically investigated with particular focus on the exit plane acceleration region near the top magnetic cusp. Time-averaged xenon ion laserinduced fluorescence measurements using the 5d[4]7/2 -6p[3]5/2 (λ = 834.72-nm air) Xe II transition have mapped the total ion velocity vectors in this region. The thruster is also simulated using the fully kinetic 3-D particle-in-cell code F3MPIC. The consistent experimental and numerical results give physical insight into the mechanisms of ion acceleration and the role of the magnetic field topology in determining ion trajectories and plume divergence. The electrons are strongly magnetized and follow the magnetic field structure, grouping near the cusps. A steep potential drop over a few millimeters near the exit plane follows the magnetic separatrix of the top cusp, and is consistent with measured ion velocity vectors. A characteristic conical region of high ion density, peak ion velocity, and visible emission is observed in the experimental and simulated plume, with an estimated divergence half-angle of 30°.

Numerical simulation of the Helicon Double Layer Thruster concept

During last years some laboratories obtained current free helicon double layers in experiments with both electropositive and electronegative gases. The current free double layer has potential application as a plasma thruster. Although considerable progress has been made, at the present time a number of aspects related to this phenomenon are still only partially understood. This paper presents results obtained by numerical simulation of double layer formation, stability and characteristics and explores the applicability of this concept to a space mission. The analysis has been conducted using a combination of 1-D and 2-D numerical codes. After the modelization of helicon source, the code is applied to simulate a simple plasma thruster.

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.