Simulation of main plasma parameters of a cylindrical asymmetric capacitively coupled plasma micro-thruster using computational fluid dynamics

Computational fluid dynamics (CFD) simulations of a radio-frequency (13.56 MHz) electrothermal capacitively coupled plasma (CCP) micro-thruster have been performed using the commercial CFD-ACE+ package. Standard operating conditions of a 10W, 1.5 Torr argon discharge were used to compare with previously obtained experimental results for validation. Results show that the driving force behind plasma production within the thruster is ion-induced secondary electrons ejected from the surface of the discharge tube, accelerated through the sheath to electron temperatures up to 33.5 eV. The secondary electron coefficient was varied to determine the effect on the discharge, with results showing that full breakdown of the discharge did not occur for coefficients less than or equal to 0.01

A POLYTROPIC MODEL for SPACE and LABORATORY PLASMAS DESCRIBED by BI-MAXWELLIAN ELECTRON DISTRIBUTIONS

Non-local electron energy probability functions (EEPFs) are shown to have an important effect on the thermodynamic behavior of plasmas in the context of solar wind and laboratory plasmas. A conservation relation is held for electron enthalpy and plasma potential during the electron transport. For an adiabatic system governed by non-local electron dynamics, the correlation between electron temperature and density can be approximated by a polytropic relation, with different indexes demonstrated using three cases of bi-Maxwellian EEPFs. This scenario differs from a local thermodynamic equilibrium having a single polytropic index of 5/3 for adiabaticity

Principle of radial transport in low temperature annular plasmas

Radial transport in low temperature annular plasmas is investigated theoretically in this paper. The electrons are assumed to be in quasi-equilibrium due to their high temperature and light inertial mass. The ions are not in equilibrium and their transport is analyzed in three different situations: a low electric field (LEF) model, an intermediate electric field (IEF) model, and a high electric field (HEF) model. The universal IEF model smoothly connects the LEF and HEF models at their respective electric field strength limits and gives more accurate results of the ion mobility coefficient and effective ion temperature over the entire electric field strength range. Annular modelling is applied to an argon plasma and numerical results of the density peak position, the annular boundary loss coefficient and the electron temperature are given as functions of the annular geometry ratio and Paschen number

Approximants to the Tonks-Langmuir theory for a collisionless annular plasma

Maclaurin series approximant and Padé rational approximant are used to solve the Tonks-Langmuir theory for an annular plasma and investigate the radial transport behavior of charged particles. Coefficients of the well-known Maclaurin approximant are given in a novel form of recurrence relations which are convenient for computation and present a lower limit for the annular ratio of inner radius to outer radius (i.e., this approximant is not applicable to annular geometries with small inner radii). The newly introduced Padé approximant extrapolates the annular ratio limit determined by the Maclaurin approximant to a lower value and hence is applicable to most annular geometries. General radial profiles of the normalized plasma density and mean drift velocity of ions are given across the annulus and they are independent of the gas type and the Paschen number of the discharge. The annular modeling is applied to an argon plasma and obtains the electron temperature as a function of the Paschen number for different annular geometries

In situ electrostatic characterisation of ion beams in the region of ion acceleration

In situ and ex situ techniques have been used to measure directional ion beams created by a sharp axial potential drop in low pressure expanding plasmas. Although Retarding Field Energy Analysers (RFEAs) are the most convenient technique to measure the ion velocities and plasma potentials along with the plasma density, they are bulky and are contained in a grounded shield that may perturb the electric potential profile of the expanding plasma. In principle, ex situ techniques produce a more reliable measurement and Laser Induced Fluorescence spectroscopy (LIF) has previously been used to characterise the spatial velocity profile of ion beams in the same region of acceleration for a range of pressures. Here, satisfactory agreement between the ion velocity profiles measured by LIF and RFEA techniques has allowed the RFEA method to be confidently used to probe the ion beam characteristics in the regions of high gradients in plasma density and DC electric fields which have previously proven difficult

Spectral measurements of inductively coupled and m =+1,-1 helicon discharge modes of the constructed plasma source

In the present context, the industrial type of Amirkabir helicon plasma source has been introduced that was designed and constructed at the Helicon Plasma Laboratory of Amirkabir University of Technology with the aim of using it in material processing applications. Helicon plasma in two m = +1 and m = −1 modes of operation was studied, and also its application was compared with the inductively coupled plasma (ICP) mode in this experimental work. This study was performed by employing two techniques including optical emission spectroscopy and imaging using a camera with polarizing filters, in which the images and spectra of ICP and m = +1, −1 helicon plasma modes were recorded under the experimental conditions. The effects of the device operational parameters on the argon plasma emission spectra were investigated in the wavelength range of 350-950 nm. It was observed from the comparison of the plasma spectra that the ionization rate increases significantly for the plasma helicon mode than ICP and also for m = +1 helicon mode of operation than m = −1. In this work, the values of device operational parameters such as the RF power delivered to the half-helix antenna, external magnetic field intensity, and the injected gas flow rate were varied in the range of 400-900 W, 100-300 mT, and 1-10 SCCM in the experiment, respectively. In addition, the optimum values of RF power, magnetic field intensity, and the injected gas flow rate for achieving the maximum ionization rate were, respectively, obtained as 900 W, 300 mT, and 3 SCCM.The research was carried out under the current budget of the Amirkabir University of Technology. Partial support from the Iran National Science Foundation through Grant No. 97004488 is acknowledged and appreciated

Density Measurements in Low Pressure, Weakly Magnetized, RF Plasmas: Experimental Verification of the Sheath Expansion Effect

This experimental study shows the validity of Sheridan’s method in determining plasma density in low pressure, weakly magnetized, RF plasmas using ion saturation current data measured by a planar Langmuir probe. The ion density derived from Sheridan’s method which takes into account the sheath expansion around the negatively biased probe tip, presents a good consistency with the electron density measured by a cylindrical RF-compensated Langmuir probe using the Druyvesteyn theory. The ion density obtained from the simplified method which neglects the sheath expansion effect, overestimates the true density magnitude, e.g., by a factor of 3 to 12 for the present experiment.This research was partially funded by the Australian Space Research Program (APT project) and the Australian Research Council Discovery Project (DP140100571)

Naphthalene as a cubesat cold gas thruster propellant

The “cubesat” form factor (multiples of 10 × 10 × 10 cm3 volume and 1.33 kg mass) has been adopted as the defacto standard for a cost effective and modular, nano-satellite platform. Many commercial options exist for nearly all components required to build such satellite; however, there is a limited range of thruster options that suit the power and size restrictions of a cubesat. This work presents the design, implementation and direct thrust measurements of a proof of concept cubesat cold gas thruster system using naphthalene (C10H8) as the propellant. The proposed design is optimized for simplicity to match the requirement of entry level cubesat missions, yet, due to the properties of naphthalene, it can achieve a total impulse in the order of tens of newton-seconds

Current-Free Electric Double Layer in a Small Collisional Plasma Thruster Nozzle Simulation

A computational fluid dynamics and plasma model of a collisional (~ a few Torr) radiofrequency (at 13.56 MHz) argon plasma capacitively coupled in a converging-diverging nozzle (applied to the optimization of electrothermal plasma thrusters for space use) shows the formation of a strong stationary current-free double layer (CFDL) at the 1.5 mm diameter nozzle throat for a downstream pressure of ~ 0.1 Torr. The cycle average magnitude of the double layer potential is ΔΦDL = 77 V and the electron temperature at the high potential edge of the double layer is kBTe = 2.64 eV, yielding a strength of ΔΦDL/(kBTe) ~ 30. The double layer is 1.2 mm wide which corresponds to ~ 90 Debye lengths. The axial electric field of the double layer accelerates ions along the nozzle to a maximum drift velocity of 17 km s−1, about 3.3 times the ion sound speed, and their kinetic energy is transferred to neutrals by ion-neutral charge exchange collisions. The ion transit time τi through the potential structure spontaneously forming at the nozzle throat is about 5 times the radiofrequency excitation period τRF. These findings are discussed in the broader context of double layer physics and the dynamics of their formation as well as in the context of electrothermal thruster optimization in which neutral propellant heating via ion-neutral charge exchange collisions is the main source of thrust