Logan T. Williams

Lifetime and Failure Mechanisms of an Arrayed Carbon Nanotube Field Emission Cathode

There is interest in the use of carbon nanotubes (CNTs) to create a field emission (FE) cathode for the neutralization of exhaust plumes of low-power (<; 500 W) electric propulsion devices since FE cathodes do not require a gas flow to operate. To incorporate CNT emitters into propulsion systems, the current emission output over the lifetime of the cathode must be understood. Multiple FE cathodes that consist of multiwalled CNT arrays have been fabricated. Seven cathodes are characterized at pressures below 10 5 T at constant voltage between the CNTs, and the gate until failure occurs. The maximum current density observed is 9.08 mA/cm2 with average current densities up to 2.52 mA/cm2, and the maximum life span is 368 h. The behavior of the cathode current emission is highly unstable and consists of oscillations and sudden shifts. Resistive heating is believed to be the primary cause for failure in two thermally assisted modes: 1) oxidative ablation at the root of the nanotube and 2) field evaporation at the tip.

Thrust Measurements of a Helicon Plasma Source

There is interest in using a helicon plasma source in propulsive applications as both an ion source and as a thruster. Developing a helicon thruster requires a performance baseline as a basis for future optimization and expansion. The thrust of a helicon is measured using an null-type inverted pendulum thrust stand at a pressure of 2x10 -5 Torr through the operating range of 215-840 W RF power, 11.9 and 13.56 MHz RF frequency, 150-450 G magnetic field strength, and 1.5-4.5 mg/s propellant flow rate for argon. Maximum thrust is found to be 6.27 mN at 142 s of specific impulse, and maximum specific impulse at 377 s at 5.55 mN. Thrust is primarily increased by increasing RF power, with smaller gains from magnetic field strength and mass flow rate. Maximum efficiency is 1.37% and is limited by a combination of low mass utilization and high beam divergence.

Estimating the Enthalpy of Gasification of Acrylonitrile-Butadiene-Styrene Hybrid Rocket Fuels

This report investigates the pyrolysis properties of both extruded and additively manufactured acrylonitrile–acrylonitrile–styrene at material surface temperatures from 350 to 725°C. These temperatures approximate the range of surface temperatures reached during combustion when acrylonitrile–acrylonitrile–styrene is employed as a hybrid rocket propellant. This temperature range is significantly higher than temperatures evaluated during previous acrylonitrile–acrylonitrile–styrene fire-safety combustion and pyrolysis studies. Linear regression rates of acrylonitrile–acrylonitrile–styrene fuel grain material specimens are measured during pyrolysis and used to derive estimates for both the molar and mass-based enthalpies of gasification. Molar values for enthalpy of gasification are estimated from the regression rate data using an Arrhenius-type process model. The mass-based enthalpy of ablation is calculated from the power duty cycle of the heating element used to produce the fuel pyrolysis. Test results co...

Ion production cost of a gridded helicon ion thruster

Helicon plasma sources are capable of efficiently ionizing propellants and have been considered for application in electric propulsion. However, studies that estimate the ion production cost of the helicon plasma source are limited and rely on estimates of the extracted ion current. The ion production cost of a helicon plasma source is determined using a gridded ion thruster configuration that allows accurate measurement of the ion beam current. These measurements are used in conjunction with previous characterization of the helicon plasma to create a model of the discharge plasma within the gridded thruster. The device is tested across a range of operating conditions: 343‐600W radio frequency power at 13.56MHz, 50‐250G and 1.5mgs −1 of argon at a pressure of 1.6 × 10 −5 Torr-Ar. The ion production cost is 132‐212 ± 28‐46eV/ion, driven primarily by ion loss to the walls and anode, as well as energy loss in the anode and grid sheaths. (Some figures may appear in colour only in the online journal)

Design and Operation of an Annular Helicon Plasma Source

*† ‡ § An annular helicon plasma source for the ionization stage of a two-stage Hall effect thruster is considered. The magnetic and electric field structures in the plasma source are computed considering classical helicon wave and Trivelpiece-Gould wave. The antenna coupling and power deposition mechanism model is developed to determine the effective antenna geometry. Methodology for selecting the antenna power, frequency, and applied magnetic field strength is presented. Using the theoretical model, the annular helicon source compatible with a 5-kW Hall thruster is sized and built.

Plume Structure and Ion Acceleration of a Helicon Plasma Source

Helicon plasma sources are capable of efficiently ionizing propellants and have been considered for application in electric propulsion. The literature suggests that the ion acceleration mechanism is a current-free double layer. Previous work shows that single-stage helicon thrusters can produce thrust in the range of 1-6 mN, but it is unknown whether the thrust contribution is due to direct ion acceleration versus thermal expansion. The ion energy and current density profiles in the plume of a helicon plasma source are measured across a range of operating conditions: 343-600 W RF power at 13.56 MHz, 50-350 G, and 1.5-mg/s Ar at a pressure of 1.6 × 10-5 torr-Ar. The plasma potential, electron temperature, and ion number density are also measured inside the discharge chamber and in the plume up to 60 cm downstream of the exit plane and 45-cm radially outward from the device axis. Ions are found to have energies in the range of 20-40 V, with total beam currents in the range of 7-47 mA. The plume has an average divergence half angle of 82°, either evenly distributed across all angles or focused at large angles to the centerline. From these measurements, it is found that the estimated thrust due to ion acceleration is far less than what has been directly measured on the same device in previous work.

Experimenal Characterization of a Carbon Nanotube Field Emission Cathode

There is interest in the use of carbon nanotubes (CNTs) to create a field emission cathode. The primary benefit of this cathode is that a gas flow is not required to create electrons, which is of critical importance for space missions that must minimize propellant mass. A field emission cathode consisting of CNT arrays is fabricated by the Georgia Tech Research Institute and the Georgia Tech High-Power Electric Propulsion Laboratory. The cathode is tested at pressures below 3 x 10 -5 Torr. The average output current density is measured over a cathode voltage range of 250-800 V relative to the gate for several CNT cathodes. The highest stable emission current density is 0. 6 mA/cm 2 . The emission current density is measured throughout a 50-hour life test at a cathode voltage of 550 V which demonstrated an overall constant emission current density of 0.51 mA/cm 2 with an emission current to input power ratio of 1.7 mA/W.

Pyrolysis of Acrylonitrile-Butadiene-Styrene ABS Under High Heat Flux Conditions

Acrylonitrile-butadiene-styrene (ABS) is a common industrial plastic that is widely used for structural and piping applications. Because ABS possesses a variety of advantageous material properties, within the past three years ABS plastic blends have been investigated as a potential fuel for hybridand solid-propelled rocket systems. Promising results have been achieved. Because ABS as a rocket propellant is a very recent development, a database describing the pyrolysis properties of ABS at the temperatures and heating rates experienced by rocket systems does not exist. Especially important is the current lack of established values for the specific enthalpy of gasification (latent heat of vaporization) for various ABS material formulations. All existing ABS pyrolysis data were collected during fire prevention studies, and were performed at heating rates nearly an order of magnitude lower than those experienced during rocket combustion. This report investigates the pyrolysis properties of ABS at high flux levels and material temperatures from 350 to 700 C. Linear regression rates are measured and used to derive estimates for the specific enthalpy of gasification based on the power duty cycle of the heating element used to produce the fuel pyrolysis. Results for both extruded and additively manufactured ABS are compared.

Experimental Characterization of Carbon Nanotube Field Emission Cathode Lifetime

There is interest in the use of carbon nanotubes (CNTs) to create a field emission cathode for small satellite applications and the neutralization of exhaust plumes of low-power electric propulsion devices since field emission cathodes do not require a gas flow to operate. As a part of the cathode’s development, the current emission output over the lifetime of the cathode must be determined. The Georgia Tech Research Institute and the Georgia Tech High-Power Electric Propulsion Laboratory have fabricated multiple field emission cathodes that consist of multi-walled CNT arrays. Seven cathodes are characterized at pressures below 10 -5 Torr at constant voltage between the CNTs and the gate until failure occurs. The maximum current density observed is 9.08 mA/cm 2 , the maximum power density is 9.08 W/cm 2 , and the maximum life-span is 368 hours. The behavior of the cathode current emission consists of oscillations and sudden shifts thought to be caused by CNT interactions. Resistive heating is thought to be the primary cause for failure.