Samuel Langendorf

Effect of secondary electron emission on the plasma sheath

In this experiment, plasma sheath potential profiles are measured over boron nitride walls in argon plasma and the effect of secondary electron emission is observed. Results are compared to a kinetic model. Plasmas are generated with a number density of 3 × 1012 m−3 at a pressure of 10−4 Torr-Ar, with a 1%–16% fraction of energetic primary electrons. The sheath potential profile at the surface of each sample is measured with emissive probes. The electron number densities and temperatures are measured in the bulk plasma with a planar Langmuir probe. The plasma is non-Maxwellian, with isotropic and directed energetic electron populations from 50 to 200 eV and hot and cold Maxwellian populations from 3.6 to 6.4 eV and 0.3 to 1.3 eV, respectively. Plasma Debye lengths range from 4 to 7 mm and the ion-neutral mean free path is 0.8 m. Sheath thicknesses range from 20 to 50 mm, with the smaller thickness occurring near the critical secondary electron emission yield of the wall material. Measured floating potenti...

Effects of wall electrodes on Hall effect thruster plasma

This paper investigates the physical mechanisms that cause beneficial and detrimental performance effect observed to date in Hall effect thrusters with wall electrodes. It is determined that the wall electrode sheath can reduce ion losses to the wall if positioned near the anode (outside the dense region of the plasma) such that an ion-repelling sheath is able to form. The ability of the wall electrode to form an ion-repelling sheath is inversely proportional to the current drawn—if the wall electrode becomes the dominant sink for the thruster discharge current, increases in wall electrode bias result in increased local plasma potential rather than an ion-repelling sheath. A single-fluid electron flow model gives results that mimic the observed potential structures and the current-sharing fractions between the anode and wall electrodes, showing that potential gradients in the presheath and bulk plasma come at the expense of current draw to the wall electrodes. Secondary electron emission from the wall electrodes (or lack thereof) is inferred to have a larger effect if the electrodes are positioned near the exit plane than if positioned near the anode, due to the difference in energy deposition from the plasma.

Characterization of Hall effect thruster propellant distributors with flame visualization

A novel method for the characterization and qualification of Hall effect thruster propellant distributors is presented. A quantitative measurement of the azimuthal number density uniformity, a metric which impacts propellant utilization, is obtained from photographs of a premixed flame anchored on the exit plane of the propellant distributor. The technique is demonstrated for three propellant distributors using a propane-air mixture at reservoir pressure of 40 psi (gauge) (377 kPa) exhausting to atmosphere, with volumetric flow rates ranging from 15-145 cfh (7.2-68 l/min) with equivalence ratios from 1.2 to 2.1. The visualization is compared with in-vacuum pressure measurements 1 mm downstream of the distributor exit plane (chamber pressure held below 2.7 × 10(-5) Torr-Xe at all flow rates). Both methods indicate a non-uniformity in line with the propellant inlet, supporting the validity of the technique of flow visualization with flame luminosity for propellant distributor characterization. The technique is applied to a propellant distributor with a manufacturing defect in a known location and is able to identify the defect and characterize its impact. The technique is also applied to a distributor with numerous small orifices at the exit plane and is able to resolve the resulting non-uniformity. Luminosity data are collected with a spatial resolution of 48.2-76.1 μm (pixel width). The azimuthal uniformity is characterized in the form of standard deviation of azimuthal luminosities, normalized by the mean azimuthal luminosity. The distributors investigated achieve standard deviations of 0.346 ± 0.0212, 0.108 ± 0.0178, and 0.708 ± 0.0230 mean-normalized luminosity units respectively, where a value of 0 corresponds to perfect uniformity and a value of 1 represents a standard deviation equivalent to the mean.

Hysteresis and mode transitions in plasma sheath collapse due to secondary electron emission

In this experiment, hysteresis is observed in the floating potential of wall material samples immersed in a low-temperature plasma as the energy of a prevalent non-thermal electron population is varied from 30–180 eV. It is indicated that the hysteresis is due to secondary electron emission from the wall material surface. Measurements are performed in a filament discharge in argon gas pressure 10−4 Torr of order 107 cm−3 plasma number density. The primary ionizing electrons from the discharge filament make up 1%–10% of the overall plasma number density, depending on discharge voltage. Immersed LaB6-coated steel and roughened boron nitride (BN) wall material samples are mounted on the face of a radiative heater, and the wall temperature is controlled from 50–400 °C such that thermionic emission from the LaB6-coated sample is not significant. The energy of the primary plasma electrons from the discharge filament is varied and the floating potentials of the material samples are monitored. The floating potent...

Velocimetry of cathode particles in a magnetoplasmadynamic thruster discharge plasma

With high-speed imaging, it is possible to directly observe the time-evolution of the macroscopic behavior of the discharge plasma in a magnetoplasmadynamic thruster (MPDT). By utilizing direct high-speed imaging capable of capturing many images over the course of a single discharge, the velocity of the cathode erosion particles can be measured, opening the possibility of a novel, noninvasive technique for discharge plasma flow field velocimetry. In this work, an 8 kA argon MPDT discharge is imaged at 26 173 fps utilizing a 0.9 neutral density filter. The camera is aligned with thruster centerline 4 m downstream of the thruster exit plane. By tracking visible particles appearing in the multiple images, the particle motion in the radial and azimuthal directions is directly imaged. Through the use of traditional techniques in digital particle image velocimetry, the cathode particles emanating from the discharge are measured to have a mean radial velocity of 44.6 ± 6.0 m/s with a 95% confidence interval and a statistically insignificant azimuthal velocity. The setup and analysis employed permits measurement of the particle velocity in orthogonal direction to the image sensor plane using a single camera. By combining a background removal subtraction technique and knowledge of the optical focal plane, the estimated mean axial velocity of the particles is 1.59 km/s. This investigation ends with a discussion of important factors to consider for future MPDT high-speed imaging particle velocimetry, such as frame-rate, image size, spatial resolution, optics, and data handling selections.

Effects of wall material, wall temperature, and surface roughness on the plasma sheath

The effect of wall material, wall temperature, and surface roughness on the near-wall plasma sheath region that exists in confined plasma devices is experimentally investigated in a multidipole-type plasma device. Smooth and rough wall material samples are immersed in a quiescent argon plasma and heated using an enclosed radiative sample heater. This will all be new data, which the authors have not yet presented or published. It is hypothesized that elevated wall material temperature may increase secondary electron emission (SEE) from the wall material due to the increased initial energy of wall electrons. SEE has been observed in previous work to decrease the magnitude of the sheath potential drop and to effect a transition to an inverse sheath potential profile. Material samples of magnesium oxide (high SEE,) HP-grade boron nitride (medium SEE,) and quartz (low SEE) are studied at temperatures from 20 to 800 °C to characterize the effect of temperature and SEE on the sheath. Plasma Debye lengths are from 0.5-5.0 mm and ion-neutral mean free paths are maintained at least two orders of magnitude greater than the Debye length in order to study the collisionless sheath regime. The potential profile of the sheaths above the wall material samples are measured using an emissive probe, employing the technique of inflection points extrapolated to zero emission. Emissive probe measurements of plasma potential are obtained at 1-5 mm intervals, and the electron current collected by the emissive probe is reported as a surrogate for electron number density in the sheath. Bulk plasma parameters are monitored using a cylindrical Langmuir probe. The derivative of the Langmuir probe characteristic is obtained to characterize the electron energy distribution function (EEDF). The plasma EEDF is manipulated by controlling the bias voltage of the emissive tungsten filaments in the plasma device, which emit and accelerate the ionizing electrons.Increased SEE wall material is seen to decrease the magnitude of the sheath potential drop at a given electron number density and EEDF, and with increasing electron energy to effect a transition to an inverse sheath potential profile.Increased wall temperature is observed to effect the transition to an inverse sheath regime at lower plasma electron energies, consistent with an increase in SEE. Increased surface roughness also allows the sheath to transition at lower plasma electron energies, believed due to increased SEE from the increased effective surface area.

Effects of surface roughness on plasma sheath

In this experiment, sheath potential profiles are measured over smooth (Ra <; 0.2 um) and rough (Ra = 9.08 um) wall material samples of AX05-grade boron nitride. Argon plasmas are generated at 1.0° 10-4 Torr-Ar using a multidipole-type plasma device. Wall material samples are positioned within the plasma and heated using an enclosed radiative sample heater. The sheath potential profiles are measured with emissive probes, and electron number densities and temperatures are measured in the bulk plasma with a planar Langmuir probe. Plasma number densities are roughly 3 × 1012 m-3. The electron energy distribution of the plasma is non-Maxwellian, with isotropic and directed energetic electron populations from 70 - 150 eV and hot and cold Maxwellian populations from 3.4 - 6.4 eV and 0.3 - 1.3 eV, respectively. Plasma Debye lengths range from 3 to 6 mm and ion-neutral mean free paths are maintained at least two orders of magnitude greater in order to study the collisionless sheath regime. Sheath thicknesses range from approximately 30 to 60 mm, and are smaller in the collapsed sheaths. For both rough and smooth samples, increased primary electron energy is seen to affect a transition in the sheath structure to a collapsed profile. Secondary electron emission (SEE) is inferred as the mechanism for the transition in accordance with prevalent theory. Increased surface roughness causes the sheath to transition at higher plasma electron energies, believed due to decreased SEE from geometrical obstruction of escaping electrons. When rough and smooth wall are in the same sheath regime, sheaths are similar within the measured precision.

Effects of Wall Temperature and Surface Roughness on the Plasma Sheath

In this experiment, plasma sheath potential profiles are measured over smooth (Ra < 0.2 μm) and rough (Ra = 10.4 μm) wall material samples of AX05-grade boron nitride over the temperature range 0 to 600 °C. Argon plasma with a number density of approximately 3 x 10 m is generated at an operating pressure 1.0 x 10 Torr-Ar using a multidipole-type plasma device. The sheath potential profile at the surface of each sample is measured with emissive probes, and electron number densities and temperatures are measured in the bulk plasma with a planar Langmuir probe. The electron energy distribution of the plasma is non-Maxwellian, with isotropic and directed energetic electron populations from 50 – 200 eV and hot and cold Maxwellian populations from 3.6 – 6.4 eV and 0.3 – 1.3 eV, respectively. Plasma Debye lengths range from 3 to 6 mm and ion-neutral mean free paths are maintained at least two orders of magnitude greater in order to study the collisionless sheath regime. Sheath thicknesses range from approximately 30 to 60 mm, and are smaller in the collapsed sheaths. For both rough and smooth samples, increased primary electron energy is seen to affect a transition in the sheath structure to a collapsed profile. Secondary electron emission (SEE) is inferred as the mechanism for the transition in accordance with prevalent theory. Increased surface roughness causes the sheath to transition at 40 ± 20 eV greater plasma primary electron energies, believed due to decreased SEE from geometrical obstruction of escaping electrons. When rough and smooth wall are in the same sheath regime, the sheaths are similar within ~ 2 V. Elevating the wall material temperature to 600 oC causes the sheath over the rough sample to collapse at 125 ± 20 eV greater primary electron energy in comparison to the unheated sample, believed due to decreased SEE. The effect of heating to 600 oC on the smooth sample was much less, causing the sheath to collapse at 10 ± 20 eV greater primary electron energy. These effects may be primarily due to removal of contaminants from the material surface rather than a decoupled effect of wall temperature.