Aimee A. Hubble

Spatially resolved study of primary electron transport in magnetic cusps

Spatially resolved primary electron current density profiles were measured using a planar Langmuir probe in the region above a magnetic cusp in a small ion thruster discharge chamber. The probe current maps obtained were used to study the electron collection mechanics in the cusp region in the limit of zero gas flow and no plasma production, and they allowed for the visualization of primary electron transport through the cusp. Attenuation coefficients and loss widths were calculated as a function of probe distance above the anode at various operating conditions. Finally, the collection mechanics between two magnetic cusps were studied and compared. It was found that primary electron collection was dominated by the upstream magnet ring.

Fast shut-down protection system for radio frequency breakdown and multipactor testing

Radio frequency (RF) breakdown such as multipactor or ionization breakdown is a device-limiting phenomenon for on-orbit spacecraft used for communication, navigation, or other RF payloads. Ground testing is therefore part of the qualification process for all high power components used in these space systems. This paper illustrates a shut-down protection system to be incorporated into multipactor/ionization breakdown ground testing for susceptible RF devices. This 8 channel system allows simultaneous use of different diagnostic classes and different noise floors. With initiation of a breakdown event, diagnostic signals increase above a user-specified level, which then opens an RF switch to eliminate RF power from the high power amplifier. Examples of this system in use are shown for a typical setup, illustrating the reproducibility of breakdown threshold voltages and the lack of multipactor conditioning. This system can also be utilized to prevent excessive damage to RF components in tests with sensitive or flight hardware.

Center conductor diagnostic for multipactor detection in inaccessible geometries

Electron collecting current probes are the most reliable diagnostic of multipactor and radiofrequency (RF) ionization breakdown; however, stand-alone probes can only be used in test setups where the breakdown region is physically accessible. This paper describes techniques for measuring multipactor current directly on the center conductor of a coaxial RF device (or more generally, on the signal line in any two-conductor RF system) enabling global multipactor detection with improved sensitivity compared to other common diagnostics such as phase null, third harmonic, and reflected power. The center conductor diagnostic may be AC coupled for use in systems with a low DC impedance between the center conductor and ground. The effect of DC bias on the breakdown threshold was studied: in coaxial geometry, the change in threshold was <1 dB for positive biases satisfying VDC/VRF0<0.8, where VRF0 is the RF voltage amplitude at the unperturbed breakdown threshold. In parallel plate geometry, setting VDC/VRF0<0.2 was necessary to avoid altering the threshold by more than 1 dB. In most cases, the center conductor diagnostic functions effectively with no bias at all-this is the preferred implementation, but biases in the range VDC=0-10V may be applied if necessary. The polarity of the detected current signal may be positive or negative depending on whether there is net electron collection or emission globally.

Characterization of dynamic and structured plasma using laser-collision induced fluorescence

Laser collision-induced fluorescence (LCIF)1 is a powerful diagnostic which can be used for making temporally and spatially resolved measurements of electron densities in a plasma discharge. The technique, which involves the measurement of optical emission emanating from higher energy excited states due to the redistribution of the lower energy laser-excited state by collisions with energetic electrons, has been readily employed to study helium discharges. In this work, we report on recent systems studied with the LCIF diagnostic and describe efforts to extend the LCIF technique to argon based plasma systems.

Experimental assessment of bulk plasma distribution in a multi-cusp device

The physics of plasma transport from the bulk plasma through the magnetic cusp to the anode remains poorly understood. A proper accounting of plasma losses to the anode is critical to accurate modeling of multicusp device performance. The objective of this work is to analyze plasma transport to the anode in multicusp discharge chambers and evaluate its impact on discharge performance. In this work, plasma transport in two magnetic circuit configurations of a 16-cm multicusp discharge chamber was studied. Each ring is covered with an electrically isolated electrode, which enables the direct measurement of current to each individual ring, as well as the discharge chamber wall. Ring electrode measurements coupled with spatially resolved plasma parameter measurements throughout the discharge chamber allow for an assessment of plasma losses to each ring in terms of an “effective loss area” which, multiplied by electron current density incident on the bulk/cusp boundary, gives the correct collected current to each ring. The effective loss area is compared with parameters such as cusp mirror ratio, collision frequency, and sheath potential. Spatially resolved maps of plasma density and temperature in the bulk region are be obtained and used to compare the location of the plasma centroid with current distribution at the cusps.

Evaluation of plasma transport in a multipole ion source and its impact on discharge performance

Summary form only given. In this work, the current distribution at the four magnet rings of a cylindrical 16-cm diameter mild steel ion source discharge chamber was studied. The current distribution measurements were made using thin, electrically isolated collector electrodes affixed to each magnet ring. Current collected at the magnetic cusps as well as the intervening anode material was quantified as a function of discharge plasma conditions. Discharge efficiency and stability were investigated as a function of current distribution. This work is aimed at obtaining a more complete understanding of plasmas losses to the wall surfaces in multipole ion sources.

Simulated beam extraction performance characterization of a 50-cm ion thruster discharge

A 50 cm ion thruster is being developed to operate at >65 percent total efficiency at 11 kW, 2700 s Isp and over 25 kW, 4500 s Isp at a total efficiency of >75 percent. The engine is being developed to address the need for a multimode system that can provide a range of thrust-to- power to service national and commercial near-earth onboard propulsion needs such as station-keeping and orbit transfer. Operating characteristics of the 50 cm ion thruster were measured under simulated beam extraction. The discharge current distribution at the various magnet rings was measured over a range of operating conditions. The relationship between the anode current distribution and the resulting plasma uniformity and ion flux measured at the thruster exit plane is discussed. The thermal envelope will also be investigated through the monitoring of magnet temperatures over the range of discharge powers investigated. Discharge losses as a function of propellant utilization was also characterized at multiple simulated beam currents. Bulk plasma conditions such as electron temperature and electron density near engine centerline was measured over a range of operating conditions using an internal Langmuir probe. Sensitivity of discharge performance to chamber length is also discussed. This data acquired from this discharge study will be used in the refinement of a throttle table in anticipation for eventual beam extraction testing.

Addressing issues in probing the magnetic cusp region

Summary form only given. There are several inherent difficulties in making spatially resolved electrostatic probe measurements of plasma properties within a current collecting magnetic cusp. It is difficult to insert a physical probe into the region without shadowing current flow to the anode as a result of the physical obstruction of field lines by the probe body. Current flow to the probe collecting surface can also be shadowed by the probe body blocking flux lines that curve away from the probe surface as a consequence of the physical magnetic field distribution in the region. Shadowing can therefore significantly reduce collected current at the probe, the anode, or both. The problem is complicated by the presence of the plasma sheath and the large magnetic field at the anode surface. Since the sheath potential varies spatially, it is not possible to simply set the voltage on a collecting probe and measure electron density when the potential is changing as a function of probe position. Unless the probe is biased at the local plasma potential at each point, the probe sheath will interfere with the local plasma conditions. A solution to these issues is proposed in the form of a joint emissive/collecting probe that will negate the shadowing issue as well. The probe will measure plasma potential and then interrogate the cusp region by actively biasing the probe in collection mode at the plasma potential. At the plasma potential, the maximum current flowing through that point is collected provided shadowing affects are accounted for.

Spatially resolved study of inter-cusp transport and containment of primary electrons in a line cusp source

Summary form only given. The physics of plasma losses at magnetic cusp surfaces in multipole ion sources remains a poorly understood problem. Loss area in these sources determines discharge ionization performance, efficiency, and stability. In previous work, electron current density profiles were obtained in the region above a magnetic cusp in a 20 cm partial conic ring cusp ion thruster discharge chamber to study electron collection mechanics in the absence of gas flow and plasma production. This work characterizes primary electron losses in a line cusp source by obtaining electron current density profiles in the region above three magnetic line cusps arrayed on a flat plate anode. The flat plate geometry will allow for the investigation of the effect that cathode/anode geometry and magnetic cusp symmetry plays in particle collection. The current density maps allow for particle transport through the cusps to be visualized. These results will be compared to the present work to study how source geometry affects particle collection.

Spatially Resolved Study of Inter-Cusp Transport and Containment of Primary Electrons

Electron current density profiles were obtained in the region above two magnetic cusps in a 20 cm partial conic ring cusp ion thruster discharge chamber. The density profiles were obtained by use of a translatable Langmuir probe and an automated motion stage and data collection system. These high-resolution density profiles allowed for the study of electron collection mechanics in the absence of gas flow and plasma production, as well as at varying levels of gas flow. This is the second stage in a series of experiments aimed at better understanding collection physics at the magnetic cusps during discharge chamber operation. The current density maps allow for particle transport through the cusps to be visualized, and see how discharge chamber geometry plays a role in collection. Transmission coefficients and loss widths as a function of probe height above the anode were calculated from the current density data, and compared at a variety of discharge currents. In addition, these current density maps were compared to those obtained using a planar line cusp source geometry to study how source geometry and magnet cusp spacing affects collection.