VLEO Research Terms

VLEO

Very Low Earth Orbit, roughly 150-350 km up (papers give slightly different ranges). The thin air there still causes heavy drag.

ABEP

Air-breathing electric propulsion. A thruster that collects thin atmosphere as propellant instead of carrying fuel.

Atomic oxygen (AO)

Single oxygen atoms, common in low orbit. Highly reactive and the main thing that erodes plastics and carbon materials on spacecraft.

Hyperthermal

Much more energetic than room-temperature atoms. Orbital atomic oxygen hits at about 8 km/s, roughly 5 eV.

Thermal atoms

Atoms with only a few tenths of an eV, like those from simple plasma ashers. Less like orbit and questioned for material testing.

eV (electron-volt)

Unit of energy for particles. A 5 eV oxygen atom moves at about 8 km/s.

Flux

Number of atoms hitting a surface per area per second, such as atoms/cm²/s. This is what you’re maximizing.

Fluence

Total atoms per area over an exposure, which is flux × time.

Acceleration factor

How much faster a lab exposes materials than orbit (ground flux ÷ orbital flux). Months of orbit in hours or days needs a big factor.

Dissociation (degree of dissociation)

Fraction of O₂ molecules that split into atoms. One of the two main ways to raise atom output.

Throughput (Q)

Amount of gas flowing through the source per second. Atom output is roughly dissociation × throughput.

sccm

Standard cubic centimeters per minute. A flow unit that counts the amount of gas, not its volume at a given pressure.

MFC (mass flow controller)

Valve plus sensor that holds a gas flow at a set value.

Plasma

Gas where some atoms are split into ions and electrons. Made here with RF power, and it glows.

RF (radio frequency)

Oscillating electric power. These sources use 13.56 MHz, a standard frequency.

ICP (inductively coupled plasma)

Plasma sustained by a coil around the tube that induces electric fields. Reaches higher density than CCP.

CCP (capacitively coupled plasma)

Plasma sustained between two electrodes. Simpler but lower atom density; the NASA memo used this type.

Matching network

Circuit that tunes the load so RF power goes into the plasma instead of reflecting back to the supply.

Reflected power

RF power that bounces back to the supply instead of reaching the plasma. Should be near zero.

Electron temperature (Te)

Measure of electron energy in a plasma, in eV. Controls how fast molecules ionize and dissociate.

Plasma density

Number of electrons and ions per volume. Higher density usually means more dissociation.

Sheath

Thin boundary layer where a plasma meets a surface. It has a voltage that pushes ions toward the surface.

Sheath expansion

The sheath growing thicker as voltage rises, which increases the effective collecting area. Central to the RF cathode paper.

Langmuir probe

Small metal tip in the plasma. Sweeping its voltage and measuring current gives electron temperature and density.

RPA (retarding potential analyzer)

Stack of charged grids that measures the energy distribution of ions.

IEDF / IVDF

Ion energy (or velocity) distribution function: how many ions have each energy.

ExB probe

Uses crossed electric and magnetic fields to sort ions by velocity, which shows species and charge state.

Faraday probe

Collects ion current to map ion current density across a beam.

QCM (quartz crystal microbalance)

Vibrating crystal whose frequency shifts as mass lands on or leaves it. Measures deposition or erosion.

Mass spectrometer (QMS)

Sorts particles by mass-to-charge ratio to identify species such as O, O₂, and CO. Used to measure dissociation.

OES (optical emission spectroscopy)

Reads the plasma’s glow color. Each species emits its own wavelengths, such as O at 777 and 844 nm.

Actinometry

Estimating atom density by comparing an oxygen emission line to a known amount of added argon (750 nm).

Catalytic probe

Metal tip that warms up when atoms recombine on it, so the temperature rise shows atom flux.

Time-of-flight (TOF)

Measures particle speed from how long it takes to cross a known distance, which gives energy.

Faraday cage

Metal shield that keeps RF energy from leaking out and disturbing sensors and electronics.

Orifice / nozzle

Small opening between regions. Its size sets throughput and the pressure drop across it.

Skimmer

Cone with a small opening that passes the center of a gas beam into the next pumping stage.

Differential pumping

Pumping in stages so a high-flow source can sit next to a low-pressure region.

Pumping speed

Volume a pump removes per second (L/s). It limits how much gas you can flow while holding low pressure.

Torr / mTorr / Pa

Pressure units. 1 Torr is about 133 Pa, and 1 mTorr is one thousandth of a Torr.

Mean free path

Average distance a particle travels before colliding. When it’s longer than the setup, atoms fly straight.

Knudsen number

Mean free path ÷ length scale. Above 1 the gas is rarefied (non-continuum), as in VLEO.

Recombination coefficient (γ)

Fraction of atoms hitting a surface that pair up into O₂ and are lost. Averages: Pyrex under 1%, quartz about 1-2%, stainless steel about 10%.

Wall recombination

Atoms pairing into O₂ on chamber surfaces. At low pressure it’s the main way atoms are lost.

Afterglow

Region just downstream of the plasma where the glow, electrons, and UV fade but atoms persist.

Neutralizer

Two meanings. In thrusters, an electron source that cancels beam charge. In AO sources, something that turns ions into neutral atoms. Confirm which one is meant.

Charge exchange

A fast ion takes an electron from a slow neutral, making a fast neutral and a slow ion.

VUV (vacuum ultraviolet)

Very short-wavelength UV, about 100-200 nm. Plasma gives it off and it can damage materials, especially together with AO.

Kapton

Polyimide plastic used on spacecraft. The standard reference for AO erosion, at about 3×10⁻²⁴ cm³ per atom.

Erosion yield

Volume of material lost per incident atom, in cm³/atom.

Witness sample

Reference material (Kapton, FEP, polyethylene, graphite) exposed alongside test samples to calibrate the exposure. See ASTM E2089.

Plume

Stream of gas or plasma leaving a source or thruster.

Cathode / anode

Electrodes. The cathode collects ions or emits electrons, and the anode collects electrons.

Borosilicate (Pyrex)

Glass used for the discharge tube. It loses few atoms to recombination.

DAQ (data acquisition)

System that reads sensors and logs the data. Yours runs in LabVIEW.