The multiprobe platform (LEFT) positioned in the vacuum tank (RIGHT) of the UPM plasma lab.

Plasma Diagnostics

• Diagnostics of high-velocity plasma streams generated is complex, as the parallel component

of the velocity is much higher than the ion sound speed.

It is therefore necessary to use different diagnostics to characterize the plasma jet

produced. A multiprobe approach has been adopted, to have a more complete plasma

beam characterization.

• To date, a four-grid retarding field energy analyzer (RFEA), a Langmuir probe (LP), and an

emissive probe (EP) have been employed (see multiprobe platform in the figure below).

Internal coordinates of the 3D positioning system

• A 3D positioning system whose cylindrical

support is concentric with the vacuum tank is

used to displace the probe platform to different

spatial positions of the vacuum tank.

• The probe platform centre is aligned with the

axis of the thruster, which forms the axial

direction (z-axis) of the 3D positioning system

(see coordinate system in the figure below).

AbstractThe characteristics of ion beams from the alternative low power hybrid ion engine (ALPHIE) – a simple DC-powered plasma accelerator with only one electron source are studied, which can

provide information on the thrust level achieved, specific impulse produced, power consumption, etc. A combination of three diagnostics (a retarding-field energy analyzer (RFEA), a Langmuir

probe (LP), and an emissive probe (EP)) on a single movable (multiprobe) platform is employed to explore the plasma expansion process for supersonic plasma jets. The experimental studies

provide information on the ion velocity distribution function (IVDF) obtained with the RFEA, plasma electron Debye length, and other plasma parameters. A mesothermal plasma flow with two-

peaked IVDFs composed of low and high speed ion groups has been detected, with the ion drift speeds controlled by one of the engine’s parameters. The IVDFs and IEDFs appear to remain

conserved downstream of the engine axis, though with ion energy losses also sometimes observed downstream the engine exhaust. Thus, electron impact ionization needs to be studied in

relation with collisional ionization processes by fast ions downstream the thruster grids. An attempt to explain the energy losses in terms of collisionless cooling by electrons is to be made, but will

require more experiments to measure the electron energy distribution functions (EEDF). Nevertheless, the thruster generates supersonic ion beams that are highly collimated.

Characterization of Plasma Streams and Ion Populations from Plasma Thrusters for Space Propulsion

Julius Damba

Dept. of Applied Physics, ETSI Aeronautica y del Espacio, Univ. Politecnica de Madrid, 28040 Madrid. Spain.

Thesis Supervisor: Dr. Conde L. Lopez

• Using the Langmuir probe, the measured electron densities ne of the background

plasma far from the thruster exit were typically ≈0.1–8.0 × 108 cm−3.

• The corresponding Debye lengths λDe are between 0.4 and 3 mm, with the

characteristic ion sound speeds in the order of 1.6–2.2 km/s.

Schematic of the ion engine of the ALPHIE. The dotted lines indicate

the cusped magnetic field lines.

A photograph of the assembled ALPHIE

The ALPHIE Thruster

ALPHIE – alternative low power hybrid ion

engine.

Key features:

• Plasma production and space charge

neutralization processes are coupled.

• Plasma is generated in the discharge

unit, using an electron beam in argon

gas.

• Ionizing electrons from the cathode are

accelerated by the voltages VAC and VEG

to energies above the ionization threshold of the neutral gas. The intricate pattern of the magnetic

field lines confines the electrons. Enhanced collisions, and thus ionization rate, of the trapped and

accelerated electrons with neutral atoms produces the engine plasma.

• The ions are magnetized and move along the electric field lines, hence are ejected as a high-

speed beam through the engine exhaust.

• The thruster is expected to yield increased thrust by plasma acceleration.

RFEA component parts

• A key parameter in characterizing ion beams is the ion energy spectrum or

distribution function (IEDF), which has been measured using an RFEA.

The Retarding Field Energy Analyzer (RFEA)

• The RFEA consists of four grids

• Plasma-facing grid is left floating at the

plasma potential.

• Plasma electrons are repelled by the

second grid, whereas the ions are

selected by the ion discriminator (ID)

grid.

• A secondary electron suppressor grid is

used to prevent any secondary electrons

resulting from high energy impacts by

the collected ions on the copper

collecting plate at the back of the

analyzer.

Above, Left: Measurements obtained by the RFEA; raw data and the Savitzky-Golay numerical approximation of the

I─V curve (left axis) and the calculated IVDF (right axis) at Z= 200 mm. Above, Right: The IVDF (open symbols) of

figure to the LEFT and the two main peaks approximated by Gaussian functions (solid curves).

Results

• Slope of the I—V characteristic curves obtained with the RFEA give rise to two IEDF

peaks, which are used to estimate the IVDF.

Above, Left: Waterfall representation of axial (z-axis) SG-smoothed I—V curves. . Above, Right: Representation

of the corresponding two-peaked IVDFs, calculated from curves on LEFT figure, against the kinetic energy.

• Energy of fast ions

increases with driving

voltage VAC.

The fast ions gain

energy and are

accelerated to

supersonic speeds.

• Maxima of the fast ion group are around 330 eV and at about 30 eV for the low-energy

populations. The low-energy ion group speeds remain essentially constant, whereas the

fast ion speeds depend on the acceleration voltage.

Presented at the II EDICION SIMPOSIO : "CUENTANOS TU TESIS“ organized by Doctorado UPM. March 2018

This work was funded by the by the Ministerio de Economía Ciencia e Innovación of Spain under Grant ESP2013-41078-R