Micro-Architectured Materials for Electric Propulsion and Pulsed Power

Nasr M. Ghoniem, P.I. (UCLA) Yevgeny Raitses (PPPL), Robert Shaefer

(Sharafat) (UCLA) Co-P.I.sInvestigators

Dan Goebel (UCLA/JPL), Igor D. Kaganovich (PPPL) Brian Williams (Ultramet)

CollaboratorsAlp Sehirlioglu (Case Western)

Richard Wirz (UCLA)

AFOSR Program on Materials and Processes Far From Equilibrium

Heat Flux & Energy Density Limits on Materials in EP & PP Applications-Performance metrics are found: Heat Flux and Energy Fluence.

New architectured materials have been developed

Textured armor: Ultramet developed new material architectures that are more resilient in ion and plasma environments. These include refractory metal surfaces with micro-rods, micro-spears, and nodular structures.

A new High Energy Flux Atmospheric Plasma Facility (HEFTY) has been developed

HEFTY is a high heat flux testing facility dedicated to studying materials in extreme thermal loading environments

Capacity to test materials in up 300MW/m^2 heat flux through the use of a plasma gun (Praxair SG-100)

Temperature gradients are generated across sample thickness so an investigation into a combined damage event consisting of heat flux and temperature gradients is possible

Gradient is achieved through impingement cooling on back side of the samples

Thermal fatigue experiments can be performed through pulsed operation of the system allowing for evaluation of materials in a more realistic operating environment

Rear view of sample

Sample holder assembly

Heat source, SG-100 plasma gun

CAD model of testing chamber

Plasma Source

Magnetic Confinement Rings Sample

manipulation and cooling

Material Sample

Parameter Range

Plasma density ≤1x1019 m-3

Electron temperature 3 to 20 eV

Ion energy 10 to 300 eV

Ion flux to target 1021 to 1023 m-2s-1

Target area ≈5 cm2

UCLA Pi – Plasma Surface Interaction Facility is developed for plasma-material interaction studies

Capabilities: Controlled studies of the erosion and redepostion of materials during high flux and fluence plasma exposure

Microstructural investigation of engineered surfacesAlp Sehirlioglu (Case Western Reserve University)

– Discovered amorphous Tungsten!!

Two main types of structures (i) Dendrites (ii) Nodules

- Dendrites contained a stem and a cap.- Stems were faceted before and after sputtering.- Caps eroded the most during sputtering.

Before Sputtering After Sputtering

Sputtering created a ~10 nm amorphous region on the surface of the caps of dendrites Before Sputtering

After Sputtering

Crystalline W

Platinum

- Pt was coated during sample prep.- Area under Pt is “real” surface, not exposed surface during sample prep.

Crystalline W

Platinum

Amorphous W

Modeling Plasma and Ion Effects on Materials – Developed phase field theory of thermomechanical damage-- Developed Ab Initio & MD models for surface diffusion in W

• The objective is develop a multiphysics computational model capable of simulating the damage in W surface and crack propagation, under severe transient heating conditions.

• The method is based on the phase-field method, in which an energy functional for the system can be defined as:

r(r) of surface crowdion indicates the high mobility and strong anistropy of its movement.

Snapshots of the bombardment of a Xe atom (KE = 100 eV) on W(001) surface at T = 200 K.

Upgraded Setup for Measurements of Secondary Electron Emission (SEE) from Micro-Engineered Materials

• Cryogenic system to maintain better vacuum(<10-8 torr) during SEE measurements• Ion source to remove surface charges

• Importantly for micro-engineered materials , the upgrade allows to minimize, outgassing, surface , contamination, etc

Experiments with micro-engineered materials immersed in non-equilibrium plasma of LTPX

Micro-engineered materials are expected to minimize SEE, but may be a source of electron field emission due to surface non-uniformities.

Electron field emission may have a similar effect on plasma-wall sheath as SEE, i.e. to weaken electrical and thermal insulating properties of the sheath

To evaluate possible effects of electron field emission and photoelectron emission, we impressed a 4” silicon wafer coated with ultrananocrystalline diamond (UNCD) in the non-equilibrium plasma of LTPX setup

Typical features of UNCD : several nm’s grains with non-uniformities of up to 100’s nm

UNCD has exceptional field emission properties

Y. Raitses and A. V. Sumant, XXI International MRC, 2012

A new regime of plasma-wall interaction with a very strong SEE > 1 is found

Qualitative differences between the potential profile, relative to the wall, of a classical sheath (a), SCL sheath (b) and the new inverse sheath (c). Note that plasma electrons are still confined by the SCL sheath, but not confined by the inverse sheath.

The main result - Disappearance of Debye Sheaths Due to SEE

Results of particle-in-cell simulations of Hall thruster discharge: a comparison of results with classical (Sim. A), E = 200 V/cm, and inverse sheath (Sim. B) E = 250 V/cm. Simulation parameters: the distance between the opposite walls is 2.5 cm, B= 100 Gauss, neutral density, na =1012 cm3, plasma density n0 = 1011 cm3, turbulent collision frequency values 1.4 106 s-1 for Sim A, and 2.8 106 s-1 for Sim B.

M. Campanell et al., Phys. Rev. Lett. 108, 255001 (2012)