Plasma Measurements Using Micromachined Retarded Potential Analyzers

SAMPLE DOCUMENT

E. V. Heubel, A. I. Akinwande, L. F. Velásquez-García
Sponsorship: NASA

Plasma diagnostic instruments are pervasive across many disciplines. They are used in aerospace to measure propulsion plasmas, in nuclear science as sensors in fusion reactors, and may even be used in microprocessing foundries to monitor plasma etchers. Devices such as Langmuir probes offer a simple design, yet they require assumptions about the plasma (confinement, distribution, etc.) to extract information. The Retarding Potential Analyzer (RPA), though a bit more complex, yields a direct measurement of the plasma energy distribution. Through batch-microfabrication we aim to lower cost and increase precision in RPA measurements.

Our previously reported RPA demonstrates improved performance over the current state-of-the-art. The hybrid-RPA, our sensor with a conventionally machined steel housing fitted with micromachined electrodes (µ-grids), achieves a threefold increase in signal strength. Furthermore, the MEMS-RPA (fully microfabricated) gains an order of magnitude over its conventional counterpart. The hybrid-RPA has four µ-grids with 100 µm apertures hexagonally packed with a 200 µm pitch and a collector plate (Figure 1). Measurements of a Helicon plasma were obtained from MIT’s Plasma Science and Fusion Center using the hybrid sensor. The test chamber is capable of generating plasmas with energies and densities comparable to reentry conditions.

With the first µ-grid floating, the quasi-neutral Helium plasma establishes a floating potential on this electrode. Two electron-repelling grids are held at -10 V while the ion-retarding grid is swept. The current measured at the collector is used to derive the energy distribution function. The normalized distribution, with a peak at about 15 V, is plotted alongside ion measurements in Figure 2. The non-physical negative distribution attributed to ion focusing and characteristic of our hybrid-RPA is still clearly apparent. Simultaneous Langmuir probe measurements provide an estimate of the floating potential around 6 V and a plasma potential of approximately 15.5 V.

Figure 1: Assembled hybrid-RPA (bottom right), exploded view of the hybrid-RPA benchmarking sensor (center) and detail of micromachined grid (left). Scale bar common to all three images [3]. / Figure 2: Ion energy distribution obtained with the hybrid-RPA in a Helium Helicon plasma showing artificial negative distribution characteristic of focusing in our hybrid device, and a peak around 15 V.

Further Reading

I. H. Hutchinson, “Plasma Particle Flux,” in Principles of Plasma Diagnostics, 2nd ed., Cambridge: Cambridge University Press, 2002, ch. 3, pp. 55-103.
J. Adámek, J. Stöckel, M. Hron, and J. Ryszawy, “A Novel Approach to Direct Measurement of the Plasma Potential,” Czechoslovak Journal of Physics, vol. 54, pp. C95-C99, 2004.
E. V. Heubel, A. I. Akinwande, and L. F. Velásquez-García, “MEMS-Enabled Retarding Potential Analyzers for Hypersonic In-Flight Plasma Diagnostics,” Technical Digest of the 2012 Hilton Head Workshop on Physical Sensors, Hilton Head, SC, pp. 324-327, Jun., 2012.

Microfabricated Ionic Liquid Electrospray Sources with Dense Arrays
of Emitters and Carbon Nanotube Flow Control Structures

Figure 1: Design of the MEMS electrospray source, consisting of an emitter die containing an array of sharpened emitter tips and an extractor grid die containing a matching array of apertures. When the two dies are assembled, each emitter tip sits centered below an extractor grid aperture.
Figure 2: Scanning electron microscope images of an array of microfabricated silicon emitters coated with a carbon nanotube forest.

F. A. Hill, P. J. Ponce de Leon, L. F. Velásquez-García
Sponsorship: DARPA

Electrospray is a process to ionize electrically conductive liquids that relies on strong electric fields; charged particles are emitted from sharp tips that serve as field enhancers to increase the electrostatic pressure on the surface of the liquid, overcome the effects of surface tension, and facilitate the localization of emission sites. Ions can be emitted from the liquid surface if the liquid is highly conductive and the emitter flowrate is low. Previous research has demonstrated successful operation of massive arrays of monolithic batch-microfabricated planar electrospray arrays with an integrated extractor electrode using ionic liquids EMI-BF4 and EMI-Im—liquids of great importance for efficient nanosatellite propulsion. The current design builds upon the previous electrospray array designs by increasing the density of the emitter tips, increasing the output current by custom-engineering suitable nanofluidic structures for flow control, and improving the ion optics to gain control of the plume divergence and exit velocity.

The MEMS electrospray source consists of an emitter die and an extractor grid die (Figure 1), both made of silicon and fabricated using deep reactive ion etching. The two dies are held together using a MEMS high-voltage packaging technology based on microfabricated springs that allows precision packaging of the two components with low beam interception. The emitter die contains dense arrays of sharp emitter tips with over 1,900 emitters in 1 cm2 (Figure 2). A carbon nanotube forest grown on the surface of the emitters transports the liquid from the base of the emitters to the emitter tips. A voltage applied between the emitter die and the extractor grid die creates the electric field necessary to ionize the ionic liquid. The present research focuses on engineering the nanofluidic structure to attain higher emitter current while maintaining good array emission uniformity, and on developing batch microfabricated advanced ion optics for control of the electrospray plume.