Erosion Measurements by Cavity Ring-Down Spectroscopy for the VHITAL Program

Abstract: We report the use of cavity ring-down spectroscopy (CRDS) as a diagnostic tool to study sputter erosion of the VHITAL (Very High I sp Thruster with Anode Layer) thruster. The lifetime of the VHITAL thruster is largely governed by sputter erosion of guard-rings and other critical components. The CRDS technique will be used to measure sputtered molybdenum in the plume of the thruster for various thruster operating conditions. Because the sputter rates of interest are relatively low (material erosion rates are ~0.1-10 µm/hour), a sensitive measurement technique such as CRDS is required. Our group has recently demonstrated the use of CRDS for sputter erosion studies. Here, we describe our bench-top sputtering setup consisting of an ion beam and target which we use for diagnostic development. We present CRDS measurements of sputtered molybdenum number density and velocity, including the dependence on number density on beam current, and comparison with a numerical sputter model. We present cavity hardware designs and bench-top testing for CRDS implementation on the VHITAL thruster. Challenges for the thruster implementation include maintaining cavity alignment and stability for the relatively long (≈3m) optical-axis, and minimizing the reduction of effective cavity finesse (and CRDS sensitivity) due to mirror contamination from condensed bismuth or sputtered products

ICEF2007-1617 DESIGN AND BENCH-TOP TESTING OF MULTIPLEXED FIBER DELIVERED LASER IGNITION SYSTEM FOR NATURAL GAS ENGINES

ABSTRACT Past research has demonstrated the feasibility of using optical sparks for engine ignition, and has shown potential benefits associated with reduced cyclic variability and increased rate of cylinder pressure rise, thus extending the lean operating limit of natural gas engines. This contribution details the design and bench-top testing of a fiber-optic delivery system for ignition of natural gas engines. The system is designed for use on a Caterpillar G3516C engine and is comprised of a single Nd:YAG laser as the energy source, a multiplexer for switching the beam between cylinders, fiber optics to deliver the laser pulses to individual cylinders, and optical plugs to couple the beam into the cylinders. The optical fibers are a critical component of the system and discussion of use of both solid core silica fibers and cyclic olefin polymer-coated silver hollow fibers is included. The multiplexer design is presented and optical testing of the multiplexed fiber delivery on the bench-top is reported. Design considerations for engine integration are introduced

Real-Tme Boron Nitride Erosion Measurements of the HiVHAc Thruster via Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy was used to make real-time erosion measurements from the NASA High Voltage Hall Accelerator thruster. The optical sensor uses 250 nm light to measure absorption of atomic boron in the plume of an operating Hall thruster. Theerosion rate of the High Voltage Hall Accelerator thruster was measured for discharge voltages ranging from 330 to 600 V and discharge powers ranging from 1 to 3 kW. Boron densities as high as 6.5 x 10(exp 15) per cubic meter were found within the channel. Using a very simple boronvelocity model, approximate volumetric erosion rates between 5.0 x 10(exp -12) and 8.2 x 10(exp -12) cubic meter per second were found

Optical Mass Gauging System for Measuring Liquid Levels in a Reduced Gravity Environment

A compact and rugged fiber-coupled liquid volume sensor designed for flight on a sounding rocket platform is presented. The sensor consists of a Mach-Zehnder interferometer capable of measuring the amount of liquid contained in a tank under any gravitational conditions, including a microgravity environment, by detecting small changes in the index of refraction of the gas contained within a sensing region. By monitoring changes in the interference fringe pattern as the system undergoes a small compression provided by a piston, the ullage volume of a tank can be directly measured allowing for a determination of the liquid volume. To demonstrate the technique, data are acquired using two tanks containing different volumes of liquid, which are representative of the levels of liquid in a tank at different time periods during a mission. The two tanks are independently exposed to the measurement apparatus, allowing for a determination of the liquid level in each. In a controlled, laboratory test of the unit, the system demonstrated a capability of measuring a liquid level in an individual tank of 10.53 mL with a 2% error. The overall random uncertainty for the flight system is higher than that one test, at +/- 1.5 mL

Boron Nitride Sputter Erosion Measurements by Cavity Ring-Down Spectroscopy

Abstract: Sputter erosion is a critically important process in many electric propulsion (EP) devices from the point of view of both lifetime assessment and contamination effects. In many Hall thrusters erosion of boron nitride (BN) is of primary interest. In this contribution we introduce the use of cavity ring-down spectroscopy (CRDS) as a diagnostic for measurement of sputtered BN. The measurement approach is based upon probing sputtered boron atoms in the region of 250 nm. We report proof of principle CRDS measurements of sputtered boron atoms from both boron and BN targets. The measurements are obtained with pulsed CRDS in a diagnostic chamber consisting of an ion beam incident on the target materials. We also outline the design of a higher sensitivity continuous-wave (cw) CRDS system using the fourth harmonic beam from an external cavity diode laser as the light source. The cw-CRDS system will be used for near real time sputter erosion measurements of thruster devices. Anticipated signal levels and signal-to-noise for the cw-CRDS system are discussed. Nomenclatur

High Power Spark Delivery System Using Hollow Core Kagome Lattice Fibers

This study examines the use of the recently developed hollow core kagome lattice fibers for delivery of high power laser pulses. Compared to other photonic crystal fibers (PCFs), the hollow core kagome fibers have larger core diameter (~50 µm), which allows for higher energy coupling in the fiber while also maintaining high beam quality at the output (M2 = 1.25). We have conducted a study of the maximum deliverable energy versus laser pulse duration using a Nd:YAG laser at 1064 nm. Pulse energies as high as 30 mJ were transmitted for 30 ns pulse durations. This represents, to our knowledge; the highest laser pulse energy delivered using PCFs. Two fiber damage mechanisms were identified as damage at the fiber input and damage within the bulk of the fiber. Finally, we have demonstrated fiber delivered laser ignition on a single-cylinder gasoline direct injection engine

Cavity Ring-Down Methane Sensor for Small Unmanned Aerial Systems

We present the development, integration, and testing of an open-path cavity ring-down spectroscopy (CRDS) methane sensor for deployment on small unmanned aerial systems (sUAS). The open-path configuration used here (without pump or flow-cell) enables a low mass (4 kg) and low power (12 W) instrument that can be readily integrated to sUAS, defined here as having all-up mass of <25 kg. The instrument uses a compact telecom style laser at 1651 nm (near-infrared) and a linear 2-mirror high-finesse cavity. We show test results of flying the sensor on a DJI Matrice 600 hexacopter sUAS. The high sensitivity of the CRDS method allows sensitive methane detection with a precision of ~10–30 ppb demonstrated for actual flight conditions. A controlled release setup, where known mass flows are delivered, was used to simulate point-source methane emissions. Examples of methane plume detection from flight tests suggest that isolated plumes from sources with a mass flow as low as ~0.005 g/s can be detected. The sUAS sensor should have utility for emissions monitoring and quantification from natural gas infrastructure. To the best of our knowledge, it is also the first CRDS sensor directly deployed onboard an sUAS

Cavity ring-down spectroscopy sensor for ion beam etch monitoring and end-point detection of multilayer structures

This contribution reports on the development of in situ sputter monitoring and end-point detection for ion beam etch systems using continuous-wave cavity ring-down spectroscopy (cw-CRDS). The demonstrated system is based on the detection of sputtered manganese atoms using a tunable external cavity diode laser in the vicinity of 403.07 nm. The cw-CRDS system is described and measurements from a manganese-iron target are presented. End-point detection is demonstrated by monitoring the time dependence of manganese concentration for a multilayer target comprised of alternating layers of manganese/iron and titanium. Detection limits are shown to be adequate for today\u27s commercial ion beam sputter systems