Pulsed Microwave Plasma Instrumentation for Investigation of Plasma-Tuned Multiphase Combustion

Instrumentation developed to support the investigation of electromagnetic wave interaction with energetic materials and flames could help develop microwave-sensitive energetic materials that produce effects such as microwave ignition, acceleration of burning rate, extinguishment, and re-ignition.

Strategies to control solid rocket propellant regression rate require a robust throttling technique applicable to high performance propellant formulations. Currently, several methods to control and throttle either motors or subscale propellant strands exist, including chamber pressure control (e.g. pintle nozzles or rapid depressurization quench), infrared laser irradiation of the burning surface to increase burning rates, development of inherently unstable combustion chamber geometries (producing either local pressure or velocity perturbations), and electrically sensitive hydroxylammonium nitrate (HAN)-based formulations in which burning rate is controlled by a voltage potential. However, these techniques are limited in that they either can only be used with low flame temperature (low specific impulse) propellants, result in low propulsion system mass fraction (pintle), are only capable of producing a single perturbation, or are formulation specific.

Illustration of possible mechanisms for burning rate enhancement. Plasma kernel seeding frequently occurs around Al particle flame structures due to high local flame temperatures and subsequently grows through the flame volume. Direct energy absorption (dielectric loss) to oxides (smoke and oxide cap) can occur. Condensed phase heating can also contribute to the burning rate enhancement.

To gain control over a combustion process, combustion plasma enhancement has been demonstrated in electrothermal-chemical (ETC) launchers, in which solid gun propellant ignition flame spread, pressurization rate, and global propellant burning rate improvements were observed. With ETC enhancement, burning rate improvement of up to 35% is possible and further enhancement is speculated to be possible with higher solid loading. However, ETC launchers (e.g. capillary plasma generation) are capable only of single plasma injections or have limited volume.

In consideration of a microwave – rather than ETC – generated plasma, the microwave transparency of many propellant ingredients may enable large volume plasma generation in complex grain shapes, and microwave plasma seeding techniques are limited in number of plasma events by only energy availability. Unlike pintle throttling techniques, modification of a motor for microwave enhancement requires no moving parts and is simplified by utilization of the motor casing as an in-situ waveguide, requiring only the addition of an RF pressure window transition for magnetron interfacing. The use of microwave seeded plasmas within motors for control may also reduce aerodynamic loss (compared to pintle) and may enable throttling of higher performance propellant formulations (e.g. aluminized composites) that have flame temperatures too high to be easily controlled with pintle nozzles.

Pulsed microwave seeded-plasma generation is a multi-shot technique that has been used to gain control over combustion processes. This technique exploits low duty cycle, high-power microwave pulses, for precise control over plasma growth. One strategy for pulsed microwave plasma generation involves operating in a subcritical regime, below the threshold for ionization in the ambient gas, where microwave energy deposition to the flame is facilitated through interaction of high field strengths produced from a ~100 kW pulsed source with weak electron populations produced from chemiionization radicals. This strategy allows for preferential coupling to regions of locally high ionization while avoiding parasitic gas breakdown and absorption at other locations. Previously, this approach was demonstrated for both laser-generated ionization and in atmospheric pressure hydrocarbon flames.

Successful attempts at microwave supported plasma enhancement of premixed gas-phase flames resulted in an increase in flame speed, a ~500°C increase in flame temperature, and extension of lean flammability limits. The high field strengths able to be produced using pulsed techniques (order 10-100 kV/m) require only very low levels of ionization to rapidly establish plasma seeding and growth. However, seeding and growth are possible at much lower field strengths through non-equilibrium thermal ionization of a small amount of an easily ionizing dopant.

The use of a novel alkali metal doping technique for efficient, targeted low-power (field strength) microwave energy deposition to the flame structure in order to seed the formation of a combustion-enhancing plasma has been demonstrated. With this technique, the propellant is doped with a small quantity of material containing easily ionizing atoms, such as alkali earth metals (e.g. sodium in the form of sodium nitrate, NaNO 3). In doing so, microwave energy can be targeted to free electrons in a propellant flame, in order to produce the formation of plasma seeding (see illustration).

Propellant containing dopant (e.g. sodium in the form of sodium nitrate, NaNO3) decomposes from thermal energy provided from combustion, producing Na+ ion and electron in the flame. During microwave radiation, these ions and free electrons in the flame become energy deposit sites, producing plasma kernels which grow throughout the flame. Plasma kernels form near combusting Al agglomerates due to high local flame temperatures. Including plasma combustion enhancement, two other rate enhancing mechanisms have been identified and are being studied: (1) condense phase heating in Maxwell- Wagner effect due to conductive particles in a non-conductive dielectric matrix (HTPB bind, AP) and (2) combustion enhancement of burning Al agglomerates in the flame.

This work was done by Travis Sippel, James Michael, Stuart Barkley and Keke Zhu of Iowa State University for the Air Force Research Laboratory. AFRL-0271

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Pulsed Microwave Plasma Instrumentation for Investigation of Plasma-Tuned Multiphase Combustion

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