Air Force

Plasma torch combining arc and microwave discharges

Portable instrument can generate a stable and sizable plasma torch independent of the gas flow rate

Energy Other

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The microwave-augmented plasma torch module is shown in this illustration.

For a typical hydrocarbon-fueled supersonic combustor startup scenario such as a scramjet, the fuel-air mixture will not auto-ignite. An ignition aid is necessary to initiate main-duct combustion and since the residence time through the combustion region is short (1 millisecond) a way to reduce the ignition delay time and to increase the rate of combustion of hydrocarbon fuels is necessary.

The igniter must provide adequate thermal energy to light the fuel, and the delivered thermal energy should be able to deeply penetrate into the supersonic cross-flow.

To date, scientists have perfected the use of dense atmospheric-pressure plasma produced through direct current, low-frequency capacitive or high-frequency inductive arc discharges. These require adding gas flows to stabilize the discharges and to carry the generated plasmas out of the discharge regions to form torches.

The inductive torch and non-transferred DC torch employ high current power supply and require very high gas flow rates to achieve stable operation. Consequently, the structures of these torches are large and unsuitable for specific applications.

A typical torch module can run in DC or low-frequency AC mode and can produce low-power (hundreds of watts) or high-power (a few kW in 60-Hz periodic mode or hundreds of kW in pulsed mode) plasma torches; however, the size of the plasma torch produced by such module is generally limited by the gap between the electrodes and depends strongly on the gas flow rate.

Air Force researchers have tackled the above problem and designed, assembled, and characterized a versatile modular plasma torch for application as an ignition aide for a hydrocarbon-fueled scramjet engine.

The unique features of this new torch module are its use of an arc torch module as a microwave adaptor of a tapered rectangular waveguide cavity, and a hollow central electrode to add a fuel injection port. In this setup, microwave couples through the torch module, as a transmission line, to the plasma torch, which is generated by the arc discharge between the electrodes of the module. An analysis shows that nearly 80 percent of the supplied microwave power is delivered to the arc plasma through this adaptor arrangement.

The added microwave power enhances the peak power delivered by the arc discharge by reducing the phase difference between the current and voltage. The addition of the magnetron to the circuit also increases the circuit efficiency enabling more power to be delivered to the gas. The added microwave triples the energy of the torch, run with a small compact power supply, to about 12 J/cycle (peak power exceeds 6 kW). It enhances the electron excitation temperature by a factor as significant as 6, measured via emission spectroscopy. These enhancements enable the torch to ignite hydrocarbon-fuels.

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