Interaction of Long Pulse Electron Beams with Gases and Flames.

The combustion rate of premixed gases in a closed vessel is increased up to 50% by injecting a high current electron beam into the gas mixture within about 20 ms of spark ignition. This effect is observed with the fuels ethylene, methane, ethane, propane, and n-butane. Experimental results provide strong evidence that e-beam excitation of the fundamental longitudinal acoustic mode of the cylindrical chamber is the mechanism of combustion enhancement. An observable combustion enhancement requires that the amplitude of the fluid velocity oscillation in this acoustic mode be greater than or approximately equal to the flame propagation speed, and is associated with a wrinkled or cellular flame structure with dimensions on the order of 1/2 cm. These results are in good agreement with values for the threshold acoustic velocity amplitude and dimension of cellular structure predicted for a periodically-accelerated flame. Current clamping (constant current behavior) is observed in measurements of the current drawn by a metal plate that acts as a beam collector for a long pulse electron beam propagating a short distance through low pressure gas (40 to 1000 mTorr). When the clamping occurs, oscillations in the beam collector are reduced, resulting in a flat pulse. Clamping behavior is in good agreement with a model which considers the impedance of the plasma produced by the electron beam in parallel with the inductance of the beam collector current path. The current clamping occurs when the plasma impedance is smaller than the inductive impedance of the beam collector current path.