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Dual-Laser-Pulse Ignition

This scheme provides a more reliable ignition source and more efficient energy delivery than a single-pulse format.

Marshall Space Flight Center, Alabama

A dual-pulse laser (DPL) technique has been demonstrated for generating laser-induced sparks (LIS) to ignite fuels. The technique was originally intended to be applied to the ignition of rocket propellants, but may also be applicable to ignition in terrestrial settings in which electric igniters may not be suitable. Laser igniters have been sought as alternatives to such conventional devices as electrical spark plugs and torch igniters for the following main reasons:

  1. A typical electric spark igniter generates sparks at its electrode near a wall, which potentially quenches combustion. Hence, more spark energy is needed to ensure ignition. A large combustion chamber would require a torch igniter, which comprises an electric-spark source, a pre-mixing chamber, and propellant valves. In contrast, the laser igniter is capable of creating sparks directly in a main chamber at specific optimal locations, which can be out away from the chamber walls, and without the need of other subsystems.
  2. Laser igniters can generate LIS with very precise timing, on the order of nanoseconds. This accurate timing precision may be helpful in certain ignition applications. Furthermore, this ignition generates significantly less electromagnetic emission noise than electrical igniters. Such noise can interfere with other electronic signals of engine sensors and control components.

Years of research on laser ignition have produced viable single-pulse laser ignition concepts; however, the transmission of high laser energy through fiber optics required by these single-pulse schemes has been problematic due to potential fiber damage and reduction in transmission efficiency. In comparison, optical energy for the DPL method can be stretched out and transmitted through multiple fiber lines, effectively reducing the energy intensity. In addition, the lifetime of the plasmas generated by use of the DPL exceeds those of plasmas generated by single-laser pulses, which increases their efficacy as an ignition source.

In the present DPL technique, the first pulse is used to generate a small plasma kernel. The second pulse is subsequently used to irradiate the plasma kernel. The transfer of laser energy into the kernel is much more efficient because the radiation-absorption characteristics of the plasma kernel are greatly enhanced, relative to the single-pulse approach. Consequently, the kernel can develop into a more effective ignition source.

Comparative experiments on single- and dual-pulse laser ignition were performed in a small test-bed rocket thrust chamber, at Marshall Space Flight Center, using gaseous oxygen and kerosene. Sapphire windows were used for optical access to the chamber. In the tests, the DPL technique was found to provide a repeatable ignition source for combustion with an optimal energy level.

This work was done by Huu Trinh of Marshall Space Flight Center, James W. Early of Los Alamos National Laboratory, Matthew E. Thomas of CFD Research Corp., and John A. Bossard, formerly of CFD Research Corp. MFS-31922