Experimental and numerical studies of laser sustained gas plasmas

Laser propulsion is the production of high specific impulse rocket thrust using a high power laser as a remote energy source. Specific impulses in excess of 1000 seconds are achievable because propellant temperatures are very high and low molecular weight gases can be used. The energy conversion mechanisms of laser sustained plasmas in pure flow argon and argon helium mixtures are examined. Experiments at very high argon mass flux (55 kg/m2s) and pressure as high as 2.5 atmospheres were performed. The results indicate that nearly all the laser power can be absorbed (greater than 97 percent), and efficiencies approaching 50 percent can be obtained. Experiments with mixtures of argon and helium indicate that the high specific heat and thermal conductivity of the helium tends to allow for more of the absorbed energy to be retained rather than reradiated to the chamber walls. This despite the fact that the very high ionization energy of helium limits the global absorption to values below that for pure argon plasmas. Fundamental research concerning laser sustained plasmas such as the independent experimental determinations of electron number density and electron temperature is required. This will allow the evaluation of the local thermal equilibrium which is needed in order to better interpret the spectroscopic and numerical results. Also required is the more accurate determination of downstream plasma exhaust gas temperature via Rayleigh scattering thermometry.