Investigation of jet curtains for chemical laser application

Abstract

A method is presented for passively containing reacting gas within the cavity of a chemical laser using a combination of cavity wall displacement and jet curtains. Suggesting several advantages over the pressure approach, the concept was evaluated in an application to a DF laser using the hydraulic analogy. Analyses were made to determine the flow characteristics of the confined and unconfined laser gas, the jet curtain requirements, and basic information for the design of the test hardware (water table). Experiments were conducted on the water table with the jet nozzle size, jet exit Froude number, and wall displacement as the variables. The results of these experiments qualitatively demonstrated the feasibility of the containment concept and demonstrated that for a given nozzle size, the ideal wall displacement is that which requires the minimum jet Froude number for containment. N a chemical laser, the power is extracted from the cavity in a transverse direction to the gas flow. This means that the optical system requires apertures in the cavity walls and that these be located downstream of the entrance to the cavity and sized to approximately span the iasing zone. The heat release within the Iasing zone will generate a static pressure rise which, when combined with the aspirating effect of the flow across the apertures, will result in a transverse pressure gradient and a lateral expansion of the laser gas. Some of this gas will enter the tunnels leading to the mirror cavities and may reach and contaminate the mirror surfaces unless measures are taken to prevent it. Reference 1 describes a method of preventing the laser gas from entering the tunnel by injecting a noncontaminating gas into the mirror cavity and thereby increasing the flow static pressure there to a point where it matches the laser gas static pressure at the aperture. This is referred to as matched pressure'* injection. This paper discusses a recently completed study of a more passive method of gas containment in which an aerodynamic barrier is used much in the same manner as described in Ref. 2. In this concept, the gas is permitted to expand freely in the lateral direction, and its capture is achieved by a lateral displacement of the cavity sidewalls. In addition, a constant Mach number inert gas jet located at each aperture and flowing across it in the direction of the laser gas serves as a barrier or curtain to prevent the latter's entrance into the mirror cavities. When wall displacement is sufficient to capture the laser gas plus a major portion of the flow from the jets, while diverting the remainder into the mirror cavity, then this containment concept exhibits the following desirable features: 1) Capturing the expanded gas rather than forcibly con- taining the unexpanded gas eliminates the introduction of shock disturbances in the Iasing zone. 2) The placement of the coflowing jet adjacent to the laser gas when coupled with an adequate wall displacement will prevent the formation of a large zone of recirculating laser gas at the tunnel entrance as a result of the latter's partial en- trance into the tunnel. Such a recirculation can bring harmful deactivants back to the start of the Iasing zone. 3) The coflowing nature of the jet minimizes its distur- bance impact upon the laser gas compared with jets used to replenish the mirror cavities for pressure injection.