Variable orifice for differential pumping in a vacuum system

Abstract

In differential pumping, the pumping speed has in the past been controlled by the size of the orifice between the vacuum chamber and the diffusion pump. This fixed orifice is usually in the valve gate of a high-vacuum valve (or in a baffle plate). To change the pumping speed, then, the valve must be removed and another valve with an orifice of a different size must be inserted. This process is time consuming and risks contamination of the system, which would require another pumpdown and bakeout. A simple shutter that controls pumping speed without requiring any change in components has been developed. The shutter consists of 20 leaves arranged as an iris that opens to a 6-in.-dia orifice or closes to 0.25 in. The shutter consists of a mounting ring, the leaves, a drive ring, and a drive gear. Each leaf is stainless steel with a pivot pin at one end and an offset pin at the other. The leaves are placed in the mounting ring and the drive ring and the drive gear are attached. Rotating the drive gear moves the leaves and changes the orifice size. One of the shutters has been used successfully with a 4-in. diffusionmore » pump system. In this system the pressure is 100 ..mu.. in the vacuum chamber and 0.0001 ..mu.. at the diffusion pump. The shutter was placed above the cold trap so it could be used for both diffusion pumping and roughing.« less