Changes to the LANL gas driven two stage gun : projectile velocity measurement and etc.

Abstract

stage gun. It was necessary to use optical methods because electrical shorting pins damaged the projectile:, turned .the projectile causing tilted impacts, and sprayed the target with bits of broken pin. The first optical method involved cutting shrzllow grooves in the sides of the projectile at precisely measured intervals. Thc projectile pilssed through a single light beam focused in such a way that the grooves would alternately block and transmit light to a sensing system. This system didn't work because the groovas filled with smoke, blocking the light at all times after the projectile first broke the hearn. The second method used light rcflectetl off the projectile at four different positions. Light from a 400 mW laser was split into four oplical fibers. Half of the light reflected from the end of each B9er 'was retutncd to it phototnulitiplier. When the projectile passed in front of a fiber the amount of returned light increased. This system had a very poor signal to noise ratio: the amount of light returned when the projectile passed in front ofthe fiber was scarcely larger than the noise on the signals. 'I'hc third system used four stations at which laser light was transmitted from one optical fiber to another. 'The projectile passed close by tlhe sending or receiving fiber, rapidly cutting off the transmitted light. This method suffered from a lasix speckle pattern which changed with time thereby giving a constiintly changing inlerisiily. The fiber optic beam splitter used to split the laser light in methods two and three was also very nnstable: the amount of light split into any particular fiber varied with teinperature, vibration, and any movement of fibers. The method which was ultimately successful used it SmW, 670 nni laser diode at each of' four positions. A small lens focused this light to a point through which Ilie projectile passed. Transmitted light was imaged into 700 micron plastic fibers which relayed thhe light to a bank of photomultipliers. 'The combination of imaging the luminous area of the laser diodc and the end of the sensing fiber onto the same plane, through which the projectile passed, piovided veiy good rejection of stray light, a very fast light cutoff as the projectile passed tlwough the focal point, and efficient use of light. Projectile velocities were measured with an accuracy of 1 part in 1,000. In addition to our optical projectile velocity measuring system, we have significantly changed our projectiles, OUI transition section diaphragms, and developed a new honing teditiique. Thesc will be briefly discuswd as well