Study of a light-gas gun for launching active transient internal probes

The Transient Internal Probe (TIP) is a diagnostic for the measurements of local parameters in high temperature plasmas. An optic probe is launched into a plasma at ∼1.8 km/s using a two-stage light gas gun, while a polarized laser light illuminates the probe and the light retroreflected after double-pass through the probe is continuously detected with an ellipsometer. The polarization state change of the light corresponds to Faraday rotation or Pockels effect depending on the probe material. Recently developed refractory clad probes and catseye retroreflecting probes are presented in this paper. One component of magnetic field or electric field profile measurements along the transit of the probe can be achieved by using the present passive optic probes.; The next step of the TIP will be active probes utilizing on-board sensors and telemetry, which will allow measurements of multiple plasma parameters, possibly including the plasma temperature and density. At issue is the survivability of the on-board electronic components under the high acceleration during launch. Experimental and numerical study of the TIP light gas gun has been performed to lower the maximum acceleration of the probe while the probe velocity is still high (≥1.5 km/s) to avoid ablation during transit through a plasma. A microwave cavity technique is used to measure the probe position versus time during launch, from which the in-bore velocity and the acceleration profiles are determined. Piezoelectric pressure sensors measure the pressure history of the gun gas at several locations. A numerical code ('SIMTIP') is used to simulate the TIP two-stage light gas gun and the code output is compared to the measurements. This paper provides an optimum gun operation condition for launching an on-board active TIP. A prototype of the active TIP has been gun-launched. The probe includes a timer, a capacitor, and a Vertical Cavity Surface Emitting Laser (VCSEL). It survived ≥160,000 g's of acceleration and achieved a velocity of 1,700 m/s.