Transient Internal Probe Experiment

SITE INDEX

Currently the TIP diagnostic is in its second generation of development, funded from Lawrence Livermore National Laboratories as an internal probe for their Sustained Spheromak Physics Experiment (SSPX). Goals to be acheived by September 1999 include completion of an improved design for the magneto-optic probe, along with a computational fluid dynamics simulation for the gas gun propulsion system. In addition, a second TIP two-stage light gas gun is being manufactured and put together for vacuum coupling with the SSPX plasma machine.

Redesigning the magneto-optic probe is focused on an increase of both its belief time and retro-reflection signal intensity. Probe belief time is being extended by cladding the faraday glass with sapphire, since it has a higher melting point and is more refractrory than the terbium doped borosillicate faraday glass. Signal intensity improvements are being tested with a spherical curvature plano-convex sapphire lens placed on the front surface of the probe, in conjunction with an aluminized coating placed on the back surface of the probe. This new optics design should retro-reflect laser light at a greater intensity than the corner-cube retrosheet material, which had previously been placed on the back of the uncladded glass probes for measurements inside the relatively cool HIT plasma. In general, increase of the belief time and return signal intensity of the probe extends the applicability of the TIP diagnostic to hotter core plasmas.

Another way to extend the the TIP diagnostic to hotter and hotter plasmas is to increase the speed of the probe transit through the core. CFD simulations of the two-stage light gas gun are addressing the issue of probe speed; however, the problem is made difficult because the acceleration of the small-mass probe, only ~3.5g with cladding, cannot be too stressful otherwise large inertial stresses will pulverize its miniature optics. Multiple design parameters are being examined, including the fill pressures for the driver and pump tubes, the burst pressure of the pump tube diaphragm, and the stripping gas pressure in the gun barrel, in order to simultaneously broaden the acceleration profile and maximize the kinetic energy transfer from the pump gas to the probe.

Measurements of the toroidal field profile inside the SSPX plasma are expected to commence during the fall of 1999.