The Transient Internal Probe Explanation

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The Transient Internal Probe (TIP) is a unique plasma diagnostic being developed here in the Aerospace and Energetics Research Program at the University of Washington. It is a technique which directly measures the internal magnetic field structure of a high temperature plasma for magnetic confinement fusion research.

In magnetic confinement controlled fusion, internal magnetic field structure is the most important variable of the hot plasma. This is because internal magnetic fields both describe and influence the plasma current profile and equilibrium state, which are each essential for generation of heat and sustainment of plasma ions at thermo-nuclear reaction density and temperature.

Unfortunately, typical plasma ion temperatures necessary for breakeven fusion power production are on the order of ~100 million degrees Kelvin! This makes any material probe of the plasma's internal magnetic field structure extremely difficult, if not impossible. Conventional probe materials will melt and vaporize in a process called ablation, before probe data can be obtained.

Today, however, most plasma heating and confinement experiments are conducted at somewhat lower temperatures. In fact, most research into the fundamental physics of magnetically confined plasmas is done between 1 and 10 million degrees. At such reduced temperatures, it becomes possible to insert a probe for a brief period of time inside the hot core plasma. Data acquisition takes place extremely fast at high sampling rates, usually over a significant fraction of the plasma lifetime.

But even 1 million degrees is still quite hot. Eventually, a conventional internal magnetic probe (like a Rogowski coil) inserted in such a plasma will produce impurities, as ablation of its surface begins to contaminate the plasma. Also, the simple placement of a coil inside the plasma machine can perturb the plasmadynamics from what it would have been without the coil, resulting in inaccurate internal magnetic field data.

The TIP diagnostic avoids the problems of probe ablation and plasma perturbation. Ablation is avoided by minimizing the transit time, and therefore heat transfer, to a probe immersed in the hot core region. Specifically, TIP uses a two-stage light gas gun to propel a magneto-optic probe up to a speed of ~2km/s, for mapping out the magnetic field component along the path of its transit through the plasma. Perturbations to the plasma from TIP are avoided since it employs a small, 1cm long probe which is magneto-optic in nature. In particular, each probe is a piece of terbium-doped borosillicate glass, which, when immersed in a magnetic field, rotates the polarization of laser light illuminated through it, by an amount linearly proportional to the ambient magnetic field, via the Faraday effect.

The TIP diagnostic is currently the most accurate and precise way to resolve spatial and temporal profiles of the toroidal magnetic field in a tokamak plasma at 1 million degrees Kelvin. Work is underway to perform a test of a sapphire clad probe in a 5 million degree spheromak plasma, along with a test of a new cat's eye retro-reflector probe which should increase magnetic field resolution of the diagnostic.