Magnetic Diagnostics at nT-Tao

At nT-Tao, we study an innovative compact-core pulsed stellarator concept that aims for fusion conditions in a magnetically confined D-T plasma, targeting ~10 keV temperature, ~10²² m⁻³ density, and confinement times of ~0.01 seconds. To heat the plasma, the required hundreds of MW of power are achieved by tight coils driven by pulsed power generators. On the research and development route, different prototypes are built. The C-2A compact pulsed power device was designed and constructed as an optimized quasi-axisymmetric stellarator using the DESC optimization code. In addition, we have built a linear theta-pinch device in which a current pulse of ~1 MA forms a hot and dense pinched plasma. In all devices, non-invasive diagnostics are a key tool in the development route, allowing a measurement of plasma properties and identification of onsets of instabilities.

Here we present our customized ensemble of magnetic diagnostics, including diamagnetic loops to measure plasma energy and excluded flux radius, Rogowski coils to measure the plasma current, voltage loops to measure plasma inductance and resistivity, and an in-situ Mirnov coil array to observe MHD modes. Our systems are characterized by a short operation time and sharp gradients, presenting significant challenges in analysis and interpretation. Nonetheless, with a combination of careful design, informative mock-up trials, and careful noise treatment, we extracted various plasma parameters from the magnetic diagnostics. Comparison between different diagnostics and interpretation methods shows a robust derivation of plasma parameters.