Magnetic Probes

For magnetic fusion energy (MFE) systems, measurements of the magnetic field and associated current are the primary diagnostic interest. Faraday's law states that a time-varying magnetic field, B(t), will induce an electric field, E(t), in a loop of wire: ∇×E = -dB/dt. For n loops of wire with cross-sectional area, A, the resulting electric potential is φ(t) = nAdB/dt (where B is the component of B along the axis of the loops). Measuring the voltage produced by such a coil will therefore give the time- variation of B, which after integration (numerical or passive) will yield B(t). Magnetic probes typically comprise many loops of magnet wire wound around a plastic form. These coils are commonly mounted around the perimeter of an experiment and designed either to measure either the slow or fast variations in B (so-called equilibrium coils or fluctuation coils). Coils can also be wound to be sensitive to multiple orthogonal components of B, measuring all at the same time. A similar type of probe called a Rogowski coil measures the time-variation of the current passing through it, which is integrated to find I(t). We have engineered various ultra-high vacuum (UHV) compatible magnetic probes for various purposes, including single coils and arrays. We have also developed Helmholtz coil arrays used to calibrate these coils either in-situ or on a bench. A good reference for magnetic probes is Hutchinson's Principles of Plasma Diagnostics. References for specific applications can be found in Magnetics References.

When requesting a quote for a magnetic diagnostic, please consider the following:

Expected magnitude of the measured quantity and digitizer dynamic range. These determine the minimum and maximum nA of the coil.
Frequency response. What is the frequency (or range of frequencies) that need to be measured? This determines the required inductance, L, and resistance, R, of the coil.
Vacuum environment. Is the probe intended to be used in air or vacuum? High vacuum (HV, 10^-3 - 10^-9 torr) or ultra-high vacuum (UHV, 10^-9 - 10^-12 torr)? This helps us determine the materials that can be used for your application.
Thermal environment. Will the probe experience high heat loads (for example, behind first wall tiles) or low heat loads? Will it experience thermal cycling? Will it be used at a high or low ambient temperature (will the chamber be baked)? This information also places restrictions on the materials that can be used.
Mounting interface. Will the probe be mounted to a vacuum flange, a custom mounting structure on the chamber wall, or something else?
Size constraints. Width of gap behind wall tiles, diameter of port through which the coil must be inserted, perturbation to the plasma, et cetera.