TJ-II is fitted with an ample set of Mirnov coils:    There are two straight arrays above the plasma column, each with 12 coils, mounted in a 0.5 mm thick stainless steel tube attached to a port flange. The 12 coils are distributed in 4 groups of 3 coils each, separated by a distance of 30 mm. The three coils in each group measure the three cylindrical components (R, φ, and Z) of the magnetic field.
- One straight array in sector B5, at φ = 232°. The corresponding signals in the TJ-II database are called 'MIB5xy', where 'x' is 'R', 'F' (φ), or 'Z', and 'y' is the coil number.
- One straight array in sector C4, at φ = 308°.
- One poloidal array with 15 coils measuring the poloidal field component. This array is located between sectors D4 and D5, at φ = 45°. The signals in the TJ-II database corresponding to these coils are called 'MID5Px', where 'x' is the coil number. This array has been enhanced by 10 additional coils (5 at each end).
- Two toroidal arrays (installed 2018)
Two Rogowski coils have been installed in sectors A5 (148.7°) and B4 (211.3°). Since the coils have to surround the plasma column completely, they are located under the protecting tiles of the groove that runs toroidally all along the vacuum vessel. For this purpose, 2 mm deep channels were machined on the vacuum side of the groove, at the above mentioned poloidal cross sections, in order to allow enough space for the coils.
The signals 'Ip_a5_' and 'Ip_b4_' in the TJ-II database provide the corresponding measurements of the net plasma current.
Two internal diamagnetic loops are installed in sector A5 (135°) and B4 (225°). They consist of a main loop which encircles the plasma column and runs under the protecting tiles in the groove region and a compensating loop which measures only the vacuum magnetic flux.
- E. Ascasíbar et al, Energy content and magnetic configuration scan in TJ-II plasmas, 12th Int. Stell. Workshop (1999)
- R. Jiménez-Gómez et al., Analysis of Magnetohydrodynamic Instabilities in TJ-II Plasmas, Fusion Science and Technology 51, 20 (2007)
- R. Jiménez-Gómez, A. Könies, E. Ascasíbar, F. Castejón, T. Estrada, L. G. Eliseev, A. V. Melnikov, J.A. Jiménez, D. G. Pretty, D. Jiménez-Rey, M.A. Pedrosa, A. de Bustos and S. Yamamoto, Alfvén eigenmodes measured in the TJ-II stellarator, Nucl. Fusion 51, 3 (2011) 033001