Fast single loop diamagnetic measurements on the TCV tokamak

Abstract

Measurement of the plasma generated diamagnetic flux on TCV is used to derive the plasma pressure, as on other magnetic confinement experiments. However specificities of the device make the measurement more difficult: for passive stabilisation of the vertical position of highly elongated plasmas, the vessel has a low electrical resistivity, leading to large image currents in the vessel. For the same reason the plasma must also be kept close to the conducting wall, so that the in-vessel double loop method usually used to compensate for these currents can not be applied. In order to achieve proper compensation, the diamagnetic measurement diagnostic on TCV uses the signal from a single loop wound outside the vessel in combination with appropriate signal processing that accurately matches the fast component of the induced vessel current. This allows extraction of the plasma diamagnetic flux with a remarkable bandwidth, of ~10kHz, and with the required accurate compensation of 0.05mWb out of 2Wb.As a result, rapid changes in plasma pressure can be studied, for example during additional heating power modulation or plasma instabilities such as ELMs.

Tokamak equilibrium reconstruction code LIUQE and its real time implementation

“…A detailed description of the measurement was given in [17]. This flux can be expressed, using the parametrisation of the magnetic field given by equation (2) …”

Diagnostics for steady state plasmas

“…The flux in the periphery of the plasma column can also be obtained by compensation loops. Fine compensation of single diamagnetic loops with a resultant diamagnetic bandwidth of 10 kHz has been demonstrated for TCV [315], being, however, a rather complex task. In a long pulse experiment the setup of a double diamagnetic loop is only feasible, if enough space is left between the vessel wall and the elements of the protecting first wall or if loops with low winding numbers are used as in TS, where a two turn loop made of 1.05 mm thick cable has been realized [296].…”

The Magnetic Diagnostic Set for ITER

“…Whereas older tokamaks installed specific Rogowski coils to measure such a loop-integrated current, poloidal field measurements are now currently used to create a "virtual" Rogowski coil by a weighted sum of the individual signals, which is also the present ITER plan. The magnetic system on the TCV tokamak is a standard example of this approach [8], [9]. Nonetheless, conventional Rogowski coils are being developed to sit inside the TF coil casings at liquid helium temperature, and a schematic of this system is shown in Fig.…”