Fast single loop diamagnetic measurements on the TCV tokamak

Measurement of the plasma generated diamagnetic flux on tokamak a configuration variable (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 stabilization 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 cannot 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 similar to10 kHz, and with the required accurate compensation of 0.05 mWb out of 2 Wb. As a result, rapid changes in plasma pressure can be studied, for example during additional heating power modulation or fast plasma instabilities. (C) 2003 American Institute of Physics.