Diamagnetic flux measurements on the STOR-M tokamak

Abstract

Diamagnetic measurements of poloidal beta have been performed in the STOR-M tokamak by a flux loop placed exterior to the vacuum chamber. Poloidal beta is defined as the ratio of plasma kinetic pressure to poloidal magentic field pressure. Compensation for the vacuum toroidal field has been performed using a non-enclosing co-planar coil, and vibrational compensation from auxiliary coils. It was found that in STOR-M conditions (20% toroidal magnetic field decay over discharge) there is significant influence on the diamagnetic flux measurements from strong residual signals, presumably from image currents being induced by the toroidal field coils, requiring further compensation. A blank (non-plasma) shot is used specifically to eliminate the residual component which is not proportional to the toroidal magnetic field. Data from normal ohmic discharge operation is presented and calculations of poloidal beta from coil data (βθ ≃ 0.5) is found to be in reasonable agreement with the values of poloidal beta obtained from measurements of electron density and Spitzer temperature with neoclassical corrections for trapped electrons. Contributions present in the blank shot (residual) signal and the limitations of this method are discussed. A pulse with Compact Toroid Injection was examined and compared to a normal ohmic discharge, and one where the Compact Toroid Injector was used to supply the tokamak with neutral gas. Soft X-Ray (SXR) measurements were taken and compared. There is a strong agreement between the profiles of the poloidal beta and the SXR measurements. The bulk plasma thermal energy was measured and found to increase by 5.6 J following the injection of a CT. The diamagnetic measurements appear to be affected by image currents induced in the chamber walls by the plasma current, and also by plasma position fluctuations. Future work outlining the possibilty of compensating these currents and improving the measurements is presented.