Electric fields in decaying CTX spheromaks in the 0.67 m radius mesh flux conserver

The effective electric field parallel to the magnetic field at the edge of decaying spheromaks has been deduced from equilibrium measurements. It is found to depend on the plasma electron density but not on the electron temperature, consistently with hydrogen ionization dominating the spheromak helicity dissipation. Although not directly measured, the plasma resistivity is expected to be much higher at the edge than at the magnetic axis, an assumption supported by the peaking of the spheromak currents towards the magnetic axis throughout the decay phase. To estimate the electric field, we assume the plasma resistivity to be appreciable only at the open magnetic flux surfaces at the spheromak edge. A floating Langmuir probe has been inserted into the spheromak plasma to qualitatively diagnose the spheromak radial electric fields. The results of these measurements are presented