Study of Low Frequency Electric Field in MST Reversed Field Pinch and Tokamak Plasma

Two innovative capacitive probes have been developed and tested to study plasma potential and electric fields in the Madison Symmetric Tours. One probe consists of 16 capacitors arranged in cubical shape that measure the plasma potential, to calculate the 3D electric field. Additionally, the ▽ x Ε can be estimated by using the finite difference method to reveal the inductive component of E. The second capacitive probe consists of 5 fingers arranged in a plus shape each with three capacitive rings. The second probe also contains a triaxial magnetic pick-up coil located in front of the capacitors in each finger to measure the local magnetic field vector, Β. In addition to measuring 3D electric field, this probe is also capable of measuring ▽ · E, by using the finite difference method, to study the local plasma charge density, σ , and investigate plasma local quasi-neutrality. This probe can also measure < E x B >. Both probes are used to study Runaway Electrons by measuring low frequency inductive toroidal electric field in low density tokamak plasma as well as inductive poloidal electric field during magnetic reconnection activity (sawtooth events) to provide critical information to understand particle transport in magnetized plasmas. In Reversed Field Pinch (RFP) plasma, magnetic fields are stochastic in the core, a positive Er develops to reduce the electron transport to the level of the ion transport. Design details and initial results from both probes will be presented.