Design Of Eddy Current Diagnostic System On KTX And Experimental Study Of Dynamo Effect In RFP Plasmas

Generation and sustainment of magnetic fields from dynamics in electrically con-ducting media,or so-called dynamo actions,is a universal physical phenomenon in nature and laboratory.Reversed-field pinch(RFP)configuration has robust and rich dynamo effects which could be driven by a wide variety of mechanisms.Dynamo in-volves the core process of re versed-field pinch plasmas relax to Woltj er-Taylor state and is the basic of magnetic self-organization and the key factor to understand the equilib-rium and instability of reversed-field pinch configuration.The evolution of eddy current on conductive shell in RFP devices could reveal the appearance,development and formation processes of plasma perturbations such as:resistive wall mode and tearing mode.Measuring spatial and temporal distribution of eddy current is conductive to aid investigating plasma equilibrium and magnetic field perturbations.Eddy current diagnostic system is suitable for research on three dimen-sional physical phenomenon in toroidal magnetic confinement devices.Though the dynamo is a spacial random event,the detection of eddy current is able to give a good indication of where the dynamo occurs,which might provide further insight into the three dimensional essence of dynamo effect.An original and practical eddy current diagnostic tool:eddy current probe array(ECPA)has been developed on Keda Torus experiment(KTX).Eddy current probe ar-ray could detect more than 3 basic physical parameters at the closed boundary of plasma and covers the whole surface of conductive shell,including two poloidal gaps on KTX.Featuring in high spatiotemporal resolution,ECPA is able to detect the polarization characteristic of eddy current.The principle of measurement,composition and distri-bution of probe array,as well as the calibration of probes are introduced in detail.A model verification and platform test of probe array are also described.The spatial and temporal distribution of eddy current on KTX's composite shell have been measured by ECPA,with both amplitude and angle of eddy current.By introducing flow func-tion,the 2D eddy current vector is converted into 1D streamline.Eddy current probe array could be applied to a variety of research fields like plasma equilibrium,instability analysis,magnetic self-organization and active control.Expernmental results of oscillating poloidal current drnve and its two main physi-cal effects:entrainment of sawtooth cycle and impurity ion(C+4)heating are present-ed.OPCD can entrain the RFP's nonlinear magnetic relaxation cycle and can therefore modify the frequency of sawtooth crash events.These crashes,which are intermittent or quasiperiodic in standard RFP plasmas without oscillating voltages,can be entrained by OPCD to become nearly periodic,repetitive only near a particular phase of the OPCD cycle.The optimum entrainment condition of the sawtooth cycle is investigated by varying OPCD amplitude and frequency,as well as plasma reversal parameter.The re-lationship between oscillating frequency and sawtooth crash frequency are also checked.We relate this to the overall magnetic relaxation process in terms of changes to magnet-ic equilibrium and its MHD stability properties as modulated cyclically by the OPCD loop voltage.Impurity ion heating induced and modulated by OPCD is measured by ion Doppler spectrometry and is ascribed to the modulation of magnetic reconnection activity.The perturbation of eddy current on Madison Symmetrnc Torus(MST)is used to in-vestigate the localized feature of dynamo.An amplitude modulation on tearing mode is found to be distributed in a narrow toroidal range and its width almost remains constant between sawtooth crash.The amplitude of modulation gradually grows and its velocity slows down before sawtooth event occurs.The mode spectrum and spatial evolution of modulation are analysed carefully.Several physical parameters are ensemble-averaged and the threshold of amplitude is given according to data statistics at particular mo-ments.At length,the trigger mechanism of dynamo is put forward on the basis of the localized modulation on eddy current perturbation.