Three-dimentional Probe Diagnostic And Characteristics Of Edge Plasma Turbulence On Keda Torus EXperiment

Keda Torus eXperiment(KTX)is a reversed field pinch device.Its engineering design and implementation are relatively simple,and it is particularly suitable for the research of basic plasma physics in colleges and universities.It can realize three kinds of discharge modes of ultra-low q,reverse field pinch and tokamak for flexible dis-charge,which are used to study the confinement state of plasma.It can also study the characteristics of electrostatic fluctuations at the edge plasma,the transport of particles and energy across the magnetic field caused by turbulence,and the improvement of confinement through current drive such as PPCD and OFCD.In order to measure the basic parameters of the edge plasma,including the electron temperature,density,floating potential and its radial distribution of fluctuations and turbulent transport,we develop three sets of scanning probe systems.We have built a bellows type and a magnetic drive type probe system that fit the installation environment in the top window OU and a mid-plane window M,respectively.In another mid-plane window O,a fast scanning probe system is developed.Among them,the OU and O windows are 90 degrees apart from each other in the poloidal direction,and the O and M windows are 30 degrees apart from each other in the toroidal direction.The mode measurement of m?2 and n?6 can be achieved.We have designed a fast scanning probe system driven by a servo motor and a linear module.The maximum speed of the system can be adjusted between 1 m/s and 4m/s to meet our needs for different speeds.Displacement is measured using an shock-resistant magnetic grating ruler.Through repeatability testing,we find that the delay time of the load between different shots is within 0.4ms.After the rapid collision stops,the difference in displacement amplitude is about 1 mm.We use a single-chip microcomputer to remotely operate the three probe systems,which can not only realize the slow movement shot by shot,but also the ability to quickly push the probe into the plasma and scan a radial profile during the plasma discharge in one shot.In the tokamak discharge mode,the comparison of the floating potential signals of the quick-action single probe and the radial rake probe shows that the two signals are in good agreement.showing that the fast scanning probe system can be used normally.We use a magnetic probe to measure the radial distribution of the toroidal field,its penetration time into the vacuum chamber and the "zero field" produced by ohmic field,verifying the design index of KTX.We also measure the plasma safety factor q distribution:ultra-low q discharge mode.q<1;tokamak discharge,q>1:reverse field discharge q<<1,which don't turn to be negative,indicating that the plasma has not entered the reverse field state.We measure the electrostatic fluctuation characteristics of the edge plasma.The electron energy distribution function calculated from the ?-? characteristic curve satis-fies the Maxwell distribution and electrostatic probe can be used in edge plasma.Then,profiles of floating potential,ion saturation current,electron temperature and density and their fluctuation are measured under three discharge modes.For ultra-low q dis-charge,the floating potential fluctuation has only a short-range correlation;in the re-verse field and tokamak discharges,the fluctuations are positively correlated in a long-range time series.The fluctuations of the three modes are concentrated within 100kHz.For the ultra-low q mode,|k?|?1cm-1,in the direction of electron diamagnetic drift,|kr| leqlcm-1;in reverse field mode,|k?|<2cm-1,in the electron diamagnetic drift direction,|kr|?1cm-1;for tokamak mode,|k?|?2cm-1,in the direction of electron diamagnetic drift,|kr|?1.5 cm-1,radially outward.The statistical dispersion relation-ship and the wavenumber spectrum width and the conditional spectrum coincide with each other.In the three modes,the radial correlation length of fluctuation is 1-4cm,and the poloidal correlation length is 1.5-8cm.We also measure the particle transport flux caused by fluctuations.In the ultra-low q mode,the particle transport flux is much larger than the other two modes;while in the reverse field and the tokamak mode,Te is equivalent,even though density fluctuations and speed fluctuations behave differently.Finally,we use a linear time-invariant system method to correct the measurments of plasma current and boundary magnetic field.The toroidal field signal interferes the Rogowski coil and thus the plasma current signal.The linear time-invariant system method can accurately predict its interference signal to the plasma current through the toroidal field current input signal,thereby obtaining a pure plasma current.Compared with traditional proportional compensation,this method will be more widely used and flexible.The method can also be applied to the correction of boundary poloidal mag-netic field and toroidal magnetic field.Especially after the correction of the toroidal field signal,the magnetic field signal that originally mislead us to believe plasma enter the reverse field state is not reversed,helping us identify the plasma state correctly.