Development Of Solid-state Diode Source Interferometer-Polarimeter On EAST Tokamak

Electron density and current density are key parameters for the stable operation of magnetic confinement fusion plasma and have important relationships with physical processes such as plasma transport and magneto-hydrodynamics（MHD）.Fast and accurate measurements of electron density,current density and their distributions are essential for experimental operation and physical studies of magnetically confined fusion plasmas.Far infrared laser interferometer-polarimeter system is an effective tool to measure tokamak electron density and Faraday rotation angle due to its high resolution and the advantage of not requiring direct contact with the plasma.By combining with the corresponding inversion algorithms,it is possible to obtain magnetic field information and thus infer the current density distribution.However,the traditional interferometer-polarimeter utilizes a gas laser,which has the problems of large laser footprint,large amount of heat generated during operation requiring a water tank for cooling and cumbersome operation in the experiment.Meanwhile,for the future International Thermonuclear Experimental Reactor（ITER）,the internal deuterium-tritium reaction will subject the components in the device to large doses of gamma and neutron radiation,so the space occupied by the diagnostic system on the device needs to be as small as possible to facilitate better radiation shielding and ensure the stable operation of the system.And with the development of terahertz radiation source technology,the research of new terahertz solid sources has made great progress.The emergence of compact size,lightweight mass,simple operation,no water cooling,stable performance,one-touch frequency adjustment,and easy radiation shielding of terahertz solid source lasers can well solve the above problems and be fully applicable to the advanced diagnostic techniques for future plasma density measurements of fusion reactors.In order to provide a new means for plasma physics research on the all-superconducting tokamak device of EAST,and to realize miniaturization,modularization,and plug-in of polarization interferometer diagnostics,a terahertz solid source vertical three-channel polarization interferometer system（SSIP）is designed for the measurement of current density,electron density,magnetic perturbation,density perturbation,plasma displacement,and other parameters of long-pulsed plasma.This paper introduces the working principle,system design,practical construction,and experimental operation of the terahertz solid source polarization interferometer system on EAST.In this paper,we firstly start from the basic concepts of polarization and interference,review the basic principles of polarization and interferometry,and introduce several major polarization interferometry methods and their respective advantages and disadvantages.Then,based on the transmission theory of Gaussian optics,the design scheme of the terahertz solid-state diode source interferometer-polarimeter system on EAST is given,including the position layout of the SSIP system,optical path design and simulation,optical component size design,etc.At the same time,the principle and advantages of the new Terahertz solid-state diode source,detector and acquisition system used in the system are introduced in detail.In the mesa test,the performance of some key components is tested.Finally,the experimental results of the EAST Terahertz solid-state diode source interferometer-polarimeter are given.The density and Faraday rotation angle measured by the SSIP system were compared with the results which obtained by the stable CO₂ dispersion interferometer system and POINT interferometer-polarimeter system on EAST.The accuracy and reliability of the density measurement of SSIP system is verified.Stable density data are also obtained under the condition of high parameters and long pulses in 100 seconds.At present,SSIP system has carried out density feedback to ensure the daily operation of the EAST device.The measurement accuracy of the SSIP system is as follows:the time resolution is 1μs,the electron density is 1x10¹⁷m^-3,the Faraday rotation Angle is 1°.