Three-wave Polarimeter-interferometer Research Articles

Recent progress on the J-TEXT three-wave polarimeter-interferometer

The J-TEXT three-wave polarimeter-interferometer system (POLARIS), which measures time-space distribution of electron density and current density, has been optimized with both the optical system and the equilibrium reconstruction method. The phase resolution of a Faraday rotation angle has been improved from 0.1 to 0.06 degree in chords from –0.18 to 0.18 m (plasma minor radius), and the sawtooth oscillation behavior has been detected by Faraday rotation angle measurement. By combining the POLARIS measured data and the equilibrium and fitting code (EFIT), an upgraded equilibrium reconstruction method has been developed, which provides a more accurate temporal and spatial distribution of current density and electron density. By means of the optimized POLARIS and improved equilibrium reconstruction, variations of profiles with increasing density have been carried out, under both Ohmic and electron cyclotron resonance heating discharges.

Measurement of density fluctuation propagation direction via the far-forward collective scattering diagnostic based on polarimeter-interferometer.

Previously, the 17-channel three-wave polarimeter-interferometer system (POLARIS) on the J-TEXT tokamak has been implemented to measure far-forward collective scattering (FFCS) from electron density fluctuations. Recently, this system has been exploited to measure the propagation direction of density fluctuation. After considering the refraction of the laser probe beam passing through plasma, the ray tracing result shows that the detector of POLARIS may receive asymmetric far-forward scattering beams. Thus, the heterodyne detection of FFCS is available to identify the propagation direction of density fluctuation by resolving the asymmetric scattering spectrum. Experimentally, the transform of the heterodyne scattering spectrum from symmetry to asymmetry has been observed, while the refraction effect becomes strong demonstrating the capacity of measuring the propagation direction of fluctuation. Furthermore, by changing the plasma potential through the use of an applied positive electrode biasing, the reverse of frequency shift for the heterodyne scattering spectrum is identified, confirming the validity of direction discrimination of density fluctuation.

Profile Aided Real-Time Plasma Electron Density Feedback Control Based on FPGA on J-TEXT

The J-TEXT newly deployed three-wave polarimeter–interferometer (POLARIS) system provides a better temporal and spatial resolution of the plasma electronic density than the that of old HCN interferometer system. The new plasma electron density feedback control system is implemented on the already existing POLARIS data acquisition system which is based on FlexRIO field-programmable gate array (FPGA). An analog output card is added to control the piezoelectric crystal valve of the gas puffing. The density feedback control system is able to extract the phase-shift information from the intermediate frequency signal using fast Fourier transform, calculate density of multiple chords, and output control signal to the piezoelectric crystal valve in real time. NI P2P technology is used to transfer data from a FlexRIO FPGAs to another in real time without using the CPU; thus, all FlexRIO FPGAs onboard can calculate the density with only one acquiring the reference chord signal. This system is also able to calculate density profile. With the density profile, it can use compensated central chord integrated or average density as control target. Also, the real-time density profile data are useful for the future plasma control system and disruption detection system.

Opto-Mechanical systems design for polarimeter-interferometer on EAST

An 11-channel Far-InfaRed (FIR) three-wave POlarimeter-INTerferometer (POINT) system has been successfully operated in 2015 EAST experimental campaign. For high accuracy measurement of POINT system, optimized optical system to reduce the stray light and crosstalk is very important. Optical design is done and improved by using ZEMAX software, in which spot size and energy distribution can be calculated in any position. The crosstalk and stray light can be reduced by optimized design of optical components and putting high extinction ratio materials in some key positions. Vibration isolation coefficient of optical platform is set to 90%. The optical platform and vibration isolation system are about 5 and 20 tons in weight respectively. To reduce vibration caused by the EAST hall, a more than 30 tons in weight stainless steel tower, filled with sand and mounted independent of the EAST machine, is constructed to ensure the stability of optics. Based on the optimized opto-mechanical design, the POINT system resolutions for Faraday rotation and line integral electron density measurements are 0.1° and 1×1016m−2, respectively.

Far-forward collective scattering measurements by FIR polarimeter-interferometer on J-TEXT tokamak.

The multi-channel three-wave polarimeter-interferometer system on J-TEXT tokamak has been exploited to measure far-forward collective scattering from electron density fluctuations. The diagnostic utilizes far infrared lasers operated at 432 μm with 17-channel vertical chords (3 cm chord spacing), covering the entire cross section of plasma. Scattering laser power is measured using a high-sensitivity Schottky planar diode mixer which can also detect polarimetric and interferometric phase simultaneously. The system provides a line-integrated measurement of density fluctuations with maximum measurable wave number: k⊥max ≤ 2 cm-1 and time response up to 350 kHz. Feasibility of the diagnostic has been tested, showing higher sensitivity to detect fluctuation than interferometric measurement. Capability of providing spatial-resolved information of fluctuation has also been demonstrated in preliminary experimental applications.

The rectangular array of magnetic probes on J-TEXT tokamak.

The rectangular array of magnetic probes system was newly designed and installed in the torus on J-TEXT tokamak to measure the local magnetic fields outside the last closed flux surface at a single toroidal angle. In the implementation, the experimental results agree well with the theoretical results based on the Spool model and three-dimensional numerical finite element model when the vertical field was applied. Furthermore, the measurements were successfully used as the input of EFIT code to conduct the plasma equilibrium reconstruction. The calculated Faraday rotation angle using the EFIT output is in agreement with the measured one from the three-wave polarimeter-interferometer system.

Density modulation experiment to determine transport coefficients on Joint-TEXT Tokamak.

Density modulation experiments have been conducted on Joint-TEXT (J-TEXT) Tokamak Ohmic discharge to investigate particle transport based on a model with constant diffusion plus inward convection. Like the HCN interferometer, the newly developed three-wave polarimeter-interferometer system (POLARIS) is used to measure the perturbed density. The comparison of results between the HCN interferometer and POLARIS is given. The consistent results indicate the validity of the analysis scheme. At lower densities, the typical particle confinement time τp is found to increase with electron density, while it saturates at higher densities.

Optical layout and mechanical structure of polarimeter-interferometer system for Experimental Advanced Superconducting Tokamak.

A Far-InfaRed (FIR) three-wave POlarimeter-INTerferometer (POINT) system for measurement current density profile and electron density profile is under development for the EAST tokamak. The FIR beams are transmitted from the laser room to the optical tower adjacent to EAST via ∼20 m overmoded dielectric waveguide and then divided into 5 horizontal chords. The optical arrangement was designed using ZEMAX, which provides information on the beam spot size and energy distribution throughout the optical system. ZEMAX calculations used to optimize the optical layout design are combined with the mechanical design from CATIA, providing a 3D visualization of the entire POINT system.

Optimization of J-TEXT three-wave polarimeter-interferometer system

A laser-based three-wave polarimeter-interferometersystem (POLARIS) is operational on J-TEXT tokamak (R=1.05 m, a=0.27 m) tomeasure plasma electron density and Faraday rotation angle simultaneously.Vibration and collinearity are two key factors which affect the accuracy ofthe measurement. Vibration at the amplitude of even 1 μm is able toinduce almost 1° error (λ=432 μm) to the measurementthrough phase detection. And misalignment of collinearity can also causesystematic error to the measurement of Faraday angle due to density gradientand path length difference. Thus the systematic error from these two sourcesshould be eliminated to obtain accurate equilibrium and fluctuationinformation of the plasma. Some work has been done on optimizing vibrationand collinearity to achieve this goal. After optimization, POLARIS now hasachieved 12 chord measurements (spacing of 3 cm) with high temporal andphase resolution. Details of the optimization and results are described inthis study.

Vibration mitigation in J-TEXT far-infrared diagnostic systems

Optical structure stability is an important issue for far-infrared (FIR) phase measurements. To ensure good signal quality, influence of vibration should be minimized. Mechanical amelioration and optical optimization can be taken in turn to decrease vibration's influence and ensure acceptable measurement. J-TEXT (Joint Texal Experiment Tokamak, formerly TEXT-U) has two FIR diagnostic systems: a HCN interferometer system for electron density measurement and a three-wave polarimeter-interferometer system (POLARIS) for electron density and Faraday effect measurements. All use phase detection techniques. HCN interferometer system has almost eliminated the influence of vibration after mechanical amelioration and optical optimization. POLARIS also obtained first experimental results after mechanical stability improvements and is expected to further reduce vibration's influence on Faraday angle to 0.1° after optical optimization.

First results from the J-TEXT high-resolution three-wave polarimeter-interferometer

A laser-based far-infrared polarimeter-interferometer system utilizing the three-wave technique has been implemented on the J-TEXT tokamak. The polarimeter determines the Faraday effect by measuring the phase difference between two collinear, counter-rotating, circularly polarized laser beams. The first results of the polarimeter-interferometer designed for J-TEXT have been obtained in the most recent J-TEXT experimental campaign. Simultaneous polarimetric and interferometric measurement is achieved, with phase resolution up to 0.1°, at bandwidth of 50 kHz. The temporal resolution, which is dependent on the laser's frequency offset, is ∼1 μs. Continual spatial measurement covering 45 cm (80% of the plasma cross-section) is realized by utilizing 1D parabolic beam expansion optics. Three initial test chords are installed and future plans call for expansion up to 30 chords with 1.5 cm chord spacing, providing high spatial resolution for measurement of electron density and current density profiles. Reliability of both polarimetric and interferometric measurement is confirmed by comparison with computation and data from a hydrocyanic acid (HCN) interferometer. With the high temporal and phase resolution, perturbations associated with the sawtooth cycle and MHD activity have been observed.

Design of far-infrared three-wave polarimeter-interferometer system for the J-TEXT tokamak

A multichannel three-wave far-infrared polarimeter-interferometer will be constructed on the J-TEXT tokamak (R=1.05 m, a=0.27 m, B(T)≤3 T, and I(P)≤350 kA) for current density profile and electron density profile measurements. The system will adopt the three-wave polarimeter configuration which was first introduced on RTP. Three 432.5 μm HCOOH lasers pumped by three CO(2) lasers separately will be adopted, which could generate high output power, nearly 50 mW at each cavity. Two of them will be counter-rotating circularly polarized to probe the Faraday angle, while the third laser will be used as a local oscillator to get the phase shift caused by electron density. Excellent port access (600×76 mm(2)) and high laser power would promise a profile measurement across the whole plasma section with good signal quality. A high-speed digital phase comparator with a few (∼2) μs temporal resolution will be developed, so that fast changes of current or density profile could be measured. Six channels will be installed in the first stage.

High-speed three-wave polarimeter-interferometer diagnostic for Madison symmetric torus

A high-speed three-wave polarimeter-interferometer diagnostic has been developed on the Madison symmetric torus reversed field pinch to provide simultaneous measurement of electron density and poloidal magnetic field profile evolution. With increased digitizer bandwidth, polarimetry noise due to aliasing and cross-talk is minimized, and time response improved. System performance is demonstrated by measurements of equilibrium profile evolution during fast events such as the sawtooth crash and pellet injection.