Polarimeter-interferometer System Research Articles

A terahertz solid-state source interferometer-polarimeter designed for long pulse discharges on EAST.

A 0.65THz Solid-state Source Interferometer-Polarimeter (SSIP) with vertical viewing has been installed for line-integrated density and Faraday rotation measurements on Experimental Advanced Superconducting Tokamak (EAST). The SSIP utilizes three independent solid-state diode sources based on frequency multiplier (X48) to provide the probing beam at a fixed frequency of 0.65THz with ∼2.5 mW output power, which provide an Intermediate Frequency (IF) with wide range, and the highest IF is up to ∼10MHz. The mixers optimized for high sensitivity, ∼750 mV/mW, are used in the SSIP system, which permits multichannel interferometer-polarimeter on EAST with a low phase noise. The resolution of the integrated density is 3.3 × 1017m-2, and the Faraday rotation is about 1.5°. The density measurement has been implemented with IF ∼0.85MHz, in a plasma with plasma current ∼300kA on EAST. The line-averaged density, 3.5 × 1019m-3, measured by the SSIP shows good agreement with the results measured by the POlarimeter-INTerferometer (POINT) system and the CO2 dispersion interferometer system in the same discharge.

A fringe jump counting method for the phase measurement in the HCN laser interferometer on EAST and its FPGA-based implementation

Electron density in fusion plasma is usually diagnosed using laser-aided interferometers. The phase difference signal obtained after phase demodulation is wrapped, which is also called a fringe jump. A method has been developed to unwrap the phase difference signal in real time using FPGA, specifically designed to handle fringe jumps in the hydrogen cyanide (HCN) laser interferometer on the EAST superconducting tokamak. This method is designed for a phase demodulator using the fast Fourier transform (FFT) method at the front end. The method is better adapted for hardware implementation compared to complex mathematical analysis algorithms, such as field programmable gate array (FPGA). It has been applied to process the phase measurement results of the HCN laser interferometer on EAST in real time. Electron density results show good confidence in the fringe jump unwrapping method. Further possible application in other laser interferometers, such as the POlarimeter-INTerferometer (POINT) system on EAST tokamak is also discussed.

First results of CO2 dispersion interferometer on EAST tokamak

A dispersion interferometer (DI) has been installed and operates on the Experimental Advanced Superconducting Tokamak (EAST). This DI system utilizes a continuous-wave 9.3 μm CO2 laser source to measure line-averaged electron densities accurately. In contrast to conventional interferometers, the DI does not require substantial vibration isolations or compensating systems to reduce the impact of vibrations in the optical path. It also employs a ratio of modulation amplitudes, ensuring it remains immune to the variations in detected intensities. Without a variation compensation system, the DI system on EAST reaches a density resolution of less than 1.8 × 10−2π rad and a temporal resolution of 20 μs. The measurements made by the POlarimeter-INTerferometer (POINT) system and the far-infrared hydrogen cyanide (HCN) interferometer are remarkably consistent with the DI’s results. The possibility of fringe jumps and the impact of refraction in high-density discharge can be significantly decreased using a shorter wavelength laser source. A rapid density change of 3 × 1019 m−3 during 0.15 s has been measured accurately in shot No. 114755 of EAST. Additionally, the DI system demonstrates dependability and stability under 305 s long-pulse discharges in shot No. 122054.

Neural network identification of the weakly coherent mode in I-mode discharge on EAST

The improved energy confinement mode (I-mode) is widely considered as an important operation regime for ITER. I-mode implementation depends on the specified basic plasma parameters and certain operation conditions, which are discovered by statistical plasma characteristics from a large number of I-mode discharges on a tokamak. The extraction process of I-mode plasma characteristics is complicated, time-consuming, and limited to the sampling rate of the measured signals. Experimental observation of the I-mode is accompanied by the appearance of a weakly coherent mode (WCM). However, it takes much time to accurately scan and quantify WCM characteristics when analyzing many I-mode discharges. Recently, a neural network identification method was developed as an I-mode detector to traverse a whole database as a replacement for manual identification. Two fully connected neural network models were trained with the spectrum of propagation velocity of density perturbation from Doppler backward scattering and the electron density measured by a polarimeter-interferometer system with the experimental advanced superconducting tokamak I-mode database. An accuracy of 98.30% in identifying WCMs in I-mode discharges is achieved with the WCM classification model. In addition, the regime classification model was also utilized to successfully distinguish between the low confinement mode (L-mode), I-mode, and high confinement mode (H-mode) with 96.03% accuracy. Finally, ablation experiments were performed on the regime classifiers, showing that there is potential for further performance improvement with future use of RNN model.

Design of line array detection scheme for Polarimeter-Interferometer system on EAST

A multi-channel POlarimeter-INTerferometer (POINT) system has been constructed in the core region of Experimental Advanced Superconducting Tokamak (EAST) plasma for electron density profile and current density profile measurements, with a spatial resolution of 8.5 cm. To explore the small spatial scale density fluctuation phenomena in the core plasma induced by MHD instabilities effects, a higher spatial resolution and higher responsivity detection is required.A novel line array detection scheme of POINT is proposed. By replacing the single Schottky mixer with a highly integrated line array HEMT detector at the central channel position, the existing single channel can be extended to 5 channels for spatial continuous measurement in the core plasma, which has a significantly simplified structure design compared with the discrete upgrade scheme. Related optical design, component selection, and bench tests have been carried out to validate the feasibility and reliability of the novel scheme.The IF signals are detected by an AlGaN/GaN line array high electron mobility transistors (HEMT) detector based on five silicon lenses integrated. The average responsivity of the line array detector elements is 40 kV/W. Compared with the existing planar diode Schottky mixer with a sensitivity of 750 V/W, the line array detector can also meet the essential requirements of coherent signal measurement. Benefiting from the highly integrated array detector, the spatial resolution will be improved to 4.5 mm in the novel scheme. Additionally, a circular Plano cylindrical lens is designed as a beam-shaping component to compress the Gaussian beam into a stripe. Bench test results displayed the excellent performance of the cylindrical lens, which is consistent with the simulation results. Furthermore, normalized responsivity coefficients of array elements are determined by test results.

Observation of electron-scale turbulence suppression under weak magnetic shear with neon seeding in EAST plasma

Improved confinement caused by Ne injection is investigated in EAST. The safety factor q profile and electron-scale turbulence are studied with a polarimeter-interferometer system and CO2 laser collective scattering system in a series of Ne injection experiments in EAST. Ne with a suitable injection rate can lead to negative magnetic shear in the plasma core, which promotes electron-scale turbulence suppression. The continuous injection of Ne under the reversed shear condition further reduces the electron-scale turbulence intensity. Core electron temperature increase and confinement improvement are observed. QuaLiKiz is applied to model the influence of an impurity and reversed q on turbulence. The combination of stabilization effects of negative magnetic shear and Ne on turbulence is much stronger than the impurity’s own effect, which could explain the experimentally observed electron temperature (T e) increase and turbulence suppression. Besides, the relationship between pedestal structure induced by Ne injection and lower hybrid wave (LHW)-driven current deposition is discussed. These results provide a new perspective to improve the confinement in the case of Ne injection, i.e. to adjust the Ne injection to modify the ideal plasma equilibrium.

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.

Study of microscale surface morphology changing of the retroreflector in EAST by means of laser speckle interferometry technique

• The surface morphology of the retroreflector served in EAST is first time reconstructed through a 3D visual exhibition. • Three internal reflection planes of the retroreflector served in EAST appear severely different surface morphology features. • Laser speckle interferometry technique is a reliable surface morphology characterization approach for PFMs. The retroreflectors in EAST (Experimental Advanced Superconducting Tokamak) are important optical components of Polarimeter-Interferometer (POINT) system, working as the first mirrors to reflect the incident far-infrared laser. The surface morphology changing of the retroreflector has a direct impact on the optical reflection accuracy and it will generate a significant influence on EAST plasma density measurement and plasma current distribution diagnosis. Therefore, it is essential to research the surface morphology changing of the retroreflector systematically. Laser speckle interferometry technique is considered as a most promising method for real-time monitoring the PFMs surface morphology changing at plasma discharge intervals in the near future and its measurement accuracy and reliability have been adequately verified in the laboratory. One retroreflector served in 2019–2020 EAST Campaign is selected as a representative, compared with a brand-new retroreflector, to demonstrate that microscale surface morphology changing occurred on the internal reflection planes after bearing a whole EAST campaign plasma interaction. Meanwhile, the surface morphology of each internal reflection plane appears severely different features. Further research and analysis of the surface morphology are more contributed to evaluating the damage states of the retroreflector and providing more suggestions for the improvement and protection of the retroreflector. The surface morphology reconstruction results are given through a 3D visual exhibition and laser speckle interferometry technique is proposing a new research perspective for PWI research.

Upgrade of far infrared three-wave polarimeter-interferometer system on Joint-TEXT.

The far infrared polarimeter-interferometer system (POLARIS) on the Joint-TEXT has been upgraded recently. A new support structure has been applied to the mixer array of the POLARIS for adjusting the position of the mixers precisely. Five detection chords have been added to the system on the low field side by optimizing the utilization efficiency of the laser beams on both sides of the beam splitting grid. The spatial resolution can be improved from 3 to 1.5cm. A better measurement on plasma electron density and current density has been obtained after the system is upgraded. The initial experimental result will be introduced along with the upgrade details.

Correlation polarimeter-interferometer in the DIII-D tokamak

A correlation polarimeter-interferometer system has been developed for the DIII-D tokamak to detect small amplitude magnetic and density fluctuations. Two collinear, frequency-offset (5-15MHz), orthogonally polarized (right- and left-handed, circularly polarized) electromagnetic waves at 650GHz probing the plasma are used to detect the Faraday effect. A third, linearly polarized, electromagnetic wave serves as the local-oscillator to provide a measure of the line-averaged density. Correlation polarimetric measurement is accomplished by correlating the Faraday effect measured along the same line of sight using two independent detectors, whereas simultaneous correlation interferometric measurement is realized by correlating the line-averaged density measured at two different intermediate frequencies. The noise floor of the correlation polarimeter-interferometer has been demonstrated to be more than one order of magnitude lower than that of a standard polarimeter-interferometer measurement. Line-averaged correlation polarimetric-interferometric measurements in DIII-D H-mode plasmas show broadband (up to 750 kHz) magnetic and density fluctuations with amplitudes as low as 0.03Gauss/kHz and 4×1013m-3/kHz, respectively.

Optimization of the POlarimeter-INTerferometer diagnostics system on EAST

A double-pass, radial-view, 11-chord POlarimeter-INTerferometer (POINT) system has been routinely operated on the Experimental Advanced Superconducting Tokamak (EAST) to provide electron density and current density information for physics research. Stray light occurs due to spurious reflections from optical elements, including lenses, waveplates, windows, and even detectors, along the optical path and can seriously deteriorate the accuracy of the Faraday rotation measurements. As a result of stray light contamination, periodic oscillations can be observed in the Faraday rotation data during the density ramp-up phase. Aiming to realize real-time current profile feedback control using Faraday effect measurements, the contamination from stray light must be removed. Some optimization methods have been explored to decrease the error induced by stray light. The results show that the Faraday angle oscillation during the density ramp-up phase is reduced from 3̂–5̂ to 0.5̂–1̂ (double-pass). Measurement of the noncollinearity error is also performed for the POINT system on EAST. This optimization allows for more accurate current profile measurements for physics research.

Development and preliminary experimental results of a high-temporal-resolution terahertz solid-source interferometer on EAST

A vertical viewing 0.65 THz Solid-state Source Interferometer (SSI) has been installed for line-integrated density measurements on Experimental Advanced Superconducting Tokamak (EAST) . The SSI utilizes two independent solid-state diode sources with 2 mW output power and a base frequency multiplier, which provides a wide range of Intermediate Frequencies (IF) . The high time resolution of 0.1 μs at a 10 MHz IF has been successfully achieved. The IF signal remains almost constant for several hours without any active frequency stabilization. A sensitivity in the integrated density of 0.77× 1016 m−2 has been obtained. The density measurement has been implemented with IF ∼ 0.85 MHz, in a plasma with Ip=600 kA and PLHW=2.1 MW on EAST . The line-averaged density, 3.5× 1019 m−3, measured by the SSI shows good agreement with the results measured by the HCN interferometer system and the POlarimeter-INTerferometer (POINT) system in the same discharge.

Laser-based Faraday effect polarimetry for real-time feedback control of plasma parameters on EAST

A primary goal for ITER and prospective fusion power reactors is to achieve long-pulse, steady-state burning plasmas. Real-time control of plasma parameters such as current profile, density profile, position is necessary to extend high-performance plasma regimes to long pulse and is required for development of ITER-like discharge scenarios. A double-pass, radially-view, 11 chords, POlarimeter-INTerferometer (POINT) system has been developed and routinely operated on the EAST Tokamak, and provides important plasma current profile, plasma density profile and the vertical position information for plasma control and physics understanding. Digital Phase Detection, which provides real-time 4 μs Faraday and density output for plasma control, has been developed, thereby allowing real time current and q profile monitoring using fast equilibrium reconstruction. Besides, the Faraday effect can measure the plasma vertical position, which provides a good tool for plasma position control.

Error correction associated with stray light for Faraday-effect polarimetry system on EAST

The polarimeter-interferometer system with 11 double-pass radial-view measurement chords has the ability to provide electron density and plasma current profiles, making it exceptionally useful in daily operation on the Experiment Advanced Superconducting Tokamak. However, due to limited optical access and intrinsic feedback, the stray lights arising from spurious reflections along the optical path (unwanted reflections from various optical components/mounts and transmissive optical elements such as windows, waveplates, and lenses as well as the detectors) distort the Faraday rotation measurements. Furthermore, the feedback light from the retro-reflector which is used to realize the double-pass configuration makes it even worse. A data processing approach to decrease the stray light influence is reported in this paper. Based on the theoretical model developed, the Faraday rotation angle is extracted by subtracting the deviation term which can be calculated with a simplified model. With this approach, the Faraday rotation oscillation during density ramp-up can be reduced from 2°-5° to 0.5°-1.5°, which reduces the Faraday rotation measurement errors significantly.

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.

Optical technologies towards improving the Far-infrared laser Polarimeter-Interferometer system on HL-2A tokamak

Since the first performance of the HCOOH laser Polarimeter-Interferometer (PIer) in 2015 HL-2A experimental campaign, several optical technologies have been applied to improve the PIer system, including: (1) Optical alignment of the HCOOH laser at the long distance was implemented by the optical setup integrated by a lens and a high-sensitivity Pyroelectric Array Camera. (2) Due to application of the orthogonal waves transmission in the PIer system, some spurious intermediate frequency (IF) might be generated in the interferometry beat signal. For this problem, an effective optical adjustment method based on the single-polarization response characteristic of the Schottky diode detector was summarized in practice. (3) For double-passing type Faraday-effect polarimeter, analysis indicated that the severe oscillation disturbance in Faraday rotation angle was mainly caused by the stray lights or the feedback lights. Experimental results on HL-2A successfully validated that the unwanted feedback lights mainly came from the detector surface and was able to be effectively suppressed by the optic-isolator which was composed of a polarizer and a quarter-wave plate.

Non-inductive vertical position measurements by Faraday-effect polarimetry on EAST tokamak.

Vertical instability control in an elongated plasma is highly desirable for a tokamak reactor. A multi-channel 694 GHz far-infrared laser-based polarimeter-interferometer system has been used to provide a non-inductive vertical position measurement in the long-pulse EAST tokamak. A detailed comparison of vertical position measurements by polarimetry and external inductive flux loops has been used to validate Faraday-effect polarimetry as an accurate high-time response vertical position sensor.

Effects of stray lights on Faraday rotation measurement for polarimeter-interferometer system on EAST.

A double-pass radially view 11 chords polarimeter-interferometer system has been operated on the experimental advanced superconducting tokamak and provides important current profile information for plasma control. Stray light originating from spurious reflections along the optical path (unwanted reflections from various optical components/mounts and transmissive optical elements such as windows, waveplates, and lens as well as the detectors) and also direct feedback from the retro-reflector used to realize the double-pass configuration can both contribute to contamination of the Faraday rotation measurement accuracy. Modulation of the Faraday rotation signal due to the interference from multiple reflections is observable when the interferometer phase (plasma density) varies with time. Direct reflection from the detector itself can be suppressed by employing an optical isolator consisting of a λ/4-waveplate and polarizer positioned in front of the mixer. A Faraday angle oscillation during the density ramping up (or down) can be reduced from 5°-10° to 1°-2° by eliminating reflections from the detector. Residual modulation arising from misalignment and stray light from other sources must be minimized to achieve accurate measurements of Faraday rotation.

EAST kinetic equilibrium reconstruction combining with Polarimeter-Interferometer internal measurement constraints

Plasma equilibrium reconstruction plays an important role in the tokamak plasma research. With a high temporal and spatial resolution, the POlarimeter-INTerferometer (POINT) system on EAST has provided effective measurements for 102s H-mode operation. Based on internal Faraday rotation measurements provided by the POINT system, the equilibrium reconstruction with a more accurate core current profile constraint has been demonstrated successfully on EAST. Combining other experimental diagnostics and external magnetic fields measurement, the kinetic equilibrium has also been reconstructed on EAST. Take the pressure and edge current information from kinetic EFIT into the equilibrium reconstruction with Faraday rotation constraint, the new equilibrium reconstruction not only provides a more accurate internal current profile but also contains edge current and pressure information. One time slice result using new kinetic equilibrium reconstruction with POINT data constraints is demonstrated in this paper and the result shows there is a reversed shear of q profile and the pressure profile is also contained. The new improved equilibrium reconstruction is greatly helpful to the future theoretical analysis.

Note: Stability control of intermediate frequencies of a three laser far-infrared polarimeter-interferometer system.

Stability of the intermediate frequency (IF) in the far-infrared polarimeter-interferometer diagnostic system is critically important for the long pulse discharge experiments on the EAST tokamak. In this note, a real-time remote/local IF stability control system is described. The measured plasma parameters, including the Faraday rotation angle, electron density, lower hybrid wave, and plasma current, are obtained with the aid of this newly developed IF stability control system.