The first results of the hydrogen cyanide (HCN) interferometer measuring experimental research apparatus for electromagnetic science (ERAES) for hypersonic vehicle plasma in near space.

The formation of a plasma sheath on the surface of spacecraft or satellites during high-speed atmospheric entry is a significant factor that affects communication and radar detection. Experimental research apparatus for electromagnetic science can simulate this plasma sheath and study the interaction mechanisms between electromagnetic waves and plasma sheaths. Electron density is a crucial parameter for this research. Therefore, in this paper, a HCN heterodyne interferometer has been designed to measure the electron densities of the device, which range from 1 × 109 to 3 × 1013 cm-3 and the pressure ranges from 50 to 1500 Pa. The light source is a HCN laser with a wavelength of 337 µm, which exhibits higher spatial resolution compared to microwave interferometers. The interferometer is configured as a Mach-Zehnder interferometer, which generates an intermediate frequency through the Doppler shift achieved by a rotating grating. The spatial and temporal resolution of the HCN interferometry reach ∼14 mm and 100 µs, respectively. Antenna-coupled ALGaN/GaN-HEMT detectors have been utilized, as they possess higher sensitivity-with a typical reduction factor responsivity of around 900 V/W-than VDI planar-diode Integrated Conical Horn Fundamental Mixers in HCN interferometry. Recently, the initial results of the HCN interferometer designed for ERAES have been obtained during an experimental campaign, demonstrating a phase resolution of up to 0.04π.

Design of a real-time frequency stabilization system for dual laser-based interferometer on EAST.

This study addresses the challenge of intermediate frequency (IF) instability in the far-infrared polarimeter/interferometer (POINT) of the Experimental Advanced Superconducting Tokamak (EAST) to ensure the accuracy and stability of electron density measurements. Sudden and extensive IF shifts of the lasers can cause instability and even measurement errors of the diagnostic system. This paper introduces a comprehensive solution for stabilizing IF fluctuations. First, analog IF is converted into digital form using an analog-to-digital converter, and the digitalized signal is processed by a digital signal system based on a ZYNQ processor. The exact value of the IF is obtained by acquiring the point of maximum amplitude through the fast Fourier transform method, while the ZYNQ processor loaded with a fuzzy control algorithm will precisely adjust the laser cavity length via piezoelectric ceramics, achieving frequency stabilization within a target range. Comparative analyses confirm the method's efficacy in managing sudden frequency shifts, maintaining stability within 850 ± 100 kHz (a central frequency of 850 kHz, fluctuating within a range of ±100 kHz), with a control speed of 0.5 s per action and robust against variations up to 270 kHz. This efficient and rapid control mechanism fulfills the critical need for IF stabilization, ensuring the stability and precision of the POINT system in the EAST discharge experiments.

Magnetic topology changes induced by lower hybrid waves and their profound effect on edge-localized modes in the EAST tokamak.

Strong mitigation of edge-localized modes has been observed on Experimental Advanced Superconducting Tokamak, when lower hybrid waves (LHWs) are applied to H-mode plasmas with ion cyclotron resonant heating. This has been demonstrated to be due to the formation of helical current filaments flowing along field lines in the scrape-off layer induced by LHW. This leads to the splitting of the outer divertor strike points during LHWs similar to previous observations with resonant magnetic perturbations. The change in the magnetic topology has been qualitatively modeled by considering helical current filaments in a field-line-tracing code.

Non-inductive plasma vertical position measurement for the 1056 s discharge on EAST.

Vertical position stability plays a crucial role in maintaining safe and reliable plasma operation for long-pulse fusion devices. In general, the vertical position is measured by using inductive magnetic coils installed inside the vacuum vessel; however, the integration drift effects are inherent for steady-state or long-pulse plasma operation. Developing a non-magnetic approach provides a fusion reactor-relevant steady-state solution that avoids the negative impact of integration drift. In this paper, we compare the non-inductively determined vertical position achieved by line-integrated interferometer and polarimeter measurements to that employing an inductive flux loop for a 1056 s discharge recently achieved on EAST (Experimental Advanced Superconducting Tokamak). Experimental results show that the non-inductive measurement is more robust than flux loops after 300 s if the integrator is not reset to suppress integrator drift. Real-time vertical position control using the non-inductive system is proposed for the next EAST experimental campaign.

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.

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.

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.

Optical configuration optimization and calibration for the POINT system on EAST.

Calibration of the polarimeter system is one of the key elements to determine the overall measurement accuracy. The anisotropic reflection and transmission properties of the mesh beam splitters can easily distort the polarization state of the circularly polarized beams. Using a rotating crystal quartz λ/2-waveplate to replace the plasma can effectively allow us to obtain the ratio of the measured Faraday rotation angle to the known rotation angle of the waveplate. This ratio is used to estimate the calibration factor for each chord to be accurately determined and help to minimize distortions introduced by the wire-mesh beam splitters. With the novel configuration optimization, the distortion of polarization state is effectively eliminated.

Initial measurements of plasma current and electron density profiles using a polarimeter/interferometer (POINT) for long pulse operation in EAST (invited).

A double-pass, radially viewing, far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique has been implemented for diagnosing the plasma current and electron density profiles in the Experimental Advanced Superconducting Tokamak (EAST). POINT has been operated routinely during the most recent experimental campaign and provides continuous 11 chord line-integrated Faraday effect and density measurement throughout the entire plasma discharge for all heating schemes and all plasma conditions (including ITER relevant scenario development). Reliability of both the polarimetric and interferometric measurements is demonstrated in 25 s plasmas with H-mode and 102 s long-pulse discharges. Current density, safety factor (q), and electron density profiles are reconstructed using equilibrium fitting code (EFIT) with POINT constraints for the plasma core.

Design of vibration compensation interferometer for Experimental Advanced Superconducting Tokamak.

A vibration compensation interferometer (wavelength at 0.532 µm) has been designed and tested for Experimental Advanced Superconducting Tokamak (EAST). It is designed as a sub-system for EAST far-infrared (wavelength at 432.5 µm) poloarimeter/interferometer system. Two Acoustic Optical Modulators have been applied to produce the 1 MHz intermediate frequency. The path length drift of the system is lower than 2 wavelengths within 10 min test, showing the system stability. The system sensitivity has been tested by applying a periodic vibration source on one mirror in the system. The vibration is measured and the result matches the source period. The system is expected to be installed on EAST by the end of 2014.

Faraday-effect polarimeter-interferometer system for current density measurement on EAST.

A multichannel far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique is under development for current density and electron density profile measurements in the EAST tokamak. Novel molybdenum retro-reflectors are mounted in the inside wall for the double-pass optical arrangement. A Digital Phase Detector with 250 kHz bandwidth, which will provide real-time Faraday rotation angle and density phase shift output, have been developed for use on the POINT system. Initial calibration indicates the electron line-integrated density resolution is less than 5 × 10(16) m(-2) (∼2°), and the Faraday rotation angle rms phase noise is <0.1°.

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.