POlarimeter-INTerferometer Research Articles

First time observation of local current shrinkage during the MARFE behavior on the J-TEXT tokamak

Multifaceted asymmetric radiation as well as strong poloidal asymmetry of the electron density from the edge, dubbed as ‘MARFE’, has been observed in high electron density Ohmically heated plasmas on J-TEXT tokamak. Equilibrium reconstruction based on the measured data from the 17-channel FIR polarimeter–interferometer indicates that an asymmetric plasma current density distribution forms at the edge region and the plasma current shrinkage locates at the MARFE affected region. Furthermore, associated with the localized plasma current shrinkage, a locked mode MHD activity is excited, which then terminate the discharge with a major disruption. Localized plasma current shrinkage at the MARFE region is considered to be the direct cause for the density limit disruptions, and the proposed interpretation is consistent with the experimental observations.

Development of real-time plasma current profile reconstruction with POINT diagnostic for EAST plasma control

This paper describes the plasma current profile reconstruction algorithms using POlarimeter-INTerferometer(POINT) measurements in GPU parallel equilibrium reconstruction code P-EFIT for EAST plasma control. Real-time POINT diagnostic data acquisition system is designed to satisfy the requirement for real-time reconstruction. The static time-slice and experimental simulation benchmark calculations using experimental magnetic plus POINT data of EAST discharge are conducted, which show that P-EFIT could provide reasonable plasma current and q profile in 0.7ms in real-time with 65×65 spatial grid resolution. These results show that P-EFIT with POINT diagnostic could provide equilibrium reconstruction results for real-time plasma current and q profile control which will be applied in the near future.

EAST equilibrium current profile reconstruction using polarimeter-interferometer internal measurement constraints

The first equilibrium reconstruction of EAST current-density profile based on internal Faraday rotation measurements provided by the POlarimeter-INTerferometer (POINT) diagnostic is demonstrated using the EFIT equilibrium reconstruction code. EFIT incorporates 11 simultaneous line-integrated density and Faraday effect measurements from POINT to self-consistently reconstruct the equilibrium toroidal current density profile using a Faraday rotation reconstruction algorithm. It is shown that the POINT measurements can be applied to improve the accuracy of core plasma current density and q profile on EAST. Comparisons of magnetic surfaces and the q profile reconstructed using external magnetic data against those using magnetic and POINT data are presented. Equilibrium reconstructions using POINT data are found to be consistent with sawtooth phenomena. The sensitivity of equilibrium reconstruction to POINT measurements indicates Faraday rotation provides important constraints for determining the current profile.

Equilibrium Reconstruction and Integration of EFIT with Diagnoses in J-TEXT Tokamak

The EFIT program is integrated with the high resolution laser polarimeter interferometer system (POLARIS), the soft X-ray imaging diagnostic system (SXR) and the electron cyclotron emission radiometer (ECE) in the J-TEXT tokamak. Then some internal information about Faraday angle and the position of safety factor q=1 can be obtained as a constraint to EFIT. The modified EFIT code is used to calculate the internal parameters such as flux function, safety factor q, pressure and current density.

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.

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.

Plasma response to m/n = 3/1 resonant magnetic perturbation at J-TEXT Tokamak

The influence of resonant magnetic perturbations (RMPs) with a large m/n = 3/1 component on electron density has been studied at J-TEXT tokamak by using externally applied static and rotating RMPs, where m and n are the poloidal and toroidal mode number, respectively. The detailed time evolution of electron density profile, measured by the polarimeter–interferometer, shows that the electron density ne first increases (decreases) inside (around/outside) of the 3/1 rational surface (RS), and it is increased globally later together with enhanced edge recycling. Associated with field penetration, the toroidal rotation around the 3/1 RS is accelerated in the co-Ip direction and the poloidal rotation is changed from the electron to ion diamagnetic drift direction. Spontaneous unlocking-penetration circles occur after field penetration if the RMPs amplitude is not strong enough. For sufficiently strong RMPs, the 2/1 locked mode is also triggered due to mode coupling, and the global density is increased. The field penetration threshold is found to be linearly proportional to neL (line-integrated density) at the 3/1 RS but to (neL)0.73 for ne at the plasma core. In addition, for rotating RMPs with a large 3/1 component, field penetration causes a global increase in electron density.

Design of geometric phase measurement in EAST Tokamak

The optimum scheme for geometric phase measurement in EAST Tokamak is proposed in this paper. The theoretical values of geometric phase for the probe beams of EAST Polarimeter-Interferometer (POINT) system are calculated by path integration in parameter space. Meanwhile, the influences of some controllable parameters on geometric phase are evaluated. The feasibility and challenge of distinguishing geometric effect in the POINT signal are also assessed in detail.

The implementation of real-time plasma electron density calculations on EAST

The plasma electron density is one of the most fundamental parameters in tokamak experiment. It is widely used in the plasma control system (PCS) real-time control, as well as plasma physics analysis. The 3-wave polarimeter-interferometer (POINT) system had been used to measure the plasma electron density on the EAST since last campaign. This paper will give the way to realize the real-time measurement of plasma electron density.All intermediate frequency (IF) signals after POINT system, in the 0.5–3MHz range, stream to the real-time density calculation system (DCS) to extract the phase shift information. All the prototype hardware is based on NI Flex RIO device which contains a high speed Field Programmable Gate Array (FPGA). The original signals are sampled at 10M Samples/s, and the data after roll-over module are transmitted to PCS by reflective memory (RFM). With this method, real-time plasma electron density data with high accuracy and low noise had been obtained in the latest EAST tokamak experiment.

Internal magnetic field measurements by laser-based POlarimeter-INTerferometer (POINT) system on EAST

A multi-channel far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique has been implemented for fully diagnosing the internal magnetic field in the EAST tokamak. Double-pass, horizontal, radially-viewing chords access the plasma via an equatorial port. The laser source consists of three CW formic acid (HCOOH) FIR lasers at nominal wavelength 432.5 μm which are optically pumped by independent infrared CO2 lasers. Output power is more than 30 mW of per cavity. Novel molybdenum retro-reflectors, can with withstand baking temperature up to 350°C and discharge duration more than 1000 s, are mounted in the inside wall for the double-pass optical arrangement. A Digital Phase Detector with 250 kHz bandwidth, which provide real-time Faraday rotation angle and density phase shift output for plasma control, have been developed for the POINT system. Reliability of both polarimetric and interferometric measurement are obtained in 22 s long pulse H mode discharge and 8 s NBI H mode discharge, indicating the POINT system works for any heating scheme on EAST so far. The electron line-integrated density resolution of POINT is less than 1 × 1016 m−2 (< 1°), and the Faraday rotation angle rms phase noise is < 0.1°. With the high temporal (∼ 1 μsec) and phase resolution (< 0.1°), perturbations associated with the sawtooth cycle and MHD activity have been observed. The current profile, density profile and safety factor (q) profile are reconstructed by using EFIT code from the external magnetic and the validation POINT data. Realtime EFIT with Faraday angle and density phase shift constraints will be implemented in the plasma control system in the future.

Study of retro reflector array for the polarimeter-interferometer system on EAST Tokamak

In this paper, we experimentally verify the feasibility of replacing individual retro reflectors (RRs) with retro reflector array (RRA) in EAST POlarimeter/INTerferometer (POINT) system, by considering mode transformation and power wastage. Being exposed to plasma environment directly, RRs have risks of deformation, erosion and deposition. RRA is preferable because it can be installed within a smaller space and provide a gap of several centimeters for the shutter design. This protective structure can reduce the cost of device maintenance and bring down system errors. According to Helmholtz-Kirchhoff integral theorem, the optimized incident diameter for the RRA, constituted by seven hexagonal RR cells, is 40 mm in POINT system. The corresponding bench tests are carried out by measuring the propagation properties of reflected beams by plane RRA for perpendicular incidence and reflected beams by terrace RRA for oblique incidence. The experimental results illustrate that RRA can be satisfactorily applied in POINT system at the optimized incident diameter. In view of the energy wastage caused by plasma film coating, it is found that RRA has more advantages for diagnostics using shorter wavelengths, such as the case in ITER.

Reconstruction of the Density Profile for the EAST Tokamak Based on Polarimeter/Interferometer and Microwave Reflectometer Systems**supported by the National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB106003, 2014GB106004 and 2014GB106002) and National Natural Science Foundation of China (Nos. 11475221 and 11105184)

A plasma density profile reconstruction procedure based on the Park matrix method has been developed for both circular and elongated plasma configuration on the Experimental Advanced Superconducting Tokamak (EAST). This method incorporates the line integrated electron density measured by the HCN interferometer and polarimeter/interferometer (POINT) system, the equilibrium fit (EFIT) based on magnetic measurements and the edge electron density profile provided by the microwave reflectometer. It is shown that when the magnetic flux surfaces are slightly corrected, the fitting error is less than 5% in comparison with the measurement data.

Algorithm Validation of the Current Profile Reconstruction of EAST Based on Polarimeter/Interferometer

The method of plasma current profile reconstruction using the polarimeter/interferometer (POINT) data from a simulated equilibrium is explored and validated. It is shown that the safety factor (q) profile can be generally reconstructed from the external magnetic and POINT data. The reconstructed q profile is found to reasonably agree with the initial equilibriums. Comparisons of reconstructed q and density profiles using the magnetic data and the POINT data with 3%, 5% and 10% random errors are investigated. The result shows that the POINT data could be used to a reasonably accurate determination of the q profile.

432-μm laser’s beam-waist measurement for the polarimeter/interferometer on the EAST tokamak

A far-infrared (FIR) polarimeter/interferometer (PI) system is under development for measurements of the current-density and the electron-density profiles in the EAST tokamak. The system will utilize three identical 432-μm CHCOOH lasers pumped by a CO2 laser. Measurements of the laser beam’s waist size and position are basic works. This paper will introduce three methods with a beam profiler and several focusing optical elements. The beam profiler can be used to show the spatial energy distribution of the laser beam. The active area of the profiler is 12.4 × 12.4 mm2. Some focusing optical elements are needed to focus the beam in order for the beam profiler to receive the entire laser beam. Two principles and three methods are used in the measurement. The first and the third methods are based on the same principle, and the second method adopts an other principle. Due to the fast and convenient measurement, although the first method is a special form of the third and it can only give the size of beam waist, it is essential to the development of the experiment and it can provide guidance for the choices of the sizes of the optical elements in the next step. A concave mirror, a high-density polyethylene (HDPE) lens and a polymethylpentene (TPX) lens are each used in the measurement process. The results of these methods are close enough for the design of PI system’s optical path.

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.

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°.

Study of the propagation properties of 432 [formula omitted]m laser reflected by retro reflector array

Some recent researches showed that retro reflector array (RRA) has installation superiorities that single retro reflector (RR) does not possess in the far-infrared (FIR) polarimeter/interferometer (PI) system for Tokamak. As a key precondition for replacing RR with RRA, the reflected light from RRA must change to Gaussian profile to guarantee the detectors get stronger signals and the threshold distance to Gaussian profile should be less than the optical path between the first wall and detectors. In this paper, the propagation properties of the reflected beams from RR and RRAs of different combinations have been measured by a beam profiler. The experimental results show that the reflected beam profiles from RRA of two retro reflectors (RRs) evolve into Gaussian distributions beyond a certain reflected optical distance and the reflected beam profiles from RRA of three RRs have a similar tendency to be Gaussian distributions with increasing reflected optical distance. The theory predictions, deriving from the integral theorem of Helmholtz and Kirchhoff, provide the same evolutions with experiments, but the threshold distances to Gaussian profile existing distinctions. These results suggest that RRA could be applied in the FIR PI system for the future ITER.

Design of FPGA based high-speed data acquisition and real-time data processing system on J-TEXT tokamak

Tokamak experiment requires high-speed data acquisition and processing systems. In traditional data acquisition system, the sampling rate, channel numbers and processing speed are limited by bus throughput and CPU speed. This paper presents a data acquisition and processing system based on FPGA. The data can be processed in real-time before it is passed to the CPU. It provides processing ability for more channels with higher sampling rates than the traditional data acquisition system while ensuring deterministic real-time performance. A working prototype is developed for the newly built polarimeter–interferometer diagnostic system on the Joint Texas Experimental Tokamak (J-TEXT). It provides 16 channels with 120MHz maximum sampling rate and 16bit resolution. The onboard FPGA is able to calculate the plasma electron density and Faraday rotation angel. A RAID 5 storage device is adopted providing 700MB/s read–write speed to buffer the data to the hard disk continuously for better performance.

Internal magnetic field structure and parallel electric field profile evolution during the sawtooth cycle in MST

Temporal dynamics of the magnetic field, parallel current density, parallel electric field and safety factor profiles in the core of a high-temperature reversed-field pinch plasma are experimentally resolved. Measurements are realized using a high-speed polarimeter–interferometer diagnostic which is employed to simultaneously determine the electron density, toroidal current density and poloidal magnetic field profiles. Combined with external magnetic measurements, these data allow determination of the equilibrium profile dynamics during individual sawtooth magnetic relaxation events. At the sawtooth crash, the E∥ profile has a large positive peak on-axis, and a negative peak of slightly smaller value near the reversal surface demonstrating the need for a dynamo to sustain the plasma equilibrium.

Improvement of interferometric measurements on far-infrared polarimeter interferometer systems

On many tokamaks the reconstruction of the magnetic field structure in the plasma is supported by polarimetric measurements. Recent proposed and realized methods are based on a far-infrared laser beam with a rotating polarization ellipse. The same instrument usually performs as an interferometer measuring the line integrated plasma density. It has been shown that the rotating polarization ellipse disturbs the interferometric measurements. A method based on the principle of a rotating polarization in which the interferometric measurement is unaffected is proposed. Bench test results are presented which show the feasibility of this method.