Polarimeter–interferometer diagnostic using terahertz solid-state sources for fluctuation measurements on Keda Torus e Xperiment(KTX)

A multi-channel polarimeter-interferometer has been developed on the Keda Torus eXperiment(KTX)for the study of equilibrium dynamics and internal magnetic fluctuations.A three-wave technique based on terahertz solid-state sources(-650 GHz)is applied for simultaneous measurements of electron density and Faraday rotation angle.The output power of the microwave source is 2 mW.Faraday rotation effect using a rotating wave plate is tested with phase noise less than 0.8°,and the density phase noise is less than 0.9°.Measurement of Faraday rotation angle and density for discharges on KTX have demonstrated high sensitivity to internal MHD activities.

High-precision X-ray polarimeter based on channel-cut crystals

We report on using synthetic silicon for a high-precision X-ray polarimeter comprising a polarizer and an analyzer,each based on a monolithic channel-cut crystal used at multiple Brewster reflections with a Bragg angle very close to 45°.Experiments were performed at the BL09B bending magnet beamline of the Shanghai Synchrotron Radiation Facility using a Si(800)crystal at an X-ray energy of 12.914 keV.A polarization purity of 8.4×10^(-9)was measured.This result is encouraging,as the measured polarization purity is the best-reported value for the bending magnet source.Notably,this is the firstly systematic study on the hard X-ray polarimeter in China,which is crucial for exploring new physics,such as verifying vacuum birefringence.

Probing the peripheral self-generated magnetic field distribution in laser-plasma magnetic reconnection with Martin-Puplett interferometer polarimeter

Magnetic reconnection of the self-generated magnetic fields in laser-plasma interaction is an important laboratory method for modeling high-energy density astronomical and astrophysical phenomena.We use the Martin-Puplett interferometer(MPI)polarimeter to probe the peripheral magnetic fields generated in the common magnetic reconnection configuration,two separated coplanar plane targets,in laser-target interaction.We introduce a new method that can obtain polarization information from the interference pattern instead of the sinusoidal function fitting of the intensity.A bidirectional magnetic field is observed from the side view,which is consistent with the magneto-hydro-dynamical(MHD)simulation results of self-generated magnetic field reconnection.We find that the cancellation of reverse magnetic fields after averaging and integration along the observing direction could reduce the magnetic field strength by one to two orders of magnitude.It indicates that imaging resolution can significantly affect the accuracy of measured magnetic field strength.

Forward modelling of the Cotton-Mouton effect polarimetry on EAST tokamak

Measurement of plasma electron density by far-infrared laser polarimetry has become a routine and indispensable tool in magnetic confinement fusion research.This article presents the design of a Cotton-Mouton polarimeter interferometer,which provides a reliable density measurement without fringe jumps.Cotton-Mouton effect on Experimental Advanced Superconducting Tokamak(EAST)is studied by Stokes equation with three parameters(s_(1),s_(2),s_(3)).It demonstrates that under the condition of a small Cotton-Mouton effect,parameter s_(2)contains information about Cotton-Mouton effect which is proportional to the line-integrated density.For a typical EAST plasma,the magnitude of Cotton-Mouton effects is less than 2πfor laser wavelength of 432μm.Refractive effect due to density gradient is calculated to be negligible.Time modulation of Stokes parameters(s_(2),s_(3))provides heterodyne measurement.Due to the instabilities arising from laser oscillation and beam refraction in plasmas,it is necessary for the system to be insensitive to variations in the amplitude of the detection signal.Furthermore,it is shown that non-equal amplitude of X-mode and O-mode within a certain range only affects the DC offset of Stokes parameters(s_(2),s_(3))but does not greatly influence the phase measurements of Cotton-Mouton effects.

Error analysis and Stokes parameter measurement of rotating quarter-wave plate polarimeter

In this paper, we present a simple Stokes parameter measurement method for a rotating quarter-wave plate polarimeter. This method is used to construct a model to describe the principle of how the magnitudes of errors influence the deviation of the output light Stokes parameter, on the basis of accuracy analysis of the retardance error of the quarter-wave plate, the misalignment of the analyzing polarizer, and the phase shift of the measured signals, which will help us to determine the magnitudes of these errors and then to acquire the correct results of Stokes parameters. The method is validated by the experiments on left-handed circularly polarized and linear horizontal polarization beams. With the improved method, the maximum measurement deviations of Stokes parameters for these two different polarized states are reduced from 2.72% to 2.68%, and from 3.83% to 1.06% respectively. Our results demonstrate that the proposed method can be used as a promising approach to Stokes parameter measurement for a rotating quarter-wave plate polarimeter.

Design and calibration of a high-sensitivity and high-accuracy polarimeter based on liquid crystal variable retarders

Polarimetry plays an important role in the measurement of solar magnetic fields. We devel- oped a high-sensitivity and high-accuracy polarimeter （HHP） based on nematic liquid crystal variable retarders （LCVRs）, which has a compact setup and no mechanical moving parts. The system design and calibration methods are discussed in detail. The azimuth error of the transmission axis of the polarizer as well as the fast axes of the two LCVRs and the quarter-wave plate were determined using dedicated procedures. Linearly and circularly polarized light were employed to evaluate the performance of the HHP. The experimental results indicate that a polarimetric sensitivity of better than 5.7 × 10-3 can be achieved by using a single short-exposure image, while an accuracy on the order of 10-5 can be reached by using a large number of short-exposure images. This makes the HHP a high-performance system that can be used with a ground-based solar telescope for high-precision solar magnetic field investigations.

Research on the principle of space high-precision temperature control system of liquid crystals based Stokes polarimeter

The magnetic field is one of the most important parameters in solar physics,and a polarimeter is the key device to measure the solar magnetic field.Liquid crystals based Stokes polarimeter is a novel technology,and will be applied for magnetic field measurement in the first space-based solar observatory satellite developed by China,Advanced Space-based Solar Observatory.However,the liquid crystals based Stokes polarimeter in space is not a mature technology.Therefore,it is of great scientific significance to study the control method and characteristics of the device.The retardation produced by a liquid crystal variable retarder is sensitive to the temperature,and the retardation changes 0.09°per 0.10℃.The error in polarization measurement caused by this change is 0.016,which affects the accuracy of magnetic field measurement.In order to ensure the stability of its performance,this paper proposes a high-precision temperature control system for liquid crystals based Stokes polarimeter in space.In order to optimize the structure design and temperature control system,the temperature field of liquid crystals based Stokes polarimeter is analyzed by the finite element method,and the influence of light on the temperature field of the liquid crystal variable retarder is analyzed theoretically.By analyzing the principle of highprecision temperature measurement in space,a high-precision temperature measurement circuit based on integrated operational amplifier,programmable amplifier and 12 bit A/D is designed,and a high-precision space temperature control system is developed by applying the integral separation PI temperature control algorithm and PWM driving heating films.The experimental results show that the effect of temperature control is accurate and stable,whenever the liquid crystals based Stokes polarimeter is either in the air or vacuum.The temperature stability is within±0.0150℃,which demonstrates greatly improved stability for the liquid crystals based Stokes polarimeter.

MOSSBAUER POLARIMETER AND MOSSBAUER FARADAY EFFECT

A Mossbauer polarimeter consists of a gamma ray source (polarizer), transmitter (sample to be analyzed), analyzer and automatic counting system. This equipment was used to observe the Mossbauer Faraday effect in non-stoichiometric Fe3O4. Experimental results demonstrated that electronic hopping above the Verwey temperature between Fe2+- Fe3+ ions on the octahedral sites is a only localized phenomenon and the recoilless fractions of 57Fe nuclei in Fe3-vO4 (v=0.02) are 0.71 for A sites and 0.62 for B sites, respectively.

Passive magnetic shielded spin polarized electron source with optical electron polarimeter

A new GaAs（100） spin polarized electron source with an optical polarimeter, which is employed in the field of polarized electron and gas atom collision, is presented in detail. The apparatus is passive-magnetic-shielded by a box and a cylinder made of nickel-iron-molybdenum soft magnetic alloy without Helmholtz coil arrangement. And a uniformly distributed residual magnetic field of less than 5 × 10^-7T is obtained near the collision area. The spin polarized electron beam is transmitted and focused onto collision point from photocathode by a set of electron optics with more than 25% transmission 95 cm distance through an 1 mm diameter aperture. Construction and operation of the apparatus, such as vacuum and magnetic shielding system, photocathode, laser optics, electron optics and polarimeter are discussed. The polarization of the spin polarized electron beam is determined to be 30.8 ±3.5% measured with a He optical polarimeter.

Recent Progress of the HL-2A Multi-Channel HCOOH Laser Interferometer/Polarimeter

A multichannel methanoic acid （HCOOH, λ=432.5 μm） laser interferometer/polarimeter is being developed from the previous eight-channel hydrogen cyanide （HCN, λ=337 μm） laser interferometer in the HL-2A tokamak. A conventional Michelson-type interometer is used for the electron density measurement, and a Dodel-Kunz-type polarimeter is used for the Faraday rotation effect measurement, respectively. Each HCOOH laser can produce a linearly polarized radiation at a power lever of -30 mW, and a power stability 〈10% in 50 rain. A beam waist （diameter d0 ≈12.0 mm, about 200 mm away from the outlet） is finally determined through a chopping modulation technique. The latest optical layout of the interferometer/polarimeter has been finished, and the hardware data processing system based on the fast Fourier transform phase- comparator technique is being explored. In order to demonstrate the feasibility of the diagnostic scheme, two associated bench simulation experiments were carried out in the laboratory, in which the plasma was simulated by a piece of polytetrafluoroethene plate, and the Faraday rotation effect was simulated by a rotating half-wave plate. Simulation results agreed well with the initial experimental conditions. At present, the HCOOH laser interferometer/polarimeter system is being assembled on HL-2A, and is planned to be applied in the 2014-2015 experimental campaign.

A high-contrast imaging polarimeter with a stepped-transmission filter based coronagraph

The light reflected from planets is polarized mainly due to Rayleigh scattering, but starlight is normally unpolarized. Thus it provides an approach to enhance the imaging contrast by inducing the imaging polarimetry technique. In this paper, we propose a high-contrast imaging polarimeter that is op- timized for the direct imaging of exoplanets, combined with our recently developed stepped-transmission filter based coronagraph. Here we present the design and calibration method of the polarimetry system and the associated test of its high-contrast performance. In this polarimetry system, two liquid crystal variable retarders （LCVRs） act as a polarization modulator, which can extract the polarized signal. We show that our polarimeter can achieve a measurement accuracy of about 0.2% at a visible wavelength （632.8 nm） with linearly polarized light. Finally, the whole system demonstrates that a contrast of 10 9 at 5A/D is achievable, which can be used for direct imaging of Jupiter-like planets with a space telescope.

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

The J-TEXT three-wave polarimeter-interferometer system(POLARIS),which measures timespace 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.

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.

YFPOL: A Linear Polarimeter of Lijiang 2.4 m Telescope

Polarimetry plays an important role in investigating physical properties for celestial objects. We present a polarimeter named YFPOL for the Cassegrain focus of the Lijiang 2.4 m Telescope(LJT) of Yunnan Observatories, Chinese Academy of Sciences. YFPOL is a traditional single-beam polarimeter with a rotating polarizer. As the focal-reducer instrument Yunnan Faint Object Spectrograph and Camera(YFOSC) is always positioned on the Cassegrain focal plane of LJT, we develop two sets of ultra-thin(thickness <12 mm) polarizer rotation control systems with wireless charging and control functions, which are suitable for mounting on the two front-wheels of YFOSC. One set is used as the polarimetric calibration unit, and the other is for the polarimetric modulation unit. Both of the polarizers have an ultra-high contrast ratio of 1,000,000:1 in the optical band. We investigate the instrumental polarization characteristics(IPCs) in the full field of view that is transferred from YFOSC. Furthermore, we identify that the IPCs change when the Cassegrain axis rotates. The spurious polarization from the IPCs can be effectively minimized by flat-fielding using the unpolarized domeflat, when the Cassegrain rotation angle is the same or nearest to that of the polarization observation. We develop a quasiautomatic pipeline for YFPOL and its effectiveness has been verified by tests of the polarimetric observation with blazar S5 0716+714. The calibration is performed by observing the zero-polarized and highly-polarized standard stars. We successfully reach high precision polarization in the 7’ field of view, and the systematic uncertainty is below 0.8% for a V = 11.68 target with a 10 s exposure. The instrument polarization angle offset is 2°. 6. YFPOL is not only a simple polarimeter, but also a spectropolarimeter with grisms that can be considered in the future.

Radiation shielding design of the CFETR polarimeter interferometer and CO_(2) dispersion interferometer

A three-wave based laser polarimeter/interferometer and a CO_(2)laser dispersion interferometer are used to determine the electron and current density profiles on a Chinese fusion engineering test reactor(CFETR).Radiation shielding is designed for the combination of polarimeter/interferometer and CO_(2)dispersion interferometer.Furthermore,neutronics models of the two systems are developed based on the engineering-integrated design of CFETR polarimeter/interferometer and CO_(2)dispersion interferometer and the major material components of CFETR.The polarimeter/interferometer and CO_(2)dispersion interferometer's neutron and photon transport simulations were performed using the Monte Carlo neutral transport code to determine the energy deposition and neutron energy spectrum of the optical mirrors.The energy depositions of the first mirrors on the polarimeter/interferometer are reduced by three orders with the whole shielding.Since the mirrors of CO_(2)dispersion interferometer are very close to the diagnostic first wall,shielding space is limited and the CO_(2)dispersion interferometer energy deposition is higher than that of the polarimeter/interferometer.The dose rate after shutdown106s in the back-drawer structure has been estimated to be 83μSv h^(-1)when the radiation shield is filled in the diagnostic shielding modules,which is below the design threshold of 100μSv h^(-1).Radiation shielding design plays a key role in successfully applying polarimeter/interferometer and CO_(2)dispersive interferometer in CFETR.

Discrimination of background events in the PolarLight X-ray polarimeter

PolarLight is a space-borne X-ray polarimeter that measures the X-ray polarization via electron tracking in an ionization chamber.It is a collimated instrument and thus suffers from the background on the whole detector plane.The majority of background events are induced by high energy charged particles and show ionization morphologies distinct from those produced by X-rays of interest.Comparing on-source and off-source observations,we find that the two datasets display different distributions on image properties.The boundaries between the source and background distributions are obtained and can be used for background discrimination.Such a means can remove over 70%of the background events measured with PolarLight.This approaches the theoretical upper limit of the background fraction that is removable and justifies its effectiveness.For observations with the Crab nebula,the background contamination decreases from 25%to 8%after discrimination,indicative of a polarimetric sensitivity of around 0.2 Crab for PolarLight.This work also provides insights into future X-ray polarimetric telescopes.

A differential-imaging polarimeterforhigh-contrast exoplanet imaging

Starlight is generally unpolarized,but the light reflected from the planet is linearly polarized as the result of the Rayleigh scattering.For ground-based exoplanet imaging,atmospheres turbulence is changing from time to time,which induces speckle noise and hampers the high-contrast imaging of the faint exoplanets.In this paper,we propose a differential-imaging polarimeter dedicated for exoplanet high-contrast imaging.The system contains a zero-order half-wave plate(HWP)located on the optical pupil plane,which can rotate to modulate the incoming light,and a Wollaston prism(WP)is used to generate two polarized images,which is used for simultaneously polarization differential imaging and thus our system is fundamentally immune to the atmospheric turbulence induced temporally-variable wavefront aberration.Our polarimeter can be inserted near the telescope image focal plane,and provide an extra contrast for the exoplanet high-contrast imaging.To achieve best differential-imaging performance,dedicated algorithm is developed,which can effectively correct the distortion and the intensity unbalance between the two differential images.The system successfully achieves an extra contrast of^30~50 times,which can be used with current adaptive optics and coronagraph system for directly imaging of giant Jupiter-like exoplanets.

Triple Range Imager and POLarimeter(TRIPOL)——a compact and economical optical imaging polarimeter for small telescopes

We report the design concept and performance of a compact, lightweight and economical imaging polarimeter, the Triple Range Imager and POLarimeter(TRIPOL), capable of simultaneous optical imagery and polarimetry. TRIPOL splits the beam in wavelengths from 400 to 830 nm into g′-, r′-and i′-bands with two dichroic mirrors, and measures polarization with an achromatic half-waveplate and a wire grid polarizer. The simultaneity makes TRIPOL a useful tool for small telescopes for the photometry and polarimetry of time variable and wavelength dependent phenomena. TRIPOL is designed for a Cassegrain telescope with an aperture of^1 m. This paper presents the engineering considerations of TRIPOL and compares the expected with observed performance. Using the Lulin 1-m telescope and 100 seconds of integration, the limiting magnitudes are g′~19.0 mag, r′~18.5 mag and i′~18.0 mag with a signal-to-noise ratio of 10, in agreement with design expectation. The instrumental polarization is measured to be^0.3% in the three bands. Two applications, one to the star-forming cloud IC 5146 and the other to the young variable GM Cep, are presented as demonstrations.

Fast Data Processing of a Polarimeter-Interferometer System on J-TEXT

A method of fast data processing has been developed to rapidly obtain evolution of the electron density profile for a multichannel polarimeter-interferometer system（POLARIS）on J-TEXT. Compared with the Abel inversion method, evolution of the density profile analyzed by this method can quickly offer important information. This method has the advantage of fast calculation speed with the order of ten milliseconds per normal shot and it is capable of processing up to 1 MHz sampled data, which is helpful for studying density sawtooth instability and the disruption between shots. In the duration of a flat-top plasma current of usual ohmic discharges on J-TEXT, shape factor u is ranged from 4 to 5. When the disruption of discharge happens, the density profile becomes peaked and the shape factor u typically decreases to 1.

线性剪切空间调制快拍成像偏振仪动态定标数据处理技术

基于动态定标的线性剪切空间调制快拍成像测偏技术能够在动态环境(如温度变化)下对目标偏振信息进行更精确的实时测量,但实验噪声会引起反演图像的条状畸变。针对这一问题,系统分析了快拍成像偏振仪动态定标技术原理和目标相位的定量求解过程,提出了结合相位差分与双线性插值算法的噪声识别与相位修正方法。实验和计算结果表明,经过相位求解和修正计算,条带状失真得到去除,反演成像质量提升。为高相位变化和低信噪比环境下空间调制快拍成像测偏技术的实时应用提供支持,推动其在动态环境下的工程实际应用进程。