W.M. Li

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°.

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.

A physics package for rubidium atomic frequency standard with a short-term stability of 2.4 × 10−13 τ−1/2

In this article, a new type of physics package with high signal to noise ratio for a rubidium atomic frequency standard is reported. To enhance the clock transition signal, a slotted tube microwave cavity with a field orientation factor of 0.93 and an absorption cell with the diameter of 30 mm were utilized in design of the cavity-cell assembly. Based on the spectral analysis of the three commonly used rubidium spectral lamps, the spectral lamp filled with Xe gas was chosen as the optical pumping source for its small line shape distortion. To suppress the shot noise of the signal, a band pass interference filter was used to filter out Xe spectral lines from the pumping light. A desk system of the rubidium frequency standard with the physics package was realized, and the short-term stability of the system was predicted and tested. The measured result is 2.4 × 10-13 τ-1/2 up to 100 s averaging time, in good agreement with the predicted one.

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.

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.

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.

Demonstration of a Physics Package with High SNR for Rubidium Atomic Frequency Standards

The frequency stability of a rubidium atomic frequency standard (RAFS) depends mainly on the signal to noise ratio (SNR) of atomic discrimination signal provided by the physics package. In order to improve further the frequency stability of our RAFS, a new physics package with high SNR was designed recently. The physics package was designed based on an improved slotted tube cavity. Compared with our previous design, the new cavity has more uniform magnetic line distribution and larger size, so that a larger resonance cell can be used. In the design the separated filter technique (SFT) was used. A Helmholtz coil was substituted for a solenoid one to create a more uniform C field. At present a prototype of the physics package has been made. A preliminary test has been performed, and a short term frequency stability of 6 × 10−13/1 s was achieved. This result indicates that the SNR of the physics package could meet the requirement for building a RAFS with frequency stability better than 1 × 10−12/τ1/2.

Study of the Physics Package for High Performance Rubidium Frequency Standards

Frequency stability of an atomic frequency standard depends mainly on the Signal to Noise Ratio (SNR) of atomic discrimination signal. For a Rubidium Atomic Frequency Standard (RAFS), the SNR is closely related to the characteristics of the cavity-cell assembly in physics package. The RAFS in our laboratory is designed based on the slotted tube cavity, and a typical stability of 1.5 × 10−12 τ−1/2 (1–1000 s) has been achieved. The current cavity used in our design is of a disadvantage of field inhomogeneity. To improve further the stability of the RAFS, a modification of the cavity has been carried out. With this modification, the field homogeneity has been greatly improved, and the cavity size was increased also, enabling to hold a larger absorption cell. According to a theoretical evaluation, the SNR of physics package was enhanced by nearly two times, meeting the need to design a RAFS with a stability better than 1.0 × 10−12 τ−1/2.