Motional Stark Effect Polarimeter Research Articles

Current Profile Reconstruction Using Motional Stark Effect Polarimeter Data on HL-2A Tokamak

In order to reconstruct the plasma current density, the Current Profile Fitting (CPF) code has been successfully developed on the HL-2A tokamak. A seven-channel motional Stark effect (MSE) diagnostic based on dual photoelastic modulators is installed to measure the pitch angle of the magnetic field, which can be used as an internal magnetic field constraint for the CPF code. Recently, the MSE polarimeter was upgraded with a real-time wavelength matching system to improve the signal-to-noise ratio. The magnetic field angle (γpitch) with a temporal resolution of 10 ms can be provided. In the CPF code, the plasma current density is described as a polynomial, and the Least-Squares method is used to determine the coefficients of the polynomial. The Finite Difference method and the Strongly Implicit Procedure method are used to solve the Grad-Shafranov equation. The code operation is stable. With the improved-quality MSE data, the CPF calculation result of shot 30782 suggests that the safety factor q profile is monotonic. The minimum q value is less than 1 on-axis during sawtooth oscillations in shot 30782. And, the position of the q = 1 surface is consistent with the sawtooth inversion radius measured by electron cyclotron emission and soft X-ray diagnostics.

Note: Real-time wavelength matching system designed for the motional Stark effect polarimeter on HL-2A tokamak.

A 7-channel motional Stark effect diagnostic based on dual photo-elastic modulators is installed and operated routinely for rather low beam energy and magnetic field on the HL-2A tokamak, with a spatial resolution of ∼3 cm and a temporal resolution of 10 ms. The instrument observes the σ component of the full energy Dα from the first or the fourth ion source of a neutral beam injector. However, the change in beam energy during a discharge causes variation of the Doppler shift with the maximum of 1 Å, which leads to the polarization fraction drop from 30%-40% to 10% and then makes the signal-to-noise ratio of the system become very poor. Therefore, a real-time wavelength matching system is designed to promote polarization fraction. The beam emission spectra are filtered by using a monochrometer in real time. And a narrowband filter is tilted by using an absolutely calibrated rotator through beam energy in order to make sure that the deviation of wavelength matching is less than 0.1 Å.

Asymmetries in the motional Stark effect emission on the DIII-D tokamak

Spectrometer measurements and filter upgrades to a motional Stark effect polarimeter measuring the outer half-radius of the DIII-D tokamak helped to identify asymmetries in the polarization angle of Stark-split emission. The measured polarization angle of the π components differs and is not orthogonal to the σ component. These differences persist over a range of densities and with low levels of background light. It is suggested that the difference in the polarization angle between components is from a change in the ellipticity of the emitted light across the Stark components coupled with imperfect polarization preservation from an in-vessel mirror.

A photoelastic-modulator-based motional Stark effect polarimeter for ITER that is insensitive to polarized broadband background reflections.

A motional Stark effect polarimeter insensitive to polarized broadband light is proposed. Partially polarized background light is anticipated to be a significant source of systematic error for the ITER polarimeter. The proposed polarimeter is based on the standard dual photoelastic modulator approach, but with the introduction of a birefringent delay plate, it generates a sinusoidal spectral filter instead of the usual narrowband filter. The period of the filter is chosen to match the spacing of the orthogonally polarized Stark effect components, thereby increasing the effective signal level, but resulting in the destructive interference of the broadband polarized light. The theoretical response of the system to an ITER like spectrum is calculated and the broadband polarization tolerance is verified experimentally.

The motional Stark effect polarimeter in the HL-2A tokamak.

A 7-channel motional Stark effect polarimeter based on four polarizers and a spectrometer has been developed in the HL-2A tokamak, which is the first time successful utilizing this kind of polarimeter on a tokamak. The accuracy of the angle can reach ±0.25° in the calibration experiments. Pilot experiments of measuring the magnetic pitch angle have been successfully carried out in the weak motional Stark effect plasma discharge with toroidal magnetic field of ~1.3 T and beam energy of ~25 keV/amu. The pitch angles of magnetic field are obtained for 7 spatial points covering 24 cm along major radius with time resolution of 40 ms; the profiles of safety factor are obtained by combining with the Equilibrium and Reconstruction Fitting Code. The core value of safety factor (q) is less than 1 during the sawtooth oscillation and the position of q = 1 surface is well consistent with the results measured by soft X-ray array.

Plasma Diagnostics in JFT-2M

The diagnostic system of JFT-2M has consisted of about 30 individual diagnostic instruments, which were used to study plasma production, control, equilibrium, stability, confinement, plasma heating by neutral beam injection and/or by radio-frequency (rf) (lower hybrid, ion cyclotron resonance frequency, electron cyclotron heating), and current drive by rf. In these instruments, the motional Stark effect polarimeter, charge-exchange recombination spectroscopy, heavy ion beam probe, time-of-flight neutral particle analyzer, etc., have helped in further understanding the improved mechanism of confinement such as H-mode and high-recycling-steady H-mode, and operational regimes of these modes. An infrared television camera system and a lost ion probe have played a very important role in investigating the heat load on the walls due to ripple lost particles and escaping ions from the core plasma region, respectively.

Multichannel motional Stark effect diagnostic discriminating radial electric field in the JFT-2M tokamak

A multichannel motional Stark effect polarimeter system, which is capable of simultaneous measurement of a radial electric field, has recently been developed on the JFT-2M. The diagnostic can measure the polarization angle at 18 radial locations, which cover a region between just inside from the magnetic axis and the outboard edge of the plasma. By viewing two neutral beam lines (one is co-parallel to the plasma current and the other is counter parallel) simultaneously and near tangentially to the toroidal magnetic field from only one spectroscopic instruments, it provides the best sensitivity in radial electric field measurements with good spatial resolution. The magnetic field pitch angle is also measured with the smallest uncertainty. Preliminary data for L-mode plasma has been obtained. It is found that the statistical uncertainty of the magnetic field pitch angle and radial electric field is about 0.1° and 4 kV/m, respectively, with a time resolution of 10 ms.

Current profile measurements with motional Stark effect polarimeter in the JT-60U tokamak

A motional Stark effect polarimeter with five viewing points has been installed to obtain current profiles in the JT-60U tokamak. Offset angles caused by Faraday rotation were calibrated by rapidly moving a plasma, keeping the q profile unchanged. Polarized background light was observed and its effect was corrected by detecting the light which had a wavelength close to that of the beam emission. Current profiles in a current rampdown discharge were measured, and the q value at the inversion radius of the sawtooth oscillation agreed with the theoretical value (unity) within 10%.

Equilibrium reconstructions of reversed-shear discharges based on motional Stark effect measurements

Internal magnetic field measurements are now routinely made in tokamak discharges with a motional Stark effect polarimeter. Using these measurements, free-boundary equilibrium reconstructions of the temporal evolution of tokamak discharges can be made. The variational moments equilibrium code (VMEC) has been upgraded with an improved plasma boundary positioning algorithm and an improved spline representation of the pressure and q profiles. The uncertainty of the reconstruction is assessed for discharges with reversed magnetic shear where q(0)≫qmin>1.

Confinement and the safety factor profile

The conjecture that the safety factor profile, q(r), controls the improvement in tokamak plasmas from poor confinement in the Low- (L-) mode regime to improved confinement in the supershot regime has been tested in two experiments on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 1, 51 (1987)]. First, helium was puffed into the beam-heated phase of a supershot discharge, which induced a degradation from supershot to L-mode confinement in about 100 ms, far less than the current relaxation time. The q and shear profiles measured by a motional Stark effect polarimeter showed little change during the confinement degradation. Second, rapid current ramps in supershot plasmas altered the q profile, but were observed not to change significantly the energy confinement. Thus, enhanced confinement in supershot plasmas is not due to a particular q profile, which has enhanced stability or transport properties. The discharges making a continuous transition between supershot and L-mode confinement were also used to test the critical-electron-temperature-gradient transport model. It was found that this model could not reproduce the large changes in electron and ion temperature caused by the change in confinement.

The multichannel motional Stark effect diagnostic on TFTR

Although the q profile plays a key role in theories of instabilities and plasma equilibrium, it has been quite difficult to measure until the recent development of the motional Stark effect (MSE) diagnostic. A multichannel motional Stark effect polarimeter system has recently been installed on the Tokamak Fusion Test Reactor (TFTR). The diagnostic can measure the magnetic field pitch angle [γp=tan−1(Bp/BT)] at ten radial locations. The doppler shifted Dα radiation from a TFTR heating beam is viewed near tangential to the toroidal magnetic field via a re-entrant front surface reflecting mirror. The field of view covers from inboard of the magnetic axis to near the outboard edge of the plasma with a radial spatial resolution of 3–5 cm. A high throughput f/2 optics system results in an uncertainty for γp of ∼0.1°–0.2° with a time resolution of ∼5–10 ms. Initial pitch angle profiles from TFTR have been obtained. The MSE data is consistent with the estimated magnetic axis position from external magnetic measurements and the q=1 radius is in good agreement with the inversion radius from the electron cyclotron emission temperature measurements.