Measurements with magnetic field in the National Spherical Torus Experiment using the motional Stark effect with laser induced fluorescence diagnostic.

The motional Stark effect with laser-induced fluorescence diagnostic (MSE-LIF) has been installed and tested on the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Lab. The MSE-LIF diagnostic will be capable of measuring radially resolved profiles of magnetic field magnitude or pitch angle in NSTX plasmas. The system includes a diagnostic neutral hydrogen beam and a laser which excites the n = 2 to n = 3 transition. A viewing system has been implemented which will support up to 38 channels from the plasma edge to past the magnetic axis. First measurements of MSE-LIF signals in the presence of small applied magnetic fields in neutral gas are reported.

Suppression of electron temperature gradient turbulence via negative magnetic shear in NSTX.

Negative magnetic shear is found to suppress electron turbulence and improve electron thermal transport for plasmas in the National Spherical Torus Experiment (NSTX). Sufficiently negative magnetic shear results in a transition out of a stiff profile regime. Density fluctuation measurements from high-k microwave scattering are verified to be the electron temperature gradient (ETG) mode by matching measured rest frequency and linear growth rate to gyrokinetic calculations. Fluctuation suppression under negligible E×B shear conditions confirm that negative magnetic shear alone is sufficient for ETG suppression. Measured electron temperature gradients can significantly exceed ETG critical gradients with ETG mode activity reduced to intermittent bursts, while electron thermal diffusivity improves to below 0.1 electron gyro-Bohms.

Observations of reduced electron Gyroscale fluctuations in national spherical torus experiment H-mode plasmas with large ExB flow shear.

Electron gyroscale fluctuation measurements in National Spherical Torus Experiment H-mode plasmas with large toroidal rotation reveal fluctuations consistent with electron temperature gradient (ETG) turbulence. Large toroidal rotation in National Spherical Torus Experiment plasmas with neutral beam injection generates ExB flow shear rates comparable to ETG linear growth rates. Enhanced fluctuations occur when the electron temperature gradient is marginally stable with respect to the ETG linear critical gradient. Fluctuation amplitudes decrease when the ExB flow shear rate exceeds ETG linear growth rates. The observations indicate that ExB flow shear can be an effective suppression mechanism for ETG turbulence.

The motional Stark effect diagnostic for ITER using a line-shift approach.

The United States has been tasked with the development and implementation of a motional Stark effect (MSE) system on ITER. In the harsh ITER environment, MSE is particularly susceptible to degradation, as it depends on polarimetry, and the polarization reflection properties of surfaces are highly sensitive to thin film effects due to plasma deposition and erosion of a first mirror. Here we present the results of a comprehensive study considering a new MSE-based approach to internal plasma magnetic field measurements for ITER. The proposed method uses the line shifts in the MSE spectrum (MSE-LS) to provide a radial profile of the magnetic field magnitude. To determine the utility of MSE-LS for equilibrium reconstruction, studies were performed using the ESC-ERV code system. A near-term opportunity to test the use of MSE-LS for equilibrium reconstruction is being pursued in the implementation of MSE with laser-induced fluorescence on NSTX. Though the field values and beam energies are very different from ITER, the use of a laser allows precision spectroscopy with a similar ratio of linewidth to line spacing on NSTX as would be achievable with a passive system on ITER. Simulation results for ITER and NSTX are presented, and the relative merits of the traditional line polarization approach and the new line-shift approach are discussed.

The motional Stark effect diagnostic on NSTX.

This work describes the implementation and recent results from the motional Stark effect (MSE) collisionally induced fluorescence diagnostic on NSTX. Due to the low magnetic field on NSTX the MSE diagnostic requires a new approach for the viewing optics and spectral filter. This has been accomplished with a novel optical design that reduces the geometric Doppler broadening, and a high throughput, high resolution spectral filter to optimize signal-to-noise ratio. With these improvements the polarization fraction is approximately 30%-40% and, combined with the large throughput, a time resolution of approximately 5 ms. The MSE diagnostic presently has 16 sight lines operating, providing measurements of the magnetic field line pitch from the plasma center to near the outboard edge of the plasma.

Simulation of the motional Stark effect diagnostic gas-filled torus calibration.

Many motional Stark effect diagnostics around the world make use of a calibration procedure in which the observed neutral beam is injected into a gas-filled torus with known vacuum fields. The instrument is calibrated by reconciling measured angles with vacuum magnetic reconstructions through a range of pitch angles. This in situ gas-filled torus calibration most closely approximates the working conditions of the diagnostic and includes effects such as beam and viewing geometries, beam voltages, Faraday and stress induced birefringence (in most cases) of the transmissive optics, as well as the polarimeter response. However, secondary neutrals, produced after ionization then reneutralization of a beam neutral, have been found to contaminate measured angles by emitting Balmer alpha with similar Doppler shifts and Stark polarizations as beam neutrals, but with different polarization angles. Simulation results that show spectral and angle behavior versus calibration parameters such as fill gas pressure will be presented. Data from NSTX and C-Mod will be compared to simulations results.

Momentum-transport studies in high E x B shear plasmas in the National Spherical Torus Experiment.

Experiments have been conducted at the National Sperical Torus Experiment (NSTX) to study both steady state and perturbative momentum transport. These studies are unique in their parameter space under investigation, where the low aspect ratio of NSTX results in rapid plasma rotation with ExB shearing rates high enough to suppress low-k turbulence. In some cases, the ratio of momentum to energy confinement time is found to exceed five. Momentum pinch velocities of order 10-40 m/s are inferred from the measured angular momentum flux evolution after nonresonant magnetic perturbations are applied to brake the plasma.

Observation of instability-induced current redistribution in a spherical-torus plasma.

A motional Stark effect diagnostic has been utilized to reconstruct the parallel current density profile in a spherical-torus plasma for the first time. The measured current profile compares favorably with neoclassical theory when no large-scale magnetohydrodynamic instabilities are present in the plasma. However, a current profile anomaly is observed during saturated interchange-type instability activity. This apparent anomaly can be explained by redistribution of neutral beam injection current drive and represents the first observation of interchange-type instabilities causing such redistribution. The associated current profile modifications contribute to sustaining the central safety factor above unity for over five resistive diffusion times, and similar processes may contribute to improved operational scenarios proposed for ITER.

Active stabilization of the resistive-wall mode in high-beta, low-rotation plasmas.

The resistive-wall mode is actively stabilized in the National Spherical Torus Experiment in high-beta plasmas rotating significantly below the critical rotation speed for passive stability and in the range predicted for the International Thermonuclear Experimental Reactor. Variation of feedback stabilization parameters shows mode excitation or suppression. Stabilization of toroidal mode number unity did not lead to instability of toroidal mode number two. The mode can become unstable by deforming poloidally, an important consideration for stabilization system design.

Observation of plasma toroidal-momentum dissipation by neoclassical toroidal viscosity.

Dissipation of plasma toroidal angular momentum is observed in the National Spherical Torus Experiment due to applied nonaxisymmetric magnetic fields and their plasma-induced increase by resonant field amplification and resistive wall mode destabilization. The measured decrease of the plasma toroidal angular momentum profile is compared to calculations of nonresonant drag torque based on the theory of neoclassical toroidal viscosity. Quantitative agreement between experiment and theory is found when the effect of toroidally trapped particles is included.