H.D. Stephens

Multipoint Thomson scattering diagnostic for the Madison Symmetric Torus reversed-field pinch.

The multipoint Thomson scattering diagnostic on the Madison Symmetric Torus (MST) is now fully operational with 21 spatial points, which cover the entire minor radius. Four full electron temperature profiles can be obtained during each MST discharge, with a variable delay between each profile. This system overcomes challenges that arise from the unique machine design, location, and plasma characteristics of MST. The machine design limits the maximum porthole diameter to 11.4 cm, requiring a compact, re-entrant, seven element lens for scattered light collection. Limited space near MST necessitates a long beam path for the two Nd:YAG lasers requiring a remote beam line adjustment system to suppress drift in the beam position due to thermal expansion of the building. Due to the remote location of the laser head, substantial design effort was put into the creation of a set of safety interlocks for the laser system. The dynamic nature of MST plasmas and the wide range of operating space require a versatile scattered light detection system consisting of filter polychromators with temperature controlled avalanche photodiode detectors. We also implement an insertable integrating sphere, which travels along the laser beam path through the vacuum vessel, for the alignment of both the fiber optics and the lasers.

Improved-confinement plasmas at high temperature and high beta in the MST RFP

We have increased substantially the electron and ion temperatures, the electron density, and the total beta in plasmas with improved energy confinement in the Madison Symmetric Torus (MST). The improved confinement is achieved with a well-established current profile control technique for reduction of magnetic tearing and reconnection. A sustained ion temperature >1?keV is achieved with intensified reconnection-based ion heating followed immediately by current profile control. In the same plasmas, the electron temperature reaches 2?keV, and the electron thermal diffusivity drops to about 2?m2?s?1. The global energy confinement time is 12?ms. This and the reported temperatures are the largest values yet achieved in the reversed-field pinch (RFP). These results were attained at a density ~1019?m?3. By combining pellet injection with current profile control, the density has been quadrupled, and total beta has nearly doubled to a record value of about 26%. The Mercier criterion is exceeded in the plasma core, and both pressure-driven interchange and pressure-driven tearing modes are calculated to be linearly unstable, yet energy confinement is still improved. Transient momentum injection with biased probes reveals that global momentum transport is reduced with current profile control. Magnetic reconnection events drive rapid momentum transport related to large Maxwell and Reynolds stresses. Ion heating during reconnection events occurs globally, locally, or not at all, depending on which tearing modes are involved in the reconnection. To potentially augment inductive current profile control, we are conducting initial tests of current drive with lower-hybrid and electron-Bernstein waves.

Electron thermal transport within magnetic islands in the reversed-field pinch

Tearing mode induced magnetic islands have a significant impact on the thermal characteristics of magnetically confined plasmas such as those in the reversed-field pinch (RFP). New Thomson scattering diagnostic capability on the Madison Symmetric Torus (MST) RFP has enabled measurement of the thermal transport characteristics of islands. Electron temperature (Te) profiles can now be acquired at 25 kHz, sufficient to measure the effect of an island on the profile as the island rotates by the measurement point. In standard MST plasmas with a spectrum of unstable tearing modes, remnant islands are present in the core between sawtoothlike reconnection events. Associated with these island remnants is flattening of the Te profile inside the island separatricies. This flattening is characteristic of rapid parallel heat conduction along helical magnetic field lines. In striking contrast, a temperature gradient within an m=1, n=5 island is observed in these same plasmas just after a sawtooth event when the m=1, n=...

Plasma behaviour at high β and high density in the Madison Symmetric Torus RFP

Pellet fuelling of improved confinement Madison Symmetric Torus (MST) plasmas has resulted in high density and high plasma beta. The density in improved confinement discharges has been increased fourfold, and a record plasma beta (βtot = 26%) for the improved confinement reversed-field pinch (RFP) has been achieved. At higher β, a new regime for instabilities is accessed in which local interchange and global tearing instabilities are calculated to be linearly unstable, but experimentally, no severe effect, e.g., a disruption, is observed. The tearing instability, normally driven by the current gradient, is driven by the pressure gradient in this case, and there are indications of increased energy transport (as compared with low-density improved confinement). Pellet fuelling is also compared with enhanced edge fuelling of standard confinement RFP discharges for the purpose of searching for a density limit in MST. In standard-confinement discharges, pellet fuelling peaks the density profile where edge fuelling cannot, but transport appears unchanged. For a limited range of plasma current, MST discharges with edge fuelling are constrained to a maximum density corresponding to the Greenwald limit. This limit is surpassed in pellet-fuelled improved confinement discharges.

Optimizing a Thomson scattering diagnostic for fast dynamics and high background

The Madison Symmetric Torus (MST) presents challenging conditions for Thomson scattering (TS) measurements. The MST plasmas are reversed-field pinches (RFPs) with electron density n(e)<3x10(13) cm(-3), typically 1x10(13) cm(-3). The TS system was designed to measure from 10 eV to 2 keV; however, six polychromators were upgraded from four to eight spectral channels to resolve to 10 keV. There is no diverter or vertical field, so wall interaction results in high background light both from ion and neutral bremsstrahlungs and from line radiation. Also during standard plasmas, the RFP exhibits regular reconnection sawteeth events during which the plasma current, density, and temperature profiles are flattened. These events are of interest both due to the reconnection physics and to their contribution to the MST equilibrium and confinement. These events occur over 100 microS and exhibit large changes in background light and fast changes in temperature. During improved confinement plasmas, there are no sawteeth; the background is low but the temperature can be over an order of magnitude higher. Data analysis of the system has been developed to accommodate both the large dynamic range of the temperature, the fast dynamics, and the fast changing, high amplitude background. Special attention has been paid to the sources of error, in particular, the contribution of the background. A response-function method reduces the measured uncertainty by a factor of 2. Numerical techniques have been developed which are extremely robust. Two methods are used, a conventional chi(2) minimization using a Levenberg-Marquardt algorithm coupled with Monte Carlo modeling for the error bar and a Bayesian statistics method. The Bayesian method computes the probability distribution for the measured photons and electron temperature and this information can be used to ensemble data and will allow future integrated data analysis efforts.

Calibration of a Thomson scattering diagnostic for fluctuation measurements

Detailed calibrations of the Madison Symmetric Torus polychromator Thomson scattering system have been made suitable for electron temperature fluctuation measurements. All calibrations have taken place focusing on accuracy, ease of use and repeatability, and in situ measurements wherever possible. Novel calibration processes have been made possible with an insertable integrating sphere (ISIS), using an avalanche photodiode (APD) as a reference detector and optical parametric oscillator (OPO). Discussed are a novel in situ spatial calibration with the use of the ISIS, the use of an APD as a reference detector to streamline the APD calibration process, a standard dc spectral calibration, and in situ pulsed spectral calibration made possible with a combination of an OPO as a light source, the ISIS, and an APD used as a reference detector. In addition a relative quantum efficiency curve for the APDs is obtained to aid in uncertainty analysis.

High-β, improved confinement reversed-field pinch plasmas at high density

In Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] discharges where improved confinement is brought about by modification of the current profile, pellet injection has quadrupled the density, reaching ne=4×1019m−3. Without pellet injection, the achievable density in improved confinement discharges had been limited by edge-resonant tearing instability. With pellet injection, the total beta has been increased to 26%, and the energy confinement time is comparable to that at low density. Pressure-driven local interchange and global tearing are predicted to be linearly unstable. Interchange has not yet been observed experimentally, but there is possible evidence of pressure-driven tearing, an instability usually driven by the current gradient in the reversed-field pinch.

Generation and confinement of hot ions and electrons in a reversed-field pinch plasma

By manipulating magnetic reconnection in Madison Symmetric Torus (MST) discharges, we have generated and confined for the first time a reversedfield pinch (RFP) plasma with an ion temperature >1keV and an electron temperature of 2keV. This is achieved at a toroidal plasma current of about 0.5MA, approaching MST’s present maximum. The manipulation begins with intensification of discrete magnetic reconnection events, causing the ion temperature to increase to several kiloelectronvolts. The reconnection is then quickly suppressed with inductive current profile control, leading to capture of a portion of the added ion heat with improved ion energy confinement. Electron energy confinement is simultaneously improved, leading to a rapid ohmically driven increase in the electron temperature. A steep electron temperature gradient emerges in the outer region of the plasma, with a local thermal diffusivity of about 2m 2 s −1 . The global energy confinement time reaches 12ms, the largest value yet achieved in the RFP and which is roughly comparable to the H-mode scaling prediction for a tokamak with the same plasma current, density, heating power, size and shape.

Determination of Z eff by integrating measurements from x-ray tomography and charge exchange recombination spectroscopy

The effective ionic charge, Zeff, is determined through the integration of soft x-ray tomography and charge exchange recombination spectroscopy impurity density measurements in the Madison Symmetric Torus. Zeff is found is be 2.3 ± 0.1 in the core of high temperature, high current, improved confinement discharges, with a slightly hollow profile peaking near mid-radius. A Bayesian probability framework, developed as part of an on-going effort in Integrated Data Analysis, was used to incorporate these two measurements. This framework provides a method to address different systematic and statistical uncertainties associated with each diagnostic and to test hypothetical contributions to Zeff against the existing data set. The combined analysis provides much higher confidence in the result than previous single-diagnostic attempts to characterize Zeff using near-infrared bremsstrahlung or x-ray spectroscopy.

Equilibrium evolution in oscillating-field current-drive experiments

Oscillating-field current drive (OFCD) is a proposed method of steady-state toroidal plasma sustainment in which ac poloidal and toroidal loop voltages are applied to produce a dc plasma current. OFCD is added to standard, inductively sustained reversed-field pinch plasmas in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)]. Equilibrium profiles and fluctuations during a single cycle are measured and analyzed for different relative phases between the two OFCD voltages and for OFCD off. For OFCD phases leading to the most added plasma current, the measured energy confinement is slightly better than that for OFCD off. By contrast, the phase of the maximum OFCD helicity-injection rate also has the maximum decay rate, which is ascribed to transport losses during discrete magnetic-fluctuation events induced by OFCD. Resistive-magnetohydrodynamic simulations of the experiments reproduce the observed phase dependence of the added current.