K.J. Caspary

Two-point motional Stark effect diagnostic for Madison Symmetric Torus

A high-precision spectral motional Stark effect (MSE) diagnostic provides internal magnetic field measurements for Madison Symmetric Torus (MST) plasmas. Currently, MST uses two spatial views--on the magnetic axis and on the midminor (off-axis) radius, the latter added recently. A new analysis scheme has been developed to infer both the pitch angle and the magnitude of the magnetic field from MSE spectra. Systematic errors are reduced by using atomic data from atomic data and analysis structure in the fit. Reconstructed current density and safety factor profiles are more strongly and globally constrained with the addition of the off-axis radius measurement than with the on-axis one only.

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.

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.

Classical confinement and outward convection of impurity ions in the MST RFP

Impurity ion dynamics measured with simultaneously high spatial and temporal resolution reveal classical ion transport in the reversed-field pinch. The boron, carbon, oxygen, and aluminum impurity ion density profiles are obtained in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] using a fast, active charge-exchange-recombination-spectroscopy diagnostic. Measurements are made during improved-confinement plasmas obtained using inductive control of tearing instability to mitigate stochastic transport. At the onset of the transition to improved confinement, the impurity ion density profile becomes hollow, with a slow decay in the core region concurrent with an increase in the outer region, implying an outward convection of impurities. Impurity transport from Coulomb collisions in the reversed-field pinch is classical for all collisionality regimes, and analysis shows that the observed hollow profile and outward convection can be explained by the classical temperature screening mechanism. The profile agrees well with classical expectations. Experiments performed with impurity pellet injection provide further evidence for classical impurity ion confinement.

Advances in time-resolved measurement of magnetic field and electron temperature in low-magnetic-field plasmas

Abstract Internal time-resolved measurement of magnetic field and electron temperature in low-field (≤ 1 T) plasmas is a difficult diagnostic challenge. To meet this diagnostic challenge in the Madison Symmetric Torus reversed-field pinch, two techniques are being developed: 1) spectral motional Stark effect (MSE) and 2) Fast Thomson scattering. For spectral MSE, the entire Stark-split Hα spectrum emitted by hydrogen neutral beam atoms is recorded and analyzed using a newly refined atomic emission model. A new analysis scheme has been developed to infer both the polarization direction and the magnitude of Stark splitting, from which both the direction and magnitude of the local magnetic field can be derived. For Fast Thomson scattering, two standard commercial flashlamp-pumped Nd:YAG lasers have been upgraded to “pulse-burst” capability. Each laser produces a burst of up to fifteen pulses at repetition rates 1–12.5 kHz, thus enabling recording of the dynamic evolution of the electron temperature profile and electron temperature fluctuations. To further these capabilities, a custom pulse-burst laser system is now being commissioned. This new laser is designed to produce a burst of laser pulses at repetition frequencies 5–250 kHz.

A Simple Technique for Increasing the Temperature Range of a Cryogenic Frozen-Gas Pellet Injector for Operation with Various Gases

Abstract On the Madison Symmetric Torus magnetic fusion plasma experiment, frozen pellet injection is an established method of depositing deuterium fuel into the core of the plasma. To freeze deuterium gas into pellets, the injector is cooled to 10 K with a cryogenic helium refrigerator. To exhaust residual frozen deuterium following injection of each pellet, the injector is warmed by resistive heating to >18.7 K, the triple point of deuterium. Motivated by the desire to inject carbon-containing pellets, the injector was modified to allow the freezing and injection of methane. The triple point of methane, 90.7 K, is well beyond the capability of the resistive heating hardware. To supplement the resistive heating, a small, steady flow of room-temperature helium was introduced as a heat source. The flow rate was optimized to provide minimum and maximum injector temperatures of 24 and 95 K, respectively, sufficient for methane pellet formation and exhaust. The flow rate can easily be optimized for other gases as well.