G. Fiksel

High confinement plasmas in the Madison Symmetric Torus reversed-field pinch

Reduction of core-resonant m=1 magnetic fluctuations and improved confinement in the Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch have been routinely achieved through control of the surface poloidal electric field, but it is now known that the achieved confinement has been limited in part by edge-resonant m=0 magnetic fluctuations. Now, through refined poloidal electric field control, plus control of the toroidal electric field, it is possible to reduce simultaneously the m=0 and m=1 fluctuations. This has allowed confinement of high-energy runaway electrons, possibly indicative of flux-surface restoration in the usually stochastic plasma core. The electron temperature profile steepens in the outer region of the plasma, and the central electron temperature increases substantially, reaching nearly 1.3 keV at high toroidal plasma current (500 kA). At low current (200 kA), the total beta reaches 15% with an estimated energy confinement time of 10 ms, a tenfold ...

Reconnection scaling experiment: A new device for three-dimensional magnetic reconnection studies

The reconnection scaling experiment (RSX), a linear device for studying three-dimensional magnetic reconnection in both collisional and collisionless laboratory plasmas, has been constructed at Los Alamos National Laboratory. Advanced experimental features of the RSX that lead to scientific advantages include the use of simple technology (commercial plasma guns) to create plasma and current channels. Physics motivations, design and construction features of the RSX, are presented. Basic plasma parameters that characterize the RSX are shown together with preliminary measurements of visible light emission during the merging of two parallel current channels.

Tokamak-like confinement at a high beta and low toroidal field in the MST reversed field pinch

Energy confinement comparable with tokamak quality is achieved in the Madison Symmetric Torus (MST) reversed field pinch (RFP) at a high beta and low toroidal magnetic field. Magnetic fluctuations normally present in the RFP are reduced via parallel current drive in the outer region of the plasma. In response, the electron temperature nearly triples and beta doubles. The confinement time increases ten-fold (to ~10 ms), which is comparable with L- and H-mode scaling values for a tokamak with the same plasma current, density, heating power, size and shape. Runaway electron confinement is evidenced by a 100-fold increase in hard x-ray bremsstrahlung. Fokker–Planck modelling of the x-ray energy spectrum reveals that the high energy electron diffusion is independent of the parallel velocity, uncharacteristic of magnetic transport and more like that for electrostatic turbulence. The high core electron temperature correlates strongly with a broadband reduction of resonant modes at mid-radius where the stochasticity is normally most intense. To extend profile control and add auxiliary heating, rf current drive and neutral beam heating are in development. Low power lower-hybrid and electron Bernstein wave injection experiments are underway. Dc current sustainment via ac helicity injection (sinusoidal inductive loop voltages) is also being tested. Low power neutral beam injection shows that fast ions are well-confined, even in the presence of relatively large magnetic fluctuations.

Measurement of core velocity fluctuations and the dynamo in a reversed-field pinch

Plasma flow velocity fluctuations have been directly measured in the high temperature magnetically confined plasma in the Madison Symmetric Torus (MST) Reversed-Field Pinch (RFP). These measurements show that the flow velocity fluctuations are correlated with magnetic field fluctuations. This initial measurement is subject to limitations of spatial localization and other uncertainties, but is evidence for sustainment of the RFP magnetic field configuration by the magnetohydrodynamic (MHD) dynamo. Both the flow velocity and magnetic field fluctuations are the result of global resistive MHD modes of helicity m = 1, n = 5--10 in the core of MST. Chord-averaged flow velocity fluctuations are measured in the core of MST by recording the Doppler shift of impurity line emission with a specialized high resolution and throughput grating spectrometer. Magnetic field fluctuations are recorded with a large array of small edge pickup coils, which allows spectral decomposition into discrete modes and subsequent correlation with the velocity fluctuation data.

Advances in neutral-beam-based diagnostics on the Madison Symmetric Torus reversed-field pinch (invited)

Innovative charge-exchange recombination spectroscopy (CHERS), motional Stark effect (MSE), and Rutherford scattering diagnostics are now in operation on the Madison Symmetric Torus (MST) reversed-field pinch (RFP). The CHERS diagnostic measures impurity ion flow and temperature, localized to 2cm with high time resolution (∼100kHz). A spectral MSE diagnostic has been in use for five years, measuring ∣B∣ down to 0.2T with high precision (∼2%) and good time resolution (10kHz). The Rutherford scattering diagnostic has demonstrated the robustness of this technique for reliable measurement of majority (D) ion temperature, also with high time resolution. MST is a large RFP (R=1.5m, a=0.52m) operated at moderate current (Ip⩽600kA), with ne typically (1–2)×1019m−3 and Te, Ti⩽2keV. Two compact and reliable diagnostic neutral beams are installed on MST. These beams are short pulse, intense, monoenergetic, and low divergence. The first, a neutral H beam, is used in combination with ultraviolet and visible spectrosco...

First charge exchange recombination spectroscopy and motional Stark effect results from the Madison Symmetric Torus reversed field pinch

We report on the first results of charge exchange recombination spectroscopy (CHERS) and motional Stark effect (MSE) measurements in the Madison Symmetric Torus reversed field pinch. A 30 keV, 4 A neutral H beam is used in combination with visible and ultraviolet spectroscopy to make the measurements. For CHERS, we find that the C VI line at 3433.69 A yields the largest charge-exchange signal/background ratio and is most clearly resolved from other nearby lines. Equilibrium ion temperature measurements have been made with an existing fast Doppler spectrometer and a higher throughput spectrometer is being designed to do velocity and temperature fluctuation measurements. MSE measurements are made by recording the Doppler shifted Hα spectrum emitted by the beam with a charge coupled device and imaging spectrometer. We have observed separation of the π components of the Stark manifold at magnetic fields of about 0.5 T and are considering options for increasing measurement accuracy.

A diagnostic neutral beam system for the MST reversed-field pinch

A diagnostic neutral beam system has been developed for the Madison symmetric torus (MST) reversed-field pinch. The system is primarily used: (1) for measurement of the majority ion equilibrium and fluctuating velocity and temperature by Rutherford scattering (RS); (2) for measurement of the impurity ion velocity and temperature, both equilibrium and fluctuating, by charge-exchange recombination spectroscopy (CHERS); and (3) for magnetic field measurement via motional Stark effect (MSE). The system consists of two neutral beam injectors, and two neutral particle analyzers. One injector creates a 20 keV, 4 A helium beam for RS. The energy spectra of the helium beam atoms scattered from the plasma ions is measured with two 12-channel, 45° electrostatic energy analyzers equipped with a hydrogen stripping cell. A second injector creates a 30 keV, 4 A hydrogen beam, which is used for the CHERS and MSE diagnostics. In each injector ions are extracted from a plasma created by an arc discharge source and, after a...

An optical probe for local measurements of fast plasma ion dynamics

A novel insertable probe for local measurements of equilibrium and fluctuating plasma ion flow velocity and temperature via Doppler spectroscopy is described. Optical radiation is collected by two fused silica fiber optic bundles with perpendicular viewlines. Spatial resolution of about 5 cm is achieved by terminating each view with an optical dump. The collected light is transported by the fiber bundles to a high-resolution spectrometer. Two components of the velocity are measured simultaneously—the radial along the insertion of the probe and a perpendicular component (which can be varied by simply rotating the probe by 90°). The accuracy of the velocity measurements is better than 1 km/s. The probe is armored by a boron nitride enclosure and is inserted into a high temperature plasma to obtain radial profiles of the equilibrium and fluctuating plasma velocity. Initial measurements have been done in Madison Symmetric Torus reversed field pinch.

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.

Impurities, temperature and density in a miniature electrostatic plasma and current source

We have spectroscopically investigated the Sterling Scientific miniature electrostatic plasma source - a plasma gun. This gun is a clean source of high-density ( - ), low-temperature (5 - 15 eV) plasma. A key result of our investigation is that molybdenum from the gun electrodes is largely trapped in the internal gun discharge; only a small amount escapes in the plasma flowing out of the gun. In addition, the gun plasma parameters actually improve (even lower impurity contamination and higher ion temperature) when up to 1 kA of electron current is extracted from the gun via the application of an external bias. This improvement occurs because the internal gun anode no longer acts as the current return for the internal gun discharge. The gun plasma is a virtual plasma electrode capable of sourcing an electron emission current density of . The high emission current, small size (3 - 4 cm diameter), and low impurity generation make this gun attractive for a variety of fusion and plasma technology applications.