[en] The United States Department of Energy (DOE) and Sandia National Laboratories (SNL) have recently proposed a set of updates that integrate data from recent site characterization studies to two conceptual models for the Waste Isolation Pilot Plant's Performance Assessment (WIPP PA) program. This paper discusses the changes to the Disturbed Rock Zone (DRZ) model, which describes the macroscopic manifestation of grain-scale microcracks and larger macro-cracks that are created by induced stresses in the salt surrounding excavations. The DRZ, as modeled in WIPP PA, is an important component of the repository system because its properties affect the quantity of available brine and its ability to enter the waste areas as well as the connectivity of panels after closure. The DOE and SNL have proposed decreasing the region that represents the DRZ in WIPP PA. Additionally, it has been proposed to make the permeability of DRZ a time-dependent quantity to reflect the long-term behavior. In this paper, the implementation of the proposed DRZ model changes is outlined, and the impact of the DRZ modifications on the long-term performance of the WIPP is discussed. The DRZ modifications generally reduced the amount of brine that entered into the repository, as well as reduced the pressure in the repository, except for scenarios in which a pressurized brine pocket was encountered. Overall, the saturation and pressure changes affected the frequency and magnitude of the direct brine and spalling volumes. (authors)

[en] Current drive using RF waves has been proposed as a means to reduce the tearing fluctuations responsible for anomalous energy transport in the RFP. A traveling wave antenna operating at 800 MHz is being used to launch lower hybrid waves into MST to assess the feasibility of this approach. Parameter studies show that edge density is a major factor in antenna/plasma coupling. Gas puffing near the antenna is shown to alter coupling without changing plasma conditions. Hard x-ray emission has been correlated to RF power and is seen to vary strongly with direction of power flow through the antenna

[en] Full text: There are two cases in MST where a central helical magnetic structure emerges and produces a region of improved confinement. One case is due to a single tearing mode dominating the core-resonant m = 1 mode spectrum. Here, runaway electrons are observed with energies > 100 keV, a feature normally absent in standard, stochastic RFP plasmas. These electrons are deduced to be confined in a locally non-stochastic region inside the dominant mode's magnetic island. The other case corresponds to an additional mode that emerges following global reconnection events. In this case, the m = 1 spectrum is fairly flat, but the electron temperature profile exhibits a local peaking corresponding to a substantially reduced electron thermal diffusivity. While neither of these cases corresponds to a substantial improvement in global confinement, recent discharges near MST's maximum toroidal plasma current exhibit very peaked tearing mode spectra. These spectra bear a striking similarity to those in the RFX-mod RFP which produce a several-fold improvement in global confinement. Helical structures in the RFP are of interest not only for their contribution to confinement improvement, but also for their connection to 3D physics in other configurations. Work supported by U.S. Dept. of Energy and National Science Foundation. (author)

[en] 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.

[en] Developing a reactor-compatible divertor has been identified as a particularly challenging technology problem for magnetic confinement fusion. Application of lithium (Li) in NSTX resulted in improved H-mode confinement, H-mode power threshold reduction, and other plasma performance benefits. During the 2010 NSTX campaign, application of a relatively modest amount of Li (300 mg prior to the discharge) resulted in a ∼50% reduction in heat load on the liquid lithium divertor (LLD) attributable to enhanced divertor bolometric radiation. These promising Li results in NSTX and related modelling calculations motivated the radiative LLD concept proposed here. Li is evaporated from the liquid lithium (LL) coated divertor strike-point surface due to the intense heat flux. The evaporated Li is readily ionized by the plasma due to its low ionization energy, and the poor Li particle confinement near the divertor plate enables ionized Li ions to radiate strongly, resulting in a significant reduction in the divertor heat flux. This radiative process has the desired effect of spreading the localized divertor heat load to the rest of the divertor chamber wall surfaces, facilitating the divertor heat removal. The LL coating of divertor surfaces can also provide a ‘sacrificial’ protective layer to protect the substrate solid material from transient high heat flux such as the ones caused by the edge localized modes. By operating at lower temperature than the first wall, the LL covered large divertor chamber wall surfaces can serve as an effective particle pump for the entire reactor chamber, as impurities generally migrate towards lower temperature LL divertor surfaces. To maintain the LL purity, a closed LL loop system with a modest circulating capacity (e.g., ∼1 l s⁻¹ for ∼1% level ‘impurities’) is envisioned for a steady-state 1 GW-electric class fusion power plant. (paper)

[en] In the general area of confinement improvement and concept advancement, recent results in the MST reversed-field pinch include 1) improved confinement at 500 kA plasma current, with copious energetic electrons and sustained elevated ion temperature greater than 1 keV; 2) raised density in improved-confinement plasmas from injection of frozen deuterium pellets; 3) sustainment of 10% of the plasma current by oscillating field current drive (ac helicity injection), in agreement with theory for the applied power; 4) wave injection from electron Bernstein wave and lower hybrid wave antennas, satisfying theoretical expectations of plasma loading at power levels of about 100 kW. Substantial progress has been made in understanding the causes and effects of magnetic reconnection in the RFP: 1) discovery that fast ions with large gyro-radii are well-confined in the standard RFP, despite the underlying stochasticity of the magnetic field; 2) determination that ion heating during reconnection events appears to be localized to the reconnection layer; 3) observation that global magnetic self-organization (characterized by sudden changes in magnetic energy, plasma momentum, and ion temperature) occurs only when spontaneous (m = 1) and driven (m = 0) reconnection are present simultaneously. Diagnostic system development continues, including charge-exchange recombination spectroscopy for fluctuation measurement, multi-point and multi-pulse Thomson scattering, spectral motional Stark effect measurement of low-field vertical bar B vertical bar, and multi-color soft x-ray detector arrays for tomographic measurement of time-resolved electron temperature. (author)