Measurements of multiple heat flux components at the divertor target by using surface eroding thermocouples (invited).

The Surface Eroding Thermocouple (SETC) is a robust diagnostic utilized in DIII-D to provide fast, edge-localized modes (ELMs) resolved heat flux measurements, in particular in geometric regions that are too shadowed for traditional infrared thermography. In order to further investigate the power dissipation in the divertor region, a combination of flush-mounted and recessed SETCs was developed to assess the effect on surface heating from non-charged particles at the divertor target. Utilizing the Divertor Materials Evaluation System sample exposure platform, the first demonstration of the feasibility of using this new method to distinguish between the heat flux from charged particles and that from neutrals and radiative heating was achieved. This paper details the process of using the combination of flush SETCs and recessed SETCs to measure the multiple heat flux components at the divertor target and further discusses how to determine two important ratios, α (ratio of heat flux from charged particles deposit on recessed SETC to that deposit on flush SETC) and ß (ratio of heat flux from non-charged particles deposit on recessed SETC to that deposit on flush SETC), in the estimation of the heat flux from non-charged particle sources. Using a time dependent ratio α, it was found that ∼50% of the total incident heat flux is attributable to the non-charged particles in the fully detached open divertor in DIII-D. Finally, the new application of similar SETC diagnostics in the Small Angle Slot divertor with a V-like configuration and partial tungsten coated surface (SAS-VW) is also introduced.

Preliminary design of a tangentially viewing imaging bolometer for NSTX-U.

The infrared imaging video bolometer (IRVB) measures plasma radiated power images using a thin metal foil. Two different designs with a tangential view of NSTX-U are made assuming a 640 × 480 (1280 × 1024) pixel, 30 (105) fps, 50 (20) mK, IR camera imaging the 9 cm × 9 cm × 2 µm Pt foil. The foil is divided into 40 × 40 (64 × 64) IRVB channels. This gives a spatial resolution of 3.4 (2.2) cm on the machine mid-plane. The noise equivalent power density of the IRVB is given as 113 (46) µW/cm2 for a time resolution of 33 (20) ms. Synthetic images derived from Scrape Off Layer Plasma Simulation data using the IRVB geometry show peak signal levels ranging from ∼0.8 to ∼80 (∼0.36 to ∼26) mW/cm2.

Evidence of Toroidally Localized Turbulence with Applied 3D Fields in the DIII-D Tokamak.

New evidence indicates that there is significant 3D variation in density fluctuations near the boundary of weakly 3D tokamak plasmas when resonant magnetic perturbations are applied to suppress transient edge instabilities. The increase in fluctuations is concomitant with an increase in the measured density gradient, suggesting that this toroidally localized gradient increase could be a mechanism for turbulence destabilization in localized flux tubes. Two-fluid magnetohydrodynamic simulations find that, although changes to the magnetic field topology are small, there is a significant 3D variation of the density gradient within the flux surfaces that is extended along field lines. This modeling agrees qualitatively with the measurements. The observed gradient and fluctuation asymmetries are proposed as a mechanism by which global profile gradients in the pedestal could be relaxed due to a local change in the 3D equilibrium. These processes may play an important role in pedestal and scrape-off layer transport in ITER and other future tokamak devices with small applied 3D fields.

Observation of edge instability limiting the pedestal growth in tokamak plasmas.

With fusion device performance hinging on the edge pedestal pressure, it is imperative to experimentally understand the physical mechanism dictating the pedestal characteristics and to validate and improve pedestal predictive models. This Letter reports direct evidence of density and magnetic fluctuations showing the stiff onset of an edge instability leading to the saturation of the pedestal on the Alcator C-Mod tokamak. Edge stability analyses indicate that the pedestal is unstable to both ballooning mode and kinetic ballooning mode in agreement with observations.

Pedestal structure model.

Predictions are developed for gradients and profiles of the electron density and temperature in tokamak H-mode pedestals that are in transport quasiequilibrium. They are based on assuming paleoclassical processes provide the irreducible minimum radial plasma transport and dominate in the steep gradient regions of pedestals. The predictions agree (within a factor of about two) with properties of a number of pedestal experimental results.

Predicting high harmonic ion cyclotron heating efficiency in Tokamak plasmas.

Observations of improved radio frequency (rf) heating efficiency in ITER relevant high-confinement (H-)mode plasmas on the National Spherical Tokamak Experiment are investigated by whole-device linear simulation. The steady-state rf electric field is calculated for various antenna spectra and the results examined for characteristics that correlate with observations of improved or reduced rf heating efficiency. We find that launching toroidal wave numbers that give fast-wave propagation in the scrape-off plasma excites large amplitude (∼kV m(-1)) coaxial standing modes between the confined plasma density pedestal and conducting vessel wall. Qualitative comparison with measurements of the stored plasma energy suggests that these modes are a probable cause of degraded heating efficiency.

Continuous improvement of H-mode discharge performance with progressively increasing lithium coatings in the National Spherical Torus Experiment.

Lithium wall coatings have been shown to reduce recycling, improve energy confinement, and suppress edge localized modes in the National Spherical Torus Experiment. Here, we show that these effects depend continuously on the amount of predischarge lithium evaporation. We observed a nearly monotonic reduction in recycling, decrease in electron transport, and modification of the edge profiles and stability with increasing lithium. These correlations challenge basic expectations, given that even the smallest coatings exceeded that needed for a nominal thickness of the order of the implantation range.

On demand triggering of edge localized instabilities using external nonaxisymmetric magnetic perturbations in toroidal plasmas.

The application of nonaxisymmetric magnetic fields is shown to destabilize edge-localized modes (ELMs) during otherwise ELM-free periods of discharges in the National Spherical Torus Experiment (NSTX). Profile analysis shows the applied fields increased the temperature and pressure gradients, decreasing edge stability. This robust effect was exploited for a new form of ELM control: the triggering of ELMs at will in high performance H mode plasmas enabled by lithium conditioning, yielding high time-averaged energy confinement with reduced core impurity density and radiated power.

Triggered confinement enhancement and pedestal expansion in high-confinement-mode discharges in the national spherical torus experiment.

We report observation of a new high performance regime in discharges in the National Spherical Torus Experiment, where the H mode edge "pedestal" temperature doubles and the energy confinement increases by 50%. The spontaneous transition is triggered by a large edge-localized mode, either natural or externally triggered by 3D fields. The transport barrier grows inward from the edge, with a doubling of both the pedestal pressure width and the spatial extent of steep radial electric field shear. The dynamics suggest that 3D fields could be applied to reduce edge transport in fusion devices.

Effect of quasihelical symmetry on trapped-electron mode transport in the HSX stellarator.

This Letter presents theory-based predictions of anomalous electron thermal transport in the Helically Symmetric eXperiment stellarator, using an axisymmetric trapped-electron mode drift wave model. The model relies on modifications to a tokamak geometry that approximate the quasihelical symmetry in the Helically Symmetric eXperiment (particle trapping and local curvature) and is supported by linear 3D gyrokinetic calculations. Transport simulations predict temperature profiles that agree with experimental profiles outside a normalized minor radius of rho>0.3 and energy confinement times that agree within 10% of measurements. The simulations can reproduce the large measured electron temperatures inside rho<0.3 if an approximation for turbulent transport suppression due to shear in the radial electric field is included.

Experimental demonstration of improved neoclassical transport with quasihelical symmetry.

Differences in the electron particle and thermal transport are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The thermal diffusivity is reduced in the QHS configuration, resulting in higher electron temperatures than in the nonsymmetric configuration for a fixed power input. The density profile in QHS plasmas is centrally peaked, and in the nonsymmetric configuration the core density profile is hollow. The hollow profile is due to neoclassical thermodiffusion, which is reduced in the QHS configuration.

Experimental evidence of reduced plasma flow damping with quasisymmetry.

Measurements of plasma flow damping have been made in the helically symmetric experiment using a biased electrode to impulsively spin the plasma. There are two time scales in the evolution of the plasma flow, for both the spin-up and relaxation. Compared to a configuration with the quasisymmetry broken, the flow in the quasisymmetric configuration rises more slowly and to a higher value at bias turn-on, and decays more slowly at bias turn-off. The decays of the flows are significantly faster than the neoclassical prediction.

Mitochondrial DNA sequence analysis of human skeletal remains: identification of remains from the Vietnam War.

Deoxyribonucleic acid (DNA) sequence analysis of the control region of the mitochondrial DNA (mtDNA) genome was used to identify human skeletal remains returned to the United States government by the Vietnamese government in 1984. The postmortem interval was thought to be 24 years at the time of testing, and the remains presumed to be an American service member. DNA typing methods using nuclear genomic DNA, HLA-DQ alpha and the variable number of tandem repeat (VNTR) locus D1S80, were unsuccessful using the polymerase chain reaction (PCR). Amplification of a portion of the mtDNA control region was performed, and the resulting PCR product subjected to DNA sequence analysis. The DNA sequence generated from the skeletal remains was identical to the maternal reference sequence, as well as the sequence generated from two siblings (sisters). The sequence was unique when compared to more than 650 DNA sequences found both in the literature and provided by personal communications. The individual sequence polymorphisms were present in only 23 of the more than 1300 nucleotide positions analyzed. These results support the observation that in cases where conventional DNA typing is unavailable, mtDNA sequencing can be used for human remains identification.

The medical investigation of airship accidents.

A review of the autopsy reports for 18 of 21 victims in 3 of the 4 nonrigid Navy airship accidents during the period 1955 to 1966 revealed that the patterns of injury, complicated by postcrash entrapment, immersion, or fire, are similar to the injuries observed in the low-speed, low-altitude crashes of rigid airships and of light aircraft. With the renewed interest in the development of airships for military purposes, there is a need for improved design related to crashworthiness and to aircrew habitability, safety, restraint, and egress in order to enhance the chance for survival in the event of an accident.

Pulmonary function in sickle cell trait.

Pulmonary function abnormalities, which have been reported to occur in persons with sickle cell trait (hemoglobin AS), could intensify the hypoxic stimulus for the systemic formation of sickle cells at high altitude. We sought to determine whether pulmonary function abnormalities occur as a result of exposure to high altitudes in persons with hemoglobin AS. In a prospective study, 13 men with hemoglobin AS ("cases") and 13 controls (hemoglobin AA) matched by age, sex, and race were exposed to five to seven altitude simulations (ranging from 1524 to 7620 m [5000 to 25,000 ft]) in a hypobaric chamber. Measurements of diffusing capacity for carbon monoxide (DLco), forced vital capacity, forced expiratory volume in one second, and forced midexpiratory flow were obtained before and after each exposure. Data before exposures did not differ statistically between cases and controls. Altitude had no systematic effect on DLco or spirometric values in cases compared with values in controls (p greater than 0.05). Individual declines in forced vital capacity or DLco of more than 10% occurred with similar frequency in both groups. Measurements made after the series of exposures showed no change from those made before. We conclude that short serial exposures to hypoxia at high altitudes does not acutely or cumulatively alter DLco or spirometric values in healthy, nonexercising persons with sickle cell trait.