Collisionless microinstabilities in stellarators II - numerical simulations

Microinstabilities exhibit a rich variety of behavior in stellarators due to the many degrees of freedom in the magnetic geometry. It has recently been found that certain stellarators (quasi-isodynamic ones with maximum-$J$ geometry) are partly resilient to trapped-particle instabilities, because fast-bouncing particles tend to extract energy from these modes near marginal stability. In reality, stellarators are never perfectly quasi-isodynamic, and the question thus arises whether they still benefit from enhanced stability. Here the stability properties of Wendelstein 7-X and a more quasi-isodynamic configuration, QIPC, are investigated numerically and compared with the National Compact Stellarator Experiment (NCSX) and the DIII-D tokamak. In gyrokinetic simulations, performed with the gyrokinetic code GENE in the electrostatic and collisionless approximation, ion-temperature-gradient modes, trapped-electron modes and mixed-type instabilities are studied. Wendelstein 7-X and QIPC exhibit significantly reduced growth rates for all simulations that include kinetic electrons, and the latter are indeed found to be stabilizing in the energy budget. These results suggest that imperfectly optimized stellarators can retain most of the stabilizing properties predicted for perfect maximum-$J$ configurations.Comment: 15 pages, 40 figure

Solar System Experiments and the Interpretation of Saa's Model of Gravity with Propagating Torsion as a Theory with Variable Plank "Constant"

It is shown that the recently proposed interpretation of the transposed equi-affine theory of gravity as a theory with variable Plank "constant" is inconsistent with basic solar system gravitational experiments.Comment: 6 pages, latex, no figures. Typos correcte

Gyrokinetic studies of the effect of beta on drift-wave stability in NCSX

The gyrokinetic turbulence code GS2 was used to investigate the effects of plasma beta on linear, collisionless ion temperature gradient (ITG) modes and trapped electron modes (TEM) in National Compact Stellarator Experiment (NCSX) geometry. Plasma beta affects stability in two ways: through the equilibrium and through magnetic fluctuations. The first was studied here by comparing ITG and TEM stability in two NCSX equilibria of differing beta values, revealing that the high beta equilibrium was marginally more stable than the low beta equilibrium in the adiabatic-electron ITG mode case. However, the high beta case had a lower kinetic-electron ITG mode critical gradient. Electrostatic and electromagnetic ITG and TEM mode growth rate dependencies on temperature gradient and density gradient were qualitatively similar. The second beta effect is demonstrated via electromagnetic ITG growth rates' dependency on GS2's beta input parameter. A linear benchmark with gyrokinetic codes GENE and GKV-X is also presented.Comment: Submitted to Physics of Plasmas. 9 pages, 27 figure

Collisionless microinstabilities in stellarators. III. The ion-temperature-gradient mode

We investigate the linear theory of the ion-temperature-gradient (ITG) mode, with the goal of developing a general understanding that may be applied to stellarators. We highlight the Wendelstein 7X (W7-X) device. Simple fluid and kinetic models that follow closely from existing literature are reviewed and two new first-principle models are presented and compared with results from direct numerical simulation. One model investigates the effect of regions of strong localized shear, which are generic to stellarator equilibria. These "shear spikes" are found to have a potentially significant stabilizing affect on the mode; however, the effect is strongest at short wavelengths perpendicular to the magnetic field, and it is found to be significant only for the fastest growing modes in W7-X. A second model investigates the long-wavelength limit for the case of negligible global magnetic shear. The analytic calculation reveals that the effect of the curvature drive enters at second order in the drift frequency, confirming conventional wisdom that the ITG mode is slab-like at long wavelengths. Using flux tube simulations of a zero-shear W7-X configuration, we observe a close relationship to an axisymmetric configuration at a similar parameter point. It is concluded that scale lengths of the equilibrium gradients constitute a good parameter space to characterize the ITG mode. Thus, to optimize the magnetic geometry for ITG mode stability, it may be fruitful to focus on local parameters, such as the magnitude of bad curvature, connection length, and local shear at locations of bad curvature (where the ITG mode amplitude peaks).Comment: v3: final version for journa

Response of Mouse Breast Cancer Cells to Anastrozole, Tamoxifen, and the Combination

The murine breast cancer cells (4T1) grown both in female BALB/c mice and in culture were treated with anastrozole (50 μg/mL), tamoxifen citrate (5 μg/mL), and the combination of the two drugs in order to determine treatment efficacies, toxic potential, and the mechanism of cell death. The in vivo treatments were evaluated by monitoring tumor growth, development, and life span. The in vitro effects were measured through cell growth kinetics, cell proliferation, mitochondrial membrane potential disruption assay, and light and scanning electron microscopy. All drug treatments extended the mean life span of the 4T1-inoculated tumor-bearing mice; however, only tamoxifen and combination treatments statistically increased the life span when compared to untreated mice. Although the most drug inhibitory effect on cell multiplication was observed in the combination treatment, both anastrozole and tamoxifen individually inhibited cell proliferation significantly at most time periods in this mouse breast cancer cell line. The mitochondrial membrane potential disruption assay demonstrated significant increase in the percent of cells undergoing apoptosis in all treatment groups. However, the combination treatment was the most effective in inducing cell death via apoptosis. Light and scanning electron microscopy of the treated cells revealed characteristics such as rounding, clumping, and shrinkage of the cells as well as formation of cell surface blebbing and apoptotic bodies suggestive of cell death via apoptotic pathway

Statistical analysis and modeling of intermittent transport events in the tokamak SOL

The turbulence observed in the scrape-off-layer of a tokamak is often characterized by intermittent events of bursty nature, a feature which raises concerns about the prediction of heat loads on the physical boundaries of the device. It appears thus necessary to delve into the statistical properties of turbulent physical fields such as density, electrostatic potential and temperature, focusing on the mathematical expression of tails of the probability distribution functions. The method followed here is to generate statistical information from time-traces of the plasma density stemming from Braginskii-type fluid simulations, and check this against a first-principles theoretical model. The analysis of the numerical simulations indicates that the probability distribution function of the intermittent process contains strong exponential tails, as predicted by the analytical theory.Comment: 16 pages, 8 figure