10 Search Results
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Evidence and modeling of turbulence bifurcation in L-mode confinement transitions on Alcator C-Mod
Analysis and modeling of rotation reversal hysteresis experiments show that a single turbulent bifurcation is responsible for the Linear to Saturated Ohmic Confinement (LOC/SOC) transition and concomitant intrinsic rotation reversal on Alcator C-Mod. Plasmas on either side of the reversal exhibit different toroidal rotation profiles and therefore different turbulence characteristics despite the profiles of density and temperature, which are indistinguishable within measurement uncertainty. Elements of this bifurcation are also shown to persist for auxiliary heated L-modes. The deactivation of subdominant (in the linear growth rate and contribution to heat transport) ion temperature gradient and trapped electron mode instabilities is identifiedmore » -
Hysteresis as a probe of turbulent bifurcation in intrinsic rotation reversals on Alcator C-Mod
Analysis and modeling of a new set of rotation reversal hysteresis experiments unambiguously show that changes in turbulence are responsible for the intrinsic rotation reversal and the linear to saturated ohmic confinement (LOC/SOC) transition on Alcator C-Mod. Plasmas on either side of the reversal exhibit different toroidal rotation profiles and therefore different turbulence characteristics despite profiles of density and temperature that are indistinguishable within measurement uncertainty. The deactivation of subdominant (in linear growth rate and heat transport) ion-temperature gradient and trapped electron mode-like instabilities in a mixedmode state is identified as the only possible change in turbulence within a quasilinearmore » -
Efficient design and verification of diagnostics for impurity transport experiments
Recent attempts to measure impurity transport in Alcator C-Mod using an x-ray imaging crystal spectrometer and laser blow-off impurity injector have failed to yield unique reconstructions of the transport coefficient profiles. This paper presents a fast, linearized model which was constructed to estimate diagnostic requirements for impurity transport experiments. The analysis shows that the spectroscopic diagnostics on Alcator C-Mod should be capable of inferring simple profiles of impurity diffusion DZ and convection VZ accurate to better than ±10% uncertainty, suggesting that the failure to infer unique DZ and VZ from experimental data is attributable to an inadequate analysis procedure rathermore » -
eqtools: Modular, extensible, open-source, cross-machine Python tools for working with magnetic equilibria
As plasma physics research for fusion energy transitions to an increasing emphasis on cross-machine collaboration and numerical simulation, it becomes increasingly important that portable tools be developed to enable data from diverse sources to be analyzed in a consistent manner. This paper presents eqtools, a modular, extensible, open-source toolkit implemented in the Python programming language for handling magnetic equilibria and associated data from tokamaks. eqtools provides a single interface for working with magnetic equilibrium data, both for handling derived quantities and mapping between coordinate systems, extensible to function with data from different experiments, data formats, and magnetic reconstruction codes, replacingmore » -
X-ray observations of $${{\rm{K}}_{\beta }}$$ emission from medium Z He-like ions in C-Mod tokamak plasmas
X-ray spectra of n = 3 to 1 transitions in He-like ions (and satellites) from calcium, argon and chlorine have been measured in the core of Alcator C-Mod tokamak plasmas using high wavelength resolution x-ray spectrometer systems. The intensity ratio of the intercombination line y3 (1s3p 3P1 - 1s2 1S0) to the resonance line w3 (1s3p 1P1 - 1s2 1S0) is found to be much larger than what is expected if collisional excitation out of the ground state is considered as the only population mechanism for the upper levels. This suggests that recombination and cascades from higher levels with nmore » -
On the ρ*scaling of intrinsic rotation in C-Mod plasmas with edge transport barriers
Here, changes in the core intrinsic toroidal rotation velocity following L- to H- and L- to I-mode transitions have been investigated in Alcator C-Mod tokamak plasmas. The magnitude of the co-current rotation increments is found to increase with the pedestal temperature gradient and $$q_{95}$$ , and to decrease with toroidal magnetic field. These results are captured quantitatively by a model of fluctuation entropy balance which gives the Mach number $$M_i \cong \rho _*/2 L_s/L_T \sim \nabla T q_{95}/B_{T}$$ in an ITG turbulence dominant regime. The agreement between experiment and theory gives confidence for extrapolation to future devices in similar operationalmore » -
High-field side scrape-off layer investigation: Plasma profiles and impurity screening behavior in near-double-null configurations
New experiments on Alcator C-Mod reveal that the favorable impurity screening characteristics of the high-field side (HFS) scrape-off layer (SOL), previously reported for single null geometries, is retained in double null configurations, despite the formation of an extremely thin SOL. In balanced double-null, nitrogen injected locally into the HFS SOL is better screened by a factor of 2.5 compared to the same injection into the low field side (LFS) SOL. This result is insensitive to plasma current and Greenwald fraction. Nitrogen injected into the HFS SOL is not as well screened (only a factor of 1.5 improvement over LFS) inmore » -
Nonlinear gyrokinetic simulations of the I-mode high confinement regime and comparisons with experimenta)
For the first time, nonlinear gyrokinetic simulations of I-mode plasmas are performed and compared with experiment. I-mode is a high confinement regime, featuring energy confinement similar to H-mode, but without enhanced particle and impurity particle confinement [D. G. Whyte et al., Nucl. Fusion 50, 105005 (2010)]. As a consequence of the separation between heat and particle transport, I-mode exhibits several favorable characteristics compared to H-mode. The nonlinear gyrokinetic code GYRO [J. Candy and R. E. Waltz, J Comput. Phys. 186, 545 (2003)] is used to explore the effects of E × B shear and profile stiffness in I-mode and comparemore »