Search NASA⌕ Search

Engineering topics

Reinke, M L

Publications and source records attributed to Reinke, M L.

13 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Kinetic neoclassical calculations of impurity radiation profiles

Modifications of the drift-kinetic transport code XGC0 to include thetransport, ionization, and recombination of individual charge states,as well as the associated radiation, are described. The code is firstapplied to a simulation of an NSTX H-mode discharge with carbonimpurity to demonstrate the approach to coronal equilibrium. Theeffects of neoclassical phenomena on the radiated power profile areexamined sequentially through the activation of individual physicsmodules in the code. Orbit squeezing and the neoclassical inwardpinch result in increased radiation for temperatures above a fewhundred eV and changes to the ratios of charge state emissions at agiven electron temperature. Analogous simulations with a neonimpurity yield qualitatively similar results.

Spherical Torus↗
DOE OSTI2012

Lower hybrid current drive at high density in the multi-pass regime

Assessing the performance of lower hybrid current drive (LHCD) at high density is critical for developing non-inductive current drive systems on future steady-state experiments. Excellent LHCD efficiency has been observed during fully non-inductive operation ({eta}=2.0-2.5 Multiplication-Sign 10{sup 19} AW{sup -1}m{sup -2} at n{sub e}=0.5 Multiplication-Sign 10{sup 20} m{sup -3}) on Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] under conditions (n{sub e}, magnetic field and topology, and LHCD frequency) relevant to ITER [S. Shiraiwa et al., Nucl. Fusion 51, 103024 (2011)]. To extend these results to advanced tokamak regimes with higher bootstrap current fractions on C-Mod, it is necessary to increase n{sub e} to 1.0-1.5 Multiplication-Sign 10{sup 20} m{sup -3}. However, the number of current-carrying, non-thermal electrons generated by LHCD drops sharply in diverted configurations at densities that are well below the density limit previously observed on limited tokamaks. In these cases, changes in scrape off layer (SOL) ionization and density profiles are observed during LHCD, indicating that significant power is transferred from the LH waves to the SOL. Fokker-Planck simulations of these discharges utilizing ray tracing and full wave propagation codes indicate that LH waves in the high density, multi-pass absorption regime linger in the plasma edge, and SOL region, where absorption near or outside the LCFS results in the loss of current drive efficiency. Modeling predicts that non-thermal emission increases with stronger single-pass absorption. Experimental data show that increasing T{sub e} in high density LH discharges results in higher non-thermal electron emission, as predicted by the models.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Laboratory calibration of density-dependent lines in the extreme ultraviolet spectral region

We have been making spectral measurements in the extreme ultraviolet (EUV) from different laboratory sources in order to investigate the electron density dependence of various astrophysically important emission lines and to test the atomic models underlying the diagnostic line ratios. The measurement are being performed at the Livermore EBIT-I electron beam ion trap, the National Spherical Torus Experiment (NSTX) at Princeton, and the Alcator C-Mod tokamak at the Massachusetts Institute of Technology, which together span an electron density of four orders of magnitude and which allow us to test the various models at high and low density limits. Here we present measurements of Fe XXII and Ar XIV, which include new data from an ultra high resolution ({lambda}/{Delta}{lambda} >4000) spectrometer at the EBIT-I facility. We found good agreement between the measurements and modeling calculations for Fe XXII, but poorer agreement for Ar XIV.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Measurement of plasma current dependent changes in impurity transport and comparison with nonlinear gyrokinetic simulation

Measured impurity transport coefficients are found to demonstrate a strong dependence on plasma current in the core of Alcator C-Mod. These measurements are compared directly with linear and nonlinear gyrokinetic simulation in an attempt to both qualitatively and quantitatively reproduce the measured impurity transport. Discharges constituting a scan of plasma current from 0.6 to 1.2 MA were performed during the 2010 run campaign. The impurity transport from these discharges was determined using a novel set of spectroscopic diagnostics available on Alcator C-Mod. This diagnostic suite allowed for the effective constraint of impurity transport coefficient profiles inside of r/a = 0.6. A decrease in the measured impurity diffusivity and inward convection is found with increased plasma current. Global, nonlinear gyrokinetic simulations were performed using the GYRO code [J. Candy and R. E. Waltz, J Comput. Phys. 186, 545 (2003)] for all discharges in the experimental scan and are found to reproduce the experimental trends, while demonstrating good quantitative agreement with measurement. A more comprehensive quantitative comparison was performed on the 0.8 MA discharge of the current scan which demonstrates that simultaneous agreement between experiment and simulation in both the impurity particle transport and ion heat transport channels is attainable within experimental uncertainties.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Production of internal transport barriers via self-generated mean flows in Alcator C-Mod

New results suggest that changes observed in the intrinsic toroidal rotation influence the internal transport barrier (ITB) formation in the Alcator C-Mod tokamak [E. S. Marmar and Alcator C-Mod group, Fusion Sci. Technol. 51, 261 (2007)]. These arise when the resonance for ion cyclotron range of frequencies (ICRF) minority heating is positioned off-axis at or outside of the plasma half-radius. These ITBs form in a reactor relevant regime, without particle or momentum injection, with Ti Almost-Equal-To Te, and with monotonic q profiles (q{sub min} < 1). C-Mod H-mode plasmas exhibit strong intrinsic co-current rotation that increases with increasing stored energy without external drive. When the resonance position is moved off-axis, the rotation decreases in the center of the plasma resulting in a radial toroidal rotation profile with a central well which deepens and moves farther off-axis when the ICRF resonance location reaches the plasma half-radius. This profile results in strong E Multiplication-Sign B shear (>1.5 Multiplication-Sign 10{sup 5} rad/s) in the region where the ITB foot is observed. Gyrokinetic analyses indicate that this spontaneous shearing rate is comparable to the linear ion temperature gradient (ITG) growth rate at the ITB location and is sufficient to reduce the turbulent particle and energy transport. New and detailed measurement of the ion temperature demonstrates that the radial profile flattens as the ICRF resonance position moves off axis, decreasing the drive for the ITG the instability as well. These results are the first evidence that intrinsic rotation can affect confinement in ITB plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Ohmic energy confinement saturation and core toroidal rotation reversal in Alcator C-Mod plasmas

Ohmic energy confinement saturation is found to be closely related to core toroidal rotation reversals in Alcator C-Mod tokamak plasmas. Rotation reversals occur at a critical density, depending on the plasma current and toroidal magnetic field, which coincides with the density separating the linear Ohmic confinement regime from the saturated Ohmic confinement regime. The rotation is directed co-current at low density and abruptly changes direction to counter-current when the energy confinement saturates as the density is increased. Since there is a bifurcation in the direction of the rotation at this critical density, toroidal rotation reversal is a very sensitive indicator in the determination of the regime change. The reversal and confinement saturation results can be unified, since these processes occur in a particular range of the collisionality.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Lower Hybrid Wave Induced SOL Emissivity Variation at High Density on the Alcator C-Mod Tokamak

Lower Hybrid Current Drive (LHCD) in the Alcator C-Mod tokamak provides current profile control for the generation of Advanced Tokamak (AT) plasmas. Non-thermal electron bremsstrahlung emission decreases dramatically at n-bar{sub e}>1{center_dot}10{sup 20}[m{sup -3}] for diverted discharges, indicating low current drive efficiency. It is suggested that Scrape-Off-Layer (SOL) collisional absorption of LH waves is the cause for the absence of non-thermal electrons at high density. VUV and visible spectroscopy in the SOL provide direct information on collision excitation processes. Deuterium Balmer-, Lyman- and He-I transition emission measurements were used for initial characterization of SOL electron-neutral collisional absorption. Data from Helium and Deuterium LHCD discharges were characterized by an overall increase in the emissivity as well as an outward radial shift in the emissivity profile with increasing plasma density and applied LHCD power. High-temperature, high-field (T{sub e} = 5keV,B{sub t} = 8T) helium discharges at high density display increased non-thermal signatures as well as reduced SOL emissivity. Variations in emissivity due to LHCD were seen in SOL regions not magnetically connected to the LH Launcher, indicating global SOL effects due to LHCD.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

ICRF Mode Conversion Flow Drive Experiments on Alcator C-Mod

We have carried out a detailed study of the dependence of ICRF mode conversion flow drive (MCFD) on plasma and RF parameters. The flow drive efficiency is found to depend strongly on the {sup 3}He concentration in D({sup 3}He) plasmas, a key parameter separating the ICRF minority heating regime and mode conversion regime. At +90 deg. antenna phasing (waves in the co-I{sub p} direction) and dipole phasing, the driven flow is in the co-I{sub p} direction, and the change of the rotation velocity increases with both P{sub RF} and I{sub p}, and scales unfavorably vs. plasma density and antenna frequency. When MCFD is applied to I-mode plasmas, the plasma rotation increases until the onset of MHD modes triggered by large sawtooth crashes. Very high performance I-mode plasmas with H{sub ITER98,y2}{approx}1.4 and T{sub e0}{approx}8 keV have been obtained in these experiments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Lower hybrid current drive in a high density diverted tokamak

Experimental observations of LHCD at high density (n-bar{sub e}>10{sup 20}m{approx}{sup 3}) on the Alcator C-Mod tokamak are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops sharply in single null discharges well below the density limit previously observed on limited tokamaks ({omega}/{omega}{sub LH}{approx}2). Modeling and experimental evidence suggest that the absence of LH driven fast electrons at high density may be due to collisional absorption in the scrape off layer. Experiments show that the expected current drive density dependence is recovered for inner wall limited discharges across the range of densities scanned (0.5x10{sup 20} m{sup -3}<n-bar{sub e}<1.5x10{sup 20} m{sup -3}). Increasing T{sub e} in the periphery of the plasma (0.8<r/a<1.0) also results in a modest increase in non-thermal electron emission at high n-bar{sub e}. Ray tracing/Fokker-Planck simulations of these discharges predict the observed sensitivity to plasma position when the effects of collisional absorption in the SOL are included in the model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Observation of Co and Counter Rotation Produced by Lower Hybrid Waves in Alcator C-Mod

Lower hybrid waves launched uni-directionally into tokamak plasmas impart momentum to the electrons. This momentum can be transferred to the ions, leading to substantial counter current rotation. Observations of LH-induced counter rotation have been previously reported [1], and the initial rate of increase has been found to be consistent with the calculated rate of wave momentum injection [2]. However, in recent experiments in Alcator C-Mod it has been found that application of LH waves to relatively low current (I{sub p}{approx}0.4-0.6 MA) plasmas can result in a co-current change of rotation, which implies a different mechanism than that described above. This appears to be linked to the so-called intrinsic rotation commonly observed in Alcator C-Mod and other tokamaks [3]. In addition to the change in direction at low current, some dependence on the magnetic configuration (USL vs. LSN) has been observed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Estimation of the ion toroidal rotation source due to momentum transfer from Lower Hybrid waves in Alcator C-Mod

Significant ion toroidal rotation (50km/s) has been measured by X-Ray spectroscopy for impurities in Alcator C-Mod during lower hybrid (LH) RF power injection. We investigate the relation between the computed toroidal momentum input from LH waves and the measured INITIAL change of ion toroidal rotation when the LH power is turned on. The relation may depend on the plasma current and magnetic configuration. Because of the fast build up time of the electron quasilinear plateau (<1 millisecond), the electron distribution function rapidly reaches steady state in which the electrons transfer momentum to the ions. The LH wave momentum input is computed from the self consistent steady state electron distribution function and a bounce-averaged quasilinear diffusion coefficient that are obtained by iterating a full wave code (TORLH) with a Fokker Plank code (CQL3D)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Rotation Reversal Bifurcation and Energy Confinement Saturation in Tokamak Ohmic L-Mode Plasmas

Direction reversals of intrinsic toroidal rotation have been observed in diverted Alcator C-Mod Ohmic L-mode plasmas following electron density ramps. For low density discharges, the core rotation is directed cocurrent, and reverses to countercurrent following an increase in the density above a certain threshold. Such reversals occur together with a decrease in density fluctuations with 2 cm{sup -1}{<=}k{sub {theta}}{<=}11 cm{sup -1} and frequencies above 70 kHz. There is a strong correlation between the reversal density and the density at which the Ohmic L-mode energy confinement changes from the linear to the saturated regime.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗