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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Simulation of High-Harmonic Fast-Wave Heating on the National Spherical Tokamak Experiment

Images associated with radio-frequency heating of low-confinement mode plasmas in the National Spherical Tokamak Experiment, as calculated by computer simulation, are presented. The AORSA code has been extended to simulate the whole antenna-to-plasma heating system by including both the kinetic physics of the well-confined core plasma and a poorly confined scrape-off plasma and vacuum vessel structure. The images presented show the 3-D electric wave field amplitude for various antenna phasings. Visualization of the simulation results in 3-D makes clear that -30 degrees phasing excites kilo-volt per meter coaxial standing modes in the scrape-off plasma and shows magnetic-field-aligned whispering-gallery type modes localized to the plasma edge.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Propagation of broadband terahertz pulses through a dense-magnetized-collisional-bounded plasma layer

The terahertz characteristics of a dense-magnetized-collisional-bounded plasma under normal incident are analyzed in this study, which is of practical significance in plasma diagnostics with electromagnetic waves. We theoretically calculate the reflection, absorption, and transmission coefficients for right- and left-handed polarized terahertz waves through a uniform, magnetized, and collisional plasma slab bounded by lossless transparent walls. The power absorption spectra in the frequency range of 0.1-2 THz are given with strong external magnetic fields and different plasma parameters such as plasma density and collisional frequency. Our numerical result is consistent with Jamison's experimental result. It is found that plasma absorption is mainly caused by the collisional absorption and electron cyclotron resonance. Furthermore, the absorption heavily depends on the polarization mode of the terahertz waves when the external magnetic field B is high enough that the election gyrofrequency is near the incident wave frequency. The relationships between the corresponding parameters of the problem are studied numerically.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Scale-selective turbulence reduction in H-mode plasmas in the TJ-II stellarator

Wavenumber spectra of density turbulence in L- and H-mode plasmas have been measured in the TJ-II stellarator by means of Doppler reflectometry. A pronounced suppression of the density fluctuation level is observed in H-mode close to the radial position of maximum radial electric field (E{sub r}) shear. Furthermore, intermediate scale density turbulence is reduced preferentially. This effect can be interpreted within the framework of vortex stretching feeding energy through Reynolds stress into zonal flows, while shear decorrelation of turbulent structures might not play a central role in TJ-II. Moreover, it is shown that in both L- and H-mode, the phase velocity of density fluctuations does not depend on the structure scale.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Hohlraum Reheating from Burning NIF Implosions

As fusion experiments at the National Ignition Facility (NIF) approach and exceed breakeven, energy from the burning capsule is predicted to couple to the gold walls and reheat the hohlraum. For example, on December 5, 2022, experiment N221204 exceeded target breakeven, historically achieving 3.15 MJ of fusion energy from 2.05 MJ of laser drive; for the first time, energy from the igniting capsule reheated the hohlraum beyond the peak laser-driven radiation temperature of 313 eV to a peak of 350 eV, in less than half a nanosecond. This reheating effect has now been unambiguously observed by the two independent Dante calorimeter systems across multiple experiments, and is shown to result from reheating of the remnant tungsten-doped ablator by the exploding core, which is heated by alpha deposition.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Measurements and monitoring of the hydrogen and deuterium contents in the plasma of the L-2M stellarator

The program of experiments on ITER includes a sequential change of the plasma isotopic composition from pure hydrogen plasma in the initial stage of research to deuterium and, then, deuterium-tritium plasma with a gradual increase in the tritium content. In this context, the influence of the plasma isotopic composition on the processes of plasma heating and confinement are being actively studied on the existing tokamaks and stellarators. The plasma isotopic composition also depends on the composition of the gas desorbed from the vacuum chamber wall in the course of recycling. Therefore, the rate of change of the plasma isotopic composition after altering the injected gas also depends on the rate of change of the isotopic composition of the gas absorbed in the wall. These effects were studied in the experiments carried out on the L-2M stellarator in which the working gas was changed from hydrogen to deuterium. Spectral measurements of the intensity ratio between the H{sub {alpha}} and D{sub {alpha}} lines made it possible to monitor the isotopic composition of the plasma in the course of cleaning of the chamber wall from earlier absorbed hydrogen and its replacement with deuterium. After returning to hydrogen, the rate of cleaning of the wall from deuterium was also determined. The results of these experiments show that the plasma isotopic composition varies exponentially with the number N of shots after transition to another isotope, {approx}exp(-N/47). Hence, the isotopic composition can be changed almost completely over 2 to 3 working days. This allows one to study the influence of the plasma isotopic composition on plasma confinement during the same experimental session.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Dynamics of stimulated L → H transitions

We report on model studies of stimulated L → H transitions [K. Miki et al., Phys. Rev. Lett. 110, 195002 (2013)]. These studies use a reduced mesoscale model. Model studies reveal that L → H transition can be triggered by particle injection into a subcritical state (i.e., P<P{sub Thresh}). Particle injection changes edge mean flow shear via changes of density and temperature gradients. The change of edge mean flow shear is critical to turbulence collapse and the subsequent stimulated transition. For low ambient heating, strong injection is predicted to trigger a transient turbulence collapse. Repetitive injection at a period less than the lifetime of the collapsed state can thus maintain the turbulence collapse and so sustain a driven H-mode-like state. The total number of particles required to induce a transition by either injection or gas puffing is estimated. Results indicate that the total number of injected particles required is much smaller than that required for a transition by gas puffing. We thus show that internal injection is more efficient than gas puffing of comparable strength. We also observe that zonal flows do not play a critical role in stimulated transitions. For spontaneous transitions, the spike of the Reynolds work of turbulence on the zonal flow precedes the spike in the mean electric field shear. In contrast, we show that the two are coincident for stimulated transitions, suggesting that there is no causal link between zonal and mean flows for stimulated transitions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Plasma response measurements of non-axisymmetric magnetic perturbations on DIII-D via soft x-ray imaging

Recent observations on DIII-D have advanced the understanding of plasma response to applied resonant magnetic perturbations (RMPs) in both H-mode and L-mode plasmas. Three distinct 3D features localized in minor radius are imaged via filtered soft x-ray emission: (i) the formation of lobes extending from the unperturbed separatrix in the X-point region at the plasma boundary, (ii) helical kink-like perturbations in the steep-gradient region inside the separatrix, and (iii) amplified islands in the core of a low-rotation L-mode plasma. In this study, these measurements are used to test and to validate plasma response models, which are crucial for providing predictive capability of edge-localized mode control. In particular, vacuum and two-fluid resistive magnetohydrodynamic(MHD) responses are tested in the regions of these measurements. At the plasma boundary in H-mode discharges with n = 3 RMPs applied, measurements compare well to vacuum-field calculations that predict lobe structures. Yet in the steep-gradient region, measurements agree better with calculations from the linear resistive two-fluid MHD code, M3D-C1. Relative to the vacuum fields, the resistive two-fluid MHD calculations show a reduction in the pitch-resonant components of the normal magnetic field (screening), and amplification of non-resonant components associated with ideal kink modes. However, the calculations still over-predict the amplitude of the measuredperturbation by a factor of 4. In a slowly rotating L-mode plasma with n = 1 RMPs, core islands are observed amplified from vacuum predictions. Finally, these results indicate that while the vacuum approach describes measurements in the edge region well, it is important to include effects of extended MHD in the pedestal and deeper in the plasma core.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Effects of the magnetic equilibrium on gyrokinetic simulations of tokamak microinstabilities

The general geometry of the experimental tokamak magnetic equilibrium is implemented in the global gyrokinetic simulation code GEM. Compared to the general geometry, the well used Miller parameterization of the magnetic equilibrium is a good approximation in the core region and up to the top of the pedestal. Linear simulations indicate that results with the two geometries agree for r/a ≤ 0.9. However, in the edge region, the instabilities are sensitive to the magnetic equilibrium in both the L-mode and the H-mode plasmas. A small variation of the plasma shaping parameters leads to large changes to the edge instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Parameters of turbulent structures at the periphery of the FT-2 tokamak

Results are presented from probe measurements carried out in the scrape-off layer of the FT-2 tokamak in the course of additional lower hybrid heating, during which an L-H transition was observed. The objective of this study was to obtain information on the parameters of blobs-turbulent structures with enhanced plasma density. The measurements were performed not only on the low-field side of the torus, but also on the high-field side, which is still poorly studied. Coherent structures with radial velocities directed both toward the vessel wall and into the plasma column were revealed at the tokamak periphery. Blobs propagating toward the vessel wall were found to prevail both before and after the L-H transition. The average radial velocity of blobs in the L- and H-modes was determined experimentally. The dependence of the radial blob velocity on the transverse size and density of the structure agrees with the ballooning mode model. It is found that the average value of the poloidal blob velocity is four to five times higher than the average radial velocity. The results of measurements carried out on both sides of the torus indicate the presence of internal poloidal polarization of blobs. The average drift velocity of such polarized structures is directed toward the vessel wall. The L-H transition is accompanied by a reduction in the radial velocity. At the same time, the average plasma density inside the structures observed on the low-field side increases appreciably during the transition. The obtained dependences of the radial blob velocity on the plasma density inside the structure generally agree with predictions of the ballooning mode model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Application of Spatially Resolved High Resolution Crystal Spectrometry to ICF Plasmas

High resolution (λ/Δ λ ~ 10 000) 1D imaging x-ray spectroscopy using a spherically bent crystal and a 2D hybrid pixel array detector is used world wide for Doppler measurements of ion-temperature and plasma flow-velocity profiles in magnetic confinement fusion plasmas. Meter sized plasmas are diagnosed with cm spatial resolution and 10 ms time resolution. This concept can also be used as a diagnostic of small sources, such as inertial confinement fusion plasmas and targets on x-ray light source beam lines, with spatial resolution of micrometers, as demonstrated by laboratory experiments using a 250-μm 55 Fe source, and by ray-tracing calculations. Throughput calculations agree with measurements, and predict detector counts in the range 10-8 -10-6 times source x-rays, depending on crystal reflectivity and spectrometer geometry. Results of the lab demonstrations, application of the technique to the National Ignition Facility (NIF), and predictions of performance on NIF will be presented.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Observation of fully detached divertor integrated with improved core confinement for tokamak fusion plasmas

Integration of divertor detachment with a high-performance (β N ~ 3, β p > 2, H 98 ~ 1.5) core plasma has been demonstrated in DIII-D high-β p (poloidal beta) plasmas associated with a sustained core internal transport barrier (ITB) and an H-mode edge transport barrier (ETB). Such good core-edge integration has been achieved for both neon and nitrogen seeding, for both favorable and unfavorable B-field directions, independently from the impurity puffing locations, though these variations play important roles on divertor characteristics. Compared to the standard H-mode plasmas, the high-β p plasma exhibits a much wider window of detachment compatible with high confinement core. Fully detached divertor plasmas with low plasma temperature (Te < 5 eV), low particle flux, and low heat flux across the entire divertor target plate were obtained by using nitrogen seeding. This detached high-β p plasma is compatible with a newly developed detachment control system which can help optimize the nitrogen gas flow rate. Several features, i.e., the high edge safety factor in the high-β p scenario, impurity injection, closed divertor and reduced heating power requirement due to the high confinement, facilitate the achievement of full divertor detachment at lower density. Instead of degrading global performance, the divertor detachment facilitates the access to an even stronger ITB at large radius with a relatively weak ETB through self-organized synergy between ITB and ETB, leading to sustained high confinement. The strengthening of the large-radius ITB compensates for the ETB degradation associated with divertor detachment. In addition, a weak ETB naturally has smaller edge localized modes (ELMs). In particular, with neon injection, a long-period no-ELM H-mode phase has been achieved simultaneously with high-performance core and partially detached divertor plasmas. Furthermore, these results demonstrate the possibility of integrating excellent core plasma performance with an effective divertor solution, an essential step toward steady-state operation of reactor-grade plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Advanced Design Studies. Final report

The ARIES-CS project was a multi-year multi-institutional project to assess the feasibility of a compact stellarator as a fusion power plant. The work herein describes efforts to help design one aspect of the device, the divertor, which is responsible for the removal of particle and heat flux from the system, acting as the first point of contact between the magnetically confined hot plasma and the outside world. Specifically, its location and topology are explored, extending previous work on the sub ject. An optimized design is determined for the thermal particle flux using a suite of 3D stellarator design codes which trace magnetic field lines from just inside the confined plasma edge to their strike points on divertor plates. These divertor plates are specified with a newly developed plate design code. It is found that a satisfactory thermal design exists which maintains the plate temperature and heat load distribution below tolerable engineering limits.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Development of KSTAR ECE imaging system for measurement of temperature fluctuations and edge density fluctuations

The ECE imaging (ECEI) diagnostic tested on the TEXTOR tokamak revealed the sawtooth reconnection physics in unprecedented detail, including the first observation of high-field-side crash and collective heat transport [H. K. Park, N. C. Luhmann, Jr., A. J. H. Donneet al., Phys. Rev. Lett. 96, 195003 (2006)]. An improved ECEI system capable of visualizing both high- and low-field sides simultaneously with considerably better spatial coverage has been developed for the KSTAR tokamak in order to capture the full picture of core MHD dynamics. Direct 2D imaging of other MHD phenomena such as tearing modes, edge localized modes, and even Alfven eigenmodes is expected to be feasible. Use of ECE images of the optically thin edge region to recover 2D electron density changes during L/H mode transitions is also envisioned, providing powerful information about the underlying physics. The influence of density fluctuations on optically thin ECE is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

On the Origin of Fast Radio Bursts (FRBs)

We derive stringent constraints on the persistent source associated with FRB 121102: size 0.3<R{sub 17.5} =(R/10{sup 17.5} cm)<3, age <10{sup 2.5} year, energy E≈10{sup 49}(ε{sub e}/0.2GeV){sup 3} erg, characteristic electron energy 0.1⩽ε{sub e}/1 GeV⩽0.5; the radiating plasma is confined by a cold plasma of mass M{sub c}<10{sup −1.5}R{sub 17.5}{sup 4} M{sub ⊙}; these properties are inconsistent with typical “magnetar wind nebulae” model predictions. The fact that ε{sub e}∼m{sub p}c{sup 2} suggests that the hot plasma was created by the ejection of a mildly relativistic, M≈E/c{sup 2}≈10{sup −5} M{sub ⊙} shell, which propagated into an extended ambient medium or collided with a pre-ejected shell. Independent of the persistent source model, we suggest a physical mechanism for the generation of fast radio bursts (FRBs): the ejection from an underlying compact object, R{sub s}=10{sup 6}R{sub s,6} cm, of highly relativistic shells with energy E{sub s}=10{sup 41}E{sub 41} erg and Lorentz factor γ{sub s}=10{sup 3}E{sub 41}{sup 1/8}R{sub s,6}{sup −3/8}, into a surrounding e − p plasma with density n∼10{sup −1} cm{sup −3} (consistent with that inferred for the persistent source). For E {sub s} similar to observed FRB energies, plasma conditions appropriate for strong synchrotron maser emission at ν{sub coh.}≈0.5E{sub 41}{sup 1/4}R{sub s,6}{sup −3/4} GHz are formed. A significant fraction of the deposited energy is converted to an FRB with duration R{sub s}/c, accompanied by ∼10 MeV gamma-rays carrying less energy than the FRB. The inferred energy and mass associated with the source suggest some type of a “weak stellar explosion,” where a neutron star is formed with relatively low mass and energy ejection. However, the current upper limit on R does not allow one to rule out M{sub c}∼1 M{sub ⊙}, or the ejection of a larger mass well before the ejection of the confining shell.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Integrated control of edge localized modes and divertor flux using mixed toroidal harmonic resonant magnetic perturbations in EAST

Mixed harmonic resonant magnetic perturbations (RMPs) for integrated edge localized modes (ELMs) and divertor flux control are demonstrated in EAST target plasmas of low input torque and normalized beta β N ~1.7–1.9, which are close to the equivalent value in ITER high Q operation. The applied RMPs are designed to combine a static harmonic of the toroidal mode number n = 3 with a static or rotating harmonic of n = 2. ELM suppression is achieved without a drop of plasma energy confinement, and tungsten concentration is effectively reduced during the application of RMPs. With mixed harmonics, the toroidal varying steady state heat and particle fluxes on the divertor target can be modified with the rotating n = 2 harmonic, which agrees with the numerical modeling of three-dimensional magnetic topology, with plasma responses being taken into account. ELM suppression correlates with the times of larger n = 3 response with mixed n = 2 and n = 3 RMPs. The mixture of harmonics and the rotating n = 2 harmonic does not require additional coil current because the variation is only in the upper-lower coil current phase space. Furthermore, these results further affirm the effectiveness of integrated ELM and divertor flux control using RMPs with mixed harmonics and improve the understanding of the role of plasma responses in ELM suppression.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Direct Measurement of Mass-Temperature Distributions in the Core of Implosions on the National Ignition Facility (Full Technical Final Report)

Inertial confinement fusion plasmas are characterized by several kilo-electron-volt (keV) electron temperatures and ion and electron densities approaching 100 to 1000 times solid. In these implosions which simulate the dynamics of our sun, spectroscopic dopants can lead to detailed exploration of the plasma gradients that are not possible in other places. This is both in the central core pressure and temperature as well as any “mix” resultant from ablation and hydrodynamic interactions. This work will show new Bayesian atomic physics models applied to ICF plasmas to give both the temperature and density of the internal hot spot to better describe the physical nature of these implosions along with trying to spatially resolve the mix coming from the ablator region. The mission impact is to demonstrate this effect and to lead to a more quantitative description of mix in inertial confinement plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1976

Measurements of uranium mass confined in high density plasmas

An X-ray absorption method for measuring the amount of uranium confined in high density, rf-heated uranium plasmas is described. A comparison of measured absorption of 8 keV X-rays with absorption calculated using Beer Law indicated that the method could be used to measure uranium densities from 3 times 10 to the 16th power atoms/cu cm to 5 times 10 to the 18th power atoms/cu cm. Tests were conducted to measure the density of uranium in an rf-heated argon plasma with UF6 infection and with the power to maintain the discharge supplied by a 1.2 MW rf induction heater facility. The uranium density was measured as the flow rate through the test chamber was varied. A maximum uranium density of 3.85 times 10 to the 17th power atoms/cu cm was measured.

Stoeffler, R. C.↗