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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

ICRH operations and experiments during the JET-ILW tritium and DTE2 campaigns

2021 has culminated with the completion of the JET-ILW DTE2 experimental campaign. This contribution summarizes Ion Cyclotron Resonance Heating (ICRH) operations from system and physics point of view. Improvements to the (ICRH) system, to operation procedures and to real time RF power control were implemented to address specific constraints from tritium and deuterium-tritium operations and increase the system reliability and power availability during D-T pulses. ICRH was operated without the ITER-Like Antenna (ILA) because water leaked from an in-vessel capacitor into the vessel on day-2 of the D-T campaign. Three weeks were required to identify and isolate the leak and resume plasma operations. Dedicated RF-Plasma Wall Interaction (PWI) experiments were conducted; tritium plasmas exhibit a higher level of Be sputtering on the outer wall and impurity content when compared to deuterium or hydrogen plasmas. The JET-DTE2 campaigns provided the opportunity to characterize ICRH schemes foreseen for the ITER operation, in the ITER like wall environment in ELMy H-mode scenarios aiming at maximizing fusion performance. The second harmonic tritium resonance heating and to a lesser extent minority 3He heating (ITER D-T ICRH reference schemes) lead to improved ion temperature and fusion performance when compared to hydrogen minority ICRH. However, these discharges suffered from a lack of stationarity and gradual impurity accumulation potentially because of a deficit of ICRH power when using JET antennas at lower frequencies. Fundamental deuterium ICRH was used in tritium-rich plasmas and with deuterium Neutral Beam Heating; this ICRH scheme proved to be very efficient boosting ion temperature and fusion performance in these plasmas.

Klepper, C. Christopher↗
NASA NTRS2012

Free-Free Transitions of the e-H System Inside a Dense Plasma Irradiated by a Laser Field at Very Low Incident-Electron Energies

The free-free transition is studied for an electron-hydrogen atom in ground state when a low-energy electron (external) is injected into hydrogenic plasma in the presence of an external homogenous, monochromatic, and linearly polarized laser field. The effect of plasma screening is considered in the Debye-Huckel approximation. The calculations are performed in the soft photon limit. The incident electron is considered to be dressed by the laser field in a nonperturbative manner by choosing the Volkov solutions in both the initial and final channels. The space part of the scattering wave function for the electron is solved numerically by taking into account the electron exchange. The laser-assisted differential and total cross sections are calculated for single-photon absorption or emission and no-photon exchange in the soft photon limit, the laser intensity being much less than the atomic field intensity. The calculations have been carried out for various values of Debye parameter, ranging from 0.005 to 0.12. A strong suppression is noted in the laser-assisted cross sections as compared to the field-free situation. A significant difference is noted for the singlet and triplet cross sections. The suppression is much more in the triplet states.

Bhatia, A. K.↗
DOE OSTI2023

Review of the 1st EUV Light Sources Code Comparison Workshop

We review the results of the 1 st Extreme Ultraviolet (EUV) Light Sources Code Comparison Workshop. The goal of this workshop was to provide a platform for specialists in EUV light source plasma modeling to benchmark and validate their numerical codes using well-defined case studies. Detailed consideration of a plethora of atomic collisional and radiative processes is required for modeling EUV light source plasmas. Eight institutions spanning four countries contributed data to the workshop. Two topics were addressed, namely (i) the atomic kinetics and radiative properties of tin plasmas under EUV-generating conditions and (ii) laser absorption in a fully ionized, one-dimensional hydrogen plasma. In this paper, we summarize the key findings of the workshop and outline plans for future iterations of the code comparison activity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Dependence of ion-cyclotron range of frequencies instabilities on species mix and fast-ion distribution: I. Stability

Ion cyclotron emission (ICE) driven by fast ions has the potential to be a reactor relevant diagnostic of confined and escaping fast ions; moreover, controlled experiments in magnetic fusion devices can shed light on similar magnetospheric instabilities below and above the ion cyclotron frequency f ci . Dedicated DIII-D experiments studied instabilities below f ci and at cyclotron harmonics l f ci by injecting thirteen different neutral beam populations into plasmas with different thermal compositions of H, D, and 3He at five different values of toroidal field. High-frequency magnetic loops diagnose mode properties. Each beam type is individually injected for an approximate slowing-down time (~100 ms), resulting in a database of 2529 conditions that are analyzed in frequency bands corresponding to ICE harmonics and < f ci global Alfvén eigenmodes. The sub-cyclotron modes are more unstable at low field and in plasmas with large H concentration. For D injection, first harmonic ICE at f cD is more unstable in hydrogen plasmas but, for H injection, the converse is true (ICE at f cH is more unstable in D), suggesting that thermal same-species ions damp fundamental ICE. ICE stability also depends upon the pitch-angle anisotropy gradient and upon the ‘bump-on-tail’ gradient ∂f/∂v. Although the frequency of unstable ICE in these plasmas always occurs near the central cyclotron frequency, the precise value also depends on the Alfvén speed and beam direction, consistent with the idea that instability occurs for modes that simultaneously satisfy ω ≃ 2πlf ci , the magnetosonic dispersion relation, and the Doppler-shifted resonance condition.

electromagnetic ion cyclotron instabilities↗
DOE OSTI2023

Effect of He seeding on hydrogen isotope permeation in tungsten by H-D mixed plasma exposure

Hydrogen (H) and deuterium (D) permeation fluxes for tungsten (W) by H-D mixed plasma were studied to clarify the hydrogen isotope effect on permeation by a newly designed plasma driven permeation (PDP) device at Shizuoka University. A mixed H-D plasma was produced by introducing H and D gasses with various ratios. The atomic ratio and permeation flux in the plasma were measured during the experiments. It was found that the H:D atomic ratio in the plasma was almost proportional to the introduced H:D gas ratio. When single species of H or D was irradiated, the H permeation rate showed approximately 1.6 times as high as that of D. Under H-D mixed plasma irradiation, the highest HD permeation rate was observed when the atomic ratio of H:D in the plasma was 50:50. In addition, the effect of He seeding on H-D mixed plasma driven permeation behavior was also evaluated. The recycling of hydrogen isotopes on the upstream side was enhanced due to the formation of He bubbles near the surface, which would prevent the dissolution of hydrogen isotopes into bulk W and enhance the reflection process during plasma exposure. H and D permeation fluxes were clearly reduced by seeding He in H-D mixed plasma. At lower temperature (~723 K), the ratio of steady permeation fluxes of H and D was about 75 : 25, confirming the high permeation flux of H . These results have demonstrated a significant hydrogen isotope effect, even if He was seeded or not. As the temperature increased, H and D permeation fluxes approached unity at 923 K. Therefore,this indicates that the hydrogen isotope effect was determined by the combination of recycling / reflection at or beneath the surface, which will be controlled by the temperature during plasma exposure.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1982

Observations of structuring in the downstream region of a large spherical model in a laboratory simulated solar wind plasma

The effects of inserting a spherical conducting model, large in comparison with the Debye length, into a free streaming high-energy 1 kV) unmagnetized hydrogen plasma are investigated in order to measure energies and compositions directly relevant to solar wind and astrophysical plasma phenomena. Holding the incident plasma parameters constant, transverse profiles of the net Langmuir probe current are plotted at various locations downstream in the model wake and are divided into three regions (the shadow, transition, and boundary). Results attributable to the use of a high-energy plasma show that enhancements in the shadow exist at downstream locations where the Mach ratio is less than one, and turbulence exists in the transition region on the shadow edges and outside in the boundary region. In addition, a small current enhancement is found in the boundary and can be attributed to the plasma/model interaction. It is concluded that many similar features observed by spacecraft downstream from planetary bodies are relatively permanent and are due to the intrinsic nature of the interaction between the solar wind plasma and the obstacle.

Intriligator, D. S.↗
DOE OSTI2024

Avoiding fusion plasma tearing instability with deep reinforcement learning

For stable and efficient fusion energy production using a tokamak reactor, it is essential to maintain a high-pressure hydrogenic plasma without plasma disruption. Therefore, it is necessary to actively control the tokamak based on the observed plasma state, to manoeuvre high-pressure plasma while avoiding tearing instability, the leading cause of disruptions. This presents an obstacle-avoidance problem for which artificial intelligence based on reinforcement learning has recently shown remarkable performance. However, the obstacle here, the tearing instability, is difficult to forecast and is highly prone to terminating plasma operations, especially in the ITER baseline scenario. Previously, we developed a multimodal dynamic model that estimates the likelihood of future tearing instability based on signals from multiple diagnostics and actuators. Here we harness this dynamic model as a training environment for reinforcement-learning artificial intelligence, facilitating automated instability prevention. We demonstrate artificial intelligence control to lower the possibility of disruptive tearing instabilities in DIII-D, the largest magnetic fusion facility in the United States. The controller maintained the tearing likelihood under a given threshold, even under relatively unfavourable conditions of low safety factor and low torque. In particular, it allowed the plasma to actively track the stable path within the time-varying operational space while maintaining H-mode performance, which was challenging with traditional preprogrammed control. This controller paves the path to developing stable high-performance operational scenarios for future use in ITER.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2012

Plasma Treatment to Remove Carbon from Indium UV Filters

The sounding rocket experiment FIRE (Far-ultraviolet Imaging Rocket Experiment) will improve the science community fs ability to image a spectral region hitherto unexplored astronomically. The imaging band of FIRE (.900 to 1,100 Angstroms) will help fill the current wavelength imaging observation hole existing from approximately equal to 620 Angstroms to the GALEX band near 1,350 Angstroms. FIRE is a single-optic prime focus telescope with a 1.75-m focal length. The bandpass of 900 to 1100 Angstroms is set by a combination of the mirror coating, the indium filter in front of the detector, and the salt coating on the front of the detector fs microchannel plates. Critical to this is the indium filter that must reduce the flux from Lymanalpha at 1,216 Angstroms by a minimum factor of 10(exp -4). The cost of this Lyman-alpha removal is that the filter is not fully transparent at the desired wavelengths of 900 to 1,100 Angstroms. Recently, in a project to improve the performance of optical and solar blind detectors, JPL developed a plasma process capable of removing carbon contamination from indium metal. In this work, a low-power, low-temperature hydrogen plasma reacts with the carbon contaminants in the indium to form methane, but leaves the indium metal surface undisturbed. This process was recently tested in a proof-of-concept experiment with a filter provided by the University of Colorado. This initial test on a test filter showed improvement in transmission from 7 to 9 percent near 900 with no process optimization applied. Further improvements in this performance were readily achieved to bring the total transmission to 12% with optimization to JPL's existing process.

Greer, Harold F.↗
NASA NTRS1972

The possibility of supersonic plasma flow in a collapsing post-sunset ionosphere.

As a result of the rapidly decreasing pressure in the topside ionosphere during twilight hours, a rapid downward flow of hydrogen plasma from the protonosphere takes place. In the case of steady state, isothermal, frictionless flow, the criterion for the existence of a critical point (transition to supersonic flow) above 1000 km is that the plasma temperature be lower than a certain limiting temperature which is a function of the field line considered. In the latitude region between 40 and 70 deg, this upper temperature limit varies from 963 to 1066 K. Since these temperatures are considerably lower than the observed temperatures, it follows that in the case of steady state, isothermal flow the velocities will always remain subsonic. When the effect of the neglected terms is examined, the temperature gradient is shown to exert the strongest influence on the nature of the flow. It is concluded that there is a definite possibility that supersonic downward flows in a post-sunset topside ionosphere may occur.

Fontheim, E. G.↗
NASA NTRS1979

Dielectronic recombination rates, ionization equilibrium, and radiative energy-loss rates for neon, magnesium, and sulfur ions in low-density plasmas

Results of detailed and systematic calculations are presented for the total dielectronic recombination rate coefficients for the ions of Ne, Mg, and S in a low-density predominantly hydrogen plasma. The new recombination rates are used to calculate solar corona ionization-equilibrium distributions of the ions. The most important effect of dielectronic recombination for ions in corona equilibrium is found to be a shift in the maximum-abundance temperatures toward higher temperatures, which are in some cases reduced from those predicted on the basis of the simple Burgess formula.

Jacobs, V. L.↗
DOE OSTI2022

Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development

An overview of recent results obtained at the tokamak ASDEX Upgrade (AUG) is given. A work flow for predictive profile modelling of AUG discharges was established which is able to reproduce experimental H-mode plasma profiles based on engineering parameters only. In the plasma center, theoretical predictions on plasma current redistribution by a dynamo effect were confirmed experimentally. For core transport, the stabilizing effect of fast ion distributions on turbulent transport is shown to be important to explain the core isotope effect and improves the description of hollow low-Z impurity profiles. The L–H power threshold of hydrogen plasmas is not affected by small helium admixtures and it increases continuously from the deuterium to the hydrogen level when the hydrogen concentration is raised from 0 to 100%. One focus of recent campaigns was the search for a fusion relevant integrated plasma scenario without large edge localised modes (ELMs). Results from six different ELM-free confinement regimes are compared with respect to reactor relevance: ELM suppression by magnetic perturbation coils could be attributed to toroidally asymmetric turbulent fluctuations in the vicinity of the separatrix. Stable improved confinement mode plasma phases with a detached inner divertor were obtained using a feedback control of the plasma β. The enhanced D α H-mode regime was extended to higher heating power by feedback controlled radiative cooling with argon. The quasi-coherent exhaust regime was developed into an integrated scenario at high heating power and energy confinement, with a detached divertor and without large ELMs. Small ELMs close to the separatrix lead to peeling-ballooning stability and quasi continuous power exhaust. Helium beam density fluctuation measurements confirm that transport close to the separatrix is important to achieve the different ELM-free regimes. Based on separatrix plasma parameters and interchange-drift-Alfvén turbulence, an analytic model was derived that reproduces the experimentally found important operational boundaries of the density limit and between L- and H-mode confinement. Feedback control for the X-point radiator (XPR) position was established as an important element for divertor detachment control. Stable and detached ELM-free phases with H-mode confinement quality were obtained when the XPR was moved 10 cm above the X-point. Investigations of the plasma in the future flexible snow-flake divertor of AUG by means of first SOLPS-ITER simulations with drifts activated predict beneficial detachment properties and the activation of an additional strike point by the drifts.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1973

Active cleaning techniques for removing contamination from optical surfaces in space

Research in developing an active cleaning technique for removing contaminants from optical surfaces in space is reported. In situ contamination/cleaning experiments were conducted on gold and platimum coated mirrors, which were contaminated by exposure to UV radiation in a 1,3, butadiene environment. Argon and oxygen plasma exposure cleaned the mirrors equally well. Silicone cleaning experiments were also conducted. Exposure of the contaminated mirrors to helium, oxygen, and hydrogen plasmas restored the reflectance at the shorter wavelengths and degraded it at the longer wavelengths.

Shannon, R. L.↗
DOE OSTI2024

Matched Guiding and Controlled Injection in Dark-Current-Free, 10-GeV-Class, Channel-Guided Laser-Plasma Accelerators

Here we measure the high-intensity laser propagation throughout meter-scale, channel-guided laser-plasma accelerators by adjusting the length of the plasma channel on a shot-by-shot basis, showing high-quality guiding of 500 TW laser pulses over 30 cm in a hydrogen plasma of density n 0 ≈ 1 X 10 17 cm -3 . We observed transverse energy transport of higher-order modes in the first ≈ 12 cm of the plasma channel, followed by quasimatched propagation, and the gradual, dark-current-free depletion of laser energy to the wake. We quantify the laser-to-wake transfer efficiency limitations of currently available petawatt-class lasers and demonstrate via simulation how control over the laser mode can significantly improve beam parameters. Using 21.3 J of laser energy, and triggering localized electron injection, we observed electron bunches with single, quasimonoenergetic peaks up to 9.2 GeV with charge extending beyond 10 GeV.

43 PARTICLE ACCELERATORS↗
DOE OSTI2020

Applications of plasma-enhanced metalorganic chemical vapor deposition

The use of plasma enhancement for growth of III-V compound semiconductor materials by metalorganic chemical vapor deposition (MOCVD) is examined, to improve control of microstructure, develop understanding of the underlying growth mechanisms, and expand the range of materials combinations for photovoltaic and solid-state lighting applications. Whereas plasma is commonly used with group-IV materials, such as silicon, few studies have examined the impact of plasma-enhanced MOCVD (PE-MOCVD) for III-V materials growth. PE-MOCVD provides improved decomposition of metalorganic precursors, which is driven by hydrogen plasma to augment hydride reactions and thermal pyrolysis, but plasma generation requires low reactor pressure. We have demonstrated elemental Al films grown by PE-MOCVD, which show distinct crystallographic texturing, and assume epitaxial microstructure upon post-growth annealing. We also demonstrate PE-MOCVD of GaAs at temperatures as low as 300 °C. With increasing radio-frequency power, the GaAs growth rate shows a transition from power limited to mass-transport limited. PE-MOCVD grown Ga 0.5 In 0.5 P shows no detectable spontaneous atomic ordering, which offers a mechanism to form order/disorder unicompositional heterostructures, without temperature change or impurity incorporation. In its current implementation, PE-MOCVD grown films have shown microstructural degradation, that we attribute to direct plasma exposure during growth. Improvements in the system design and growth sequences are at the focus of on-going efforts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Self‐consistent transport simulation of boron dust particle injection in the peripheral plasma in Large Helical Device

Abstract The trajectories and the ablation positions of boron dust particles dropped from an impurity powder dropper in the peripheral plasma in the Large Helical Device (LHD) were calculated using a three‐dimensional edge plasma simulation code (EMC3‐EIRENE) and a dust transport simulation code (DUSTT). The simulation shows that the trajectory of the boron dust particles is deflected at the upper divertor leg due to the effect of the hydrogen plasma flow, and the ablation positions of the dust particles in an ergodic layer change toward the outboard side of the torus for higher plasma densities. The effect of the boron ion flow in the divertor leg on the deflection is investigated by coupling the two codes self‐consistently. The simulation predicts that the boron ions in the divertor leg, which are produced by sputtering on the divertor plates, which do not affect the change in the ablation positions. It also shows that the ablation positions move toward the inboard side and approach the Last Closed Flux Surface (LCFS) in case of increased boron dust drop rates, which is caused by the lowered plasma flow in the upper divertor leg due to the lowered electron temperature by radiation cooling by the dropped dust particles.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1983

Steady-state turbulence with a narrow inertial range

Coupled two-dimensional wave equations are solved on a computer to model Langmuir wave turbulence excited by a weak electron beam. The model includes wave growth due to beam-plasma interaction, and dissipation by Landau damping. The inertial range is limited to a relatively small number of modes such as could occur when the ratio of masses between the negative and positive ions is larger than in a hydrogen plasma, or when there is damping in long wavelength Langmuir waves. A steady state is found consisting of quasistable, collapsed wave packets. The effects of different beam parameters and the assumed narrow inertial range are considered. The results may be relevant to plasma turbulence observed in connection with type III solar bursts.

Weatherall, J. C.↗
NASA NTRS1980

Parametric dependence of ion temperature and electron density in the SUMMA hot-ion plasma using laser light scattering and emission spectroscopy

Hot-ion plasma experiments were conducted in the NASA Lewis SUMMA facility. A steady-state modified Penning discharge was formed by applying a radially inward dc electric field of several kilovolts near the magnetic mirror maxima. Results are reported for a hydrogen plasma covering a wide range in midplane magnetic flux densities from 0.5 to 3.37 T. Input power greater than 45 kW was obtained with water-cooled cathodes. Steady-state plasmas with ion kinetic temperatures from 18 to 830 eV were produced and measured spectroscopically. These ion temperatures were correlated with current, voltage, and magnetic flux density as the independent variables. Electron density measurements were made using an unusually sensitive Thomson scattering apparatus. The measured electron densities range from 2.1 x 10 to the 11th to 6.8 x 10 to the 12th per cu cm.

Snyder, A.↗
DOE OSTI2021

Plasma radiation behavior approaching high-radiation scenarios in W7-X

The W7-X stellarator has so far performed experiments under both limiter and divertor conditions. The plasma is mostly generated by ECR-heating with powers up to 6.5 MW, and the plasma density is usually limited by the radiation losses from low-Z impurities (such as carbon and oxygen) released mainly from the graphite targets. The present work first summarizes the radiation loss fractions f rad achieved in quasi-stationary hydrogen plasmas in both operational phases, and then shows how impurity radiation behaves differently with the two different boundary conditions as the plasma density increases. The divertor operation is emphasized and some beneficial effects (with respect to impurity radiation) are highlighted: (1) intensive radiation is located at the edge (r/a > 0.8) even at high radiation loss fractions, (2) the plasma remains stable up to f rad approaching unity, (3) the reduction in the stored energy is about 10% for high f rad scenarios. Moreover, effects of wall boronisation on impurity radiation profiles are also presented.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗