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Regimes of cosmic-ray diffusion in Galactic turbulence
No full textCosmic-ray transport in astrophysical environments is often dominated by the diffusion of particles in a magnetic field composed of both a turbulent and a mean component. This process, which is two-fold turbulent mixing in that the particle motion is stochastic with respect to the field lines, needs to be understood in order to properly model cosmicray signatures. One of the most important aspects in the modeling of cosmic-ray diffusion is that fully resonant scattering, the most effective such process, is only possible if the wave spectrum covers the entire range of propagation angles. By taking the wave spectrum boundaries into account, we quantify cosmic-ray diffusion parallel and perpendicular to the guide field direction at turbulence levels above 5% of the total magnetic field. We apply our results of the parallel and perpendicular diffusion coefficient to the Milky Way. We show that simple purely diffusive transport is in conflict with observations of the inner Galaxy, but that just by taking a Galactic wind into account, data can be matched in the central 5 kpc zone. Further comparison shows that the outer Galaxy at >5 kpc, on the other hand, should be dominated by perpendicular diffusion, likely changing to parallel diffusion at the outermost radii of the Milky Way.</p
Physics of the high specific impulse alternative low power hybrid ion engine (alphie): Direct thrust measurements and plasma plume kinetics
No full textThe Alternative Low Power Ion Engine (alphie) is a high specific impulse plasma thruster different from conventional gridded ion engines (GIEs). It uses only one external cathode and ions and electrons flow through the open spaces of its two grids, whereas only ions are transported through the GIE ion optics. Ionizing electrons from the cathode move inward to the alphie ionization chamber and ions, which are neutralized by electrons from the same cathode, exit along the opposite direction. These currents together with the voltages applied to the grids produce a self-consistent electric field that accelerates the charges. The one-dimensional ion velocity distribution and the electron energy spectra in the collisionless alphie plasma plume are studied along its axial axis of symmetry. The thruster produces a mesothermal plasma flow with a non-monotone plasma potential profile along the axial direction. The ion populations observed are of those accelerated by the self-consistent electric field and a low velocity group that results from the charge exchange collisions in the thruster. Both populations remain essentially unaltered in the plasma flow. Conversely, the two electron groups observed merge along the axial direction of the plume following the changes in the plasma potential. The temperatures of ion populations are high by the neutral gas heating inside the thruster by high-energy ionizing electrons. The direct measurement of thrusts of 0.8–3.5 mN for argon gives 13 900–20 000 s specific impulses. These high values might be explained by the additional contribution to the thrust by the remaining non-ionized hot neutral gas effusion through the apertures of grids
Efficient organic solar cells with small energy losses based on a wide-bandgap trialkylsilyl-substituted donor polymer and a non-fullerene acceptor
No full textInsight into CO2 Dissociation Kinetics in Microwave Plasma using Laser Scattering
No full textEmbargo 1 year, pdf open access 1-3-202
Integrated modelling and multiscale gyrokinetic validation study of ETG turbulence in a JET hybrid H-mode scenario
No full textPrevious studies with first-principle-based integrated modelling suggested that ETG turbulence may lead to an anti-GyroBohm isotope scaling in JET high-performance hybrid H-mode scenarios. A dedicated comparison study against higher-fidelity turbulence modelling invalidates this claim. Ion-scale turbulence with magnetic field perturbations included, can match the power balance fluxes within temperature gradient error margins. Multiscale gyrokinetic simulations from two distinct codes produce no significant ETG heat flux, demonstrating that simple rules-of-thumb are insufficient criteria for its onset.</p
Power balance analysis at the L-H transition in JET-ILW NBI-heated deuterium plasmas
No full textThe understanding of the physics underlying the L-H transition has strong implications for ITER experimental reactor and demonstration power plant (DEMO). In many tokamaks, including JET, it has been observed that, at a particular plasma density, n e,min , the power necessary to access H-mode P L-H is minimum. In the present work, L-H transitions of JET deuterium plasmas heated by neutral beam injection (NBI) are studied for the first time by means of a power balance analysis to characterize the main contributions in the transition, through integrated transport modelling. In the pulses analysed, we do observe a minimum of the L-H power threshold in density, indicating the presence of density branches and of n e,min . Electron and ion heat fluxes at the transition are estimated separately. The electron/ion equipartition power results in favour of the ions, as shown by QuaLiKiz quasilinear gyrokinetic simulations, which predict a larger ion transport that causes T e > T i. The resulting edge ion heat flux also shows a clear change of slope below n e,min , similarly to ASDEX-Upgrade (AUG) NBI pulses (Ryter et al 2014 Nucl. Fusion 54 083003). JET NBI data are compared to radio-frequency heated AUG and Alcator C-mod pulses (Schmidtmayr et al 2018 Nucl. Fusion 58 056003), showing a different trend of the power, coupled to ions at the L-H transition with respect to the linearity observed in the radio-frequency heated plasmas. The presence of n e,min and the role of the ion heat flux is discussed in the paper, although it seems it is not possible to explain the presence of a P L-H minimum in density by a critical ion heat flux and by the equipartition power for the JET NBI-heated plasmas analysed.<br/
Surface-limited deuterium uptake of Ru films under plasma exposure
No full textBlister formation has been an emerging research topic for extreme ultraviolet (EUV) mirrors exposed to hydrogen plasmas. Similar to plasma-facing materials in nuclear fusion reactors, it has been reported that blister formation in EUV mirrors is initiated by hydrogen uptake due to hydrogen ion or atom bombardment. However, the research so far has focused on Mo/Si multilayers exposed to only hydrogen ions or atoms, while the EUV mirror typically has a Ru capping layer facing hydrogen plasmas. We present experimental work to measure plasma-induced hydrogen uptake of Ru films. We bombarded our designed Ru-capped target with a low-temperature deuterium plasma and measured the deuterium retention using elastic recoil detection. Contrary to ion-driven deuterium uptake, the deuterium uptake rate of the Ru film had no dependence on the deuterium ion flux or energy after a period of plasma exposure. A reaction–diffusion model has been built to calculate the time evolution of deuterium retention, which well fits the experimental data. Based on this model, we conclude that the surface composition of the Ru film is the limiting factor for the deuterium uptake, which is seriously weakened when the surface is covered by Ru oxide. After the Ru oxide is reduced by the plasma, the uptake rate is predominantly driven by the deuterium surface coverage on metallic Ru. Our model also indicates that at the deuterium-populated Ru surface, deuterium has a low absorption barrier to penetrate the surface, which is supported by previously reported computational work
Thrust measurements and mesothermal plasma plume of the Alternative Low Power Hybrid Ion Engine (alphie)
No full textThe high specific impulse Alternative Low Power Ion Engine (alphie) is a gridded plasma thruster different from conventional (Kaufman) ion engines. In this disruptive concept, the ionization of the propellant neutral gas and the neutralization of ion outflow is achieved with only one cathode located in front and outside of the thruster. Electrons and ions move under the self-consistent field created by the DC voltage applied to its two planar grids together with the currents of charges flowing through them, unlike to conventional ion engines, where only ions move through its ion optics system. The stationary mesothermal flow of ions and electrons in the plasma plume is characterized with a retarded field energy analyzer in conjunction with Langmuir and emissive probes. The ion velocity distribution functions and the electron energy spectra for different operating conditions of the alphie thruster are discussed. The observed high ion temperatures are explained by the collisional interaction between the fast ionizing electrons and the neutral atoms that increases their average kinetic energy. Finally, the alphie delivers 0.8-3.5 mN throttleable thrusts giving specific impulses in the range of 14000-20000 s with estimated thruster efficiencies between 8% and 40%
The 2022 Plasma Roadmap: Low temperature plasma science and technology
No full textThe 2022 Roadmap is the next update in the series of Plasma Roadmaps published by Journal of Physics D with the intent to identify important outstanding challenges in the field of low-temperature plasma (LTP) physics and technology. The format of the Roadmap is the same as the previous Roadmaps representing the visions of 41 leading experts representing 21 countries and five continents in the various sub-fields of LTP science and technology. In recognition of the evolution in the field, several new topics have been introduced or given more prominence. These new topics and emphasis highlight increased interests in plasma-enabled additive manufacturing, soft materials, electrification of chemical conversions, plasma propulsion, extreme plasma regimes, plasmas in hypersonics, data-driven plasma science and technology and the contribution of LTP to combat COVID-19. In the last few decades, LTP science and technology has made a tremendously positive impact on our society. It is our hope that this roadmap will help continue this excellent track record over the next 5-10 years.</p