Physics

A realistic description of partially-ionized matter in extreme thermodynamic states is critical to model the interior and evolution of the multiplicity of high-density astrophysical objects. Current predictions of its essential property, the ionization degree, rely widely on analytical approximations that have been challenged recently by a series of experiments. Here, we propose a novel ab initio approach to calculate the ionization degree directly from the dynamic electrical conductivity using the Thomas-Reiche-Kuhn sum rule. This Density Functional Theory framework captures genuinely the condensed matter nature and quantum effects typical for strongly-correlated plasmas. We demonstrate this new capability for carbon and hydrocarbon, which most notably serve as ablator materials in inertial confinement fusion experiments aiming at recreating stellar conditions. We find a significantly higher carbon ionization degree than predicted by commonly used models, yet validating the qualitative behavior of the average atom model Purgatorio. Additionally, we find the carbon ionization state to remain unchanged in the environment of fully-ionized hydrogen. Our results will not only serve as benchmark for traditional models, but more importantly provide an experimentally accessible quantity in the form of the electrical conductivity.

Driving laser wakefield acceleration with extremely short, near single-cycle laser pulses is crucial to the realisation of an electron source that can operate at kHz-repetition rate while relying on modest laser energy. It is also interesting from a fundamental point of view, as the ponderomotive approximation is no longer valid for such short pulses. Through particle-in-cell simulations, we show how the plasma response becomes asymmetric in the plane of laser polarization, and dependent on the carrier-envelope phase (CEP) of the laser pulse. For the case of self-injection, this in turn strongly affects the initial conditions of injected electrons, causing collective betatron oscillations of the electron beam. As a result, the electron beam pointing, electron energy spectrum and the direction of emitted betatron radiation become CEP-dependent. For injection in a density gradient the effect on beam pointing is reduced and the electron energy spectrum is CEP-independent, as electron injection is mostly longitudinal and mainly determined by the density gradient. Our results highlight the importance of controlling the CEP in this regime for producing stable and reproducible relativistic electron beams and identify how CEP effects may be observed in experiments. In the future, CEP control may become an additional tool to control the energy spectrum or pointing of the accelerated electron beam.

Atmospheric pressure plasma (APP) generates highly reactive species that are useful for surface activations. We demonstrate a fast regeneration of iron oxides, that are popular catalysts in various industrial processes, using microwave-driven argon APP under ambient condition. The surface treatment of hematite powder by the APP with a small portion of hydrogen (0.5%) lowers the oxide's reduction temperature. A near-infrared laser is used for localized heating to control the surface temperature. Controlled experiments without plasma confirm the catalytic effect of the plasma. Raman, XRD, SEM, and XPS analyses show that the plasma treatment changed the chemical state of the hematite to that of magnetite without sintering.

It is widely believed that magnetic reconnection plays an important role in various eruptive phenomena of space and astrophysical plasmas. The mechanism of anomalous resistivity, however, has been an open and unsolved problem. The chaos-induced resistivity proposed by Yoshida (1998) is one of possible mechanisms for anomalous resistivity. By use of the test particle simulation, the present work studies the chaos-induced resistivity for different configurations of reconnection magnetic fields and its distribution in different chaos regions of reconnection current sheets. The results show that the chaos-induced resistivity can be 6-7 orders of magnitude higher than the classical Spitzer resistivity in the X-type chaos regions and 5 orders of magnitude in the O-type chaos regions. Moreover, in the X-type chaos regions the chaos-induced resistivity of the magnetized case is higher by a factor of 2 to 3 times than that of the unmagnetized case, but in the O-type chaos regions the chaos-induced resistivity of the magnetized case is close to or lower than that of the unmagnetized case. The present work is helpful to the understanding of the dynamics of reconnection current sheets, especially of the generation mechanism of the anomalous resistivity of collisionless reconnection regions.

Two magnetic configurations of Multi-cusp Plasma Device (MPD) have been explored to obtain high quiescence level, large uniform plasma region with nearly flat mean density and temperature profiles. In particular, properties of plasma in a six-pole six magnet (SPSM) and twelve pole six magnets (TPSM) cusp configurations are rigorously compared and reported here. It is found that more uniform plasma with nearly flat profiles is found in TPSM along with increased quiescence level. Findings are verified across various magnetic field strengths for both configurations.

In this work, solid targets made from boron and boron nitride (BN) materials are ablated by a nanosecond pulsed laser at sub-atmospheric pressures of nitrogen and helium gases. Excited species in the ablation plume from the target are probed with spatiotemporally resolved optical emission spectroscopy (OES). Evaluation of chemical composition in the plasma plume revealed that for both boron-rich targets, emission from BN molecules is always observed in nitrogen-rich environments. In addition, BN molecules also present when ablating a boron nitride target in helium gas -- an indication that BN molecules in the plume may originate from the solid target. Furthermore, the ablation of BN target features emission of B2N molecules, regardless of the pressure and surrounding gas. These results suggest that the ablation of the BN target is more favorable for the generation of complex molecules containing boron and nitrogen species and possibly hint that BN is also more favorable feedstock for high-yield BN nanomaterial synthesis. Plasma parameters such as electron temperature (peak value of 1.3 eV) and density (peak value of 2x10^18 cm^-3) were also investigated in this work in order to discuss the chemical dynamics in the plume.

In this work we present the first measurements obtained by the V-band Doppler reflectometer during the second operation phase of Wendelstein 7-X to discuss the influence in the velocity shear layer and the radial electric field, E r , of several plasma parameters such as magnetic configuration, rotational transform or degree of detachment. In the first place, we carry out a systematic characterization of the turbulence rotation velocity profile in order to describe the influence of density and heating power on E r under the four most frequent magnetic configurations. The | E r | value in the edge is found to increase with configurations featuring higher ι , although this does not apply for the high mirror configuration, KJM. As well, the E r value in the SOL and the velocity shear near the separatrix are found to display a clear dependence on heating power and density for all configurations. For a number of relevant cases, these results are assessed by comparing them to neoclassical predictions obtained from the codes DKES and KNOSOS, finding generally good agreement with experimental results. Finally, the evolution of E r at the edge is evaluated throughout the island-divertor detachment regime achieved for the first time in the 2018 campaign. After detachment, | E r | is reduced both at the SOL and edge, and the plasma column shrinks, with the shear layer seemingly moving radially inwards from the separatrix.

Charge state distributions in hot, dense plasmas are a key ingredient in the calculation of spectral quantities like the opacity. However, they are challenging to calculate, as models like Saha-Boltzmann become unreliable for dense, quantum plasmas. Here we present a new variational model for the charge state distribution, along with a simple model for the energy of the configurations that includes the orbital relaxation effect. Comparison with other methods reveals generally good agreement with average atom based calculations, the breakdown of the Saha-Boltzmann method, and mixed agreement with a chemical model. We conclude that the new model gives a relatively inexpensive, but reasonably high fidelity method of calculating the charge state distribution in hot dense plasmas, in local thermodynamic equilibrium.

The ambipolar electrostatic potential rising along the magnetic field line from the grounded wall to the centre in the linear gas dynamic trap, rules the available suppression of axial heat and particle losses. In this paper, the visible range optical diagnostic is described using the Doppler shift of plasma emission lines for measurements of this accelerating potential drop. We used the room temperature hydrogen jet puffed directly on the line of sight as the charge exchange target for plasma ions moving in the expanding flux from the mirror towards the wall. Both bulk plasma protons and H e 2+ ions velocity distribution functions can be spectroscopically studied; the latter population is produced via the neutral He tracer puff into the central cell plasma. This way, potential in the centre and in the mirror area can be measured simultaneously along with the ion temperature. A reasonable accuracy of 4÷8% was achieved in observations with the frame rate of ?? kHz . Active acquisitions on the gas jet also provide the spatial resolution better than 5~mm in the middle plane radial coordinate because of the strong compression of the object size when projected to the centre along the magnetic flux surface. The charge exchange radiation diagnostic operates with three emission lines: H- α 656.3~nm, He-I 667.8~nm and He-I 587.6~nm. Recorded spectra are shown in the paper and examples for physical dependences are presented. The considered experimental technique can be scaled to the upgraded multi-point diagnostic for the next generation linear traps and other magnetic confinement systems.

Massive gas injection (MGI) experiments have been carried out in many tokamaks to study disruption dynamics and mitigation schemes. Two events often observed in those experiments are the excitation of the m = 2,n = 1 magnetohydrodynamic (MHD) mode before the thermal quench (TQ), and the formation of cold bubble structure in temperature distribution during the TQ. Here m is the poloidal mode number, n the toroidal mode number. The physics mechanisms underlying those phenomena, however, have not been entirely clear. Recent NIMROD simulationsofMGIprocessinatokamakhavereproducedmainfeaturesofbothevents, which has allowed us to examine and establish the causal relation between them. In particular, the 3/1 and 2/1 islands are found to form successively after the arrival of impurity cold front at the corresponding q = 3 and q = 2 rational surfaces. At the interface between impurity and plasma, peaked poloidal magnetic perturbation along with a thin current sheet moves inward following the gas cold front. This eventually leads to the formation of an inner 2/1 mode structure in the region between q = 2 and q = 1 surfaces, which has an opposite phase to the dominant 2/1 mode on the q = 2 surface. ItisthroughtheO-pointoftheinner 2/1 modethattheimpurityfrontfurther penetrates inside the q = 1 surface, and enables the formation of a cold bubble at the beginning of TQ. In addition, a 1/1 mode appears inside the q = 1 surface after the impurity penetration, which dominates the subsequent start of current quench (CQ).