Leveraging HPC Profiling & Tracing Tools to Understand the Performance of Particle-in-Cell Monte Carlo Simulations

Large-scale plasma simulations are critical for designing and developing next-generation fusion energy devices and modeling industrial plasmas. BIT1 is a massively parallel Particle-in-Cell code designed for specifically studying plasma material interaction in fusion devices. Its most salient characteristic is the inclusion of collision Monte Carlo models for different plasma species. In this work, we characterize single node, multiple nodes, and I/O performances of the BIT1 code in two realistic cases by using several HPC profilers, such as perf, IPM, Extrae/Paraver, and Darshan tools. We find that the BIT1 sorting function on-node performance is the main performance bottleneck. Strong scaling tests show a parallel performance of 77% and 96% on 2,560 MPI ranks for the two test cases. We demonstrate that communication, load imbalance and self-synchronization are important factors impacting the performance of the BIT1 on large-scale runs.Comment: Accepted by the Euro-Par 2023 workshops (TDLPP 2023), prepared in the standardized Springer LNCS format and consists of 12 pages, which includes the main text, references, and figure

Tritiation of amorphous and crystalline silicon using T <inf>2</inf> gas

Incorporation of tritium in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) at 250 °C using tritium (T 2) gas at pressures of up to 120 atm is reported. The tritium is stored in a surface layer which is approximately 150 and 10 nm for a-Si:H and c-Si, respectively. The concentration of tritium occluded in planar and textured c-Si is linearly dependent on the total surface area. The tritium is stable and the dominant tritium evolution occurs at temperatures above 300 °C. The concentration of tritium locked in a-Si:H and c-Si was 20 and 4 at. %, respectively. Self-catalysis appears to be important in the tritiation process. © 2006 American Institute of Physics

Plasma potential probes for hot plasmas: A review and some news

Plasma probes are well established diagnostic tools. They are not complicated, relatively easy to construct and to handle. The easiest and fastest accessible parameter is their floating potential. However, the floating potential of a cold probe is not very significant. Much more important and relevant is the plasma potential. But in most types of plasmas, consisting mainly of electrons and only positive ions, the floating potential is more negative than the plasma potential by a factor proportional to the electron temperature. Obviously this is due to the much higher mobility of the electrons. We present a review on probes whose floating potential is close to or ideally equal to the plasma potential. Such probes we name Plasma Potential Probes (PPP) and they can either be Electron Emissive Probes (EEP) or so-called Electron Screening Probes (EPS). These probes make it possible to measure the plasma potential directly and thus with high temporal resolution. An EEP compensates the plasma electron current by an electron emission current from the probe into the plasma, thereby rendering the current-voltage characteristic symmetric with respect to the plasma potential and shifting the floating potential towards the plasma potential. Only the simplest case of an EEP floating exactly on the plasma potential is discussed here in which case no sheath is present around the probe. An ESP, principally operable only in strong magnetic fields, screens off most of the plasma electron current from the probe collector, taking advantage of the fact that the gyro radius of electrons is usually much smaller than that of the ions. Also in this case we obtain a symmetric current-voltage characteristic and a shift of the probe’s floating potential towards the plasma potential. We have developed strong and robust EEPs and two types of ESPs, called BUnker Probes (BUP), for the use in the Scrape-Off Layer (SOL) of Medium-Size Tokamaks (MST), and other types of strongly magnetized hot plasmas. These probes are presented in detail

Sex difference and intra-operative tidal volume: Insights from the LAS VEGAS study

BACKGROUND: One key element of lung-protective ventilation is the use of a low tidal volume (VT). A sex difference in use of low tidal volume ventilation (LTVV) has been described in critically ill ICU patients.OBJECTIVES: The aim of this study was to determine whether a sex difference in use of LTVV also exists in operating room patients, and if present what factors drive this difference.DESIGN, PATIENTS AND SETTING: This is a posthoc analysis of LAS VEGAS, a 1-week worldwide observational study in adults requiring intra-operative ventilation during general anaesthesia for surgery in 146 hospitals in 29 countries.MAIN OUTCOME MEASURES: Women and men were compared with respect to use of LTVV, defined as VT of 8 ml kg-1 or less predicted bodyweight (PBW). A VT was deemed 'default' if the set VT was a round number. A mediation analysis assessed which factors may explain the sex difference in use of LTVV during intra-operative ventilation.RESULTS: This analysis includes 9864 patients, of whom 5425 (55%) were women. A default VT was often set, both in women and men; mode VT was 500 ml. Median [IQR] VT was higher in women than in men (8.6 [7.7 to 9.6] vs. 7.6 [6.8 to 8.4] ml kg-1 PBW, P &lt; 0.001). Compared with men, women were twice as likely not to receive LTVV [68.8 vs. 36.0%; relative risk ratio 2.1 (95% CI 1.9 to 2.1), P &lt; 0.001]. In the mediation analysis, patients' height and actual body weight (ABW) explained 81 and 18% of the sex difference in use of LTVV, respectively; it was not explained by the use of a default VT.CONCLUSION: In this worldwide cohort of patients receiving intra-operative ventilation during general anaesthesia for surgery, women received a higher VT than men during intra-operative ventilation. The risk for a female not to receive LTVV during surgery was double that of males. Height and ABW were the two mediators of the sex difference in use of LTVV.TRIAL REGISTRATION: The study was registered at Clinicaltrials.gov, NCT01601223

Modeling of beta conductivity in tritiated amorphous silicon

grantor: University of TorontoFor the first time a model was developed to explain the time evolution of the electrical conductivity in tritiated amorphous silicon samples using data obtained earlier by Mr. Tome Kosteski and Dr. Franco Gaspari. The model takes into account thermally activated conductivity in the extended states, hopping conductivity in the conduction band tail localized states and beta conductivity in the extended states. The variable parameters are neutral and positively charged dangling bond concentrations, the quasi-Fermi energy for electrons and the hopping distance. Results show that the main factor affecting the conductivity is the concentration of neutral dangling bonds. The time evolution of the total dangling bond concentration is in agreement with the tritium nuclear decay process. The values found for the Fermi energy and for the hopping distance are in agreement with the literature and support the time evolution of the dangling bond concentration.M.A.Sc

Analysis of ion orbits in front of a negative planar electrode immersed in an oblique magnetic field

The orbital motion approach is used to analyze the ion impact on a negatively biased planar wall immersed in a strongly magnetized plasma. It is assumed that the given homogeneous magnetic field forms a small angle with a planar negatively biased electrode, while the inhomogeneous electric field is perpendicular to the electrode. Spatial dependence of the electric field is modeled in such a way that the electric field exhibits two-scale behavior, which is characteristic of plasma sheath problems. The equation of motion of a singly charged deuterium ion is solved for a large variety of parameters and initial conditions. The effects of electrode bias, magnetic flux density, magnetic field angle, initial velocity, electric field scaling, and electrode bias are investigated. It is found that the impact angles of ions are distributed over a surprisingly wide range, and in a vast majority of cases, the angle of impact is several times larger than the magnetic field angle. The study is relevant for the analysis of ion flow to the electrodes in fusion plasmas, i.e., divertors of tokamaks

One-dimensional, multi-fluid model of the plasma wall transition. I. Hot electrons

The plasma-wall transition in a plasma containing singly charged positive ions and two groups of electrons is studied with a one-dimensional steady-state multifluid model, which is presented in some detail. When the temperature and the initial density ratio between the two groups of electrons are varied, a transition between the two types of solutions to the model equations is observed. When the density and temperature of the hot electrons are above certain critical values, a high solution is observed. If the ion mass is decreased, these critical values increase. However, this effect only occurs with artificially small ion masses, which are significantly lower than the proton mass. In the high solution, the potential drop is determined by the hot electrons and is greater in absolute terms than in the low solution, where it is determined by the base electron population. The transition between the low and high solutions is very sharp if a neutrality condition is imposed. However, if the neutrality condition is replaced by the Poisson equation, the transition becomes blurred and the solutions exhibit oscillations. The temperature profiles of the ions are analyzed, and it is confirmed that the ion sound and the ion fluid velocity become equal at the breaking point of the plasma neutrality. It is shown how the ion source term, the initial ion velocity, and the initial electric field are found to be self-consistent. The density profiles of the negatively biased particles resulting from the fluid equations deviate very little those of from the Boltzmann-distributed particles, even if the corresponding source terms are quite large

One-dimensional, multi-fluid model of the plasma-wall transition

The plasma-wall transition is investigated by a one-dimensional steady-state multifluid model, which was presented in detail in Part I [T. Gyergyek et al., AIP Adv. 14, 045201 (2024)]. In this work, the plasma-wall transition is analyzed for the case where the plasma consists of singly charged positive ions, electrons, and singly charged negative ions. When the temperature and initial density of the negative ions are varied, a transition between two types of solutions of the model is observed. We call them the low and high solution, with respect to the absolute value of the potential drop. When the density and temperature of the negative ions are above a critical value, the low solution is observed. As the mass of the positive ions increases, these critical values also increase, but only until the ion mass is below about 1000 electron masses. With larger ion masses, the critical density of the negative ions and the temperature no longer change. In the low solution, the potential drop in front of the sheath is determined by the negative ions and is smaller in absolute terms than in the case of the high solution, where the potential drop in front of the sheath is determined by the electrons. If the problem is analyzed on the pre-sheath scale, the transition between the low and high solution is very sharp. However, when the neutrality condition is replaced by the Poisson equation, this transition becomes blurred and the solutions of the model equations exhibit oscillations. The role of the smallness parameter is highlighted. It is shown how the initial electric field is determined. Deviation of the negative ion density profile from the Boltzmann relation is discussed