Call for Papers on Special Issue of Nuclear Materials and Energy

A new Special Issue of the journal Nuclear Materials and Energy, Elsevier, on the topic of Modeling and Experimental Validation of Plasma Material Interactions is now accepting papers. The scope of this special issue focuses on both modeling and simulation as well as experimental research of plasma material interactions for fusion energy devices. You are invited to submit your manuscript at any time before the submission deadline 31st May 2022. For any inquiries about the appropriateness of contribution topics, please contact Professor Davide Curreli via For additional information, read more at the following link:


Final Doctoral Defense – Mikhail Finko

Mikhail Finko, Ph.D. Candidate
Dr. Davide Curreli, Director of Research
January 12  11:00am – 1:00pm

Constraining Chemical Kinetics of Uranium Oxides in Extreme Environments

ABSTRACT: Current understanding of metallic chemistry in extreme environments, such as nuclear fireballs, remains limited due to the multitude of physical processes and timescales involved. In particular, many chemical and plasma chemical reaction pathways responsible for forming uranium molecular species remain either unknown or unverified. In recent years, this longstanding knowledge gap has been the target of an increasing number of experimental characterizations, which this work aims to leverage to produce an updated model of uranium oxide (UOx) formation in extreme environments.

To this end, a Monte Carlo Genetic Algorithm (MCGA) is utilized to calibrate a UOx reaction mechanism using measurements from a plasma flow reactor (PFR). In addition, laser ablation modeling capabilities are assessed for use in future chemical validation studies. These two systems cover a complimentary range of detonation-relevant flow regimes and cooling timescales. Bench-top laser ablation systems produce high temperature (>11,600 K) rapidly cooling (ns-µs) uranium plumes following laser-induced vaporization and shock expansion. The PFR, on the other hand, features a steady flow of uranium through a plasma torch (<10,000 K) cooling over longer (ms) timescales. The PFR is uniquely suited for the initial calibration of the reaction mechanism due to the relative ease of modeling the system. Thus, the MCGA optimization is limited to the PFR system in this work. Laser ablation, on the other hand, provides a potential test of the resulting mechanism over a wider range of detonation-relevant conditions. Performing such a test, however, requires first developing a predictive ablation model capable of capturing both the complex fluid dynamics and plasma chemistry of an ablation plume. Therefore, we evaluate the suitability of current modeling tools towards this problem and subsequently propose a coupled modeling approach for investigating ablation plume dynamics and chemistry. Lastly, synthetic diagnostics of emission and absorption spectroscopy signals are used to facilitate the characterization of both systems by enabling direct comparisons between simulations and measurements.

The optimization of a UOx reaction mechanism using PFR measurements is the primary scientific result of this work. The MCGA is used to identify dominant reaction channels and corresponding rate coefficients that produce the best agreement with available PFR data. The resulting reaction mechanism is compared against a previously constructed UOx mechanism, and differences in reaction rates and favorable reaction pathways are identified through a sensitivity analysis. Finally, recommendations for an updated UOx reaction mechanism are made, with considerations based on the limited constraining dataset.

The secondary scientific result of this work is the development of a one-way coupled radiation hydrodynamics and reactive CFD modeling approach for simulating ns duration pulsed laser ablation in reactive atmospheres. These simulations are used to study the interplay between fluid dynamics and chemistry in low-Z (aluminum and carbon) ablation plumes over ns to µs timescales. As a validation exercise, the ablation model results are compared against high fidelity plume imaging, time-of-flight expansion velocities, and spectroscopic molecular formation measurements. Reasonable agreement is observed across these comparisons and potential future refinements to the modeling approach are identified.


Final Doctoral Defense – Shane Keniley

Shane Keniley, Ph.D. Candidate
Dr. Davide Curreli, Director of Research
December 16, 2021  1:00pm – 3:00pm

Multiscale Fluid Modeling of Direct-Current Plasma-Water Interactions

ABSTRACT: One of the most promising emerging fields in plasmas is that of plasma-water interactions. Electrons in the plasma facilitate the production of both short- and long-lived reactive oxygen and nitrogen species, which have been shown to disinfect surfaces, induce cancerous cell death, and open up reaction pathways for high value chemical synthesis. A plasma in direct electrical contact with water will induce all of these effects in both the gas and liquid phases. Charged and neutral species may transport through the liquid interface, which changes the chemical composition of both phases and forms a tightly coupled electro-chemical. These conditions are difficult to characterize both numerically and experimentally because of the disparate spatial and temporal scales at play: solvated electrons are expected to drive chemical reactions in a region only nanometers thick at the interface, and reactive species lifetimes range from microseconds to hours.

This work is focused on the multiscale modeling of a direct-current (DC) plasma with a liquid water counterelectrode. A new open source plasma chemistry software, Crane, was developed in the MOOSE finite element framework in order to study the detailed chemical reactions that exist in plasma-water systems. Crane was coupled to a MOOSE application dedicated to plasma transport, Zapdos, which was upgraded to facilitate the modeling of multispecies plasmas. Both codes were verified against known global and 1D plasma transport problems and compared to existing software. A fully coupled plasma-water interface model was developed using the combined software, including electron transport across the interface, neutral species solvation and evaporation, and tightly coupled chemical reaction networks in both plasma and liquid phases.

The coupled plasma transport and chemistry models allow for the analysis of the electronic and chemical structure of the plasma-water interface. Results show that the chemical composition of the water is dramatically affected by the polarity of the driven electrode in the plasma phase. During anodic plasma treatment, a strong electric field forms in the cathode fall region over the water surface, which facilitates the production of near-surface reactive oxygen species (ROS). These species, along with positive ions, dissolve into the water and lead to the production of hydrogen peroxide (H2O2(aq)), ozone (O3(aq)), and hydroxyl radicals (OH(aq)). In contrast, the primary species injected into the water during cathodic plasma treatment are the highly reactive solvated electrons (e(aq)), which react with and inhibit the accumulation of ROS.

Simulations were supported by optical emission spectroscopy and chemical probe measurements in an equivalent electrochemical cell. The numerical model predicted that the disparate H2O2(aq) concentrations seen between anodic and cathodic plasma operation are the result of solvated electrons degrading any H2O2(aq) before it is allowed to accumulate. Experiments carried out in support of this prediction demonstrated that the H2O2(aq) concentration increased as solvated electron scavengers were added, showing strong agreement with simulation results. These results emphasize the tightly coupled nature of the plasma- water interface and may be used to inform future experimental work on water disinfection and chemical production using plasma electrolysis systems.


63rd APS Division of Plasma Physics 2021

63rd Annual Meeting of the APS Division of Plasma Physics

Monday–Friday, November 8–12, 2021; Pittsburgh, PA

GP11.00006 Recent Results from the SciDAC Partnership for Simulation of Fusion Relevant RF Actuators
GP11.00010 Development of a non-linear rf sheath benchmark suite
JO08.00002 Modeling Microsecond Timescale Molecular Formation in Laser Ablated Plasma Plumes
JO08.00015 Porting the PUMImbbl library to GPUs and integration in the hPIC2 Particle-in-Cell code
JP11.00020 Zapdos-CRANE implementation of a lithium vapor shielding model
JP11.00039 Particle-in-Cell study of impurity production from RF sheaths in front of ICRH actuators
NM09.00003 Integrated Multi-Scale Modeling of Impurity Migration and Plasma-Facing Material Evolution in Present and Future Tokamaks
NM09.00006 hPIC2: a GPU-accelerated, hybrid particle-in-cell code for plasma-material interactions in complex geometries  
ZP11.00018 Effect of ion acoustic instabilities on the energy-angle distributions of the ions impacting on the wall of a finite-size plasma


Final Doctoral Defense – Moutaz Elias

Moutaz Elias, Ph.D. Candidate
Dr. Davide Curreli, Director of Research
August 10, 2021  10am-12pm

Kinetic Characterization of Enhanced Impurity Sputtering Due to Ion Cyclotron Radio-Frequency Heating

ABSTRACT: ICRH devices are a cornerstone in the auxiliary heating requirement of future fusion devices as they are the most advanced and cost-effective option to heat the plasma. However, RF sheaths have been a major concern accompanying the use of ICRH systems. The presence of RF sheaths has been experimentally and theoretically linked to the enhancement of the impurity flux sputtered from the Plasma Facing Components. It is a pivotal task to minimize the impurity emission from the PFC of the ICRH system. Several mitigation strategies have been developed and tested on smaller scale devices experimentally. Previous attempts to model RF sheaths and PMI are limited to electromagnetic simulation and at best a fluid description of the plasma without any PMI simulations. RF sheaths require a detailed kinetic ion simulation that captures the ion dynamics in order to provide an accurate description of the IEAD at the PFC, particularly a Particle-In-Cell simulation would be advantageous. Using Maxwell-Boltzmann electrons would allow to overcome some of the limitations connected to the fast electron physics, but in order to avoid spurious electrostatic oscillations, it would require to enforce global charge conservation for transient and RF sheath plasma simulations. In this work we developed a new charge conservation scheme enabling the treatment of RF sheaths, and other type of transients, in hybrid Particle-in- Cell codes having kinetic ions and Maxwell-Boltzmann electrons. We report numerical tests on magnetized Radio-Frequency plasma sheath designed to test the stability and ability of the scheme to capture important RF sheath phenomena. An extensive benchmarking comparison of time-averaged and time dependent profiles with fluid codes is also reported. The developed hPIC model is used to analyze the dependence of the kinetic IEAD impacting on the RF antenna at various RF sheath parameters. Furthermore, a simulation case representing the latest JET campaign was analyzed. We found that in typical tokamak conditions of grazing magnetic field incidence, the IEAD of the ions impacting on the surface of the RF actuator exhibits a “phase-space cusp”, which can be explained as an effect due to finite ion Larmor radius.

In order to quantify material emission consequent to ion bombardment, the hPIC framework was interfaced to the RustBCA sputtering code. RustBCA is a previously-developed binary-collision-approximation code, which can be used to simulate material sputtering in time resolved conditions. Time resolved coupling allowed us to inspect changes in sputtering yield during one RF cycle. We found that the yield has a highly non-linear evolution during the RF cycle, which is a consequence of the exponential dependence of sputtering vs. energy across the sputtering threshold.

Finally, we performed a preliminary validation using experimental data taken at the RF limiter on the WEST tokamak at CEA, France, with the goal of comparing the results from hPIC-RustBCA against experimental measurements. In order to allow the comparison against OES (Optical Emission Spectroscopy) acquisitions, we converted the sputtered fluxes calculated by the code into absolute spectral radiance. We found reasonable agreement between the calculated values and the experimental measurements of the tungsten W I optical line emission at 400.9 nm, representative of the amount of sputtered tungsten. The multiple sources of uncertainties affecting the validation have been discussed, namely the high variability of the inverse photon efficiency S/XB, the actual impurity composition of the plasma (O, C, F, Cu, etc.), and the effect of higher charge states (O+, O2+, O3+, …, O8+). A systematic analysis of the different sources of uncertainty has been reported.

NPRE Graduate Student Spotlight: Moutaz Elias

See where Moutaz Elias is now: LinkedIn


New Collaborative Project with WEST Tokamak in Europe funded by DOE

NPRE Prof. Davide Curreli is collaborating on an international project to simulate operations of the WEST tokamak in France, with an end goal of advancing development of the ITER fusion device.

The WEST (W Environment in Steady-state Tokamak) device’s main mission is to test the ITER (International Thermonuclear Experimental Reactor) tungsten divertor technology at scale, including high heat flux components in a steady state tokamak environment. A tokamak’s divertor allows the online removal of waste material from the plasma while the reactor is in operation. Curreli’s team will join WEST in characterizing the tokamak performance from the wall surface to the tokamak edge through both measurements and modeling.

The objective of Curreli’s computational modeling will be to increase understanding of and improve use of tungsten as a plasma-facing material in fusion energy reactors. Among techniques to be examined will be the concept of “real-time boronization,” in which boron is added to a tokamak in real time for wall-conditioning as the plasma discharge is taking place.

Among challenges to fusion reactor technology is determining the best materials to use to face the plasma along the inside of a reactor vessel’s walls. Plasma facing components (PFCs) need to withstand extreme heat and particle loads to protect in-vessel structures and last sufficiently under steady-state erosion/re-deposition conditions.

Worldwide, many tokamak reactors use tungsten as a PFC. The International Thermonuclear Experimental Reactor ITER, whose first plasma is expected in 6 years from now, will also have a tungsten divertor. Tungsten has both the highest melting point and highest boiling point of any element, is among the least chemically reactive of elements, and has very low sputtering yield, meaning that it emits few impurities during plasma bombardment. However, tungsten has a high atomic number ( is “high-Z”), so the few impurities it emits significantly interfere with the plasma.

Scientists have learned that tungsten tokamaks work best when the wall is covered with a thin layer of a material with a low atomic number, such as boron. However, erosion of the boron layer exposes the tungsten underneath, degrading the benefit from low-Z operations. The boronization technique calls for boron to be continuously replenished into the vessel as the reaction is in progress and while the plasma is charged. Among phenomena, Curreli will determine through simulations whether the injection of boron powder is able to continuously replenish and reconstitute the surface with new boron.

Curreli’s work is supported by a three-year grant from the U.S. Department of Energy Office of Science. The work will be performed in collaboration with scientists at Oak Ridge National Laboratory, Princeton Plasma Physics Laboratory, and at the University of Tennessee Knoxville. (Susan Mumm)