Michael Armstrong (14-ERD-077)
We have developed an inductively coupled plasma flow reactor to investigate chemical kinetics relevant to Inertial Confinement Fusion energy reactor design. For fusion reactors, carbon-based materials are exposed to harsh environments that can cause the degradation of plasma-exposed components and impact the reprocessing of target materials. To obtain the high-temperature chemical kinetics data required to mitigate these issues, the reactor we developed creates an environment similar to what would be observed in a fusion reactor, exposing target reactants to extremely high temperatures, on the order of 10,000K. At this temperature, all reactants dissociate into their constituent atoms. Molecular species formed as the plasma cools are characterized through spectroscopic techniques. We report here on the design, simulations, and operating parameters for the reactor, spectroscopic techniques deployed to characterize chemical kinetics, and measurements. We also briefly discuss the use of the plasma reactor to address the chemical kinetics of radionuclides relevant to nuclear forensics.
Background and Research Objectives
Non-combustion-related chemical and physical transformations on timescales in the millisecond to sub-second range are relevant for a number of applications where (1) nonequilibrium conditions (i.e., chemical kinetic rates) influence the concentrations of final products; (2) the relevant reactant density may be substantially lower than ambient density, increasing the time to equilibrium; (3) the temperatures of relevant reactions may be substantially higher than those found in combustion; (4) slower processes like condensation are important; or (5) processes involve exotic reactions or rare elements. Under these circumstances, assumptions that are valid in the context of combustion or standard conditions may not apply, and fundamental quantities such as reaction cross-sections may not be available.
Although inductively coupled plasma generation1 is often used for determining elemental chemical concentrations via emission or mass spectroscopy,2 the investigation of fundamental properties of plasmas,3 and material synthesis,4,5 this method is also useful for investigating longer timescale chemical kinetics starting from very high temperature, where molecules are dissociated to their constituent atoms. In particular, inductively coupled plasma flow reactors generate a continuous, steady flow of reactants, which has advantages compared to transient methods like combustion. These advantages include high signal-to-noise, allowing low signal intensities to be integrated over long times, flexibility in controlling the distribution of analytes, and the ability to simultaneously observe qualitatively disparate phenomena such as chemical kinetics and condensation processes over several orders of magnitude in time. Also, because the states of interest are continuously generated, time-dependent diagnostics such as short-pulse lasers are not necessary—the observation of time-dependent effects is implicit in the design of the instrument.
Inertial Confinement Fusion requires the very rapid compression of a target comprising a spherical shell containing a deuterium–tritium mixture.6 Rapid extreme compression and heating of this mixture results in deuterium–tritium nuclear fusion7 and the generation of heat that may ultimately be used for commercial power generation. Schemes for realizing commercial Inertial Confinement Fusion energy envision a large chamber analogous to the target chamber used at Livermore's National Ignition Facility,8 into which fusion targets would be injected at a relatively high rate, on the order of 10 Hz, resulting in a continuous input stream of target materials including carbon and tritium. As such, the removal of target waste material is a critical issue for fusion energy schemes, because radioactive hydrocarbons can condense as solids on the inner wall of the target chamber, creating handling and contaminated waste problems, complicating reprocessing, and degrading the mechanical integrity of the chamber wall.8
One proposed solution to the accumulation of graphitic carbon in a fusion energy chamber is the addition of oxygen to the chamber with the goal of removing carbon as carbon dioxide gas. But given the operating conditions of a fusion energy chamber (i.e., continuous ~100-ms-period cyclic intense heating followed by rapid cooling) chemical kinetics rather than equilibrium chemistry may play a limiting role in determining the dominant products. In terms of timescale, temperature range, and experimental convenience, an inductively coupled plasma-based gas flow reactor is well suited to determine the significance of rapid heating and cooling on the formation of hydrocarbons versus carbon dioxide, and to more generally address the question of rapid chemical kinetics in fusion energy applications.
Generally, the formation of complex hydrocarbons is preceded by acetylene,9 whose optical properties are well characterized. Here we present results addressing this question by examining the formation of diatomic carbon radicals and acetylene following extreme dissociative heating and subsequent cooling of acetylene and oxygen in an inductively coupled plasma-based flow reactor over a timescale ranging from milliseconds to less than a second.
Scientific Approach and Accomplishments
We modified a commercially available, inductively coupled plasma flow reactor and optical emission spectrometer (Profile Model ICP, Teledyne Leeman Instruments) by removing the optical emission spectrometer and inserting a flow reactor at the output of the inductively coupled plasma generator. A schematic of the inductively coupled plasma generator and the flow reactor is shown in Figure 1.
Analyte as either a gas mixture or nebulized solution, along with a buffer gas, typically argon, is injected into the gas flow upstream of the plasma generator and heated to approximately 10,000K in the plasma generator, which dissociates analyte molecules into their constituent atoms. Downstream flow into the flow reactor after the "ring flow" injector cools on a less than 10-ms timescale, allowing dissociated atoms to recombine into molecules. Analyte volume is typically much smaller (10-4) than the total buffer volume. Cool buffer gas is injected into an outermost ring flow, in part, to keep the gas temperature within a safe range for the instrument, and to enable, via variation of the flow rate, some control of the temperature of the gas downstream from the plasma generator. A water-cooled copper sleeve surrounds the flow reactor. Notches are cut into the sleeve to allow access to optical probes.
An optical diagnostic bench is mounted onto an automated translation stage adjacent and parallel to the flow reactor. This bench allows optical access along the length of the flow reactor, and currently implements an acetylene diagnostic and an optical emission spectrometer. The bench is also used to mount sample and temperature probes into the flow reactor.
Total gas flow through the reactor is typically between 20 to 30 L/min, and the analyte flow may be as low as some milliliters per minute. Ideally, the analyte flow rate should be as low as detectable to extend the time scale of chemical kinetics by reducing the density of analyte, and thus the frequency of collisions between reactants. The flow velocity through the reactor varies, scaling with temperature from 1 to 10 m/s, so the timescale of reactions proceeding in the flow scales to about 1 to 10 mm of reactor length per millisecond of time. As such, particularly for regions of flow at high temperature, we can obtain sufficient time resolution to investigate chemical kinetics on the timescale of the problem, about 100 ms.
Two types of optical diagnostics were implemented: infrared line absorption for identifying acetylene, and optical emission spectroscopy, which can also be used to identify molecular diatomic carbon (at high temperatures) and make temperature measurements. In the downstream portion of the flow reactor, a high-temperature thermocouple was used to directly measure the temperature of the gas flow.
The detection of stable acetylene downstream of the reactor was accomplished through laser absorption diagnostics. Acetylene has a strong absorption band in the infrared region around 3,300 cm-1. A fixed-wavelength laser absorption diagnostic was developed by Stranic and Hanson10 in this wavelength region for measuring acetyleneconcentration during the pyrolysis of higher-order hydrocarbons.
Here, a continuous wave distributed feedback inter-band cascade laser (Nanoplus DFB ICL) was used for measuring acetylene absorption. The wavelength output of the diode was varied over an approximately 2-nm range around the center wavelength by rapidly scanning the drive current and comparing the total absorption of acetylene with and without the inductively coupled plasma flow reactor on (Figure 2). Because the lines of this molecule in this wavelength region have very narrow linewidths, these scanned wavelength measurements had enough bandwidth to spectrally resolve the absorption features of acetylene. The absorption measurement has a sensitivity of better than 3 ppm.
Optical emission spectroscopy was used to detect hot diatomic carbon radicals downstream from the plasma generator. This method is very sensitive to the presence of diatomic carbon at high temperatures, and allows estimates of the temperature.11 An example of the emission is shown in Figure 3.
Modeling included a flow component to determine the flow and temperature conditions of the reactor, and a chemical kinetics component. Because the energetics of the system are dominated by the inductively coupled plasma flow reactor and buffer flow, and not the chemistry of the reactants, simulations of the buffer flow only were done first to determine the temperature and density conditions in the flow reactor followed by chemical kinetics simulations to determine concentrations. Generally, these simulations are consistent with our observations (see Figure 4). Specifically, consistent with a more accurate, kinetically limited model of acetylene formation in the absence of oxygen (and inconsistent with an equilibrium model), we observed acetylene far downstream of the inductively coupled plasma in the flow reactor. Also, in the presence of oxygen, COx formation rapidly consumes all carbon, also consistent with our observations.
Generally, the kinetics of all carbon formation are substantially faster than the relevant timescale envisioned for fusion energy applications (100 ms). In the absence of oxygen, optical emission spectroscopy showed very rapid recombination of carbon within centimeters of exiting the high-temperature plasma, indicating reformation of diatomic carbon on a sub-10-millisecond timescale. At lower temperatures (and longer timescales) downstream, acetylene reformed at concentrations consistent with simulations, which include chemical kinetics, indicating that equilibrium calculations are not sufficient to describe chemistry in this system. Diatomic carbon emission downstream from the plasma generator is shown in Figure 5.
Injection of oxygen stoichiometric for carbon dioxide formation completely eliminated all diatomic carbon emission within approximately 1 cm (1 ms) of the high-temperature plasma region. This is a strong indication that the chemical kinetics of carbon formation are very fast—essentially the chemistry of carbon–carbon bond formation is not significantly limited by kinetics even for very fast (<10 ms) cooling rates. Nonetheless, the formation of COx still dominates the chemistry when stoichiometric oxygen is present, on a timescale as fast or faster than the formation of diatomic carbon. The lowest carbon number densities of this study (~10 ppm at standard temperature and pressure and ~1014 carbon atoms per cubic centimeter) were approximately 10 times larger than the density envisioned in fusion energy schemes, but scaling the reaction rate for density using conventional models does not change the expected timescale of the reaction by more than a factor of 10, for the formation of carbon monoxide. Conservatively, the conversion to COx under these conditions would remain less than 10 ms.
In conclusion, assuming quantities of oxygen stoichiometric with carbon for carbon dioxide formation are added to an inertial fusion energy chamber, chemical kinetics will not limit the formation of COx on a 100-ms (and possibly much shorter) timescale.
Impact on Mission
The success of our effort will help establish technical foundations to support the LLNL strategic focus area in inertial fusion science and technology and core competency in nuclear, chemical, and isotopic science and technology, and will support a potential future role in the increasingly important plasma-based processing industry. Our research has provided two principal impacts. First, we have concluded that stoichiometric oxygen added to an Inertial Confinement Fusion chamber designed for energy production with a repetition rate of about 10 Hz will be effective in removing carbon as gaseous COx. In particular, the chemical kinetics of COx formation at the relevant densities are fast enough to avoid the formation of graphitic solid carbon on a less than 10-ms timescale. Second, the environment prepared by the inductively coupled plasma flow reactor for investigation of chemical kinetics in fusion energy is also ideal for the investigation of uranium oxide chemical kinetics relevant to nuclear forensics. A current LDRD research project (16-ERD-008) employs the inductively coupled plasma flow reactor as its principal instrument for creating this environment, and for investigating physical and chemical causes of uranium fractionation in fallout. This issue is an impediment to modeling the formation of fallout, and empirical data derived from these studies is expected to significantly contribute to the accuracy of these models.
This project has answered the question that motivated it. Specifically, in an Inertial Confinement Fusion-based energy generation scheme, the addition of oxygen will remove target-injected waste carbon as gas, and avoid the accumulation of tritium-containing graphitic solids in the chamber—this process is not limited by chemical kinetics on a sub-10-ms timescale. Follow on research will include a significant nuclear forensics component. There is broad interest in the empirical data generated by this instrument, which is currently one of a kind, for the empirical calibration of models of fallout formation.
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