High-pressure Noble Gas Research Articles

GanESS: detecting coherent elastic neutrino-nucleus scattering with noble gases

The recent detection of the coherent elastic neutrino-nucleus scattering (CEνNS) opens the possibility to use neutrinos to explore physics beyond standard model with small size detectors. However, the CEνNS process generates signals at the few keV level, requiring very sensitive detecting technologies for its detection. The European Spallation Source (ESS) has been identified as an optimal source of low energy neutrinos offering an opportunity for a definitive exploration of all phenomenological applications of CEνNS.GanESS will use a high-pressure noble gas time projection chamber to measure CEνNS at ESS in gaseous Xe, Ar and Kr. Such technique appears extraordinarily promising for detecting the process, although characterization of the response to few-keV nuclear recoils will be necessary. With this goal, we are currently commissioning GaP, a small prototype capable of operating up to 50 bar. GaP will serve to fully evaluate the low energy response of the technique, with a strong focus on measuring the quenching factor for the different noble gases that will later be used at GanESS. An overview of the GanESS project with a focus on the status of GaP and its short-term plans is presented.

Noble gas (He, Ne, and Ar) solubilities in high-pressure silicate melts calculated based on deep-potential modeling

Noble gas solubility in silicate melts has provided important insights into how these gases are incorporated into the Earth's interior and how they can be used as tracers of geochemical processes. However, previous studies have shown conflicting results regarding the noble gas solubility in silicate melts at high pressures. Some suggest that a drastic drop in solubility occurs above 5 GPa, while others indicate that maximum solubility is reached at this pressure but the solubility keeps unchanged when the pressure further increases. To clarify this discrepancy and gain a better understanding of the underlying mechanisms, we conducted a solubility investigation using the thermodynamic integration (TI) method and molecular dynamics (MD) simulations based on a machine-learning based force-field called deep potential (DP), which was trained by density functional theory (DFT) calculations. The effects of temperature, pressure and melt composition were investigated up to ∼30 GPa, 2500 K for the melt of model basalt, olivine and enstatite. Our results showed that the solubility of all noble gases (He, Ne and Ar) increased with temperature, while they increased with pressure until maximum solubility was reached at around 5 GPa. Beyond this pressure, solubility gradually decreased as pressure continued to increase. The structural and dynamic analysis of silicate melts revealed an abnormal increase in the self-diffusivity of oxygen atoms in silicate melts from ambient pressure to ∼5–8 GPa, which can be attributed to the formation of Si/Al-O5-6 in the melts upon compression. Interestingly, the upper limit pressure (5–8 GPa) for this abnormal increase in oxygen diffusivity was also the pressure at which noble gas solubilities reached their maximum. Our findings suggest that the maximum noble gas solubility at around 5 GPa may be a universal phenomenon in silicate melts. However, the specific values and changing trends of solubility depend on the specific melt compositions and noble gases. The complexity of noble gas solubility in silicate melts at high temperatures and pressures suggests that there is significant potential for improving the high-pressure noble gas partition coefficients between minerals, silicate melts, and even the Earth's core. Such improvements would extend our understanding of the behavior of noble gases in the Earth's interior and their role in tracing geodynamic processes.

Noble Gases in Solid Compounds Show a Rich Display of Chemistry With Enough Pressure.

In this review, we summarize the rapid progress that has been made in the study of noble gas chemistry in solid compounds under high pressure. Thanks to the recent development of first-principles crystal structure search methods, many new noble gas compounds have been predicted and some have been synthesized. Strikingly, almost all types of chemical roles and interactions are found or predicted in these high-pressure noble gas compounds, ranging from cationic and anionic noble gases to covalent bonds between noble gas atoms, and to hydrogen bond-like noble gas bonds. Besides, the recently discovered He insertion reactions reveal a unique chemical force that displays no local chemical bonding, providing evidence that research into noble gas reactions can advance the frontier of chemistry at the very basic level.

High voltage insulation and gas absorption of polymers in high pressure argon and xenon gases

High pressure gas time projection chambers (HPGTPCs) are made with a variety of materials, many of which still await proper characterization in high pressure noble gas environments. As HPGTPCs increase in size toward ton-scale detectors, assemblies become larger and more complex, creating a need for detailed understanding of how structural supports and high voltage insulators behave. This includes identification of materials with predictable mechanical properties and without surface charge accumulation that may lead to field deformation or sparking. This paper explores the mechanical and electrical effects of high pressure gas environments on insulating polymers PTFE, HDPE, PEEK, POM and UHMW in argon and xenon, including studying gas absorption, swelling and high voltage insulation strength.

Disruption mitigation with high-pressure helium gas injection on EAST tokamak

High pressure noble gas injection is a promising technique to mitigate the effect of disruptions in tokamaks. In this paper, results of mitigation experiments with low-Z massive gas injection (helium) on the EAST tokamak are reported. A fast valve has been developed and successfully implemented on EAST, with valve response time ⩽150 μs, capable of injecting up to particles, corresponding to 300 times the plasma inventory.Different amounts of helium gas were injected into stable plasmas in the preliminary experiments. It is seen that a small amount of helium gas () can not terminate a discharge, but can trigger MHD activity. Injection of 40 times the plasma inventory impurity () can effectively radiate away part of the thermal energy and make the electron density increase rapidly. The mitigation result is that the current quench time and vertical displacement can both be reduced significantly, without resulting in significantly higher loop voltage. This also reduces the risk of runaway electron generation. As the amount of injected impurity gas increases, the gas penetration time decreases slowly and asymptotes to (∼7 ms). In addition, the impurity gas jet has also been injected into VDEs, which are more challenging to mitigate that stable plasmas.

A new in-gas-laser ionization and spectroscopy laboratory for off-line studies at KU Leuven

The in-gas laser ionization and spectroscopy (IGLIS) technique is used to produce and to investigate short-lived radioactive isotopes at on-line ion beam facilities. In this technique, the nuclear reaction products recoiling out of a thin target are thermalized and neutralized in a high-pressure noble gas, resonantly ionized by the laser beams in a two-step process, and then extracted from the ion source to be finally accelerated and mass separated. Resonant ionization of radioactive species in the supersonic gas jet ensures very high spectral resolution because of essential reduction of broadening mechanisms. To obtain the maximum efficiency and the best spectral resolution, properties of the supersonic jet and the laser beams must be optimized. To perform these studies a new off-line IGLIS laboratory, including a new high-repetition-rate laser system and a dedicated off-line mass separator, has been commissioned. In this article, the specifications of the different components necessary to achieve optimum conditions in laser-spectroscopy studies of radioactive beams using IGLIS are discussed and the results of simulations are presented.

The dominant role of impurities in the composition of high pressure noble gas plasmas

We present in this letter how a molecular gas such as nitrogen at different levels of impurity dominates the ionic composition of an atmospheric pressure noble gas plasma such as in helium. The positive charge in the discharge is only determined by helium ions if the discharge gas contains less than 1ppm of impurity. Above this impurity level, the positive charge is completely determined by the impurity nitrogen. The higher the relative nitrogen concentration, the more N4+ dominates over N2+. If the impurity level is between 1 and about 20ppm, N2+ is clearly the most abundant positive ion but for higher levels of impurity, N4+ almost completely determines the positive charge.

Gas jet disruption mitigation studies on Alcator C-Mod and DIII-D

High-pressure noble gas jet injection is a mitigation technique which potentially satisfies the requirements of fast response time and reliability, without degrading subsequent discharges. Previously reported gas jet experiments on DIII-D showed good success at reducing deleterious disruption effects. In this paper, results of recent gas jet disruption mitigation experiments on Alcator C-Mod and DIII-D are reported. Jointly, these experiments have greatly improved the understanding of gas jet dynamics and the processes involved in mitigating disruption effects. In both machines, the sequence of events following gas injection is observed to be quite similar: the jet neutrals stop near the plasma edge, the edge temperature collapses and large MHD modes are quickly destabilized, mixing the hot plasma core with the edge impurity ions and radiating away the plasma thermal energy. High radiated power fractions are achieved, thus reducing the conducted heat loads to the chamber walls and divertor. A significant (2 × or more) reduction in halo current is also observed. Runaway electron generation is small or absent. These similar results in two quite different tokamaks are encouraging for the applicability of this disruption mitigation technique to ITER.

Effect of recombination in a high quantum efficiency prototype ionization‐chamber‐based electronic portal imaging device

The quantum efficiency (QE) of an imaging detector can be increased by utilizing a thick, high-density detection medium to increase the number of quantum interactions. However, image quality is more accurately described by the detection quantum efficiency (DQE). If a significant fraction of the increase in the number of detected quanta from a thick, dense detector were to result in useful imaging signal, this represents a favorable case where enhanced QE leads to increased DQE. However, for ionization-type detectors, one factor that limits DQE is the recombination between ion pairs that acts as a secondary quantum sink due to which enhancement in QE may not result in higher DQE depending on the extent of the signal loss from recombination. Therefore, an analysis of signal loss mechanisms or quantum sinks in an imaging system is essential for validating the overall benefit of high QE detectors. In this paper, a study of ion recombination as a secondary quantum sink is presented for a high QE prototype ion-chamber-based electronic portal imaging device (EPID): the kinestatic charge detector (KCD). The KCD utilizes a high pressure noble gas (krypton or xenon at 100 atm) and an arbitrarily large detector thickness (of the order of centimeters), resulting in a high QE imager. Compared with commercial amorphous silicon flat panel imagers that provide DQE(0) approximately 0.01, the KCD has much higher DQE. Studies indicated that DQE(0) = 0.20 for 6.1 cm thick, 100 atm (rho = 3.4 g/cm3) xenon chamber, and DQE(0)=0.34 for a 9.1 cm thick chamber. A series of experiments was devised and conducted to determine the signal loss due to recombination for a KCD chamber. The measurements indicated a fractional recombination loss of about 14% for a krypton chamber and about 18% for a xenon chamber under standard operating conditions (100 atm chamber pressure and 1275 V/cm electric field intensity). A theoretical treatment of the effect of recombination on imaging signal-to-noise ratio was applied to quantify the loss in DQE. These calculations indicated that recombination had a limited effect (<2%) on DQE under standard operating conditions. This was validated by good agreement between experimentally measured DQE and that obtained using Monte Carlo simulations that did not account for recombination.

Gas jet disruption mitigation studies on Alcator C-Mod

Damaging effects of disruptions are a major concern for Alcator C-Mod, ITER and future tokamak reactors. High-pressure noble gas jet injection is a mitigation technique which potentially satisfies the operational requirements of fast response time and reliability, while still being benign to subsequent discharges. Disruption mitigation experiments using an optimized gas jet injection system are being carried out on Alcator C-Mod to study the physics of gas jet penetration into high pressure plasmas, as well as the ability of the gas jet impurities to convert plasma energy into radiation on timescales consistent with C-Mod's fast quench times, and to reduce halo currents given C-Mod's high-current density. The dependence of impurity penetration and effectiveness on noble gas species (He, Ne, Ar, Kr) is also being studied. It is found that the high-pressure neutral gas jet does not penetrate deeply into the C-Mod plasma, and yet prompt core thermal quenches are observed on all gas jet shots. 3D MHD modelling of the disruption physics with NIMROD shows that edge cooling of the plasma triggers fast growing tearing modes which rapidly produce a stochastic region in the core of the plasma and loss of thermal energy. This may explain the apparent effectiveness of the gas jet in C-Mod despite its limited penetration. The higher-Z gases (Ne, Ar, Kr) also proved effective at reducing halo currents and decreasing thermal deposition to the divertor surfaces. In addition, noble gas jet injection proved to be benign for plasma operation with C-Mod's metal (Mo) wall, actually improving the reliability of the startup in the following discharge.

Analysis of the kinestatic charge detection system as a high detective quantum efficiency electronic portal imaging device

Megavoltage x-ray imaging suffers from reduced image quality due to low differential x-ray attenuation and large Compton scatter compared with kilovoltage imaging. Notwithstanding this, electronic portal imaging devices (EPIDs) are now widely used in portal verification in radiotherapy as they offer significant advantages over film, including immediate digital imaging and superior contrast range. However video-camera-based EPIDs (VEPIDs) are limited by problems of low light collection efficiency and significant light scatter, leading to reduced contrast and spatial resolution. Indirect and direct detection-based flat-panel EPIDs have been developed to overcome these limitations. While flat-panel image quality has been reported to exceed that achieved with portal film, these systems have detective quantum efficiency (DQE) limited by the thin detection medium and are sensitive to radiation damage to peripheral read-out electronics. An alternative technology for high-quality portal imaging is presented here: kinesatic charge detection (KCD). The KCD is a scanning tri-electrode ion-chamber containing high-pressure noble gas (xenon at 100 atm) used in conjunction with a strip-collimated photon beam. The chamber is scanned across the patient, and an external electric field is used to regulate the cation drift velocity. By matching the scanning velocity with that of the cation (i.e., ion) drift velocity, the cations remain static in the object frame of reference, allowing temporal integration of the signal. The KCD offers several advantages as a portal imaging system. It has a thick detector geometry with an active detection depth of 6.1 cm, compared to the sub-millimeter thickness of the phosphor layer in conventional phosphor screens, leading to an order of magnitude advantage in quantum efficiency (>0.3). The unique principle of and the use of the scanning strip-collimated x-ray beam provide further integration of charges in time, reduced scatter, and a significantly reduced imaging dose, enhancing the imaging signal-to-noise ratio (SNR) and leading to high DQE. While thick detectors usually suffer from reduced spatial resolution, the KCD provides good spatial resolution due to high gas pressure that limits the spread of scattered electrons, and a strip-collimated beam that significantly reduces the inclusion of scatter in the imaging signal. A 10 cm wide small-field-of-view (SFOV) prototype of the KCD is presented with a complete analysis of its imaging performance. Measurements of modulation transfer function (MTF), noise power spectrum (NPS), and DQE were in good agreement with Monte Carlo simulations. Imaging signal loss from recombination within the KCD chamber was measured at different gas pressures, ion drift velocities, and strip-collimation widths. Image quality for the prototype KCD was also observed with anthropomorphic phantom imaging in comparison with various commercial and research portal imaging systems, including VEPID, flat-panel imager, and conventional and high contrast film systems. KCD-based imaging provided very good contrast and good spatial resolution at very low imaging dose (0.1 cGy per image). For the prototype KCD, measurements yielded DQE(0)=0.19 and DQE(1 cy/mm)=0.004.

A gas cell for thermalizing, storing and transporting radioactive ions and atoms. Part II: On-line studies with a laser ion source

The application of a gas cell filled with noble gas (helium or argon) for thermalizing, storing and transporting trace radioactive ions and atoms has been studied in on-line conditions. Radioactive ions produced in nuclear reactions and stable energetic ions have been resonantly re-ionized by laser light via a two-step resonant process after thermalization and neutralization in high-pressure noble gas. The influence of the ion–electron density created by the projectile beam on the recombination of exotic ions has been investigated in different experimental conditions, including DC and RF electrical fields in the gas cell. Results for the laser ion source efficiency and selectivity for heavy-ion induced fusion reaction products are given. For the Rh isotopes the efficiency reaches up to 12%.

A gas cell for thermalizing, storing and transporting radioactive ions and atoms. Part II: On-line studies with a laser ion source

The application of a gas cell filled with noble gas (helium or argon) for thermalizing, storing and transporting trace radioactive ions and atoms has been studied in on-line conditions. Radioactive ions produced in nuclear reactions and stable energetic ions have been resonantly re-ionized by laser light via a two-step resonant process after thermalization and neutralization in high-pressure noble gas. The influence of the ion–electron density created by the projectile beam on the recombination of exotic ions has been investigated in different experimental conditions, including DC and RF electrical fields in the gas cell. Results for the laser ion source efficiency and selectivity for heavy-ion induced fusion reaction products are given. For the Rh isotopes the efficiency reaches up to 12%.

Disruption mitigation with high-pressure noble gas injection

High-pressure gas jets of neon and argon are used to mitigate the three principal damaging effects of tokamak disruptions: thermal loading of the divertor surfaces, vessel stress from poloidal halo currents and the buildup and loss of relativistic electrons to the wall. The gas jet penetrates as a neutral species through to the central plasma at its sonic velocity. The injected gas atoms increase up to 500 times the total electron inventory in the plasma volume, resulting in a relatively benign radiative dissipation of >95% of the plasma stored energy. The rapid cooling and the slow movement of the plasma to the wall reduce poloidal halo currents during the current decay. The thermally collapsed plasma is very cold (∼1–2 eV) and the impurity charge distribution can include >50% fraction neutral species. If a sufficient quantity of gas is injected, the neutrals inhibit runaway electrons. A physical model of radiative cooling is developed and validated against DIII-D experiments. The model shows that gas jet mitigation, including runaway suppression, extrapolates favorably to burning plasmas where disruption damage will be more severe. Initial results of real-time disruption detection triggering gas jet injection for mitigation are shown.

Status of the laser ion source at the Leuven isotope separator online

An online laser ion source has been used at the Leuven isotope separator online for the production of pure beams of exotic nuclei. The operational principle of the ion source is based on the element-selective multistep laser resonance ionization of nuclear reaction products thermalized and neutralized in a high-pressure noble gas. A number of improvements has been carried out to obtain stable and reproducible operation. The ion source has been optimized for the production of beams of exotic nuclei, created in proton-induced fission reactions. The efficiency of the ion source has been improved by incorporating the sextupole ion guide to separate laser-produced ions from the gas jet and to transport them to the acceleration stage of the mass separator. A gas purification system has been installed to purify the noble gas down to ppb level. High selectivity and efficiency of the ion source allowed to collect nuclear spectroscopic information for the neutron deficient Ni54 and neutron-rich Ni68–74 isotopes.

Effective secondary emission coefficient in a high-pressure noble gas

The back-scattering of secondary electrons toward the cathode in a high-density noble gas and the resulting effective secondary (electron) emission coefficient, ESEC, is evaluated using a kinetic approach. The behaviour of the ESEC in a mixture of noble gases is also discussed. The obtained results can be applied to different sources of secondary electrons - ions, photons, etc. We provide a comparison of obtained expressions with experiments.

Laser ion source for the Leuven Isotope Separator On-Line

An on-line laser ion source has been developed and coupled to an on-line isotope separator for the production of isotopically and isobarically pure beams of radioactive ions. It is based on element-selective resonant laser ionization of nuclear reaction products thermalized and neutralized in a high-pressure noble gas after their recoil out of the target. Pure beams of 54,55Ni and 54Co, produced in a light-ion-induced fusion-evaporation reaction, and of 113Rh, produced in a proton-induced fission of 238U, were obtained. A preliminary efficiency of the ion source of ∼6% for fusion reactions and of ∼0.2% for fission reactions has been obtained. A selectivity of the ion source of 300 for fusion and 50 for fission reactions limited by the evacuation of non-neutralized reaction products has been achieved.

High-Pressure Noble Gas Broadening of the Cs Resonance Lines

The noble gas broadening (Ar and Kr) of the Cs resonance lines 8944 and 8521 Å (62S1/2-62P1/2, 3/2) was measured in absorption up to perturber pressures of about 20 atm. The experimental shapes of the 8944 Å line induced by multiperturber interaction are compared with theoretical shapes calculated by means of the quasistatic theory assuming additivity of the A2Π1/2 - X2Σ1/2 difference potential energies

Investigations of ionization of sodium vapour by resonant laser light

The ionization of dense sodium vapour in presence of various buffer gases by high power density light tuned to 3S → 3P transition was investigated. The by Measures proposed process of impact ionization by electrons heated in superelastic collisions was considered to be predominant. It was found, that the decrease of the ionization rate by a high pressure noble gas takes place due to the shortening of the effective lifetime of 3P-excited sodium atoms or due to formation of sodium-noble gas quasimolecules. The decrease of the time of imprisonment of the resonance radiation with the increase of the electron density was observed.

Photodissociative generation of population inversion in alkali noble gas excimer systems

Alkali noble gas excimer molecules are produced by photodissociation of alkali halogen vapor in a high-pressure noble gas atmosphere. Experimental details for NaI-Xe and CsI-Xe systems excited by 193 nm ArF or 249 nm KrF laser radiation are given, and the possibility of realizing an optically pumped alkali noble gas excimer laser is discussed.