Neutral Pressure Research Articles

Wisconsin In Situ Penning (WISP) gauge: A versatile neutral pressure gauge to measure partial pressures in strong magnetic fields.

A new type of in-vessel Penning gauge, the Wisconsin In Situ Penning (WISP) gauge, has been developed and successfully operated in the Wendelstein 7-X (W7-X) island divertor baffle and vacuum vessel. The capacity of the quantitative measurements of the neutral reservoir for light impurities, in particular, helium, is important for tokamaks as well as stellarator divertors in order to avoid fuel dilution and radiative energy loss. Penning gauges assisted by spectroscopy are a powerful tool to obtain the total neutral pressure as well as fractional neutral pressures of specific impurities. The WISP gauge is a miniaturized Penning gauge arrangement, which exploits the ambient magnetic field of magnetic confinement fusion experiments to establish the Penning discharge. Then, in situ spectroscopy is conducted to separate the fractional neutral pressures of hydrogen, helium, and possibly also other impurities. The WISP probe head was qualified using the magnetic field of the Magnetized Dusty Plasma Experiment at Auburn University between 0.25 T and 3.5 T [E. Thomas et al., J. Plasma Phys. 81, 345810206 (2015)]. The in-depth quantitative evaluation for hydrogen and helium will be shown as well as an exploration of nitrogen, argon, and neon. A power law scaling between current I and pressure p, I = f(Gas,V) · pn(Gas, B), was shown. The factor f is gas and anode potential dependent, while n is gas and magnetic field strength dependent. Pressure measurements from 0.1 mbar and down to 1 × 10-5 mbar were achieved, demonstrating a reliable operating range for relevant pressure levels in the divertor and main vessel regions in current and future fusion devices, with a time resolution of up to 1 kHz. The lowest achievable pressure measurement increases with an increase in B and can be shifted with the anode potential V. At W7-X, the WISP probe head was mounted on an immersion tube setup that passes through the cryostat and places the probe head close to the plasma. Two probe heads were positioned in different divertor pump gaps, top and bottom, and one close to the plasma on the midplane in one module. The gauges were in situ calibrated together with the ASDEX pressure gauges [G. Haas and H.-S. Bosch, Vacuum 51, 39 (1998)]. Data were taken during the entire operation phase 1.2b, and measurements were coherent with other neutral gas pressure gauges. For the spectroscopic partial pressure measurements, channels of a spectroscopic detection system based on photo-multipliers, a so-called filterscope [R. J. Colchin et al., Rev. Sci. Instrum. 74, 2068 (2003)], provided by the Oak Ridge National Lab were used.

Application of dynamic mode decomposition to rotating structures in detached linear plasmas

Dynamic mode decomposition (DMD) was applied to time-series snapshots of dynamic behavior in detached plasmas with a fast framing camera in the linear plasma device NAGDIS-II. The DMD extracted radial plasma ejection and Er×B rotation structures, which are associated with blob-like plasma structures. Besides, we investigated the influences from neutral gas pressure on the growth rates of the DMD modes. By increasing the neutral gas flow rate in the detached plasma, the growth rate of rotation mode became larger while the frequency decreased. The results indicated that the ejected plasma existed for a longer time in the periphery region. It is likely due to the fact that the ratio of the radial velocity to the rotation velocity of the ejected plasma decreased.

Construction of a linear plasma device for studying helicon plasmas relevant to plasma-wakefield accelerators

The magnetized anisotropic ion apparatus (MARIA) was constructed to study the density buildup and particle balance in helicon plasmas. This device will help address key questions surrounding the dynamics of the neutral argon population and the role it plays in defining the ultimate density achieved. Insights gained from this research are particularly important for meeting the demanding high density and high uniformity requirements of plasma based wakefield accelerator concepts. A key feature of the MARIA device is it is fully transparent borosilicate glass chamber which enables the flexible use and development of optical techniques for measuring plasma parameters like electron temperature and density. Whistler-mode behavior featuring very high on-axis ionization fraction and a linear relationship between electron density and magnetic field strength up to 5 × 1018 m−3 has been measured with an RF compensated Langmuir probe in argon with 700 W RF power, 650 G at the antenna, and 2 mTorr neutral pressure. Laser induced fluorescence measurements, which build on the Langmuir probe measurements, are able to fully constrain the 2D particle balance by enabling ion and neutral sources and sinks to be spatially resolved. Initial flow velocity measurements in the axial direction are presented.

Study of Initial Pressure Rise in Multi Grain Solid Propellant Rocket Motor

AbstractIn the present research, an experimental and numerical investigation has been performed to study initial pressure rise in multi grain solid rocket motor. A cluster of seven uninhibited tubular grains of double base propellant is analysed to get near neutral pressure in rocket motor. It is experimentally observed that in some static firings initial pressure rise in first 100 ms is higher than the predicted value of ∼7.5 MPa. The average pressure in the rocket motor is ∼7 MPa and total burning time of these grains is ∼2 s. ANSYS software is used for numerical analysis to investigate the effects of various % openings of nozzle end (NE) metal grid on pressure rise in the rocket motor with and without peripheral opening. The analysed results are compared with experiments. It is found that, as the %opening of NE metal grid increases, the initial pressure in the rocket motor decreases. The peripheral opening is also important to reduce pressure rise in rocket motor. This study helps in reducing the initial pressure rise in the combustion chamber which is essential for safe working of the rocket motor.

SOLPS-ITER modelling of ITER edge plasma with drifts and currents

Using the new version of the SOLPS plasma boundary code package, SOLPS-ITER, this paper presents the first ever simulations of the ITER burning baseline H-mode edge plasma with drifts and currents activated. Neon (Ne) seeded discharges for divertor power dissipation are considered. The results for divertor and scrape-off layer (SOL) parameters with and without drifts are compared, both for the SOLPS-ITER simulations and against the earlier SOLPS-4.3 modelling (which did not include a drift description) constituting the bulk of the existing ITER divertor simulation database. Whereas the drift effect on the equatorial midplane (main chamber) density and temperature profiles is moderate, drifts increase the peak heat flow to the outer divertor target. This effect is more pronounced for regimes with low sub-divertor neutral pressure, when even drift-free SOLPS4.3 simulations find strong out-in target power asymmetries. An important conclusion is thus that if ITER operates as expected with partially detached divertor targets, drifts should not influence the power handling, but that in the case of divertor reattachment, they will act to worsen the target loading, increasing the need for development of reliable schemes for detachment control. Comparing SOLPS-4.3 and SOLPS-ITER results for the key peak target heat flux versus sub-divertor neutral pressure operating domain, SOLPS-ITER with drifts predicts a narrower operational window for the divertor pressure.

Mechanical Stability of Filaments for ITER Diagnostic Pressure Gauges Relating to Creep and Fatigue

Diagnostic pressure gauges (DPGs) shall provide measurements of the neutral gas pressure in various locations of the International Thermonuclear Experimental Reactor (ITER) vacuum vessel. This parameter is essential for the basic control of ITER operation as well as for input to physics models of the plasma boundary. The hot cathode (filament) is the component of the DPG sensor that is exposed to most demanding loads. In order to reach the required electron emission from the filament, it has to be heated to high temperatures by a direct current. As a result, thermal stresses appear, and in the presence of a magnetic field, additional mechanical stresses caused by Lorentz forces arises. On the one hand, due to a large number of pulses foreseen in ITER, this load will be cyclic, which may result in filament failure caused by fatigue. On the other hand, the longest pulses in ITER are expected to be in the order of 30 min, and thus, filaments could fail due to creep. In order to verify that the filament of the DPG sensor can withstand fatigue and creep dedicated experiments have been conducted. Results of these tests are discussed in this article.

Evidence for neutrals carrying ion-acoustic wave momentum in a partially ionized plasma

An experimental study of Ion Acoustic (IA) wave propagation is performed to investigate the effect of neutral density for Argon plasma in an unmagnetized linear plasma device. The neutral density is varied by changing the neutral pressure, which, in turn, allows the change in ion-neutral, and the electron-neutral collision mean free path. The collisions of plasma species with neutrals are found to modify the IA wave characteristics such as the wave amplitude, the velocity, and the propagation length. Unlike the earlier reported work where neutrals tend to heavily damp the IA wave in the frequency regime ω<νin (where ω is the ion-acoustic mode frequency and νin is the ion-neutral collision frequency), the experimental study of the IA wave presented in this paper suggests that the collisions support the wave to propagate for longer distances as the neutral pressure increases. A simple analytical model is shown to qualitatively support the experimental findings.

Experimental investigation of sheath effects on I–V traces of strongly electron emitting probes

I–V traces of strongly emitting emissive probes are investigated in a multidiople filament discharge. It is found that at sufficiently high neutral pressure and emitting current, the variation of the I–V traces and their associated inflection points no longer follow the previous predictions of space charge limited (SCL) models. A new, steep slope region of the I–V trace appears near the plasma potential when the probe is strongly emitting, causing the inflection point and the floating potential to increase towards the plasma potential as emission current increases, rather than staying constant. This is, to our knowledge, the first experimental evidence that the effects predicted by Campanell et al’s inverse sheath theory (2017 Physics of Plasmas 24 057101) not only affect the floating potential but also a region in the I–V trace of an emissive probe. It is also found that the double inflection point structure when the probe is biased below the ionization energy of the working gas is highly likely to be an emission retardation effect from enhanced virtual cathode formation due to the increased local electron density. The implications of these findings on hot cathode sources are briefly discussed.

Subsolar Electron Temperatures in the Lower Martian Ionosphere

AbstractMartian subsolar electron temperatures obtained below 250 km are examined using data obtained by instruments on the Mars Atmosphere Evolution Mission (MAVEN) during the three subsolar deep dip campaigns and a one‐dimensional fluid model. This analysis was done because of the uncertainty in MAVEN low electron temperature observations at low altitudes and the fact that the Level 2 temperatures reported from the MAVEN Langmuir Probe and Waves instrument are more than 400 K above the neutral temperatures at the lowest altitudes sampled (~120 km). These electron temperatures are well above those expected before MAVEN was launched. We find that an empirical normalization parameter, neutral pressure divided by local electron heating rate, organized the electron temperature data and identified a similar altitude (~160 km) and time scale (~2,000 s) for all three deep dips. We show that MAVEN data are not consistent with a plasma characterized by electrons in thermal equilibrium with the neutral population above 100 km. Because of the lack data below 120 km and the uncertainties of the data and the cross sections used in the one‐dimensional fluid model above 120 km, we cannot use MAVEN observations to prove that the electron temperature converges to the neutral temperature below 100 km. However, the uncertainty of the electron temperature altitude profile below 120 km does not impact our understanding of the role of electron temperature in determining ion escape rates because ion escape is determined by electron temperatures above 180 km.

Global transmission and evolutionary dynamics of the Chikungunya virus.

Chikungunya virus (CHIKV) is a re-emerging pathogen of global importance. We attempted to gain an insight into the organisation, distribution and mutational load of the virus strains reported from different parts of the world. We describe transmission dynamics and genetic characterisation of CHIKV across the globe during the last 65 years from 1952 to 2017. The evolutionary pattern of CHIKV was analysed using the E1 protein gene through phylogenetic, Bayesian and Network methods with a dataset of 265 sequences from various countries. The time to most recent common ancestor of the virus was estimated to be 491 years ago with an evolutionary rate of 2.78 × 10-4 substitutions/site/year. Genetic characterisation of CHIKV strains was carried out in terms of variable sites, selection pressure and epitope mapping. The neutral selection pressure on the E1 gene of the virus suggested a stochastic process of evolution. We identified six potential epitope peptides in the E1 protein showing substantial interaction with human MHC-I and MHC-II alleles. The present study augments global epidemiological and population dynamics of CHIKV warranting undertaking of appropriate control measures. The identification of epitopic peptides can be useful in the development of epitope-based vaccine strategies against this re-emerging viral pathogen.

Deuterium retention in Be-D co-deposits formed over an ITER relevant parameter space

Be-D co-deposit samples are produced over an extended range of plasma-material interaction parameters reflective of the conditions expected in ITER deposition dominant locations. Deposition temperature, D impact energy, atomic Be deposition rate, and D2 neutral pressure were varied in the ranges 373–623 K, ∼2–100 eV, 1–10 × 1019 m2 s−1, and 0.27–13.3 Pa respectively. Thermal desorption spectrometry was carried out on samples, subsequently, to measure the D release profile and retention. It was found that the D retention in samples agrees well with the existing GDT D/Be scaling over the extended parameter space, and that increased D impact energy and D2 pressure can lead to the observation of a sharp low-temperature release peak at ∼440 K in desorption data. Desorption data without the presence of the sharp release mechanism are modeled with TMAP-7 utilizing three traps of de-trapping energies, 0.8, 0.9 and 1.0 eV.

温度差換気を採用する高層建物の自然換気制御効果の推定法

As natural ventilation significantly contributes to energy savings and enhances comfort, it has been adopted in many buildings. However, to maintain a comfortable room environment, including the lower limit control for room temperature, it is necessary to control the natural ventilation rate by adjusting the opening area or opening time (referred to as opening ratio). Thus, we proposed a new airflow balance simulation tool, based on Proportional-Integral-Derivative (referred to as PID) control theory, to evaluate the performance of natural ventilation control in office buildings. The proposed tool can approximately solve the thermal and airflow balance by simply adding to the existing thermal load calculations. We focused on buoyancy driven ventilation, and analyzed the following parameters using the proposed simulation tool: change in neutral pressure level, thermal load reduction, and room environment.

The effect of voluntary head movements on postural kinetics in the standing cat.

BackgroundAlthough the postural instability accompanying bilateral vestibular loss in human and quadrupeds during lateral head movements are well-known, it is still unclear whether or not lateral head turns would indeed activate the postural control system to maintain balance. This study aimed to examine the kinetic parameters in freely standing intact cats during head movements in order to further answer the above question.MethodsSix intact cats were trained to stand, unrestrained on a force plate and perform voluntary head movements to the left and right positions in response to visual cues. Each trial was divided into two phases, quiet standing with the cat’s head maintaining a straight forward and lateral head position after voluntary head movements. Kinetic parameters including peak pressure and contact area under each limb as well as center of pressure (COP) displacements of the whole body were measured.ResultsCompared to the neutral head position, peak pressure and contact area of the left head position were significantly smaller for the left forelimb while greatly larger for the right forelimb. An exact opposite case of peak pressure and contact area in the forelimbs was found between the right and neutral head positions. In addition, the COP displacements altered oppositely to the head movements, and presented a significantly right shift in the left position and a significantly left shift in the right position.ConclusionThese results demonstrate that the lateral displacement of the head in standing intact cats does activate the postural adjustment to maintain balance, which is consistent with the concept that vestibular input can contribute to postural balance during voluntary head turns.

Performance of new crystal cathode pressure gauges for long-pulse operation in the Wendelstein 7-X stellarator.

To improve the reliability of the ASDEX pressure gauges in the plasma vessel of the Wendelstein 7-X stellarator, nine of them were equipped with a LaB6 crystal electron emitter for the first time. These crystal cathode pressure gauges were operated during the last campaign in 2018 (operation phase 1.2b) with only 2 A heating current for over 40 h in a magnetic field of about 2.1 T without failure. Owing to this excellent performance, we have decided to equip all pressure gauges with crystal cathodes for the next campaign of Wendelstein 7-X (operation phase 2). We report on a pretest in a superconducting magnet, show a measurement of the neutral pressure in Wendelstein 7-X, and demonstrate the long-term stability of the crystal cathode pressure gauges.

Floral volatiles identification and molecular differentiation of Osmanthus fragrans by neutral desorption extractive atmospheric pressure chemical ionization mass spectrometry

Floral volatiles are commonly present only at trace amounts and can be degraded or lost during vapor collection, which is often challenging from the analytical standpoint. Osmanthus fragrans Lour. is a widely cultivated plant known for the highly distinct fragrance of its flowers. The identification of specific volatile organic compounds (VOCs) and molecular differentiation of O. fragrans without any chemical pretreatment and VOC collection are important. Twenty-eight VOCs released by the flowers from ten different cultivars of O. fragrans were identified using neutral desorption extractive atmospheric pressure chemical ionization mass spectrometry (ND-EAPCI-MS) without any chemical pretreatment or VOC collection. Chemical identification was performed by high-resolution MSn analysis and whenever possible was confirmed by the analysis of standards. According to our literature search, nine of the identified VOCs, 3-buten-2-one, cyclohexadiene, 2-methylfuran, 3-allylcyclohexene, cuminyl alcohol, hotrienol oxide, epoxy-linalool oxide, N-(2-hydroxyethyl) octanamide, and 3-hydroxy-dihydro-β-ionone, have not been reported in O. fragrans in earlier studies. Confident differentiation between ten different cultivars of O. fragrans was achieved by the principal component analysis of the mass spectrometric results. The results of our ND-EAPCI-MS analysis substantially increase our knowledge about the chemistry of the O. fragrans floral fragrance and demonstrate the power of this technique for direct molecular profiling for plant recognition or in biotechnological applications.

Hybrid ventilation of a room: A theoretical model for the combined effects of mechanically-imposed and buoyancy-induced driving pressures

A supply fan and an extract fan, at identical settings in identical rooms, do not necessarily have the same effect on the ventilation. For a heated room in which the floor-level vents are larger than the ceiling-level vents, we show that an extract fan provides better ventilation than a supply fan. If the ceiling-level vents are larger, we show that a supply fan is more effective; this for the same, constant fan airflow rate. We investigate the hybrid ventilation of a room in which the, otherwise buoyancy-driven, ventilation is augmented by a forced volume flux, provided by a supply or extract fan. In hybrid ventilation, naturally-occurring and mechanically-imposed pressure differences combine to determine the resulting ventilation airflow. Herein, a mathematical model is developed which enables prediction of: the inflow and outflow volume fluxes; the vertical position of the neutral pressure level; and the steady, uniform temperature, within a single, isolated room in hybrid ventilation. The physical problem is reduced to a mathematical model with two controlling parameters; namely, the effective vent area ratio, R*, and a ratio of forced and natural volume fluxes, QF/QN. We demonstrate that the volume flux through an open vent can be controlled remotely, by mechanically imposing the volume flux through an entirely separate vent.

Influence of external magnetic field on dust acoustic waves in a capacitive RF discharge

AbstractThis paper reports experiments on self‐excited dust acoustic waves (DAWs) and its propagation characteristics in a magnetized rf discharge plasma. The DAWs are spontaneously excited in dusty plasma after adding more particles in the confining potential well and found to propagate in the direction of streaming ions. The spontaneous excitation of such low‐frequency modes is possible due to the instabilities associated with streaming ions through the dust grain medium. The background E‐field and neutral pressure determine the stability of excited DAWs. The characteristics of DAWs strongly depend on the strength of external magnetic field. The magnetic field of strength B < 0.05 T only modifies the characteristics of propagating waves in dusty plasma at moderate power and pressure, P = 3.5 W and p = 27 Pa, respectively. It is found that DAWs start to be damped with increasing the magnetic field beyond B > 0.05 T and get completely damped at higher magnetic field B ∼ 0.13 T. After lowering the power and pressure to 3 W and 23 Pa respectively, the excited DAWs in the absence of B are slightly unstable. In this case, the magnetic field only stabilizes and modifies the propagation characteristics of DAWs while the strength of B is increased up to 0.1 T or even higher. The modification of the sheath electric field where particles are confined in the presence of the external magnetic field is the main cause of the modification and damping of the DAWs in a magnetized rf discharge plasma.

Scrape-off layer transport and filament characteristics in high-density tokamak regimes

A detailed cross-device investigation on the role of filamentary dynamics in high-density regimes has been performed within the EUROfusion framework, comparing the ASDEX Upgrade (AUG) and TCV tokamaks. Both devices run density ramp experiments at different levels of plasma current, keeping the toroidal field or q95 constant in order to disentangle the role of the parallel connection length and the current. During the scan at a constant toroidal field, in both devices the scrape-off layer (SOL) profiles tend to develop a clear SOL density shoulder at a lower edge density whenever the current is reduced. Different current behaviour is substantially reconciled in terms of the edge density normalized to the Greenwald fraction. During the scan at constant q95 AUG exhibits similar behaviour, whereas in TCV no upstream profile modification signature has been observed at lower current levels. The latter behaviour has been ascribed to the lack of target density rollover. The relation between the upstream density profile modification and detachment condition has been investigated. For both devices the relation between blob size and the SOL density e-folding length is found independent of the plasma current, with the observation of a clear increase in blob size and the edge density normalized to a Greenwald fraction. ASDEX Upgrade has also explored filamentary behaviour in the H-mode. The experiments in AUG have focused on the role of neutrals, performing discharges with and without cryogenic pumps, highlighting how high neutral pressure, not only in the divertor but also at the midplane, is needed in order to develop an H-mode SOL profile shoulder in AUG.

Link between divertor conditions and high-field-side/low-field-side midplane density profiles in H-mode plasmas at ASDEX Upgrade

The connection between midplane and divertor conditions is studied on ASDEX Upgrade (AUG) for H-mode scenarios. H-mode discharges with variations in fuelling, heating power and impurity seeding are analysed, enabling to disentangle their impact on the evolution of midplane density profiles. The O-mode reflectometer installed on AUG provides unique insights on the midplane density profiles thanks to the ability to measure simultaneously at the high-field-side (HFS) and the low-field-side (LFS). At the inner divertor, with the onset of detachment, a region of high density is formed (HFS high density front, HFSHD) that expands into the HFS midplane, leading to strong poloidal asymmetries in the main chamber scrape-off layer (SOL) density. A good agreement is observed between the evolution of the density profiles at the midplane and that of the divertor volume density confirming the strong influence of divertor conditions on the midplane density profiles at the HFS. Our results confirm the existence of a relationship between plasma confinement, the shift in the midplane LFS density profile and the presence of the HFSHD with respect to changes in seeding and fuelling. It is established that the separatrix density at the LFS is better correlated with the neutral pressure at the outer target while the HFS SOL density follows the neutral pressure at the inner divertor. A comprehensive characterisation of the edge-localised mode (ELM) evolution at the midplane and divertor is performed demonstrating that also during the ELM cycle the divertor conditions have a strong effect on the SOL density at the midplane. The most striking result is the observation of a HFSHD at the HFS midplane just after the ELM crash associated with strong inner divertor detachment.

Volumetric measurement of the synchronization and desynchronization of the dust acoustic wave with an external modulation

A spatio-temporal measurement showing the volumetric nature of the phase synchronization of a naturally occurring dust acoustic wave to an external modulation and the relaxation from the driven wave mode back to the naturally occurring wave mode (phase desynchronization) is presented. It is shown that the phase synchronization and desynchronization occur behind a propagating synchronization/desynchronization front that travels at a slower speed than the phase velocity of the wave and that the speed of this synchronization/desynchronization front decreases with increasing neutral gas pressure. It is also observed that volume of the wave that is synchronous depends on the frequency of the external modulation and the neutral gas pressure.