Neutral Pressure Research Articles

Investigation of reliability of the cutoff probe by a comparison with Thomson scattering in high density processing plasmas

A “cutoff probe” uses microwaves to measure the electron density in a plasma. It is particularly attractive because it is easy to fabricate and use, its measurement is immune to surface contamination by dielectric materials, and it has a straightforward analysis to measure electron density in real time. In this work, we experimentally investigate the accuracy of the cutoff probe through a detailed comparison with Thomson scattering in a low temperature, high density processing plasma. The result shows that the electron density measured by the cutoff probe is lower than that by Thomson scattering and that the discrepancy of the two results becomes smaller as the gap between the two tips increases and/or the neutral gas pressure decreases. The underestimated electron density found by the cutoff probe can be explained by the influence of the probe holder, which becomes important as the pressure increases and the gap gets closer.

Electromagnetic diagnostics of ECR-Ion Sources plasmas: optical/X-ray imaging and spectroscopy

Magnetoplasmas in ECR-Ion Sources are excited from gaseous elements or vapours by microwaves in the range 2.45–28 GHz via Electron Cyclotron Resonance. A B-minimum, magnetohydrodynamic stable configuration is used for trapping the plasma. The values of plasma density, temperature and confinement times are typically ne= 1011−1013 cm−3, 01 eV<Te<10 keV, 0.001<tc<1 s. At INFN-LNS several diagnostics tools have been developed for probing the electromagnetic emission of such plasmas, in the optical/X-ray domain. Fast Silicon Drift detectors with high energy resolution of 125 eV at 5.9 keV have been used for the characterization of plasma emission at 02<E<3 keV . In the domain 0.4–17 keV an X-ray pin-hole camera technique has allowed space resolved X-ray spectroscopy with a spatial resolution down to 30 μm and an energy resolution down to 140 eV at 5.9 keV . In parallel, imaging in the optical range and spectroscopic measurements have been carried out. Relative abundances of H/H2 atoms/molecules in the plasmas have been measured for different values of neutral pressure, microwave power and magnetic field profile (they are critical for high-power proton sources).

A Comparative Study on the Influence of Ventilation on Weather- and Fire-Induced Stack Effect in the Elevator Shafts of a High-Rise Building

This work assessed the impact of ventilation on both weather- and fire-induced stack effect in an 18-story high-rise office building. Elevator shafts are considered the main route of vertical air movement. Pressure distribution induced by cold weather within the elevator shafts was calculated theoretically. Computational fluid dynamics simulations of fire in the same high-rise building under different ventilation conditions were carried out with a fire dynamics simulator. It was found that ventilation exerted a more complex impact on fire than the weather-induced stack effect. For the weather-induced stack effect, the ventilation condition of the building only affected the height of the neutral pressure plane; in fire situations, it did not only affect the height of the neutral pressure plane in a similar manner to the weather-induced stack effect, but also influenced temperature and pressure distributions in the elevator shafts. The smoke movement and the distributions of temperature and pressure in elevator shafts are also learned. The smoke movement in high rises experienced four typical stages after ignition. The ventilation condition of the fire floor influences gas flow into elevator shafts, while that of the upper floors impacts the smoke rise speed in vertical shafts. When the stack effect finally reaches steady state, the gas temperature in the shaft decreases exponentially with height. Based on this assumption, a theoretical model was presented to characterize the fire-induced stack effect in typical high rises. Results showed that the model successfully predicts the pressure distribution in high-rise buildings.

Theoretical and experimental studies of a planar inductive coupled rf plasma source as the driver in simulator facility (ISTAPHM) of interactions of waves with the edge plasma on tokamaks

This research aims to design and build a planar inductive coupled RF plasma source device which is the driver of the simulator project (ISTAPHM) of the interactions between ICRF Antenna and Plasma on tokamak by using the AMPICP model. For this purpose, a theoretical derivation of the distribution of the RF magnetic field in the plasma-filled reactor chamber is presented. An experimental investigation of the field distributions is described and Langmuir measurements are developed numerically. A comparison of theory and experiment provides an evaluation of plasma parameters in the planar ICP reactor. The objective of this study is to characterize the plasma produced by the source alone. We present the results of the first analysis of the plasma characteristics (plasma density, electron temperature, electron-ion collision frequency, particle fluxes and their velocities, stochastic frequency, skin depth and electron energy distribution functions) as function of the operating parameters (injected power, neutral pressure and magnetic field) as measured with fixed and movable Langmuir probes. The plasma is currently produced only by the planar ICP. The exact goal of these experiments is that the produced plasma by external source can exist as a plasma representative of the edge of tokamaks.

A PIC-MCC code RFdinity1d for simulation of discharge initiation by ICRF antenna

Discharges produced and sustained by ion cyclotron range of frequency (ICRF) waves in absence of plasma current will be used on ITER for (ion cyclotron-) wall conditioning (ICWC, eV, ). In this paper, we present the 1D particle-in-cell Monte Carlo collision (PIC-MCC) RFdinity1d for the study the breakdown phase of ICRF discharges, and its dependency on the RF discharge parameters (i) antenna input power Pi, (ii) RF frequency f, (iii) shape of the electric field and (iv) the neutral gas pressure . The code traces the motion of both electrons and ions in a narrow bundle of magnetic field lines close to the antenna straps. The charged particles are accelerated in the parallel direction with respect to the magnetic field BT by two electric fields: (i) the vacuum RF field of the ICRF antenna and (ii) the electrostatic field determined by the solution of Poisson’s equation.The electron density evolution in simulations follows exponential increase, . The ionization rate varies with increasing electron density as different mechanisms become important. The charged particles are affected solely by the antenna RF field at low electron density ( , ). At higher densities, when the electrostatic field is comparable to the antenna RF field , the ionization frequency reaches the maximum. Plasma oscillations propagating toroidally away from the antenna are observed. The simulated energy distributions of ions and electrons at correspond a power-law Kappa energy distribution. This energy distribution was also observed in NPA measurements at ASDEX Upgrade in ICWC experiments.

Diamagnetism and neutrals depletion in a plasma

Recent experimental and theoretical findings [Shinohara et al., Phys. Plasmas 23, 122108 (2016)] regarding the pressure balance between a cylindrical plasma, an axial magnetic field, and neutral gas are explored further theoretically. The length of the cylinder is assumed much larger than its radius, so that axial losses are small and cross-field transport is dominant. Conditions for either magnetic pressure or neutral pressure balancing the plasma pressure and an associated coupling parameter, which were identified in the above-mentioned recent study, are examined further. In addition, a second coupling parameter is identified which determines which is larger, the relative change in the magnetic field or the relative change in neutral density. An unexpected nonmonotonic variation of the plasma density with the plasma particle flux is demonstrated. It is shown that for plasma beta close to unity, as plasma generation and plasma particle flux increase, the plasma density surprisingly decreases. This decrease follows a decrease in plasma confinement due to an increased plasma diamagnetism. The effect of the magnetic field on neutral depletion is examined. It is shown that an increase in the magnetic field as the plasma density is kept constant results in a decrease in neutral depletion, while an increase in the magnetic field as the plasma particle flux is kept constant results in constant neutral depletion.

Effect of error field correction coils on W7-X limiter loads

In the first campaign Wendelstein 7-X was operated with five poloidal graphite limiters installed stellarator symmetrically. In an ideal situation the power losses would be equally distributed between the limiters. The limiter shape was designed to smoothly distribute the heat flux over two strike lines. Vertically the strike lines are not uniform because of different connection lengths. In this paper it is demonstrated both numerically and experimentally that the heat flux distribution can be significantly changed by non-resonant perturbation field of the order of . Numerical studies are performed with field line tracing. In experiments perturbation fields are excited with five error field trim coils. The limiters are diagnosed with infrared cameras, neutral gas pressure gauges, thermocouples and spectroscopic diagnostics. Experimental results are qualitatively consistent with the simulations. With a suitable choice of the phase and amplitude of the perturbation a more symmetric plasma-limiter interaction can be potentially achieved. These results are also of interest for the later W7-X divertor operation.

Advanced neutral gas diagnostics for magnetic confinement devices

For the study of particle exhaust in nuclear fusion devices the neutral pressure must be measured in strong magnetic fields. We describe as an example the neutral pressure gauges in the Wendelstein 7-X stellarator. Two types are used: hot cathode ionization gauges (or ASDEX pressure gauges) and Penning gauges. We show some results from the first experimental campaign. The main problems were runtime effects and the failure of some ASDEX pressure gauges. To improve the reliability we integrated a new LaB6 electron emitter into the ASDEX pressure gauges. In addition, a special Penning gauge without permanent magnets was developed in order to operate Penning gauges near the plasma edge. These new pressure gauges will be used in the upcoming campaign of Wendelstein 7-X.

Measurement of linear polarization in the HeI 21P–31D emission line in an electron cyclotron resonance discharge plasma

The magnitude and direction of linear polarization in the HeI 21P–31D emission line are simultaneously measured using a beam-splitting-type polarization spectroscopy system. The polarization measured in the steady-state phase of an electron cyclotron resonance (ECR) discharge produced in a simple cusp field is directed closely perpendicular to the magnetic field at the ECR surface, and the degree of polarization increases up to 8% when the neutral pressure is reduced to less than 23 mPa. These results suggest that the cause of the polarization is attributable to impact excitation by high-energy electrons produced at the ECR surface.

Dynamics of spinning particle pairs in a single-layer complex plasma crystal.

Spontaneous formation of spinning pairs of particles, or torsions, is studied in a single-layer complex plasma crystal by reducing the discharge power at constant neutral gas pressure. At higher gas pressures, torsions spontaneously form below a certain power threshold. Further reduction of the discharge power leads to the formation of multiple torsions. However, at lower gas pressures the torsion formation is preceded by mode-coupling instability (MCI). The crystal dynamics are studied with the help of the fluctuation spectra of crystal particles' in-plane velocities. Surprisingly, the spectra of the crystal with torsions and MCI are rather similar and contain hot spots at similar locations on the (k,ω) plane, despite very different appearances of the respective particle trajectories. The torsion rotation speed is close (slightly below) to the maximum frequency of the in-plane compressional mode. When multiple torsions form, their rotation speeds are distributed in a narrow range slightly below the maximum frequency.

Application of optical emission spectroscopy to high current proton sources

Optical Emission Spectroscopy (OES) represents a very reliable technique to carry out non-invasive measurements of plasma density and plasma temperature in the range of tens of eV. With respect to other diagnostics, it also can characterize the different populations of neutrals and ionized particles constituting the plasma. At INFN-LNS, OES techniques have been developed and applied to characterize the plasma generated by the Flexible Plasma Trap, an ion source used as "testbench" of the proton source built for European Spallation Source. This work presents the characterization of the parameters of a hydrogen plasma in different conditions of neutral pressure, microwave power and magnetic field profile, along with perspectives for further upgrades of the OES diagnostics system.

Impact of divertor geometry on H-mode confinement in the JET metallic wall

Recent experiments with the ITER-like wall have demonstrated that changes in divertor strike point position are correlated with strong modification of the global energy confinement. The impact on energy confinement is observable both on the pedestal confinement and core normalised gradients. The corner configuration shows an increased core density gradient length and ion pressure indicating a better ion confinement. The study of neutral re-circulation indicates the neutral pressure in the main chamber varies inversely with the energy confinement and a correlation between the pedestal total pressure and the neutral pressure in the main chamber can be established. It does not appear that charge exchange losses nor momentum losses could explain this effect, but it may be that changes in edge electric potential are playing a role at the plasma edge. This study emphasizes the importance of the scrape-off layer (SOL) conditions on the pedestal and core confinement.

Development of a Helicon Plasma Source for Neutral Beam Injection System of the Alborz Tokamak

Amirkabir Helicon plasma source (AHPS) has been designed and developed at the fusion laboratory of Amirkabir University of Technology for the neutral beam injection (NBI) system of the Alborz tokamak. The design parameters of the Helicon plasma source are determined to get the $$ n_{e} \cong 10^{19} \,{\text{m}}^{ - 3} $$ plasma density. However, the Nagoya type III antenna was used to excite the Argon plasma by the applied magneto-static field of 750 G and RF power up to 2 kW at the frequency of 13.56 MHz. Preliminary experiments are performed to confirm the Helicon mode operation of the AHPS. Mode changing from the ICP to the Helicon mode was proved by using the optical emission spectroscopy and polaroid photography technique of the plasma “blue core”. Spectroscopy measurements results show that the emission line intensity of the $$ {\text{Ar}}\;{\text{II}} $$ at 434.8 nm increases as the RF power increases in the range of values of 300–1000 W at the neutral background pressure of 0.7 mTorr. Also, we found that, in Helicon mode, the increase of the magneto-static field from 350 to 750 G not only increases the plasma density besides the efficiency of the RF power absorption increases.

Overview of ASDEX Upgrade results

The ASDEX Upgrade (AUG) programme is directed towards physics input to critical elements of the ITER design and the preparation of ITER operation, as well as addressing physics issues for a future DEMO design. Since 2015, AUG is equipped with a new pair of 3-strap ICRF antennas, which were designed for a reduction of tungsten release during ICRF operation. As predicted, a factor two reduction on the ICRF-induced W plasma content could be achieved by the reduction of the sheath voltage at the antenna limiters via the compensation of the image currents of the central and side straps in the antenna frame. There are two main operational scenario lines in AUG. Experiments with low collisionality, which comprise current drive, ELM mitigation/suppression and fast ion physics, are mainly done with freshly boronized walls to reduce the tungsten influx at these high edge temperature conditions. Full ELM suppression and non-inductive operation up to a plasma current of MA could be obtained at low plasma density. Plasma exhaust is studied under conditions of high neutral divertor pressure and separatrix electron density, where a fresh boronization is not required. Substantial progress could be achieved for the understanding of the confinement degradation by strong D puffing and the improvement with nitrogen or carbon seeding. Inward/outward shifts of the electron density profile relative to the temperature profile effect the edge stability via the pressure profile changes and lead to improved/decreased pedestal performance. Seeding and D gas puffing are found to effect the core fueling via changes in a region of high density on the high field side (HFSHD).The integration of all above mentioned operational scenarios will be feasible and naturally obtained in a large device where the edge is more opaque for neutrals and higher plasma temperatures provide a lower collisionality. The combination of exhaust control with pellet fueling has been successfully demonstrated. High divertor enrichment values of nitrogen have been obtained during pellet injection, which is a prerequisite for the simultaneous achievement of good core plasma purity and high divertor radiation levels. Impurity accumulation observed in the all-metal AUG device caused by the strong neoclassical inward transport of tungsten in the pedestal is expected to be relieved by the higher neoclassical temperature screening in larger devices.

Influence of He and Ar injection on ammonia production in N2/D2 plasma in the medium flux GyM device

Nitrogen used to reduce the power load onto the divertor in tokamak fusion devices (JET, AUG) has the drawback of ammonia formation. Non-negligible quantity of tritiated ammonia could be a serious concern for ITER, since it cannot be reduced through the presently designed fuel cycle loop.In this paper we report a study of the production of ND3 as a function of the electron temperature (Te) and neutral pressure in a N2/D2 plasma mixture in the linear machine GyM. The nitrogenized compounds were monitored by Optical Emission Spectroscopy (OES) and Mass Spectrometry (MS). Measurements were performed at different values of Te ranging from 3eV to 6eV by varying the microwave power (2.45GHz, up to 0.6kW cw) that sustains the plasma, and for different neutral pressure at a constant ratio of nitrogen and deuterium partial pressures. The effect of introduction of He or Ar in the N2/D2 mixtures has been also investigated.The ND3 produced during plasma experiments has been quantified with a dedicated setup based on an in-line LN2 trap and Liquid Ion Chromatography (LIC).Mass-spectrometry results showed that ND3 is formed only during the plasma phase of the experiment while LIC showed that ammonia production increases with Te and with the total neutral pressure. Optical Emission Spectroscopy confirms the presence of ND species in all the cases studied. The addition of He and Ar in the N2/D2 plasma, not modifing Te and ne, causes a reduction of ammonia formation, that is negligible in the case of Ar addition, while in the case of He reaches 80%. This suggests that He modifies the physical chemical process occurring at the wall, where the adsorbed He inhibits the reactions leading to ammonia formation at the metallic surface of the vessel. These results confirm the active role of a metallic surface in ammonia production and indicate He injection as a promising solution to limit the formation of the tritiated ammonia in the N seeded plasma of ITER.

Simulation of neutral gas flow in the JET sub-divertor

The present work presents a numerical study of the neutral gas dynamics in the JET sub-divertor. A complex model of the sub-divertor geometry is implemented and successful comparisons between corresponding numerical and experimental data have been performed. The experimental data represent the neutral gas pressure obtained by a sub-divertor pressure gauge. The recently developed Divertor Gas Simulator (DIVGAS) which is based on the Direct Simulation Monte Carlo (DSMC) method is applied. DIVGAS is able to predict the behaviour of the flow including macroscopic quantities of practical interest as for instance the pressure, temperature and bulk velocity. The non-linear feedback of the sub-divertor gas flow on the divertor plasma vicinity is not taken into account. For all presented plasma cases, the deduced flow pattern is non-isothermal and covers the free molecular up to the transition flow regime. Furthermore, for low intermediate and high divertor density simulations, recirculation effects occur through gaps between the vertical target tiles, which seem to be two order of magnitude less compared with the recycling ion flux onto the divertor walls.

Molecular activated recombination in divertor simulation plasma on GAMMA 10/PDX

In the tandem mirror GAMMA 10/PDX, molecular activated recombination (MAR) leading to plasma detachment has been observed by additional hydrogen gas injection to the divertor simulation plasma (i.e. end loss plasma) which is exposed to the V-shaped target in the divertor simulation experimental module (D-module). The temperature near the corner of the V-shaped target decreased from ∼23eV to ∼2eV as the neutral pressure in the D-module increased. A clear density rollover was observed at ∼2Pa. A position of the density maximum moves to upstream of the plasma with increase in the neutral pressure and the density near the corner of the target decreases to detach the plasma from the target. After the occurrence of the density rollover, the Balmer β intensity decreases as with the density but the Balmer α intensity continues to increase, indicating the dissociative attachment process in MAR is more dominant than the ion conversion process although the rate coefficient of the former process is lower than that of the latter one, which is calculated by using a collisional radiative model. This would be caused by the MAR process related to triatomic hydrogen molecules which significantly contributed to the detachment process.

Experiments on actuator management and integrated control at ASDEX Upgrade

The established field of integrated multivariable control promises improved performance and stability for strongly coupled processes, given a control-oriented model of the system. Fusion plasmas are strongly coupled, but there is currently no model which accurately reflects the nature of these complex interactions. Therefore, experiments were performed specifically to investigate coupling between controlled parameters, as a step towards designing integrated controllers in the future. The parameters chosen were core density, divertor neutral pressure and divertor temperature. For control of the plasma pressure and Neoclassical Tearing Modes, where a simple model of the coupling is known, it will be shown that linking the two controllers gives reliably good plasma performance.An additional complication with integrated control is that limited actuator resources are often oversubscribed when trying to control multiple parameters simultaneously. In order to achieve the optimum result, some form of actuator management is a pre-requisite for integrated control. An algorithm has been developed to automatically allocate Electron Cyclotron Resonant Heating gyrotrons to targets, by evaluating a cost function in real-time. Results will be shown to demonstrate the flexibility of this routine to changes in plasma state, gyrotron availability and the goals of the physics experiment.

Scalings and universality for high-frequency excited high-pressure argon microplasma

The breakdown transition mechanism, scaling law for transition frequency, and universal law for breakdown voltage of a Ramsauer gas Ar with various ranges of neutral gas pressure and micron gap distance between parallel electrodes are examined. The electron kinetics in argon gas is analyzed to understand the reason of the abrupt transition of breakdown voltage partitioning γ- and α-regimes. The quiver motion of electron in high frequency source implies that the breakdown voltage drastically drops when the oscillating amplitude of an electron becomes smaller than its critical value. The scaling law, which reveals that the transition frequency is inversely proportional to the neutral gas pressure and the gap distance to the fractional power, supports the conjecture about the transition mechanism, and this is confirmed by particle-in-cell incorporating Monte Carlo collision (PIC/MCC) simulations. Breakdown voltage as a function of the product of the neutral gas pressure and gap distance, the ratio of the driving frequency and neutral gas pressure, secondary electron emission coefficient induced by the ion bombardment, and the ratio of gap distance over the radius of electrodes is expressed by the universal law which, as well, are confirmed by the PIC/MCC and fluid simulations. Furthermore, no universality is observed at the plasma size of 3 μm with field emission under diversified neutral gas pressure.

Hydrogen Plasma Characterization at Low Pressure in 2.45 GHz Electron Cyclotron Resonance Proton Ion Source

The plasma parameters in the 2.45 GHz electron cyclotron resonance (ECR) proton ion source have been measured and optimized at low pressures. These parameters are important as they are related to the beam parameters of the ion source. A detailed plasma characterization study has been conducted using hydrogen as a working gas in the operating pressure and power range $10^{-4}$ – $10^{-3}$ mbar, 400–1000 W of the ion source, respectively. The plasma parameters have been measured in the ECR zone of the plasma chamber. An automatic Langmuir probe and data acquisition system has been used to measure these parameters. In the above-mentioned power and pressure range, the plasma density, plasma electron temperature, and plasma potential measured are in the range $4 \times 10^{11}$ – $6.5 \times 10^{11}$ cm $^{-3}$ , 4–11 eV, and 30–40 V, respectively. The variation of plasma parameters with neutral gas pressure is studied for different microwave powers. A 0-D (global) model has been developed to compare the measured plasma parameter data with the calculated ones. The ion species content in the plasma was measured after extraction using a magnetic mass analyzer and a Faraday cup. A plasma-based model has been developed to compare the measured ion species fraction data with the calculated ones by taking the measured ECR plasma parameters as input.