Increasing Plasma Pressure Research Articles

Magnetic holes between Earth and Mercury: BepiColombo cruise phase

Context.Magnetic holes are ubiquitous structures in the solar wind and in planetary magnetosheaths. They consist of a strong depression of the magnetic field strength, most likely in pressure balance through increased plasma pressure, which is convected with the plasma flow. These structures are created through a plasma temperature anisotropy, where the perpendicular temperature (with respect to the magnetic field) is greater than the parallel temperature. The occurrence rate of these magnetic holes between Earth and Mercury can give us information about how the solar wind conditions develop on their way from the Sun to the outer Solar System. They also give information about basic plasma processes such as diffusion of magnetic structures.Aims.In this study we investigate the occurrence, size, and depth of magnetic holes during the cruise phase of BepiColombo and compare them with earlier studies.Methods.The BepiColombo magnetometer data were used to find the magnetic holes. We determined the size in seconds, the depth with respect to the background field, and the rotation angle of the background field across the structure. Minimum variance analysis delivers the polarization state of the magnetic holes. A direct comparison is made to the results obtained from the MESSENGER cruise phase.Results.We find an almost constant occurrence rate for magnetic holes between Mercury and Earth. The size of the holes is determined by the plasma conditions at the location where they are created and they grow in size, due to diffusion, as they move outwards in the Solar System. The greater the rotation of the background magnetic field across the structure, the larger the minimum size of the magnetic hole is.

Dependence of the boundary heat flux width on core and edge profiles in Alcator C-Mod

This work presents new evidence that the heat flux width, λ q , in the Alcator C-Mod tokamak scales with the edge electron pressure, as observed in the ASDEX Upgrade (AUG) tokamak (Silvagni et al 2020 Plasma Phys. Control. Fusion 62 045015), but the scaling with volume-averaged pressure, , from the plasma stored energy, found by Brunner et al (2018 Nucl. Fusion 58 094002), is a better predictor of λ q in Alcator C-Mod than the edge electron pressure. These previous studies, which find that λ q decreases with increasing plasma pressure, imply that a high performance core at high pressure will lead to challenging heat and particle exhaust due to very small λ q . This concern has led to our significant enlargement of the C-Mod database with the electron density, temperature, and pressure profile data from the Thomson scattering and electron cyclotron emission diagnostics. Using the C-Mod database augmented with new profile data, we find that λ q decreases with increasing edge electron pressure as , similar to results from AUG, and showing the strength of cross-machine comparisons. We also find that , consistent with the original finding from C-Mod that the heat flux width scales as (Brunner et al 2018 Nucl. Fusion 58 094002). The scalings of λ q with separatrix pressure and gradient scale length are found to match the AUG results qualitatively. The C-Mod scalings with edge plasma quantities have more scatter than the scaling, and, importantly, show different trends for H-modes relative to L- and I-mode. Investigating the source of this discrepancy presents an opportunity for further study that may improve our ability to predict the heat flux width in different confinement scenarios in the pursuit of optimizing core-edge performance in future reactors.

Numerical study of the effect of kinetic damping on resistive wall modes with plasma toroidal rotation in CFETR

Herein, the effect of kinetic damping on the resistive wall mode (RWM) instability in the China nuclear fusion engineering test reactor (CFETR) is numerically studied using the magnetohydrodynamic resistive spectrum-kinetic (MARS-K) code. A CFETR hybrid equilibrium with a plasma current of Ip = 13 MA is adopted. The sum of the electron diamagnetic, plasma rotation, and mode frequencies increases at q = 2 rational surface with increasing plasma rotation frequency. When the sum of the frequencies is less than 0, kinetic damping has a stabilizing effect on RWM. In contrast, when the sum of the frequencies is greater than 0, the kinetic damping has a destabilizing effect on RWM, causing a second unstable branch to appear. Additionally, the effects of plasma pressure and wall distance on the two unstable branches are investigated. The mode frequency is excited with increasing plasma pressure and wall distance. Consequently, the first branch is stabilized and the second branch is destabilized.

Dynamic oxidation mechanism of SiC fiber reinforced SiC matrix composite in high-enthalpy plasmas

A plasma wind tunnel was utilized to explore the dynamic oxidation mechanism of SiC fiber reinforced SiC matrix composite in high-enthalpy plasmas. The results suggest the occurrences of active and passive oxidations on SiC fiber/matrix with atomic oxygen at 900∼1600 °C and 1∼6 kPa. Increasing plasma pressure could retard the active oxidation and promote the passive oxidation. Severe corrosion of SiC fibers due to active oxidation is highlighted. The as-formed SiO2 layers cannot fully seal the open porosities and the interfacial gaps formed by oxidation of pyrocarbon interphases. This led to penetration of oxidized species and failure of fiber bundles directly exposed to heat flows. In addition, the spatial heat flux and temperature distributions were not homogeneous on the oxidized surface, which triggered early ‘temperature jump’ at ≈1600 °C (≈4.5 kPa) and severe localized ablation. If sealing the composite surface with SiC coating, the ablation resistance was significantly improved.

Hypertensive nephropathy as an outcome of unilateral nephrectomy in kidney cancer

Aim. To study the adaptive mechanisms of structural and functional changes in a single kidney after nephrectomy for kidney cancer.
 Materials and Methods. A total of 179 operations of two types were performed: nephrectomy and kidney resection in patients with cancerous lesions. Postoperative ultrasound was performed size control and dopplerography of the vessels of the contralateral single kidney, monitoring-control of blood pressure.
 Results. In case of kidney resection, the adaptive mechanisms controlling the volume of functioning tissue are preserved. The load on the organ remains minimal and physiological, and is not redistributed, blood pressure remains close to baseline. Nephrectomy does not lead to functional changes in a single kidney, but to adaptive and pathophysiological structural damage as a result of increased plasma pressure, organ reboot, its vicarious hypertrophy, which is accompanied by venous edema of interstitium as a pressure factor on the tissue, increased tone of arterioles, the development of secondary organ ischemia, circulatory hypoxia and increased blood pressure. All this fits into the clinical picture of hypertensive nephropathy.
 Conclusions. The potential risk of hypertension and hypertensive nephropathy in patients undergoing nephrectomy, compared with patients after organ-saving surgery, is significantly higher. One of the most important manifestations of hypertension in the elderly is a violation of the structure and function of target organs, which include: the brain, heart, blood vessels, kidney. Nephrectomy forms a pathological vicious circle, contributing to the development and further progression of renal and cardiovascular failure.

Investigation on the effect of pressure on turbulent transports of the IR-T1 Tokamak plasma

Investigation on the effects of limiter biasing and applying external electric fields (positive and negative voltage of 200 V) under various pressures of 1.9, 2.3 and 2.7 Torr on the IR-T1 Tokamak plasma parameters such as plasma current, loop voltage, radial electric field, poloidal electric field and Reynolds stress and turbulent transports, was done in this work. The results show that the ramp time of the plasma current decreased and the change rate of loop voltage raised by increasing plasma pressure. The power of radial electric field is more than the power of poloidal electric field. According to the applied positive voltage to the plasma, it is observed that the Reynolds stress of IR-T1 Tokamak plasma has experienced a more significant reduction at the pressure of 2.3 Torr than other pressures. The results Also show that the applying positive voltage of 200 V to the IR-T1 Tokamak plasma at the pressure of 2.3 Torr (compared to other pressures) has more effect on increases and decreases the poloidal and radial turbulent transport, respectively.

Pressure-driven amplification and penetration of resonant magnetic perturbations

We show that a resonant magnetic perturbation applied to the boundary of an ideal plasma screw-pinch equilibrium with nested surfaces can penetrate inside the resonant surface and into the core. The response is significantly amplified with increasing plasma pressure. We present a rigorous verification of nonlinear equilibrium codes against linear theory, showing excellent agreement.

Three‐Dimensional Effects on Stochasticity in Non‐Axisymmetric Tori

AbstractThree‐dimensional (3D) effects on the stochasticity of magnetic field lines in helical devices are investigated. The stochastization due to increased plasma pressure is an intrinsic property of the stellarator/heliotron because the pressure‐gradient driven currents are the source of perturbation fields. The stochastization depends on the plasma pressure profile. In an advanced stellarator, the stochastization due to the 3D effects is smaller. However, magnetic islands in the edge are affected by 3D effects. 3D effects in a tokamak plasma are studied,too, and differences from the vacuum approximation are found (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stabilization and prevention of the 2/1 neoclassical tearing mode for improved performance in DIII-D

The m = 2/n = 1 neoclassical tearing mode (NTM) has been observed to strongly degrade confinement and frequently lead to a disruption in high β discharges in DIII-D if allowed to grow to a large size. Stabilization of grown NTMs by the application of a highly localized electron cyclotron current drive (ECCD) at the island location has led to operation at an increased plasma pressure, up to the no-wall kink limit. After the NTM is stabilized by the ECCD, the correct location for the current drive is maintained using information from real-time equilibrium reconstructions which include measurements from the motional Stark effect diagnostic. This same process is used alternatively to prevent the mode from ever growing, leading to performance at the pressure limit in high performance hybrid discharges with β above 4%. Modelling using the modified Rutherford equation shows that the required power is in close agreement with the experimental threshold for the prevention of the 2/1 NTM.

Electron Bernstein wave-bootstrap current synergy in the National Spherical Torus Experiment

Current driven by electron Bernstein waves (EBW) and by the electron bootstrap effect are calculated separately and concurrently with a kinetic code to determine the degree of synergy between them. A target β=40% NSTX [M. Ono, S. Kaye, M. Peng et al., Proceedings of the 17th IAEA Fusion Energy Conference, edited by M. Spak (IAEA, Vienna, Austria, 1999), Vol. 3, p. 1135] plasma is examined. A simple bootstrap model in the collisional-quasilinear CQL3D Fokker–Planck code (National Technical Information Service document No. DE93002962) is used in these studies: the transiting electron distributions are connected in velocity space at the trapped-passing boundary to trapped-electron distributions that are displaced radially by a half-banana-width outwards/inwards for the co-passing/counter-passing regions. This model agrees well with standard bootstrap current calculations over the outer 60% of the plasma radius. Relatively small synergy net bootstrap current is obtained for EBW power up to 4 MW. Locally, bootstrap current density increases in proportion to increased plasma pressure, and this effect can significantly affect the radial profile of driven current.

Flow burst–induced large‐scale plasma sheet oscillation

On 12 August 2001 the Cluster spacecraft measured a rapid flux transport event, consisting of a strong perpendicular earthward flow burst combined with a dipolarization of the magnetic field after a strong compression of the magnetotail. Combining the Cluster data with those from ground‐based magnetometers, we find that this event is related to patchy reconnection taking place in the tail. After the event the magnetotail is locally evacuated of magnetic field, and an increased plasma pressure takes over from the magnetic pressure. This situation lasts for 15 min, after which a new equilibrium is sought, resulting in an oscillating magnetic field with a period of 20 min. The rapid flux transport observed with Bz and vx is shown to be in agreement with Bx variation using Maxwell's equations. The oscillation period agrees well with what is theoretically predicted. Our results show how a damped eigenoscillation of the magnetotail can be initiated by fast flows.

Effects of oxygen plasma treatment on the formation of ohmic contacts to GaN and AlGaN

AbstractThe effects of oxygen plasma treatment prior to metal contact deposition have been studied for both n‐ and p‐GaN, as well as n‐AlGaN. In n‐GaN, the as‐deposited Ti/Al contacts change from rectifying to ohmic, and further improvements are observed after rapid thermal annealing (RTA). A specific contact resistivity better than 10–7 Ω cm2 was obtained using a plasma treatment of 20 s at 30 W and 0.2 mbar, followed by RTA at 500 °C in argon. The I–V characteristics of the Ti/Al contacts degraded when plasma treatments were performed for a longer time and/or at increased plasma pressure. However, in contrast to n‐GaN, the electrical properties of the Ni/Au contacts to p‐GaN and Ti/Al contacts to n‐Al0.15Ga0.85N deteriorated following oxygen plasma treatment. X‐ray photoelectron spectroscopy (XPS) was used in order to help elucidate the mechanisms behind these effects. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of oxygen plasma treatments on the formation of ohmic contacts to GaN

Oxygen plasma treatments have been performed prior to contact deposition on both n - and p-type GaN, and the effects of plasma pressure, rf power and treatment time on the contact characteristics are discussed. By exposing the surface of n -type GaN to an oxygen plasma prior to metal deposition, the as-deposited Ti/Al contacts change from rectifying to ohmic, and further improvements are observed after rapid thermal annealing (RTA). A specific contact resistivity better than 10 −7 Ω cm 2 was obtained using a plasma treatment of 20 s at 30 W and 0.2 mbar, followed by RTA at 500 °C in argon. The I–V characteristics of the Ti/Al contacts degraded when plasma treatments were performed for a longer time, at increased plasma pressure, or at higher rf power. However, unlike in the case for n -type GaN, oxygen plasma treatment prior to metal deposition deteriorated the electrical properties of the Ni/Au contacts to p-type GaN. X-ray photoelectron spectroscopy (XPS) was used in order to help elucidate the mechanism behind these effects.

Relaxed plasma-vacuum states in cylinders

The variational principle for relaxed toroidal plasma-vacuum systems with pressure is applied to axially periodic circular cylinders. More precisely, equilibria with cylindrical symmetry are investigated for their potential to be a relaxed state. Such equilibria are characterized by their pinch ratio μ, the jump δ of the pitch angle of the magnetic field across the plasma-vacuum interface, a constant plasma pressure β, and the ratio l of wall radius over interface radius. In the limit of an infinitely long cylinder, the necessary and sufficient condition for the equilibrium to be a relaxed state defines one or two intervals of allowed values of the pinch ratio μ, which depend still on the other parameters. These intervals are contained in the interval known from Taylor's theory, but are generally smaller. They shrink with increasing plasma pressure β or increasing radius ratio l. In particular, in the field-free limit case β = 1, for I exceeding the critical value l c 4.983, or for a vanishing δ, these intervals are zero.

Nano-Structured Amorphous Carbon Films Synthesised Using Decr Plasma

ABSTRACTA Distributed Electron Cyclotron Resonance plasma reactor powered by a microwave generator operating at 2.45 GHz was used to deposit ta-C:H (Diamond-Like Carbon, DLC) thin films at RT. A graphite sputtering target immersed in an argon plasma was used as carbon source. The Ar plasma density was about 5×1010 cm-3. Single crystal <100> Si substrates were RF biased to a negative voltage of -80 V. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), nuclear reaction analysis (NRA) using the resonance at 6.385 MeV of the reaction: 15N + 1H-→ 12C + 4He + γ, elastic recoil detection analysis (ERDA) and Rutherford backscattering (RBS) were used to investigate the early phase of the growth. The morphology of the films grown at low pressure (0.3 mTorr) is shown to be dominated by stress-mediated nucleation leading to formation of basket-like clusters of circular hillocks 20 nm high surrounded by a planar, mostly sp2 bonded film ∼8 nm thick. With increasing plasma pressure the spatial frequency of the hillocks becomes random and the growth is dominated by the Stranski-Krastanov mode. The XPS data taken at decreasing emergence angles show that the structure of the hillocks is dominated by sp3 bonded carbon. The XPS argon signal disappears at 10° emergence angle indicating that integration of argon occurs mainly within the sp bonded regions. The NRA and ERDA analysis show that the amount of integrated hydrogen decreases with increasing substrate current density. RBS data indicate that increasing bias enhances argon integration.

Amorphous carbon films deposited by direct current-magnetron sputtering: Void distribution investigated by gas effusion and small angle x-ray scattering experiments

Amorphous carbon films were deposited by direct current-magnetron sputtering onto p-doped (100) silicon crystals and onto ultrapure aluminum foils at different argon pressures, ranging from 0.17 to 1.4 Pa. The film density was determined by the combination of the areal density, obtained from ion beam analysis, and the film thickness measured by a stylus profilometer. Film density decreased when the argon pressure used during deposition was increased. Gas effusion measurements indicated that the films deposited at low pressures are more compact than the films deposited at higher pressures. In the case of the latter, C2Hn effusion at temperatures as low as 250 °C indicated that they have an open structure that allows the evolution of large molecules. Small angle x-ray scattering results revealed an increase of the void density with increasing plasma pressure. Guinier plots show that these voids have a broad distribution of sizes, ranged from 7 to 26 Å, which is nearly independent of the plasma pressure. A direct correlation between film density and the open volume fraction in the films was found. These different film microstructures could be explained by the existence of different bombardment regimes during film growth: films deposited at lower plasma pressures are hard and dense, while soft films grown at higher pressures have an open microstructure.

Physics of the L-H Transition and Type III-ELMs Phenomena (Scaling Properties and Dimensionless Analysis)

The stability theory of Alfven drift-waves shows that with increasing plasma pressure electron drift waves become strongly coupled with Alfven waves thus damping radial perturbations. The Alfven drift turbulence suppression at the plasma edge was suggested as a triggering mechanism for the L to H transition [1]. After the transition the resistive interchange in-stability emerges as a dominant phenomenon which reveals itself in the form of Type III ELMs. Magnetic flutter provides the resistive driver for the interchange instability and, together with the stability condition caused by the electric shear stabilisation, creates the right dependence of the marginal temperature on density and other plasma parameters seen in experiments.

Atomic force microscopy and high-resolution RBS investigation of the surface modification of magnetron sputter-etched Si(111) in an argon plasma at different pressures

The surface morphology and metallic contamination of magnetron sputter-etched Si(111) was investigated by atomic force microscopy (AFM) and high-resolution Rutherford backscattering spectroscopy (RBS) as a function of Ar plasma pressure. The root-mean-square roughness (Rrms) of plasma-etched Si decreased monotonically from 36 ± 28 Å at 2 × 10−3 mbar to 13 ± 6 Å at 2 × 10−2 mbar. High-resolution RBS showed that metallic impurity contamination increased with increasing plasma pressure, whereas Ar atom incorporation in the near-surface region of etched Si followed the opposite pressure dependence. The barrier height of Pd Schottky contacts fabricated on the etched n-Si also decreased monotonically with decreasing Ar pressure, showing that the extent of barrier height modification was not affected by metallic impurity contamination. High-resolution RBS combined with channelling experiments showed that the topmost layers of the plasma-etched samples were disordered. The thickness of the damaged layers decreased with increasing plasma pressure. Copyright © 1999 John Wiley & Sons, Ltd.

Characteristics of a new class of transport-related MHD modes in JET H-mode plasmas

A new type of magnetohydrodynamic (MHD) mode, provisionally termed the wash board (WB) mode, has been observed during H-mode plasmas in JET. It occurs in all types of H-mode discharges, but is not seen during L-mode even at high values of . The WB mode appears to be linked with saturation in the plasma confinement and central plasma temperatures. These modes have high m and n numbers and are localized in the outer part of the plasma, typically from the q = 2 surface to the plasma edge. They rotate with the electron diamagnetic frequency and have a strong ballooning character. There is a good correlation between increasing plasma pressure and the growth of both the spectral extent and amplitude of the WB modes. Changes in the electron temperature profile also correlate well with changes in the amplitude of these modes. They are, therefore, regarded as a possible candidate to play a role in the confinement degradation of H-mode plasmas. Although these modes appear in all types of H-modes in JET, most of the analysis has been carried out during the hot-ion H-mode period of the 100% T discharges, since these modes are practically the only MHD activity left and the small-scale density fluctuations can, therefore, be analysed.

Self-sustained steady state tokamak plasmas

Self-sustained steady-state tokamak equilibria for National Spherical Torus Experiment (NSTX) and Spherical Torus-Volume Neutron Source (ST-VNS) are presented. It is demonstrated that the amount of the pressure gradient driven current which includes the bootstrap current and the diamagnetic current is comparable to the designed current specification in these devices. A possible startup sequence by increasing plasma pressure is also illustrated.