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This paper highlights the most important results achieved at the spherical tokamak Globus-M2 with a high magnetic field. This paper also covers the most important topics of fusion research: thermal energy confinement in regimes with neutral beam injection, toroidal Alfvén eigenmode and correspondent fast ions confinement issues, L-H transition, turbulence suppression and edge-localized modes' behavior, experimental and theoretical study of regimes with nitrogen seeding that allow to significantly reduce thermal loads on the divertor plates, and experiments and simulations of lower hybrid current drive. The research results provide the basis for the next step toward a fusion neutron source—the development of the Globus-3 spherical tokamak. [ABSTRACT FROM AUTHOR]

Two recent papers, P. H. Yoon and G. Choe, Phys. Plasmas 28, 082306 (2021) and Yoon et al., Phys. Plasmas 29, 112303 (2022), utilized in the derivation of the kinetic equation for the intensity of turbulent fluctuations the assumption that the wave spectra are isotropic, that is, the ensemble-averaged magnetic field tensorial fluctuation intensity is given by the isotropic diagonal form, ⟨ δ B i δ B j ⟩ k = ⟨ δ B 2 ⟩ k δ i j . However, it is more appropriate to describe the incompressible magnetohydrodynamic turbulence involving shear Alfvénic waves by modeling the turbulence spectrum as being anisotropic. That is, the tensorial fluctuation intensity should be different in diagonal elements across and along the direction of the wave vector, ⟨ δ B i δ B j ⟩ k = 1 2 ⟨ δ B ⊥ 2 ⟩ k (δ i j − k i k j / k 2) + ⟨ δ B ∥ 2 ⟩ k (k i k j / k 2). In the present paper, we thus reformulate the weak magnetohydrodynamic turbulence theory under the assumption of anisotropy and work out the form of nonlinear wave kinetic equation. [ABSTRACT FROM AUTHOR]

During the flight of hypersonic vehicle, air will be decomposed and ionized due to the "friction" under ultra-high speed, thus forming a plasma layer. Because the plasma has the ability to absorb and reflect electromagnetic waves, the "black barrier" phenomenon is formed. In addition, when the hypersonic vehicle passes through the atmosphere, the surface temperature rises sharply due to aerodynamic heating, and the surface material undergoes a series of complex changes to form ablation. In this paper, the finite volume method and the laminar finite rate model are used to study the flow field velocity, pressure distribution, flow field temperature, and spatial distribution of each component of the aircraft at different Mach numbers, angles of attack, and heights. In the flow field of supersonic aircraft, N and O are mainly concentrated in the tail of the aircraft, NO is mainly concentrated in the head of the aircraft, and N2 and O2 are full of the whole space. Because of the accumulation of NO+ and O2+ in the tail of the aircraft, the charge accumulation is formed, which will further interfere with the electromagnetic wave signal. The mass fraction of N and O increases with the increase in Mach numbers, while the mass fraction of O2 decreases with the increase in Mach numbers. Different angles of attack will affect the asymmetry of the shock wave of the aircraft. In this paper, the evolution mechanism and distribution characteristics of aircraft plasma are revealed, which lay a theoretical foundation for solving the problem of black barrier and ablation. [ABSTRACT FROM AUTHOR]

A novel design methodology based on a current-tunable magnetic focusing system that can flexibly switch between Brillouin and immersive flow focusing modes is presented in this paper. Such a magnetic system, which comprises cascaded solenoid coils and pole pieces, could be used in beam optics systems of vacuum tubes, particle accelerators, and free-electron lasers. Although the profile of this magnetic system is not brand new, the novel way proposed in this paper to establish and manipulate such a system to fit versatile purposes has never been reported in the literature to the best of the authors' knowledge. The specific structure of the magnetic system should be optimized, starting from the immersive flow focusing mode together with the electron gun design, which will be determined on successful optimization. According to our analysis, such systems could be transformed into the Brillouin bunching mode by simply adjusting the coil currents without modifying the hardware. To verify such an approach, single-beam and multi-beam optical systems are demonstrated in this paper. Moreover, the beam radius in such systems could also be conveniently adjusted via a similar technique. [ABSTRACT FROM AUTHOR]

This paper presents a novel coupling of a kinetic description of neutrals with a fluid description of a fusion plasma. The code, plasma interacting super-atoms and molecules (PISAM), employs a grid-free Cartesian geometry and a direct simulation Monte Carlo approach to solve the kinetic equations of deuterium atoms and molecules. The grid-free geometry and the parallel nature of the neutral dynamics, in the absence of neutral–neutral interactions, allow for an unlimited and work-efficient parallelization of PISAM that always ensures a balanced workload. The highly optimized Python implementation obtains good performance while securing easy accessibility to new users. The coupling of PISAM with the edge turbulence model HESEL is outlined with emphasis on the technical aspects of coupling Message Passing Interface-parallelized Python and C++ codes. Furthermore, the paper presents and analyzes simulation results from running the coupled HESEL-PISAM model. These results demonstrate the impact of radial neutral transport and plasma–neutral dynamics perpendicular to the magnetic field. Specifically, they illustrate how the inward flow of neutral kinetic energy and the inhibition of radial electric shear, resulting from poloidal momentum transfer between atoms and ions, can affect the energy containment time. By comparing the results of the HESEL-PISAM model with those obtained from coupling HESEL with a diffusive-fluid-neutral model, the capabilities of diffusion models in predicting neutral transport in the plasma edge and scrape-off layer are elucidated. [ABSTRACT FROM AUTHOR]

The plasma weak turbulence theory is a perturbative nonlinear theory, which has been proven to be quite valid in a number of applications. However, the standard weak turbulence theory found in the literature is fully developed for highly idealized unmagnetized plasmas. As many plasmas found in nature and laboratory are immersed in a background static magnetic field, it is necessary to extend the existing discussions to include the effects of ambient magnetic field. Such a task is quite formidable, however, which has prevented fundamental and significant progresses in the subject matter. The central difficulty lies in the formulation of the complete nonlinear response functions for magnetized plasmas. The present paper derives the nonlinear susceptibilities for weakly turbulent magnetized plasmas up to the third order nonlinearity, but in doing so, a substantial reduction in mathematical complexity is achieved by the use of Bessel function addition theorem (or sum rule). The present paper also constructs the weak turbulence wave kinetic equation in a formal sense. For the sake of simplicity, however, the present paper assumes the electrostatic interaction among plasma particles. Fully electromagnetic generalization is a subject of a subsequent paper. [ABSTRACT FROM AUTHOR]

Abstract-dynamic plasma sheath can severely interfere with the communication of hypersonic vehicles during atmospheric reentry. Theoretical and experimental results show that low-frequency (LF) electromagnetic (EM) waves could penetrate the plasma sheath, building a feasible method to solve the "radio blackout" problem. This paper discovers that the propagation of LF EM waves in plasmas is still influenced by parasitic modulation effects. Compared to microwave frequencies, the impact of parasitic modulation effects on signal modulation patterns is more distinct for LF EM waves. In contrast to the microwave frequency range, where the rotation direction of QPSK signal constellation points changes with the ratio of plasma frequency to electromagnetic wave frequency, in the LF range, the constellation points undergo limited clockwise rotation. This phenomenon can be attributed to the unique magnetic field propagation mechanism of LF EM waves in dynamic plasmas. This paper analyzes the mechanism of this specific parasitic modulation effect and discovers a sinusoidal transformation relationship between amplitude attenuation and phase shift. Meanwhile, the experimental and simulation results proved that the time-varying plasma could cause the parasitic modulation effect of LF EM wave, resulting in a limited clockwise rotation of orthogonal phase-shift keying constellation points, which is consistent with the theoretical analysis. [ABSTRACT FROM AUTHOR]

This document serves as a preface to a special topic collection in the journal "Physics of Plasmas" that focuses on the study of relativistic plasma in supercritical fields. The collection includes papers from a mini-conference held at the 2022 meeting of the American Physical Society Division of Plasma Physics. The objective of the mini-conference was to bring together researchers from different physics communities to address the physics of matter in extreme astrophysical objects and high-power laser systems. The papers in the collection cover topics such as electron-positron pair production, extreme astrophysics, radiation reaction, and vacuum physics. The mini-conference aimed to facilitate knowledge exchange and inspire future research in this emerging field of plasma physics. The authors of this document, Alec G. R. Thomas and Stepan S. Bulanov, declare that they have no conflicts of interest to disclose and were supported by the U.S. Department of Energy Office of Science Offices of High Energy Physics and Fusion Energy Sciences. [Extracted from the article]

Lightning is a major cause of tripping faults in overhead transmission lines. Multi-chamber lightning arresters are widely used to protect high-voltage lines from lightning because of their reliable arc-quenching performance. This paper analyzes the development and quenching process of an arc after a lightning breakdown arrester and studies the quenching mechanism of a multi-chamber arc structure based on its structural characteristics. We built an impulse-current test platform and carried out an impulse-current test for the multi-chamber arrester. The arc motion was captured by a high-speed camera, and the height, speed, and temperature of the arc ejection were obtained by signal processing. The multi-chamber arrester structure is optimized by using the maximum arc ejection height and the maximum ejection speed as metrics to find the optimal combination of the quenching-hole diameter and depth and the gap distance of the multi-chamber structure. [ABSTRACT FROM AUTHOR]

Aligning pedestal models and associated magnetohydrodynamic codes with experimental data is an important challenge in order to be able to generate predictions for future devices, e.g., ITER. Previous efforts to perform calibration of unknown model parameters have largely been a manual process. In this paper, we construct a framework for the automatic calibration of JOREK. More formally, we reformulate the calibration problem into a black-box optimization task, by defining a measure of the discrepancy between an experiment and a reference quantity. As this discrepancy relies on JOREK simulations, the objective becomes computationally intensive and, hence, we resort to batch Bayesian optimization methodology to allow for efficient, gradient-free optimization. We apply this methodology to two different test cases with different discrepancies and show that the calibration is achievable. [ABSTRACT FROM AUTHOR]

Microwave plasma jets have garnered significant attention due to their unique advantages and wide applications in numerous fields. However, the frequent collisions between electrons and neutral particles at atmospheric pressure make it challenging to generate a room-temperature plasma jet and maintain a high energy efficiency at the same time. This paper introduces a new microwave plasma jet based on a rectangular waveguide, which utilizes the surface wave propagating along the plasma jet to sustain the gas discharge. It features a simple structure, low power consumption, high energy efficiency, and strong controllability without the need for any external tuning measures. The minimum power required to ignite and stably maintain the plasma jet at atmospheric pressure is as low as 25 W and the measured lowest gas temperature at the plasma jet tail is approximately 295 K. Meanwhile, experiments show the microwave energy efficiency can be higher than 90% in a large range of input powers and gas inflow rates, while the gas temperature is maintained close to room temperature. These characteristics of our proposed surface wave plasma jet demonstrate tremendous potential in fields such as hemostasis, sterilization, wastewater treatment, semiconductor cleaning, and material processing. [ABSTRACT FROM AUTHOR]

It was recently shown that the use of Coulomb logarithms appropriate for bremsstrahlung radiation (rather than transport processes) along with corrections for the Langdon effect and ion screening reproduced measurements of collisional absorption in well-characterized underdense plasmas [D. Turnbull et al., Phys. Rev. Lett. 130, 145103 (2023)]. However, it was recognized at the time that the use of the standard absorption-reduction factor from Langdon's seminal paper was inconsistent with the use of Coulomb logarithms that are thermally averaged over a Maxwellian electron-velocity distribution function. A more accurate approach would be to average over the expected super-Gaussian distribution function while accounting for the Gaunt factor's velocity dependence, which somewhat mitigates the Langdon effect; however, at that time, this theory matched the data less well. This conflict is now eliminated with the additional insight that the ionization state of our mid-Z ion species (when present) was lower than had been assumed, as evidenced by the Thomson-scattering data and time-dependent Cretin simulations. We are now able to show that an improved treatment of the Langdon effect provides the best match to data. Otherwise, the prior conclusions remain unchanged. We also show an example of the substantial expected impact to the absorption rate in calculations of indirect-drive hohlraums. [ABSTRACT FROM AUTHOR]

Proton radiography is a widely used experimental method to diagnose the electric and magnetic (EM) fields in high-energy-density plasmas. In proton radiography, the probe protons are typically assumed to be deflected only by the EM fields, whereas the Coulomb scattering caused by the charged particles in the target plasmas is generally ignored. However, at high plasma densities, the presence of Coulomb scattering could reduce the proton flux perturbations recorded on the detector and influence the inversion of the EM fields from experiments. In this paper, a theoretical model is developed for the first time to describe the proton flux distribution on the detector when the EM field deflections and Coulomb scattering coexist in deflecting the probe proton trajectories. Our theory indicates that the Coulomb scattering could decrease the signal contrast of the probed EM fields, which is determined not only by the strengths of the EM field deflections and Coulomb scattering but also by the spatial gradient of the EM fields. Monte Carlo simulations are also conducted to validate our theoretical model. It would be helpful to interpret the proton radiography experiments quantitatively. [ABSTRACT FROM AUTHOR]

Achieving ignition in inertial confinement fusion requires significant heating and compression of the thermonuclear fuel. One of the most efficient ways to achieve such conditions is spherical compression of the target initiated by specially profiled laser pulses. Any irradiation asymmetries and target imperfections break the symmetry of the compression and are seeds for the growth of hydrodynamic instabilities. As a result, the initial small amplitudes grow significantly and break the effective target compression, which is critical for successful ignition. The paper numerically studies the evolution of ice–ablator small perturbations in a direct drive target. The simulations consider the target dynamics with two different numerical hydrocodes. As a result of Rayleigh–Taylor instability development during both acceleration and deceleration phases, the perturbations grow significantly and could violate the ignition if the initial amplitude is larger than the critical value of several hundred nm: for successful ignition, the amplitude should be compared or smaller than a 100 nm. The effect of perturbations propagating from the distributed interface to the internal smooth layers is also observed. [ABSTRACT FROM AUTHOR]

Recent indirect drive inertial confinement fusion implosions on the National Ignition Facility (NIF) [Spaeth et al., Fusion Sci. Technol. 69, 25 (2016)] have crossed the threshold of ignition. However, performance has been variable due to several factors. One of the leading sources of variability is the quality of the high-density carbon (HDC) shells used as ablators in these experiments. In particular, these shells can have a number of defects that have been found to correlate with the appearance of ablator mix into the hot spot and a degradation in nuclear yield. These defects include pits on the ablator surface, voids in the ablator bulk, high-Z debris from the Hohlraum wall that adheres to the capsule surface, and finally the inherent granular micro-structure of the crystalline HDC itself. This paper summarizes high-resolution modeling of each of these mix sources in two recent high-performance NIF implosion experiments. The simulated impact from a range of individual capsule defects is found to be broadly consistent with the trends seen in experiment, lending credence to the modeling results and the details of the mixing process that they reveal. Interestingly, modeling of the micro-structure inherent to HDC shows that this perturbation source results in considerable mixing of the deuterium–tritium fuel with ablator material during the implosion. The reduction in fuel compression from this mix results in an approximately factor of two reduction in neutron yield in current implosions and emphasizes the importance of mitigating this significant performance degradation. [ABSTRACT FROM AUTHOR]

In modeling the charged alpha particle transport in hot-spot plasmas of inertial confinement fusion, the energy-losing rate is a major concern in the Monte Carlo simulations of alpha particle transport of a radiative-hydrodynamic code. However, the traditionally used energy stopping-power only describes the averaged energy-losing rate of the incident charged particles, whereas the variance of the energy exchange with the background particles is generally ignored. In this paper, the variance of charged particle collisions is studied by both analytical derivation and Monte Carlo simulations. An expression of the divergence of the charged particle energy-losing rate is given for the first time, which can be directly used for practical estimations. It indicates that when the areal density of the target particles along the incident particle path length is low, the divergence of the lost energy would be much larger than the average value, and the traditionally used energy stopping-power would be no longer sufficient to describe the charged particle Coulomb collisions. It helps to obtain a more comprehensive understanding about the charged particle transport in plasmas. [ABSTRACT FROM AUTHOR]

The dynamics of an inertial confinement fusion capsule, or of a stellar or planet interior, obey a very similar set of equations: magneto-radiative-hydrodynamic equations. The solutions of these equations, however, depend entirely on the transport properties associated with the different materials at play. To properly model the dynamics of these systems, it is necessary to determine with high accuracy the transport coefficients of several materials over a large range of thermodynamic conditions. Experimental capabilities in this respect are still limited due to the nature of the microphysics at play and the extreme conditions involved. Numerical simulations are thus necessary, and in this respect, molecular dynamics simulations based on density functional theory offer exquisite possibilities to constrain the transport properties in the warm to hot dense matter regime. In this paper, we report the methodology used to extract different transport properties based on molecular dynamics performed with the software Abinit. The examples shown are based on the specific cases identified for the purpose of the second charged-particle transport code comparison workshop. [ABSTRACT FROM AUTHOR]

In its basic form, phase contrast imaging (PCI) provides line-integrated measurements of electron density fluctuations in plasmas. As turbulent fluctuations in magnetically confined plasmas have wave vectors almost perpendicular to the background magnetic field, the signals scattered by fluctuations from different parts of the PCI line-of-sight (LoS) are spatially separated in focal planes of the plasma. This allows localized PCI measurements by placing a mask in such a plane, to only permit signals from specific parts of the LoS to reach the PCI detectors. The present paper describes modeling and design of localization masks for the PCI system at the Wendelstein 7-X (W7-X) stellarator as well as the first results obtained using the masks in the recent long-pulse W7-X experimental campaign. During this project, we have extended the theory describing the mask response within the Fraunhofer diffraction model. As a novel development, we show from first principles that the mask response is determined by the fraction of power of the scattered beam spots that passes the mask. These insights have been used to select the W7-X mask design, consisting of a circular cutout, allowing the unscattered beam spot to pass the mask, with wedges covering a fixed angular range outside the central cutout. In the recent W7-X experimental campaign, the masks have verified the location of the main turbulence features observed by the PCI system and provided new information about the location of short-wavelength magnetohydrodynamic modes. [ABSTRACT FROM AUTHOR]

A theoretical investigation is presented to explain the formation and characteristics of relaxed equilibrium structures in a three-component dusty plasma within Saturn's atmosphere, composed of negatively charged dust particles, electrons, and ions. The Quadruple Beltrami equation is derived by utilizing the vortex dynamic equations along with the current density. Solutions for the higher Beltrami states are obtained in two different modes, a simple rectangular geometry and a coplanar rectangular geometry, to explore the characteristics of relaxed structures within the Saturn magnetosphere and its rings. The solutions are depicted through some plots by varying the Beltrami parameters and the densities of the plasma species. It is observed that only paramagnetic structures are formed in the coplanar geometry, while variations in the Beltrami parameters and plasma species densities significantly affect the magnetic characteristics of the relaxed structures in a simple rectangular geometry. This paper will provide an important contribution to understand the atmospheric vortical structures developed in different astronomical bodies that have double or more than double configurations, such as Saturn's rings, Jupiter magnetosphere, Uranus, Neptune, etc. [ABSTRACT FROM AUTHOR]

Quantum electrodynamic (QED) plasmas, describing the intricate interplay of strong-field QED and collective pair plasma effects, play pivotal roles in astrophysical settings like those near black holes or magnetars. However, the creation of observable QED plasmas in laboratory conditions was thought to require ultra-intense lasers beyond the capabilities of existing technologies, hindering experimental verification of QED plasma theories. This paper provides a comprehensive review of recent studies outlining a viable approach to create and detect observable QED plasmas by combining existing electron beam facilities with state-of-the-art lasers. The collision between a high-density 30 GeV electron beam and a 3 PW laser initiates a QED cascade, resulting in a pair plasma with increasing density and decreasing energy. These conditions contribute to a higher plasma frequency, enabling the observation of ∼ 0.2 % laser frequency upshift. This solution of the joint production-observation problem should facilitate the near-term construction of ultra-intense laser facilities both to access and to observe the realm of strong-field QED plasmas. [ABSTRACT FROM AUTHOR]

Stellarator plasmas are externally controlled to a degree unparalleled by any other fusion concept, magnetic or inertial. This control is largely through the magnetic fields produced by external coils. The development of fusion energy could be expedited by carrying out remarkably straight-forward computations to define strategies for exploiting this external control. In addition to these computations, which have a reliability limited only by competence, certain physics areas that affect the development of stellarator power plants should have a more intense study. The low cost and speed with which computations can be carried out relative to experiments have implications for the development of fusion. Computations should be used to develop a strategy that to the extent possible allows major issues to be circumvented. Required computations for this strategy are the subject of this paper. [ABSTRACT FROM AUTHOR]

Initial arc formation in high-current drawn vacuum arc was investigated experimentally and analyzed quantitatively in this paper. Experiments were conducted in a demountable vacuum chamber. Characteristics of molten metal bridges evolution and arc voltage indicated that initial arc formation could be divided into three stages. The correlation between the rupture of molten metal bridges and the fluctuation of arc voltage in stage 2 was analyzed. In addition, characteristics in stage 2 under different arc currents and opening speeds were analyzed quantitatively. [ABSTRACT FROM AUTHOR]

This paper investigates the self-pulsing of dielectric barrier discharges (DBDs) at low driving frequencies. In particular, (a) the dependence of current on the product pd of gas pressure p and the gas gap length d, (b) the effects of lossy dielectrics (in resistive discharges) and large dielectric permittivity (in ferroelectrics) on current dynamics, (c) the transition from Townsend to a dynamic capacitively coupled plasma (CCP) discharge with changing pd values, and (d) the transition from Townsend to a high-frequency CCP regime with increasing the driving frequency. A one-dimensional fluid model of argon plasma is coupled to an equivalent RC circuit for lossy dielectrics. Our results show multiple current pulses per AC period in Townsend and CCP discharge modes which are explained by uncoupled electron–ion transport in the absence of quasineutrality and surface charge deposition at dielectric interfaces. The number of current pulses decreases with an increasing applied frequency when the Townsend discharge transforms into the CCP discharge. The resistive barrier discharge with lossy dielectrics exhibits Townsend and glow modes for the same pd value (7.6 Torr cm) for higher and lower resistances, respectively. Finally, we show that ferroelectric materials can amplify discharge current in DBDs. Similarities between current pulsing in DBD, Trichel pulses in corona discharges, and subnormal oscillations in DC discharges are discussed. [ABSTRACT FROM AUTHOR]

Prompt redeposition is a process in which a neutral atom sputtered from a divertor target or the first wall of a tokamak returns on the surface right after ionization, before experiencing any collisions with the background plasma. This paper presents analytical solutions of kinetic equations for sputtered neutrals and resulting ions. Using obtained distribution functions, the redeposition coefficient and average impact angle are derived under assumptions of thin Debye sheath. Our expression for the prompt redeposition coefficient explicitly depends on the angular and energy distributions of sputtered atoms and, therefore, is more universal than often used Fussmann equation valid only for cosine distribution. The effect of the angular distributions of sputtered particles on the prompt redeposition efficiency and parameters of redeposited ions is analyzed. [ABSTRACT FROM AUTHOR]

This paper presents a correlation between time evolution of ions and electrons with soft and hard x-rays emitted from argon plasma. The plasma setup comprises of two copper electrodes connected with a dc power supply. Faraday cups were used to monitor time evolution of ions, to extract their energy, temperature, and flux. Double Langmuir probe was employed to determine electron temperature, energy, and density. To explore time-resolved emission of soft x-rays, PIN photodiodes filtered with 24 μm aluminum, 90 μm Mylar, 80 μm copper, and 10 μm silver have been used. To evaluate temporal evolution of hard x-rays, a scintillator–photomultiplier system was utilized. The plasma was generated using argon gas at atmospheric pressure 760 Torr and constant flow rate 5 L/min at 7, 9, and 11 kV. The flux, energy, and temperature of Ar ions increase with the increase in the operating potential. Soft x-ray signals last much longer (3000 ns) than those of electrons' and ions' signals (300 ns). The plasma operated at 11 kV permits highest emission of ions, electrons, and x-rays. The ions, electrons, and hard and soft x-ray irradiation on silicon wafer exhibited the presence of damaged trails. Ion irradiation showed the presence of latent damaged trails. Electron irradiation caused more damage to the Si surface compared to ion irradiation due to higher flux and density. Soft x-rays had a lesser effect as compared to Si exposed to hard x-rays due to higher energy of the hard x-rays. In the case of hard x-rays irradiated Si, erupted volcano-like structure is formed. [ABSTRACT FROM AUTHOR]

The Brillouin flow is a rectilinear, sheared electron fluid flow in a crossed electric field (E) and magnetic field (B), in the E × B direction with zero flow velocity and zero electric field at the surface with which the flow is in contact. It is broadly considered as the equilibrium electron flow in high power crossed-field devices including the magnetron and magnetically insulated transmission line oscillator. This paper provides an examination of Brillouin flow in two dimensions, in a cylindrical geometry where the anode radius changes abruptly at a single axial location, while the cathode surface has a constant radius. Our simulation confirms the proof that there is no equilibrium Brillouin flow solution for such a geometry. It further reveals that this change in the anode radius introduces novel bunching of the electrons within the Brillouin hub. This bunching occurs at low frequencies and is very pronounced if the Brillouin flow is from the small gap region to the large gap region, but is minimal if the Brillouin flow is from the large gap region to the small gap region. New insights are provided into the physical processes that initiate and sustain the bunching processes that are unique for a crossed-field diode, as compared with a non-magnetized diode. We argue that this enhanced bunching, and its concomitant formation of strong vortices, is not restricted to an abrupt change in the anode–cathode gap spacing. [ABSTRACT FROM AUTHOR]

In proton acceleration from laser-irradiated thin foil targets, adding foams on the front surface or connecting a helical coil on the rear surface of the foil has proven to be an effective scheme to enhance proton energy. In this paper, we make the first attempt to incorporate the above two enhancement schemes for laser-proton acceleration by simultaneously adding foams and connecting a helical coil to a thin foil target. By utilizing such integrated targets in the experiment, focused beams were generated. The maximum proton energy and the number of energetic protons are apparently enhanced. Moreover, quasi-monoenergetic peaks were formed at the high-energy end of the spectra. Particle-in-cell plasma simulations and electromagnetic beam dynamics simulations show that the double-layer target not only enhances the energy of protons but also leads to a multiple-fold increase in the number of escaped electrons, which results in an enhanced post-acceleration in helical coil subsequently. [ABSTRACT FROM AUTHOR]

Public engagement stands at the core of fostering meaningful connections between physicists and the public, amidst growing public skepticism toward science and scientists. Increasing representation requires a transformative approach to public engagement, emphasizing a collaborative and bidirectional interaction that transcends conventional outreach models. In this paper, we articulate and promote the principles of effective public engagement for physicists. We highlight the successful collaboration between the American Physical Society (APS) Public Engagement and the APS Division of Plasma Physics (DPP) members in developing plasma kits for PhysicsQuest. By sharing insights, examples, and lessons learned, we aim to inspire a cultural shift within the physics community, fostering more inclusive and impactful engagement with diverse audiences. [ABSTRACT FROM AUTHOR]

High energy density physics (HEDP) and inertial confinement fusion (ICF) research typically relies on computational modeling using radiation-hydrodynamics codes in order to design experiments and understand their results. These tools, in turn, rely on numerous charged particle transport and relaxation coefficients to account for laser energy absorption, viscous dissipation, mass transport, thermal conduction, electrical conduction, non-local ion (including charged fusion product) transport, non-local electron transport, magnetohydrodynamics, multi-ion-species thermalization, and electron-ion equilibration. In many situations, these coefficients couple to other physics, such as imposed or self-generated magnetic fields. Furthermore, how these coefficients combine are sensitive to plasma conditions as well as how materials are distributed within a computational cell. Uncertainties in these coefficients and how they couple to other physics could explain many of the discrepancies between simulation predictions and experimental results that persist in even the most detailed calculations. This paper reviews the challenges faced by radiation-hydrodynamics in predicting the results of HEDP and ICF experiments with regard to these and other physics models typically included in simulation codes. [ABSTRACT FROM AUTHOR]

Radio-frequency capacitively coupled plasmas (CCPs) are one of the key technologies enabling the latest etching processes in 3D NAND and FinFET manufacturing. These processes rely crucially on the precise control of the uniformity of ion/radical fluxes and ion angular and energy distribution function (IAEDF) in CCPs. The plasma behavior and scaling properties are dependent on the plasma chemistry in these processes, e.g., electro-positive Ar plasmas vs highly electro-negative O2 plasmas. With the large number of process and design parameters influencing the plasma properties, computational modeling has become an important tool in conjunction with experimental diagnostics in understanding the intricate physical mechanisms in CCPs. In this paper, a 2D particle-in-cell plasma model is used to study the kinetic behavior of low-pressure (<5 Pa) CCPs in two different representative chemistries: Ar and O2. The low-frequency RF source is at 1.356 MHz while 27.12 MHz is used for the high frequency. Simulations show a shift of the peak in the plasma density from the center of the chamber to the edge as the pressure increases from 0.3 to 2.6 Pa. The computed magnitude and spatial profile of electron density compare reasonably with experimental measurements over a range of pressure. Comparison between electro-positive and electro-negative plasmas are discussed. Modeling results for the dual frequency CCP highlight the effect of plasma uniformity on the IAEDF, especially near the outer edge of the electrodes. Collisions in the sheath increase the population of low-energy ions as the pressure is increased to 2.6 Pa. [ABSTRACT FROM AUTHOR]

In recent years, interest in striation phenomena in radio frequency (rf) discharges has risen due to the availability of new experimental data and the implementation of new computational models. Depending on the conditions, different mechanisms of discharge striations are realized. These are the ionization instability, the instability due to the electron attachment to electronegative gases, or the instability due to thermoelectric transport. Although the first two mechanisms were modeled quite extensively in recent years, the understanding of the influence of the Dufour effect originating from plasma density gradients on the stability of radio frequency discharges in long tubes remains poor. In this paper, the influence of this mechanism on the longitudinal striations of radio frequency discharge is presented using a one-dimensional model of argon discharge driven with rf excitation under intermediate pressure conditions of 0.5 Torr. It is found that striation formation is sensitive to the value of the thermoelectric heat transport coefficient in the low electron temperature range. The critical value of this coefficient necessary for the instability onset is derived using the linear stability analysis. [ABSTRACT FROM AUTHOR]

This paper addresses the challenge of extending the pulse duration of high-power microwave (HPM) sources, a critical focus in HPM technology. Transit-time oscillators (TTOs) typically modulate electron beams by forming standing waves through the cutoff of the TM01 mode in the modulation cavity. This study proposes enhancing the power capacity of TTOs by widening the drift channel. To enhance modulation capability without compromising power capacity, front and rear reflectors are strategically introduced. In particle-in-cell simulations, the device exhibits an output power of 1.65 GW at a frequency of 12.6 GHz, with an efficiency of 33.6%. The saturation time for the output microwave is 30 ns, and the maximum internal electric field strength is 860 kV/cm. The presented TTO, with design improvements, offers a promising solution for applications requiring prolonged pulse durations in high-power microwave sources. [ABSTRACT FROM AUTHOR]

The analysis of Z-pinch implosion dynamics plays one of the most important roles in the study of pulsed power discharges. At the same time, it is difficult to determine the density distribution together with the current density (current coupling to the imploding layer) to provide more detailed information about the dynamics. Numerical simulations can now provide high-resolution results that are almost unattainable in experiments. The challenge, however, is to obtain reliable results that are close enough to the experimental data to describe individual physical phenomena. In this paper, we show that it is possible to use a combination of experimental data and magnetohydrodynamic (MHD) simulations to verify and identify the physical processes during the stagnation of a Z-pinch. We focus on the analysis of the density profile from experimental data of the mega-ampere plasma focus PF-1000 and its reconstruction using an extended MHD code. Thanks to multi-frame interferometry, we recorded a total of 29 interferometric images of two shots, each in a 200 ns time window around the pinch phase. We were then able to obtain density profiles and observe the reflection of the shock wave from the axis. By the appropriate choice of initial conditions and boundary values in the simulation, we were able to obtain reasonable agreement with the experimental values. We also evaluated the possible shortcomings of the 1D simulation, such as mass loss and current flow at the periphery. [ABSTRACT FROM AUTHOR]

A theory of the three-wave coupling of triplets of tearing modes in toroidal pinches [i.e., either reversed field pinches (RFPs) or tokamaks] was proposed by R. Fitzpatrick [Phys. Plasmas 6, 1168 (1999)]. However, this theory only applies to toroidal pinches with negligible equilibrium plasma pressure gradients. Such a limitation is particularly inappropriate to RFPs. This paper generalizes the analysis of R. Fitzpatrick [Phys. Plasmas 6, 1168 (1999)] in order to take the equilibrium pressure gradient into account. However, for the sake of simplicity, a stepped pressure profile, rather than a continuous profile, is employed. In the limit in which the number of steps becomes very large, the results obtained from the generalized theory are presumably equivalent to those that would have been achieved using a continuous pressure profile. The generalized theory is used to investigate the formation of the characteristic toroidally localized pattern of phase-locked m = 1 and m = 0 tearing modes in RFP plasmas that is known as the "slinky" pattern. The incorporation of the equilibrium plasma pressure into the analysis is found to be of crucial importance when determining the properties of the pattern. This is the case because the plasma pressure controls the number of unstable m = 1 and m = 0 tearing modes, and also significantly affects the strength of three-wave coupling, as well as the phase relation between the phase-locked m = 1 and m = 0 modes. [ABSTRACT FROM AUTHOR]

The experiments on medium-size stellarator Uragan-2M (U-2M) in Kharkiv, Ukraine, are carried on in support of the Wendelstein 7-X (W7-X) experimental program. The scenario ion cyclotron frequency range (ICRF) plasma production at the hydrogen minority regime had been experimentally tested on U-2M and was qualified at the Large Helical Device (LHD). The paper presents the results of further research on the ICRF plasma production. The ICRF discharge studies were carried out in a H2 + He mixture with a controlled hydrogen concentration ranging from few percents to 75%. The two-strap like antenna mimicks the W7-X antenna operated in monopole phasing. The applied RF power was in the range of ∼100 kW. Relatively dense plasma of up to Ne ∼ 1019 m−3 was produced near the first harmonic of the hydrogen cyclotron frequency. The maximum temperature of the electrons and ions was not more than a few tens of electron volt. The characteristic features of RF plasma production and the propagation of electromagnetic waves in the experimental conditions are discussed. The experiments on U-2M and LHD indicate that the minority scenario of ICRF plasma production appears to be scalable and could be used in large stellarator machines. This is, in particular, important for the future experiments ICRF production of target plasma in W-7X in conditions where electron cyclotron resonance heating start-up is not possible. [ABSTRACT FROM AUTHOR]

Flows and rotation, particularly E × B rotation, are critical to improving plasma performance, and waves are a primary tool of plasma control. Thus, it is paramount to understand under what conditions waves can drive E × B flows in plasmas. In this didactic review, an invited paper accompanying the 2023 Marshall N. Rosenbluth Doctoral Thesis Award, this question is answered in the context of momentum-conserving quasilinear theory. There are two primary frameworks for momentum-conserving quasilinear theories that can handle both resonant and nonresonant particles: Eulerian averaging theories and oscillation-center Hamiltonian theories. There are also two different paradigmatic wave problems: plane-wave initial value problems, and steady-state boundary value problems. Here, it is shown that each of these frameworks "naturally" works better with a different problem type. By using these theories, one finds a great difference in the behavior of time- vs space-dependent waves. A time-evolving plane wave can only drive flow if the electromagnetic momentum of the wave, given by the Poynting flux, changes. This result precludes flow drive by any planar electrostatic wave. In contrast, a steady-state spatially evolving wave can drive flow whenever there is divergence in the flux of Minkowski momentum, a completely different physical quantity. This review aims to provide a high-level, intuitive understanding of the very different behaviors observed for these two types of problem. [ABSTRACT FROM AUTHOR]

In the previous studies of nonlinear saturation of the Buneman instability caused by high electron drift velocity relative to ions, the phase-space holes and the plateau on the electron velocity distribution function were identified as features of the saturation stage of instability [notably in the paper by Omura et al., J. Geophys. Res. 108, 1197 (2003)]. We have performed a much longer simulation of the Buneman instability and observed a secondary instability. This secondary instability generates fast electron-acoustic waves. By analyzing the phase-space plot of ions and electrons, we show that the fast electron heating and the formation of the plateau of electron velocity distribution function are not due to the quasi-linear diffusion but due to the nonlinear interaction of ion- and electron-acoustic solitary waves (phase-space holes) by exchange of trapped electrons in each wave. We also report the details on the intermittent and bursty nature of turbulence driven by this instability. [ABSTRACT FROM AUTHOR]

This paper presents an overview of experimental results of a laser-produced plasma expanding into a background gas, immersed within a large range of highly uniform magnetic fields (of up to 3 T), that are transverse to the expanding plasma. We used intensified gated imaging to capture the expansion of the plasma across and along the magnetic field lines to observe the spatiotemporal expansion dynamics for different magnetic field strengths. We observe changes in the perpendicular and parallel dynamics of the laser-produced plasmas expansion at high magnetic field. In addition, our results have also indicated the presence of electron-ion hybrid instabilities at relatively high pressures (100 mTorr) and relatively high magnetic field strengths (2 T), in accordance with theoretical calculations. [ABSTRACT FROM AUTHOR]

The explosive emission processes that occur at electrode surface microprotrusions may have harmful effects in a variety of electrodynamic and acceleration systems exposed to high-power radio frequency electromagnetic waves. This paper presents the results of a radiative magnetohydrodynamic simulation of the explosion of copper conductors that occur under conditions inherent in the explosion of electrode microprotrusions, i.e., at current densities of the order of 109 A/cm2. Explosions occurring under quasi-stationary and radio frequency conditions (hereinafter referred to as quasi-stationary and radio frequency explosions, respectively) were considered. It was shown that in all the considered cases, the explosion occurred at high temperatures, so that the energy deposited in the conductor by the time of explosion exceeded the sublimation energy of the conductor material. It turned out, however, that the energy deposited in the conductor under radio frequency conditions, regardless of the frequency of current oscillations, was more than two times less than that deposited under quasi-stationary conditions. The explosion time was also virtually independent of the frequency, and it was approximately three times longer than that calculated for quasi-stationary conditions. For a radio frequency explosion, the specific current action integral was somewhat less (by about 25%) than that for a quasi-stationary explosion, and its value was actually independent of frequency. At the same time, in the radio frequency regime, the radiation power coming out of the conductor substance drops strongly, and it is almost two orders of magnitude smaller compared to the radiation power in the quasistationary regime. [ABSTRACT FROM AUTHOR]

The use of non-sinusoidal waveforms in low pressure capacitively coupled plasmas intended for microelectronics fabrication has the goal of customizing ion and electron energy and angular distributions to the wafer. One such non-sinusoidal waveform uses the sum of consecutive harmonics of a fundamental sinusoidal frequency, f0, having a variable phase offset between the fundamental and even harmonics. In this paper, we discuss results from a computational investigation of the relation between ion energy and DC self-bias when varying the fundamental frequency f0 for capacitively coupled plasmas sustained in Ar/CF4/O2 and how those trends translate to a high aspect ratio etching of trenches in SiO2. The fundamental frequency, f0, was varied from 1 to 10 MHz and the relative phase from 0° to 180°. Two distinct regimes were identified. Average ion energy onto the wafer is strongly correlated with the DC self-bias at high f0, with there being a maximum at φ = 0° and minimum at φ = 180°. In the low frequency regime, this correlation is weak. Average ion energy onto the wafer is instead dominated by dynamic transients in the applied voltage waveforms, with a maximum at φ = 180° and minimum at φ = 0°. The trends in ion energy translate to etch properties. In both, the high and low frequency regimes, higher ion energies translate to higher etch rates and generally preferable final features, though behaving differently with phase angle. [ABSTRACT FROM AUTHOR]

This paper presents spatially resolved laser Thomson scattering measurements in a cathode presheath and describes a new approximation for the electric field at the plasma-sheath boundary that accounts for collisional and ionization effects. The approximation is derived from a 1D sheath model using asymptotic theory, and the approximation is validated against the exact solution of the 1D model. The approximation of the electric field at the plasma-sheath boundary is examined with experimentally measured profiles of electron properties in the cathode presheath and negative glow of a glow discharge. The measurements are made with a noninvasive laser Thomson scattering system applied to a 20 kV pulsed plasma with an electrode gap of 26.53 mm at neutral pressures of 2, 2.5, and 25 Torr for argon, krypton, and helium, respectively. Peak electron densities for the He, Ar, and Kr plasmas are around 9 × 1018, 2.4 × 1019, and 7.5 × 1019 m−3, respectively. For all the gases, the electrons in the negative glow are approximately isothermal at electron temperatures between 1 and 2 eV, and for He and Ar, the electrons were not isothermal in the cathode presheath. The electron density profile in the cathode presheath was nonmonotonic, and calculations of the ionization rate indicate that a sharp increase in the ionization rate may produce a nonmonotonic density profile in the cathode presheath. The insights gained from spatially-resolved noninvasive measurements of electron properties in a cathode sheath reveal the need for more detailed cathode sheath models. [ABSTRACT FROM AUTHOR]

Kr L-shell spectroscopy modeling results are discussed in this paper, focusing on the n = 4 to n = 2 line transitions of Be- and Li-like Kr ions. Collisional radiative atomic kinetic and Stark-broadened spectral line shape calculations show electron temperature T e and density n e sensitivity in the spectrum. The combination of the T e dependence due to the relative intensity of Be-like to Li-like line emissions in the range from 1.5 to 3 keV and the n e sensitivity from the Stark broadening effect on the line shapes in the range from 5 × 10 23 to 2 × 10 24 / cc results in a spectrum that can be employed to diagnose T e and n e . Two different collisional radiative atomic kinetic models i.e., Prismspect [J. J. MacFarlane, et al., Int. Fusion Sci. Appl. Conf. Proc. 457 (2003)] and ABAKO [Florido, et al., PRE, 80, 056402 (2009)] produce similar results in level populations and spectra. In x-ray spectroscopy of implosion cores, this Kr L-shell spectrum may prove useful in an intermediate T e range in which Ar is too ionized for its K-shell to be of diagnostic value and Kr is not ionized enough for its K-shell emission to be useful. [ABSTRACT FROM AUTHOR]

This paper presents an extended interaction klystron (EIK) operating at 0.22 THz based on a four-sheet-beam orthogonal interconnection structure (OIS). The proposed OIS comprises four vertically positioned trapezoidal line structures, significantly enhancing the coupling efficiency between cavities and achieving optimal interactions with the TM81 mode. This study investigates the dispersion characteristics and field distribution numerically, while the optimal working parameters and output structure were analyzed using the particle-in-cell (PIC) studio. The parameters of each cavity and the selection of five intermediate cavities were finalized by calculating the beam conductance, external quality factor, and stagger-tuning. The EIK generated a continuous 480 W output power with a four-sheet-beam at 17.5 kV and a current density of 200 A/cm2 from an input drive signal of 20 mW. The gain was approximately 43.8 dB. [ABSTRACT FROM AUTHOR]

Due to their cost, experiments for inertial confinement fusion (ICF) heavily rely on numerical simulations to guide design. As simulation technology progresses, so too can the fidelity of models used to plan for new experiments. However, these high-fidelity models are by themselves insufficient for optimal experimental design, because their computational cost remains too high to efficiently and effectively explore the numerous parameters required to describe a typical experiment. Therefore, traditionally, ICF design has relied on low-fidelity modeling to initially identify potentially interesting design regions, which are then subsequently explored via selected high-fidelity modeling. In this paper, we demonstrate that this two-step approach can be insufficient: even for simple design problems, a two-step optimization strategy can lead high-fidelity searching toward incorrect regions and consequently waste computational resources on parameter regimes far away from the true optimal solution. We reveal that a primary cause of this behavior in ICF design problems is the presence of low-fidelity optima in different regions of the parameter space far away from high-fidelity optima. To address this issue, we propose an iterative multifidelity Bayesian optimization method based on Gaussian Process Regression that leverages both low- and high-fidelity models simultaneously. We demonstrate, using both two- and eight-dimensional ICF test problems, that our algorithm can effectively utilize both low-fidelity and high-fidelity models to refine the designs. This approach proves to be more efficient than relying solely on high-fidelity modeling for optimization. [ABSTRACT FROM AUTHOR]

In direct-drive inertial confinement fusion, target offset from the target chamber center (or center of beam convergence) may lead to significant implosion asymmetry and fusion yield degradation. In addition, cross-beam energy transfer (CBET) has been shown to be a significant source of laser energy scattering and leads to a reduction in implosion velocity and yield. To improve energy coupling and implosion performance, several techniques for CBET mitigation have been proposed. Recent simulations, however, have shown that CBET also substantially mitigates the effect of target offset on implosion asymmetry and yield [Anderson et al., Phys. Plasmas 27, 112713 (2020)]. Furthermore, the inclusion of CBET models in radiation-hydrodynamics codes was shown to greatly improve agreement between simulations and experiments involving substantial target offset distances. This paper explores the intensity dependence of this CBET–offset effect. In addition, it is shown that enhanced sensitivity to target offset can be expected when CBET-mitigation techniques are used in direct-drive implosions. This is shown through simulations of two such CBET-mitigation techniques on the OMEGA laser: (1) decreased beam-to-target radius, and (2) beam-to-beam frequency detuning. For the typical target offset distances (<15 μm) observed in experiments on OMEGA, however, overall yield is still anticipated to be substantially higher when CBET-mitigation techniques are employed. [ABSTRACT FROM AUTHOR]

A large number of magnetohydrodynamic (MHD) equilibrium calculations are often required for uncertainty quantification, optimization, and real-time diagnostic information, making MHD equilibrium codes vital to the field of plasma physics. In this paper, we explore a method for solving the Grad–Shafranov equation by using physics-informed neural networks (PINNs). For PINNs, we optimize neural networks by directly minimizing the residual of the partial differential equation as a loss function. We show that PINNs can accurately and effectively solve the Grad–Shafranov equation with several different boundary conditions, making it more flexible than traditional solvers. This method is flexible as it does not require any mesh and basis choice, thereby streamlining the computational process. We also explore the parameter space by varying the size of the model, the learning rate, and boundary conditions to map various tradeoffs such as between reconstruction error and computational speed. Additionally, we introduce a parameterized PINN framework, expanding the input space to include variables such as pressure, aspect ratio, elongation, and triangularity in order to handle a broader range of plasma scenarios within a single network. Parameterized PINNs could be used in future work to solve inverse problems such as shape optimization. [ABSTRACT FROM AUTHOR]

In the context of KSTAR plasma research, the discovery of the fast ion regulated enhanced mode is noteworthy due to its remarkable ability to maintain ion temperatures exceeding up to 10 keV for a few tens of seconds, avoid impurity accumulation, and keep low loop voltages. This new plasma operating scenario is achieved in a diverted configuration plasma by avoiding the H-mode transitions with sufficient additional power for the transition. Keeping the density low is the primary method for the avoidance. Additionally, adjustments to other parameters (plasma shape, neutral beam injection, and toroidal magnetic field) associated with the H-mode threshold power are applied to inhibit the transition process. This paper includes an experimental analysis and discussion of these findings. [ABSTRACT FROM AUTHOR]

Nonlinear kinetic structures, called electron phase-space holes (EHs), are regularly observed in space and experimental magnetized plasmas. The existence of EHs is conditioned and varies according to the ambient magnetic field and the parameters of the electron beam(s) that may generate them. The objective of this paper is to extend the 3D Bernstein–Greene–Kruskal model with cylindrical geometry developed by L.-J. Chen et al. ["Bernstein–Greene–Kruskal solitary waves in three-dimensional magnetized plasma," Phys. Rev. E 69, 055401 (2004)] and L.-J. Chen et al., ["On the width-amplitude inequality of electron phase space holes," J. Geophys. Res. 110, A09211 (2005)] to include simultaneously finite effects due to (i) the strength of the ambient magnetic field B 0 , by modifying the Poisson equation with a term derived from the electron polarization current, and (ii) the drift velocity ue of the background plasma electrons with respect to the EH, by considering velocity-shifted Maxwellian distributions for the boundary conditions. This allows us to more realistically determine the distributions of trapped and passing particles forming the EHs, as well as the width-amplitude relationships for their existence. [ABSTRACT FROM AUTHOR]

The process of numerical thermalization in particle-in-cell (PIC) simulations has been studied extensively. It is analogous to Coulomb collisions in real plasmas, causing particle velocity distributions (VDFs) to evolve toward a Maxwellian as macroparticles experience polarization drag and resonantly interact with the fluctuation spectrum. This paper presents a practical tutorial on the effects of numerical thermalization in 2D PIC applications. Scenarios of interest include simulations, which must be run for many thousands of plasma periods and contain a population of cold electrons that leave the simulation space very slowly. This is particularly relevant to many low-temperature plasma discharges and materials processing applications. We present numerical drag and diffusion coefficients and their associated timescales for a variety of grid resolutions, discussing the circumstances under which the electron VDF is modified by numerical thermalization. Though the effects described here have been known for many decades, direct comparison of analytically derived, velocity-dependent numerical relaxation timescales to those of other relevant processes has not often been applied in practice due to complications that arise in calculating thermalization rates in 1D simulations. Using these comparisons, we estimate the impact of numerical thermalization in several examples of low-temperature plasma applications including capacitively coupled plasma discharges, inductively coupled plasma discharges, beam plasmas, and hollow cathode discharges. Finally, we discuss possible strategies for mitigating numerical relaxation effects in 2D PIC simulations. [ABSTRACT FROM AUTHOR]

Zonal flows are known to diminish turbulent transport in magnetic fusion. Interestingly, there is an adiabatic invariant that implies the emergence of zonal flow. This paper shows that if this invariant is decreasing (due to some external factors), then the emerging zonal flow is better. It is also shown that the plasma inhomogeneity can lead to the decrease in the adiabatic invariant. A simple condition for such decrease is found. [ABSTRACT FROM AUTHOR]