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A general approach of how to construct plasma equilibrium in a tokamak with n = 1 violation of toroidal symmetry is proposed. For an arbitrary axisymmetric tokamak plasma equilibrium, there exists the small n = 1 deformation of the initial magnetic configuration that keeps the nesting of the magnetic surfaces (as in the initial configuration) and provides plasma equilibrium; such deformation and final equilibrium configuration are calculated analytically. The asymmetric analogue of the Solov'ev's equilibrium with non-degenerated plasma pressure and current density profiles is presented as an example of the application of the developed algorithm. [ABSTRACT FROM AUTHOR]

High-beta plasma equilibria are realized in a number of physical systems, from planetary magnetospheres, sunspots, and magnetic holes to fusion laboratory experiments. When plasma pressure becomes large enough to completely expel the magnetic field from its volume, the particle trajectories cannot be considered any more as circular gyro-orbits, and plasma pressure ceases to be gyrotropic. These non-gyrotropic effects require kinetic description and are actively studied for a long time in the magnetic reconnection problem. In this paper, we will show that non-gyrotropy of plasma pressure makes it possible to markedly exceed the limit β = 1 dictated by the magnetohydrodynamics for finite-size plasmas, which may be attractive for some fusion schemes such as mirror and cusp configurations. As a first step, we study how these effects manifest themselves in a simple classical problem of confining a cylindrical plasma column by a uniform vacuum magnetic field. Using particle-in-cell simulations, we show that the equilibrium of the diamagnetic bubble type with zero internal magnetic field is formed with an electron-produced current layer of sub-ion scale and found that the gas-kinetic pressure of the central plasma exceeds the pressure of the vacuum magnetic field by 15%. [ABSTRACT FROM AUTHOR]

A set of experiments using controlled, skin depth-sized plasma pressure filaments in close proximity have been carried out in a large linear magnetized plasma device. Two- and three-filament configurations have been used to determine the scale of cross field nonlinear interaction. When the filaments are separated by a distance of approximately five times the size of a single filament or less, a significant transfer of charge and energy occurs, leading to the generation of inter-filament electric fields. This has the effect of rotating the filaments and influencing the merging dynamics. Nonlinear gyrokinetic simulations using seeded filaments confirm the presence of unstable drift-Alfvén modes driven by the steep electron temperature gradient. When the filaments are within a few collisionless electron skin depths (separations twice the size of a single filament), the unstable perturbations drive the convective mixing of the density and temperature and rearrange the gradients such that they maximize in the region surrounding the filament bundle. [ABSTRACT FROM AUTHOR]

Following Greene et al. [Phys. Fluids 14, 671 (1971)] and Connor et al. [Phys. Plasmas 31, 577 (1988); Plasma Phys. Control. Fusion 34, 161 (1992); and Nucl. Fusion 33, 1533 (1993)], the Grad-Shafranov equation for an axisymmetric tokamak plasma equilibrium is solved via an expansion in the, supposedly small, inverse aspect-ratio of the plasma, ϵ. The displacements of equilibrium magnetic flux-surfaces due to plasma shaping are assumed to be O (ϵ) smaller than the minor radii of the surfaces, but no other restriction is placed on the nature of the shaping. The solution of the Grad-Shafranov equation is matched to a vacuum solution that extends to infinity, and consists of an expansion in toroidal functions. The external poloidal magnetic field generated by a finite set of discrete external poloidal magnetic field-coils is calculated, and incorporated into the toroidal function expansion. In this manner, the shape of a large aspect-ratio tokamak plasma is directly related to the currents flowing in the external poloidal field-coils. Finally, a pedestal in the plasma pressure, and the associated spike in the bootstrap current, are incorporated into the model. [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]

In this work, linear neoclassical resistive instabilities are investigated in general toroidal plasmas using standard perturbation theory. Using a neoclassical fluid model, we derive the singular layer equations modified by bootstrap currents and also obtain the dispersion relation of the resistive interchange mode and the neoclassical tearing mode (NTM), respectively. Additionally, we determine the stability criteria D R b s and Δ c b s for bootstrap current-modified resistive modes. The resistive interchange mode is stable when D R b s < 0 and the NTM is stable when Δ ′ < Δ c b s , where Δ ′ is the stability index of the tearing mode. It is found that, in tokamak plasmas with a positive magnetic shear, bootstrap currents have a destabilizing effect on resistive interchange modes, which not only increases the value of the stability criterion ( D R b s ) but also enhances the growth rate. In addition, bootstrap currents have a stabilizing effect on the growth rate of the NTM in a low growth rate region. However, bootstrap currents can also decrease the critical value Δ c b s . In plasmas with negative magnetic shear, the opposite holds. Furthermore, the coupling effect between bootstrap currents and Pfirsch–Schlüter currents via magnetic field curvature is determined for the first time in this work. This coupling always has a stabilizing influence on the resistive interchange mode and can increase the value of Δ c b s . The coupling is also independent of the sign of the magnetic shear and can be enhanced in low-aspect-ratio tokamaks (such as spherical tokamaks) or in plasma regions with low magnetic shear (as used in ITER hybrid scenarios). Our results are valid for low-n resistive instabilities in toroidal plasmas with arbitrary aspect ratios and β, where n is the toroidal mode number and β represents the ratio of the plasma pressure to the toroidal magnetic pressure. Overall, this investigation has broad parametric applications and deepens our understanding of the physical mechanisms underlying the influence of neoclassical effects on resistive instabilities. [ABSTRACT FROM AUTHOR]

We present an analytical model of the self-consistent equilibrium of a magnetic flux rope, which is obtained in cylindrical geometry. The equilibrium quantities, namely, the azimuthal magnetic field and plasma pressure, are determined in a self-consistent way through the current density, which is derived as a solution of a nonlinear equation. By minimizing the energy functional, it was shown that the constrained equilibrium state is stable. The obtained results are also applicable to the cylindrical tokamak magnetic configurations. It is shown that the analytically predicted radial profiles of equilibrium quantities are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Numerical analysis of the coupling between the Alfvén and slow azimuthally small scale modes in the dipolar model of the magnetosphere is performed. The field line curvature, inhomogeneity both across the magnetic shells and along the field lines, and finite plasma pressure are taken into account. The field line curvature causes coupling Alfvén and slow modes, while the contribution from the fast mode is neglected due to the assumed small scale of the oscillations in the azimuthal direction. The plasma pressure decreases with distance from the Earth, which is a characteristic for the ring current region. The wave's transverse dispersion, that is, dependence of the radial wave vector kr on the frequency ω, was studied. It was found that the wave can exist in two frequency ranges. Both ranges are bounded by resonant frequency on one side and the cutoff frequency on the other. The higher frequency range corresponds to the Alfvén mode. However, its properties are modified due to the coupling with the slow mode. For example, the divergence of the plasma displacement and the magnetic field compressional component appear. In the slow magnetosonic region, in contrast, the cutoff frequency is always smaller than the resonant one. If the pressure gradient is strong and negative, the slow mode cutoff frequency can disappear, that is, the radial wave vector squared even for the zero frequency. It means that the kr value goes to zero at imaginary frequency. Mode structure along the field line for different plasma pressure values and its pressure gradients was calculated. [ABSTRACT FROM AUTHOR]

The Alfvén-ion cyclotron (AIC) mode is an instability that can be driven in magnetized plasmas with anisotropic pressure. Its chief deleterious effect is the driving of enhanced pitch-angle scattering of ions. Although the AIC mode has been observed in several mirror devices, it has not yet been observed in FRC devices developed by TAE Technologies [H. Gota et al., Nucl. Fusion 61, 106039 (2021)]. Previous theoretical work [T. Tajima et al., Phys. Rev. Lett. 39, 201 (1977)] has suggested that sufficient axial inhomogeneity, quantified by a critical axial plasma length, can stabilize this mode. This stabilization mechanism is examined in fully kinetic particle-in-cell simulations with one spatial dimension modeling a simplified magnetic mirror geometry for a plasma with β ∼ 1. A fast-ion population provides the driving anisotropy for the AIC mode, and the resulting effect on the fast-ion pitch angle distribution is examined. The severity of mode activity is recorded for a scan of plasma lengths for multiple fast-ion injection angles. This scan yields critical lengths that show good qualitative agreement with those from the past theoretical work. [ABSTRACT FROM AUTHOR]

Energetic particle (EP)-driven instabilities will be of strongly increased relevance in future burning plasmas as the EP pressure will be very large compared to the thermal plasma pressure. Understanding the interaction of EPs and bulk plasma is crucial for developing next-generation fusion devices. In this work, the JOREK magnetohydrodynamic code and its full-f kinetic particle-in-cell module are extended by an anisotropic pressure coupling model to allow for the simulation of EP instabilities at high EP pressures using realistic plasma and EP parameters. Furthermore, a diagnostic is implemented to allow for the visualization of phase-space resonances. The resulting code is first benchmarked linearly for the International Tokamak Physics Activity-toroidal Alfvén eigenmodes as well as the experiment-based ASDEX-Upgrade-NonLinear Energetic particle Dynamics cases, obtaining good agreement with other codes. Then, it is applied to a high energetic particle pressure discharge in the ASDEX Upgrade tokamak using a realistic non-Maxwellian distribution of EPs, reproducing aspects of the experimentally observed instabilities. Non-linear applications are possible based on the implementation, but will require dedicated verification and validation left for future work. [ABSTRACT FROM AUTHOR]

The origin of intermittent fluctuations in an experiment involving several interacting electron plasma pressure filaments in close proximity, embedded in a large linear magnetized plasma device, is investigated. The probability density functions of the fluctuations on the inner and outer gradient of the filament bundle are non-Gaussian and the time series contain uncorrelated Lorentzian pulses that give the frequency power spectral densities an exponential shape. A cross-conditionally averaged spatial reconstruction of a temporal event reveals that the intermittent character is caused by radially and azimuthally propagating turbulent structures with transverse spatial scales on the order of the electron skin depth. These eruption events originate from interacting pressure gradient-driven drift-Alfvén instabilities on the outer gradient and edge of the filament bundle. The temporal Lorentzian shape of the intermittent structures and exponential spectra are suggestive of deterministic chaos in the underlying dynamics; this conclusion is supported by the complexity–entropy analysis (CH-plane) that shows the experimental time series are located in the chaotic regime. [ABSTRACT FROM AUTHOR]

Sheath expansion is a distinctive feature of the double flush mounted probe because of the embedded configuration. Previously, the sheath expansion effect was usually neglected in weakly ionized plasma dominated by collisions between charged particles and neutrals. In this work, we investigated the sheath expansion effect of the double flush mounted probe in weakly ionized plasma. Results indicate that measurements using the double flush mounted probe were also influenced to a certain extent by the sheath expansion effect in weakly ionized plasma. To eliminate the influence, an empirical analytical formula has been presented to eliminate the influence of sheath expansion. In addition, a fitting curve is given based on experimental data, which indicates that sheath expansion should be considered in processing the measured data when the plasma pressure is lower than 200 Pa. In summary, this work indicates that the ion–neutral collision is a crucial factor that affects sheath expansion in addition to the radius parameter and probes' bias, which can be extended to double flush mounted probe diagnostics in collisional plasma such as the reentry plasma sheath and high-powered plasma thruster. [ABSTRACT FROM AUTHOR]

A new model for electron temperature gradient (ETG) modes is developed as a component of the multi-mode anomalous transport module [Rafiq et al., Phys Plasmas 20, 032506 (2013)] to predict a time-dependent electron temperature profile in conventional and low aspect ratio tokamaks. This model is based on two-fluid equations that govern the dynamics of low-frequency short- and long-wavelength electromagnetic toroidal ETG driven drift modes. A low collisionality NSTX discharge is used to scan the plasma parameter dependence on the ETG real frequency, growth rate, and electron thermal diffusivity. Electron thermal transport is discovered in the deep core region where modes are more electromagnetic in nature. Several previously reported gyrokinetic trends are reproduced, including the dependencies of density gradients, magnetic shear, β and gradient of β (β ′) , collisionality, safety factor, and toroidicity, where β is the ratio of the plasma pressure to the magnetic pressure. The electron heat diffusivity associated with the ETG mode is discovered to be on a scale consistent with the experimental diffusivity determined by power balance analysis. [ABSTRACT FROM AUTHOR]

Quasi-symmetry can greatly improve the confinement of energetic particles and thermal plasma in a stellarator. The magnetic field of a quasi-symmetric stellarator at high plasma pressure is significantly affected by the bootstrap current, but the computational cost of accurate stellarator bootstrap calculations has precluded use inside optimization. Here, a new efficient method is demonstrated for optimization of quasi-symmetric stellarator configurations such that the bootstrap current profile is consistent with the geometry. The approach is based on the fact that all neoclassical phenomena in quasi-symmetry are isomorphic to those in axisymmetry. Therefore, accurate formulas for the bootstrap current in tokamaks, which can be evaluated rapidly, can be applied also in stellarators. The deviation between this predicted parallel current and the actual parallel current in the magnetohydrodynamic equilibrium is penalized in the objective function, and the current profile of the equilibrium is included in the parameter space. Quasi-symmetric configurations with significant pressure are thereby obtained with self-consistent bootstrap current and excellent confinement. In a comparison of fusion-produced alpha particle confinement across many stellarators, the new configurations have significantly lower alpha energy losses than many previous designs. [ABSTRACT FROM AUTHOR]

Electromagnetic microturbulence at finite normalized plasma pressure β = n 0 T 0 / (B 0 2 / 2 μ 0) (n0 is the equilibrium density, T0 the equilibrium temperature, B0 the equilibrium magnetic field, and μ0 the permeability of free space) is investigated within a local gradient-driven gyrokinetic framework. The focus lies on the well-known high β turbulence runaways [R. E. Waltz, Phys. Plasmas 17, 072501 (2010)] that have been proposed to set a nonlinear upper threshold β c , also known as the nonzonal transition [M. J. Pueschel, Phys. Rev. Lett. 110, 155005 (2013)]. In this paper, it is shown that persistent mesoscale zonal flow patterns, developing self-consistently on long time scales, mitigate high β turbulence runaways. The application of such mesoscale zonal flow states as initial conditions allows for the access of an improved β-regime, that is, β > β c , in which no turbulence runaway occurs. Various aspects of those mesoscale zonal flow-dominated improved states are investigated, such as (i) the stability constraints with respect to the mesoscale zonal flow level for the triggering of turbulence runaways, (ii) the influence of mesoscale zonal flows on magnetic stochasticity [W. M. Nevins, Phys. Rev. Lett. 106, 065003 (2010)], and (iii) the transfer processes connected to the drive and damping of mesoscale zonal flows. This work implies that β c does not set the upper limit in the normalized plasma pressure for stationary operation, provided persistent mesoscale zonal flow patterns can develop. Since variations of β occur on the energy confinement time, which is large compared to the time required for mesoscale zonal flows to develop, the reported mesoscale zonal flow-dominated improved β-regime is expected to be the experimentally relevant branch. Furthermore, this work highlights the need for sufficiently long simulation time traces of at least a few ∼ 10 3 R 0 / v th , i (R0 is the major radius, and v th , i is the ion thermal velocity), also within kinetic electron frameworks, to capture non-negligible long-term dynamics. [ABSTRACT FROM AUTHOR]

A plasma glow discharge tube, a versatile device widely employed in several scientific and industrial applications, is also a useful tool for many basic plasma studies in the laboratory. Anode glow oscillations are well-known phenomena in such devices that arise from an instability of the plasma glow around a small positively charged electrode. Depending upon the gas pressure, the applied DC voltage, and the distance between the electrodes, these oscillations can display a rich dynamical behavior. Over a certain parametric regime, these nonlinear oscillations exhibit a stable limit cycle behavior that has been modeled in the past by a Van der Pol like equation. While such a model equation provides a qualitative description of the observations, it lacks quantitative agreement and does not have any predictive capability. We employ the sparse identification of nonlinear dynamics (SINDy) method to obtain a model equation directly from a time series of the experimental data. Our model captures well the main features of the experimental data in a quantitative manner. It also shows a significant deviation from the Van der Pol model due to additional contributions that are akin to nonlinear damping in a Rayleigh oscillator. Such a hybrid Van der Pol–Rayleigh oscillator model could provide a useful paradigm for future explorations of the nonlinear dynamics of this system. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The features of an unexpected, large event that arises spontaneously during a basic heat transport experiment are presented. It consists of the sudden collapse of the radial plasma pressure profile, akin to disruption events observed in toroidal magnetic confinement devices. The experiment is performed on the Large Plasma Device at the University of California, Los Angeles (UCLA). It uses a LaB6 thermionic emitter of annular shape to induce off-axis heating of a cold, afterglow plasma, in a linear magnetic geometry. The temporal evolution consists of three regimes. During an early, quiescent period, classical heat transport along and across the magnetic field arises from Coulomb collisions. After significant pressure gradients develop, drift-Alfvén waves become unstable. Upon reaching large amplitude, they trigger avalanche events that flatten the outer part of the heated region, which, in turn, quenches the instability. Due to the sustained heating, the pressure profile rebuilds and the process repeats, leading to a relatively long, second regime that displays multiple avalanches, but suddenly, the annular pressure profile is observed to collapse. After this collapse, the system enters a third regime with large fluctuations. Before the collapse, a rapid, runaway heating environment arises whose time evolution exhibits a self-similar dependence on the applied voltage. The time evolution, morphology, and scaling of the collapse event are presented, and an examination is made of the underlying mechanisms. [ABSTRACT FROM AUTHOR]

Magnetic field fluctuations are observed in current-carrying stellarator plasmas when the rotational transform is close to a rational value at the edge of the plasma. At low plasma pressure, these fluctuations are associated with perturbed currents parallel to the equilibrium magnetic field lines. A model for these magnetohydrodynamic modes in a low-β, three-dimensional stellarator equilibria has been developed. A set of helical current filaments are constrained to mimic the structure of magnetic field lines on rational surfaces derived from three-dimensional (3D) equilibrium reconstructions. Transformation to straight field line coordinates then allows fitting of the poloidal magnetic sensor data to a single harmonic function, which fixes the modeled toroidal mode structure via the field line flow geometry. The developed procedure accurately captures phase and amplitude variation for m/n = 3/2, 3/1, and 4/1 modes in the 3D equilibria of the compact toroidal hybrid experiment. [ABSTRACT FROM AUTHOR]

Results are presented from basic heat transport experiments using a magnetized electron temperature filament that behaves as a thermal resonator. Using a small cathode source, low energy electrons are injected along the magnetic field into the afterglow of a pre-existing plasma forming a hot electron filament embedded in a colder plasma. A series of low amplitude, sinusoidal perturbations are added to the cathode discharge bias that creates an oscillating heat source capable of driving large amplitude electron temperature oscillations. Langmuir probes are used to measure the amplitude and phase of the thermal wave field over a wide range of driver frequencies. The results are used to verify the excitation of thermal waves, confirm the presence of thermal resonances, and demonstrate the diagnostic potential of thermal waves through measurement of the parallel thermal diffusivity. [ABSTRACT FROM AUTHOR]

We propose, test, and analyze a new step-wise excitation approach to quickly obtain, or determine with high accuracy, the resonance frequency of a microparticle confined in the space charge region of a low pressure plasma operated above a horizontal surface. The local value of this resonance frequency can directly be related to the charge of such a plasma-immersed microparticle and is, therefore, essential to be obtained with respect to the interpretation of many dusty and complex plasma laboratory experiments. The presented approach applies a step-wise change in plasma power, after which the damped harmonic oscillatory motion of the plasma-confined microparticle toward its "new" equilibrium confinement position is monitored temporally resolved and fitted with the theory describing this damped harmonic oscillator. Proof-of-principle experiments demonstrate that the results from this method match the results of the traditional frequency sweep resonance method within 5%. Comparing to existing—for instance, phase-resolved or single particle injection—methods to determine the resonance frequency, this approach offers the experimenter a unique possibility to choose the trade-off between reduced measurement time—essential when working with particles that are vulnerable to plasma etching—and measurement accuracy. [ABSTRACT FROM AUTHOR]

Electromagnetic gyrokinetic simulation of high temperature plasma is required to predict confinement in magnetic fusion devices and has posed challenges for existing codes. In this paper, we demonstrate successful global gyrokinetic simulation of the ion temperature gradient-driven mode-kinetic ballooning mode transition in a toroidal fusion plasma test case using the mixed variables/pullback transformation (MV/PT) scheme with the particle-in-cell codes XGC and ORB5, and compare to results from a conventional continuum code from the literature. The MV/PT scheme combines explicit time integration with mitigation of the well-known electromagnetic gyrokinetic "cancelation problem." We calculate eigenmodes in the electrostatic and parallel vector potentials, and find good agreement in growth rate, real frequency, and the normalized plasma pressure of mode transition. [ABSTRACT FROM AUTHOR]

Driven resonant current amplification in uniform λ = μ o J B , self-organized plasma configurations with plasma pressure confinement is studied. This work extends previous analyses of force-free states (J ∥ B) to equilibria with plasma pressure gradients (∇ P = J × B ≠ 0). An analytical treatment shows that in equilibria with uniform λ and a plasma pressure gradient that varies linearly with poloidal magnetic flux, driven resonances are either downshifted or upshifted from their force-free values. Additionally, a modification of off resonance current amplification is observed in pressure confining equilibria. Computational results add support to analytical predictions, and also include driven resonance behavior with equilibrium profiles not considered analytically. These results suggest not only persistence, but also modification of driven resonant current amplification in sustained spheromak configurations with plasma pressure confinement when compared to force-free states. By providing a basis for the existence of driven, high current amplification, pressure confining equilibria, this work provides insight into the accessibility of sustained spheromak states of most interest for potential fusion energy applications. [ABSTRACT FROM AUTHOR]

The effect of electron temperature on the space–time evolution of nonlinear plasma oscillations in an inhomogeneous plasma is studied using a one-dimensional particle-in-cell code. It is observed that, for an inhomogeneous plasma, there exists a critical value of electron temperature beyond which the wave does not break. These simulation results, which are in conformity with the purely theoretical arguments presented by Trines [Phys. Rev. E 79, 056406 (2009)], represent the first numerical elucidation of the effect of plasma pressure on wave breaking amplitude. The results presented are of relevance to experiments where moderate to large amplitude plasma waves are excited, e.g., laser–plasma interaction experiments. [ABSTRACT FROM AUTHOR]

Time and spatially resolved electric field measurements by Stark polarization spectroscopy in a nanosecond pulsed atmospheric pressure helium jet operating in ambient air and impinging on a indium tin oxide coated glass slide are reported. An automatic fitting procedure of the Stark shifted spectra taking into consideration constraints regarding Stark components' positions and intensities as well as molecular nitrogen emission subtraction was implemented. This allowed electric field vector component measurements both in the gas phase and at the interface when the jet impinges on the substrate and during the development of a surface ionization wave. The obtained results show an increase in the axial electric field in the jet effluent in the gas phase with a peak magnitude from 12 to 18 kV cm−1 before the ionization wave impinges on the substrate. A steep electric field enhancement to a peak value of about 24 kV cm−1 was observed when the ionization wave impinges on the surface. A peak radial electric field of about 27 kV cm−1 was measured off-axis in the surface ionization wave. These results are consistent with previously reported modelling predictions. While Stark polarization spectroscopy is limited to electric field measurements from regions with emission, we illustrate that the capability to measure near surface electric fields in helium makes it a valuable complementary technique for the electric field-induced second harmonic (EFISH) technique. [ABSTRACT FROM AUTHOR]

The nonlinear evolution of the m/n = 2/1 double tearing mode (DTM) is investigated by the toroidal resistive magnetohydrodynamic code CLT. It is found that the m/n = 2/1 DTM can lead to either a core pressure crash or an off-axis pressure crash. Unlike the core pressure crash, the plasma pressure at the magnetic axis remains almost unchanged during the off-axis pressure crash. The pressure crash only occurs in the annular region during the off-axis crash, and the on-axis plasma pressure slowly reduces after the crash, which is consistent with TFTR observations. A series of simulations are carried out to investigate the influence of the radial position of the inner resonant surface r 1 , the magnetic shear at the inner resonance surface, and the spatial separation between the two resonant surfaces on nonlinear behaviors of DTMs. We find that r 1 plays a dominant role in the nonlinear DTM behaviors. It is more likely for the DTM to lead to the core pressure crash with a smaller r 1 . It is also found that the magnetic shear at the inner resonant surface and the spatial separation between the two resonant surfaces can also largely influence the nonlinear evolution of the DTM. A simple theoretical formula of the transition criterion between the two pressure crashes is proposed, which agrees well with the simulation results. [ABSTRACT FROM AUTHOR]

In this study, the effects of small nitrogen impurity on the behaviors of helium APPJs propagating into nitrogen surrounding gas are investigated numerically based on a two-dimensional fluid model. The results show that the addition of N2 impurity can change the APPJ length and there exists an optimal N2–helium ratio which enables the plasma jet to reach the maximum length. This is attributed to the variation of helium mole fraction and the ionization rate in the helium flow channel resulting from N2 impurity. Like the APPJ propagating into ambient air, with the addition of nitrogen, the profile of APPJ exposed to a nitrogen environment varies from an annular shape to a nearly uniform structure, though their reaction chemistries inside the jet effluent differ remarkably. Also, under different impurity concentrations, the density distribution and evolution of active species throughout the effluent of APPJ, as well as their dominant reaction pathways, behave differently. The physics mechanisms underlying these behaviors are discussed in detail. [ABSTRACT FROM AUTHOR]

The maximum stable pedestal pressure has been shown to increase with core pressure and, in combination with core transport effects, this can lead to a positive feedback mechanism. However, the effect is shown to saturate for a high β in ASDEX-Upgrade simulations [Wolfrum et al. "Impact of wall materials and seeding gases on the pedestal and on core plasma performance," Nucl. Mater. Energy 12, 18 (2017)]. In this paper, it is numerically investigated whether this effect appears in ITER plasmas, using ideal MHD numerical codes HELENA and MISHKA for different ITER scenarios, in a range of plasma conditions: two inductive scenarios at 7.5 MA/2.65 T and 15 MA/5.3 T and one steady-state scenario at 10 MA/5.3 T. For all scenarios, reference cases for ITER plasmas were taken as a starting point. No pedestal pressure saturation is found for the inductive scenarios, gradually growing up to the global βN limit, which is determined by the Troyon limit. On the contrary, for the 10 MA/5.3 T steady-state scenario, the maximum stable pedestal pressure does not depend on the total β and it is limited by low-n kink-peeling modes, as opposed to high-n peeling-ballooning modes that limit the maximum attainable pedestal height in the inductive scenarios. This core-edge MHD stability feedback loop has been investigated for two assumptions regarding the scaling of the pedestal width with β p , ped ¯ , using either a constant pedestal width or when scaling it as Δ ψ N ∝ β p , ped ¯ 1 / 2 . A stronger core-edge MHD stability feedback is observed for the varying pedestal width for the inductive plasma scenarios, which is closer to the experimental results from JET [Challis et al. "Improved confinement in JET high plasmas with an ITER-like wall," Nucl. Fusion 55(5), 053031 (2015)], but not for the steady-state one. Finally, the pressure achieved according to this core-edge feedback stability analysis is compared to the plasma pressure achievable on the basis of the energy confinement IPB98(y,2) scaling for various assumptions regarding the scaling of core plasma confinement with heating power. [ABSTRACT FROM AUTHOR]

The nonlinear evolution of electromagnetic instabilities in reversed shear plasmas is investigated by means of global gyrokinetic simulations. It is found that the kinetic infernal mode (KIM), which is a pressure-driven instability with low to intermediate toroidal mode number excited in a region of low magnetic shear, is unstable at high β, while the ion temperature gradient mode is unstable at low β, where β is the ratio of the plasma kinetic pressure to the magnetic pressure. The β threshold of the KIM is much lower than that of the kinetic ballooning mode (KBM) appearing in a normal shear plasma, while both the KIM and KBM are strong at the unfavorable curvature region, and the KIM has the same parity as the KBM. Nonlinear simulations show that the KIM gets saturated by exciting strong zonal flows and fluctuations of low toroidal mode number. The amplitude of the KIM turbulence is similar to that of the KBM turbulence in spite of the fact that the linear growth rate of the KIM is much higher than that of the KBM. This is because the excitation of zonal flows and fluctuations at low toroidal mode number is stronger in the reversed shear plasma than that of the normal shear plasma. On the other hand, the energy flux and particle flux due to the KIM turbulence are about two or three times larger than those by the KBM turbulence. [ABSTRACT FROM AUTHOR]

Microtearing (MT) turbulence affects plasma confinement and profile evolution in many different magnetic-confinement scenarios, ranging from tokamak core plasmas to the H-mode pedestal and from spherical tokamaks to reversed-field pinches. Thus, an urgent need exists to design and test reduced models of MT turbulence and transport. Here, a heuristic quasilinear model is composed and verified by comparing its predictions against nonlinear MT simulations. It is found to yield good predictions of magnetic flutter transport as key parameters are varied: the collision frequency, the electron temperature gradient, and the normalized plasma pressure. [ABSTRACT FROM AUTHOR]

The helium (He) non-thermal atmospheric pressure plasma jet (APPJ) source was configured, and the He spectra were measured by applying AC power to the source. A He collisional-radiative (CR) model was developed to investigate the He spectra obtained from the APPJ source. Different atmospheric pressure (AP) processes were evaluated, and the dominant processes among them that contribute to the He spectra were determined. The plasma parameters of the APPJ were determined using the He CR model, and the He spectra were successfully reproduced. The electron density was determined to be ∼ 2 × 10 12 cm − 3 between the electrodes of the APPJ, and ∼ 9 × 10 11 cm − 3 outside the quartz tube, for a frequency and bias of an AC voltage source of 5 kHz and 10 kV p. p , respectively. A parametric study was performed to explain the behavior of the electron temperature and electron density of APPJ. [ABSTRACT FROM AUTHOR]

The fueling efficiency of supersonic molecular beam injection (SMBI) depends on injection parameters. In this article, we report on simulations of SMBI into turbulent edge fusion plasmas. Density continuity equations, which describe the dynamics of molecules and Franck-Condon atoms, are coupled with the plasma equations in the hot-edge-sol-electrostatic (HESEL) drift-fluid model. Numerical simulations, with parameters relevant to experimental conditions, are performed in order to investigate the dependency of the fueling efficiency of SMBI on the molecular beam density, velocity, and beam width. In all conditions, the efficiency grows with the increase in the molecular beam velocity. The variation in efficiency with respect to the beam density depends on the plasma pressure and the ratio of the density source, produced by ionization of neutrals, to the plasma density source provided by core fueling. The effect of the molecular beam width is pronounced only if the beam density is high and the plasma, into which the beam is injected, is relatively dense and hot. The influence of plasma fluctuations on SMBI is weak; however, a strong neutral injection leads to suppression of a turbulence transport due to relaxation of pressure gradient profiles. [ABSTRACT FROM AUTHOR]

A hysteresis loop has been observed during the E–H mode transition in an inductively coupled plasma at high pressures. The cause of the hysteresis has been reported as a nonlinearity of the transferred and dissipated powers due to capacitive coupling, multi-step ionization, the change in the electron energy probability function (EEPF), and so on [M. M. Turner and M. A. Lieberman, Plasma Sources Sci. Technol. 8(2), 313–324 (1999) and H. C. Lee and C. W. Chung, Sci. Rep. 5, 15254 (2015)]. However, when a coil current reduction is considered, the previous interpretation of hysteresis cannot explain the observations in the intermediate pressure region, where the coil current reduction occurs, but hysteresis is not observed. In this work, the E–H mode transition and its hysteresis are discussed in three pressure regions, the low, intermediate, and high pressure regions, whether or not the coil current and the hysteresis are observed. The power transfer efficiency, transferred power, EEPF, and total energy loss are obtained at 10, 100, and 300 mTorr. Hysteresis is only observed at 300 mTorr, and the coil current reduces at 100 and 300 mTorr during the E–H mode transition. The mechanism of hysteresis is explained in a power balance diagram that includes the transferred power and the dissipated power by considering the power transfer efficiency in the E and H modes. In addition, the conditions of hysteresis and coil current reduction are revealed and classified in this work. [ABSTRACT FROM AUTHOR]

We present here the detailed measurements of radial distribution of the magnetic field in a gas-puff z-pinch plasma at the final stages of the implosion phase and at stagnation. While the measurements are chordal, the radial distribution of different charge states was utilized to measure the magnetic field locally for certain radii, so that unlike chordal measurements in general, the magnetic field radial distribution was obtained with no need for the Abel inversion of the data. The distribution was measured using the Zeeman effect via a novel spectroscopic technique, at several axial locations, and demonstrates striking features such as the peak field remaining at a radius much larger than the stagnation radius at all times. Furthermore, while the distribution observed is sometimes monotonic with respect to the radius, it is often not, a behavior that can be linked to 2D features in the plasma column resulting from the Rayleigh–Taylor instability. The current flowing through the stagnating plasma was found to be a small fraction of the total current, resulting in clearly insufficient magnetic pressure to balance the plasma pressure at stagnation. The magnetic field data, taken over several axial positions, are used to obtain the true inductance in the imploding plasma for the first time; it is found that the data cannot explain the current turnover at stagnation. A simulation with the MACH2-Tabular Collisional-Radiative Equilibrium magnetohydrodynamics code in the r–z plane shows that the peak of the magnetic field pinches to a much smaller radius than is observed in the spectroscopic data. Furthermore, the depth of the computed current turnover at stagnation is smaller than the measured one. The two observed features of a radially extended magnetic field at stagnation together with a deep current turnover are a challenge to match in simulations. Various calculations and estimates of the inductive and resistive load voltages are examined to ascertain if they are responsible for the observed current notch. The results demonstrate that the knowledge of the true inductance in the driven load requires such magnetic-field-distribution measurements and that imaging data or electrical measurements are insufficient. [ABSTRACT FROM AUTHOR]

The two-field (pressure/density) model for the L→H transition is extended and analyzed qualitatively. In its original form the model is ambiguous as to the location of the transition within the range of bistability of particle and thermal fluxes. Here, the model is regularized by including (i) hyperdiffusion, (ii) time dependence, and (iii) curvature of the pressure profile. The regularizations (i)-(ii) agree and indicate that the Maxwell rule for the forward and back transition applies, as opposed to the maximum flux forward and minimum flux backward transition rules (which yields hysteresis) as suggested previously. Regarding (i)-(ii), simple models suggest that for a pressure gradient driven electric field shear bifurcation, the basic scale of the pedestal is inexorably tied to the particle fueling depth, which normally is the neutral penetration depth. There is no hysteresis predicted by the local model of transport suppression. However, the effect of pressure profile curvature (iii) changes these results substantially. When it dominates, the curvature effect reduces the transition threshold to the lower end of the range of heating power, which falls within the phase coexistence region for both forward and back transitions. This softens the transition threshold requirements. In this limit, the model with pressure curvature also predicts transitions which occur in regimes of flat density and driven exclusively by the temperature gradient. This allows the pedestal to extend beyond the fueling depth, and also allows some decoupling of density and pressure profiles. In a parameter range where the pressure curvature is less important the transition occurs somewhere between the above two limits. © 2008 American Institute of Physics.

To achieve a tolerable heat and particle flux to the divertor target of fusion reactors, the so-called plasma detachment is essential to be set up and controlled. Impurity seeding facilitates the achievement of such a regime, mostly due to the enhanced plasma radiation led by the excitation-relaxation cycle of such species. Little is known about the impurity-induced plasma chemical processes occurring in the divertor region during detachment operation. In this work, the influence of three different impurities, i.e., N2, Ar, and He, on detachment performance is studied. To do so, experimental campaigns on the linear plasma machine Magnum-PSI have been carried out. Results highlight the beneficial role of N2 + H2 seeding, decreasing the plasma pressure in front of the target, leading to a reduced heat load compared to the pure H2 seeding case. An opposite trend has been found concerning He and Ar puffing. In fact, injection of H2 + He and H2 + Ar gas mixtures led to an increased heat flux. To address the importance of different plasma-chemical reaction paths, global plasma models have been used. The resulting reduced reaction schemes for Ar + H2, He + H2, and N2 + H2 have been implemented in B2.5-Eunomia, a coupled code consisting of a Monte Carlo code treating the transport of neutrals and a fluid code solving plasma equations. Simulation results qualitatively reproduce the favorable effect of N2, while confirming the deteriorating effect of He and Ar on a detachedlike hydrogen plasma. We point the synergetic role of H2 + N2 to be due to molecular-driven ion recombination, i.e., N-molecular-assisted recombination (MAR). A direct comparison of the collision frequency between N-MAR and MAR is showed, highlighting the crucial importance of the former in reducing the ion and heat flux to the target plate. [ABSTRACT FROM AUTHOR]

The stability of contact discontinuities formed by the relaxation of two Maxwellian plasmas with different number densities but the same plasma thermal pressure is studied by means of a one-dimensional electrostatic full Vlasov simulation. Our simulation runs with various combinations of ion-to-electron ratios of the high-density and low-density regions showed that transition layers of density and temperature without jump in the plasma thermal pressure are obtained when the electron temperatures in the high-density and low-density regions are almost equal to each other. However, the stable structure of the contact discontinuity with a sharp transition layer on the Debye scale is not maintained. It is suggested that non-Maxwellian velocity distributions are necessary for the stable structure of contact discontinuities. [ABSTRACT FROM AUTHOR]

Previously [E. Kawamura et al., Plasma Sources Sci. Technol. 25, 054009 (2016) and E. Kawamura et al., J. Phys. D: Appl. Phys. 50, 145204 (2017)], one dimensional (1D) particle-in-cell (PIC) simulations of 1 to 4 mm gap, He/2%H2O atmospheric pressure plasma discharges showed an ionization instability resulting in bulk striations. Assuming that the ionization rate coefficient Kiz is related to the root mean square electric field E by Kiz ∝ Eq, a striation theory showed that q < 0 is a necessary condition for the instability. A local calculation yielded q > 0, implying that nonlocal electron kinetics are required for the instability. Wider gaps can fit in a wider range of wavelengths, resulting in multi-mode striations. Previously, we assumed a single mean q value for each discharge, and did not calculate q for each mode separately. Here, we develop a wavelength resolved global striation model and apply it to 1D PIC simulations of 4 mm gap discharges with input currents Jrf = 0.04 to 0.30 A/cm2. We first examine a base case at 0.23 A/cm2 and observe a mixture of unstable modes within a window of wavelengths λ. At shorter λ, the modes are suppressed by diffusion. At longer λ, we observe a transition to locality in which q becomes less negative with increasing λ, approaching its local positive value and stabilizing the modes. The window of unstable modes shifts to shorter λ with increasing Jrf, causing the modes to be suppressed by diffusion at higher Jrf. At lower Jrf, the decrease in bulk plasma density with decreasing Jrf suppresses the striations. [ABSTRACT FROM AUTHOR]

This paper presents the results of experimental and theoretical studies of the electric discharge characteristics and the concentration dependences of water isotopomers (H2O, HDO, and D2O) in a glow discharge in mixtures of helium with water vapor and molecular deuterium depending on the initial concentrations of H2O and D2. Measurements of the H2O and HDO concentrations were carried out by absorption diode laser spectroscopy in parallel with measurements of the axial electric field. The calculations were performed using a homogeneous kinetic model including balance equations for neutral and charged particles, which are solved together with the Boltzmann equation for the electron energy distribution function in the two-term approximation and the equation for the external electric circuit. A comparison was made of the experimental and calculated values of the concentrations of H2O and HDO, the stationary values of the electric field strength, and the discharge current. All abovementioned experimental parameters showed satisfactory agreement with the calculations. It was found that in self-consistent calculations, as well as in the experiment, the ratio of the concentrations of H2O and HDO was the same for different proportions of the initial concentrations of H2O and D2, and this ratio remained the same during the discharge action. [ABSTRACT FROM AUTHOR]

The spatial distribution of floating potential for charging distribution at the wafer-level is measured by using a wafer-type probe array, and the experiment is performed in an inductively coupled oxygen plasma. At low pressures, the floating potential distribution exhibits a convex shape. As the pressure increases, the profile gradually evolves into a concave shape, indicating more negative charge at the center, which is unusual for electropositive plasmas. A similar evolution is observed for positive ion flux under the same discharge conditions. It is noteworthy that the charging distribution at the wafer position in process plasmas at high pressures can be reversed due to negative ions compared to that in electropositive plasmas. In addition, the correlation between the floating potential and electronegativity is also presented. In order to investigate the effect of negative ions on floating potential distribution, a 2D fluid simulation with an electron-heating model is conducted, and the experimental results are in good agreement with those from the 2D fluid simulation. [ABSTRACT FROM AUTHOR]

An analytical theory based on the MHD model is presented for understanding the coupling between the shear Alfvén wave and the slow magnetosonic wave (or the so called slow mode) in the frequency range below the Toroidal Alfvén Eigenmode (TAE) continuum spectrum gap. In tokamak plasmas, coupling of the Alfvén m poloidal harmonic and the slow mode m ± 1 harmonics causes the Alfvén m harmonic continuous spectrum to be uplifted from the uncoupled Alfvén spectrum by an amount related to the plasma pressure and the geodesic magnetic field curvature. However, the frequency of one of the slow mode m ± 1 harmonics is reduced by the same pressure-curvature coupling effect. But the frequency of the other slow mode harmonic is relatively unaffected. As a result, the Alfvén-slow (AS) mode continuous spectrum is broken up into two Alfvén-Slow (AS) gaps below the TAE continuum gap. Coupling of the Alfvén m harmonic with the slow mode m ± 2 harmonics also creates additional AS gaps if their frequencies cross each other. The AS gap widths increase with the plasma pressure. The creation of AS gaps allows the existence of several new types of Alfvén-Slow Eigenmodes (ASEs). The radial structure of these new ASEs does not intersect with the continuous spectrum and thus does not suffer from continuum damping. The Beta-induced Alfvén-Acoustic Eigenmode (BAAE) is one type of ASEs with frequency in the lowest AS frequency gap. However, the BAAE usually interacts with the Alfvén-Slow continuum and suffers from continuum damping. The existence of ASEs is ubiquitous for normal and reserve safety factor profiles, broad range of plasma β values, different plasma shapes, and many different toroidal mode numbers. The newly discovered ASEs provide explanation for the experimentally observed Beta-induced Alfvén Eigenmodes destabilized by fast ions with frequencies below the TAE frequency gap. [ABSTRACT FROM AUTHOR]

Energy transfer in ion-temperature-gradient-driven (ITG) turbulence and its role in modeling transport are examined for finite normalized plasma pressure β for a number of test cases and experimental discharges. The analysis shows that like the zero-β case, finite-β ITG turbulence saturates by nonlinear energy transfer to stable modes mediated by a zonal flow. Electromagnetic effects reliably increase stable mode amplitudes but affect heat fluxes only at the ≈5% level. The most important change with increased β is an increase in the correlation time of the triplet interaction of the unstable mode, stable mode, and zonal flow, thus providing a heightened nonlinear energy transfer efficiency, which allows the instability to saturate at lower amplitude. The heat flux is examined in connection with nonlinear electromagnetic stabilization, the phenomenon where the flux falloff with β is more pronounced than the falloff predicted by quasilinear transport models. The inclusion of the triplet correlation time in the quasilinear model captures most of the nonlinearly enhanced stabilization for the configurations studied here. [ABSTRACT FROM AUTHOR]

The temporal evolution of a low pressure argon plasma driven by a combination of radio frequency and kilohertz frequency power sources was characterized by phase resolved optical emission and laser absorption spectroscopy. The compact, low pressure plasma was formed in a hybrid surface/jet geometry and powered by a high voltage power supply operating at 31 kHz and by a lower voltage RF supply operating at 13.56 MHz. An accurate evaluation of the 1s state density dynamics was performed by means of laser absorption measurements of the 1s5 and 1s4 sublevels. The emission spectra were recorded with a submicrosecond temporal resolution and evaluated using an extended Corona model. In comparison with the modeling results, the phase dependent ratio of different Ar(2p) → Ar(1s) emission lines was used to determine temporal profiles of electron temperature and relative electron density. The results were compared with the results from time averaged emission measurements by performing an identical analysis. Strong spatial and temporal variations of plasma parameters were observed when the plasma was driven solely by the 31 kHz source, resulting in nonequilibrium conditions for long living states. Inconsistent observations in the reconstructed temporal development of plasma energy and density were further investigated by illustrating full spatiotemporal reconstruction of plasma parameters. These results permit clear identification of strong spatial gradients through the tube. [ABSTRACT FROM AUTHOR]

The effects of deeply trapped energetic ions (DTEIs) on the stability of tearing modes through stability criterion Δ ′ are studied. In contrast to the stabilizing adiabatic effect of background plasma pressure, the adiabatic contribution of the DTEI destabilizes due to the direct coupling between the bad curvature and asymmetrical pressure of energetic ions. When the orbit width is much larger than the island width, the nonadiabatic effect of the DTEI stabilizes, which counteracts the major destabilizing adiabatic contribution. DTEIs are shown to have a net destabilizing effect on the tearing mode. The main physics of destabilization of Δ ′ comes from the modification of the Mercier index. [ABSTRACT FROM AUTHOR]

Atmospheric pressure helium plasma jets driven by a low-frequency bipolar microsecond-pulsed voltage were characterized. Some characteristic features of plasma jets were discussed with an emphasis on the temporal evolution of discharge current and light emission. The electrical and optical characteristics of the jet exhibited quite a strong dependence on the pulse width and oxygen gas admixture. We investigated the effects of the pulse width and oxygen admixture on the production of reactive species in gas and liquid phases. The concentrations of reactive species were measured at the plasma-liquid surface and inside the plasma-treated liquids using ultraviolet absorption spectroscopy, the chemical probe method, and UV-VIS spectrophotometry. In the range of pulse widths of 1.5–5.5 μs, the 3.5 μs case was observed to exhibit higher values in discharge current and optical emission intensity. The OH densities estimated at the plasma-liquid interface and inside the plasma-treated liquid were observed to vary with the pulse width, and it exhibited a similar trend of change to those of discharge current, optical emission intensity, and the concentrations of the long-living reactive species, H2O2 and NO3−, in the plasma-treated liquids. Around the oxygen admixture of 10 SCCM (=1% vol. %), the discharge current and wavelength-integrated optical intensity were maximal, but the estimated OH density inside the plasma-treated liquid exhibits a slight rise with the increasing oxygen admixture, reaching the highest at 20 SCCM, and then decreases slightly. [ABSTRACT FROM AUTHOR]

Formation of a plasma sheath is studied in collisional and magnetized low pressure plasma. The magnetic field has a tilted configuration. The model provides plasma profiles for a wide range of magnetic field strengths and tilting angles. It is shown that the sheath entry velocity in the magnetized case should be chosen for all three components of ion velocity rather than a single component (which is directed along the sheath). It is found in the collisionless case that the net magnitude of the ion velocity gains the characteristic ion speed (Bohm speed). This, in turn, is found to retain the monotonic variation of the ion density. Ionization is considered with constant ionizing frequency. Comments are made on the sheath thickness and the retention of the Debye sheath at a low angle, for low pressure magnetized plasma. [ABSTRACT FROM AUTHOR]

Rayleigh–Taylor instability (RTI) is a primary hurdle for many different fusion approaches, most of which rely on external pressure to stabilize the plasma by impeding plasma displacement. In this paper, we report a novel method that utilizes a rotating magnetic field (RMF) to drive an azimuthal electron current to reduce the charge separation caused by RTI. The fluctuation measured in the central cell of the mirror device, approximately half a device length away from the RMF, is identified as the m = 1 mode and is suppressed by the RMF in the plug cell. The azimuthal electric fields of the fluctuation are found to decrease to almost zero, and the radial confinement is improved by more than a factor of ten. The separation of the RMF region from the central cell makes this stabilization method unique because the RMF, which can complicate the local magnetic field lines, has little influence on the magnetic field configuration in the central cell. This study may shed light on the use of resonant magnetic perturbations in tokamaks as well as on stabilization methods for many other fusion experiments. [ABSTRACT FROM AUTHOR]

This paper describes first results of a novel compression method for a plasma by acceleration into a glass cone. The plasma is generated by a pulsed-power-driven coaxial plasma accelerator, prefilled with 2% hydrogen in a helium mixture. The pulse forming network of the experiment has a total capacitance of 27 μF at a maximum voltage of 10 kV. The low inductance of 130 nH leads to an average frequency of about 86 kHz and high current rise rates in the 10 11 A s range. Due to the high current of up to 150 kA, the Lorentz force accelerates the plasma depending on pressure and applied voltage to velocities in the 10 4 m s order. After ejection, the plasma moves into several cone geometries, placed at the end of the electrodes. The conducted studies focus on the Stark-broadening of spectral lines to determine the electron density changes due to the compression. Because of the wide broadening at high electron densities, the well-known Hβ line broadening method is not eligible for cone compressed plasmas. Alternatively, the broadening of the Hα line and a copper line has been used. The achieved electron densities could be increased approximately two orders of magnitude from 1015 cm−3 without the cone to 1017–1018 cm−3 with cone compression. [ABSTRACT FROM AUTHOR]

We have investigated electron beam transport at an elevated forevacuum pressure of tens of Pascals of helium. The continuous electron beam (6–14 keV, 300 mA) is generated by a forevacuum-pressure plasma-cathode electron source utilizing a hollow-cathode discharge. A beam-plasma discharge is generated in the beam transport zone, which is characterized by increased plasma density in the region of the most intense beam-plasma interaction. We find that the location and distribution of the beam-plasma discharge depend on the electron beam energy and current density. Under certain conditions, we observe that the beam plasma is stratified, with a periodic variation of plasma density and luminosity along the direction of electron beam propagation. [ABSTRACT FROM AUTHOR]

We describe our experimental research on the stable generation of large-radius pulsed high-current electron beams using a forevacuum-pressure plasma-cathode electron beam source with cathodic arc plasma generation. We find that increased gas pressure and the use of gas with a higher ionization cross-section considerably decrease the stable emission current due to loss of electric strength of the acceleration gap (breakdown). Increased pulse duration results in a decrease in the emission current, but the influence of the gas species and its pressure remains for all pulse durations. There is an extremum in the dependence of emission current on the acceleration gap length and the optimum gap length depends on the gas pressure and the kind of gas. The experiment results can be explained by the effect of back-streaming ion flow, which charges dielectric inclusions on the emission electrode (anode grid) and thereby amplifies the electric field. [ABSTRACT FROM AUTHOR]