Session PO06: Fundamental: Basic Plasma and Waves

Particle, Charge, and Energy Rearrangement in Rotating Magnetized Plasma

Rotating plasmas are useful for a number of applications, including nuclear fusion and plasma mass filters. Moreover, rotating plasmas -- especially rotating plasmas with several ion species -- often behave quite differently from plasmas that do not rotate. In this thesis talk, we discuss a number of different ways in which rotation can be used to facilitate the rearrangement of particles, charge, and energy. These include cross-field impurity and ash transport; cross-field conductivity; ways of controlling the dissipation pathways to drive temperature differences between species; and fundamental limitations on the efficiency with which energy can be transferred through different classes of reversible and irreversible processes. In uncovering new phenomena, insights into the behavior of neutral fluids can often inform on the behavior of more complicated rotating plasma systems.   

Statistical analysis of fluctuations and transport in multiple entangled plasma pressure filaments

Steep thermal gradients in a magnetized plasma can induce a variety of spontaneous low frequency excitations such as drift-Alfven waves and vortices. We present results from basic experiments on heat transport in magnetized plasmas with multiple heat sources in close proximity. The experiments were carried out at the upgraded Large Plasma Device (LAPD) operated by the Basic Plasma Science Facility at the University of California, Los Angeles. The setup consists of three biased probe-mounted CeB6 crystal cathodes that inject low energy electrons along a strong magnetic field into a pre-existing cold afterglow plasma forming three electron temperature filaments. A triangular spatial pattern is chosen for the thermal sources and multiple axial and transverse probe measurements allow for determination of the cross-field mode patterns and axial filament length. When the three sources are placed within a few collisionless electron skin depths, a non-azimuthally symmetric wave pattern emerges due to the overlap of drift-Alfven modes forming around each filament. This leads to enhanced cross-field transport from nonlinear convective (E×B) chaotic mixing and rapid density and temperature profile collapse in the inner triangular region of the filaments. Steepened thermal gradients form in the outer triangular region, which spontaneously generates quasi-symmetric, higher azimuthal mode number drift-Alfven fluctuations. A steady-current model with emissive sheath boundary predicts the plasma potential and shear flow contribution from the sources. A statistical study of the fluctuations reveals amplitude distributions that are skewed which is signature of intermittency in the transport dynamics.

Effects of Magnetic Field Gradients on Inertial Alfvén Waves

We have performed laboratory experiments on the Large Plasma Device (LAPD) to measure the transmission and reflection of Inertial Alfvén waves (electron thermal speed << Alfvén speed) through a parallel magnetic field gradient. Alfvén waves are important in natural systems, such as the solar corona and the terrestrial magnetosphere, as well as in laboratory experiments. The reflection and transmission of Alfvén waves through inhomogeneities in the background plasma are important for understanding wave propagation, turbulence, and heating of thermonuclear plasmas. In a recent laboratory experiment, it was observed that Kinetic Alfvén waves (thermal speed >>Alfvén speed) lose energy after passing through a magnetic field gradient. Surprisingly, though, no reflection from the magnetic field gradient was observed. One possibility is that wave energy is dissipated rapidly in the gradient region. To better understand the effects of gradient, we are now carrying out an analysis of Alfvén wave propagation under similar conditions, but in the inertial regime. The propagation of these waves has been studied for different magnetic field gradients and for different wavelengths. We will present our measurements of the reflection and transmittance of Inertial Alfvén waves through a magnetic field gradient. This work is supported, in part, by grants from the NSF Solar-Terrestrial Program under grant AGS-1834822 and the U.S. Department of Energy, Office of Science, Office of Fusion Energy Science under Award Numbers DE-SC0016602 and DE-SC0021261. The experiments were performed at the Basic Plasma Science Facility (BaPSF), which is supported by the DOE and NSF, with major facility instrumentation developed via an NSF award AGS–9724366.   

Excitation of an Ion-Acoustic Soliton in quiescent argon plasma confined by multipole line cusp magnetic field

Multi Cusp plasma Device (MPD) with six electromagnets has the feature of producing different multi-pole cusp magnetic field configurations.  In this system with filament produced argon, the electrostatic fluctuations are found to be less than 1% (δn/n<1%), a characteristic of quiescent plasma. In this plasma condition, Ion acoustic waves were excited and their propagation was studied in detail by varying the plasma conditions along with the multi-pole cusp field values.  After this, large amplitude potential perturbations were given to the grid immersed in the plasma in the field free region.  These large potentials bring in nonlinear characteristics with coherence which helps in identifying solitary waves. These solitary waves are then characterized experimentally and compared with 1-D KdV Ion Acoustic soliton. The results of these studies will be discussed in detail along with possible explanation.   

Ion cyclotron emission and fast wave destabilization by a proton beam on the Large Plasma Device

Resonant interaction between energetic-ions and plasma waves is a fundamental topic of importance in space and laboratory plasma physics (e.g., interaction of protons and alpha particle with Alfven-ion-cyclotron and fast-magnetosonic waves in the solar wind, aurora and magnetic fusion plasmas). We report results on the spontaneous generation of fast Alfvén waves and high-harmonic electrostatic waves in the lower-hybrid range of frequencies by an intense proton beam. A proton beam (15 keV, 10 A) has been injected into a large magnetized plasma (n ≈ 10¹⁰ – 10¹³ cm^-3, T_e ≈ 5.0-15.0 eV, B = 0.6–1.8 kG, He⁺ and H⁺ ions, 19 m long, 0.6 m diameter) for performing fast-ion studies on the Large Plasma Device (LAPD). The beam forms a helical orbit (pitch-angle ≈ 7°–55°) and propagates with an Alfvénic speed (beam-speed/Alfvén-speed = 0.2–3.0). The role of resonant processes in destabilizing these waves were examined by recording the mode-structure of these waves and relevant parameters of plasma using a variety of diagnostic tools (retarding-field energy analyzer, three-axis magnetic-loop, Dipole, and Langmuir probes). This presentation will particularly focus on the excitation of waves with right-handed polarization that cover a broad spectrum above the beam gyro-frequency.   

Magnetorotational instability breaks rotational symmetry in the laboratory

The standard magnetorotational instability (SMRI) has been regarded as the sole viable instability responsible for the turbulence required to explain the fast accretion observed across the Universe. Nonetheless, SMRI remains unconfirmed even for its existence long after its proposal, despite its widespread applications in modeling including recent black hole imaging. Its direct detection has been hindered in observations due to its microscopic nature at astronomical distances, and in the laboratory due to stringent requirements and interferences from other processes. Here we report the first direct evidence showing that SMRI indeed exists in a novel laboratory setup where a uniform magnetic field is imposed along the axis of a differentially rotating flow of liquid metal confined radially between concentric cylinders and axially by copper endrings. As predicted the observed SMRI exists only at sufficiently large rotation rates and moderate field strengths, but surprisingly with its symmetry broken about the rotation axis. The nonaxisymmetric nature of SMRI is important in generating large-scale magnetic fields, as detected recently. Our results show that the axisymmetric presumption is oversimplified in past studies on SMRI, which calls for future investigations.   

Preliminary Results from the NRL Plasma Surface Wave Test Setup

A radiofrequency plasma source has been constructed to investigate the propagation of surface waves and currents along a cylindrical column of plasma. The source consists of a argon filled simple glass tube with a short solenoidal antenna wrapped around the exterior. The antenna is connected to a variable power and frequency RF power supply for ionizing the plasma, while secondary broadband antennas are used to excite the surface waves. Measurements of phase changes over broad frequency ranges and spatial distances are used to numerically construct dispersion relations for surface wave modes, which are compared against electron density values inferred from Langmuir probe measurements and microwave interferometry. Preliminary results on transmission properties for different surface modes will also be presented.   

Finite size effects on the dynamics of long wavelength modes in inhomogeneous one dimensional Vlasov plasmas

Collisionless damping of electron plasma waves is a well-known example of wave particle interaction phenomenon also termed as Landau damping [1] usually addressed for uniform plasmas. In the past, in an attempt linear Landau damping in 1D, periodic, inhomogeneous plasma was addressed using fluid model at large perturbation scales [2]. It was demonstrated that the presence of finite amplitude ion density background inhomogeneity efficiently manages to couple the electron plasma wave of long wavelength perturbations leading to damping of high phase velocity waves with v_Φ=ω/k, even though the resonance conditions are not satisfied [2]. Also, it was illustrated that in this cold plasma limit, the amplitude of the coupled modes will evolve in time t according to the Bessel function J_n(At/2) depending on the strength of inhomogeneity, where n is related to strength of inhomogeneity A [2]. 

In light of the above, we have studied linear Landau damping in a periodic inhomogeneous collisionless 1D plasma using a high resolution Vlasov-Poisson solver i.e VPPM-OMP 1.0 [3] without any approximations. In the work presented here, we ask ourselves as to what would happen to J_n(At/2) scaling, if the long wavelength limit is relaxed? Interestingly, for the parameters considered, it is found that for a given inhomogeneity strength A, the perturbation amplitude does not anymore follow J_n(At/2), for systems with finite size (L_x). We numerically demonstrate that at large L_x, our findings correctly asymptote to that of Ref [2]. In the following, we will address the various regimes of perturbation scales in an inhomogeneous plasma with kinetic electrons and immobile ions [4]. The details of this work will be presented.

References

[1] L. Landau, J. Phys. USSR 10, 25 (1946).

[2] P. K. Kaw, A. T. Lin, and J. M. Dawson, Phys. Fluids 16, 1967 (1973).

[3] Sanjeev Kumar Pandey, Rajaraman Ganesh, AIP Advances 11, 025229 (2021).

[4] Sanjeev Kumar Pandey, Rajaraman Ganesh, Under Preparation (2021).   

Effect of controlled ion population on the evolution of a quiescent low aspect ratio toroidal pure electron cloud : A 3D PIC approach

A quiescent quasi-steady inhomogeneous, equilibrium state of axisymmetric toroidal electron cloud in a tight aspect ratio device, has been found¹. This numerical study was performed using a 3D3V PIC code PEC3PIC² where a maximum entropy mean field solution was used as “seed" solution to the solver. The resultant quasisteady equilibrium of the electron cloud satisfies full equations of motion with negligible presence of m = 1 Diocotron motion resulting in a quiescent toroidal electron plasma with superior confinement properties.

In experimental studies of toroidal electron clouds in large³ and small⁴ aspect ratio partial devices, small amount of backgound ions lead to ion resonance instability. In the present study, the effect of ions on the above-said quiescent quasi-steady equilibrium with nearly zero center of charge motion has been investigated. A minuscule or small ion fraction introduced in the device at a particular time, is found to result in systematic destabilization of the electron cloud. The resulting electron cloud exhibits systematically linear, followed by quasi-linear and nonlinear center of charge oscillation or toroidal Diocotron mode, with increasing ion fraction value upto a certain limit. Quantitative analysis of the wall probe current reveals interesting features⁵ of this phenomena. Initial observation shows that the electron cloud destabilization process depends on the mass of ion species as well as the strength of the toroidal magnetic field. These and other details will be presented in this paper.

1. S. Khamaru, R. Ganesh, and M. Sengupta, Phys. Plasmas 28, 042101(2021). 2. S. Khamaru, M. Sengupta, and R. Ganesh, Physics of Plasmas 26, 112106 (2019). 3. M. R. Stoneking et al, Phys. Plasmas 9, 766 (2002). 4. L. Lachvani et al, J. Phys.: Conf. Ser. 390, 012047 (2012). 5. S. Khamaru, R. Ganesh and M. Sengupta, "3D3V PIC Simulation of ions-driven destabilization of a toroidal electron cloud ", Manuscript Under Preparation.   

Spatiotemporal Evolution Of Relativistic Upper-Hybrid Waves In A Cold Magnetized Plasma

The space-time evolution of relativistic upper-hybrid oscillations and waves in cold homogeneous magnetized plasma has been studied analytically and numerically using Dawson Sheet Model [1]. It is found that in case of stationary oscillations, due to relativistic variation of electron mass, the oscillation frequency becomes function of space, which results in phase mixing[1, 2, 4], whereas the traveling wave solution [3] which is like Akhiezer-Polovin mode in unmagnetized cold plasma[5] exhibit phase mixing only when longitudinally perturbed. The sensitivity of the relativistic upper hybrid mode to initial conditions has been analytically studied and an expression for breaking time has been derived. Numerical 1-1/2 D code, based on Dawson Sheet Model has been developed to simulate the evolution of relativistic upper-hybrid mode and the breaking time has been verified for an entire range of input parameters. These results are of relevance to laboratory and astrophysical plasmas where the magnetic field plays an important role.

[1] J. M. Dawson, Phys. Rev. 113, 383 (1959)

[2] C. Maity et.al. PRL 110, 215002 (2013)

[3] M. Karmakar et.al. POP 23, 064503 (2016)

[4] S. Sengupta et.al. Phys. Rev. E 79, 026404 (2009)

[5] A. I. Akhiezer and R. V. Polovin, Sov. Phys. JETP., 3, 696 (1956).   

Fully kinetic simulations of collisional plasma shocks

We discuss fully kinetic simulations of steady-state high Mach number planar shocks in a Hydrogen plasma with kinetic electrons and ions, and compare to hybrid simulations with kinetic ions coupled to a fluid electron model [1]. The simulations are independently performed by two different fully kinetic codes, namely the Vlasov-Fokker-Planck code iFP and the particle-in-cell code VPIC. We find that, while kinetic electrons do not change the shock structure observed in the hybrid simulation at a fundamental level, the preheat layer experiences an appreciable rearrangement. This modification is associated with kinetic electron effects, namely nonlocal electron heat transport. We find that the kinetic electron heat flux exhibits significant nonlocality, and agrees well with the Luciani-Mora-Virmont nonlocal electron heat flux closure [2]. Finally, the fully kinetic simulations show a significant collapse in the ‘precursor’ electric-field shock at the edge of the preheat layer, which contrasts with the classical hydrodynamic picture [3].

References:

[1]  S. E. Anderson, et. al, “Fully kinetic simulations of strong steady-state collisional planar plasma shocks”, arXiv:2106.13108 (2021), preprint submitted to Phys. Rev. Lett.

[2]  J. F. Luciani et. al, “Nonlocal heat transport due to steep temperature gradients,” Phys. Rev. Lett. 51, 1664 (1983)

[3]. M. Y. Jaffrin and R. F. Probstein, “Structure of a plasma shock wave,” Phys. Fluids 7, 1658 (1964)   

Electron Weibel instability-generated magnetic fields in laser-produced plasmas

The Biermann battery is understood to be the mechanism for generating seed magnetic fields in interstellar plasma, however it is hard to explain how these fields could have been amplified to observed levels [RM Kulsrud & EG Zweibel RPP 71 (2008)]. The Weibel instability is another candidate for field generation and amplification. Predicted by PIC simulations to exist in plasmas with sufficiently strong thermal anisotropy, the Weibel instability can generate magnetic fields which exceed the amplitude of the Biermann fields [Schoeffler et al. PRL 112 (2014)]. Relevant plasmas can be produced in the laboratory with inertial confinement fusion lasers: these ~ns-duration, ~kJ-energy pulses set up strong density and temperature gradients which support the Biermann battery (as has been established by prior work) and a thermal anisotropy which supports the electron Weibel instability. This work reports on data from the OMEGA laser spanning a range of densities and temperatures that is used to to explore the dynamics of the observed magnetic field generating filamentary instability, whose properties are consistent with PIC-predicted properties of the electron Weibel instability.   

Nonlinear frequency shift, wave breaking and saturation via sideband instability of electron plasma waves in the adiabatic approximation.

Electrons trapped in the wave troughs often lead to plasma instabilities, either self-consistent such as the beam plasma instability or externally driven such as stimulated Raman scattering. These instabilities generally arise at wave amplitudes large enough so that common linearization techniques do not give quantitative results anymore. It is what we call the nonlinear regime.

Here we will show a method to self consistently compute the electron distribution function in the nonlinear regime, when the nonlinearity is due to electron trapping in the wave troughs. This method applies whatever the variations of the wave amplitude, provided that these variations are slow enough and is valid even when the plasma is inhomogeneous and non stationary. This requires a self-consistent description of the electron plasma wave scalar and vector potentials as well as of the nonlinear frequency. It accounts for various nonlinear effects such as the change in phase velocity making the wave frame non-inertial, the transition probabilities from one region of phase space to the other when an orbit crosses the separatrix, the possible change in direction of the wavenumber and others. The relative importance of each of these effects will be discussed in detail and a discussion of the wave breaking because of the impossibility to solve the dispersion relation above certain amplitude presented.

A validation against numerical simulations by test particles in order to check self-consistency and comparison with previous results will be provided.

The obtained distribution function will then be propagated in time by a Cheng and Knorr Vlasov-Poisson solver with cubic spline interpolation in order to observe the growth of sidebands. A theoretical method for computing the growth rate of these sidebands will be given and compared with previously obtained results such as those of Kruer or Dodin.