In linear induction accelerators, high-brightness electron beams are accelerated by voltages applied at gaps at discrete locations along the accelerator. At high currents, the beam itself induces wakefields in the accelerating gaps which can then feedback and distort the beam. The coupling of the gap-cavity modes and the beam can be characterized by frequency-dependent quantities known as the parallel and perpendicular gap impedances. Assessing the effects of instabilities resulting from these interactions requires accurate knowledge of these quantities. In this paper, we describe how a 3D finite-difference time-domain particle-in-cell code can be used to calculate both parallel and perpendicular gap impedances. We also demonstrate good agreement between full particle-in-cell simulations and results from a beam transport code making use of gap impedances to model beam centroid deflections. In practice, ferrite materials are often used in accelerating gaps to dampen cavity modes and reduce the severity of the resulting instabilities. We describe an implicit recursive convolution algorithm used to model the linear response of dispersive ferrite materials.

The first fully kinetic particle-in-cell (PIC) simulations of sheared flow stabilized Z-pinch plasmas show the suppression of the sausage instability by shear, ∂rvz ≠ 0, with flow Mach numbers ≲1, consistent with experimental observations. Experimental investigations of sheared-flow stabilized Z-pinches demonstrated stability for 10 s of microseconds, over 1000 Alfvén radial transit times, in quasi steady-state plasmas that are an intermediate between conventional inertial and magnetic confinement systems. The observed stability coincides with the presence of radial shear in axial flow profiles with peak speeds less than Mach 1, and experiments are underway to validate scaling this design to fusion conditions. The experimentally observed stability agrees with models of m = 1 kink mode suppression by sheared flows, but existing models of the m = 0 sausage mode underestimate the efficacy of sheared flow stabilization. These models rely on fluid approximations and find that stabilization requires flows ranging from Mach 1.7 to 4.3, and in some cases, stabilization is not reproduced in the models. This is faster than the measured flows in long-lived plasmas and would necessitate substantial energy convection out of the Z-pinch and the need to drive and sustain supersonic flows in future devices. The MHD models typically used in the literature are invalid in the high-temperature, high-current environments desirable for many Z-pinch applications, and they ignore large Larmor radius effects and viscous dissipation which are known to impact Z-pinch stability. PIC simulations can capture all these effects as well as kinetic instabilities that could influence the performance of high-temperature sheared flow stabilized Z-pinch plasmas. The PIC simulations presented here show the suppression and damping of m = 0 modes by sheared flows ∂rvz = 0.75vA/r0 with flow Mach numbers ≲1. Equivalent stability occurs under plasma conditions ranging from the limits of present-day experimental capabilities to the projected conditions of a sheared flow stabilized Z-pinch reactor.

The walls of the hohlraum used in experiments at the national ignition facility are heated by laser beams with intensities ∼1015 W/cm2, a wavelength of ∼1/3 μm, and pulse lengths on the order of a ns, with collisional absorption believed to be the primary heating mechanism. X-rays generated by the hot ablated plasma at the gold walls are then used to implode a target in the hohlraum interior. In addition to the collisional absorption of laser energy at the walls, non-linear laser-plasma interactions (LPI), such as stimulated Raman scattering and two plasmon decay, are believed to generate a population of supra-thermal electrons which, if present in the hohlraum, can have a deleterious effect on target implosion. We describe results of hohlraum modeling using a hybrid particle-in-cell code. To enable this work, new particle-based algorithms for a multiple-ion magneto-hydrodynamic (MHD) treatment, and a particle-based ray-tracing model were developed. The use of such hybrid methods relaxes the requirement to resolve the laser wavelength, and allows for relatively large-scale hohlraum simulations with a reasonable number of cells. But the non-linear effects which are believed to be the cause of hot electron generation can only be captured by fully kinetic simulations with good resolution of the laser wavelength. For this reason, we employ a two-tiered approach to hohlraum modeling. Large-scale simulations of the collisional absorption process can be conducted using the fast quasi-neutral MHD algorithm with fluid particle species. From these simulations, we can observe the time evolution of the hohlraum walls and characterize the density and temperature profiles. From these results, we can transition to smaller-scale highly resolved simulations using traditional kinetic particle-in-cell methods, from which we can fully model all of the non-linear laser-plasma interactions, as well as assess the details of the electron distribution function. We find that vacuum hohlraums should be stable to both two plasmon decay and stimulated Raman scattering instabilities for intensities ≤1015 W/cm2. In gas-filled hohlraums, shocks may be induced in the blowoff gold plasma, which leads to more complex density and temperatures profiles. The resulting effect on LPI stability depends strongly on the details of the profile, and it is possible for the gas-filled hohlraum to become unstable to two plasmon decay at 1015 W/cm2 if the quarter-critical surface reaches temperatures exceeding 1 keV.

Low-frequency electrostatic drift waves are studied in an inhomogeneous dust magnetoplasma containing dust with components of opposite polarity. The drift waves are driven by the magnetic-field-aligned (parallel) sheared flows in the presence of electrons and ions. Due to sheared flow in the linear regime, the electrostatic dust drift waves become unstable. The conditions of mode instability, with the effects of dust streaming and opposite polarity, are studied. These are excited modes which gain large amplitudes and exhibit interactions among themselves. The interaction is governed by the Hasegawa-Mima (HM) nonlinear equation with vector nonlinearity. The stationary solutions of the HM equation in the form of a vortex chain and a dipolar vortex, including effects of dust polarity and electron (ion) temperatures, are studied. The relevance of the present work to space and laboratory four component dusty plasmas is noted.

Numerical simulations of a vacuum post-hole convolute driven by magnetically insulated vacuum transmission lines (MITLs) are used to study current losses due to charged particle emission from the MITL-convolute-system electrodes. This work builds on the results of a previous study [E.A. Madrid et al. Phys. Rev. ST Accel. Beams 16, 120401 (2013)] and adds realistic power pulses, Ohmic heating of anode surfaces, and a model for the formation and evolution of cathode plasmas. The simulations suggest that modestly larger anode-cathode gaps in the MITLs upstream of the convolute result in significantly less current loss. In addition, longer pulse durations lead to somewhat greater current loss due to cathode-plasma expansion. These results can be applied to the design of future MITL-convolute systems for high-current pulsed-power systems.

A novel algorithm for the simulation of cathode plasmas in particle-in-cell codes is described and applied to investigate cathode plasma evolution in magnetically insulated transmission lines (MITLs). The MITL electron sheath is modeled by a fully kinetic electron species. Electron and ion macroparticles, both modeled as fluid species, form a dense plasma which is initially localized at the cathode surface. Energetic plasma electron particles can be converted to kinetic electrons to resupply the electron flux at the plasma edge (the “effective” cathode). Using this model, we compare results for the time evolution of the cathode plasma and MITL electron flow with a simplified (isothermal) diffusion model. Simulations in 1D show a slow diffusive expansion of the plasma from the cathode surface. But in multiple dimensions, the plasma can expand much more rapidly due to anomalous diffusion caused by an instability due to the strong coupling of a transverse magnetic mode in the electron sheath with the expanding resistive plasma layer.

We have developed conceptual designs of two petawatt-class pulsed-power accelerators: Z 300 and Z 800. The designs are based on an accelerator architecture that is founded on two concepts: single-stage electrical-pulse compression and impedance matching [Phys. Rev. ST Accel. Beams 10, 030401 (2007)]. The prime power source of each machine consists of 90 linear-transformer-driver (LTD) modules. Each module comprises LTD cavities connected electrically in series, each of which is powered by 5-GW LTD bricks connected electrically in parallel. (A brick comprises a single switch and two capacitors in series.) Six water-insulated radial-transmission-line impedance transformers transport the power generated by the modules to a six-level vacuum-insulator stack. The stack serves as the accelerator's water-vacuum interface. The stack is connected to six conical outer magnetically insulated vacuum transmission lines (MITLs), which are joined in parallel at a 10-cm radius by a triple-post-hole vacuum convolute. The convolute sums the electrical currents at the outputs of the six outer MITLs, and delivers the combined current to a single short inner MITL. The inner MITL transmits the combined current to the accelerator's physics-package load. Z 300 is 35 m in diameter and stores 48 MJ of electrical energy in its LTD capacitors. The accelerator generates 320 TW of electrical power at the output of the LTD system, and delivers 48 MA in 154 ns to a magnetized-liner inertial-fusion (MagLIF) target [Phys. Plasmas 17, 056303 (2010)]. The peak electrical power at the MagLIF target is 870 TW, which is the highest power throughout the accelerator. Power amplification is accomplished by the centrally located vacuum section, which serves as an intermediate inductive-energy-storage device. The principal goal of Z 300 is to achieve thermonuclear ignition; i.e., a fusion yield that exceeds the energy transmitted by the accelerator to the liner. 2D magnetohydrodynamic (MHD) simulations suggest Z 300 will deliver 4.3 MJ to the liner, and achieve a yield on the order of 18 MJ. Z 800 is 52 m in diameter and stores 130 MJ. This accelerator generates 890 TW at the output of its LTD system, and delivers 65 MA in 113 ns to a MagLIF target. The peak electrical power at the MagLIF liner is 2500 TW. The principal goal of Z 800 is to achieve high-yield thermonuclear fusion; i.e., a yield that exceeds the energy initially stored by the accelerator's capacitors. 2D MHD simulations suggest Z 800 will deliver 8.0 MJ to the liner, and achieve a yield on the order of 440 MJ. Z 300 and Z 800, or variations of these accelerators, will allow the international high-energy-density-physics community to conduct advanced inertial-confinement-fusion, radiation-physics, material-physics, and laboratory-astrophysics experiments over heretofore-inaccessible parameter regimes.

The Los Alamos suite of relativistic atomic physics codes is a robust, mature platform that has been used to model highly charged ions in a variety of ways. The suite includes capabilities for calculating data related to fundamental atomic structure, as well as the processes of photoexcitation, electron-impact excitation and ionization, photoionization and autoionization within a consistent framework. These data can be of a basic nature, such as cross sections and collision strengths, which are useful in making predictions that can be compared with experiments to test fundamental theories of highly charged ions, such as quantum electrodynamics. The suite can also be used to generate detailed models of energy levels and rate coefficients, and to apply them in the collisional-radiative modeling of plasmas over a wide range of conditions. Such modeling is useful, for example, in the interpretation of spectra generated by a variety of plasmas. In this work, we provide a brief overview of the capabilities within the Los Alamos relativistic suite along with some examples of its application to the modeling of highly charged ions.

Electrostatic solitons in an unmagnetized pair-ion plasma comprising adiabatic fluid positive and negative ions and non-isothermal electrons are investigated using both arbitrary and small amplitude techniques. An energy integral equation involving the Sagdeev potential is derived, and the basic properties of large amplitude solitary structures are investigated. Various features of solitons differ in different existence domains. The effects of ion adiabaticity, particle concentration, and resonant electrons on the profiles of Sagdeev potential and corresponding solitary waves are investigated. The generalized Korteweg-de Vries equation with mixed-nonlinearity is derived by expanding the Sagdeev potential. Asymptotic solutions for different orders of nonlinearity are discussed for solitary waves. The present work is applicable to understanding the wave phenomena and associated nonlinear electrostatic perturbations in pair/bi-ion plasmas which may occur in space and laboratory plasmas.

The available experimental cross-section data on electron-impact dissociative excitation and ionization processes of methane family hydrocarbon molecular ions have been evaluated and represented by three-parameter analytic fit functions appropriate for use in plasma modeling codes. The accuracy of the fit functions is well within the uncertainties of experimental data.

Electron power flow in two radial magnetically insulated transmission lines (MITLs) coupled to a vacuum post-hole convolute is studied using 3D particle-in-cell simulations. At sufficiently high voltages, electron emission upstream of the convolute results in a portion of the current carried by the transmission lines to flow in an electron sheath along the cathode surfaces. The simulations show that at 50-200 TW, the transition from the individual MITLs to the convolute results in a portion of the MITL sheath current being lost to both anode and cathode structures. The losses are identified as a function of radius and correlated with Poynting vector stream lines which can be followed by individual electrons. For a fixed MITL-convolute geometry, the difference between the current in the system upstream of the convolute and current delivered to the load (defined as the loss current) increases with both operating voltage and load impedance. The effects of space-charge-limited (SCL) ion emission from anode surfaces are considered for several specific cases in both steady-state and time-dependent operating modes. The impact of cathode plasma formation on the loss current is also considered for the time-dependent simulation results. Collectively, these simulation results are being used to help formulate design criteria for high-power convolute-MITL systems.

Quasiequilibrium power flow in two radial magnetically insulated transmission lines (MITLs) coupled to a vacuum post-hole convolute is studied at 50 TW-200 TW using three-dimensional particle-in-cell simulations. The key physical dimensions in the model are based on the ZR accelerator [D. H. McDaniel, et al., Proceedings of 5th International Conference on Dense Z-Pinches, edited by J. Davis (AIP, New York, 2002), p. 23]. The voltages assumed for this study result in electron emission from all cathode surfaces. Electrons emitted from the MITL cathodes upstream of the convolute cause a portion of the MITL current to be carried by an electron sheath. Under the simplifying assumptions made by the simulations, it is found that the transition from the two MITLs to the convolute results in the loss of most of the sheath current to anode structures. The loss is quantified as a function of radius and correlated with Poynting vector stream lines which would be followed by individual electrons. For a fixed MITL-convolute geometry, the current loss, defined to be the difference between the total (i.e. anode) current in the system upstream of the convolute and the current delivered to the load, increases with both operating voltage and load impedance. It is also found that in the absence of ion emission, the convolute is efficient when the load impedance is much less than the impedance of the two parallel MITLs. The effects of space-charge-limited (SCL) ion emission from anode surfaces are considered for several specific cases. Ion emission from anode surfaces in the convolute is found to increase the current loss by a factor of 2-3. When SCL ion emission is allowed from anode surfaces in the MITLs upstream of the convolute, substantially higher current losses are obtained. Note that the results reported here are valid given the spatial resolution used for the simulations.

Advanced z-pinch accelerators require precise timing of multiple mega-ampere drivers to deliver terawatt power. The triggering of these drivers is now largely initiated by laser ionization of gas switches. In this paper, we discuss detailed fully kinetic simulation of the Z laser-triggered gas switch involving detailed finite-difference time-domain particle-in-cell Monte Carlo modeling of the trigger section of the switch. Other components of the accelerator from the Marx bank through the pulse-forming line are described as circuit elements. The simulations presented here build on a recently developed model of electro-negative gas breakdown and streamer propagation that included photons produced from de-excited neutrals. New effects include multi-photon ionization of the gas in a prescribed laser field. The simulations show the sensitivity of triggering to laser parameters including focal plane within the anode-cathode gap of the trigger section of the switch, intensity at focus, and laser pulse length. Detailed electromagnetic simulations of the trigger section with circuit modeling of the upstream and downstream components are largely in agreement with Z data and demonstrate a new capability.

Getting reliable atomic and molecular data for applications is a major challenge. In some instances, data can be obtained using analytical functions as proposed earlier. There are also some atomic and molecular codes available through the Internet, which can generate data online. However, the major trend currently is to download the information directly through the Internet. Web-based atomic and molecular databases have the advantage, when available, to readily provide reliable original data and possibly evaluated and/or recommended data. In this chapter, the current state of atomic and molecular and online calculation tools in early 2011 is described, with special focus on databases with a convenient user web-interface, as well as information distributed through data centers.

A frequency-dependent impedance model for laminated ferromagnetic cores is presented and analyzed. The model assumes a multiple-winding ferromagnetic induction core composed of multiple thin layers with linear material response. This model builds on the analysis presented by Rose et al. [Phys. Rev. ST Accel. Beams 13, 090401 (2010)], that determined an equivalent time-dependent resistance that was used to successfully model the loss currents in a linear transformer device cavity containing ferromagnetic cores. The new core impedance model is more general and has been implemented as a surface-impedance boundary condition [K. S. Oh and J. E. Schutt-Aine, IEEE Trans. Antennas Propag. 43, 660 (1995)] which is suitable for use in multidimensional finite-difference time-domain codes.