Diamagnetic loops (DML) provide a non-invasive method for measurement of beam size in electron beam accelerators that use solenoidal magnetic transport. The loop fundamentally measures the magnetic flux excluded by a diamagnetic object. A comprehensive theory relates the rms beam radius to the excluded flux measured by the DML. We have built and calibrated a DML apparatus. Recently, this DML has been used to measure the size of the electron beam near the final focus of the DARHT-I flash-radiography accelerator. Results agree with beam transport code predictions. In this article, we review and summarize the construction, calibration, and electron-beam testing of this DML.
Recent works have shown that strongly magnetized plasmas characterized by having a gyrofrequency greater than the plasma frequency exhibit novel transport properties. One example is that the friction force on a test charge shifts, obtaining components perpendicular to its velocity in addition to the typical stopping power component antiparallel to its velocity. Here, we apply a recent generalization of the Boltzmann equation for strongly magnetized plasmas to calculate the ion-electron temperature relaxation rate. Strong magnetization is generally found to increase the temperature relaxation rate perpendicular to the magnetic field and to cause the temperatures parallel and perpendicular to the magnetic field to not relax at equal rates. This, in turn, causes a temperature anisotropy to develop during the equilibration. Strong magnetization also breaks the symmetry of independence of the sign of the charges of the interacting particles on the collision rate, commonly known as the "Barkas effect." It is found that the combination of oppositely charged interaction and strong magnetization causes the ion-electron parallel temperature relaxation rate to be significantly suppressed, scaling inversely proportional to the magnetic field strength.
In moderately coupled plasmas, a significant fraction of the internal energy resides in electric fields. As these plasmas are heated or compressed, the shifting partition of energy between particles and fields leads to surprising effects, particularly when ions and electrons have different temperatures. In this work, quasi-equations of state (quasi-EOS) are derived for two-temperature moderately coupled plasma in a thermodynamic framework and expressed in a simple form. These quasi-EOS readily yield expressions for correlation heating, in which heating of the electrons causes a rapid increase in ion temperature even in the absence of collisional energy exchange between species. It is also shown that, remarkably, compression of moderately coupled plasma drives a temperature difference between electrons and ions, even when the species start at equal temperatures. These additional channels for ion heating may be relevant in designing ignition schemes for inertial confinement fusion.
The formation of striations in a low temperature plasma column has been known for more than a century and a considerable number of papers have been devoted to experimental and theoretical studies of these striations. Due to the large variety of regimes and to the complexity of the physics involved, our understanding of these instabilities is still limited. In this presentation we focus on the formation of striations under low current and low pressure conditions, i.e. when stepwise ionization or Coulomb collisions are negligible.
Nonlinear Alfvén waves are subject to parametric instabilities. The parametric decay instability (PDI) is of special interest as it produces backward propagating Alfvén waves which may trigger turbulence development and generates compressible waves which may cause plasma heating. Despite the significant implications, the PDI process has not been directly verified. Recent experiments on the Large Plasma Device (LAPD) showed a modulational-like instability [1] . However, the PDI was missing despite its significant growth rates under the conditions investigated. To resolve the puzzle, we are developing hybrid simulation capabilities to investigate the LAPD-type experiments with realistic geometry and physics conditions. As a first step, we relax the usual periodic boundary conditions and study the PDI of an Alfvén wave packet in a large system under an absorption boundary [2] . We further consider the LAPD-like wave injection and outline the conditions required for observing the PDI in a laboratory plasma [3] . These results may guide the first laboratory verification of the fundamental PDI physics and shed lights on several problems in the heliosphere such as corona/solar-wind heating.
Understanding the physics of ion stopping in warm dense matter (WDM) is of great interest both for fundamental science and inertial confinement fusion. The theoretical description of the ion stopping power in WDM is challenging notably due to electron coupling and degeneracy, and the experimental database is essentially missing. In particular, the low velocity stopping power regime where v p (ion velocity) ~ v th (electron thermal velocity), near the Bragg peak, where the largest modeling uncertainties are reported, remains virtually unexplored.
Magnetically insulated line oscillators (MILOs) are crossed-field devices capable of generating intense bursts of high power microwaves. A typical MILO operates with a low (5-10 Ohm) impedance, and uses a large drive current (10s of kA) to generate a self-insulating magnetic field. Through analytic theory [1] , simulation, and experimental work [2] , researchers at the University of Michigan have developed and tested higher-impedance 1 GHz and 2 GHz MILOs.
Pulsed power diodes that generate bremsstrahlung x-rays are of interest as sources for radiation-matter interaction studies. The cylindrical reflex triode (CRT) is one such diode that is notable for its ability to produce low-endpoint x-ray spectra and its design is favorable to combine multiple CRTs to increase x-ray output, in either series or parallel configurations [1] . CRTs in various configurations are fielded on NRL’s Gamble II generator. This work marks the first particle-in-cell (PIC) simulations of a CRT and will focus on two CRTs in parallel in support of recent Gamble II experiments. The PIC simulations will be used to predict current measurements for the inner and outer triodes as well as report direct voltage measurements near the anode-cathode gap. Radiation production calculations based on the PIC simulation particle outputs will be made to make predictions of the time dependent x-ray production, near field radial dose distribution, and x-ray source distribution. These calculations will be validated against Gamble II experiments. Successful confidence in predictive capability with the model will allow for the development of synthetic x-ray spectra diagnostics.
Experimental measurements of the azimuthal magnetic field in cylindrically imploding plasmas reveals how plasma-current coupling is affected by various parameters such as electrode geometry or initial gas density profile. The efficacy of low impedance, fast linear transformer drivers as Z-pinch accelerators has not been thoroughly studied. Experiments carried out on the CESZAR driver (500 kA peak current, 180 ns rise) at UC San Diego have examined plasma-current coupling in liner-on-target gas-puff implosions using a polarization-based spectroscopic technique to determine azimuthal magnetic field (B θ ). The initial conditions were varied across multiple shots, namely, the plenum pressure and valve timing with respect to the current driver. The measured B θ values were compared with Ampere’s law. It was found that with relatively high plenum pressure and short valve opening duration, ~70% of the expected value of B θ were measured during the implosion phase, compared to 50% for low pressure shots. Additionally, two different cathode structures were examined using the same diagnostic technique. The modified cathode with an array of metallic screws improved shot-to-shot reproducibility and current coupling compared to the flat cathode. Lastly, the effect of an axial magnetic field strength was found to have minimal impact on the current coupling but did improve reproducibility. The results are compared with MHD simulations.
Coupled-cavity travelling wave tubes (CC-TWTs) are important to applications which require broad-band amplification and high gain. Classical Pierce theory [1] has treated the beam-circuit interaction as being continuous and uniform in the axial direction, which is taken to extend to infinity. However, it is important to test the limit of this treatment of beam-circuit interaction in real systems where the tubes are finite in length and discrete cavity effects may become important. Discrete cavity analysis allows us to treat individually tunable cavities, which was not possible with classical Pierce’s analysis. In this paper, we extend the previous preliminary work on the discrete cavity model by Wong et al [2] . An eigen-analysis method is used to obtain the cold-tube dispersion relationship for multi cavity systems, where the effects of cavity number, quality factor, and cavity coupling will be examined. The model will then be extrapolated to include the electron beam, and the hot-tube results will be examined. The relationship between this type of analysis and the classical Pierce analysis will also be discussed.
Recently, there is a rapidly growing body of work studying plasma-based water treatment for applications within the medical, environmental, and agriculture sectors. Atmospheric pressure plasmas produced by high voltage pulses with 10’s of nanoseconds in duration are well suited for treatment of liquids 1 , 2 , 3 and surfaces. Modeling of nanosecond pulsed plasma is challenging due to plasma complexity, as well as different reaction time-scales; from nanoseconds to seconds. Moreover, the plasma is affected by sheath formation 4 and reactions of aqueous species with gas phase plasma species at the plasma/liquid interface 5 . We present a time-dependent global model to study the decay of various plasma species in helium carrier gas and obtain the trends with changing the pulse frequency and pulse duration. The model in conjunction of voltage measurements and OES diagnostics provide a useful tool for such analyses.
The neutralization of an ion beam pulse directly by electron injection excites the two-stream instability of neutralizing electrons and produces an electrostatic solitary wave (ESW) after the instability saturates. The ESW propagates stably and reflects back and forth in the potential well of the ion beam pulse. Through a two-dimensional Particle-in-Cell code, we numerically simulated the whole neutralization process of the ion beam pulse as well as the excitation and propagation of the ESW. It is found that the reflections of the ESW at the edges of the ion beam pulse cause a lot of neutralizing electrons to be thrown out, forming runaway electrons released in a pulsed manner. Each reflection of the ESW accelerates the reduction of the neutralization degree and brings a small disturbance to the ESW. The accumulative effect of multiple reflections results in rapid collapse of the ESW at the end. The reflections of the ESW at the head and tail of the beam pulse cause the ion beam to lose more neutralizing electrons than the slow attenuation of the ESW inside the ion beam.
Recently, vacuum transistors, miniature vacuum triodes fabricated on a chip by microfabrication technologies, have rekindled many researchers’ interest because of stability in harsh environment, high velocity of electrons transporting in a vacuum and reliability for high-power and high-frequency devices. Most of the reported devices over the past decades have mainly focused on field emission vacuum transistors, which are so sensitive to the microstructures of emitters and the absorption of ambient molecules onto emitters that suffer from unstable electron emission, poor uniformity and high requirement for operating vacuum. To overcome these problems, we propose a vacuum transistor based on field-assisted thermionic emission from a Joule-heated multiwalled carbon nanotube, fabricated by microfabrication technologies. The field-assisted thermionic emission vacuum transistor exhibits an ON/OFF current ratio as high as 10 4 and a subthreshold slope of ~4 V•dec -1 .
A pinched-beam diode (PBD) consists of a thin annular cathode which emits electrons that are accelerated toward a planar anode. Once the energy deposited by the electrons on the anode is sufficiently large, a plasma forms on the anode and the electrons are strongly pinched toward the axis of the diode. An intense ion beam with current of 100’s of kA’s is also produced. This effort builds upon previously presented material and shows new particle-in-cell simulation results of a PBD where the focus is on the quality of the ion beams produced. The simulations provide additional detail to the formation of “hot spots” in the electron beam flow that produce regions of locally high charge and current density. As ions flow through the electron space charge cloud, the hot spots attract ions producing a non-uniform ion current distribution. The length of the cavity behind the cathode tip influences both the number and amplitude of the hot spots. Results will be presented to show that longer cavity lengths increase the number of hot spots but significantly reduces the amplitude producing smoother, more uniform ion beams than when the length of the cavity is short. Additional results will be presented that show the net current and ion bending angles are also significantly smaller when the length of the cavity is long.
We present a fully-kinetic numerical investigation of charging of irregularly-shaped dust particulates in low temperature collisionless plasmas. The recently developed Parallel Immersed-Finite-Element Particle-in-Cell (PIFE-PIC) code is utilized to self-consistently resolve the plasma environment and charging of immersed materials. This model explicitly includes the materials property (dielectric constant) of dust grains. Multiple dust grain shapes/configurations will be considered and compared to find how single and multiple dust grains are charged in a collisionless plasmas.
Charged particle beam transport through matter is a fundamental process in many high energy density science scenarios, including applications such as isochoric heating, secondary particle generation, and inertial confinement fusion. The propagation of intense charged particle beams through solid density plasmas can induce resistive electromagnetic fields, triggering collective effects such as resistive collimation. While these effects have been theorized 1 and observed with hot electron beams 2 , laser-driven proton beams are now approaching current density regimes (~10 10 A/cm 2 ) in which collective effects become significant. Here, we present a theoretical model for the proton beam-driven generation of resistive magnetic fields in solid density plasmas. A comparison of the essential heating mechanisms which play a crucial role in field generation is made between intense proton beams and hot electron beams. Using an analytic model for resistivity 3 and stopping power 4 that span cold to hot plasma regimes, the theoretical evolution of the magnetic field profile is compared with 2-D hybrid-PIC simulations and shows good agreement. Finally, the roles of various beam parameters are explored for both monoenergetic and Maxwellian proton beams, and a simple model to compute the maximum magnetic field is given.
Cold atmospheric pressure plasma (CAP) has shown potential to completely remove biofilms from surfaces [1] , [2] . The goal of this study is to employ the autofluorescence nature of bacterial biofilms to guide the removal of these biofilms from wounds using a CAP scalpel. Pseudomonas fluorescens biofilms which produce a green fluorescence under 405 nm UV light were grown on 12 chicken samples. The wound model (chicken tissue) is placed on a motorized X-Y stage with the plasma discharge device directly facing the sample. An image of the fluorescent biofilm region is captured using a Mightex BCN-B013-U monochrome camera and 560 nm green filter. The captured image then guides the X-Y stage to move such that only the fluorescent region is treated with CAP. CAP treatment of biofilm regions was carried out using a 1.37 lpm Ar/H 2 O plasma device with 39.5 x 54 mm dimension and a 1.5 mm tip. The discharge voltage and current are 3.24 kV and 1.2 mA respectively. The average speed of the plasma discharge over the substrate is 1 mm/s and the gap between the substrate and the discharge is 2 mm. To evaluate the action of CAP on the sample, before and after-fluorescent images were compared, CFU counts were taken, and 3D view of the effects were observed using a confocal microscope.
