Michael P. Perkins

Advances in x-ray framing cameras at the National Ignition Facility to improve quantitative precision in x-ray imaging

We describe an experimental method to measure the gate profile of an x-ray framing camera and to determine several important functional parameters: relative gain (between strips), relative gain droop (within each strip), gate propagation velocity, gate width, and actual inter-strip timing. Several of these parameters cannot be measured accurately by any other technique. This method is then used to document cross talk-induced gain variations and artifacts created by radiation that arrives before the framing camera is actively amplifying x-rays. Electromagnetic cross talk can cause relative gains to vary significantly as inter-strip timing is varied. This imposes a stringent requirement for gain calibration. If radiation arrives before a framing camera is triggered, it can cause an artifact that manifests as a high-intensity, spatially varying background signal. We have developed a device that can be added to the framing camera head to prevent these artifacts.

Computational studies of X-ray framing cameras for the national ignition facility

Summary form only given. The NIF is the worlds most powerful laser facility and is used for inertial confinement fusion experiments. One hundred and ninety two laser beams are used to compress a small capsule. X-ray framing cameras are an important diagnostic used to help characterize the dynamics of the capsule. The gated x-ray framing cameras consists of several key components including a pin hole array, microstrip/microchannel plate, pulsed phosphor, and either film pack or CCD for recording images1. The pin hole array is a thin piece of tantalum with small holes used to shield most of the incident x-rays, but allows some to be projected onto a microstrip/microchannel plate. When photons strike the microstrip/microchannel plate photoelectrons are created which can be accelerated through pores in the microchannel plate by pulsed voltages on the microstrips. The electrons are amplified in the pores by a secondary electron cascade. At the output of the microchannel plate the electrons are accelerated to a phosphor screen where the output can be recorded. The x-ray framing cameras have provided excellent information. As the yields at NIF have increased and the data provided by the framing cameras have been further resolved, some “streak” artifacts were discovered that needed further understanding2. A theory was proposed as to the origin of these artifacts2, as well as a mitigation strategy2. In this presentation we will discuss the results of electrostatic, full wave electromagnetic, and particle-in-cell simulations used to further understand the streaks in the data as well as simulation results for the mitigation strategy2 used to help correct the problem. We will also discuss some simulation results that illustrate potential enhancements for future framing cameras.

Investigation and suppression of artifacts in x-ray framing cameras due to advance radiation incident on microchannel plates

We present evidence of an artifact in gated x-ray framing cameras that can severely impact image quality. This artifact presents as a spatially-varying, high-intensity background and is correlated with experiments that produce a high flux of x-rays during the time before the framing camera is triggered. Dedicated experiments using a short pulse UV laser that arrives before, during, and after the triggering of the framing camera confirm that these artifacts can be produced by light that arrives in advance of the voltage pulse that triggers the camera. This is consistent with these artifacts being the result of photoelectrons produced uniformly at the active area of the camera by early incident light and then selectively trapped by the electromagnetic (EM) fields of the camera. Simulations confirm that the EM field above the active surface can act to confine electrons produced before the camera is triggered. We further present a method to suppress these artifacts by installing a conducting electrode in front of the active area of the framing camera. This device suppresses artifacts by attracting any electrons liberated by x-rays that arrive before the camera is triggered.

Analysis of a Small Loop Antenna With Inductive Coupling to Nearby Loops

This paper analyzes the inductive coupling that occurs when a loop antenna is near other conductive objects that form complete loops and are excited by incident low-frequency magnetic fields. The currents developed on the closed loops from the time changing magnetic fields generate their own magnetic fields that alter the voltage received by nearby open loop antennas. We will demonstrate how inductance theory can be used to model the system of loops. Using this theory, time domain circuit models are developed to find the open circuit voltage of a loop near one closed loop and for the open circuit voltage of one loop near two closed loops. We will show that the model is in good agreement with measurements that have been made in a TEM cell. One important application of this work is for electroexplosive device safety. It is necessary to ensure that if lightning strikes a facility that the electromagnetic fields generated inside do not have strong enough coupling to a detonator cable to cause initiation of explosives. We will show how the model can be used to analyze magnetic field coupling into a detonator cable attached to explosives in one typical type of work stand.

Simulations for initiation of vacuum insulator flashover

The vacuum/dielectric interface of insulators is often the weakest part in high voltage and pulsed power systems. Surface flashover can occur for electric field values much lower than that of bulk breakdown through the material. Although much empirical data and many theories can be found in the literature, there are no models that can be used to optimally design insulators and reliably predict when flashover will occur. In this presentation we will discuss the results of a FDTD-PIC code that is being used to model physics phenomena common to many flashover theories. In order to simulate the initiation of vacuum insulator flashover, VORPAL [1] is being used on the Linux clusters at LLNL. In [2] we presented the results for implementing physics modules that included the effects of field distortion due to the dielectric, Fowler-Nordheim field emission, low energy secondary emission, insulator charging, and magnetic fields. We have extended our previous work to include a thin gas layer near the surface of the insulator. Electrons may cause ionization depending on their energies and the collision cross section of the gas. The inclusion of these physics effects leads to a more complete model and better understanding of vacuum insulator flashover.

Two-dimensional phase unwrapping to help characterize an electromagnetic beam for quasi-optical mode converter design

An improved two-dimensional phase unwrapping procedure is discussed that uses a weighted least-squares algorithm, a congruence operation, and a filter to unwrap the phase distribution of an electromagnetic beam. These improvements make possible several advances for mirror designs used in gyrotron quasi-optical mode converters. The improved phase unwrapping procedure is demonstrated by applying it to a measured beam and a simulated beam that are used to design mirrors. The unwrapping procedure produces a smooth unwrapped phase that does not change the characteristics of the beam. The smooth unwrapped phase distribution is also used to find an estimate for the wavenumber vector distribution that is needed to design the mirrors.

Analysis of conductor impedances accounting for skin effect and nonlinear permeability

It is often necessary to protect sensitive electrical equipment from pulsed electric and magnetic fields. To accomplish this electromagnetic shielding structures similar to Faraday Cages are often implemented. If the equipment is inside a facility that has been reinforced with rebar, the rebar can be used as part of a lighting protection system. Unfortunately, such shields are not perfect and allow electromagnetic fields to be created inside due to discontinuities in the structure, penetrations, and finite conductivity of the shield. In order to perform an analysis of such a structure it is important to first determine the effect of the finite impedance of the conductors used in the shield. In this paper we will discuss the impedances of different cylindrical conductors in the time domain. For a time varying pulse the currents created in the conductor will have different spectral components, which will affect the current density due to skin effects. Many construction materials use iron and different types of steels that have a nonlinear permeability. The nonlinear material can have an effect on the impedance of the conductor depending on the B-H curve. Although closed form solutions exist for the impedances of cylindrical conductors made of linear materials, computational techniques are needed for nonlinear materials. Simulations of such impedances are often technically challenging due to the need for a computational mesh to be able to resolve the skin depths for the different spectral components in the pulse. The results of such simulations in the time domain will be shown and used to determine the impedances of cylindrical conductors for lightning current pulses that have low frequency content.

Progress on simulating the initiation of vacuum insulator flashover

Vacuum insulators are critical components in many pulsed power systems. The insulators separate the vacuum and non-vacuum regions, often under great stress due to high electric fields. The insulators will often flashover at the dielectric vacuum interface for electric field values much lower than for the bulk breakdown through the material. Better predictive models and computational tools are needed to enable insulator designs in a timely and inexpensive manner for advanced pulsed power systems. In this article we will discuss physics models that have been implemented in a PIC code to better understand the initiation of flashover. The PIC code VORPAL [1] has been ran on the Linux cluster Hera at LLNL. Some of the important physics modules that have been implemented to this point will be discussed for simple angled insulators. These physics modules include field distortion due to the dielectric, field emission, secondary electron emission, insulator charging, and the effects of magnetic fields. In the future we will incorporate physics modules to investigate the effects of photoemission, electron stimulated desorption, and gas ionization. This work will lead to an improved understanding of flashover initiation and better computational tools for advanced insulator design.

Bonded penetration analysis for a severe lightning strike to a facility

Lightning strikes pose a serious threat to facilities and their subsystems. If a facility takes a direct strike, large amounts of pulsed electromagnetic (EM) energy can radiate into the interior of the facility. This energy can couple into electronic systems causing failures. Often, proper shielding of the facility can reduce the radiated energy by an order of magnitude. In an attempt to reduce pulsed EM energy, facilities are built to resemble a Faraday cage. However, most facilities have several imperfections which limit the effectiveness of their shielding capabilities. Penetrations into the facility are a type of imperfection that allows EM fields to be produced in the interior of the facility. Therefore, penetrations must be connected to the Faraday cage through bond wires to maintain the shields integrity and protect sensitive components. Finite element computer simulations have been performed to determine the effects of bonded penetrations, using 6 AWG bond wires. In an attempt to offer guidelines, which optimize the facilitys shielding effectiveness; several bond wire configurations have been investigated. Bond wire lengths, bond wire orientation, single and multiple bond wire configurations and varying the angle between bond wires have been investigated. Simulation results have shown that multiple bond wires result in greater than 40dB reduction of pulsed EM fields in the interior of the facility and a spacing of greater than 45° is optimum for bond wire spacing, for the simulated facility. In addition, the penetration current diverted by the bond wire was monitored. For severe direct lightning strikes, i.e. Ipeak=200 kA and dI/dt=400 kA/μs, the simulation suggest greater than 90% of the lightning current is diverted through the bond wire into the Faraday cage for the configurations examined. The high current nature of the severe lightning pulse produces large Lorentz forces on the bond wire. Laboratory experiments are being developed at the LLNL pulsed power lab to ensure that bond wires maintain proper connection when exposed to high currents, ensuring desired shielding throughout a direct strike.

A low-frequency model for E-field and B-field coupling into a folded antenna with two gaps

Electric field coupling into electrically small monopoles/dipoles and magnetic field coupling into electrically small loop antennas has been investigated extensively due to their applicability to a wide range of applications. However, under certain conditions electrically small folded antenna structures exist in which both coupling mechanisms must be included simultaneously in order to perform an accurate system analysis. In this paper we present a low frequency model that includes both electric and magnetic field coupling simultaneously for a folded antenna with two gaps. Values for a circuit model are found using an electrostatic finite element code and a full wave frequency domain finite element code. The circuit model is then validated by a full wave finite difference time domain code. For the time domain analysis the antenna structure is excited by fields from a lightning pulse. The time domain simulation has excellent agreement with the circuit model that is presented.