Mark L. Kiefer

This article describes our effort to measure the back-streaming ions emitted from the target X-ray convertor and thus estimate the ion contribution to the A-K gap bipolar current flow. Our plans were first to measure the total ion current using B-dot monitors, Rogowski coils, and Faraday cups and then to utilize filtered Faraday cups and time of flight techniques to identify and measure the various ionic species. LSP simulations found that the most prominent ions are protons and carbon single plus (C $^{+}$ ). For an 8-MV A-K maximum voltage, the estimated proton current back-streaming through a 1 cm in diameter hollow cathode tip was on the average 3 kA and the carbon current 0.7 kA. Since only a small fraction of the ions makes it through the hollow cylinder of the cathode tip, the corresponding total ion currents were calculated to be respectively 25 kA for proton and 7 kA for carbon ions, a quite substantial contribution to the total bipolar beam current. Unfortunately, the diagnostic cables connecting the B-dot monitors, Rogowski coil, and Faraday cup to the screen room scopes experienced a large amount of charge pick-up (Compton electrons) that obliterated our effort to directly measure those relative to the large total beam current small ion currents. In the following sections we present some typical cable pickup results and our efforts to verify that the observed “current” scope traces were indeed not ion currents but instead cable charge pickup. Interestingly enough, we also discovered that the appearance of those “currents” are in coincidence with the A-K gap impedance variation (collapse) and the MITL sheath current retrapping. Hence, the diagnostic cable current pickup could be utilized as diode behavior diagnostics.

Radiographic diodes focus an intense electron beam to a small spot size to minimize the source area of energetic photons for radiographi c interrogation. The self - magnetic pinch (SMP) diode has been developed as such a source and operated as a load for the RITS - 6 Inductive Voltage Adder (IVA) driver. While experiments support the generally accepted conclusion that a 1:1 aspect diode (cathod e diameter equals anode - cathode gap) delivers optimum SMP performance, such experiments also show that reducing the cathode diameter, while reducing spot size, also results in reduced radiation dose, by as much as 50%, and degraded shot reproducibility. An alyzation of the effective electron impingement angle on the anode converter with time made possible by a newly developed dose - rate array diagnostic indicates that fast - developing oscillations of the angle are correlated with early termination of the radia tion pulse on many of the smaller - diameter SMP shots. This behavior as a function of relative cathode size persists through experiments with output voltages and currents up to 11.5 MV and 225 kA, respectively, and with spot sizes below ~ few mm. Since simu lations to date have not predicted suchmore » oscillatory behavior, considerable discussion of the angle - behavior of SMP shots is made to lend credence to the inference. There is clear anecdotal evidence that DC heating of the SMP diode region leads to stabiliza tion of this oscillatory behavior. This is the first of two papers on the performance of the SMP diode on the RITS - 6 accelerator . « less

Our research was mainly concentrated on the physics of semiconductor diodes and especially on measuring the reverse recovery times and currents. In addition, we explored the effect of the reverse bias pulses on a diode still under reverse recovery times. Therefore, we tested the diode cartridges only up to 40% of the scale of that of LANL. We utilized our Component Test Stand (CTS-I) facility modified to power two diode cartridges connected in parallel to a common load (CTS-II). We varied the charge voltage and interpulse separations between the two diode cartridges. In this article, we will describe the modifications of CTS leading to CTS-II and present and analyze the obtained results.

The objectives of this tutorial are as follows: 1) to help students and researchers develop a basic understanding of how pulsed-power systems are used to create high-energy-density (HED) matter; 2) to develop a basic understanding of a new, compact, and efficient pulsed-power technology called linear transformer drivers (LTDs); 3) to understand why LTDs are an attractive technology for driving HED physics (HEDP) experiments; 4) to contrast LTDs with the more traditional Marx-generator/pulse-forming-line approach to driving HEDP experiments; and 5) to briefly review the history of LTD technology as well as some of the LTD-driven HEDP research presently underway at universities and research laboratories across the globe. This invited tutorial is part of the Mini-Course on Charged Particle Beams and High-Powered Pulsed Sources, held in conjunction with the 44th International Conference on Plasma Science in May of 2017.

Optical spectroscopic measurements in pulsed power diodes have been performed to estimate plasma parameters and local magnetic fields. Initial experiments on the Self-Magnetic Pinch diode at the RITS-6 accelerator (5-11 MV, 100-200 kA) at Sandia Labs used lens coupled fiber arrays that were imaged onto a high resolution gated spectrometer. Local magnetic fields (3-4 T) were estimated from Zeeman split C IV lines, which yielded current profiles at the anode surface. Additionally, electron density profiles were estimated from the Stark widths of several ion and neutral spectral lines. More recent experiments have focused on using similar fiber arrays coupled to streak spectrometers tom image MITL regions on the Z machine in order to better understand current loss due plasma formation in the A-K gap.

Summary form only given. Self-Magnetic Pinch (SMP) diode experiments are in progress at the RITS-6 accelerator (5-11MV, 100-200kA) at Sandia National Laboratories. Te SMP diode is a flash x-ray radiography source with a radiation pulse length of 30-45 ns, dependent on the diode configuration. A hollow cathode emits electrons through an approximately 1cm vacuum A-K gap onto a high Z material. The high current (~150kA) compresses the electron beam to sub 3mm spot sizes ¹ . Visible spectra of diode plasma have been taken along the anode surface by both directly imaging onto a 0.3 m spectrometer and by using a lens coupled 11×1 fiber array, imaged onto a 0.15 m spectrometer. During the radiation pulse, the spectra contains carbon and aluminum lines, along with H-alpha. The H-alpha line has been used previously to characterize the plasma density as between 10 ¹⁷ -10 ¹⁸ cm ^-3 , during the peak of the radiation pulse, dependent on distance from the axis of the diode.The goal of these experiments is to identify lines that can reliably characterize the magnetic field through Zeeman splitting. The 580.1 and 581.2 C IV lines in particular have a small Stark width and may be a good candidate to measure splitting in 10-20 T B-fields. Planned experiments include using a streak camera in conjunction with a 1 m, high resolution (0.1 Angstrom) spectrometer to determine the time evolution of the spectral lines. Recent results will be presented.

The RITS-6 accelerator (5-11MeV, 100-200kA) at SNL is being used to evaluate the Self-Magnetic Pinch (SMP) diode as a flash x-ray radiography source1. This diode utilizes a small, hollowed, metal cathode and a planar high atomic number anode to produce a focused electron beam (<; 3mm) which generates high energy Bremsstrahlung x-rays. During this process, electrode plasmas form and propagate in an approximately 1cm A-K vacuum gap. These plasmas are measured spectroscopically using high resolution (<;1 Angstrom) Czerny-Turner spectrometers with ICCD and/or streak camera outputs. This paper explores using a novel plasma spectroscopy method to measure the magnetic fields (1-25 Tesla) within the SMP diode. With this technique, the magnetic field effects on line shapes and fine structure features can be measured even when the line shapes are dominated by Stark or Doppler broadening. This provides a great advantage, over other techniques such as Faraday rotation or polarization spectroscopy. Several suitable line species have been identified and experimentally measured. Recent results with inferred B-fields will be presented.

Summary form only given. A Monte-Carlo code (MCSwarm) has been developed to compute transport coefficients in a weakly ionized gas. The MCSwarm code includes rotational and vibrational effects as well as excitation, ionization, and attachment reactions to determine the electron mobility, mean electron energy, and ionization and attachment rates. The MCSwarm code is time accurate and correctly handles both electric and magnetic fields with arbitrary orientation. A wide variety of common gases have been implemented including N₂, O₂ , Ar, Ne, SF₆, H₂, etc. The output of the MCSwarm code is a table of steady-state transport coefficients for a specified range of E/p and B/p values where E is the electric field, B is the magnetic field, and p is the gas pressure. When coupled with a particle-in-cell (PIC) code, this table of transport coefficients can be used to model time-dependent charge and current neutralization processes when an electron beam is injected into a gas. This is a reasonable approach provided the collisional mean-free path is small compared to any length scales of interest and the energy equilibration time is short compared to the time scales of interest. The PIC code Quicksilver (QS) has been modified to use the transport tables from the MCSwarm code. This newly modified QS is being used to simulate recent experiments at the Naval Research Laboratory where ~100 keV electrons with current densities between 10 A/cm² and 1 kA/cm² are injected into various pressures of either N₂ or dry air. Comparisons of code results with this data will be presented

The light ion fusion program at Sandia National Laboratories is reviewed. The program is based on the indirect‐drive target concept and has the long‐term objective to obtain high yield in a microfusion facility. The near‐term technical effort will focus on increasing intensity and demonstrating 100 eV temperature in an ion hohlraum. Some concepts were tested experimentally confirming the accessibility of anticipated goals. (AIP)