Timothy Jerome Renk

The self-magnetic pinch (SMP) diode is a type of radiographic diode used to generate an intense electron beam for radiographic applications. At Sandia National Laboratories, SMP was the diode load for the six-cavity radiographic integrated test stand inductive voltage adder (IVA) driver operated in a magnetically insulated transmission line (MITL). The MITL contributes a flow current in addition to the current generated within the diode itself. Extensive experiments with a MITL of 40 Ω load impedance [T. J. Renk et al., Phys. Plasmas 29, 023105 (2022)] indicate that the additional flow current leads to results similar to what might be expected from a conventional high-voltage interface driver, where flow current is not present. However, when the MITL flow impedance was increased to 80 Ω, qualitatively different diode behavior was observed. This includes large retrapping waves suggestive of an initial coupling to low impedance as well as diode current decreasing with time even as the total current does not. A key observation is that the driver generates total current (flow + diode) consistent with the flow impedance of the MITL used. The case is made in this paper that the 80 Ω MITL experiments detailed here can only be understood when the IVA-MITL-SMP diode is considered as a total system. The constraint of fixed total current plus the relatively high flow impedance limits the ability of the diode (whether SMP or other type) to act as an independent load. An unexpected new result is that in tracking the behavior of the electron strike angle on the converter as a function of time, we observed that the conventional cIV^x “Radiographic” radiation scaling (where x ∼ 2.2) begins to break down for voltages above 8 MV, and cubic scaling is required to recover accurate angle tracking.

Radiographic diodes focus on an intense electron beam to a small spot size to minimize the source area of energetic photons for radiographic interrogation. The self-magnetic pinch (SMP) diode has been developed as such a source and operated as a load for the six-cavity radiographic integrated test stand (RITS-6) inductive voltage adder driver. While experiments support the generally accepted conclusion that a 1:1 aspect diode (cathode 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. Analysis 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 radiation 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 approximately few millimeters. Since simulations to date have not predicted such 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 stabilization of this oscillatory behavior. This is the first of two papers on the performance of the SMP diode on the RITS-6 accelerator.

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

Intense pulsed neutron fluences are generated by a high-energy (>10 MeV) proton beam using the beam-target method on the HERMES III facility at Sandia National Laboratories [J. J. Ramirez et al., in Proceedings of the 7th International Conference on High Power Particle Beams (Kernforschungszentrum, Karlsruhe GmbH, Karlsruhe, Germany, 1988), p. 148]. In order to generate the high-energy proton beam, a radial ion diode previously developed and fielded at the 6-MeV level in negative polarity was extended in performance to the 15-MeV level. This performance increase is described along with the development of a more durable hardware set to withstand the much more potent 15-MeV proton beam. An extensive series of simulations is developed to characterize the neutrons produced by the proton–target interaction. Particle-in-cell simulations describe the electron and ion dynamics, while Monte Carlo simulations characterize the neutron output. Due to differing estimates of proton beam voltage and current between the respective simulations, we make an approximate estimate of 13.5-MeV to 15-MeV and 120-kA ion beams at peak power in a 40 ns FWHM pulse. Simulations indicate that a total of 1.7 × 10¹³ neutrons are generated into 4π. Comparison of the neutron output predictions with a limited set of neutron flux measurements suggests a flux level of ∼1 × 10¹⁰ neutrons/cm² to 10 × 10¹⁰ neutrons/cm² over an approximately few tens of cm² area at the relevant application location. This effort also contributes to physics understanding of the use of inductive voltage adder platforms to drive ion-beam diode loads.

The goal of this Project is to produce an intense neutron pulse on HERMES III using the beam-target method with an intense proton beam. The potential advantage of proton use is that the generated neutron spectrum contains significantly more high-energy neutrons than that produced by electron-beam generated photoneutrons using the same facility. And compared to (D,T) facilities such as NIF, no tritium (or deuterium) is required for this process. To achieve the mid 10 10 neutrons/cm 2 at a test object location listed as the goal in the Proposal, it was proposed that a radial ion diode previously developed and fielded at the 6 MeV - level be extended in performance to the full-power level on HERMES, with proton energies in the neighborhood of 15 MeV. This Report details the successful development of the radial ion diode at full power, which required more durable hardware which could be fielded at a one shot/day basis with minimal debris and activation (an important concern), and which could be substituted quickly into the normal negative-polarity bremsstrahlung source experiments without compromising the main HERMES validation mission. As direct measurement of proton beam characteristics proved challenging, the Project relied on an extensive series of simulations,more » LSP for beam dynamics and MCNP to characterize neutron output. Simulation results will be discussed, including the conclusion that neutron measurements made are consistent with an MCNP-predicted proton beam of 16 MeV peak energy, and 200 kA peak current. This Project also contributes to physics understanding of the use of inductive voltage adder (IVA) platforms to drive diode loads. Since such diodes operate independently of the physics of IVAs, the IVA-diode coupling requires matching of the MITL flow to the requirements of ion diode operation. « less

Intense electron beams striking a high-atomic number target produce high-output pulsed photon fluxes for flash x-ray experiments. Without an external guide field, such beams are subject to the dynamics of high-current electron beam propagation, including changes to electron trajectories either from self-fields or from development of beam instabilities. The bremsstrahlung output (dose-rate) scales approximately as IV^x, where I is the beam current, V the electron energy, and x is in the range 2.0–2.65 and depends upon the electron angle on the converter. Using experimental beam data (dose-rate, I and V), this equation can be solved for x, a process known as “inverting the radiographer’s equation.” Inversion methods that rely on thermoluminescent dosimeters, which are time-integrated, yield no information about evolution of the electron beam angle in time. We propose here an inversion method that uses several dose-rate monitors at different angles with respect to the beam axis. By measuring dose-rates at different angles, one can infer the time-dependent beam voltage and angle. This method compares well with estimates of corrected voltage and results in a self-consistent picture of beam dynamics. Techniques are demonstrated using data from self-magnetic pinch experiments at the RITS-6 facility at Sandia National Laboratories.