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Direct irradiation of materials by electron beams (e-beams) has been used to study material response.2,3 The desire to utilize high-power (∼ TW) generators to achieve higher specific energy deposition over larger areas has led to several approaches. One approach utilizes a monolithic e-beam diode with an external magnetic field (B field). The external B field allows the diode to operate in the bipolar,...
Previous proof-of-principle experiments at NRL used the Mercury IVA facility to test a self-magnetic-pinch (SMP) diode in conjunction with a pulsed resistive heating treatment that cleaned the SMP's anode surface. This heating treatment was tested as a method for mitigating the negative effects of low-Z ions, such as post-shot activation and potentially reduced diode impedance. These low-Z ions can...
The Gamble II generator at the Naval Research Laboratory produces ∼100 ns pulse duration, relativistic-electron beams with peak energies on the order of 1MV and peak currents of about 800 kA with annular beam areas between 40–80 cm2. This gives peak current densities ∼10 kA/cm2. For many different applications, a nitrogen gas in the 1 Torr range is used as a charge- and current-neutralizing background...
Magnetic field penetration in electron-magneto-hydrodynamics (EMHD) can be driven by density gradients through the Hall term. Here we describe the effect of electron inertia on simplified one- and two-dimensional models of a magnetic front. Nonlinear effects due to inertia cause the 1D model to develop peaked solitary waves, while in 2D a shear-driven Kelvin-Helholtz (KH) like instability causes the...
Measurements of the breakdown field of vacuum gaps have been made with a 1-MV, 50-ns test stand [1]. The first set of experiments was performed with planar, bare metal electrodes [2]. This geometry had an enhanced anode edge that affected measurements. Here, we report on a new set of measurements using anodes without field enhancement. Diagnostics include current and voltage probes, x-ray detector,...
High-power diodes that generate intense electron beams are useful in many applications such as generating intense microwave radiation, pumping lasers for driving inertial fusion targets, and producing bremsstrahlung x-rays for flash radiography and nuclear weapon effects simulations. The core component of a high-power diode consists of a two-electrode acceleration gap in which a high voltage pulse...
The behavior of intense electron beams (those with current densities on the order of hundreds of kA/cm2 and beam rise times on the order of 100 ns) traveling through gaseous media depends strongly on the transport properties of the media. For example, the conductivity of a gas, which depends sensitively on its ionization state and temperature, has a strong influence on the beam behavior through the...
Understanding the interaction of a strong magnetic field with a plasma is one of the fundamental problems in plasma physics. In this talk we report on a new systematic study using two-dimensional particle-in-cell simulations designed to explore the interplay between magnetic pushing1 (MHD time scales) and magnetic field penetration due to the Hall effect2. A two-ion-species plasma that consists of...
Magnetic field penetration in current-carrying plasmas is being studied in a plasma opening switch geometry. Several Marshall guns1 are used to inject single or multi-species plasmas between coaxial conductors connected to the output of NRL's Hawk pulsed-power generator. Following injection of the plasma, the generator is used to apply an electrical pulse with a peak current of 700 kA, a peak voltage...
The current models of the plasma formation processes are relatively simple. Most PIC codes allow for space-charge limited electron emission once an electric-field threshold is exceeded but do not self-consistently model the physics of the cathode surface or the subsequent expansion of the cathode plasma. The model used for ion emission from the anode enables emission once the temperature rise from...
Intense pulsed active detection (IPAD, [1]; also see presentations at this conference by B.V. Weber, et al., D.P Murphy et al., S.B. Swanekamp et al. and J.C. Zier, et al.) has been proposed as a means of detecting contraband fissile material from a distance. In this approach, an intense bremsstrahlung pulse is used to induce photo fission, the products of which are detected. In this work, we report...
Mercury, a 2-TW inductive voltage adder located at the Naval Research Laboratory in Washington, DC, had previously been converted from negative to positive polarity output by rotating each of the cells[1]. Positive polarity was needed to field an ion-beam diode.[2] However, rotating the cells takes about 2 to 3 weeks and is very labor intensive. So, when we next needed to operate in positive polarity,...
The self-magnetic-pinch (SMP) electron beam diode is being developed for 4 to 10 M V, 30 to 50 ohms, 50 ns, flash radiography of explosively driven objects by AWE1, SNL2, and NRL3. The goal is a reproducible < 2-mm FWHM diameter radiographic spot at several hundred Rads (Si) at 1 m. In this talk a new approach is proposed where the outer diameter of an ∼50-ohm vacuum impedance MITL is tapered down...
Intense pulsed active detection (IPAD) has been proposed as a means of detecting contraband fissile material using an intense bremsstrahlung pulse to induce photofission. (S.B. Swanekamp, et al., IEEE Trans. Nucl. Sci. 58, 2011, pp. 2047–2056; also see presentations at this conference by B.V. Weber, et al., S.B. Swanekamp, et al., and J.C. Zier, et al.) A number of pulsed power technologies were evaluated...
For many applications, control and manipulation of the electron orbits in a high-current electron beam is desirable. This is especially true when a weakly-self-pinched, multi-MV electron-beam is used to make bremsstrahlung radiation. In this case, the radiation pattern is highly peaked along the direction that the electron beam makes when it strikes the x-ray target. Therefore, to maximize the number...
High energy-density plasma (HEDP) is created using a plasma-filled rod pinch diode (PFRP)1 on the Gamble II generator (2 MV, 0.7 MA, 60 ns with a matched 3-Ohm load). The PFRP consists of a 1-mm diameter tungsten rod that extends 25 mm through a hole in a grounded cathode plate. Plasma is injected from six cable guns 1 microsecond prior to firing the generator. The plasma acts like a short circuit...
Intense pulsed active detection (IPAD, S.B. Swanekamp, et al., IEEE Trans. Nucl. Sci. 58, 2011, pp. 2047–2056; also see presentations at this conference by B.V. Weber, et al., S.B. Swanekamp, et al., and J.C. Zier, et al.) has been proposed as a means of detecting contraband fissile material. In this approach, an intense bremsstrahlung pulse is used to induce photofission, the products of which are...
There is ongoing interest in the use of an intense bremsstrahlung pulse to induce photofission in fissionable material.1, 2 To optimize the radiation for inducing fissions in the forward direction, electrons should approach normal incidence and their charge on the anode converter should be maximum. Here, these are optimized on the 8-MeV, 200-kA, 50-ns Mercury inductive voltage adder3 by varying the...
High energy-density plasma (HEDP) is created using a plasma-filled rod pinch diode (PFRP) [1] on the Gamble II generator. HEDP is produced by a 1–2 MeV, 500 kA electron beam that deposits energy at the end of a tungsten rod. When the rod is tapered to a point, the energy density is estimated to reach 2.4 MJ/cm3 in about 10 ns, before rapid expansion decreases the energy density. Recent measurements...
Mercury, a 2-TW inductive voltage adder located at the Naval Research Laboratory in Washington, DC, had previously been converted from negative to positive polarity output by rotating each of the cells1. Positive polarity was needed to field an ion-beam diode.2 However, rotating the cells takes about 2 to 3 weeks and is very labor intensive. So, when we next needed to operate in positive polarity,...
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