P. F. Ottinger

Theoretical modeling of the plasma erosion opening switch for inductive storage applications

A theoretical model for the plasma erosion opening switch (PEOS) is presented which predicts its voltage, current and impedance history as a function of the input waveforms, geometry, and switch parameters. Scaling relations for the switch operation are developed from this model. System requirements for pulse compression and power multiplication using inductive storage are derived from a simple lumped circuit analysis and a transmission line analysis. These requirements are shown to be satisfied using the PEOS as a fast opening, vacuum switch in a configuration relevant for existing high‐power accelerators. The switch model is incorporated into a transmission line code for comparison with recent inductive storage experiments. Code results agree well with the data showing conduction times of ∼60 ns and switching times of ∼10 ns with peak currents of ∼600 kA.

Theoretical modeling and experimental characterization of a rod-pinch diode

The rod-pinch diode consists of an annular cathode and a small-diameter anode rod that extends through the hole in the cathode. With high-atomic-number material at the tip of the anode rod, the diode provides a small-area, high-yield x-ray source for pulsed radiography. The diode is operated in positive polarity at peak voltages of 1 to 2 MV with peak total electrical currents of 30–70 kA. Anode rod diameters as small as 0.5 mm are used. When electrode plasma motion is properly included, analysis shows that the diode impedance is determined by space-charge-limited current scaling at low voltage and self-magnetically limited critical current scaling at high voltage. As the current approaches the critical current, the electron beam pinches. When anode plasma forms and ions are produced, a strong pinch occurs at the tip of the rod with current densities exceeding 106 A/cm2. Under these conditions, pinch propagation speeds as high as 0.8 cm/ns are observed along a rod extending well beyond the cathode. Even f...

Plasma Erosion Opening Switch Research at NRL

This paper is a review of plasma erosion opening switch (PEOS) research performed at the Naval Research Laboratory (NRL). Several experimental and theoretical results are described to illustrate the present level of understanding and the best switching results obtained to date. Significant power multiplication has been achieved on the Gamble II generator, producing 3.5 TW with less than 10-ns rise time. Switching after nearly 1-¿s conduction time has been demonstrated on Pawn, producing a 0.2-TW 100-ns pulse. Scaling the switch to higher current, power, and conduction time should be possible based on theoretical analysis and the favorable results of scaling experiments performed thus far.

Characterization of a microsecond-conduction-time plasma opening switch

This paper presents data and analyses from which emerges a physical picture of microsecond‐conduction‐time plasma opening switch operation. During conduction, a broad current channel penetrates axially through the plasma, moving it toward the load. Opening occurs when the current channel reaches the load end of the plasma, far from the load. During conduction, the axial line density in the interelectrode region is reduced from its value with no current conduction as a result of radial hydrodynamic forces associated with the current channel. A factor of 20 reduction is observed at opening in a small, localized region between the electrodes. When open, the switch plasma behaves like a section of magnetically insulated transmission line with an effective gap of 2 to 3 mm. Increasing the magnetic field in this gap by 50% results in an improvement of 50% in the peak load voltage and load current rise time, to 1.2 MV and 20 nsec, respectively. An erosion opening mechanism explains the inferred gap growth rate using the reduced line density at opening. Improved switch performance results when the maximum gap size is increased by using a rising load impedance.This paper presents data and analyses from which emerges a physical picture of microsecond‐conduction‐time plasma opening switch operation. During conduction, a broad current channel penetrates axially through the plasma, moving it toward the load. Opening occurs when the current channel reaches the load end of the plasma, far from the load. During conduction, the axial line density in the interelectrode region is reduced from its value with no current conduction as a result of radial hydrodynamic forces associated with the current channel. A factor of 20 reduction is observed at opening in a small, localized region between the electrodes. When open, the switch plasma behaves like a section of magnetically insulated transmission line with an effective gap of 2 to 3 mm. Increasing the magnetic field in this gap by 50% results in an improvement of 50% in the peak load voltage and load current rise time, to 1.2 MV and 20 nsec, respectively. An erosion opening mechanism explains the inferred gap growth rate u...

Particle-in-cell simulations of high-power cylindrical electron beam diodes

Particle-in-cell (PIC) simulations are presented that characterize the electrical properties and charged-particle flows of cylindrical pinched-beam diodes. It is shown that there are three basic regimes of operation: A low-voltage, low-current regime characterized by space-charge-limited (SCL) flow, a high-voltage, high-current regime characterized by a strongly pinched magnetically limited (ML) flow, and an intermediate regime characterized by weakly pinched (WP) flow. The flow pattern in the SCL regime is mainly radial with a uniform current density on the anode. In the ML regime, electrons are strongly pinched by the self-magnetic field of the diode current resulting in a high-current-density pinch at the end of the anode rod. It is shown that the diode must first draw enough SCL current to reach the magnetic limit. The voltage at which this condition occurs depends strongly on the diode geometry and whether ions are produced at the anode. Analytic expressions are developed for the SCL and ML regimes a...

Plasma opening switch conduction scaling

Plasma opening switch (POS) experiments performed on the Hawk generator [Commisso et al., Phys. Fluids B 4, 2368 (1992)] (750 kA, 1.2 μs) determine the dependence of the conduction current and conduction time on plasma density, electrode dimensions, and current rise rate. The experiments indicate that for a range of parameters, conduction is controlled by magnetohydrodynamic (MHD) distortion of the plasma, resulting in a low density region where opening can occur, possibly by erosion. The MHD distortion corresponds to an axial translation of the plasma center‐of‐mass by half the initial plasma length, leading to a simple scaling relation between the conduction current and time, and the injected plasma density and POS electrode dimensions that is applicable to a large number of POS experiments. For smaller currents and conduction times, the Hawk data suggest a non‐MHD conduction limit that may correspond to electromagnetohydrodynamic (EMH) field penetration through the POS plasma.

Ultra-high electron beam power and energy densities using a plasma-filled rod-pinch diode

The plasma-filled rod-pinch diode is a new technique to concentrate an intense electron beam to high power and energy density. Current from a pulsed power generator (typically ∼MV, MA, 100 ns pulse duration) flows through the injected plasma, which short-circuits the diode for 10–70 ns, then the impedance increases and a large fraction of the ∼MeV electron-beam energy is deposited at the tip of a 1 mm diameter, tapered rod anode, producing a small (sub-mm diameter), intense x-ray source. The current and voltage parameters imply 20–150 μm effective anode-cathode gaps at the time of maximum radiation, much smaller gaps than can be used between metal electrodes without premature shorting. Interferometric diagnostics indicate that the current initially sweeps up plasma in a snowplow-like manner, convecting current toward the rod tip. The density distribution is more diffuse at the time of beam formation with a low-density region near the rod surface where gap formation could occur. Particle simulations of the...

Rescaling of equilibrium magnetically insulated flow theory based on results from particle-in-cell simulations

By relaxing an assumption on the electron density in the flow layer used in magnetically insulated transmission line (MITL) theory, the theory is rescaled to match particle-in-cell (PIC) simulation results, providing a more accurate determination of the line voltage from the measurement of anode and cathode currents over a broad range of parameters. Results from the PIC simulations also show that self-limited flow is not determined by either a minimum-current or a minimum-energy condition, but rather is closer to saturated flow. In addition, analytic expressions are obtained for the first time for the self-limited flow impedance ZfSL(V)∕Z0 and the self-limited anode and cathode currents Z0IaSL(V) and Z0IcSL(V), where Z0 is the vacuum impedance of the line and V is the voltage. Similar expressions for both minimum-current flow and minimum-energy flow are also obtained. Results are compared with other models for MITL flow and show that this rescaled MITL flow model is most consistent with the PIC simulation...

Propagation of intense ion beams in straight and tapered z-discharge plasma channels

A preformed z‐discharge plasma channel can be used to transport focused ion beams appropriate for a pellet fusion device. During transport, the beam can be compressed axially by time‐of‐fight bunching when appropriate ion accelerating voltage waveforms are employed. Single‐particle orbits in such channels are expressible in terms of simple harmonic functions for small ion injection angles. In this work, orbit analysis is used to investigate how nonuniformities or tapering of the channel and electric fields present in the channel affect radial beam confinement and power multiplication by bunching.

Numerical simulation of a low‐density plasma erosion opening switch

Current conduction through a low density (∼1012 cm−3) collisionless plasma injected between two coaxial conducting cylinders is simulated using a 2 (1)/(2) ‐D, electromagnetic particle‐in‐cell code. Plasma is injected through the anode towards the cathode with flow velocity, VF, and is assumed to be azimuthally symmetric. Current is driven through the plasma so that the 100 kA level is reached in ≂5 nsec. The opening process, when current is diverted to a load, is also treated. Electrons are found to carry current in a narrow current channel across the plasma by E×B drift. A large electric field is established by charge separation in the plasma in order to provide the drift. The motion of the anode end of the current channel controls the time of opening and is found to be independent of VF and to depend strongly on density and length.