Jesse M. Neri

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 the Velocity Skin Effect in the Surface Layer of a Railgun Sliding Contact

We present a characterization of contact velocity skin effect (VSEC), which is a major velocity- and efficiency-limiting effect at a railguns sliding contact. Despite enormous contact forces, the armature remains separated from the rail by a thin layer, 4 to 12 A thick. Evidence suggests VSEC is also the primary mechanism responsible for the contact voltage drop. VSEC effects are seen in both electromagnetic launcher (EML) efficiency and breech voltage. We compare theoretical predictions of system efficiency and breech voltage to experimental measurements for both a conventional and an augmented railgun. The characterization of VSEC extends our previous theoretical work in this area and provides new insights into the physics of EML operation, especially with regards to the armature and sliding contact. VSEC is a significant energy loss mechanism and heat source, possibly contributing to contact erosion and transition. We propose a similar VSEC mechanism to explain velocity saturation and efficiency roll-off in plasma and hybrid armature railguns, as well as arc restrike.

Efficiency and Scaling of Constant Inductance Gradient DC Electromagnetic Launchers

We present efficiency and scaling relationships for dc (i.e., noninduction) constant inductance gradient electromagnetic launchers.We derive expressions for electromagnetic force, efficiency, back-voltage, and kinetic power in terms of electrical circuit parameters. We show that launcher efficiency is a simple function of armature velocity and the launcher’s characteristic velocity. The characteristic velocity characterizes the launcher and is the product of two new parameters: the mode constant and launcher constant. Mathematically, the launcher must operate at its characteristic velocity for 50% maximum efficiency. The mode constant reflects the manner in which the launcher is powered and its maximum efficiency. The launcher constant reflects the geometry of the launcher. We consider two modes of operation: constant current and zero exit current operation. We develop the ideal electromagnetic launcher concept and define it as operation at 100% maximum efficiency at all velocities.We also develop the concept of same-scale comparisons, that is, that electromagnetic launcher comparisons should be done with equal bore diameter, launcher length, projectile mass, and velocity. Finally, we present a comparative analysis based on experimental data of same-scale constant gradient electromagnetic launchers for conventional railgun, augmented railgun, and helical gun launchers in terms of the launcher constant, inductance gradient, bore diameter, bore length, system resistance, and armature (i.e., projectile) velocity.

The Maximum Theoretical Efficiency of Constant Inductance Gradient Electromagnetic Launchers

The maximum theoretical efficiency of constant inductance gradient electromagnetic launchers (EMLs) is analyzed and discussed. The maximum theoretical efficiency is a parameter needed to calculate the EMLs efficiency. Constant inductance gradient EMLs include the conventional railgun, the augmented railgun, and the conventional helical launcher. The maximum theoretical efficiency of an EML is dependent on its geometry and the manner, or mode, in which it is powered. In the lossless case, the conventional railgun, the augmented railgun, and the conventional helical launcher are capable of 50% maximum efficiency when operating in constant current (CC) mode. Conventional and augmented railguns can achieve 100% maximum efficiency when operating in zero exit-current mode. While zero exit-current mode promotes high efficiency, this mode can reduce EML lifetime since it requires current levels much higher than those found in CC mode. The high-efficiency helical launcher, presented and analyzed here for the first time, combines 100% maximum theoretical efficiency with the low-current benefits of constant-current mode.

High-efficiency, medium-caliber helical coil electromagnetic launcher

Research progress in the development of a 40 mm/spl times/750 mm helical-coil electromagnetic launcher (HCEL) is presented and discussed. Significant technical problems that have been solved in this research include efficient stator commutation methods and the ability to simultaneously implement high-inductance gradient armatures. The HCEL is able to launch a 525-gram projectile to a velocity of 140 m/s. Power for the HCEL is derived from a 62.5 kJ sequentially fired pulse forming network (PFN) of 900 V (maximum) electrolytic capacitors. The experimentally measured HCEL efficiency of 18.2% is substantially greater than a conventional or augmented railgun of similar scale (i.e., equivalent mass, bore-size, and velocity). The HCELs high launch efficiencies result from its 150 /spl mu/H/m inductance gradient, which is approximately 300 times greater than the inductance gradient of a conventional railgun. HCEL computer model predictions are given and compared to experimentally measured HCEL and PFN parameters including peak current, inductance gradient, acceleration time, parasitic mass ratios, and electrical-to-kinetic conversion efficiency. Scaling relationships for the HCEL are also presented and used to predict launcher operation at higher velocity and with a larger diameter bore size.

Solid-Projectile Helical Electromagnetic Launcher

Helical electromagnetic launchers (HEMLs) can operate at significantly lower currents and higher efficiency in comparison to conventional railgun and induction coilgun launchers. The HEMLs versatility is due, in part, to its large inductance gradient which is typically two to three orders of magnitude greater than a conventional railgun and can be tailored to almost any value in that range. Historically, however, HEMLs were not considered practical since they consisted of a hollow projectile (i.e., armature that is accelerated on the outside of a stator coil). This investigation demonstrates, for the first time, a 40-mm bore times 750-mm length solid-projectile HEML, where the armature is accelerated on the inside of a stator coil. The goal of this paper is to demonstrate the practicality of solid-projectile HEML concept and to measure its performance. Numerous successful tests were conducted. The highest velocity measured for a 170-g projectile was 64 m/s.

Intense ion‐beam‐transport experiments using a z‐discharge plasma channel

A z‐discharge plasma channel is used to confine and transport an intense proton beam. A pinch‐reflex ion diode on the NRL Gamble II accelerator focuses a proton beam onto the entrance aperture of a 2.5 cm diam, 1.2 m long z‐discharge transport system. The beam ions are charge and current neutralized in the discharge plasma, and execute betatronlike orbits in the magnetic field of the discharge. Ion beam diagnostics include shadowbox imaging and prompt‐γ radiation measurements from LiF targets. Under appropriate conditions, 95% particle transport and 90% energy transport are observed, with the only energy loss attributed to classical stopping in the channel gas. The transverse phase‐space distribution of the beam measured by the shadowbox is consistent with full charge and current neutralization of the transported beam.

Ballistic transport and solenoidal focusing of intense ion beams for inertial confinement fusion

Light‐ion inertial confinement fusion requires beam transport over distances of a few meters for isolation of the diode hardware from the target explosion and for power compression by time‐of‐flight bunching. This paper evaluates ballistic transport of light‐ion beams focused by a solenoidal lens. The ion beam is produced by an annular magnetically insulated diode and is extracted parallel to the axis by appropriate shaping of the anode surface. The beam propagates from the diode to the solenoidal lens in a field‐free drift region. The lens alters the ion trajectories such that the beam ballistically focuses onto a target while propagating in a second field‐free region between the lens and the target. Ion orbits are studied to determine the transport efficiency ηt (i.e., the fraction of the beam emitted from the diode which hits the target) under various conditions relevant to light‐ion inertial confinement fusion. Analytic results are given for a sharp boundary, finite thickness solenoidal lens configura...

Self-pinched transport of an intense proton beam

Ion beam self-pinched transport (SPT) experiments have been carried out using a 1.1-MeV, 100-kA proton beam. A Rutherford scattering diagnostic and a LiF nuclear activation diagnostic measured the number of protons within a 5 cm radius at 50 cm into the transport region that was filled with low-pressure helium. Time-integrated signals from both diagnostics indicate self-pinching of the ion beam in a helium pressure window between 35 and 80 mTorr. Signals from these two diagnostics are consistent with ballistic transport at pressures above and below this SPT pressure window. Interferometric measurements of electron densities during beam injection into vacuum are consistent with ballistic transport with co-moving electrons. Interferometric measurements for beam injection into helium show that the electron density increases quadratically with pressure through the SPT window and roughly linearly with pressure above the SPT window. The ionization fraction of the helium plateaus at about 1.5% for pressures abov...

Initialization and Operation of Mercury, A 6-MV MIVA

Mercury became operational in a stepwise manner to test the machine components after modifications and reassembly at NRL. To avoid damaging the MIVA, extensive testing of the laser and PFL output switches was performed using dummy loads. Finally, the PFLs were connected to the MIVA and Mercury was fired into a simple cylindrical diode load with a Marx charge voltage up to 75 kV. Measured MIVA currents and voltages compare well with a circuit model of the MIVA fed by the measured PFL outputs and with PIC simulations of the MIVA and the diode load.