James H. Degnan

Electromagnetic-implosion generation of pulsed high-energy-density plasma

The generation of pulsed high‐energy‐density plasmas by electromagnetic implosion of cylindrical foils (i.e., imploding liners or hollow Z pinches) has been investigated experimentally and theoretically at the Air Force Weapons Laboratory. The experimental studies involve discharging a 1.3‐μsec 1.1‐MJ capacitor bank through 7‐cm‐radius 2‐cm‐tall 3–30‐mg cylindrical foil liners. Typical discharge parameters are 7–12‐MA peak current and 1–1.5‐μsec current rise time. Current and voltage waveforms indicate strong coupling of the load to the capacitor bank, and analysis of the waveforms indicates good implosion of the current sheath. Optical‐ and magnetic‐probe measurements are consistent with 1–2‐cm thickness of the imploding plasma shell and with final implosion velocities ∼15–20 cm/sec. Radiation‐diagnostic measurements indicate ultrasoft x‐ray yields ∼50–100 kJ with the FWHM of the photon pulse ∼80–100 nsec. The radiation data is consistent with a quasiblackbody spectrum (T∼30–50 eV) comprising most of the...

Inductive Store Pulse Compression System for Driving High Speed Plasma Implosions

The Air Force Weapon Laboratory has investigated and developed inductive pulse compression techniques with fuse opening switches for driving high speed plasma implosions. Experiments have demonstrated the delivery of 7.5 MA to a 5-nH load in < 200 ns from an initial 1.9-MJ 2-¿s capacitor bank via inductive pulse compression. Circuit considerations dictate the overall energy efficiency while MHD considerations dictate overall implosion stability and thermalization time. Theoretical considerations along with initial experiment results are presented in this paper.

Generation of high-energy x-radiation using a plasma flow switch

Experiments with coaxial plasma guns at currents in excess of ten megamperes have resulted in the production of high‐voltage pulses (0.5 MV) and hard x radiation (10–200 keV). The x‐radiation pulse occurs substantially after the high‐voltage pulse suggesting that high‐energy electrons are generated by dynamic processes in a very high speed (≳106 m/s), magnetized plasma flow. Such flows, which result from acceleration of relatively low‐density plasma (10−4 vs 1.0 kg/m3) by magnetic fields of 20–30 T, support high voltages by the back electromotive force‐u×B during the opening switch phase of the plasma flow switch. A simple model of classical ion slowing down and subsequent heating of background electrons can explain spectral evidence of 30‐keV electron temperatures in fully stripped aluminum plasma formed from plasma flows of 1–2 × 106 m/s. Similar modeling and spectral evidence indicates tungsten ion kinetic energies of 4.5 MeV and 46 keV electron temperatures of a highly stripped tungsten plasma.

Design, fabrication, and operation of a high-energy liner implosion experiment at 16 megamperes

We discuss the design, fabrication, and operation of a liner implosion system at peak currents of 16 MA. Liners of 1100 aluminum, with initial length, radius, and thickness of 4 cm, 5 cm, and 1 mm, respectively, implode under the action of an axial current, rising in 8 /spl mu/s. Fields on conductor surfaces exceed 0.6 MG. Design and fabrication issues that were successfully addressed include: Pulsed Power-especially current joints at high magnetic fields and the possibility of electrical breakdown at connection of liner cassette insulator to bank insulation; Liner Physics-including the angle needed to maintain current contact between liner and glide-plane/electrode without jetting or buckling; Diagnostics-X-radiography through cassette insulator and outer conductor without shrapnel damage to film.

Fast, large‐signal, free‐standing foil bolometer for measuring ultrasoft x‐ray burst fluence

A fast ( approximately 300 ns), large-signal ( greater, similar1 V), free-standing foil bolometer was developed for measuring ultrasoft x-ray burst fluences. The results of bolometer measurements of the radiation output of an imploding foil liner plasma indicate yields of several tens of kJ, assuming isotropic emission. This is in substantial agreement with filtered metal photocathode (x-ray diode) measurements. The bolometer design, response function, and comparison with x-ray photodiode data are discussed. This type of bolometer is particularly applicable to radiation measurements of high-energy, destructive pulsed plasmas such as high-energy imploding liner plasmas.

Diagnostics for a magnetized target fusion experiment

We are planning experiments using a field reversed configuration plasma injected into a metal cylinder, which is subsequently electrically imploded to achieve a fusing plasma. Diagnosing this plasma is quite challenging due to the short timescales, high energy densities, high magnetic fields, and difficult access. We outline our diagnostic sets in both a phase I study (where the plasma will be formed and translated), and phase II study (where the plasma will be imploded). The precompression plasma (diameter of only 8–10 cm, length of 30–40 cm) is expected to have n∼1017 cm−3, T∼100–300 eV, B∼5 T, and a lifetime of 10–20 μs. We will use visible laser interferometry across the plasma, along with a series of fiber-optically coupled visible light monitors to determine the plasma density and position. Excluded flux loops will be placed outside the quartz tube of the formation region, but inside of the diameter of the θ-pinch formation coils. Impurity emission in the visible and extreme ultraviolet range will b...

Current delivery and radiation yield in plasma flow switch-driven implosions

Vacuum inductive-store, plasma flow switch-driven implosion experiments have been performed using the Shiva Star capacitor bank (1300 {mu}f, 3 nH, 120 kV, 9.4 MJ). A coaxial plasma gun arrangement is employed to store magnetic energy in the vacuum volume upstream of a dynamic discharge during the 3- to 4-{mu}s rise of current from the capacitor bank. Motion of the discharge off the end of the inner conductor of the gun releases this energy to implode a coaxial cylindrical foil. The implosion loads are 5-cm-radius, 2-cm-long, 200 to 400 {mu}g/cm{sup 2} cylinders of aluminum or aluminized Formvar. With 5 MJ stored initially in the capacitor bank, more than 9 MA are delivered to the implosion load with a rise time of nearly 200 ns. The subsequent implosion results in a radiation output of 0.95 MJ at a power exceeding 5 TW (assuming isotropic emission). Experimental results and related two-dimensional magnetohydrodynamic simulations are discussed. 10 refs., 12 figs.

Multimegajoule Electromagnetic Implosion of Shaped Solid-Density Liners

Electromagnetic implosions of shaped cylindrical aluminum liners that remain at solid density are discussed. The approximate liner parameters have an initial radius of 3 to 4 cm, are 4 cm in height, and are nearly 0.1 cm thick. The liners are driven by the Shiva Star 1300-{mu}f capacitor bank at an 84-kV charging voltage and an nearly 30-nH total initial inductance (including implosion load). The discharge current travels along the length of the liner and rises to 14 MA in nearly 8 {mu}s. The implosion time is nearly 12 {mu}s. Diagnostics include inductive current and capacitive voltage probes, magnetic probes, and radiography. Both right-circular cylinder and conical liner implosion data are displayed and discussed. Radiography indicates implosion behavior substantially consistent with two-dimensional magnetohydrodynamic calculations, which predict inner surface implosion velocities exceeding 20 km/s, and compressed density of two to three times solid density. Less growth of perturbations is evident for the conical liner (nearly 1% thickness tolerance) than for the right-circular cylindrical liner (nearly 3% thickness tolerance). 12 refs., 8 figs.

Puff‐gas coaxial‐injected electromagnetic coaxial plasma gun

We have designed, built, and tested a pulsed gas‐injection coaxial plasma gun driven by a 72‐μF capacitor bank, with operating voltages of 60–80 kV (stored energy 130–230 kJ), and 1.6‐MA, 1‐μs rise‐time current discharges. Using deuterium gas, we have obtained reliable delivery of all current to the gas and neutron yields of greater than 109. Current, voltage, inductance, and current position data are discussed and are compared with circuit solver models. Magnetic‐probe and filtered scintillator photomultiplier detector array data on high‐energy photon spectra were taken. A second puff‐gas valve near the anode (inner electrode) axis at the muzzle end substantially improved the neutron yield.

Operation of cylindrical array of plasma guns

An experiment to combine many moderate-energy plasma gun discharges into one higher-energy discharge is described. Operated in a cylindrical array were 12 to 24 plasma guns with individual currents of up to 300 kA and individual discharge energies of 25 to 80 kJ. They were directed radially inward. They used separate refractory insulators. Reusable operation was achieved at up to a 1-MJ, 3-MA composite discharge level, and fast photography indicated that the separate discharges combined to form a single, symmetric, cylindrically converging discharge.