PHERMEX

PHERMEX. PHERMEX (Pulsed High Energy Radiographic Machine Emitting X-rays) was a pioneering linear induction accelerator developed at Los Alamos National Laboratory in New Mexico. Operational from 1963, it was a cornerstone of the United States nuclear weapons program during the Cold War, designed to produce intense, short pulses of X-rays for hydrodynamic testing. The machine provided crucial data on the behavior of materials under extreme conditions, supporting the science-based Stockpile Stewardship Program without requiring full-scale nuclear testing.

Overview

PHERMEX represented a significant technological advancement in the field of flash radiography, enabling the study of fast, dense events. It was conceived and built by a team at Los Alamos National Laboratory, a key facility managed by the University of California for the Atomic Energy Commission. The primary mission was to support the United States Department of Energy in understanding the implosion dynamics of nuclear weapon primaries. By generating a high-energy electron beam focused onto a tantalum target, PHERMEX produced a brilliant X-ray source that could penetrate dense materials, capturing radiographic "snapshots" of experiments conducted at the nearby Nevada Test Site.

Design and Operation

The core technology of PHERMEX was its linear induction accelerator design, which differed from conventional radio frequency accelerators like those at SLAC National Accelerator Laboratory. It operated by passing a high-current electron beam through a series of ferrite-cored induction cells, each adding a pulsed acceleration voltage. This design allowed for exceptionally high beam currents—up to 2,000 amperes—at a final energy of approximately 30 megaelectronvolts. The beam was then focused onto a small, high-atomic number target, typically made of tantalum or tungsten, generating a point source of Bremsstrahlung X-rays. Key diagnostic systems, such as scintillator screens and image intensifiers, recorded the resulting shadowgraph images.

Applications and Experiments

PHERMEX was integral to the Subcritical experiment and hydrodynamic testing campaigns conducted under the umbrella of the Stockpile Stewardship Program. Its primary application was radiographing the implosion of mock nuclear weapon components, often using materials like uranium, plutonium, and high explosives such as HMX. Experiments provided vital data on equation of state, material strength, and shock wave propagation. The machine also supported broader research in high-energy-density physics, contributing to studies relevant to inertial confinement fusion programs like Nova at Lawrence Livermore National Laboratory and astrophysical phenomena modeled at the Sandia National Laboratories Z Pulsed Power Facility.

Historical Context and Development

The development of PHERMEX was driven by the geopolitical pressures of the Cold War and the 1963 signing of the Partial Nuclear Test Ban Treaty, which limited atmospheric testing. To maintain the reliability of the United States nuclear arsenal, scientists like those at Los Alamos National Laboratory required new tools for subsurface investigation. PHERMEX was conceived in the late 1950s, with construction led by figures such as Nicolas Christofilos, who also contributed to the Astron (device) project. It became operational in 1963, preceding other major radiographic facilities like Cyclotron-based systems at the Lawrence Berkeley National Laboratory and the later Dual-Axis Radiographic Hydrodynamic Test Facility (DARHT) at Los Alamos.

Technical Specifications

PHERMEX achieved a peak electron energy of 30 megaelectronvolts with a beam current of approximately 2,000 amperes. The accelerator was roughly 30 meters in length, consisting of over 100 induction cells. It produced X-ray pulses with durations on the order of 60 nanoseconds, enabling the capture of multiple radiographic frames per experiment through sophisticated timing systems. The resulting X-ray dose at one meter from the target was estimated in the range of several hundred roentgens, providing sufficient penetration for dense materials. Its resolution capabilities were critical for imaging features in experiments conducted for the Joint Actinide Shock Physics Experimental Research (JASPER) facility at the Nevada National Security Site.

Category:Particle accelerators Category:Los Alamos National Laboratory Category:Nuclear weapons of the United States Category:Cold War military equipment of the United States