Obituary of Frederic H. Coensgen

Frederic H. Coensgen, a pioneer of magnetic fusion research, passed away November 18, 2007, age 88, following a short illness.

During a forty-five year career at the Lawrence Livermore National Laboratory, Fred served as group leader and scientific inspiration for a succession of experiments employing the magnetic mirror principle to confine hot plasmas. In the mid 1970 s, he led the 2XIIB experiment that established the principles for stabilizing mirror-confined plasmas that inspired the invention of the tandem mirror at Livermore and Novosibirsk. Based on intense energetic neutral beams to heat the plasma, the launching of 2XIIB in the early 1970’s also led to the development at Lawrence Berkeley Laboratory of the 50 ampere neutral beam sources used to heat the DIIID tokamak at General Atomics and the TFTR at Princeton that achieved the world’s first definitive demonstration of controlled fusion energy in the mid 1990’s.

Fred was a superb experimental physicist, greatly admired by colleagues for his leadership, dry humor and boldness in tackling new technologies needed for progress. Novel magnetic coils to achieve compressional heating of plasmas led to his first success, culminating in the achievement of temperatures of 3.6 KeV in the Toy Top II device, reported at an international conference at Salzburg in 1961. However, in that same experiment it was found that, following compression, the plasma was terminated by MHD modes of instability. Characteristically, Fred took this setback in stride, and moved on by modifying the shape of the magnetic fields to create a stable magnetic well, a new technique demonstrated by the Soviet plasma physicist Ioffe.

By the early 1970 s, Fred again shifted course, to add neutral beam heating to replace compression. This led to the pace-setting 2XIIB experiment mentioned above. Again, technological innovation played a vital role. As the target plasma for injection, 2XIIB employed the washer stack plasma gun previously used as the starting point for compression. Again, there was first a setback with the occurrence of loss cone instabilities peculiar to mirror machines. Then news arrived from Ioffe in Russia that he had also stabilized these modes, using a principle developed theoretically by Post at Livermore, wherein a supply of cold ions partially fills the loss cone. Realizing at once that improving the vacuum removed cold ions, Fred set out to modify his plasma gun to provide a continuous stream of cold plasma to fill the loss cone during neutral beam injection.

The stream-stabilized 2XIIB was an instant success, yielding in 1975 ion temperatures approaching 20 KeV. This was the highest temperature yet achieved in a controlled fusion experiment of any kind, preceding by three years a similar achievement in the PLT tokamak at Princeton, also using neutral beam injection. Even more impressive was the fact that the combination of a magnetic well to stabilize low frequency MHD instabilities and the plasma stream to stabilize loss cone instabilities led to plasmas stable in every sense. It was these results, together with an analysis of how the stream gun worked, that led Dimov in Novosibirsk and, independently, Fowler and Logan in Livermore, to propose the tandem mirror. By 1976, the remarkable stability of the 2XIIB plasma led to the achievement of plasma pressures in the magnetic well at the ultimate theoretical limit with plasma pressure comparable to magnetic pressure. This inspired a different idea, called the Field Reversed Mirror.

In the 1970’s and 1980 s, the U. S. magnetic fusion program exploited successes in 2XIIB to pursue Field Reversed Mirrors in a device called Beta II, and a succession of Tandem Mirror devices beginning with the highly successful TMX experiment at Livermore and the TARA at MIT; the TMX-U employing thermal barriers to enhance confinement; and the large MFTF-B tandem mirror completed in 1986. Coensgen was physics leader for all of those projects that were carried out at Livermore. Though mirror experiments on this scale are no longer pursued in the U. S., tandem mirror programs in Japan and Russia continue Coensgen’s legacy.

Fred Coensgen was born in Great Falls, Montana, February 10, 1919. He received his Ph.D. in physics from the University of California, Berkeley. He spent his entire career at the Lawrence Livermore National Laboratory, retiring in 1994. After retirement, he became an accomplished photographer, and enjoyed hiking and family. He is survived by his wife Charlene of Pleasanton, California, 6 children and 9 grandchildren.