13.1 Introduction

This book primarily discusses NPL research in Russia. The Russian program has been larger and remains active whereas work in the United States and elsewhere has dwindled. Shortly after the end of the Cold War, two circumstances combined to cause stoppage of NPL work. While the military need ceased, public opposition to nuclear power mounted following the Three Mile Island Nuclear reactor accident in the United States. Such concerns clearly carried over to a reactor pumped laser, i. e. to NPLs. Here we briefly review earlier work is the United States, prior to a build-up of effort as part of the “Star Wars” program as cold war tension grew. For completeness, the earlier history of NPL research in the United States is briefly reviewed here, largely adapted from earlier review articles [1]. Additionally, some content also appears in another previous review of NPLs [2] while some new material has been added, particularly concerning theory.

NPLs are driven by the reaction products of fission, fusion, or radioisotopes, providing the direct conversion of nuclear energy to directed optical energy. Aside from scientific aspects, research on NPLs was initially motivated because it has the potential to introduce many exciting new applications for fission reactors. This is especially true for systems where a nuclear energy source is a logical choice. The large volume made possible by using neutron excitation leads to ultra-large, steady — state power or intense pulsed-energy outputs that cannot be matched by other laser systems. Because nuclear energy is directly converted to excited states and subse­quently to lasing, a relatively high efficiency (for traditional lasers) is achieved. Furthermore, the integrated reactor-laser system can be compact because no inter­mediate conversion equipment is needed. Consequently, researchers interested in military type laser began to take note of NPLs and a classified research program was mounted.

Chapter 1 of this book reviews work in the United States quite well from the Russian point of view, and this chapter provides a brief review from the American

© Springer Science+Business Media New York 2015 S. P. Melnikov et al., Lasers with Nuclear Pumping, DOI 10.1007/978-3-319-08882-2_13

perspective as I (George H. Miley) experienced it. Due to limitations on length, this chapter cannot do justice to the major effort done in the reactors at Sandia National Laboratory and earlier work at the Idaho National Laboratory [3—9]. These projects originated by Victor George at Lawrence Livermore National Laboratory under “Star Wars” funding and were classified (interested readers may now be able to find declassified versions of some reports). It appears that the project was aimed at KW-level demonstration results when funding stopped at the end of the Cold War. Thus, this chapter might be viewed as one from the perspective of academia. Admittedly, it is also weighted toward my work at the University of Illinois that my students, and former colleagues were involved in.

Reflections on early work at Illinois are briefly noted here to bring out some of the thinking and problems faced by early NPL researchers. In addition to my early work, other early U. S. pioneers in this field included Lloyd Herwig, David McArthur, Richard Schneider, Philip J. Ebert, Herbert Helmick, and Joe Verdeyen. Indeed, follow-on work by researchers noted in this chapter has been in a sense equally pioneering in this new unexplored field.

My experiments were uniquely enabled by access to the pulsed TRIGA reactor at the University of Illinois. These studies were enhanced by special facilities at the Illinois TRIGA reactor—a graphite thermal column, a horizontal “through-port,” and a vertical access to the center core region. All three were used to great advantage at various times. Further, because I had earned a TRIGA reactor operator’s license from the NRC, we could schedule laser experiments at conve­nient times, often at nights when the lab was relatively free from complications presented when other experiments were running. Also at these times the reactor experimental bay could be darkened to aid visual observations of the optics involved.

My interest in the field of NPLs began when I attended a faculty summer school at the Idaho National Reactor site in 1963. This frequently involved a one-hour bus ride out in the desert to the historic Fast Breeder Reactor Experiment. To pass the time, I read a book about the invention of the laser, which was discovered only a few years earlier, which inspired the thought, “Why not pump the laser with nuclear energy instead of electrical?” I then mastered the physics of laser threshold calcu­lations from the book, and decided that the CO2 laser with its long wavelength in the IR, hence low threshold, would be ideal for nuclear pumping. When I returned to the University of Illinois that fall, I discussed this with Prof. Joe Verdeyen, a well — known laser researcher in the Gaseous Electronics lab at the University of Illinois. Verdeyen had already done some pioneering work on the Transversely Excited Atmospheric Pressure (TEA) CO2 Laser, and was extremely knowledgeable (he later went on to write a classic textbook on lasers, Laser Electronics, now in its third edition). He agreed with my assessment that CO2 could easily be pumped in the TRIGA reactor, even with the low neutron fluxes available with steady state operation giving neutron fluxes of ~1014 n/cm2 s. Pulsing would be “overkill.” A boron coating on the laser tube would generate alpha particles from neutron-alpha reactions, and the charged alphas entering the CO2 gas would deposit the energy needed for pumping the laser. Thus I teamed up with Verdeyen, along with several

students, and began experiments. Unfortunately, this led to several years of frustration.

Though I initially thought I had invented the idea on that Idaho bus, I soon discovered that Lloyd Herwig at Northrop Grumman in California had already proposed the concept and had actually done experiments. It is hard to be first in this age of fast-moving technology! More importantly, the University of Illinois research team slowly learned two physics phenomena that represented significant challenges to the research. First, the nuclear radiations dissociated CO2 molecules and other molecules involved in the lasing reaction. Because that effect was not included in my threshold calculations, it threw everything off. (Indeed, this has prevented nuclear pumping of CO2 to this day. Amazingly, David MacArthur did succeed with CO, and that was the first experimental NPL in the United States.)

Next Illinois researchers discovered a new problem. The alpha heating of the gas gave a radial temperature profile that peaked near the wall, causing a thermal lensing effect that defocused the internal beams and threw the cavity optics off. Indeed, when using pulses, the effect was dynamic, such that the beam deflection was large and varied with time during the pulse. The effect was vividly illustrated when a cylindrical tube open to the air but with a boron coating on the tube’s interior wall was used one night. A reference He-Ne laser beam was set up to pass through the boron coated tube located in the through beam port and projected on the building wall about 30 feet away. During the reactor pulse, the beam underwent huge spiral-circular gyrations on the wall, ultimately settling back to its original steady-state position. The effect of the gas lenses was thus abundantly obvious. The question was what to do about it to avoid this effect from spoiling the gain in the NPL cavity? Verdeyen proposed an over-focused cavity design (focal spots for internal beams were located at a point behind the cavity mirrors). That seemed to overcome the problem, although not without sacrificing some gain. (This effect was later studied in some detail by Maria Petra in her thesis work at Illinois, as discussed later.)

With this new knowledge my team continued trying to obtain lasing with CO2, in desperation going to yet higher pulse sizes. Still, all attempts failed due to the dissociation effect already noted. We did achieve and publish papers on a radiation- enhanced, electrically pumped TEA CO2 laser, i. e., this electrically driven laser output was increased by pulsing a version using a boron-coated tube. The effect could be attributed to the increased electrical conductivity created by the auxiliary ionization from the alpha particles. However, this was not the “pure” NPL we desired. So we next turned to study a possible He-Ne laser. The logic was that this medium would be less prone to dissociation interference, and again approximate threshold calculations predicted lasing could be achieved with TRIGA reactor pulses, maybe even with steady state operation. However, after over a year of frustrating results, it became evident that once again the threshold calculations were off, and achieving threshold was exceedingly challenging.

The reader may think that progress with NPL research was very slow. In contrast, research elsewhere on electrically pumped laser was making great strides. To understand the differences it must be realized that reactor-pumped laser experiments are quite different from normal lab bench laser research typical of electrically pumped lasers. The NPL is remotely located inside a tube deep within the reactor, making alignment adjustments difficult. Only a limited number of pulses could be obtained per day, and the reactor experiment schedule might only provide a limited number of days a month. Laser cavity components become activated, preventing immediate access. Also, radiation can damage optical compo­nents and detectors. Indeed, shielding is required around the detector array and in addition the laser beam must enter into the detector array without a straight path that radiation could follow. Thus, reflectors are needed to form a zigzag route. After a steep learning curve, my research group (and others elsewhere working in this field) became adept at overcoming these many obstacles. While their knowledge and technique had grown significantly, they had not yet achieved a NPL when the CO NPL was achieved by David MacArthur at Sandia National Laboratory (discussed later in this chapter). Despite this frustration, my group returned to work, even more determined to find new types of NPLs. The CO achievement was truly monumental, but due to the cooling requirements, this did not appear to be a practical result. New types of electrically pumped lasers were being reported quite frequently in those days, so why should not a variety of NPLs be found? This turned out to be the case, and, as seen from the following discussion, the University of Illinois was prolific in discovering new types of NPLs.