Lasers with Nuclear Pumping

This section briefly examines the last 50 years of research in the field of transformation of the kinetic energy of products of nuclear reactions into laser radiation, that is, nuclear-pumped lasers (NPL). NPLs include lasers in the optical spectral range, which are excited directly by nuclear radiation or by using intermediate devices, for example, nuclear-optical converters (nuclear-excited plasma, scintillators).

Virtually all of the experimental investigations to seek and study the characteristics of NPLs excited by the products of nuclear reactions were carried out on pulsed nuclear reactors. Pulsed nuclear reactors are distinguished for the composition and structure of their core, the duration and fluence of the neutron pulse, the volume and configuration of the space for the irradiation, and the pulse repetition frequency. Some aspects of the use of pulsed reactors for laser pumping were examined in the survey [1]. This section uses some of the data from study [1], supplemented by information from other sources.

Presently, known gas NPLs radiate in a wide band of the optical spectrum of 390–5,600 nm using approximately 50 transitions of atoms Xe, Kr, Ar, Ne, C, N, Cl, O, Hg, Cd, and I; the ions Cd⁺, Zn⁺, and Hg⁺; the molecule CO; and the molecular ion N ₂ ⁺ .

At present, direct nuclear pumping is implemented for gas laser media in which the populating of lasing levels occurs as a result of processes occurring in a high-pressure low-temperature plasma formed by ionizing radiation. Sometimes this plasma is called nuclear-excited plasma.

This chapter examines the kinetics of plasmochemical processes and the lasing mechanisms of various types of gas NPLs pumped using neutrons from pulsed nuclear reactors. The lasers considered here typically have Continuous Wave, i.e. CW operation (or, more accurately, quasi-CW) lasing mechanisms, because the characteristic times of basic plasma processes are much less than the durations of pulsed reactor pumping pulses (≥50 μs). To determine the lasing mechanism, it is necessary to ascertain the processes of populating and quenching both of the upper and of the lower lasing levels. Information about the specific features and the constants of the most important plasmochemical processes is contained in a number of studies (see [1–4], for example).

In Chap. 2 we examined rather simple NPL designs for searching for and studying characteristics of laser active media, but there are also more complex nuclear laser devices. As a rule, they are structural elements of multichannel reactor lasers operating in stationary or pulsed modes, and are intended to study the specifics of their operation. This chapter examines some of these devices, whether in operation or still being designed.

As was noted in Sect. 1.2 of Chap. 1, the nuclear reactions that can be used for pumping gas NPLs in experiments with pulsed reactors are: When reaction (7.1) is used, the laser medium is excited by protons and tritium ions. In this case, ³He plays the role of a buffer component of the laser mixture and is simultaneously a volume pumping source. Calculations of the specific energy deposition to the gas medium when the reaction (7.1) is used were carried out in the studies [1, 2]. The gas pressure and the transverse dimension of the laser cell were varied (it was assumed that the length of the cell greatly exceeded its transverse dimension). It was shown that for each pressure value there is an optimal transverse dimension of the cell. When the dimensions are less than optimal, the losses of reaction (7.1) products on the walls of the cell become great. When the dimensions are greater than optimal, the decrease of the neutron flux in the direction from the cell boundary to the depth of the gas volume starts to have an effect. Some results of calculations are provided in Figs. 7.1 and 7.2. It should be noted that the model [2] makes it possible to perform calculations for different neutron spectra when more precise deceleration principles of charged particles are used. The results of the calculations in studies [1, 2] for the case of a mono-energetic flux of thermal neutrons differ by 10–15 % (Fig. 7.1).

The short range of fission fragments causes a specific energy input inhomogeneities which, along with excited gas heat exchange with laser cell walls, leads to density spatial redistribution, i.e., to the violation of active medium optical homogeneity. As noted in paper [1], with a reference to the 1980 report of A. N. Sizov and Yu. N. Deryugin, numerical and theoretical investigations of the dynamics of optical inhomogeneity origination and development in nuclear pumping lasers first began at the VNIIEF. During phase one, these investigations only pertained to the phenomena occurring in sealed lasers excited by pulsed neutron fluxes. This important aspect of NPLs is discussed in this chapter.

Investigations of optical inhomogeneities in sealed NPLs have revealed that flowing of the gas medium through the laser cavity is a necessary condition for achieving CW lasing in the stationary and quasi-stationary excitation modes. Gas heating is a no less important factor that requires the implementation of gas flowing.

Chapter 6 gives a brief overview of designs for nuclear laser setups with a pulse duration of ~10 ms (OKUYAN and LIRA setups). Their operation is based on the “initiating reactor–subcritical laser block” scheme. The literature has repeatedly suggested different design schemes for stationary nuclear laser facilities or reactor-laser (RL) based on gas laser media. The term RL was apparently first proposed in article [1], where the potential of removing energy in the form of laser radiation from the reactor core during the relaxation stage was noted in principle. The most interesting version of the RL is a device where, as a consequence of spatial overlapping of the nuclear fuel and laser medium, a significant portion of the nuclear energy is converted into laser radiation bypassing the intermediate thermal energy stage.

The most progress in searching for active media for NPLs, studying their characteristics, and developing various nuclear-laser devices was achieved when high-pressure gas media were used. However, there have been repeated attempts in Russia and the United States to pump different condensed media with nuclear radiation and carry out feasibility studies on developing powerful nuclear-laser devices based on these media.

In previous chapters, we have reviewed pumping methods for gas-medium and condensed-medium lasers using nuclear reactors as neutron sources. We also reviewed the characteristics of these lasers and the potential to use them in different nuclear-laser devices. The highest specific power depositions for gas media (q ≤ 5 kW/cm³) can be obtained if a laser cell that has a uranium layer or is filled with ³Не is placed inside the core of a pulsed reactor at the minimal possible pulse length for pulsed reactors, ~100 μs. However, these specific power depositions are sometimes not sufficient to reach the lasing threshold of certain active media.

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.