Direct nuclear pumped (DNP) lasers

LASERS 1 magnitude relative to longitudinally excited HCN lasers of similar size. The energy per pulse is 1 - 1 0 mJ which m a y serve for comparison with the optically p u m p e d CH3F lasers. F u r t h e r i m p r o v e m e n t of our laser is in progress.

B5

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References [1] B. Adam, F. Kneubiihl, Appl. Physics 8 (1975) 2 8 1 - 2 9 1 . [2] E.A.J. Marcatili, R.A. Schmeltzer, Bell Syst. Tech. J. 43 (1964) 1783. [31 H. Steffen, F. KneubiJhl, 1EEE J. Quant. Electr. QE-4 (1968) 992. [4] P.W. Smith, Appl. Phys. Letters 19 (1971) 132.

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DIRECT N U C L E A R PUMPED (DNP) LASERS* G.It. MILEY and W.E. WELLS

Nuclear Engineering Program, University o f lllinois, Urbana, Illinois 61801, USA Recently three laboratories have produced DNP lasers [ 1 - 4 ] . These evidences and the available n e u t r o n source technology which can provide tens of megajoules of p u m p i n g energy have d e m o n s t r a t e d the feasibility of DNP lasers and suggest the possibility of radiation-induced, large-volume, high-power lasers. Thc awaiting applications [5,6] of such lasers to laserfusion, laser-isotope separation, etc., indicate that the physics of the radiation induced plasma will become a rapidly developing new field of research in gaseous electronics. DNP lasing or gain been d e m o n s t r a t e d in He-N2-CO (5.6 ~, H e - X e (3.5 u), N e - N 2 (0.8629 ~z and 0.9393 ~) and H e - N e 0 2 (0.8446 ~z). Detailed modeling of these lasers are still incomplete, however studies [7,8] o f the resulting electron distribution functions created in the radiation induced plasma provides a great deal of insight into the plasma kinetics involved. These studies have shown non-maxwellian distribution functions which can be characterized by a bulk of lowenergy quasi-maxwellian electrons with a high-energy tail extending to keV energies. This type o f distribution has several advantages over a maxwellian (or Druyvesteyn) for producing population inversions. The high-energy tail produces excitation and ionization which exceeds by several orders o f magnitude that produced by the equivalent energy maxwellian. Because o f the bulk of low energy electrons, processes such as recombination also proceed at significantly higher rates. The resulting plasma can be characterized as a sustained afterglow. If the recombination process populates an upper level of a laser transition, the inherently large excitation and loss rates can produce large population rates for the laser, resulting in increased p u m p i n g efficiencies. In addition to the rec o m b i n a t i o n lasers, the possible development o f DNP-type charge exchange and excimer lasers are under extensive in-

*ThisresearchsponseredbytheDivisionofPhysicalResearch,

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Fig. 1. Lasing o u t p u t at 8629 and 9393 A. The neutron pulse and total s p o n t a n e o u s o u t p u t are also shown. vestigation because high efficiencies are also indicated for these. These effects are d e m o n s t r a t e d in a new neutron-driven laser that utilizes a boron-10 coating, N e - N 2 gas, and provides simultaneous o u t p u t at 8629 and 9393 A in nitrogen. This laser has several distinctive features; namely, the n e u t r o n flux, i.e., power input at threshold requirement is the lowest reported to date for nuclear p u m p i n g and the wavelengths fall into an attractive region in the near infrared. The experiment employed a 2.5-cm i.d. quartz laser tube containing a 68-cm section of a l u m i n u m tubing, coated on the inner surthce with boron-10. The cell was placed next to the core of the Illinois T R I G A reactor such that thermal n e u t r o n s bombarding the boron coating caused emission of MeV ~ and Li-ions. These ions, in turn ionized and excited the lasing gas. Lasing occurred at pressures from 75 to 350-torr Ne with partial pressures of N 2 from ~ 4 - 1 0 mtorr. Fig. 1 is a d e m o n s t r a t i o n of the lasing o u t p u t with 150 torr of N e - N 2. The laser cavity was chopped by m e a n s of a small electric m o t o r with fan blades, which alternatively blocked and unblocked the back mirror. The large change in unblocked to blocked signal, a m o n g other things, clearly verifies that lasing occurred. The lasing threshold took place at 1.0 X 1015 neutrons/cm2-sec, the lowest threshold of any nuclear p u m p e d laser to date. Since the n e u t r o n pulse width is m u c h longer than the atomic and molecular processes taking place in the gas, this can be viewed as a near steady-state laser. The peak total laser o u t p u t was 1.5 mW (2.4 × 10 - 5 J / l ) at 150 torr Ne with ~ 4 m t o r r N 2. The n e u t r o n interaction physics will be discussed and electron distribution functions for helium mixtures given, as well as comparisons to the equivalent maxweUian distributions for the processes of ionization and recombination. T h e modeling status of the N e - N 2 and H e - N e - 0 2 lasers will be presented in detail. In addition the predictions of new lasers based on the distribution function analysis will be discussed.

ERDA. 13

Volume 18, n u m b e r 1

OP'[ICS COMMUNICATIONS

References [ 1 ] D.A. McArthur and P.B. Tollefsrud, Appl. Phys. Ltrs, 26 (1975) 187. [2] H.H. Helmick, J. Fuller and R.T. Schneider, Appl. Phys. Ltrs. 26 (1975) 327. [3] R.J. DeYoung, W.E. Wells, J.T. Verdeyen and G.H. Miley, IEEE/OSA CLEA 13.A10, Washington, D.C. (May 1975). [4] R.J. DeYoung, M.A. Akerman, W.E. Wells and G.H. Miley, 2nd IEEE Conf. Plasma Science, 75CH0987-8-NPS, (May 1975). [5] G.H. Miley Trans. Am. Nuclear Soc. 15 (1972) 633. [6] W.E. Wells, 2nd IEEE Conf. Plasma Science, 75CH09878-NPS (May 1975). [7] R.H. Lo and G.H. Miley, IEEE Trans. Plasma Sci. PS-2 (1974) 198. [ 8] G.H. Miley, J.T. Verdeyen and W.E. Wells, ERDA Rept. No. COO-2007-63, (1975).

B6

N U C L E A R FISSION F R A G M E N T EXCITED LASER SYSTEMS D.C. LORENTS and C.K. RHODES

Molecular Physics Center, Stanford Research Institute, Menlo Park, California 94025, USA Optical fission fragment excited laser systems require efficient coupling of the fission fragment energy to the laser m e d i u m . The basic fission process of n+ 235U~ Z 1 +Z 2

(1)

releases ~ 1 7 5 MeV of kinetic energy in typical fragments for which Z 1 ~ 90 and Z 2 = 140. For thermal neutrons, the cross section a f - 580 barns. These fission fragments degrade energetically by a multiplicity of processes including charge exchange, collisional stripping, and direct ionization, all o f which lead to ionization o f the host material. Calculations of Leffert et al. [1] indicate that to a first a p p r o x i m a t i o n the fraction of fission fragment energy that generates ionization and excitation is similar (~50%) to that of energetic electrons. This is reasonable physically because m o s t of the ionization and excitation in b o t h cases is produced by secondary electrons. Therefore, we can apply the knowledge gained recently from energy transfer studies of e-beam excited rare gases and rare gas mixtures. In general we desire a m e d i u m whose properties are such that it (1) efficiently couples the upper laser level to the primary ionization generated in the m e d i u m by the fission fragments, and (2) is fundamentally insensitive to the kinetic temperature o f t h e m e d i u m . We observe that losses producing a rise in the temperature o f the m e d i u m often establish a fundamental limitation on the specific energy o u t p u t (joules/liter) of a given system. Therefore, from the outset it is desirable to minimize the kinetic losses through efficient eoupling while simultaneously utilizing physical m e c h a n i s m s that are basical-

14

,luly 1976

ly insensitive to the kinetic temperature. The main limitations on the gain m e d i u m include gas heating, optical absorption of ambient species, and m e d i u m homogeniety. The heating limitation relates the deposited energy Ed, the optical efficiency n, the m e d i u m density p, and the m a x i m u m temperature rise ol the medium /Xl'g by the expression E d ~<

3pk A Tg 2(1-7)

(2)

This relationship can be used to establish a reasonable limit on the energy deposition in the m e d i u m . For example, we assume ATg ~< 1000 K and a host material consisting of argon at a density PAr ~- ( ~ 5 0 atm). These conditions permit an energy deposition E d of about 2 X 104 joules/liter which for an efficiency ~ ~ 0.05 yields t03 J/liter of laser energy. We can now estimate the density of 235U (possibly present in the form of UF 6) required to yield this deposition with the neutron pulse from a m o d e r n pulsed reactor. Assuming a n e u t r o n cross section of 5.8 X 10 - 2 2 cm - 2 and a n e u t r o n flux of 6 X 1019 n t / c m 2 sec in a pulse of ~ 5 0 usec, we find the required Ut: 6 density is ~ 2 X 1018 cm - 3 . We note additionally that the low n e u t r o n cross section of argon, ~ 0 . 8 barns, m e a n s that the n e u t r o n transport is not appreciably affected by the high density of argon present. Recent data of DePoorter and Rofer-DePoorter [2] d e m o n strate that optical absorption of UF 6 may be a d o m i n a n t factor for wavelengths below 3000 A and that the m o s t favorable regions are at ~ 3 4 0 0 A and > 4100 A. Two attractive candidates in the 3400 A region are 12 and XeF, both of which have exhibited stimulated emission [3,4] under electron beam excitation. For the latter, mixtures of Ar/Xe/UF 6 involving the kinetic chain. Ar~ + Xe ~, 2Ar + Xe*,

(3)

Xe* + UF 6 ~ XeF* + U F s ,

(4)

could efficiently generate XeF* w i t h o u t the need for additional additives. An evaluation of specific systems suitable for excitation by pulsed reactors including excitation mechanisms, optical absorption, and data on kinetic processes will be given. Particular emphasis will be accorded to kinetic limitations and the behavior of u r a n i u m bearing c o m p o u n d s under conditions of high excitation levels. References [ 1 ] C.B. Leffert, D.B. Rees and F.E. l a m e r s o n , J. Appt. Phys. 37 (1966) 133. [2 ] G.L. DePoorter and C.K. Rofer-DePoorter, LA-UR-75-792, Los Alamos Sci. Lab., Los Alamos, New Mexico (1975). [3] M.V. McCusker, R.M. Hill, D.L. Huestis, D.C. Lorents, R.A. Gutcheck and H.H. Nakano, Appl. Phys. Lett. 27 (1975) 363; J.J. Ewing and C.A. Brau, Appl. Phys. Lett. (to be published); E.R. Ault, R.A. Bradford, Jr., and M.L. Bhaumik, Appl. Phys. Lett. (to be published).