I I £ S" 00 ?^ I RT/TIB/84/39 T. LETARDt, A. RENIERI THE LIGHT FOR THE FUTURE: EXCIMER AND FREE-ELECTRON LASERS COMITATO NAZIONALE PER LA RICERCA E PER LO SVILUPPO DELL'ENERGIA NUCLEARE E DELLE ENERGIE ALTERNATIVE COMITATO NAZIONALE PER LA RICERCA E PER LO SVILUPPO DELL'ENERGIA NUCLEARE E DELLE ENERGIE ALTERNATIVE THE LIGHT FOR THE FUTURE: =><* AND FREE-ELECTRON LASERS T. LETARDI, A. REHIERI Dipartimento TIB, Divisioue Fisica Applicata, C.R.E. Frascati g^CA;-. mYTIB/84/39 Testo pervenuto in dicembre 1984 This report has been prepared by: Servizio Studi e Docuaientazione • ENEA, Centro Ricerrhe Energia Fraacati, C.P. 65 - 00044 Frascati, Hone, Italy. Thia Office vili be glad to aend further copiea of thia report on requeat. The technical and scientific contents of theae reporta expreaa the opinioni o.' the authora but not necessarily "lose of ENEA Sjhmittrd to New Technologies in Neurosurgery ABSTRACT The status of art of two interesting kind of coherent radia­ tion sources (excimer and free-electron lasers) is described. The working principles of exciser lasers is analyzed together with the most relevant characteristics (active media, operating wave­ length, average and peak power, spectrum and time structure). As to the free-electron laser, whose development is at the very first stage, theoretical and experimental aspects are described and perspectives for the future are discussed. RIASSUNTO Viene descritto lo stato dell'arte di due tipi di sorgente laser che grande interesse hanno sollevato in questi ultimi anni, il laser ad eccimeri ed il laser ad ad elettroni liberi. Per il laser ad eccimeri, assieme a principi di funzionamento, vengono descritte le più importanti caratteristiche (mezzi attivi, lun­ ghezze d'onda di emissione, potenza media e di picco, caratteri­ stiche spettrali e temporali). Per quanto riguarda il laser ad elettroni liberi, il cui sviluppo è ancora ai primissimi stadi, vengono esaminate in un certo dettaglio, assieme agli aspetti teorici e sperimentali, le prospettive di sviluppo futuro. 1 I. INTRODUCTION In these last years a great effort has been done in order to develop new tunable, powerful and efficient laser sources operat­ ing both in the infrared (IR) and ultraviolet (UV) and vacuus ultraviolet (VUV) spectral regions. Two different laser schemes appear now as the best candi­ dates for the realization of these sources. Namely, for the short wavelength region (UV and VUV), the "excimer lasers", while, for IR (and hopefully, in the future, UV, VUV and X too), the "free- -electron lasers" (FEL). In this article we briefly describe the operating principles and the state of the art of these sources. Namely Sect. II is de­ voted to exciaer lasers, whose technology is now at a noticeable level of development, while theoretical and experimental aspects of FEL devices (which are at the very first stage of investiga­ tion, with only six operating sources in all the world!) are discussed in Sect. III. The list of the symbols more frequently utilized throughout the text is reported in Table 1. II. EXCIMER LASERS: INTRODUCTION The existence of molecular systems which are bound in the excited state and dissociated in the ground state was observed for the first time by Lord Rayle-igh [1], while Stevens and Hutton [2] suggested the term "excimer" to distinguish them from normal excited states, the importance of these molecular systems as laser media is evident: being the gain G = a (n2 - nj) propor­ tional to the difference between the upper state population n2 and the lower one nlf in these systems it reaches the maximum value, because it is always nj = 0. In the case of a molecular system formed by two atoms, the curves of potential energy vs interatomic distance will have the general form shown in Fig. 1, where the curve 1 refers to the ground state, and the curve 2 refers to the electronically ex- ? cited state. The transition energy AE, which defines the emission wavelength, is in the region of few e.V. (A from visible to ul­ traviolet). Experimental evidence of laser emission from excimer systems was observed for the first time in pure noble gas by Basov et al. [3]. In the following time, other important excimer systems were made to lase, as the Rare Gas Halogen (RGH) excimers (Searles et al. [4]), and the metal vapor lasers (Bazhulin et al. [5], Eden [6]). In the following, the general working scheme of these sys­ tems will be shown, comprising the excitation methods, than the main characteristics (wavelength, efficiency, power and energy per pulse) will be detailed, and finally particular methods to extend the capabilities of excimer lasers (wavelength, peak power, pulse length) will be described. II.1 - Pumping Requirements In laser systems, very high pumping levels are required as far as the emission wavelength is decreased. Indeed (Hutchinson [7]) the stimulated emission cross section can be written as (for the symbols see Table 1) 8nxAAc' where T is the lifetime of the excited state and AA is the emis­ sion bandwidth. Taking into account that the upper level mean population, in stationary conditions, is linked to the pump power P by n2 Of P T, we can conclude that the pumping power density, for gain thre­ shold conditions, scales as A-5. High power pump density can be achieved by means of particle beams, usually electrons, but also protons (Baranov et al. [8]), 3 by fast electric discharges, after U.V., X-ray or electron beam preionizat'.on, and also by means of microwave devices (Mendelsohn et al. [9]). Even if in these systems the dissociation of the lower level gets the condition An = n2 - nx > 0 always true, other mecha­ nisms, such as instabilities of the discharge, heating of the medium, growing of absorbing species, do not allow the achieving of laser pulses longer than ~ 1 usee, while in most cases only few tens of nanoseconds are achieved. II.2 - Lasing Systems Excimer molecules, ccndidates as laser media, form a very large family (Rhodes [10]) which span in wavelength from VUV to visible, but not all have succeeded to lase, and only the class of R.G.H. excimer lasers has, up to cow, reached the stage of commercial development. Anyway it is worthwhile to mention the class of pure noble gas excimer lasers, which have the shortest wavelength. Laser emission has been observed in Xe (Koehler et al. [il]) (A = 172 run), Krypton (Hoff et al. [12]) (X = 146 nm), Argon (Hughes et al. [13]) (X = 126 nm). The very high pumping power needed, due to the very short wavelength, can be achieved only by means of electron beams. Indeed laser pulses of 10 J, 400 MW have been ob­ tained (Ault et al. [14]), using Xe. Anyway, the complexity of the electron pumping apparatus limits the development of this class of lasers to laboratory systems. Only recently, in the class of metal vapors excimers, the systems operating with HgBr have reached very actractive performances. The laser wavelength is 502 nm, and fast electric discharge, after UV or X-ray preionization, can be used as pump­ ing system. High energy per pulse (3.2 J) and high overall effi­ ciency (2%) have been obtained (Fisher et al. [15]). The R.G.H. excimer lasers use, as active medium, an excited molecule (RgH) formed by a noble gas atom Rg (Ar, Kr, Xe) and a halogen atom H (F, CI, Br). As general scheme, few torrs of the 4 two species Rg and H are nixed together, and some ataospheres of a noble gas (Ar, Ne) are added to absorb the energy froa the puaping systea and trasfer it to the reacting species. Different puaping devices can be used: electron beans, proton beans, dis­ charge sustained by electron beans, self-sustained discharge after UV or X-ray preionization, microwave devices. In Table 2 it is shown the RGH systens in which laser action has been ob­ served, with the corresponding wavelength in nn (Lakoba and Yakovlenko [16]). If we define the intrinsic efficiency as the laser pulse energy divided by the energy deposited in the gas, the highest value (14%) has been obtained with KrF (Salesky and Kinura [17]) while with XeCl it has been obtained 5% (Chaapagne [18]) in both cases in electron-bean punped systens. The overall efficiency, it is laser pulse energy divided by the energy furnished by the power supply for the pump system (be it a discharge or an elec­ tron bean) is always lower, because it takes into account the transfer efficiency and the coupling to the gas systen. Anyway, very recently (Long et al. [19]) discharge systems for XeCl have been developed in which an overall efficiency of 4%, very close to intrinsic efficiency, has been obtained. The maximum values of energy per pulse have been achieved in electron-beam pumped systems: 850 J for KrF (Goldhar et al. [20]) and 100 J for XeCl (Baranov et al. [21]). The scalability for this configuration has been confirmed. Self-sustained discharge systems have volume instabilities which limit the scalability. Only for XeCl output energy of 42 J per pulse have been obtained (Guallorenzi and HcHahon [22]) in a self-sustained discharge system, while a much lower energy (~ 5 J/pulse) has been obtained (Watanabe and Endoh [23]) with KrF. The laser energy which can be extracted from a given active volume (laser energy density) depends, also for a given type of laser, from the pumping method. For electron beam pumping, 40 J/liter (Tisone et al. [24]) for KrF and 9 J/liter (Tisone and Hoffman [25]) for XeCl have been extracted from the active volu- 5 ce, while in self-sustained discharge systems 4 J/liter is the maximum value for XeCl and 5 J/liter for KrF (Baranov [26]).