E-beam-induced absorption in various grades of quartz

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1 E-beam-induced absorption in various grades of quartz P. B. Sergeev*', I.I.Cheremisin', TA. Ermolenkob, I.K.Evlampievc, S.A. Popovb, MS. Proninab, P. K. Turoverov', A.P.Sergeev', V.D.Zvorykina apn Lebedev Physical Institute, Division of Quantum Radiophysics, Leninsky prospect, 53, Moscow, , Russia. biv Grebenshikov Silica Chemistry Institute ofras, Laboratory ofhigh Puree Silica Glass, 24, bild 2, Odoevskogo street, St.Petersburg, , Russia. c000 "Silica Glass Products" Co., Ltd, 24, bud 2, Odoevskogo street, St.Petersburg, , Russia. dmoscow State Engineering Physics Institute (Technical University), Kashirskoe Shosse 31, Moscow, Russia. ABSTRACT The comprehensive results are presented on the behaviour of high purity synthetic quartz glasses under the action of intensive ionising radiation (x-rays and energetic electrons) and UV laser radiation with 248-nm wavelength. They are concerned to the application of e-beam-pumped large-size KrF-laser as a driver for the Inertial Fusion Energy. Keywords: quartz glasses, ionising radiation, KrF-laser, induced absorption 1. INTRODACTION High-purity fused silica glass commonly used for UV laser optics is also a basic material for manufacturing of largescale windows of powerful e-beam-pumped KrF lasers that are presently developed for Inertial Fusion Energy (IFE) application [1]. In IFE power plant such windows should survive during 108 laser shots being exposed to intensive UV laser light, bremsstrahlung X-rays and fast electrons originating from e-beam deceleration and scattering in a working gas [2-3]. They all produce electron-hole pairs, which subsequent relaxation forms various structural defects in the matter resulted in transient (short-lived) and residual (long-lived) decrease in the transparency. This is a reason for detailed investigation of different types of quartz glass in respect of its stability to ionizing radiation (IR) and mechanisms of W-induced optics degradation. The final goal of such researches would be the models describing a behaviour of optical materials (OM) under influence of JR and laser radiation (LR). The previous experience with MgF2 and CaF2 crystals testifies that the knowledge of effectiveness of formation in OM both short-lived and long-lived colour centres from primary electron-hole pairs is necessary for a successful solution of the problem [4,51. Experimental results permitting to obtain the required information for quartz glasses are submitted in the given report. These are results on the transparency behavior of quartz glass of trademarks QU-1, KS-4V, Corning 7940, as well as crystalline quartz, in dependence on absorbed IR doses accumulated for multiple e-beam pulses at EL-i high-current density (200 A/cm2, 80 ns) electron accelerator [6]. Two modes of samples irradiation by electrons of 280-keV and 100-keV energy were used. It allows to separate the contribution from the impact and relaxation mechanism of defect formation in OM. For glass QU- 1 the linear dependence of shot-lived absorption of excimer laser radiation at wavelengths 353 (XeF-laser), 248 (KrF) and 193 nm (ArF) on power density of JR are demonstrated too. Characteristics of quartz samples were compared with the similar data for high-purity fluorite. The distribution of the absorbed dozes of e-beam along the thickness of irradiated OM has been measured for EL-i installation to compare it with induced absorption values. This distribution characterizes influence of JR on a material of KrF-EBL windows. It will be useful at interpretation and of other experiments, where the behaviour of OM under action of e-beam was investigated. *psergeev@x4u.lebedev.ru; phone: (095) Nonresonant Laser-Matter Interaction (NLMI-11), edited by Mikhail N. Libenson, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 2004) X/04/$15 doi: /

2 2. CHARACTERISATION OF IONISING RADIATION OF EL-i INSTALLATION The studies of ionising-radiation source based on electron gun of EL-i laser installation [6] pursue the aim to find distribution of IR energy deposition along the thickness of irradiated materials. The average energy of electrons in e- beam ofel-1 is about 280 kev, a current density - up to 200 A/cm2, a pulse duration - 80 ns. The total e-beam energy is J at the area of 4x22 cm2. Two different methods have been used to construct such distribution. One of them used a calorimeter to measure the energy of e-beam and X-rays penetrated through various foils of different thickness. The calorimeter with a set of foil filters was placed nearby the exit foil window of electron gun. So the dependence of e-beam energy density passed through the filter with a certain area mass density was determined. By numerical differentiation of this curve we have received the distribution of absorbed dozes D as a function of the depth 1 in the material taking for the concreteness the density mass p=3 g/cm3. Eight experimental dots of the D(l) dependence starting with the maximal value D(O)=56000 Gy nearby the surface and to the depths up to 0.3 mn-i were obtain by this method (Fig. 1). The other dots for larger depths were measured with the help of solid-state thermo-luminescence dosimeters. They were the glass discs of 8-mm diameter and 1-mm thickness and had the density p=3 g/cm3. A set of such dosimeters consisting of 25 discs was mounted inside a thick aluminum plate, which was closed by 0.2-mm thickness titanium foil. After e- beam irradiation near the exit of electron gun with controllable energy density during 5-10 shots the dosimeters were read out. The absolute calibration of dosimeter readings were obtained by averaging over several runs and comparing them with calorimetric measurements. This method has allowed to determine the dependence 0(I) with I changing within the limits of 1-25 mm. The combined distribution of 0(I) in a material with the density 3 g/cm3, which is irradiated by one pulse of e-beam gun with energy density 2. 1 i/cm2 and electron energy of about 280 kev is presented on Fig. 1. Note that for depths more than 0.2 mm the absorbed dozes were caused mostly by bremsstrahlung X-ray radiation. This part of of distribution D(l) is typical for windows of KrF-EBL. The absolute value of D should depend on a particular laser construction, but it would increase with scaling of laser installation and its energy [2,3] , mm Fig. 1. Combined distribution of absorbed dozes versus the depth in material with p=3 g/cm3. 82 Proc. of SPIE Vol. 5506

3 3. THE BEHAVIOUR OF QUARTZ GLASSES UNDER IONISING RADIATION The work on experimental comparison of serviceability of the e-beam--pumped KrF-laser windows was carried out on installation EL-i [6]. For a large term of operation windows of various materials were tested: a quartz glasses, MgF2, CaF2, leucosapphire. The main experimental outcomes will be presented below. The laser windows of the EL- 1 installation of 60-mm diameter and aim thickness were exposed to simultaneous action of LR (the intensity at a level 5 MW/cm2) and ionising radiation with energy density on a surface about 0, 1 J/cm2 and pulse duration of 80 ns. The distribution of absorbed IR doses along the thickness of windows was like as in Fig. 1 after the filter with a thickness of about 0,2 mm. Being in contact with a working gas mixture of the laser the surface of windows was undergone also to chemical etching by fluorine. The installation EL-i worked in these experiments with repetition rate approximately one shot per 3 minutes E-beam-induced residual absorption in silica. Fused silica samples manufactured in Russia under trademark quartz glass QU-1 are analogous to Western Suprasil- 1 trademark. This type of pure quartz glass with a high content of the hydroxyl OH group (-.0. 1%) is especially transparent in the UV spectral range. The samples of 15-mm thickness were polished up to II class of cleanliness and tested as laser windows. Their initial transmittance at 248 nm wavelength was 92%. During 7-day operation 172 laser shots have been done with the total energy fluence delivered to the windows by scattered electrons and soft X-rays E=7 J/cm2. After such trial the transmittance of the window has fallen down to 84%. A thin layer of modified material was observed on the sample surface, which was in contact with working gas mixture. The light scattering due to this surface damage did not exceed 1 %. All these became a reason for more comprehensive investigation of the radiation stability of OM suitable for manufacturing of windows for powerful KrF- and others excimer EBL. For the search of the most stable OM for IR a comparative investigation of a number of quartz samples were carried out at installation EL-i. These are glasses of trademarks QU- 1, KS-4V, Corning 7940, as well as crystalline quartz. Russian newly developed high-purity KS-4V silica contains <0.2 ppm OH and <20 ppm Cl. According to manufacturer characteristics it possesses higher durability than QU- 1. Corning 7940 characteristics are close to QU-i. In given experiments the samples were placed into aluminium template covered by Ti foils. This foil served as additional filter decreasing the e-beam at the sample surface up to a required level. The assembly of samples was placed in air near an exit of the electron gun.. Maximal density of e-beam energy for one pulse on sample surface was 2 J/cm2. Distribution of D(l) in samples is shown on Fig. 1. In such mode I the irradiation of samples was carried out for about 4 months with the control of the induced residual absorption after each series with E1=i000 J/cm2. Total energy fluence that the samples of OM received during this irradiation reached 9 kj/cm2. The results of last measurements of their transmission after such irradiation are presented in a Fig. 2 and 3. H0 0n7940 KS.4V QU O 5o nrn Fig.2. Transmission of KS-4V, Corning 7940 and QU-l samples with thickness 2,5 and 4mm after irradiation by e-beam in mode 2 with E=1447 J/cm2. Proc. of SPIE Vol

4 A L KS-4V(O) KS-4V(1) 0 QU-1(O) QU-1( E; A,nm B Si02(Cr) E X,nm Fig.3. Ttransmission of KS-4V, QU-1 samples with thickness 2,5 and 4 mm (A) and crystal Si02 with thickness 3 mm (B) before (0) and after irradiation (mode 1) with E1= 9674 i/cm2, 7735 i/cm2 and 9674 J/cm2 accordingly. 84 Proc. of SPIE Vol. 5506

5 Transmission of quartz glasses after irradiation by e-beam, with electron energy about 70 kev (mode 2) is shown on Fig.2. The given samples have been closed by the additional filter from Ti foil with thickness 80 tm. The energy density of e-beam per one pulse on samples surface was about 0,3 J/cm2. At such mode of an irradiation formation of defects due to displacement of atoms as a result of direct collision with fast electrons is excluded. Here mechanisms of defects formation same, as well as at ionization of a material due to two-photon absorption of LR. As the proof of it serves that the similar degradation of quartz glass was observed under the only laser radiation with wavelengths 248, 193 and 157 nm [7-9]. it will essentially simplify interpretation of the received data. Besides this mode maximum close models operating conditions of window materials in excimer EBL. This is important and from the practical point of view. Transmission of KS-4V, QU-1 and Si02 crystal samples before (0) and after irradiation in the basic mode 1 (electron energy 280 kev) are submitted on Fig.3. The energy density of e-beam equals in this case about 2 J/cm2 per pulse. Small distinctions at a level of 2 % in initial transmission of non irradiated samples of KS-4V and QU-1, seen on fig. 3, are caused by distinction in processing their surfaces. The submitted results show that among different glasses KS-4V is the best material for windows of powerful EBL with wavelengths exceeding 190 nm. Windows of KS-4V in KrF-lasers with the output energy 1 U where the energy density of IR at their surface will be about 10 mj/cm2, can work not less than 1 O pulses with a loss of transparency at thelevel of I 0 %. Crystal quartz under radiating characteristics also is good for windows of excimer EBL but there are exist problems with its birefringence, purity, and also in cost and an opportunity of manufacturing of samples with big ( 0,5 m) sizes. Among others OM, suitable for manufacturing of excimer EBL windows, now high-purity fluorite possesses excellent radiation durability. At samples of the given crystal irradiated by e-beam in a mode 1 with E=9 kj/cm2 the transmission has decreased approximately on 2 % only in the field of 390 and 550 nm [ 10]. High-purity CaF2 essentially surpasses all grades of quartz glasses on radiation durability The transient absorption in silica. The described above outcomes show residual modifications of optical parameters of materials after IR influence. But it represents only a little part from a short-lived absorption appearing in optical materials during JR action, when laser radiation goes throughout the windows. Thefore, it is important to know a transient absorption in various OM and its correlation with JR As was shown in [11-13], it could be approximated by a linear dependence with a coefficient of proportionality k, defined as a ratio of JR-induced optical denseness (A=ln(Io/I)ln(Eo/E)) to a specific power of JR incident on a surface of a sample and completely absorbed by it (P=E1/'c): k=a/p=y/p. (1) Here lo and I incident and past through a sample intensity of LR, and E0 and E - appropriate values of a specific density of LR energy for impulse, y is JR-induced absorption coefficient of OM in that place of a sample, where the specific power of an absorbed doze equals P, 'r is a pulsewidth of IR, E,, is an energy density of JR. The first part of expression ( 1 ) reflects a method of a measurement of k, second reflects its physical essence. The measurements of k for OM have been done at installation EL- 1 in a scheme shown in Fig.5 [1 1-13]. For fused silica QU- 1 the magnitudes of k at 193, 248 and 353 nm and at LR intensities 1 MW/cm2 were equal accordingly 8, 4.6 and 0.5 cm2/gw. Close values of k are observed and for other grades of quartz [ 12]. At such k short-liven absorption in silica windows of KrF-lasers will be <1%. Under same scheme measurements of k have been carried out and for high-purity CaF2. At X=248 and 193 nm the magnitudes of k at LR intensities 1 MW/cm2 equal accordingly 64±6 and 72±7 cm2/gw [10]. If in KrF- or ArF- EBL with pulse duration 100 ns the energy density of x-ray radiation at windows will be about 100 mj/cm2, then P= 1 0 GW/cm2. The losses of LR because of a transient JR induced absorption in CaF2 windows in this case will be about 7%. This effect should be taken into account at creation of powerful e-beam-pumped excimer lasers. Proc. of SPIE Vol

6 CONCLUSION lonising radiation is an inalienable factor of powerful excimer EBL. By acting on the windows of the laser chamber it can influence on the efficiency of these lasers and their durability. The results of the work show, that the available highpurity silica allow already now to reach for excimer UV EBL with energy in a pulse up to -1 kj the resource of windows not less than lo pulses. Experimental results on influence of IR on silica submitted in this paper will be useful to developers of repetition-rate excimer lasers and to the physicists engaged in problems of radiating durability of silica. ACKNOWLEDGEMENTS This work was partially supported by The Ministry of Industry, Science and Technologies of RF, Contract N and the Naval Research Laboratory program. REFERENCES 1. J.D Sethian, S.P. Obenschain, et al., Fusion Engineering anddesign, 44, 371 (1999). 2. V.S.Barabanov, N.V.Morozov, P.B.Sergeyev J. ofnon-crist. Solids, 149, (1992). 3. V.D.Zvorykin, S.V.Arlantsev, V.G.Bakaev, et.all, Proc. SPIE, Vol. 4184, (2001). 4. S.V.Kurbasov, P.B.Sergeev, Quant. Electr. 30(8), (2000). 5. P.B.Sergeev, Quant. Electr. 32(4), (2002). 6. P.B.Sergeev J. of Soviet Laser Research, V.14, N.4, (1993). 7. S.Thomas, B.Kuehn, Proc.SPIE, Vol. 2966, (1997). 8. Y.Ikuta, S.Kikugawa, et.all, Proc.SPIE, Vol. 4347, (2000). 9. H.Hosono, T.Kinoshita, et.all, Proc.SPIE, Vol. 4347, (2000). 10. l.a. Mironov, V.M.Reiterov et al., To be published. 11. V.S.Barabanov, N.V.Morozov, P.B.Sergeev, J. of Soviet Laser Research, V.14, N.4, (1993). 12. A.V.Amosov, V.SBarabanov et.all. Quant. Electr. 23(10), (1993). 13. V.SBarabanov, P.B.Sergeev, Quant. Electr. 25(7), (1995). 86 Proc. of SPIE Vol. 5506