Electron-beam-induced absorption in quartz glassesP. B. Sergeev, V. D. Zvorykin, and A. P. Sergeev

P. N. Lebedev Physics Institute, Russian Academy of Sciences, Moscow

T. A. Ermolenko, S. A. Popov, M. S. Pronina, P. K. Turoverov, and I. I. Cheremisin

I. V. Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg

I. K. Evlampiev

OOO Silica Glass Products, St. Petersburg~Submitted December 17, 2003!Opticheski� Zhurnal71, 93–97~June 2004!

Electron-beam-induced absorption in quartz glasses of types KS-4V, KU-1, and Corning 7940has been experimentally investigated in the 150–1000-nm region. Samples of opticalmaterials were irradiated in regimes similar to the operating conditions of the windows of electron-beam excimer lasers, in particular, powerful KrF lasers for laser thermonuclear synthesis. Itis shown that the electron-beam-induced absorption in all the quartz glasses that were testedreaches a steady-state level during irradiation that is determined by the mean specificpower density of the electron beam. Under identical irradiation conditions, the steady-stateabsorption level in KS-4V glass in the UV region is about a factor of 4 less than in KU-1 glassand a factor of 2 less than in Corning 7940. ©2004 Optical Society of America

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INTRODUCTION

High-purity quartz glasses intended to operate in theregion are the base materials for the large windows of perful KrF electron-beam lasers~KrF EBLs! now being devel-oped as drivers for laser thermonuclear synthesis~LTNS!.1

The window of such a laser must sustain up to 108 pulsesduring a year. In doing so, intense laser radiation withquantum energy of 5 eV will act on them, as well as x-rbremsstrahlung and fast electrons scattered from the pbeam.2,3 These ionizing radiations create material electrohole pairs in the window whose relaxation results in the fmation of short-lived and long-lived absorbing centers. Tmakes it crucial to experimentally study the radiatistrength of various sorts of modern quartz glasses thatpromising for fabricating the windows of powerful excimEBLs in the UV range.

The new KS-4V high-purity Russian quartz glass4 hasbecome the first in this series. It was developed at the IGrebenshchikov Institute of Silicate Chemistry, RussAcademy of Sciences, and is now being introduced into wproduction. This is a type-IV glass.4,5 The total concentrationof metallic impurities in it~of fifteen elements! is no morethan 0.5 ppm, the concentration of OH groups is less thanppm, and the Cl concentration is about 20 ppm.4 The glasspossesses high radiation strength. Glass of type KU-1well as many of its foreign counterparts, in particular Coing 7940, which is a type-III glass,5 are widely used in theoptics of the UV region. There is much information conceing these glasses, but it does not make it possible to pretheir behavior under different regimes involving the actionlaser radiation and ionizing radiation.

The goal of this paper was to experimentally studybehavior of quartz glasses KS-4V, KU-1, and Corning 79under the action of an electron beam~EB! and to subse-

415 J. Opt. Technol. 71 (6), June 2004 1070-9762/2004/0604

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quently determine the efficiency of forming in them stabcolor centers accompanying the decay of electronic exctions ~EEs!. To produce EEs in the glasses, we chose EBsthe most powerful and accessible source of ionizing radtion. An important consideration in this case was the circustance that the operating conditions of the windows of poerful EBLs, particularly KrF drivers for LTNS, are directlmodelled when EBs are used.1

In order for an EB to produce only EEs without creatindefects by directly knocking atoms into intersticial positionthe energy of the electrons that are used must be lowerthe threshold of such processes.6 This was taken into accounwhen we set up our experiments. Such specificity of theperiments made it possible to use the results obtained forradiation strength of glasses to model their long-term behior under the action of intense laser radiation in the UV aVUV regions, which is a powerful source of EEs in opticmaterials because of two-photon processes.

EXPERIMENTAL TECHNIQUE

The samples of glasses were tested under the actioEBs on an E´LA apparatus with electron-beam excitation7

The laser chamber was removed from the apparatus in texperiments, and a special assembly with the test samwas placed in air at the separator foil of the electron bea

The electron gun of the E´LA apparatus operated in thpulsed regime with a frequency of about 5 MHz. When texperiments were carried out, from 50 to 150 shots wmade in the course of a workday. The electron energyyond the foil of the electron gun is about 280 keV, the crent density of the beam is as much as 200 A/cm2, and thepulse width is 80 ns. The total energy of the EB beyondseparator foil of the electron gun is about 250 J whencross sectional area is 4322 cm2, while the energy density

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of the EB per pulse (F1) reaches 3 J/cm2. To eliminate in-homogeneities of the EB, the assembly with the samplesplaced 25 mm from the separator foil. ThenF1 was about2.5 J/cm2, while the cross sectional area of the EB with sua fluence~energy density! is about 70 cm2.

For the EB used here, according to the techniquescribed in Ref. 8, we measured the distribution of absordoseD averaged over a large number of shots over the thnessl of a flat target made from a material with a density3 g/cm3. The resulting dependence is presented in Fig. 1shows theD( l ) distribution in the test samples, the densof most of which is close to 3 g/cm3.

As the number of pulses and the total fluenceF increase,the relative shape of theD( l ) distribution does not changeOnly the amplitude of this function increases proportionato F. The given distribution will be needed when interpretithe experimental results and when finding from themformation efficiencies of stable color centers.

The test samples were placed in a special assembly.is a thick duralumin plate with recesses milled out for easample. On the side on which the EB is incident, the samwere covered by one common foil made from titaniummm thick. The samples behind this foil were irradiated in tso-called first regime, in which the electron energy was ab280 keV, whileF1 was about 2 J/cm2. The second part of thesamples were covered by an additional filter made from tnium foil about 100mm thick. At the surface of thesesamples, irradiated in the so-called second regime, the etron energy of the beam did not exceed 100 keV, whileF1

was varied within the limits 0.2– 0.8 J/cm2. The glasssamples were separated into two groups to reveal posdifferences in the defect-formation efficiency by electrowith different energies.

The chosen placement geometry of the irradiasamples inside the metallic recesses was specially useorder to minimize the electric fields that appear in dielectrwhen they are irradiated by beams of charged particl9

These minimized the influence of the EB-induced elecfields on the formation efficiency of long-lived color cente

The assembly was attached in such a way that thediated surface of the samples was 25 mm from the foil of

FIG. 1. Absorbed doseD vs thicknessl in a material with density 3 g/cm3

subjected to irradiation by a single pulse of an electron beam from an´LAapparatus withF152.1 J/cm2.

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electron gun. The samples were placed over the area oassembly where the maximum homogeneous energy dedistribution of the EB was observed.

A calibration series of shots was carried out periodicaafter 500–1000 pulses to check the accuracy with whichF1

was measured. In this case, a BKDM calorimeter~producedby OKB FIAN! was installed in place of the assembly withe samples. A duralumin plate 5 mm thick with openinalong the area where the samples were placed was mouin front of it. These openings were covered with a filterfront of the samples—i.e., a titanium foil 14mm thick for thefirst regime or the corresponding set of foils for the secoirradiation regime. The EB energy density on the surfacethe samples was determined in the calibration series of sfrom the readings of the VKDM calorimeter and was corrlated with the readings of a second calorimeter of tyVChD-5 ~produced by OKB FIAN!. This calorimeter con-stantly tracked the operation of the electron gun in each sboth during calibration and during irradiation. Its readinwere used to determine the value ofF1 for each shot andthen the total fluenceF on the surface of the samples for thseries of pulses.

The transmission or absorption spectra of the sampwas periodically recorded after the corresponding irradiatin both the visible range and the VUV region. The prolight in this case passed over the samples along the proption direction of the EB.

ELECTRON-BEAM-INDUCED LONG-LIVED ABSORPTION INQUARTZ GLASSES

Under the conditions described above, in the courseabout a year and a half, the E´LA apparatus was used to carrout tests of several samples of KS-4V and KU-1 in the fiand second irradiation regimes in parallel, while one samof Corning 7940 glass was tested in only the second irration regime. More than 6000 shots were made on the samin this time. The total EB energy density on the surface of

FIG. 2. Transmittance spectra of samples of KS-4V 2.5 mm thick befirradiation—1 and after irradiation in regime 1 withF59674 J/cm2—2;samples of KU-1 4 mm thick before irradiation—3 and after irradiation inregime 1 withF57735 J/cm2—4.

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samples irradiated in the first regime was aboutF'10 kJ/cm2, while the value ofF for the second regime waan order of magnitude less.

The characteristic changes in the transmission spectrthe enumerated samples after EB irradiation can be judfrom Figs. 2 and 3. The spectral region on them is cut of400 nm, since the transmittance of the sample after irration did not change all the way to 1100 nm, remaining alevel of 92%.

We should point out that the transmission spectra ofsamples shown in Fig. 3 were obtained after EB irradiatwith an energy less than 100 keV~regime 2!. Here the valueof F1 did not exceed 0.3 J/cm2. In such a irradiation regimethe formation of defects in glass because of the displacemof atoms as a result of direct collision with fast electronsexcluded. Here the defect-formation mechanism, as whenglass is ionized by x-rays or strong laser radiation, is demined by two-photon processes. This regime models theerating conditions of the windows of EB lasers as closelypossible. It unambiguously follows from Fig. 3 that KS-4glass exceeds KU-1 and Corning 7940 glasses in radiastrength.

We noted during the investigations that, asF increases,the rate of change of the transmittance of the samples

FIG. 3. Transmittance spectra of samples of KS-4V 2.5 mm thick—1, Corn-ing 7940 4 mm thick—2, and KU-1 4 mm thick—3 after irradiation inregime 2 withF51447 J/cm2.

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creases. This compelled us to analyze the experimentasults in order to reveal how the induced absorption depeon the energy density.

From a physical viewpoint, it is more logical to associaF with the induced absorption or, as in our case of inhomgeneous distribution of absorbed dose over the thicknesthe samples, the induced optical density~OD!. For the re-quired wavelength, it is possible to calculate, starting frothe available data on transmissionT:

OD5 ln~T0 /T!. ~1!

HereT0 andT are the transmittance of the sample before aafter irradiation.

As is well known, the principal absorption bandsquartz glasses lie in the region of 250 and 215 nm,10 and thisis confirmed by our results. We therefore calculated thefor the test samples of glasses at the two wavelengthsand 250 nm (OD215 and OD250 respectively! and constructedthe dependence of OD onF. In this case, its own graph waconstructed for each of the irradiation regimes. We had tsuch regimes: with an energy density per pulse of ab0.2 J/cm2 ~indicated by the number 2 after the glass typethe label of the graph! and about 2 J/cm2 ~indicated by thenumber 1!. The resulting dependences are shown in Fi4–6.

The following conclusion can be drawn from these dpendences:

~1! In each of the test glasses, as the absorbed dosecreases, the induced absorption goes to saturation.

~2! In KU-1 glass, the absorption in the saturation regimeproportional to the mean power density of the irradtion. In KS-4V glass, with an almost tenfold differencin irradiation powers for the first and second regimes,saturated absorptions differ by about a factor 1.5. Thievidence that, even in the more rigorous first irradiatiregime, the contribution to defect formation due to tdirect displacement of atoms following elastic collisiowith electrons is negligible.

~3! Under identical conditions of EB irradiation, the induceabsorption in the saturation regime in samples of KS-

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FIG. 4. OD215 and OD250 vs F in KU-1 glass for the first irradiation regime~1!—a and the second~2!—b irradiation regime of the samples with an electrobeam.1—OD215, 2—OD250.

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FIG. 5. OD215 and OD250 vs F in KS-4V glass for the first irradiation regime~1!—a and the second~2!—b irradiation regime of the samples with an electrobeam.1—OD215, 2—OD250.

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glass in the 180–330-nm region is about a factor olower than in KU-1 and a factor of 2 lower than in Coring 7940.

In all the types of glass that were studied, the procesproducing long-lived color centers under the action ofionizer is nonlinear in time. It cannot be described bysingle ‘‘center-productivity’’ parameter. At the same time ththe centers are being produced, they are relaxing. In4–6, several extreme points are obtained for the identmaximum values ofF when the transmittanceT is measureda week and three months after the end of the irradiatThey show that there is appreciable relaxation of thewith time. According to our estimates, the OD-relaxatitime ~falloff by a factor ofe) in the KS-4V samples irradi-ated in the first regime is about 1 yr. For a comprehensdescription of these processes, numerical models need tcreated for the relaxation kinetics of the electronic exctions produced under the action of the ionizers in each ofglasses studied here. However, this will be the topic of otpapers.

CONCLUSION

This paper has experimentally checked the behaviohigh-purity quartz glasses KS-4V, KU-1, and Corning 79under the prolonged action of an electron beam with anergy less than 280 keV. Such an electron energy was be

FIG. 6. OD215 and OD250 vs F in Corning 7940 glass for the second~2!irradiation regime with an electron beam.1—OD215, 2— OD250.

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the threshold for glass destructurizing by the direct dislodment of lattice atoms. The distribution of the absorbed dover the thickness of the samples was measured for the etron beam used here, and this makes it possible to assothe dose with the induced absorption. Such specificationthe experiments made it possible for the first time to useresults to set up numerical models of the behavior of glasboth under the action of an electron beam and underaction of intense laser radiation in the UV and VUV region

It was found that the induced absorption in all thglasses went to saturation as the absorbed dose increUnder identical irradiation conditions, the electron-beainduced absorption in the saturation regime in samplesKS-4V glass in the 180–330-nm region is about a factor oless than in KU-1 and a factor of 2 less than in Corni7940.

The experimental results obtained in this paper areportant for understanding the physics of radiation procesin quartz glasses. They will be useful for the producersoptical materials and for the developers of lasers and osources of UV and VUV radiation.

This work was carried out with the partial support of thMinistry of Commerce, Science, and Technologies ofRussian Federation~Contract No. 40.020.1.1.1157!, as wellthe Naval Research Laboratory, USA.

Email: psergeev@x4u.lebedev.ru

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