Laser plasma as a source of soft x rays

Laser plasma as a source of soft x rays O.B. Anan'in, Yu. A. Bykovskii, V. L. Kantsyrev, Yu. P. Kozyrev, and A. M. Raspopin Engineering-Physics

Institute,

Moscow

(Submitted December 30, 1975; resubmitted December 20, 1976) K v a n t o v a y a E l e k t r o n . (Moscow) 4, 9 6 5 - 9 6 9 ( M a y

1977)

T h e results are reported of an investigation of soft x rays emitted from a laser plasma. T h e aim was to study the physics of the phenomenon and to develop a source of soft x rays for practical applications. Some parameters of the laser plasma were determined, including the efficiency of conversion of the laser energy into soft x rays, spectral and spatial distributions of the radiation emitted by the laser plasma, and size of the region emitting this radiation. It was found that, within a certain range, it was possible to control the size of the radiating part of the laser plasma. A comparison was made of the resolution of xray photographic films in the wavelength range corresponding to soft x rays and in the visible range. neodymium

glass

A

laser was used. Soft x rays were recorded by means of films sensitive to x rays.

Potential applications of the laser plasma as a pulse source of soft x rays are considered and

some

requirements to be satisfied by such a source are formulated. P A C S numbers: 5 2 . 7 5 . - d , 0 7 . 8 5 . + n, 52.50.Jra

Extensive investigations have been made of x rays emitted by hot l a s e r plasmas. X - r a y radiation has been studied mainly as a potential source of information on l a s e r p l a s m a s / " ' X - r a y spectra of multiply charged ions in l a s e r plasmas have been recorded. However, no systematic studies have been made of the laser p l a s ma as a source of x rays for practical applications. It is reported in Ref. 6 that the laser plasma can be used as a pulse source of soft x rays in radiography and advantages of the l a s e r plasma over other soft-x-ray sources are demonstrated. A source of this kind can be described by many parameters and the most important among them a r e : 1) the conversion efficiency TJ ( i . e . , the ratio of the energy emitted by a plasma in the form of soft x rays to the energy supplied to the plasma) and the intensity of the emitted radiation; 2) the spectral distribution of the radiation intensity; 3) the spatial distribution of the radiation intensity in a given spectral interval; 4) the size of the radiation source; 5) the time characteristics of the radiation pulses. The present paper reports a study of soft x rays emitted from a l a s e r plasma. The a i m was to study the physics of the phenomenon and the laser plasma as a source of soft x r a y s . The measurements were timeintegrated (the time characteristics of soft x - r a y e m i s sion from l a s e r plasmas have been investigated quite thoroughly and extensive experimental material is a l ready available'). We used a neodymium glass l a s e r . * The l a s e r r a d i a tion was focused on the surface of a target by a lens. The targets and lens were in an evacuated chamber. The targets could be interchanged without disturbing the vacuum in the chamber. Soft x rays were recorded on U F - V R photographic films placed in light-tight cassettes located inside the chamber. The necessary parts of the soft x - r a y spectrum were selected and the U F - V R films were protected from the visible l a s e r - p l a s m a radiation 541

Sov. J. Q u a n t u m E l e c t r o n . , V o l . 7, N o . 5, May 1977

by thin A l foils and aluminum-coated polyethylene t e r e phthalate films. In a l l our experiments, the l a s e r energy density reaching the target surface was 5x10^' W/ cm^. The l a s e r was operated in the single-pulse r e gime. We studied the x - r a y wavelength range shorter than 1 . 4 - 1 . 6 nm. In this range, there were no recombination radiation discontinuities and there was no line s p e c trum (because T^^lOO eV for ^ = 5 x 1 0 " W/cm^—Ref. 7). Thus, we studied the continuous soft x - r a y s p e c trum of the l a s e r plasma. 1 . We investigated the dependence of the conversion efficiency on the nature of the target (C, A l , F e , Cu, Mo, In, Pb). Control experiments demonstrated that the intensity of the soft x - r a y radiation was not affected by the gases absorbed in the surface layers of a target or by oxide films on the target surface. The soft X rays were recorded on a film inside a cassette with several windows and each of these w i n dows was covered by the same set of aluminum foils and polyethylene terephthalate films. In each experiment, we irradiated four targets in turn with the l a s e r r a d i a tion and exposed one frame per target. F o r each target, the exposure corresponded to s e v e r a l laser pulses. Thus, information on four targets was recorded on the same photographic film. The results were analyzed on the basis of information on the U F - V R films and the properties of the filters, taken from Refs. 8 - 1 3 , and on the basis of the results of our own calibration of the U F VR film. We analyzed the results obtained by recording X rays transmitted by an A l filter, 11 thick, or by an aluminum-coated polyethylene terephthalate filter, 10 M thick. The intensity of the radiation emitted by a copper target was the stroi^est and most stable and, therefore, all the data were normalized to the copper radiation i n tensity. Figure 1 gives the dependences of the relative intensity of the x - r a y radiation on the chemical composition of the targets (reduced to one laser pulse). There was a clear maximum for elements whose atomic number was about 30. The results obtained were in qualitative C o p y r i g h t © 1 9 7 7 A m e r i c a n I n s t i t u t e o f Physics

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^ rel. units

F I G . 1. Dependence of the relative intensity of x - r a y radiation on the c h e m i c a l composition of the target, represented by the atomic number of the element Z j : A) aluminum filter, 11 Ai thick; •) polyethylene terephthalate filter, 10 M thick, coated with an A l film, 0.2 M thick. S

n

C

A

HIS Ft [11

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PS

agreement with the predictions of the relative intensity of soft X rays emitted by targets of different chemical composition made in a theoretical paper'* for targets interacting with neodymium laser radiation in the form of pulses of 30 nsec half-width. In our case, the size of the spot in which the laser radiation was focused (130 u ) was less than the optimal value for heavy e l e ments." 2. The electron temperature of the l a s e r plasma was determined by the method of filters. We used A l filters. The results were analyzed using information given in Refs. 8 - 1 3 . The temperature T„ was found to be constant for a l l the targets and a l l pairs of filters (this corresponded to a Maxwellian distribution of the plasma electron v e l o c i ties) within the limits of the experimental e r r o r and its value was about 100 eV. 3. The spatial distribution of the x - r a y intensity was determined using several cassettes arranged inside the chamber in such a way as to record the radiation emitted at various angles (from 0 to 90°) relative to the t a r get surface. In the investigated range of wavelengths, the l a s e r plasma emitted isotropically in a solid angle of 27r s r . 4. The size of the x - r a y - e m i t t i i ^ region was determined with a camera obscura with one-dimensional r e s olution. A slit was placed between the l a s e r plasma and a cassette containing the U F - V R film. The long side of the slit was either perpendicular to the target surface (this gave the size of the emitting region parallel to the target surface) or parallel to the target (this gave the size at right-angles to the target). Aluminum foils were placed in the cassette in such a way that, in each case, the dimensions of the emittit^ region were determined simultaneously in three spectral intervals. The magnification was about 8. The resolution in the l a s e r p l a s ma plane was 40 \i. and the diameter of the spot in which the l a s e r radiation was focused was 130 ^. The target was made of copper. Within the limits of experimental e r r o r , the size of the emitting region was the same at right-angles to the target and parallel to it. T h i s size (determined using aluminum filters) was of the same order a s the focal 542

Sov. J . Quantum Electron., V o l . 7, No. 5, May

1977

spot of the l a s e r radiation. The l a s e r plasma region emitting radiation of shorter wavelengths was smaller than the region emitting long-wavelength radiation, in agreement with the results reported in Ref. 15. F o r e x ample, the size of the l a s e r plasma region emitting r a diation transmitted by an A l filter, 11 thick, was twice the size of the region emitting radiation transmitted by a 26 thick A l filter. This was true for exposures ranging from a few l a s e r pulses to 15 pulses; the image became saturated when the exposure was extended b e yond 15 pulses. In practical applications, it i s desirable to control the source s i z e . We attempted to do this by altering the pressure in the vacuum chamber. The absorption of soft x rays by the residual a i r in the p r e s s u r e range 10"'-10"' T o r r did not vary by more than a few percent. Our measurements indicated that the source size decreased with rising p r e s s u r e ; the spectrum of the radiation and its specific intensity ( i . e . , the intensity per unit volume) were not affected. The contraction of the source could be explained by a more rapid cooling of the plasma at higher p r e s s u r e s because of the collision of the plasma particles with the gas molecules. Thus, the size of the l a s e r - p l a s m a region emitting soft X rays could be controlled within a certain range by altering the pressure of the ambient medium. This should be particularly important in microradiography. As demonstrated in Ref. 6, the contact microradiography method i s more convenient when a l a s e r plasma is employed as a source of soft x r a y s . The resolution then depends on the resolution of the x - r a y - s e n s i t i v e photographic film. However, it is frequently impossible to ensure full contact between a sample and an x ray film. The half-shadow broadening of the image b e gins to play a role and this is governed by the size of the radiation source as well as by the mutual positions of the film, sample, and source. In our case (a contact scheme with a s m a l l gap between the sample and film), the resolution in the plane of the sample was e s timated to be l^sa/b, where s was the size of the emitting region, a was the distance from the film to the s a m ple, and b was the distance from the emitting region to the film. The parameters a and b could be varied only within a certain range (limited, for example, by the fall in l u m i nosity with increasing b) and, consequently, the r e s o l u tion could be improved by r e d u c i i ^ the source s i z e . One should s t r e s s that the resolution of the U F - V R and U F - 4 films in the soft-x-ray range was different from their resolution in the visible range. A comparison of the size of the structure elements of biological objects (determined using an optical microscope) and x ray photographs obtained in the soft range* indicated that the resolution of the U F - V R film in the latter range was approximately twice the resolution in the visible range. We obtain the same result for the U F - 4 film. T h i s d i s crepancy between the resolutions could be explained by the different nature of the absorption of soft x rays and visible radiation in photographic emulsions. F o r e x -

A n a n ' i n ef al.

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ample, soft x rays did not give rise to such effects as reflection of the radiation from the film substrate or scattering by silver halide grains. Although the work on the development of a laser-plasma source of soft x rays has only just begun, it is possible to identify some of its potential applications, which include a laboratory source (providing comparison spectra of multiply charged ions'^ and used to calibrate spectroscopic instruments and radiation detectors), highspeed pulse microradiography with applications to biology, medicine, and high-precision technology, ^'^^ and a pulse source of soft x rays for studies of fast processes. A source of general use should have controllable principal parameters, and be compact as well as simple in use. In the case of a laser plasma source, this means that a small commercial laser should be used. At present, lasers of this kind can produce energy densities of 10^^-10^^ W/cm^ on a target surface. Our own investigations were carried out in this range of laser radiation densities. The authors are grateful to M. R. Shpol'skii for sup-plying UF-VR and UF-4 films and for valuable discussions, and to S. V. Perfil'ev for his help in the calibration of the UF-VR films. 'N. G. Basov, V . A . Boiko, V. A. Gribkov, S. M. Zakharov, O. N. Krokhin, and G. V. Sklizkov, Pis'ma Zh. Eksp. Teor. Fiz. 9, 520 (1969) [JETP Lett. 9, 315 (1969)]. ^O. N. Krokhin, Yu. A. Mikhariov, V. V. Pustovalov, A. A. Rapasov, V. P. SUin, G. V. Sklizkov, and A. S. Shikanov, Pis'ma Zh. Eksp. Teor. Fiz. 20, 239 (1974) [JETP Lett. 20, 105 (1974)]. ' E . V. Aglitskir, V. A. Boiko, A. V. -Vinogradov, and E. A. Yukov, Kvantovaya Elektron. (Moscow) 1, 579 (1974) [Sov. J. Quantum Electron. 4, 322 (1974)].

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""N. G. Basov, V. A. Boiko, Yu. P. VoTnov, E. Ya. Kononov, S. L. Mandel'shtara, and G. V. Sklizkov, Pis'ma Zh. Eksp. Teor. Fiz. 5, 177 (1967) UETP Lett. 5, 141 (1967)]. *E. V. AgUtskii, V. A. Boiko, S. A. Pikuz, andA. Ya. Faenov, Kvantovaya Elektron. (Moscow) 1, 1731 (1974) [Sov. J. Quantum Electron. 4, 956 (1975)]. ^O. B. Anan'in, Yu. A . Bykovskir, V. L. Kantsyrev, and Yu. P. Kozyrev, Pis'ma Zh. Tekh. Fiz. 1, 366 (1975) [Sov. Tech. Phys. Lejtt. 1, 172 (1975)]. 'v. A. Borko, O. N. Krokhin, and G. V. Sklizkov, Tr. Fiz. Inst. Akad. Nauk SSSR 76^ 186 (1974). ' E . V. Agiitskir, V. A. Borko, T. A. Kalinkina, A. N. Oshurkova, S. A. Pikuz, V. M. Uvarova, A. Ya. Faenov, and M. R. Shpol'skii" Prib. Tekh. Eksp. No. 4, 207 (1975). ' T . F . Stratton, in: Plasma Diagnostic Techniques (ed. by R. Huddiestone and S. L. Leonard), Academic.Press, New York (1965). '"A. A. Kologrivov, Yu. A. MikhaUov, G. V. Sklizkov, S. I. Fedotov, A. S. Shikanov, and M. R. Shpol'skii, Preprint No. 76 [in Russian], Lebedev Physics Institute, Academy of Sciences of the USSR, Leningrad (1975). "A. E . Sandstrbm, in: X Rays (ed. by M. A. Blokhin) [Russian translation]. Part II, IL, Moscow (1960). '^B. L. Henke, R. White, and B. Lundberg, J. Appl. Phys. 28, 98 (1957). '^P. Bogen, in: Plasma Diagnostics (ed. by W. Lochte-Holtgreven), American Elsevier, New York (1968). '^D. Colombant and G, F. Tonon, J. Appl. Phys. 44, 3524 (1973). '^V. A. Borko, S. A. Pikuz, andA. Ya. Faenov, Kvantovaya Elektron. (Moscow) 2, 1216 (1975) [Sov. J. Quantum Electron. 5, 658 (1975)1. " E . V. AgUtskii, V. A. Boiko, S. M. Zakharov, N. A. Konoplev, S. A. Pikuz, and A. Ya. Faenov, Preprint No. 163 [in Russian], Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow (1973). "O. B. Anan'in, Yu. A. Bykovskii, V. L. Kantsyrev, Yu. P. Kozyrev, and P. G. Pleshanov, Abstracts of Papers presented at Scientific-Technical Conference on Applications of Lasers in Modern Technology, Leningrad, 1975 [in Russian). Translated by A. Tybulewicz

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