Review of the doctoral dissertation of Ismail Saber titled: Spectral investigation of extreme ultraviolet induced plasmas

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1 Prof. dr hab. inż. Tadeusz Pisarczyk Institute of Plasma Physics and Laser Microfusion. 23 Hery St., Warsaw. Warsaw, November 21, 2018r. Introduction: Review of the doctoral dissertation of Ismail Saber titled: Spectral investigation of extreme ultraviolet induced plasmas Applies to the scientific degree of doctor of philosophy. Plasma produced by photoionizing gases, creates new cognitive abilities in relation to the plasma produced by the electric discharge path or the irradiation of gas with high power lasers. Its main advantage is the fact that the photoionization mechanism does not require exceeding any intensity threshold - different ionization states can be achieved depending on the energy of ionizing, also low energy, photons. Photoionization plasma does not occur in normal conditions on Earth, but it is common in space, and they is created as a result of X-rays interaction (commonly occurring in space) with gas media. In particular, this occurs in the double stars with the accretion disk, in the intergalactic space, in the star-forming clouds, but also in the upper layers of the planetary atmospheres. The photoionization of the upper atmosphere layers leads to the so-called photoionization dissociation leading, among others, to escape molecular gases from the atmosphere. Soft X-ray (SXR) and extreme ultraviolet (EUV) radiation also allows for various types of chemical reactions that lead to the formation of complex molecules, which is the basis for life in the universe. From this reason the creation and investigation of the photoionized plasma is important for astrophysics. Research of photoionization plasma in laboratory conditions enables better understanding of phenomena occurring in space and will allow to test computer models of such plasma in controlled laboratory conditions. Until recently, studies of photoionization plasma were mainly conducted on the High Energy Density (HED) facilities in which the source of photon ionizing gas was plasma produced in an electric discharge, mainly z-pinch type. A very interesting alternative to the production of photoionization plasma is the use of laser-plasma sources SXR / EUV radiation, based on the interaction of laser pulses with a gas-puff targets. This new idea of generation of the

2 photoionization plasma has been initiated and they is intensively developed at the Institute of Optoelectronics, Military University of Technology. The results of the photoionization plasma research obtained with the participation of Ismail Sabier are the subject of consideration of this doctoral dissertation. The scope of dissertation: The doctoral dissertation consists of 8 chapters. In the first chapter the goal and thesis are formulated, taking into account six literature references related to the subject of the dissertation. The aim of this dissertation is to study the low-temperature, high electron density plasmas created as a result of gas irradiation in a vacuum chamber using intense EUV radiation pulses, from LPP sources, based on a double stream gas puff target. The subject of the research was the plasma generated from atomic and molecular gases by lighting them with XUV radiation impulses from LPP sources based on a two-stream gas-puff target and two Nd: YAG laser systems with different pulse parameters, operating at the basic wavelength, 1064 nm. To obtain information and temperature, electron density and ionization degree of the photoionized plasma, were registered emission spectra in the EUV, UV (ultraviolet) and visible (VIS) range using appropriate spectrometers working in the EUV and UV / VIS range. It was expected that spectral measurements of photoionization plasma, combined with numerical simulations using collision and radiation numeric codes, will allow to supplement existing knowledge regarding: the differences between EUV-induced plasmas and other kinds of plasmas (laser-produced and discharge-produced plasmas), the thermodynamic state and the degree of ionization of EUV-induced plasmas, as well as knowledge of the parameters of EUV-induced plasmas created in atomic and molecular gases. In the opinion of the author of the dissertation, obtaining answers to the above questions will be useful to choose the right model for the description of the investigated photoionized plasma. In Chapter 2, different methods of the plasma generation are briefly described by means of intense radiation pulses, in especially plasma produced by laser interaction with solids and with gases. Much space has been devoted to the characterization of X-ray and EUV sources which are applied to creation of the photoionized plasma. Applications of the laser-plasma EUV sources in science and technology were also given in the chapter. This chapter is edited on the

3 basis of 78 publications, many of which (20 item) were written by a team from the IOE MUT and 2 item co-authors is I. Saber. The chapter 3 presents the basics of plasma spectroscopy. The atomic and molecular processes occurring in plasma are described, such as ionization, excitation and absorption processes, in particular processes of photoionization and photodissociation occurring both in atoms and in molecules, which are important taking into account the subject of dissertation. The most important models describing the population distribution and thermodynamic state of plasma are also presented, and which were used to interpretation results of dissertation. This chapter is edited based on 33 important publications from world literature exclusively. In the chapter 4, LPP EXUV sources based on two Nd: YAG laser systems and a doublestream gas puff target built in IOE MUT are described, and which were used in the presented experiments in the next chapters. Two kinds of collectors (multifoil and ellipsoidal) are characterized, which were used to collect to EUV radiation from a LPP EUV source and focusing this radiation on different gases. The method of synchronization the LPP EUV source with the gas injection system for photoionized plasma is also explained. As regards diagnostic, two spectrometers are described, which have been used to measure the spectra of the photoionized plasma: (i) UV/VIS commercial spectrometer (Echelle spectral analyzer ESA 4000), which allows measurement in the spectral range of: nm, and the spectrum can be recorded with a time gate about 20 ns, and (ii) EUV spectrometer (McPherson, Model 251), which allows the measurement of the spectrum in the range of nm. In this chapter, the description of the plasma modeling tools used to simulate the experimental emission lines in the photoionized plasma are discussed. To interpret the results of experimental investigations, the following commercially available software was used: the collisional radiative spectral code (PrismSPECT and the population kinetics code (FLYCHK) for modeling plasma emissions for a wide range of temperatures under LTE and non-lte conditions as well as programs: LIFBASE and SPECAIR, which are capable of modeling a molecular spectrum from different molecular band systems. The literature for this chapter includes 15 references. 6 key items refer to the apparatus built by the team from IOE MUT, in which a PhD student works. Chapters 5, 6 and 7 are an essential part of the doctoral dissertation, in which the results of the experiments and their interpretation are presented..

4 In the chapter 5 the spectral measurements of the EUV-induced photoionized plasma created in different gases (neon, argon and nitrogen) are presented. There are described experiments, which were conducted with the participation of a doctoral student and published in 4 papers. The co-author of two publications is I. Saber. To generate of the photoionized plasmas from different gases, the LPP EUV source based on the a double-stream gas puff target, irradiated by a Nd: YAG laser (1064 nm) with 10 J energy in the pulse duration of 10 ns operating with repetition of 10 Hz was applied. The grazing incidence multifoil EUV collector was used to obtain the sufficient power density of the EUV irradiation (~10 8 W/cm 2 ) for creation of the photoionized plasma. The spectra of the photoionized gases were measured in the UV/VIS region by means the Echelle Spectra Analyzer ESA 4000 spectrograph, equipped with the ICCD Kodak KAF 1001 camera. The registered spectra identified emission lines in the UV/VIS range consisted of neutral atoms and ions up to the single-charged states. The plasma electron temperature was determined using the Boltzmann plot method from the plasma species, while the electron density was estimated from the Stark broadening line profile in the isolated atoms or ions and from the hydrogen Hβ line. To formulate conclusions regarding, the thermodynamic condition of the plasma, the collisional-radiative code PrismSPECT was used to further interpret the theoretical spectrum. In the case of neon, numerous emission lines of the neutral atom, Ne I, and the single-charged ion, Ne II, were subject of analysis. Quantitative results from the Ne I and NeII shows that partial LTE condition is a reasonable description for the low-temperature EUV-induced Ne plasma. In the case of argon, most of the emission lines were detected in the wavelength range of nm, and the strongest emission lines originated from the single-charged Ar ions. Theoretical spectra were calculated using PrismSPECT in a non-lte model. The electron density of cm -3 and an electron temperature of 1.85 ev were shown to be the best-fit parameters to reproduce the experimental spectra. In the case of nitrogen, the dominant molecular band transitions in the plasma were the electronic transitions from the 2PS (C3Πu - B3Πg) and 1NS (B2Σ+u - X2Σ+g) systems. To reproduce the experimental spectra for interpretation, the molecular simulation software, LIFBASE and SPECAIR. The calculated emission lines using SPECAIR showed good agreement with the experimentally measured spectrum.

5 In the chapter 6 spectral investigations of photoionized plasma created in Kr/Ne/H2 mixture are presented, and they have been published in Physics of Plasmas and Ismail Saber is the first author. The photoionized plasmas were created by irradiation of this gas mixture using EUV pulses from the EUV source which based on a double stream gas puff target (Xe/He) irradiated with the Nd: YAG (1064 nm) laser with parameter: the energy of 0.8 J, pulse duration of 4 ns, and 10 Hz repetition rate. The EUV radiation from the Xe/He plasma was then focused into the Kr/Ne/H2 mixture using a gold-plated grazing-incidence ellipsoidal collector. The mixture was injected into the interaction region via an auxiliary electromagnetic valve. Irradiation of the gas mixture injected into the interaction region synchronously with the EUV pulses, led to ionization and the plasma formation. The plasma emissions are recorded using spectral UV/VIS spectrometer. The time-resolved plasma emissions were measured. Numerous emission lines of neutral atoms and singly ionized with various intensities were detected. The plasma electron temperature was determined using the Boltzmann plot method with the spectral characteristics of singly-ionized and neutral atoms. from the plasma species, while the FWHM values of the Stark broadening of the intense isolated lines in singly charge states and neutral atoms were used to evaluate the electron density of the plasma. The electron temperature found in the range of between 0.5 and 2 ev. The electron density was of the order of In cm-3 the case of the Hβ line, the maximum ne reached cm -3. Based on the temporal dependence of both Te and ne the local LTE was not achieved in the plasma.. Additionally, the measured and simulated results for the dependence of the plasma emission line intensity on the delay time were also studied. A gradual decrease in the line intensities with increasing delay time was observed. These intense lines were dominated by the neutral and singly charged atoms. Analysis of the emission lines showed that the radiative transitions in the single ions mostly contributed to the spectra in the photoionized plasma. In the chapter 7 the spectral investigation of the low-temperature plasma in the UV/VIS and EUV spectral range creating by irradiation of the atomic and molecular gases with unfocused EUV radiation were carried out. The aim of this investigation was to study the influence of the unfocused radiation on atomic and molecular gases and, and to know the importance of this experimental configuration. Plasma emissions from different atomic, molecular and gas mixtures were measured and the simulation of these emissions has helped to assign the origin of vibrational spectra emitted from the molecular plasma. The electron densities of the plasma induced using unfocused EUV radiation have shown the same order magnitude with the electron densities measured in the photoionized plasma produced using the EUV radiation. For

6 different cases, the plasma emissions contain the spectral lines from the laser-produced He plasma. The emissions from the He plasma are independent of the distance between the gas injected and LPP. Most importantly, the advantage of this experimental configuration is the production of the low-temperature plasma without the necessity of using the EUV collector. The research presented in this chapter concerns two papers published by the team from IOE MUT, but their co-operative is not J. Saber. Evaluation of the doctoral dissertation Coming to the review of the dissertation, I would like to emphasize that the work is interesting, but unconventionally edited. The layout of the work is more like a monograph, because chapters 2-4 are descriptive and edited on the base of literature knowledge contained in books and scientific journals, many of them published by IOE MUT team, working on laser plasma spectroscopy. Such an arrangement of the work seems to be justified when one considers that Ismail Saber came to the MUT doctoral program and joined the IOE MUT team that has an established scientific position in the world, concerning laser plasma spectroscopic studies. Having a cutting-edge diagnostic equipment for spectroscopic measurements and a team of high-class specialists, the PhD student did not have to deal with its construction, but he could concentrate on acquiring knowledge in the field of plasma spectroscopy, participating in ongoing experiments conducted by IOE MUT. Such work system, accepted by both promoters, gives the reviewers an additional opportunity to evaluate knowledge of plasma spectroscopy, gained by the student while working in IOE MUT. In addition, the doctoral dissertation, after appropriate changes, can be published in the form of a book or script useful for students. Evaluating the chapters 5-7, which are describing plasma research carried out with participation of the PhD student, I would like to note that they are edited in a very careful and understandable way for a reader. Each chapter ends with a well-done summary of the results obtained and an adequate list of literature. I did not find any substantive errors within the chapters, while reading and analyzing them. First of all, I would like to emphasize that some of the material contained in the work has been published in peer-reviewed journals, and therefore it has already been the subject of a review. From the bibliography presented on page 8 it results that I. Saber is the first author in 5 out of 7 publications, which clearly defines his contribution presented in these chapters. However, the PhD student does not explicitly state his contribution to the research

7 presented in each of the chapters, nor in the final conclusions, and this raised my doubts about the PhD student's contribution to the research included in chapters 5 and 7. In case of chapter 5, 4 papers of the IOE MUT team (among 46 references) are related to the content of the chapter, but only one (number 46, without the student s authorship), about photoionized plasma generated in nitrogen, is consistent with the subject of the chapter. The lack of references about photoionized plasma produced from neon or argon suggests that the studies have not been published yet, although they are presented in the chapter. In case of chapter 7, the reference list shows that only 2 publications by IOE MUT (but without the student s authorship) concern the research presented in this chapter. According to the above, the reviewer asks for more-detailed explanation (at the time of the defense) of the PhD student s contribution in the studies presented in chapters 5 and 7. Except the above doubts, I found a few typing errors language short comings, mentioned below: - page 15: Figure 4.5 there should be photoionized instead of photionized - page 39: in Ref. [5] before the name of the journal there is forgotten sign " - page 40: in Ref. [19] there should be N. Kroll and K. M. Watson instead of N. Karol and K. M. Waston - page 40: in Ref. [24] there should be D. Attwood instead of D. Atwood - page 44: in Ref. [56] there should be Ł. Węgrzyński instead of Ł. Wegrzynsśki - page 45: in Ref. [63] there should be Ł. Węgrzyński instead of Ł. Wegrzynsśki - page 46: in Ref. [74] there should be B. Korczyc and H. Fiedorowicz instead of B. Kroczyc and H. Fieodorowicz - page 67: Figure 4.5 there should be "photoionized" instead of "photionized" - page 74: wrong order in Ref. [5] - page 103: in Ref. [14] there should be C. Stehlé instead of C. Sthelé - page 104: in Ref. [20] there should be W. Skrzeczanowski instead of W. Wkrzeczanowski and Ł. Węgrzyński instead of Ł. Wegrzynsśki - page 113; Chapter 6.4; paragraph 2; line 9: there should be Echelle instead of echelle - page 113; Chapter 6.4; paragraph 2; line 10: there should be Echelle instead of echelle

8 - page 121: in Ref. [2] there should be W. Skrzeczanowski instead of W. Wkrzeczanowski and Ł. Węgrzyński instead of Ł. Wegrzynsśki - page 121: in Ref. [3] there should be Ł. Węgrzyński instead of Ł. WeRgrzynski - page 148: in Ref. [7] there should be J. Czwartos instead of J czwartos - page 149: wrong order in Ref. [21] However, these typing errors do not affect the final PhD thesis evaluation. Summarizing this dissertation, I conclude that the scientific material obtained as part of the work is a measurable contribution to the supplementation of knowledge about photoionized plasma, produced from atomic and molecular gases with the help of LLP sources of EUV irradiation. I consider that doctoral dissertation presented by mgr. Eng. Ismail Saber: "Spectral investigation of extreme ultraviolet induced plasmas", meets all the substantial and formal requirements resulting from the current regulations on academic degrees, therefore, I apply for admission of the thesis to a public defense.