Ion pinhole imaging diagnostics on fast ion source in ... - OSA Publishing

Vol. 25, No. 14 | 10 Jul 2017 | OPTICS EXPRESS 16419

Ion pinhole imaging diagnostics on fast ion source in femtosecond laser plasma of cluster targets SERGEY MAKAROV,1,2 SERGEY PIKUZ,1,3 ANATOLY FAENOV,1,4,* TATIANA PIKUZ,1,5 YUJI FUKUDA,6 IGOR SKOBELEV,1,3 IRINA ZHVANIYA,1,7 SERGEY VARZAR,2 MASAKI KANDO,6 AND RYOUSUKE KODAMA4,5,8 1Joint

Institute for High Temperatures RAS, Moscow 125412, Russia of Physics of M.V. Lomonosov Moscow State University, Moscow 119991, Russia 3National Research Nuclear University MEPhI, Moscow 115409, Russia 4Open and Transdisciplinary Research Initiatives Institute, Osaka University, Suita, Osaka 565-0871, Japan 5Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan 6Kansai Photon Science Institute, National Institutes for Quantum and Radiological Science and Technology, Kizugawa, Kyoto, Japan 7International Laser Center of M.V. Lomonosov Moscow State University, Moscow 119991, Russia 8Photon Pioneers Center, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871 Japan *[email protected] 2Faculty

Abstract: The spatial configuration of the ion source generated under femtosecond laser interaction with clusters is investigated. While intense laser pulses (36 fs, 60 mJ, intensity of 4 × 1017 W/cm2) propagated in CO2 cluster (~0.22 μm in diameter) media, the shape of the obtained plasma ion source was registered for the first time by means of pinhole imaging method. The remarkable decrease in fast ion yield in the vicinity of the assumed best laser focus near the gas cluster jet axis is observed. Such observed anisotropy of the ion source is suggested to originate from the influence of the laser prepulse destroying clusters in advance to the arrival of the main pulse. The assumption is confirmed by optical shadowgraphy images of the plasma channel and is important for further development of an efficient laser-plasmabased fast ion source. Following the observed geometry of the ion source, the laser intensity limit allowing to accelerate ions to ~100 keV energy range was estimated. © 2017 Optical Society of America OCIS codes: (020.2649) Strong field laser physics; (280.5395) Plasma diagnostics.

References and links 1. 2. 3. 4. 5.

6.

7.

V. P. Krainov and M. B. Smirnov, “Cluster beams in the super-intense femtosecond laser pulse,” Phys. Rep. 370(3), 237–331 (2002). T. Ditmire, T. Donnelly, A. M. Rubenchik, R. W. Falcone, and M. D. Perry, “Interaction of intense laser pulses with atomic clusters,” Phys. Rev. A 53(5), 3379–3402 (1996). H. M. Milchberg, S. J. McNaught, and E. Parra, “Plasma hydrodynamics of the intense laser-cluster interaction,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 64(5), 056402 (2001). T. Fennel, K.-H. Meiwes-Broer, J. Tiggesbäumker, P.-G. Reinhard, P. M. Dinh, and E. Suraud, “Laser-driven nonlinear cluster dynamics,” Rev. Mod. Phys. 82(2), 1793–1842 (2010). Y. Fukuda, A. Ya. Faenov, M. Tampo, T. A. Pikuz, T. Nakamura, M. Kando, Y. Hayashi, A. Yogo, H. Sakaki, T. Kameshima, A. S. Pirozhkov, K. Ogura, M. Mori, T. Zh. Esirkepov, J. Koga, A. S. Boldarev, V. A. Gasilov, A. I. Magunov, T. Yamauchi, R. Kodama, P. R. Bolton, Y. Kato, T. Tajima, H. Daido, and S. V. Bulanov, “Energy increase in multi-MeV ion acceleration in the interaction of a short pulse laser with a cluster-gas target,” Phys. Rev. Lett. 103(16), 165002 (2009). Y. Fukuda, H. Sakaki, M. Kanasaki, A. Yogo, S. Jinno, M. Tampo, A. Ya. Faenov, T. A. Pikuz, Y. Hayashi, M. Kando, A. S. Pirozhkov, T. Shimomura, H. Kiriyama, S. Kurashima, T. Kamiya, K. Oda, T. Yamauchi, K. Kondo, and S. V. Bulanov, “Identification of high energy ions using backscattered particles in laser-driven ion acceleration with cluster-gas targets,” Radiat. Meas. 50, 92–96 (2013). H. Daido, M. Nishiuchi, and A. S. Pirozhkov, “Review of laser-driven ion sources and their applications,” Rep. Prog. Phys. 75(5), 056401 (2012).

#292637 Journal © 2017

https://doi.org/10.1364/OE.25.016419 Received 17 Apr 2017; revised 14 Jun 2017; accepted 27 Jun 2017; published 3 Jul 2017


8. 9.

10.

11.

12.

13.

14.

15. 16.

17.

18.

19. 20.

21.

22. 23.

24.

25.

26.

27.

S. V. Bulanov, J. J. Wilkens, T. Zh. Esirkepov, G. Korn, G. Kraft, S. D. Kraft, M. Molls, and V. S. Khoroshkov, “Laser ion acceleration for hadron therapy,” Phys. Uspekhi 57(12), 1149–1179 (2014). A. Ya. Faenov, T. A. Pikuz, S. A. Pikuz, Jr., Y. Fukuda, M. Kando, H. Kotaki, T. Homma, K. Kawase, T. Kameshima, A. Pirozhkov, A. Yogo, M. Tampo, V. Kartashev, I. Yu. Skobelev, S. V. Gasilov, A. S. Boldarev, V. A. Gasilov, A. Magunov, S. Kar, M. Borghesi, A. Giulietti, C. A. Cecchetti, M. Mori, H. Sakaki, Y. Hayashi, P. Bolton, T. Nakamura, H. Daido, T. Tajima, Y. Kato, and S. V. Bulanov, “Ionography of submicron foils and nanostructures using ion flow generated in FS-laser cluster plasma,” Plasma Phys. 49(7-8), 507–516 (2009). A. Ya. Faenov, T. A. Pikuz, and R. Kodama, “Laser-Driven Particle Acceleration Towards Radiobiology and Medicine,” in High Resolution Ion and Electron Beam Radiography with Laser-Driven Clustered Sources (Springer Heidelberg, 2016). S. Dobosz, M. Schmidt, M. Perdrix, P. Meynadier, O. Gobert, D. Normand, K. Ellert, T. Blenski, A. Ya. Faenov, A. I. Magunov, T. A. Pikuz, I. Yu. Skobelev, and N. E. Andreev, “Observation of ions with energies above 100 keV produced by the interaction of a 60-fs laser pulse with clusters,” J. Exp. Theor. Phys. 88(6), 1122–1129 (1999). G. C. Bussolino, A. Faenov, A. Giulietti, D. Giulietti, P. Koester, L. Labate, T. Levato, T. Pikuz, and L. A. Gizzi, “Electron radiography using a table-top laser-cluster plasma accelerator,” J. Phys. D Appl. Phys. 46(24), 245501 (2013). Y. L. Shao, T. Ditmire, J. W. G. Tisch, E. Springate, J. P. Marangos, and M. H. R. Hutchinson, “Multi-keV Electron Generation in the Interaction of Intense Laser Pulses with Xe Clusters,” Phys. Rev. Lett. 77(16), 3343– 3346 (1996). Y. Fukuda, Y. Akahane, M. Aoyama, N. Inoue, H. Ueda, Y. Kishimoto, K. Yamakawa, A. Ya. Faenov, A. I. Magunov, T. A. Pikuz, I. Yu. Skobelev, J. Abdallah, G. Csanak, A. S. Boldarev, and V. A. Gasilov, “Generation of X rays and energetic ions from superintense laser irradiation of micron-sized Ar clusters,” Laser Part. Beams 22(03), 215–220 (2004). I. Alexeev, T. M. Antonsen, K. Y. Kim, and H. M. Milchberg, “Self-focusing of intense laser pulses in a clustered gas,” Phys. Rev. Lett. 90(10), 103402 (2003). G. S. Sarkisov, V. Yu. Bychenkov, V. N. Novikov, V. T. Tikhonchuk, A. Maksimchuk, S. Y. Chen, R. Wagner, G. Mourou, and D. Umstadter, “Self-focusing, channel formation, and high-energy ion generation in interaction of an intense short laser pulse with a He jet,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 59(6), 7042–7054 (1999). C. Stenz, V. Bagnoud, F. Blasco, J. R. Roche, F. Salin, A. Y. Faenov, A. I. Magunov, T. A. Pikuz, and I. Y. Skobelev, “X-ray emission spectra of the plasma produced by an ultrashort laser pulse in cluster targets,” Quantum Electron. 30(8), 721–725 (2000). A. Ya. Faenov, A. I. Magunov, T. A. Pikuz, I. Yu. Skobelev, S. Stagira, F. Kalegari, M. Nisoli, S. De Silverstri, L. Poletto, P. Villoresi, and A. A. Andreev, “Generation of Fast Ions in Femto- and Picosecond Laser Plasmas at Low Intensities of the Heating Radiation,” JETP Lett. 84(6), 308–313 (2006). K. Krushelnick, E. L. Clark, Z. Najmudin, M. Salvati, M. I. K. Santala, M. Tatarakis, and A. E. Dangor, “MultiMeV Ion Production from High-Intensity Laser Interactions with Underdense Plasmas,” PRL 22, 737 (1999). Y. Fukuda, A. Ya. Faenov, T. Pikuz, M. Kando, H. Kotaki, I. Daito, J. Ma, L. M. Chen, T. Homma, K. Kawase, T. Kameshima, T. Kawachi, H. Daido, T. Kimura, T. Tajima, Y. Kato, and S. V. Bulanov, “Soft X-ray source for nanostructure imaging using femtosecond-laser-irradiated clusters,” Appl. Phys. Lett. 92(12), 121110 (2008). A. Ya. Faenov, T. A. Pikuz, Y. Fukuda, M. Kando, H. Kotaki, T. Homma, K. Kawase, T. Kameshima, A. Pirozhkov, A. Yogo, M. Tampo, M. Mori, H. Sakaki, Y. Hayashi, T. Nakamura, S. A. Pikuz, Jr., I. Yu. Skobelev, S. V. Gasilov, A. Giulietti, C. A. Cecchetti, A. S. Boldarev, V. A. Gasilov, A. Magunov, S. Kar, M. Borghesi, P. Bolton, H. Daido, T. Tajima, Y. Kato, and S. V. Bulanov, “Submicron ionography of nanostructures using a femtosecond-laser-driven-cluster-based source,” Appl. Phys. Lett. 95(10), 101107 (2009). A. S. Boldarev, V. A. Gasilov, A. Ya. Faenov, Y. Fukuda, and K. Yamakawa, “Gas-cluster targets for femtosecond laser interaction: Modeling and optimization,” RSI 77, 083112 (2006). S. Jinno, Y. Fukuda, H. Sakaki, A. Yogo, M. Kanasaki, K. Kondo, A. Ya. Faenov, I. Yu. Skobelev, T. A. Pikuz, A. S. Boldarev, and V. A. Gasilov, “Characterization of submicron-sized CO2 clusters formed with a supersonic expansion of a mixed-gas using a three-staged nozzle,” Appl. Phys. Lett. 102(16), 164103 (2013). S. Jinno, Y. Fukuda, H. Sakaki, A. Yogo, M. Kanasaki, K. Kondo, A. Ya. Faenov, I. Yu. Skobelev, T. A. Pikuz, A. S. Boldarev, and V. A. Gasilov, “Mie scattering from submicron-sized CO2 clusters formed in a supersonic expansion of a gas mixture,” Opt. Express 21(18), 20656–20674 (2013). A. Ya. Faenov, S. A. Pikuz, A. I. Erko, B. A. Bryunetkin, V. M. Dyakin, G. V. Ivanenkov, A. R. Mingaleev, T. A. Pikuz, V. M. Romanova, and T. A. Shelkovenko, “High-performance X-ray spectroscopic devices for plasma microsources investigations,” Phys. Scr. 50(4), 333–338 (1994). F. Blasco, C. Stenz, F. Salin, A. Ya. Faenov, A. I. Magunov, T. A. Pikuz, and I. Yu. Skobelev, “Portable, tunable, high-luminosity spherical crystal spectrometer with X-Ray charge cuple device, for high-resolution XRay spectromicroscopy of clusters, heated by femtosecond laser pulses,” Rev. Sci. Instrum. 72(4), 1956–1962 (2001). I. Yu. Skobelev, A. Ya. Faenov, A. I. Magunov, T. A. Pikuz, A. S. Boldarev, V. A. Gasilov, J. Abdallach, Jr., G. C. Junkel-Vives, T. Auguste, S. Dobosz, P. d’Oliveira, S. Hulin, P. Monot, F. Blasco, F. Dorchies, T. Caillaud,


C. Bonte, C. Stenz, F. Salin, P. A. Loboda, I. A. Litvinenko, V. V. Popova, G. V. Baidin, and B. Yu. Sharkov, “X-ray spectroscopy diagnostic of a plasma produced by femtosecond laser pulses irradiating a cluster target,” J. Exp. Theor. Phys. 94(5), 966–976 (2002). 28. G. A. Mourou, T. Tajima, and S. V. Bulanov, “Optics in the relativistic regime,” Rev. Mod. Phys. 78(2), 309– 371 (2006).

1. Introduction At present time, there are a number of investigations devoted to the study of gas clusters as a unique media for the femtosecond laser-matter interaction [1–4]. One of the most attractive applications of femtosecond laser-induced plasma is to provide a source of the high energy ion [5–7] for various fields of science and life such as cancer therapy [8], proton and electron radiography [9,10] and others. The target consisting of gas clusters becomes increasingly common in experiments on the interaction of intense (I > 1015 W/cm2) laser radiation with matter. The benefits of cluster targets include the possibility of efficient generation of ions [11], electrons [12,13], X-ray radiation under femtosecond laser excitation [14]; the absence of debris that can damage optics, quick renewal of the initial target parameters to each act of laser action; and the possibility to reach high absorption rate of laser radiation up to 90%. Also, the clustered environment has unique nonlinear optical properties that result in filamentation of femtosecond laser radiation [15], formation of a plasma channel [16], self-focusing of laser radiation [16] and generation of its harmonics. It is very important question about the distribution of ion source in cluster targets. Imaging of the ion emission may be quite useful to determine the direction and cone angle of ion emission from various regions of the target. Knowledge of the processes occurring in the plasma can be used to optimize the method of ion radiography, as well as for development of fundamental physical theories and models of interaction of lasers with cluster targets. However, the spatial distribution of the ion source formed by the interaction of laser radiation with gas-cluster targets is still poorly investigated. There is a variety of diagnostics common for laser-plasma interaction studies such as interferometry, shadowgraphy and so on, but none of them gives complete information about laser-cluster interaction or are particular about processes of ion ionization and acceleration. In order to understand better the mechanisms leading to the emission of ionizing radiation from laser-plasma interaction, commonly used imaging diagnostics such as interferometry and shadowgraphy have to be supplemented by methods providing the data on the most dense plasma / most intense laser region. One of the possible solution is in use of pinhole camera technique proving an self-emission image of laser plasma in visible or X-rays [17]. Furthermore, the similar approach but focused on fast ion emission from the plasma can be used providing the spatially resolved image of the ion source in plasma [18,19]. The scope of our work is to apply, for the first time, the ion pinhole imaging diagnostic method to reveal the spatial configuration of the ion source formed in the interaction of femtosecond laser pulses with gas cluster media and to provide a new information about the mechanisms leading to the fast ion generation. 2. Experimental description The experiment has been performed using a JLITE-X Ti:Sa laser facility at Kansai Photon Science Institute (JAEA), which generates 36 fs pulses with energy of 160 mJ (see for details [20]). The experimental setup is shown in Fig. 1. The laser beam was focused by an off-axis f/13 parabola at the spot size around 50 μm, what corresponds to laser intensity in vacuum ~4 × 1017 W/cm2 [21]. Especially designed supersonic gas jet nozzle driven at 50 bar pressure with input and output diameters of 0.5 mm and 2.0 mm, respectively, and 75 mm length was used [22]. The laser beam was focused about 1.5 mm below the nozzle outlet. The nozzle allowed producing clusters with a big size from CO2 and mixture of 90% He and 10% CO2 gases. For the latter case, as simulated in [23,24], CO2 clusters of up 0.22 μm diameter containing 5 × 108 molecules each are embedded in the He gas.


The laser propagation in the cluster jet was monitored by shadowgraphy images made by the probe laser beam at the same duration (see Fig. 1). Two pinholes with diameter 5 and 25 μm were placed symmetrically to the laser beam propagation direction at angles ± 45° and at 50 mm distance from the plasma source. Fast ions that were generated in the plasma were detected using CR-39 polymer detectors placed at distance of 150 mm from the pinhole. The pits formed in CR-39 were established following 9-hours of etching in 6N-KOH solution of 70 C° temperature, and then read out by optical scanner. In order to control X-ray emission of the plasma, the spatially resolved X-ray spectra were measured using focusing spectrometers with spatial resolution (FSSR) [25]. FSSR spectrometer (see Fig. 1) was equipped by spherically bent mica crystal as a dispersive element and by Andor 420 back illuminated CCD as X-ray detector and installed normally to the direction of laser beam propagation at a distance of 400 mm from the laser plasma source [26].

Fig. 1. Experimental setup to study the process of ion acceleration in fs laser interaction with gas cluster media at JLITE-X facility.

3. Results and discussion In experiment, several laser focusing regimes introduced in Fig. 2 were realized for a more detailed analysis of the processes occurring in the interaction of laser radiation with gas-cluster medium. As clearly appeared from CR-39 data there, for all the cases the source shape possesses a complicated geometry and cannot be considered as isotropic. In order to explain the observed inhomogeneity in the spatial configuration of the ion source, let’s first consider the case of the laser focusing to the center of the gas cluster jet (X = 0). Pinhole images of the ion source, corresponding ion image density distribution and optical shadowgrapy images are given in Fig. 3. It is clearly seen there that the images of plasma structure obtained by ion pinhole and by optical shadowgraphy are quite similar having the feature in the vicinity of the best laser focus, or jet axis. The observed decrease of ion generation efficiency in the vicinity of the best focus point is opposite to what might be expected from the laser intensity and cluster density distributions. It has been suggested the


fact might be explained by a gap in cluster concentration there due to the impact of laser prepulse destroying clusters in advance to the arrival of the main laser pulse. Note, in [27] it was shown that the flux density of a few to 1012 W/cm2 in a prepulse becomes enough for CO2 cluster destruction. Thus, with the laser pulse contrast of about 105, as expected for JLITE-X laser facility, and the peak intensity of 4 × 1017 W/cm2, at least partial cluster destruction is mostly expected at best laser focusing area.

Fig. 2. Pinhole images detected by CR-39 detector for different focus position of the laser.

Fig. 3. (a) Pinhole images on CR-39 for the two diameters of aperture: 5um and 25um. (b) Spatial distribution of the ions intensity on pinhole images (the value x = 0 mm corresponds the center of nozzle). (c) Shadowgraph image for a mixture of He gas and CO2 clusters for different time. The red line shows the initial atom density profile [5].

To verify the assumption, it is possible to refer on optical shadowgraphy data showing the plasma channel formation with ps time resolution. As seen in upper part of Fig. 3(c), the plasma ionization threshold was indeed exceeded at the laser beam axis and the plasma channel was formed several tens ps earlier then the arrival of the main laser pulse. Though the channel lasts


across the full area of gas/cluster cloud above the nozzle, the cluster destruction threshold is appeared to be exceeded in the center of the jet only. Such inhomogeneity in the central zone is seen at times after the main pulse propagation as well (bottom part of Fig. 3(c)). Note, the laser prepulse may play an important positive role in the mechanism of ions acceleration providing a strong dipole vortex plasma structure at the rear side of the jet and consecutive generation of 10-20 MeV/amu ions with a small full-angle divergence of 4° [5]. There is another interesting feature in the shape of the ion source as shown in Fig. 3(b). The spatial distribution of the ion source features with a higher yield behind the best focusing plane than in front of that, which reflects the effect of the laser beam self-focusing in gas cluster media after the intensity reaches 1017 W/cm2 range. The minimum estimate [24] for the electron density in target media is of 1x1020 cm−3, which corresponds to ~0.05 of the critical density ncr. The power critical for the relativistic self-focusing of the laser radiation in the target may be estimated as [28]  w me2 c 5 w2 Pcr =≈ 17  2 2 w e wp  p

2

 n 1.8*1021 (1) ≈ 0.3 TW 17 cr = 17 *  [GW ] = ne 1020  Due to the power of the laser main pulse in the experiment was of 4 TW, the self-focusing effect is expected to play significant role in the laser propagation in the gas-cluster media leading to an enhancement in the peak laser intensity, and to the changes in plasma channel and ion source geometry. So we may conclude that the optimal conditions for the ion generation was provided 200 μm behind the supposed best laser focusing plane, with the combination of the still high laser intensity and cluster surviving. Also, two pinholes installed at orthogonal directions to the laser beam axis demonstrate practically the same structure and sizes of the ion source. The fact is consistent with the concept of ion generation in laser-cluster plasma due to Coulomb explosion of clusters. Here, the signal from 5 μm pinhole is multiplied by 25 times normalizing the relative optical efficiency of 5 and 25 μm pinholes. The less signal behind 25 μm pinhole then associated with the readout scanner saturation. The next case studied was to focus the laser beam further away by 2.2 mm in respect to the nozzle axis (position Xnozzle = −2.2 mm in Fig. 2). The processes occurring inside the target is similar to the situation discussed above. In this case, the optical shadowgraphy data shows that the plasma channel was formed with the length of about 3 mm (bottom part of Fig. 4(c)). Note, the filamentation effect is well protruded in the plasma channel. As seen from ion pinhole images and profiles in Fig. 4(a,b) there is an asymmetry in the spatial distribution of the ion source along the laser axis too. The longitudinal size of the ion source is measured of 1.5 mm, which is less than the nozzle orifice. It is clear that the right edge of the ion source is limited by the size of the cluster jet, while the shape of the left boundary in ion source profile is associated with the decrease of the laser intensity. Taking into account the estimation on laser beam geometry focused by off-axis parabola with f/13 factor, it is possible to reveal the laser intensity limit allowing to accelerate ions up to the energies detectable by CR-39 detector, typically above 100 keV range. At 2.3 mm away from the laser beam waist the laser intensity should drop down approximately 17 times, which returns the estimation for the threshold for the fast ion generation in CO2 cluster media of about 2 × 1016 W/cm2. The estimate corresponds well to the transverse size of the ion source measured around the best focusing to be about 150-180 μm in radius. In the case of the laser focus to the center of the nozzle, the size of the ion source along the laser axis was measured to be of ~2.5 mm, which corresponds well to the diameter of the gas nozzle orifice, i.e. the full distribution of the ion yield along the laser axis is mostly determined by the concentration of cluster media alone.


The last case of laser focusing before the center of the nozzle (see Fig. 2. Xnozzle = + 2.3mm) provides the data consistent with that one for the focusing 2.2 mm after the nozzle axis, but of a bit less ion yield. Furthermore, the energy of the imaged ions was considered. When 1 μm polypropylene foil was placed in contact with CR-39 the intensity of the ion image decreased by ~30%, due to the ions with energies lower than 300 keV were filtered out. Assuming the initial Maxwell ion energy distribution, the average energy of imaged ions was about 400-500 eV.

Fig. 4. The same as Fig. 3 except that the focus point of the laser shifted by 2.2 mm.

The study of the ion source geometry was accompanied by the measurements of X-ray yield performed using focusing spectrometers with spatial resolution. The spectrometer was aligned to record the emission of H-like Oxygen ions in the range of 0.5 – 1 keV photon energy. As seen in Fig. 5(a), obtained when the laser beam is focused at the jet axis (Xnozzle = 0 mm), X-ray source size was about 400 micron at FWHM, and no gap in the spatial distribution near the best laser focusing is observed. The intensity of X-rays integrated over the observed range depending on the laser focal position inside the target media is given in Fig. 5(b). As it was expected, the maximum integrated intensity of X-rays was achieved when the laser was focused at the center of the nozzle. The similar correspondence was observed for both total fast ion and X-ray yields, regardless of the ion source shape demonstrating the local gap in generation efficiency in the vicinity of the best focus. X-ray yield drops slowly with the nozzle displacement further toward the laser beam direction, which is the evidence of the laser self-focusing in gas cluster media.


Fig. 5. Data obtained using FSSR: (a) Longitudinal x-ray distribution for case when the laser beam focused at the jet axis (Xnozzle = 0 mm). (b) X-ray yield in CO2 cluster target in the photon energy range 0.5 – 1 keV depending on the laser focusing positions related to the gas nozzle axis. When the position of nozzle center is equal to zero, the laser focal position corresponds to the nozzle center.

4. Conclusion The ion pinhole images of the plasma created in gas cluster media by an intense femtosecond laser pulses were obtained. The data taken at orthogonal directions shows remarkable similarity confirming wide-angle ion generation due to Coulomb explosion mechanism. It is demonstrated that the ion source is expanding for few mm along the laser beam propagation in cluster media, while the source cannot be considered as purely isotropic one. When the laser is focused to the center of the cluster jet, the ion source shape is found to feature with a local gap in ion yield around the best laser focusing, most likely associated with the destruction of clusters by the impact of laser prepulse of ~1012 W/cm2 intensity. Such explanation is well consistent with time-resolved optical shadowgraphy data showing the formation of the plasma channel before the arriving of the main laser pulse to the target media. Further, the asymmetry in the ion yield before and after the assumed best laser focusing position is observed associated with the laser self-focusing effect. Also, according to ion pinhole images the size of the ions source is measured to be of (300 ÷ 400) × (1500 ÷ 2500) μm in transverse and longitudinal directions, correspondingly. Based on these values and considering the laser beam geometry in CO2 gas cluster jet, it is revealed that the ions effectively accelerated to 100 keV and above energies while the laser intensity exceeds ~2 × 1016 W/cm2. The data obtained in the research is important for further development of an efficient and compact laser-plasma-based fast ion source. Funding Fast ion and X-ray diagnostics and full data analysis were made at JIHT RAS with financial support of the Russian Science Foundation (grant #14-50-00124). The work at JLITE-X facility was partially supported by a Grant-in-Aid for Scientific Research (A) No. 26247100 by JSPS.