IOP View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences Plasma Physics and Controlled Fusion Plasma Physics and Controlled Fusion Plasma Phys. Control. Fusion Plasma Phys. Control. Fusion 57 (2015) 075011 (7pp) doi:10.1088/0741-3335/57/7/075011 57 Enhancement of laser to x-ray conversion 2015 by a double-foil gold target © 2015 IOP Publishing Ltd Z Y Ge1, R Ramis2, X H Yang1, T P Yu1, B B Xu1, Y Zhao1, H B Zhuo1, Y Y Ma1, W Yu3 and X J Peng4 PPCF 1 College of Science, National University of Defense Technology, Changsha 410073, People’s Republic of China 075011 2 E.T.S.I. Aeronáuticos, Universidad Politécnica de Madrid, Madrid 28040, Spain 3 Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, Z Y Ge et al People’s Republic of China 4 Institute of Applied Physics and Computational Mathematics, Beijing 100088, People’s Republic of China Enhancement of laser to x-ray conversion by a double-foil gold target E-mail: [email protected] and [email protected] Printed in the UK Received 30 January 2015, revised 10 May 2015 Accepted for publication 19 May 2015 Published 18 June 2015 PPCF Abstract A novel double-foil configuration is proposed to improve the laser to x-ray conversion 10.1088/0741-3335/57/7/075011 efficiency from laser irradiating a solid target. One-dimensional radiation hydrodynamic simulations show that the total x-ray conversion efficiency for the double-foil target is as high as 54.7%, which has a 10% improvement compared with the normal target. The improvement Papers is mainly due to the enhanced soft x-ray emissions. Influences of the target geometry parameters on the x-ray conversion efficiency are investigated. Detailed energy distributions 0741-3335 and the individual contributions of the two foils to the thermal and kinetic energy terms are presented. It is found that the main energy terms are mostly determined by the first foil, and the enhancement of radiation is attributed to the lower ion kinetic energy of the double-foil target. 7 Keywords: laser-plasma interaction, x-ray generation, radiation hydrodynamics (Some figures may appear in colour only in the online journal) 1. Introduction any kind of x-ray photons in the energy range of 0.1 keV to 10 keV can be obtained by changing the target materials. Laser-produced plasmas as intense x-ray sources have However, the CE of the x-ray generated by a solid target is attracted much interest in recent decades due to their wide limited due to the narrow emission region [21, 22]. Gas targets applications in many fields, such as advanced lithography [1], and doped low-density foam targets have been proposed as x-ray backlighter imaging [2], high energy density physics [3] more efficient x-ray sources owing to their very large emis- and inertial confinement fusion (ICF) [4–6]. One of the major sion region [23–29]. However, the gas targets are restricted goals in these applications is to enhance the x-ray conver- to a small range of photon energy, and the doped low-density sion efficiency (CE) and therefore the x-ray emission inten- foam targets are limited to very few materials because of the sity. Considerable work has been carried out by varying laser complex fabrication techniques. X-ray emissions can be also parameters such as wavelength, pulse profile, pulse duration enhanced by utilizing the vacuum hohlraum targets in ICF and intensity, or using various target materials and structures because the hot, underdense coronal plasma is confined by the in order to achieve higher x-ray radiation [7–17, 18]. hohlraum [30–32]. However, this type of target is not always The conventional method for generating x-rays is the nano- practical since the generated x-rays are also confined inside second laser irradiation of high-Z solid targets, because the the hohlraum. Pre-exploded thin foil can also provide an effi- fabrication of a solid target is very simple and high-Z mate- cient x-ray source that is very close to those obtained from rials exhibit a higher x-ray CE [19, 20]. Moreover, almost gas or doped low-density foam targets [33, 34]. However, the 0741-3335/15/075011+7$33.00 1 © 2015 IOP Publishing Ltd Printed in the UK Plasma Phys. Control. Fusion 57 (2015) 075011 Z Y Ge et al Figure 1. Scheme of the double-foil target. laser parameters and foil thickness should be chosen carefully, those obtained from the normal solid targets [19, 20]. Very otherwise the x-ray emission can only be enhanced by a small long time plasma expansion and detailed non-local thermody- margin, since this scheme changes the target ablation process namic equilibrium (NLTE) atomic physics make a pure ana- from one-sided expansion (thick foil) to two-sided expansion lytical calculation and optimization of this configuration very (pre-exploded thin foil). Rarefaction waves coming from two difficult and not precise enough for designing experiments. sides of the thin foil will destroy the emission region quickly For that purpose, numerical simulations are performed with [35]. To depress the rarefaction wave of the hot coronal plasma the one-dimensional multi-group radiation hydrodynamics generated from the solid targets and enhance the laser to x-ray code Multi-1D [36, 37]. The hydrodynamic equations are CE, new approaches or solutions are required. solved in a Lagrangian formulation with coupled radia- In this paper, a novel double-foil gold target is proposed tion, electron thermal transport and laser energy deposition in order to enhance the laser to x-ray CE further. The double- mechanism of inverse bremsstrahlung. The electron thermal foil target consists of a thin foil and a thick foil as illustrated conduction is described by the interpolation between the in figure 1, which combines the one-sided (second thick foil) Spitzer’s regime and the flux limited regime. The flux limiter and two-sided (first thin foil) expansion together to a three- is set as f = 0.08, which is the same as that used for the anal- sided expansion. The presence of the first thin foil provides ysis of previous experiments [38]. MPQeos code provides the an additional two-sided expanding plasma x-ray source and data of equation of state (EOS) for Au in a tabular form [39]. avoids the direct irradiation of laser pulse on the second foil. Tabulated NLTE opacities divided into 100 energy groups in The second thick foil is mainly ablated by the x-ray radiation the range of 0.1–5 keV are calculated with the atomic physics generated at the rear side of the first thin foil. Collision of the code SNOP [40]. inner-expanding plasmas in the gap between the two foils In the simulations, 1 ns flat-top pulse, p-polarized lasers 14 2 will depress the rarefaction waves in the two foils. Ion kinetic with an intensity of 5 × 10 W cm are supposed to normally energy is then suppressed and converted to electron internal incident on the double-foil gold target from the left side as energy and radiation energy, thus the x-ray CE is enhanced shown in figure 1. The laser wavelength is 351 nm, which remarkably. Compared with the pre-exploded thin foil method, represents the typical frequency-tripled Nd:glass laser in the double-foil target exhibits a higher x-ray CE, and a prepulse experiments (short-wavelength laser always exhibits a higher is not needed anymore. One can also avoid the complex fab- laser to x-ray CE). The thickness of the first thin foil isd 1 = 0.3 rication technique for a gas or doped low-density foam target. μm, and the distance between the two foils is d2 = 400 μm. Au Furthermore, this double-foil target also offers the ability to is chosen as the target material for its high laser to x-ray CE generate a wide range of photon energies as the solid target. and wide applications, especially in ICF. Both the first thin This paper is structured as follows. In section 2, the detail foil and second thick foil (10 μm) have an initial density of 3 7 3 radiation hydrodynamics simulations are presented. Section 3 19.2 g cm− . A 1.92 × 10− g cm− low-density Au gas is filled deals with the influences of the target geometry parameters on in the gap between the two foils because vacuum is not per- the x-ray CE. Finally, we summarize the results and give the mitted in the simulation. For comparison, the laser irradiating conclusions in section 4. normal solid Au target is also simulated. Figure 2 shows the motion of the Lagrangian interfaces of 2. Numerical simulations the double-foil target. The red, blue and black lines represent the first thin foil, the second thick foil and the low-density The construction of a double-foil target can lead to under- Au gas, respectively. The laser pulse coming from the bottom dense plasma conditions that are much more complex than impinges on the target front side and deposits its energy in the 2 Plasma Phys. Control. Fusion 57 (2015) 075011 Z Y Ge et al 0.05 As shown in figure 3(a), for the normal solid target, after getting energy from the incident laser via the inverse 0.04 bremsstrahlung, electron internal energy increases very rap- idly. Then electrons convert their energies to ions primarily 0.03 through coulomb collision, atoms are thus ionized and excited, resulting in the generation of x-ray radiation.