22F01
High Resolution Spectral Features of the Hollow Ions and Inner-Shell X-RayEmission Produced by 10-4 - 10-10 Contrast Femtosecond-Pulse Laser
Irradiation of Argon Clusters
コントラスト比 10-4~10-10 のフェムト秒レーザーとアルゴンクラスターの相互作用により生成される中空イオンと内殻 X 線放射の高分解
能スペクトル線の特長	 
Anatoly Faenov1,2, Tatiana Pikuz1,2, Igor Skobelev2, James Colgan3, Joe Abdallah, Jr.3, Yuji
Fukuda1, Yukio Hayashi1, Alexander Pirozhkov1, Keigo Kawase1, Takuya Shimomura1, Kiriyama
Hiromitsu1, Yoshiaki Kato4, Sergei Bulanov1, Masaki Kando1
1

Quantum Beam Science Directorate, Japan Atomic Energy Agency, 8-1 Umemidai Kizugawa, Kyoto 619-0215, Japan
2
Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow 125412, Russia
3
Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
4
The Graduate School for the Creation of New Photonics Industries, Hamamatsu, Shizuoka 431-1202, Japan

A study is made of the ultra-short laser pulse irradiation of Ar cluster targets. Experiments have been performed withlarge cluster sizes and using very high laser contrasts, which have allowed clear and unambiguous observation of exotic
inner-shell transitions in near-neutral Ar ions. The interaction of the main laser pulse with the unperturbed target is anecessary requirement for observing these lines. Our measurements are supported by kinetics calculations in which avery detailed atomic model is used. The calculations predict all of the spectral features found experimentally, andsupport the notion that the X-ray emission arises from many ion stages of the Ar plasma, from near-neutral throughHe-like ions, and from a range of plasma temperatures and densities. X-ray spectral methods have been proposed todetermine the parameters of the plasma formed at the early stages of its evolution. It has been shown that the spectra ofhollow ions are the most informative in the first moments of the heating of a cluster, whereas the diagnostics of the latestages can be performed using the conventional lines of multicharged ions.

1. Introduction
The use of clusters as targets for high-power
ultra-short laser pulses is an active area ofresearch. Cluster targets absorb laser pulse energy
more efficiently than gas or solid targets, and sohave the potential to create high-density plasmas.
Previous studies [1], as illustrated in Fig.1, havefound that in the case of using micron clusters of Ar,irradiated by 30 fs Ti:Sa laser pulses withrelativistic intensities of > 1018 W/cm2, a change incontrast from of ~ 10-2 to 4x10-6 can noticeablychange the ionization balance of Ar. Fs laser pulses
with a super-high contrast of ~ 1010 have recently
become available and used in our investigations ofradiation properties of clusters, irradiated by suchlaser beams.
2. Experimental set up.
A Ti:Sa CPA laser system J-KAREN with energyof ~ 1 J, pulse duration of ~ 40 fs was focused in
the spot size of ~ 30 µm, which corresponds themaximum of laser intensity on the Ar cluster targetsof 3.5x1018 W/cm2. Using OPCPA approach, anadditional Pokkels cell and saturable absorber, thecontrast of the prepulse to the main laser pulse has
been improved up to 10-10. The laser contrast canalso be decreased by up to 10-8 by switching off theFig. 1. Variation of the observed emission spectra withthe low and medium laser contrasts

Pokkels cell and saturable absorber. Ar gas at thereservoir pressure 60 MPa was expanded throughspecially designed nozzle [2] and Ar clusters withthe mean size of ~ 1.5 µm is produced. The
spatially resolved X-ray spectra were measured
with spectral resolution λ/δλ > 3000 using focusing
spectrometers with spatial resolution (FSSR-1D)
and back illuminated X-ray CCD as a detector.
Typical X-ray spectra of He-like Ar spectra arepresented in Fig. 2.

Fig. 2. Variation of the observed emission spectra withthe high laser contrast.

2. Experimental results and discussions.
First of all, we find that for spectra obtained witha higher contrast the satellite spectral lines in therange 4.0 to 4.15 Å, which belong to lower-chargedions (B-, C-, N-, O-, F-like Ar) are weaker. At thesame time, the intensity of lines arising fromtransitions in He-like ions is increased. Suchbehavior is similar to previous findings [1] whichobserved similar trends when the laser contrast wasincreased from 10-2 to 10-6. We also observe new,very pronounced structures near the K lines of Ar,
which are belong to KL Hollow ions (see Fig. 2).
With increasing laser pulse duration for both
positive and negative directions, the total intensityof spectra in the vicinity of the He line of Ar isdecreased (and is more intense for negative laser
pulses). At the same time, the intensity of theHollow ions spectral lines relative to the intensitiesof all other lines arising from He-like ions clearly
grows for longer pulse duration, especially fornegative chirp laser pulse duration.
The modeling of Ar spectra was made by use ofthe Los Alamos suite of atomic physics codes [1].
The results of such modeling and comparison themwith experimental spectra are presented in Fig.3
and demonstrated that the main observed spectral
are formed at quite different times. The KL Hollow
ions spectra arise near the beginning of the main
pulse interaction with the cluster, at temperatures
around 10 - 20 eV and high electron densities
between 1022 - 1023 cm-3. A significant hot electroncomponent is present at these early times. The moreusual Li-like through O-like satellites, and high-ntransitions of He-like Ar ions, are formed at slightly
later times when the electron temperature grows to100 - 500 eV and the electron density decreases toaround 1021 - 1022 cm 3. The hot electron influenceon these lines is still considerable. At later times (~1 ps) contributions to the He and intercombinationlines increase, when the temperature exceeds 1 keV
ααα

Fig. 3. ATOMIC calculations compared to the measured
spectra of Fig. 2. The upper-left panel shows ATOMIC FS
calculations at a temperature of 2 KeV and electron density of1020 cm-3 and the upper-right panel shows ATOMIC FS
calculations at a temperature of 600 eV and density of 1021 cm-3.
The lower - left panel shows ATOMIC MUTA calculations at atemperature of 100 eV and electron density of 5x1021 cm-3, andthe lower - right panel shows ATOMIC MUTA calculations attemperatures of 10 and 50 eV and at electron densities of5x1022 cm-3. All calculations include a 1% hot electroncomponent.

and the density drops to around 1020 cm-3.
If to conclude, the very high contrast of theultra-short J-KAREN laser pulse used in ourexperiments has allowed to measure spectra, whichcontaining features arising from neutral Ar throughHe- like Ar, underlining the richness of the
dynamical processes which occur when intense
ultra-short laser pulses are focused on gas cluster
targets. For modeling such complicated spectra, it
was found that a detailed NLTE atomic model isrequired, which includes collision processes
spawned by highly energetic electron fractions. Ourinvestigations show that the spectral lines ofHe-like ions and their satellites, including spectral
lines from Hollow ions can be used for thediagnostics of the plasma parameters at the different
stages of ultra-short laser interaction with clusters.
Acknowledgments
This work was supported by the RussianFoundation for Basic Research (grants 10-02-91174- GFEN-a and 10-02-00345-a), by the RAS
Presidium Program of Basic Research 2 and 21 andby the U.S. Department of Energy Contract No.
DE-AC5206NA25396.
References[1] J. Colgan, J. Abdallah, Jr., A. Ya. Faenov et al, HighEnergy Density Phys. 7, 77 (2011)[2] A.S. Boldarev, V.A. Gasilov, A.Ya. Faenov, et al,Rev. Sci. Instrum 77, 083112 (2006)