Andreev, N. E.

Chefonov, O. V.

Jacoby, J.

Kostenko, O. F.

Neumayer, P.

Ovchinnikov, A. V.

Rosmej, O. N.

Schoenlein, A.

Wiechula, J.

GSI Repository

Characteristic X-rays from silver foils for backlighting of WDMO.F. Kostenko1, N.E. Andreev1, O.V. Chefonov1, A.V. Ovchinnikov1, O.N. Rosmej2, A. Scho¨nlein3,J. Wiechula3, P. Neumayer3, and J.Jacoby41JIHT RAS, Moscow, Russia; 2GSI, Darmstadt, Germany; 3EMMI, Germany; 4Goethe Universita¨t, Frankfurt, GermanyThe goal of experiments carried out at GSI with high in-tensity laser system PHELIX is the investigation of mecha-nisms leading to effective production of photons with ener-gies above 20 keV required for monochromatic backlight-ing of Warm Dense Matter (WDM). In experiments, 1ω,500fs, 100J laser pulses were used for irradiation of 3 mmthick Ag targets and thin foils deposited on Al and plasticsubstrates. The laser intensity was varied between 1018 and4 × 1019 W/cm2 by changing the laser focal spot size. Inthis report we present a comparison of numerical simula-tions of the silver Kα-photon yield with experimental re-sults obtained by means of a single-hit CCD technique [1].In simulations, the Kα-photon yields from Ag foils ingiven direction into a unit of solid angle per laser pulseenergy, Nk, were calculated according to the model [2],which takes into account dependencies of the conversionefficiency of laser energy into hot electrons η(IL) [1] andaverage energy of hot electrons Th(IL) [3] on the laserpulse intensity IL(r, t), as well as a self-absorption of22.1 keV Kα photons in a foil of arbitrary thickness. Inthe case of Gaussian laser pulse, IL(υ) = I0 exp(−υ),υ = r2/r20 + t2/t20, we getNk =2√π∫ ∞0√υ dυη(υ) e−υT 2h (υ)××∫ ∞EkdE0 exp[− E0Th(υ)]dNem(E0)dΩ,were dNem(E0)/dΩ is the number of photons per stera-dian, emitted by an electron, normally incident with initialenergy E0, from the front side of the foil in given direction.Theoretical dependencies Nk(I0), calculated with theassumption of suppression of hot electron refluxing, de-scribe well features revealed in experiments: sharp increaseof Kα-photon yield in the intensity range (1.5–2)×1018 Wcm−2, and then relatively small decrease of Nk with growthof the intensity up to 3.4×1019 W cm−2 (Fig. 1(a)). TheKα-photon yield increases up to 3 times with increase offoil thickness from 10 to 100 μm (cf. Figs. 1(a) and (b)).The last two features confirm the assumption about sup-pression of hot electron refluxing in foils deposited on thebulk substrates, even at high laser intensities. The Kα yieldfrom the foil of 10 μm thickness with refluxing electrons,calculated for intensity I0 ≈ 2.5 × 1019 W cm−2 [2], ex-ceeds shown in Fig. 1(a) value for the foil with single-passelectrons about 44 times, so only very small input fromrefluxing electrons could cause insignificant deviations ofthe experimental value from calculated one (see Fig. 1(a)).Figure 1: The Kα photon yield vs laser pulse intensity.Solid lines correspond to calculated values with Ag foils ofthicknesses: (a) 10 μm and (b) 100 μm. Points correspondto measured values, multiplied by a factor 3: (a) Ag foilof 10 μm thickness deposited on bulk plexiglass (squares)and bulk Al (triangles); (b) Ag foil of 100 μm thicknessdeposited on bulk plexiglass.It is important to point out that strong suppression of hotelectron refluxing takes place as well for non-conductivesubstrates like a plexiglass. The last allows supposing thepresence of plasma channels in dielectric substrates whichoccur due to ionization caused by the self-consistent elec-tric field of the electron bunch [4]. Systematic overesti-mation by a factor about 3 of calculated Kα-photon yieldsover measured absolute values will be a subject of futureanalysis (see also [5]).References[1] P. Neumayer et al., Phys. Plasmas 17, 103103 (2010).[2] O.F. Kostenko, N.E. Andreev, Quntum Electron. (submitted).[3] S.C. Wilks et al., Phys. Rev. Lett. 69, 1383 (1992).[4] V.T. Tikhonchuk, Phys. Plasmas 9, 1416 (2002).[5] M.N. Quinn et al., Plasma Phys. Control. Fusion 53, 025007(2011).GSI SCIENTIFIC REPORT 2012 PNI-INHOUSE-EXP-07375

Characteristic X-rays from silver foils for backlighting of WDM

http://repository.gsi.de/record/52268/files/PNI-INHOUSE-EXP-07.pdf

Characteristic X-rays from silver foils for backlighting of WDM

Abstract

Similar works

Full text

Available Versions

GSI Repository