spectroscopy of high-temperature matter heated by laser plasma x-rays _review article.pdf

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Plasma Sources Sci. Technol. 4 (1995)242-249. Printed in theUK

Spectroscopy of high-temperature

matter heated by laser plasma x-rays

lstvkn B Foldest

KFKI, Research Institute for Particle and Nuclear Physics, H-1525 Budapest,PO B 49, Hungary

Received 26 August 1994, in final form 2 November 1994

Abstract. A review of x-ray spectroscopy of radiatively heated matter is giventogether with a summary of the possible methods for x-ray conversion of laserradiation. Gold microcavity targets proved to be applicable to carry out absorptionspectroscopy of radiatively heated CH foils from I eV to 100eV temperature.Targets with embedded boron layers were also applied for investigatingthe

radiative bumthrough both with transmission and emission spectroscopy.The results obtained with both methods are in good agreement with the Mum

hydrocode with SNOP opacities and with th e simple consideratlons for thedegenerate state in th e early stage of heating.

1. Introduction

The study of laser produced plasm as has been m otivatedin recent decades mainly by the search for inertiallyconfined fusion plasmas. Over recent years applications

for studying properties, such as opacities of high-temperature matter, have also attracted interest mainlybecause of the astrophysical applications. High-power lasers can produce plasmas with temperaturesabove 1 keV and densities near to that of solidmatter. Laser generated plasmas are, however, farfrom homogeneous-they generally have steep densityand temperature gradients. Non linear laser-plasmainteractions also destroy the homogeneous illuminationof a target, which prevent-together with the Rayleigh-Taylor instability-compression of the fusion capsuletoward fusion conditions. In contrast, x-ray heated

matter is free from strong gradients, a homogeneousplasma can be created and thus x-ray heated matteris more appropriate for investigating high-temperatureopacities. Also, instead of the directly driven laserfusion, a more uniform illumination of the fusion targetcan be obtained by illuminating it indirectly, i.e. withx-rays after conversion of the laser radiation.

Radiative heating of matter can thus be usedfor (indirectly driven) inertial confinement fusion aswell as for studying properties such as opacities of

high-temperature matter. This can be done becausehigh conversion of laser radiation to x rays can be

obtained in high-Z plasmas. The reason why high-Z matter is appropriate for efficient x-ray conversionlies in the fact that the opacity, i.e. the frequency

t Also at the Max-Planck-Institut fur Quantenoptik, D-85748Gaxhing, Germany (visiting fellow 1992-93).

0963-0252/95/020242+08$19.500 1995 IOP Publishing Ltd

averaged photoabsorption coefficient, increases w ith Z.Consequently a thin layer of high-Z matter is alreadyopaque for the x-rays generated in the plasma; they willtherefore be reabsorbed and again ree mi tted in it leadingto the prevalence of radiation heat conductivity and to

an efficient conversion.In this paper we first summarize the possibleschemes for x-ray conversion. Examples for opacitymeasurements and experiments for studying the radiativebumthrough using the converter foil method are thengiven. The main aim of this paper is to give a summaryof recent experiments carried out with the high powerAsterix iodine laser, obtaining high x-ray temperature

(T 100 eV) with the use of a Hohlraum converter.Results on x-ray bum through experimentson plastic~foilsand on layered targets are given.

2. X-ray co nversion

The first reports on the possibility of an efficientconversion of laser radiation to x-rays go back to1978 [l]. The early works, however, either gave aqualitative spectrum of the converter, or they gave asingle quantitative value for the conversion efficiency.In the latter case measurements were generally takenwith the use of filtered photodiodes, i.e. without spectralresolution. Quantitative data with spectral resolution,i.e. absolutely calibrated spectra in the 1-25 nm range

were given eight years later [2] . In that work absolutelymeasured x-ray spectra were compared for differentelements at moderate, 3 x 1013W laser intensitiesat the wavelength of 0.53 pm . As seen in figure 1,by increasing the atomic number Z the conversionefficiency will reach 10%first for AI. It reaches a much



XRS of high-temperaturematter

temperatures well ab ove 100 eV. Radiation temperaturesup to 300 eV have recently been obtained [12]. Theclosed configuration provides a high luminosity due to

the absorption and re-emission of x rays on the cavitywall. The sample to be investigated is illuminatedfrom several directions, thus also providing a uniformillumination. As the x-ray source iesimilarly to thethick converter-the front side of the conv erter, caremust be taken to avoid the negative effects of laser

reflection and pla sma blow-off 1131.

Be C A I Ti Cu M o S n W A u P bb b b b b b d Z W

0.6 ' ' ' ' .L 1

o.2 I-I L

0.011 ' ' ' "" 1 , t , 1 1 1

4 6 10 20 40 60 100

atomic iiuiiiber Z

Figure 1. X-ray energy per solid angle emitted between 1and 25 nm a s a function of the atomic number of th e targetmaterial. Th e total conversion energy (right sca le) wasobtained by integration over the full solid angle. Intensity,3 x 1013W cm-'; pulse duration, 3 ns; wavelength,0.53 g m [2].

higher, (40%) conversion at higher 2 numbers, i.e. forSn and W with 2 > 40.

Even higher x-ray conversion can be obtainedby heating thick Au targets using lasers of shorterwavelengths. Conversion to thermal x-ray radiation upto SO% could be obtained by using h = 0.35 p m[3] and 0.26 p m [4] laser radiation. The obtained x-

rays were thermal becauseas these works show-theconversion to the harder M-band radiation is negligibleup to an intensity of lO l 4 W The x-ray conversionexperiments can be well described either by computersimulations [4,5] or by analytical models [6,7]. Theadvantage of using thick targets is the high availableconversion. The drawback , however, is the existence ofth e plasma blow-off and reflected laser radiation into thesame direction.

In order to eliminate these problems and to obtainmore homogeneous x-ray source, experiments werecam ed out by the so called converter foil method [S,9].In this case a thin Au foil of 1W300 nm thickness is

irradiated by the laser beam. If the converter foil isthick enough to absorb all the laser radiation but thinenough to transmit a significant part of the x-ray fluxtoward its rear side, its rear side emission can well beused as x-ray source free of laser light [lo ]. This simpleand most often used method leads to a 30% conversionat low laser intensities with a several nanosecond pulseduration [8], whereas less than 10%conversion of thehigh-power 1 ns pulse of the NOVA laser was achieved,corresponding to a brightness temperature of 100eV [9].The advantage of this arrangement is that it can providea hom ogeneous Lam bertian x-ray source.

A complementary arrangement is to carry out x-ray spectroscopy in closed microcavities [ l l ] . Thisarrangement gives the highest x-ray brightness with

3. Experiments with converter foils

In o rder to gain a deep insight into the properties of x-ray heated matter, investigations have been c arried outwith low -2 matter. The simple electronic structure ofthe low -2 elements offers a good possibility to compare

experimental results with theoretical calculations. Thedynamics of x-ray heating of low-2 matter wasinvestigated in a previous ex periment with the converterfoil method [I41 in the modest temperature range of30 eV with the use of a 15 J Nd laser.

The experimental set-up is shown in figure 4(u) itwill be compared with the microcavity configuration inthe next section). A 300 nm thick Au foil on a 1 p msubstrate was used as an x-ray converter. The absorberfoil was situated at a distance d behind the converter.The temporal development of the radiative bumthroughof a thin Be foil showed how the cold, solid Be is

transformed into a hot ionized plasma. Figure 2 showsstreaked spectra of x-ray heated Be. In figure 2(u) theAu source spectrum-which could be well approxim atedby a 50 eV Planckian-can be seen. This is comparedin figure 2(b) to the transmission spectrum of 0.5 p m

cold Be which was situated far away ( d = 350 mmdistance) from the source. The foil is fairly transparentbelow the K-edge of cold solid Be at 112 eV, and it

is opaque above it. Figure 2(c) shows the case whenthe absorber is brought so close to the source that the

heating x-ray flux is S, = 5 x 10" W cm-'. In thiscase it can be followed how the cold, soIid material is

transformed to a highly ionized gas. In the beginningthe K-edge seems to be shifted toward energies belowthe cold K-edge which moves to higher energies withsubsequent heating. This observation was explained inthe paper as the thermal smoothing of the Fermi edge.We shall retum to this point when discussing carbondata. In the later stage of heating, i.e. at t =2 ns,the region above the K-edge becomes transparent withan absorption spectrum characteristic to ionized Be.Note that the He-like absorption lines dominate in thespectrum at 124 and at 140 eV. A comparison withsimulations using the MULTI [161multigroup hydrocodewith SO opacity groups [I71 refers to a Be temperatureof 25 eV by this time. Th e particular importance of th eprecise treatment of line broadening mechanisms wasshown therein because radiation transport takes placebetween the absorption lines in a region where the line

wings overlap. A good agreement between experiment

243



converter thermal x rays low-Z material

( d y p h r a g m

spec trometer

Figure 4. Schemes of set-ups for the measurement ofabsorption spectra with the converter foil method [I51 a)and with the Hohlraum arrangement (b).

4. Experiments with A u microcavities

To study radiation properties of matter at even highertemperatures, a more efficient method for convertingthe laser energy to x-rays must be applied. This canbe done if the laser light is converted to x-rays afterinjection into a gold m icrosphere, a so-called Hohlraum .The cavity wall is sufficiently thick that the emissiontakes place mainly toward the interior of the cavity , i.e.the heated side of the wall radiates. Clearly enough,the x-ray emission of a thick gold target is strongerfrom the front side than through the rear side, the x-ray conversion in the front side may reach 80% of theincident laser energy. Additionally in a closed cavity

the radiation is absorbed and re-emitted by the cavitywalls thus confining the radiation inside the Hohlraum.Similarly to the well known Ulbricht sphere the radiationcan be efficiently transferred to an absorber [ll]. Thispossibility allows on e to use the Hohlraum configurationfor indirectly driven inertial confinement fusion, i.e.heating a DT fusion capsule by the radiation up to fusionconditions, or by using th is highly concentrated radiationfor x-ray spectroscopy. Hohlraum configurations enableus to study properties of low-2 matter radiatively heatedabove 100 eV.

Figure 4(b) shows the arrangement for radiative

bumthrough measnrement as compared with the con-verter foil method in figure 4(u). A laser energy of

200 J in a 400 ps pulse on the 0.44 p m wavelength(third harmonic of the Asterix IV laser) was focusedinto the gold microcavity of 1 mm diameter. The ab-sorber foil was glued onto the 400 p m diagnostic hole.

XRS of high-temperature matter

Thus i t is heated not only by the x-rays from the directlaser spot but also by x-rays from the other wall elementsof the cavity. A plastic diaphragm served to shield thegold emission from the edge of the diagnostic hole. A5000 1mm-' transmission grating spectrom eter with anx-ray streak cam era was used as a detector. T he spectralresolution was 0.07 nm and the tem poral resolution was30 ps. The temperature in the cavity was monitored ateach laser shot by a separate spectrometer. Th e radiationtemperature of the g old wall of the cavity was found to

be between 100 and 120 eV .We chose carbon containing plastics as the subject

of ou r investigations, partly because of the still simpleelectronic structure of carbon and partly because of itsapplicability in ICF as an ablator material for the DT

pellet. Bum through experiments for carbon were carriedout using 1-4 p m thick plastic absorbers. Note thateven in the case of radiative bumthrough measurement

the plasma density and temperature becomes stronglyuniform as a consequence of radiative heating. Themain purpose of these experiments was however theinvestigation of the dynamics of bumthrough and notthe direct opacity measurement with a single veryhomogeneous layer. Th e comparison with the opacitymodels follows in this case through hydrocodes.

Figure 5 shows the time evolution of x-raytransmission through a 1.9 p m (i.e. 0.2 mg cmT2)parylene foil within the spectral range of ou r observation,i.e. 250 to 600 eV. Note that the transmission of cold C His practically zero (about above the cold K-edge

(283.84eV) and it remains less than 1% below 400eV x-ray energy. Transmission is obtained by normalizing theabsorption spectra to th e corresponding source spectra.It can be seen that x-ray heating of the material isalready started near to the K-edge 100 ps before thepeak of the heating pulse. As the material is heated upand carbon becomes strongly ionized, a characteristictransmission window can he observed between the He-like C4 +ls 2- ls2p and 1s'-ls3p absorp tion lines. Thetransmission in this window strongly increases reachingmore than 60% within the next 200 ps. By this time,i.e. 100 ps after the peak of the heating pulse, the H-

like absorption lines also app ear, referring to the strongradiative heating of carbon. Calculations of the spectrumwith the M U L n hydrocode together with a post-processor[I61 are in good agreement with the observed spectrum.The ap plied approximations were the same as in the caseof the above mentioned Be experim ent. The simulationsrefer to a temperature near to 100 eV in the radiativelyheated foil.

Figure 6 shows the case of a thicker 3 fi m parylenefoil which remains colder than the thinner one. Th etemporal variation (shift) of the K-edge was investigatedhere, comparing it to the data for a cold foil. In figure

6(a) a typical case is shown for a cold CH foil. In thiscase the p lastic foil was far away from the target in orderto remain cold for the whole pulse duration. The sharpK-edge shows that the fo il practically do es not transm itthe gold radiation above it. The K-edge is red shifted atthe initial stage of the pulse, i.e. I00 ps before the peak

245



i B Foides

80 .

60 .

40 .

20 .

._1010.-E

e10C

I-

0 \ , , ,

0 4 I

t = o p s

40

20

0

RC

0

1010

._

._-E

*)

C

W

Energy (keV)

Energy (keV)

Figure 5. X-ray transmission of 200 pg cm-2 carbon.Each spectrum is averaged for a 100ps duration. Timezero is the peak of the x-ray pulse of the Au wall emission.

as seen in figure 6 ( b ) . This red shift strongly decreaseswithin the next'200 ps and becomes a blue shift at laterstage as the CH layer is heated up. This behaviour issimilar to that of x-ray heated beryllium [14] mentioned

above.For the understanding of the early red shift o f ~ t h e

K-edge we try to apply here the model of the thermalsmoothing of the Fermi edge [14]. In this early stageof plasma evolution the target has a low temperature inthe 1 eV range and a high density near to that of thesolid matter. Numerical simulations [17] show that the

carbon is mainly shock wave heated by this time to atemperature of 2 eV. Thus the plasma is in a partialdegenerate state. The method, which takes into accountthis degeneracy at early times, starts from the levelstructure of cold solid C. In contrast to the sharp 1s

state the 2s.2~ nd higher levels form a broad energyband which is filled at zero temperature with electronsup to the Fermi edge. Its distance to th e 1s level isEF=284 eV corresponding to the K-edge of the coldmatter. The dom inant absorption mechanism is photo-ionization which can be described by a simple formula

246



XRS of high-temperature matter

E2'(ls22s-1s2snp)

~ ~ ' ( 1 ~ 2 - 1 s n p )

~, , , ,~ , ,E L + 1s-np) ,

kT =5 eV for the transmission at t = -100 ps asillustrated with the broken curve in figure 6(b). Theaccuracy of the fit is better than il eV. We mustmake som e remarks here. This very simple calculationneglects spatial variations of the temperature and density

in the absorber foil with all of their effects onto the

level structure and the opacity. The observed goodagreement is even more surprising if we note that theobtained temperature is significantly higher than theprevious 1eV for Be [14] and the temperature expectedfrom the simulations. Clearly, for full understanding

of the phenomenon, a model is necessary which takesinto account the properties of degenerate matter and alsoth e line-broadening mechanisms of the free ions. As

the temperature increases the shape of the spectra willbe more and more determined by the latter, i.e. by theelectron-impact broadening of the red wing of the K-

to L-shell transitions which is applied in the hydrocode

simulations w ith SNOP opacities [14, 171.In order to obtain more direct information about the

front of the temperature wave, experiments were carriedout in layered bumthrough foils. We must mention herethat similar experiments on CH plastic with the use ofchlorinated tracer layer have been carried out already byEdwards et ai [21]. We chose a boron tracer becauseof its simple electronic stmcture and because its linesare spectrally well separated from that of carbon. In

the case of figure 7 a 48 &g cm-* thick B K layerwas situated behind 222 p g cm-' CH foil. The boronabsorption lines appear rather soon. As time goes on

the boron absorption feature is dominated by the H-like absorption (B4+). This is illustrated at t = 0,

i.e. at the peak of the laser pulse where besides thedominating H-like feature the He-like absorption linesare strong as well. In the later stage, i.e. at 300 ps, thesituation strong ly changes. The 'transmission' valueswell above one refer in fact to the self-emission ofboron. By this stage the appearance of the self-emissionof Li-like boron refers again to the high temperaturementioned above. It should be noted that in the carbon

absorption spectra the absorption feature still dominatesand self-emission of carbon is negligible even in the

case of higher temperatures according to the simulationsand the measurem ents. Nevertheless, the appea ranceof self-emission shows the limits of the applicability ofabsorption spectroscopy at high temperatures.

Layered targets have also been used to demonstrateradiative bum throug h by x-ray re-emission spectroscopy[13,22]. The diagnostic equipment was the same asdescribed above. The difference is that in the locationof the absorber foil in figure 4(b) here'is a diagnostichole. The re-emitter is situated on the wall on theopposite side of the Hoh lraum, thus the re-emitted fluxwas observed by looking through the cavity. There is a300 ps time window before the plasma fills up the cavitythus preventing observation of the re -emitted x-rays [22].Before this time, however, the contrast of the experimentwas very good and re-emission could well be studiedin the Hohlraum configuration. A boron tracer layer

was also used in the re-emission configuration. This is

1=3cQps

0

Energy (keV)

0

Energy (keV)

Figure 7. X-ray transmission spectra of48 w gB4C ayered over 222 i g cm-' carbon. Each spectrum isaveraged for a 100 ps duration. Time zero is th e peak ofthe x-ray pulse of th e Au wall emission.

possible because the carbon em ission strongly decreasesabove 4.2 nm, i.e. the hot carbon is optically thin to the

boron line radiation. Boron-like carbon-has a simpleelectronic structure and can thus be used as a temperaturemonitor.

Figure 8 shows the re-emission of a 35 pg cm-'thick B4C layer embedded in a depth of 120 p g cm-'thick carbon. The heating x-rays are incident onto thecarbon layer. First the carbon re-emission lines appear.The carbon re-emission is immediately dominated bythe H-like Lyman (Y radiation corresponding to the hightemperature near I0 0 eV with the He-like lines to

be clearly seen. The boron re-emission lines appearafter a significant delay. This delay was determineddirectly from the time dependent deconvolved x-raystreak camera spectrum and it appeared to be 160=!=30 ps.If the B4C layer is directly coupled to the radiation,i.e. in the case of zero carbon layer thickness, theboron lines app ear in the same time as the carbon lines.Boron re-emission is immediately dominated by the H-

247



I B FSldes

Energy (keV)

Figure 8. X-ray re-emission of B4C ayered on th e rearside of 1201.19 cm-> carbon. Each spectrum is averagedfor a 100 ps duration. Time zero is the peak of the x-raypulse of Au wall emission.

like boron line radiation. In the early stage He-likeboron (B3+) can also.be observed. The hotter the re-emitter becomes, the stronger the H - l i e boron featuredominates. The ObServatiOn is in good agreem ent withthe MULTI simulations [16] as they suggest that, bythe time of the observed start of boron emission, the

expected temperature reaches 70 eV in the depth of theboron layer.

5. Summary

In this paper we tried to illustrate the methods ofspectroscopy of x-ray heated matter with som e examples.First we briefly summarized the possible methods of

conversion of laser radiation into x-rays. The mainsubject of the paper is the investigation of the propertiesof x-ray heated matter. Examples from previous workswith the clean converter foil method were given forradiative bumthrough and opac ity measurements with x-ray absorption spectroscopy.

248

The main subject of our work was to summarizethe results with another x-ray conversion method, i.e.by using spherical gold microcavities (Hohlraums).The Hohlraums proved to be a very good choiceto obtain very high conversion efficiency (and alsocoupling efficiency for the sample under investigation),thus obtaining radiation temperatures above 100 eV.

Although the conditions in a microcavity are, inprinciple, not as clean as behind the converter foil,the obtained clean and explainable spectra show thatthe price one has to pay for the higher temperatureis not too high. Closed geometry has the advantagethat it can be used both for the study of the earlystage of the bumthrough of solid matter to highlyionized plasma, and for opacity measurements of high-temperature matter above 1MK.

Absorption spectroscopy of x-ray heated thin CHplastic foil targets follows the time evolution of thesample from a few eV in the beginning up to a

temperature of 100 eV. It was found that the K-edgeof cold carbon is red shifted in the early stage of heatingwhich is similar to the earlier finding for Be in the case ofmuch weaker x-ray heating [14]. This observation wasexplained by the thermal smoothing of the Fermi edgeof the solid matter applying the model of Schwanda andEidmann [14] for degenerate matter up to a temperatureof 5 eV. As the temperature increases the spectrumbecomes dominated more and more by the absorptionspectra of highly ionized carbon with intense He- andH-like features.

Experiments with layered targets by using boron

tracer layers show the limitations of the m ethod of pureabsorption spectroscopy,i.e. spectra were taken until theself-luminosity of the sample exceeded the luminosity ofthe backlighter. Exam ples of the transmission spectra oflayered targets are given.

Finally x-ray bumthrough experiments were carriedout using the complementary method of emissionspectroscopy. Here again boron tracer layers in acarbon sample were used. The delayed emission ofthe highly ionized boron is good agreement with theMULTI simulations and with the results of absorptionspectroscopy.

Acknowledgments

The work in the Max-Planck-Institut fur Quantenoptikwas supported by the Commission of the EuropeanCommunities in the framework of the Euratom-IPP Association. The support of the HungarianOTKA Foundation (contract No T007254) is alsoacknowledged. The author is especially thankful toK Eidmann for valuable discussions, suggestions andreading of the manuscript.

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