Bender, M.

Hoffmeister, G.

Jaeger, R.

Kollmus, H.

Roth, M.

Wagner, F.

Wengenroth, M.

Zielbauer, B.

GSI Repository

Target surface cleaning by fs laser desorption at PHELIXG. Hoffmeister∗1, M. Bender2, H. Kollmus2, R. Jaeger1, F. Wagner1, M. Wengenroth2, B. Zielbauer2,3,and M. Roth11Institut fu¨r Kernphysik, TU Darmstadt, 64289 Darmstadt; 2GSI Helmholtzzentrum fu¨r Schwerionenforschung GmbH,64291 Darmstadt; 3Helmholtz Institut Jena, 07743 JenaIn laser ion acceleration experiments like the target nor-mal sheath acceleration (TNSA) the target design has anessential influence on the resulting ion spectra. Apart fromstudies about elaborate geometries like cone or hemispher-ical targets [1] the target surface structure plays a crucialrole. In almost all laser proton acceleration experiments itis the contamination on top of the target surface -mainlyconsisting of hydrocarbons and oxygen- which is ionizedand accelerated.The fs laser desorption constitutes a promising techniquewhich enables to remove these surface contaminants suc-cessively, thereby learning more about their amount andcomposition. Due to the ultrashort laser pulse duration theenergy transfer from the laser to the target is nonthermali.e. melting is almost no issue [2]. The target surface re-mains undamaged and smooth. Combining this methodwith a sophisticated target design like a characterized mul-tilayer target could allow to scan through the TNSA fielddistribution giving information about its penetration depthand about the TNSA field strength.Experimental setup We performed a fs laser desorp-tion experiment using the PHELIX laser frontend at GSIHelmholtzzentrum fu¨r Schwerionenforschung GmbH. ThePHELIX laser frontend delivered pulses of 400 fs pulseduration with 10 Hz repetition rate. The pulse energieswere varied between 0.1 mJ and 3 mJ. With a focal spotsize of 560µm this led to laser intensities between (1010 -1012) W/cm2.The laser irradiated 1 mm thick aluminum, copper and goldfoils which were mounted in a special ultrahigh vacuum(UHV) chamber at a pressure of 10−10 mbar realized by atitanium sublimation pump and an ion pump. The UHVwas necessary to observe even small pressure increases.The analysis of the gas composition was performed by aresidual gas analyzer (RGA) represented by the quadrupolemass spectrometer QMA 200 by Pfeiffer Vacuum.Results The intention of this experiment was to deter-mine the threshold intensity for plasma formation as well asto investigate the target cleaning in dependence of the laserparameters. At intensities of I = (5 − 10) × 1010 W/cm2there were no explicit desorption effects visible. The to-tal pressure rise was negligible. With increasing intensityin the mid 1011 W/cm2 regime we observed clear signalsof target surface cleaning. Figure 1 shows the total pres-sure rise during the continuous laser irradiatiation at 10 Hzon a 1 mm thick gold foil. In the left image, three irradia-∗ g.hoffmeister@gsi.desurface cleaning successful!1st cycle 2nd cycle3rd cycleFigure 1: Total pressure as function of time of laser desorption at 10 Hzwith I = 5.9× 1011 W/cm2 (left) and I = 8.5× 1011 W/cm2 (right).tion cycles with 1000 s, 2100 s and 500 s, respectively, wereapplied. The first two cycles show an initial pressure risedue to the desorption of surface contaminants. Probablythe laser spot was slightly shifted between those two cyclessince the UHV conditions prevent the re-establishment ofa contamination layer on the desorbed surface. In the thirdirradiation cycle this pressure rise did not occur any more,indicating that the irradiated region had been cleaned andthere were no more surface contaminants to be removed.Additionally, the laser intensity was still low enough suchthat plasma formation was no issue. In contrast, the rightimage of Figure 1 demonstrates that at a laser intensityof I = 8.5 × 1011 W/cm2 the total pressure rise shows acompletely different behavior. Instead of decreasing after ashort time of laser irradiation it increased more and more.Plasma formation set in and the laser intensity sufficed toimmediately ionize the contaminants as well as the goldatoms.Similar results were obtained by irradiating aluminum andcopper foils. While for copper the plasma threshold turnedout to be at the same intensitiy as it was observed for goldfoils, for aluminum we determined a minor threshold inten-sity for the plasma formation at I = 3.5× 1011 W/cm2.With this desorption experiment at the PHELIX laser wecould experimentally determine a plasma threshold inten-sity for gold, copper and aluminum targets. Furthermore,we found clear evidence of a complete target surface clean-ing in the intensity regime of I = (2− 6)× 1011 W/cm2.AcknowledgementsThis work was funded by the BMBF, support code06DA9044I, and HIC4FAIR.References[1] P. K. Patel et al., Phys. Rev. Lett., 91, 125004 (2003)[2] R.E. Russo et al., J. Anal. At. Spectrom. 17, 1072 (2002)GSI SCIENTIFIC REPORT 2012 PNI-INHOUSE-EXP-14383

Target surface cleaning by fs laser desorption at PHELIX

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Target surface cleaning by fs laser desorption at PHELIX

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