Bagnoud, V.

Brabetz, C.

Busold, S.

Cowan, T. E.

Deppert, O.

Eisenbarth, U.

Götte, S.

Kester, O.

Schumacher, D.

Zielbauer, B.

GSI Repository

GSITemplate2007 Hollow Beam creation with continuous diffractive phase mask at PHELIX* C. Brabetz2, V. Bagnoud1, S. Busold4, T. E. Cowan3, O. Deppert4, U. Eisenbarth1, S. Götte1,           O. Kester1,2, D. Schumacher1, B. Zielbauer1 1GSI Darmstadt, Germany; 2Johann Wolfgang Goethe University, Frankfurt, Germany;                                               3HZDR, Helmholtz center Dresden Rossendorf, Germany; 4TU Darmstadt, Darmstadt, GermanyOverview In the framework of the Laser Ion Generation Handling and Transport (LIGHT) project, the reduction of the di-vergence of the laser accelerated ions is a central issue. One solution relies on engineering the electron sheath used in standard laser-driven proton acceleration (target normal sheath acceleration, TNSA) for reducing the ini-tial divergence of the ion beam. In 2012, we conducted an experimental campaign in which “donut” focal spot have been used to drive proton acceleration. From that we have found two interesting features: • One sees a qualitative effect of the focal spot beam shape on the ion beam divergence as expected, and • The energy cut-off in the proton spectrum was nearly higher when a donut focus was applied, although this re-sulted in contradiction with the scaling law of TNSA. Report on the 2012 beamtime During the beamtime, we did a first run on laser-ion ac-celeration with engineered beams. We also focused par-ticularly on avoiding strong wavefront distortion in the laser amplifier (astigmatism) because we saw how crucial it is for that kind of experiment. In 2012 a new PHELIX off-axis parabolic mirror was installed and produced a good hollow focus profile during alignment. However, we found that the on-shot aberrations also strongly alter the beam quality. Therefore a bending mechanism was in-stalled to the main-mirror 1 in the main amplifier of PHELIX to correct for the thermal aberrations happening on shot and measured with a wavefront sensor. Using this pre-compensation, the alignment- beam profile looks dis-torted but the on-shot profile looks more promising.  Results on ion acceleration with engineered la-ser beam In total we had 29 successful high-energy experiment shots on gold foils with different thicknesses with and without hollow beam. In comparison to a standard Gaussian beam (blue dots in Figure 1) the focal spot diameter of the hollow beam focus increases by a factor of about 2 and therefore the peak intensity drops nearly by a factor of 4. A troubling feature is that the maximum proton energy for the hollow beams (in red) does not depend on the laser intensity con-trarily to what laser-ion acceleration scaling laws predict. For higher-order hollow beams (green dots), the focal spot was heavily distorted and resulted in a speckle-like energy distribution at low intensity. In this case a signifi-cant reduction of the proton energies was observed.  Figure 1: Proton Energy dependency on laser intensity. From an angular distribution stand-point, the proton beams created with the hollow laser beam clearly show a systematic trend to lower divergences and higher proton yields. Taking the more aberrated shots into account we observe other effects on the ion beam. Some type of shots with a hollow beam profile broke down to 2 similar strong focal spots shows a divergence reduction in only on dimension while other shots with the hollow beam phase on thick targets shows unexpected high proton en-ergies. 3w on-shot focus diagnos-tics We set up a new imaging di-agnostic for measuring the laser focal spot on target during the shot [1]. The idea was to look at the relativistic oscillation plas-ma surface that generates har-monics. Then filter for only the 3w light that is specular reflect-ed from the target surface and is linked directly to the laser focal spot and the TNSA source size. As the compo-nents were not available in the beginning of the experi-ment it was just installed in the last two days. Therefore there was not enough time for optimization of the diag-nostic. We know that the imaging system was strongly affected by astigmatism (duo to passing through a thick glass substrate). But still we can say that the source size was smaller than 80x80 µm2 (Figure 2). References [1] B. Dromey et al., Laser and Particle Beams 27, 243-248 (2009).  * Work supported by HIC for FAIR. 0 1 2 3 4 5x 101910152025303540Proton Energy (MeV)Laser mean Intensity (W/cm2)  reference shots2π  phase mask shots>2π  phase mask shotsFigure 2: on-shot fo-cus in 3w. [µm][µm]on target laserfocus in 3ω. [a.u.]20 40 60204060GSI SCIENTIFIC REPORT 2012 PNI-IONS-EXP-27335

Hollow Beam creation with continuous diffractive phase mask at PHELIX

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Hollow Beam creation with continuous diffractive phase mask at PHELIX

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