S. Gaillard 1@ , N. Renard-Le Galloudec 1 , J. Fuchs 2 and T.E. Cowan 1

1. 1 Nevada Terawatt Facility (NTF), University of Nevada, Reno (UNR), USA 2 Laboratoire pour l’Utilisation des Lasers Intenses (LULI), Ecole Polytechnique, Palaiseau, France LIMITATIONS OF THE USE OF CR39 DETECTORS IN HIGH-ENERGY SHORT-PULSE LASER EXPERIMENTS S. Gaillard1@, N. Renard-Le Galloudec1, J. Fuchs2 and T.E. Cowan1 @sandrine@unr.edu IV. EXPERIMENTAL SET-UP, CONCEPT OF SATURATION, & VALIDITY REGION OF THE USE OF CR39 I. ABSTRACT The short-pulse high-energy laser field offers many interesting possible applications in a wide range of scientific disciplines, and has been under high investigation over the last decade. In short-pulse high-energy laser experiments, both CR39 and RCF detect the particles emitted when the laser interacts with a target, and sometimes record ring shape structures. Since now, these rings have been interpreted in three controversial ways, where the interpretation was based only upon the physical properties of the beam. This work exposes a new interpretation of these rings: We show that rings can arise as an artifact of the CR39 material, caused by saturation effect, either because of over-exposure to a well-known source of quasi mono-energetic particles and/or of over-etching. As the optical density response of CR39 for fluences above ~ 108 particles/cm2 is very non-monotonic, the only way one can distinguish a real ring from an artifact ring, at the relevant fluences for laser experiments (~ 1011 particles/cm2), is by carefully analyzing the CR39 detector under microscope. Longer etching allows to see the data more clearly CR39 can safely be used in a validity region, depending on both etching time and fluence. Outside this region, care must be taken before reaching any conclusion. ** * II. LASER ION ACCELERATION PHYSICS& DETECTION V. CR39 IS FIRST ANALYZED VIA OPTICAL SCANNING Acceleration of ions and electrons from both the front and the rear surfaces, due to the interaction of a high-energy short-pulse laser and a thin foil target Expanding electron sheath at the rear surface, with ions of different energies propagating normal to the sheath, at different divergence angles For the low fluences, up to about 5×107/cm², the detector behaves as expected At higher fluences, starting around 108/cm², the detector’s response is highly non-linear Transparency optical scans of the detectors of fluences (horizontally) from 5×106 α/cm² (exposure time of 100 seconds) to 5×1010 α/cm² (exposure time of 106 seconds) for etching times (vertically) of ~ 18 to ~ 78 minutes Main differences between CR39 and RCF A stack of RCF and/or CR39 detectors separates the energies of the ion beam The CR39 material’s response for high fluences, especially those reached in laser experiments, of ~ 1011 particles/cm², is highly non linear. The result depends on the development’s conditions: ring structures, and even bull’s eye structures, can be produced at high enough fluences, for certain etching times, from a well-characterized particle source. VI. CR39 IS THEN ANALYZED UNDER MICROSCOPE: COMPARISON BETWEEN ARTIFACT RING AND REAL RING III. THREE POSSIBLE SCENARIOS MAY EXPLAIN THE RINGS ON DETECTORS Detector exposed 106 s (fluence 5×1010 α/cm²) for ~ 78 minutes of etching time (×100 and ×400) Microscope scan across a real ring etched for ~24 and ~ 45 minutes 1. Ring-like Structures may simply be explained by the Divergence of the Perpendicular Component to the Electron Sheath at the Back Side of the Target Ion trajectories with respect to the ion energy, with a stack of RCF collecting ions with energy greater than 1 MeV Example of two shots at different laser energy ranges, leading to a ring structure recorded by the two types of detectors (CR39 and RCF) The etching procedure needs to be performed in short time steps The clumps obtained in the white ring region, however, need not be mistaken for individual particle tracks. In doubt, AFM pictures, providing with a topographical map of the scanned zone, are very helpful. Artifact RingReal Ring ?Real Ring * ** AFM pictures showing particle tracks (depth ~ 600 nm) and clumping features (depth ~ 800 nm) ** 3. [00CKD] Proton Front Emission in a Ring-like Structure is due to > 30 Mega-gauss Magnetic Fields in Ultra-Intense Laser Irradiated Solids 2. [01MKS] Proton Rear Emission in a Ring-like Structure possibly due to Giga-gauss Scale Magnetic Fields from Ultra-Intense Laser Illuminated Plastic Target To prove that a ring is real (only due to the physical properties of the beam, and not due to the artificial response of the CR39 material), a thorough microscope analysis is required. VII. REFERENCES This work was supported by the US DoE (Department of Energy) under the Grant N° DE-FC52-01NV14050 at UNR [05GFR] S. Gaillard, J. Fuchs, N. Renard-Le Galloudec, T.E. Cowan Comment on “Measurements of Energetic Proton Transport through Magnetized Plasma from Intense Laser Interactions with Solids” submitted to Physical Review Letters (May 2005). [82LSC] H.W. Lefevre, R.M. Sealock, R.C. Conolly Response of CR39 to 2 MeV microbeams of H, He and NeReview of Scientific Instruments Vol.53, p.1221-1227 (1982). [01MKS] Y. Murakami, Y. Kitagawa, Y. Sentoku, M. Mori, R. Kodama, K.A. Tanaka, K. Mima, T. Yamanaka Observation of proton rear emission and possible Giga-gauss scale magnetic fields from ultra-intense laser illuminated plastic targetPhysics of Plasmas Vol.8, p.4138-4143 (2001). [00CKD] E.L. Clark, K. Krushelnick, J.R. Davies, M. Zepf, M. Tatarakis, F. Beg, A. Machacek, P.A. Norreys, M.I.K. Santala, I. Watts, A.E. Dangor Measurement of energetic proton transport through magnetized plasma from intense laser interactions with solidsPhysical Review Letters Vol.84, p.670-674 (2000). * ** Acknowledgements to Gilliss Dyer at UTA (University of Texas, Austin). K. Krushelnick, F.N. Beg, E.L. Clark et al.Private communication (Dec 2004).