S. Gaillard N. Renard-Le Galloudec 1, J. Fuchs 2 and T.E. Cowan 1 LIMITATIONS OF THE USE OF CR39 DETECTORS IN HIGH-ENERGY SHORT-PULSE LASER EXPERIMENTS I. ABSTRACT 1 Nevada Terawatt Facility (NTF), University of Nevada, Reno (UNR), USA 2 Laboratoire pour l’Utilisation des Lasers Intenses (LULI), Ecole Polytechnique, Palaiseau, France III. THREE POSSIBLE SCENARIOS MAY EXPLAIN THE RINGS ON DETECTORS 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 ~ 10 8 particles/cm 2 is very non- monotonic, the only way one can distinguish a real ring from an artifact ring, at the relevant fluences for laser experiments (~ particles/cm 2 ), is by carefully analyzing the CR39 detector under microscope. This work was supported by the US DoE ( Department of Energy) under the Grant N° DE-FC52-01NV14050 at UNR II. LASER ION ACCELERATION PHYSICS& DETECTION II. LASER ION ACCELERATION PHYSICS & DETECTION 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 VII. REFERENCES A stack of RCF and/or CR39 detectors separates the energies of the ion beam 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) 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 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 3. [00CKD] Proton Front Emission in a Ring-like Structure is due to > 30 Mega- gauss Magnetic Fields in Ultra-Intense Laser Irradiated Solids IV. EXPERIMENTAL SET-UP, CONCEPT OF SATURATION, & VALIDITY REGION OF THE USE OF CR39 VI. CR39 IS THEN ANALYZED UNDER MICROSCOPE: COMPARISON BETWEEN ARTIFACT RING AND REAL RING COMPARISON BETWEEN ARTIFACT RING AND REAL RING V. CR39 IS FIRST ANALYZED VIA OPTICAL SCANNING Main differences between CR39 and RCF Transparency optical scans of the detectors of fluences (horizontally) from 5×10 6 α/cm² (exposure time of 100 seconds) to 5×10 10 α/cm² (exposure time of 10 6 seconds) for etching times (vertically) of ~ 18 to ~ 78 minutes Detector exposed 10 6 s (fluence 5×10 10 α/cm²) for ~ 78 minutes of etching time (×100 and ×400) AFM pictures showing particle tracks (depth ~ 600 nm) and clumping features (depth ~ 800 nm) Microscope scan across a real ring etched for ~24 and ~ 45 minutes For the low fluences, up to about 5×10 7 /cm², the detector behaves as expected At higher fluences, starting around 10 8 /cm², the detector’s response is highly non-linear 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. [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 Ne Review of Scientific Instruments Vol.53, p (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 target Physics of Plasmas Vol.8, p (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 solids Physical Review Letters Vol.84, p (2000). The CR39 material’s response for high fluences, especially those reached in laser experiments, of ~ 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. The etching procedure needs to be performed in short time 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. 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. Outside this region, care must be taken before reaching any conclusion. 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. K. Krushelnick, F.N. Beg, E.L. Clark et al. Private communication (Dec 2004). Artifact Ring Real Ring ? Real Ring * ** * * Acknowledgements to Gilliss Dyer at UTA (University of Texas, Austin). **