Fast electron transport and heating in solid-density matter E. MARTINOLLI, 1,2 D. BATANI, 1 E. PERELLI-CIPPO, 1 F. SCIANITTI, 1 M. KOENIG, 2 J.J. SANTOS, 2 F. AMIRANOFF, 2 S.D. BATON, 2 T. HALL, 3 M. KEY, 4 A. MACKINNON, 4 R. SNAVELY, 5 R. FREEMAN, 5 C. ANDERSEN, 5 J. KING, 4 R. STEPHENS, 6 M. RABEC LE GLOAHEC, 7 C. ROUSSEAUX, 7 and T.E. COWAN 7 1 Dipartimento di Fisica “G. Occhialini,” and INFM, Universitá degli Studi di Milano–Bicocca, Piazza della Scienza 3, 20126 Milano, Italy 2 Laboratoire pour l’Utilisation des Laser Intenses, Ecole Polytechnique, Palaiseau, France 3 University of Essex, Colchester, UK 4 Lawrence Livermore National Laboratory, Livermore, CA, USA 5 Department of Applied Science, University of California, Davis, CA 95616, USA 6 General Atomics Inc., San Diego, CA, USA 7 Commissariat à l’Energie Atomique, Bruyères-le-Châtel, France ~Received 15 November 2001; Accepted 12 December 2001! Abstract Two experiments have been performed to investigate heating by high-intensity laser-generated electrons, in the context of studies of the fast ignitor approach to inertial confinement fusion ~ ICF !. A new spectrometer and layered targets have been used to detect K a emission from aluminum heated by a fast electron beam. Results show that a temperature of about 40 eV is reached in solid density aluminum up to a depth of about 100 mm. Keywords: Fast electron beam; Fast ignitor; K a spectroscopy; X-ray spectrometer 1. INTRODUCTION In the fast ignitor scheme for inertial confinement fusion ~ ICF !, the phase of compression of the nuclear fuel is sep- arated from the phase of ignition. Heating and ignition are caused by a fast ~relativistic! electron beam travelling in the fuel, generated by a short-pulse, high-energy laser beam focused on the target at high irradiance. Various experiments have been done to investigate the propagation of the fast electron beam into solid density materials. These demonstrated good conversion efficiency from laser energy to electrons, and high electron tempera- tures in agreement with the scaling law T ~ keV! = 100 ~ Il 2 ! 103 ~ Beg et al., 1997!, where I is in units of 10 17 W0cm 2 and l in microns. As an example, at a laser irradiance of about 10 19 W0cm 2 at LULI, Pisani et al. ~2000! obtained a con- version efficiency of about 25% with an average fast elec- tron energy of about 500 keV and a penetration depth l 0  230 mm in solid aluminum targets ~r = 2.7 g 0cm 3 !. Such penetration was in good agreement with collisional models, aluminum being a good conductor, which implies negligible electric field inhibition, as predicted by Bell et al. ~1997!. Until now, no experiment really investigated the heating effect of fast electron beams, a crucial point in fast ignition. With this goal, we carried out two experiments with the LULI and VULCAN lasers, with an energy on target of about 20–25 J and 100–120 J, respectively, and intensity on target of the order of 10 19 W0cm 2 . In particular, we wanted to have as accurate as possible an indication of the temper- ature reached in the metal, at solid density and at different depths, due to heating by the electrons. As a diagnostic for the temperature, we used K a emission from aluminum. In many recent experiments K a emission from cold atoms has been used as a diagnostic for fast elec- tron penetration ~see, e.g., Beg et al., 1997; Pisani et al., 2000!. Here we have exploited the fact that ionized atoms emit shifted K a lines ~the wavelengths depend on the ion- ization stage of the emitting atom; House, 1968!. This effect is due to the reduced screening in the ionized atom, due to the lower number of electrons. In our case, ionization arises because of strong heating of the material ~to temperatures of several electron volts! induced by the passage of the fast electron beam. Then, using appropriate models, the shifted Address correspondence and reprint requests to: E. Perelli-Cippo, Dipartimento di Fisica “G. Occhialini,” and INFM, Universitá degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy. E-mail: duperel@tin.it Laser and Particle Beams ~2002!, 20, 171– 175. Printed in the USA. Copyright © 2002 Cambridge University Press 0263-0346002 $12.50 DOI: 10.10170S0263034602202037 171