Hot-electron energy coupling in ultraintense laser-matter interaction A. J. Kemp, 1 Y. Sentoku, 2 and M. Tabak 1 1 Lawrence Livermore National Laboratory, Livermore, California 94551, USA 2 University of Nevada, Reno, Nevada 89577, USA Received 12 March 2009; published 22 June 2009 We investigate the hydrodynamic response of plasma gradients during the interaction with ultraintense energetic laser pulses using kinetic particle simulations. Energetic laser pulses are capable of compressing preformed plasma gradients over short times, while accelerating low-density plasma backward. As light is absorbed on a steepened interface, hot-electron temperature and coupling efficiency drop below the pondero- motive scaling and we are left with an absorption mechanism that strongly relies on the electrostatic potential caused by low-density preformed plasma. We describe this process, discuss properties of the resulting electron spectra and identify the parameter regime where strong compression occurs. Finally, we discuss implications for fast ignition and other applications. DOI: 10.1103/PhysRevE.79.066406 PACS numbers: 52.57.Kk, 52.65.Pp I. INTRODUCTION While intense short laser pulses offer many interesting applications for high energy-density physics 1, coupling, and transport of energy into dense plasma in the ultrarelativ- istic intensity regime are poorly understood due to the com- plex dynamics near the absorption point, and difficult to model due to the several orders of magnitude between the dense-plasma response time and corresponding length and the scales of the laser spot size and pulse duration. An addi- tional problem is the large scale, low-density blow-off plasma in front of the actual solid target found in high- energy short-pulse experiments. It is created before the ar- rival of the main pulse by amplified spontaneous emission processes in the laser that cannot be easily suppressed and is dense enough as to prevent light propagation several mi- crometers away from the target. As the formation of the preformed plasma occurs on a nanosecond time scale, it cannot be self-consistently in- cluded in the kinetic models that are currently used for short- pulse interaction because those are limited to picosecond time scales for technical reasons. Instead, the preformed plasma is usually modeled in separate hydrodynamic simu- lations. For a recent high-energy short-pulse experiment, scale lengths of about 0.5–1.0 m between solid density and a fraction of the critical density n c = 1.1  10 21 cm -3 were found preceded by longer scale length plasma 2. In future fast ignition experiments, one expects scale lengths of up to 10 m depending on the energy in the prepulse. This paper addresses the short-pulse laser-driven dynam- ics of preformed plasma in the limit of ultraintense, ener- getic pulses over a picosecond. At intensities I L = 1.37  10 20 W / cm 2 the vacuum-energy density of light corre- sponds to 30 Gbar at 1 m wavelength light. Such a pres- sure can cause ions to move over several microns in less than 1 ps 3. Many early works on absorption consider idealized step-function density profiles, relatively short density gradi- ents, or large volumes 4,5, effectively neglecting the large- scale ion motion. We characterize the response of plasma gradients in the limit of normal incidence with one spatial and three velocity 1D3V degrees of freedom in a fully rela- tivistic kinetic description. Our approach allows us to isolate 1D “hydrodynamic”–from purely multidimensional effects such as beam filamentation 6, hole boring 7, and defor- mations of the plasma surface through Rayleigh-Taylor like instabilities 8. We ignore refluxing of hot-electrons, which can occur due to electrostatic confinement in thin foils. First we consider two plasma density gradients with different scale lengths. Both cases show how the laser- generated ponderomotive pressure near the relativistic criti- cal density causes a strong compression of the preplasma toward higher densities and acceleration into vacuum of plasma at lower densities, followed by a drop in absorption and hot-electron temperature. To understand this, we study simple step-function density profiles where the ion motion is suppressed. Here we find key properties of the laser- generated electron distribution at ultrarelativistic intensities; expressions for cut-off energies and temperatures are given. Combined with an analytical description of the compression, which depends on plasma scale length, ion charge-to-mass ratio and laser intensity, wavelength and pulse duration, this is useful for defining plasma parameters at which absorption remains high over the laser pulse duration and electron tem- perature is within the parameter band for fast ignition or other applications 1. Finally we study the generation of the “hot” tail of the electron distribution functions found in all our simulations. We demonstrate that the hot tail originates in the underdense plasma in front of the dense-plasma interface. We then dis- cuss the relationship between the in situ distribution of elec- trons in the underdense plasma and the hot tail we find inside the solid density plasma; and discuss qualitatively how elec- trons are accelerated to energies far beyond the laser pon- deromotive potential in quasiexponential distributions. Simulations were performed with a 1D3V particle-in-cell code PIC9 using a resolution of 500–1000 cells per laser wavelength and up to 300 particles per species and cell, thus resolving the plasma’s Debye length at the highest density and initial temperature of 10 keV and verified with an up to 3 higher resolution. Resistive processes are not included in our study, which is merely aimed at characterizing the laser- PHYSICAL REVIEW E 79, 066406 2009 1539-3755/2009/796/0664069 ©2009 The American Physical Society 066406-1