Relative importance of mega electronvolt-electron energy deposition by collisions and field effects in fast ignition

Abstract

The rates of energy deposition of MeV electrons by return-current Ohmic heating and Coulomb collisions are compared in the context of the fast ignition scheme of inertial confinement fusion. A criterion for distinguishing the relative importance of the two heating mechanisms is presented. It depends on the kinetic energy of the relativistic electrons, the temperature of the background plasma, as well as the ratio between the densities of the two groups of electrons. A critical density ratio is also found. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4704818]Since its inception, the fast igniter (FI) scheme for inertial confinement fusion (ICF) has triggered much interest in the interaction of short intense laser pulses with dense plasmas. 1 Unlike in conventional direct heating, 2 in FI the capsule compression and hotspot formation processes are separated, and the compression and energy input requirements are considerably reduced, resulting in easier ignition and higher gain. In FI, an ultra-intense ($ 10 20 W=cm 2 ) and ultra-short ($ 10 ps) laser pulse generates a relativistic electron beam (REB) with a suitable energy spectrum, which propagates into the pre-compressed dense core region of the target and deposits its energy there. In particular, the transport of suprathermal electrons in a cold background plasma has been examined. 3 However, in the context of fast ignition, a return current and the corresponding self-fields are generated as the fast electron beam propagates in the precompressed plasma. As a result, both return-current Ohmic heating and Coulomb collisions can affect the propagation and energy deposition of the REB through the dense coronal plasma of the imploding target. 4-10 When the density n f of the REB is comparable to that n e of the background plasma, the REB is usually filamented because of micro instabilities and the self-fields. When the core density satisfies n e ) n f , the REB will be collisionally slowed down. That is, the particle stopping/scattering dominates for dense background plasmas, and anomalous stopping dominates for lower density background plasmas.REB transport in dense plasmas has also been investigated using the electromagnetic hybrid fluid-particle-in-cell (PIC) simulation techniques, 5,9 where the return currents and the self-generated electric and magnetic fields are calculated by self-consistently solving the fluid equations for the background plasma and the Maxwell equations. A practical hybrid scheme has been developed by Bell et al. 11,12 and Gremillet et al.,13 where the relativistic motion of the fast electrons is followed by the PIC simulation, and the background plasma is represented by the return current. A model Fokker-Planck equation is used to describe the effects of Coulomb collisions, the displacement current and charge separation effects are neglected, and the Ohm's law is used to obtain the self electric field. Following this approach, two-as well as three-dimensional codes (EBT2D and EBT3D) for investigating the transport of...

Physical studies of fast ignition in China

“…Collisions between fast electrons are ignored and the background particles give only drag and random angular scattering terms. The Brown, Preston and Singleton (BPS) model [45] for the Coulomb logarithm including quantum and coupling effects for a wide range of plasma conditions has been supplemented in the code [46]. These algorithms are much simpler and enable us to do larger-scale simulations.…”