Fast Ignitor Concept with Light Ions

A short-laser-pulse driven ion flux is examined as a fast ignitor candidate for inertial confinement fusion. Ion ranges in a hot precompressed fuel are studied. The ion energy and the corresponding intensity of a short laser pulse are estimated for the optimum ion range and ion energy density flux. It is shown that a lightion beam triggered by a few-hundreds-kJ laser at intensities of *1021 W/cm2 is relevant to the fast ignitor scenario. © 2001 MAIK “Nauka/Interperiodica”. In the fast ignitor scenario [1], which is a milestone of the concept of inertial confinement fusion (ICF), a relativistic electron beam is considered to be the most suitable source for igniting a hot spot much smaller than the dense compressed DT core. Studies of the feasibility of fast ignition with relativistic electrons are now being carried out at many laboratories [2–5]. In addition, a 15-GeV bismuth ion beam from an external source instead of an electron beam generated directly in the target corona was also examined [6]. Over the past year, there have been several observations of multi-MeV ion beams generated by high-intensity ultrashort laser pulses in the interaction with solid targets [5, 7–9]. In this context, the present paper aims to provide insight into the feasibility of the fast ignition concept with high energy beams of light ions generated in laser–plasma interaction. Apart from the standard studies about the electron fast ignitor concept for ICF, our main concern is to prove that a light-ion beam is capable of igniting a hot spot on a reasonable laser energy scale. In contrast to relativistic electron beams, ions are much less influenced by collective plasma phenomena and have straight-line trajectories. Light ions, similar to electrons, can be generated due to laser– plasma interaction in a target, while a heavy ion beam must be produced by an external driver and transported to the target. Ion transport is not inhibited so much by the self-consistent electric field because the ions accelerated by the charge separation field at a near-critical density are much heavier and propagate inertially inside the target together with the electrons as a chargecompensated neutral beam. Below, the optimum parameters of an ion beam and laser pulse that are suitable for an ignition spark in a hot precompressed DT fuel are estimated as a rough guide. The mechanism for ion acceleration is charge separation in a plasma due to high-energy electrons driven by the laser inside the target [9] and/or an inductive 1063-780X/01/2712$21.00 © 1017 electric field as a result of the self-generated magnetic field [10]. These electrons can be accelerated up to multi-MeV energies due to several processes, such as stimulated Raman scattering [11], resonant absorption [12], laser wakefield [13], ponderomotive acceleration by standing [14] and propagating [15] laser pulses, “vacuum heating” due to the V × B Lorentz force [16] or Brunel effect [17], and betatron resonance provided by laser pulse channeling [18]. It is unlikely that the ponderomotive mechanism [14] at laser intensities higher than 1018 W/cm2 can produce ions with the observed energies (see [9]). The maximum proton energy in experiments with foils at a high-contrast intensity ratio was explained by acceleration in the charge-separation field arising due to “vacuum heating” [9]. However, for the fast ignitor scheme, this mechanism is inapplicable because of the extended plasma corona at the front of the dense target. Recent experiments carried out at the Center for Ultrafast Optical Science [19] demonstrated a significant increase in the ion energy (as compared to [9]) if the laser intensity contrast ratio decreases. Thus, one may identify a preformed plasma as a source of enhanced electron generation and, hence, enhanced electrostatic field that efficiently accelerates the ions. We believe that the Raman scattering mechanism for electron forward acceleration [11] together with the laser channeling effects [18] are the most likely processes at the corona of an ICF target which produce a strong sheath electrostatic field and are responsible for ion beam generation by short laser pulses at laser intensities of >1018 W/cm2. Hot electrons, accelerated in an underdense plasma (with a density ne comparable to the critical density nc) up to the energy ee, penetrate into the target at a distance on the order of the Debye length λDe ∝ and create a strong sheath electrostatic field, which accelerates ions forward. Acceleration gradients of sevee/ne 2001 MAIK “Nauka/Interperiodica” 1018 BYCHENKOV et al . eral tens of GeV/cm are expected for MeV electrons. As electrons are decelerated, their kinetic energy transforms into the electrostatic field energy and the electric potential should be expected to be at the level of the hot electron energy ee . Correspondingly, the magnitude of the electric potential determines the ion energy e ~ Zeφ ~ Zee , where e and Ze are the electron and ion charges. Most of the measurements suggest that protons and deuterons are the major species of laser triggered particle emission, although heavy ions have recently been identified [20]. Their energy is proportional the charge number Z, which is consistent with the electrostatic process of ion acceleration. Clearly, an evaluation of the feasibility of fast ignition with energetic ions must be based on the conversion efficiency of the laser light into ion beam energy and scaling of the beam parameters versus laser characteristics. A systematic investigation of both of these issues has only begun. However, the data on the ~6% conversion efficiency into ions of several MeV energy [8] and the square root dependence of the proton energy on the laser intensity inferred from the latest experiments are very promising. The general approach to fast ignition involves a powerful external unspecified source and aims to define the ignition parameters for a beam and a core. The first study of fast ignitor parameters was presented by Tabak et al. [1]. As was pointed out in [21], the original fast ignitor concept [1] dealt with a nearly isobaric fuel configuration and underestimated the energy required for ignition, which is more relevant to a nearly isochoric process and is somewhat larger than first proposed. The results of [21] roughly agree with those presented in [6] and predict a larger ignition energy than that given by the analytical model of Piriz and Sanchez [22]. According to [21], the optimum particle range is R = 0.6 g/cm2, while the model [22] predicts R = 0.25 g/cm2. Regardless of the differences between [21] and [22], we consider a wide enough domain of the particle ranges to include both of these estimations. Similar to [23], where the physics of the electron fast ignitor was discussed, our key issue includes an estimation of the ion penetration depth into the dense compressed DT core with a density of ~300 g/cm3 and temperature of ~10 keV. For ions with energies higher than one-hundred keV, the penetration depth is determined by their collisions with electrons; i.e., fast ions heat electrons of the core and lose energy in accordance with the equation [24]