Progress and prospects of ion-driven fast ignition

Abstract

Fusion fast ignition (FI) initiated by laser-driven ion beams is a promising concept examined in this paper. FI based on a beam of quasi-monoenergetic ions (protons or heavier ions) has the advantage of a more localized energy deposition, which minimizes the required total beam energy, bringing it close to the ≈10 kJ minimum required for fuel densities ∼500 g cm−3. High-current, laser-driven ion beams are most promising for this purpose. Because they are born neutralized in picosecond timescales, these beams may deliver the power density required to ignite the compressed DT fuel, ∼10 kJ/10 ps into a spot 20 µm in diameter. Our modelling of ion-based FI include high fusion gain targets and a proof of principle experiment. That modelling indicates the concept is feasible, and provides confirmation of our understanding of the operative physics, a firmer foundation for the requirements, and a better understanding of the optimization trade space. An important benefit of the scheme is that such a high-energy, quasi-monoenergetic ignitor beam could be generated far from the capsule (⩾1 cm away), eliminating the need for a reentrant cone in the capsule to protect the ion-generation laser target, a tremendous practical benefit. This paper summarizes the ion-based FI concept, the integrated ion-driven FI modelling, the requirements on the ignitor beam derived from that modelling, and the progress in developing a suitable laser-driven ignitor ion beam.

Fast ignition by quasimonoenergetic ion beams

“…We report here our progress in ion fast ignition (IFI), first proposed by Tabak et al [1,2], and more specifically on fast ignition driven by mononenergetic ion beams [3]. These ions can be generated by either 'laser-breakout afterburner' (BOA) [4][5][6], radiation pressure acceleration' (RPA) [7] or ion soliton (IS) [8] schemes, where very thin foils, <100 nm thick, are illuminated by sub-picosecond laser pulses with irradiances of 10 21 , 10 22 and 10 20 W/cm 2 , respectively.…”

“…Most of the energy deposition in the high density fuel has a hollow cone shape. The dense fuel located in this zone expands and launches a strong shock wave that propagates towards the axis, where the shocks collide and compress further the fuel to very high densities (≈1200 g/cm 3 ), as shown in Fig. 4(c).…”

“…We assume perfectly collimated cylindrical ion beams (constant flux within its cross section) impinging on an ideal configuration of compressed DT fuel with a peak density of 500 g/cm 3 . The simulation box used in our simulations is shown in Fig.…”

“…Instantaneous emission of the beam ions is assumed because the time-of-flight spread (≈3 ps) is much longer than the ion acceleration times found for the BOA scheme. Yin et al [3,4] have shown that enhanced ion acceleration takes place between the time of the foil relativistic transparency, n e ≈ n c , and the time when the foil becomes undercritical, n e,peak ≈ n c , n e and n c being the electron density and the critical density, respectively, and the relativistic Lorentz factor. For a 100 nm thick diamond like carbon (DLC) foil illuminated by a peak laser irradiance of 5.…”

“…Assuming a laser intensity consistent with the BOA scheme I L = 10 21 W/cm 2 and a laser-to-ion conversion efficiency = 10%, the laser power is P L = NI L S, the laser pulse energy E L = E ig / and the laser pulse duration = E ig /(NI L S ). For E ig = 6 kJ, one obtains P L (PW) = 13N , E L = 60 kJ 03013-p. 3 Figure 4. Ion density (top panels) and pressure (bottom panels) evolution of the target shown in Fig.…”