[Paper Review] A new ignition scheme using hybrid indirect-direct drive for inertial confinement fusion

Zhengfeng Fan, Mo Chen+5 more|arXiv (Cornell University)|Mar 6, 2013

Laser-Plasma Interactions and Diagnostics3 citations

TL;DR

This paper proposes a novel hybrid indirect-direct-drive ignition scheme for inertial confinement fusion that combines x-ray-driven implosion with simultaneous direct laser irradiation to form a double-ablation-front structure. The resulting high-density plateau suppresses rarefaction waves and enhances drive pressure, enabling a 4.3×10⁷ cm/s implosion velocity, a low convergence ratio (~25), and a 10-fold reduction in hydrodynamic instability growth—especially at the fuel/hot-spot interface—compared to conventional indirect drive.

ABSTRACT

A new hybrid indirect-direct-drive ignition scheme is proposed for inertial confinement fusion: a cryogenic capsule encased in a hohlraum is first compressed symmetrically by indirect-drive x-rays, and then accelerated and ignited by both direct-drive lasers and x-rays. A steady high-density plateau newly formed between the radiation and electron ablation fronts suppresses the rarefaction at the radiation ablation front and greatly enhances the drive pressure. Meanwhile, multiple shock reflections at the fuel/hot-spot interface are prevented during capsule deceleration. Thus rapid ignition and burn are realized. In comparison with the conventional indirect drive, the hybrid drive implodes the capsule with a higher velocity ($\sim4.3 imes10^7$ cm/s) and a much lower convergence ratio ($\sim$25), and the growth of hydrodynamic instabilities is significantly reduced, especially at the fuel/hot-spot interface.

Motivation & Objective

• To address the limitations of conventional indirect-drive inertial confinement fusion, particularly low ablation pressure due to rarefaction waves and severe hydrodynamic instabilities during capsule deceleration.
• To reduce the growth of hydrodynamic instabilities—especially at the fuel/hot-spot interface—caused by multiple shock reflections during the deceleration phase.
• To achieve rapid ignition with higher implosion velocity and lower convergence ratio than conventional indirect drive, improving robustness and ignition feasibility.
• To demonstrate that simultaneous direct and indirect drive can stabilize the critical surface and suppress laser imprinting while enhancing drive pressure.
• To validate the scheme through 2D simulations comparing instability growth against a standard point-design indirect-drive target.

Proposed method

• The hybrid scheme uses a cryogenic DT capsule in a high-Z hohlraum, initially compressed by indirect-drive x-rays from laser beams entering through two laser-entrance holes (LEHs).
• Six clusters of direct laser beams are incident simultaneously through the LEHs and four additional symmetric holes at the hohlraum waist, creating a double-ablation-front (DAF) structure with a radiation ablation front (RAF) and electron ablation front (EAF).
• The DAF structure forms a nearly steady high-density plateau between the RAF and EAF, which suppresses rarefaction waves and increases drive pressure to ~450 Mbar.
• The critical surface is maintained at a proper distance from the capsule to smooth direct-laser imprints and reduce asymmetries.
• 2D single-mode simulations with perturbations seeded at the ablator outer surface are performed on wedge geometries to evaluate instability growth, using a modified Lindl formula for Rayleigh-Taylor instability (RTI) growth.
• The scheme is benchmarked against a conventional indirect-drive point-design target (PT) with similar laser energy (~1.35 MJ), radiation temperature profile, and capsule size.

Experimental results

Research questions

• RQ1 Can a hybrid indirect-direct-drive scheme suppress rarefaction waves at the radiation ablation front and thereby enhance drive pressure?
• RQ2 How does the formation of a double-ablation-front structure affect implosion velocity and convergence ratio?
• RQ3 To what extent does the hybrid scheme reduce hydrodynamic instability growth, particularly at the fuel/hot-spot interface, compared to conventional indirect drive?
• RQ4 Can the simultaneous use of direct and indirect drive stabilize the critical surface and mitigate laser imprinting effects?
• RQ5 Does the hybrid scheme significantly reduce multiple shock reflections at the fuel/hot-spot interface during deceleration?

Key findings

• The hybrid-drive scheme achieves an implosion velocity of ~4.3×10⁷ cm/s, significantly higher than the ~3.8×10⁷ cm/s of the conventional indirect-drive point-design target.
• The convergence ratio is reduced to ~25, substantially lower than in conventional indirect drive, indicating less compression and lower risk of fuel mixing.
• The double-ablation-front structure creates a high-density plateau that suppresses rarefaction waves at the radiation ablation front, increasing drive pressure to ~450 Mbar.
• Hydrodynamic instability growth at the fuel/hot-spot interface is reduced by nearly an order of magnitude for the L=16 mode, with smaller bubble and spike structures observed in density contours at stagnation.
• The modified Lindl formula shows that the reduced Atwood number (average ~0.63) at the RAF contributes to suppressed RTI growth during the acceleration phase.
• The collision of the rarefied shock with the rebounded main shock delays the reversal of the pressure gradient, stabilizing the deceleration phase and reducing RTI growth.