Burning plasma achieved in inertial fusion

Abstract

Obtaining a burning plasma is a critical step towards self-sustaining fusion energy¹. A burning plasma is one in which the fusion reactions themselves are the primary source of heating in the plasma, which is necessary to sustain and propagate the burn, enabling high energy gain. After decades of fusion research, here we achieve a burning-plasma state in the laboratory. These experiments were conducted at the US National Ignition Facility, a laser facility delivering up to 1.9 megajoules of energy in pulses with peak powers up to 500 terawatts. We use the lasers to generate X-rays in a radiation cavity to indirectly drive a fuel-containing capsule via the X-ray ablation pressure, which results in the implosion process compressing and heating the fuel via mechanical work. The burning-plasma state was created using a strategy to increase the spatial scale of the capsule^2,3 through two different implosion concepts^4-7. These experiments show fusion self-heating in excess of the mechanical work injected into the implosions, satisfying several burning-plasma metrics^3,8. Additionally, we describe a subset of experiments that appear to have crossed the static self-heating boundary, where fusion heating surpasses the energy losses from radiation and conduction. These results provide an opportunity to study α-particle-dominated plasmas and burning-plasma physics in the laboratory.

Fig. 1. Schematic of the indirect-drive inertial confinement approach to fusion.

Centre, A typical indirect-drive target configuration with key engineering elements labelled. Laser beams (blue) enter the hohlraum through laser entrance holes at various angles. Top left, A schematic pie diagram showing the radial distribution and dimensions of materials in diamond (high-density carbon, HDC) ablator implosions. Bottom left, The temporal laser power pulse-shape (blue) and associated hohlraum radiation temperature (green). Right, At the centre of the hohlraum, the capsule is bathed in X-rays, which ablate the outer surface of the capsule. The pressure generated drives the capsule inward upon itself (an implosion) which compresses and heats the fusion fuel during the implosion process.

Fig. 2. Simple metrics for assessing a burning plasma.

a, Total fuel gain versus Lawson-like parameter; G_fuel > 5 corresponds to the burning-plasma regime. b, Probability distributions for G_fuel for high-performing experiments. In these plots the width of the shaded region is proportional to the probability distribution and the solid lines mark the 16th, 50th and 84th percentiles of the distribution c, Total α-heating energy versus fuel kinetic energy, E_α/KE_fuel > 1 corresponds to Q_α > 1. d, Probability distributions in E_α/KE_fuel criteria for high-performing experiments. Error bars in a, c are 1 standard deviation (s.d.) and are shown only for the I-Raum and Hybrid-E points. Historical data are from refs. ^,,,–,–.

Fig. 3. ICF-specific burning-plasma metrics.

a, Criteria on temperature and hot-spot ρR established by Hurricane et al.. Previous experiments are shown as points, and the present four experiments are shown as full probability distributions (red, N201101; blue, N201122; purple, N210207; grey, N210220), with contours enclosing 80% of the distribution. A single contour of equation (1) for v_imp = 385 km s⁻¹ is given by the solid black line. b, Probability distribution for experiments exceeding the Hurricane criterion, >1 is a burning plasma. c, Criteria on α-heating and PdV work from a previous work, including estimates from data inferences (solid symbols) and from 2D simulations (open symbols). d, Probability distribution for experiments exceeding the Betti criteria. For these experiments distributions are shown for data-inferred E_PdV,hs (blue) and using 2D simulations (orange). Error bars in a, c are 1 s.d. and are shown only for the I-Raum and Hybrid-E points. Historical data are from refs. ^,,,–,–.

Fig. 4. Parameter space relevant for proximity to ignition.

Left, hot-spot pressure and energy. The product P²E_hs is representative of proximity to ignition; contours of this metric relative to N210207 are shown by the dashed grey curves. Right, yield amplification (Y_amp) versus ITFX_nα. These are the highest performing ICF experiments so far and the closest to ignition. The inset shows these experiments in detail with both inferred (solid) and simulated (open) Y_amp. Error bars are 1 s.d. and are shown only for the I-Raum and Hybrid-E points, plus shot N180128. Historical data are from refs. ^,,,–,–.