At the National Ignition Facility, inertial confinement fusion experiments aim to burn and ignite a hydrogen plasma to generate a net source of energy through the fusion of deuterium and tritium ions. The energy deposited by α-particles released from the deuterium–tritium fusion reaction plays the central role in heating the fuel to achieve a sustained thermonuclear burn. In the hydrodynamic picture, α-heating increases the temperature of the plasma, leading to increased reactivity because the mean ion kinetic energy increases. Therefore, the ion temperature is related to the mean ion kinetic energy. Here we use the moments of the neutron spectrum to study the relationship between the ion temperature (measured by the variance in the neutron kinetic energy spectrum) and the ion mean kinetic energy (measured by the shift in the mean neutron energy). We observe a departure from the relationship expected for plasmas where the ion relative kinetic energy distribution is Maxwell–Boltzmann, when the plasma begins to burn. Understanding the cause of this departure from hydrodynamic behaviour could be important for achieving robust and reproducible ignition.
This is a preview of subscription content, access via your institution
Subscribe to Nature+
Get immediate online access to Nature and 55 other Nature journal
Subscribe to Journal
Get full journal access for 1 year
only $8.25 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Get time limited or full article access on ReadCube.
All prices are NET prices.
The simulation codes used in this manuscript are not available to the general public.
Lindl, J. Development of the indirect drive approach to inertial confinement fusion and the target physics basis for ignition and gain. Phys. Plasmas 2, 3933–4024 (1995).
Christopherson, A. R. et al. Theory of alpha heating in inertial fusion: alpha-heating metrics and the onset of the burning-plasma regime. Phys. Plasmas 25, 072704 (2018).
Zylstra, A. B. et al. Burning plasma achieved in inertial fusion. Nature 601, 542–548 (2022).
Kritcher, A. L. et al. Design of inertial fusion implosions reaching the burning plasma regime. Nat. Phys. 18, 251–258 (2022).
Brysk, H. Fusion neutron energies and spectra. Plasma Phys. 15, 611–617 (1973).
Ballabio, L., Källane, J. & Gorini, G. Relativistic calculation of fusion product spectra for thermonuclear plasmas. Nucl. Fusion 38, 1723–1735 (1998).
Appelbe, B. & Chittenden, J. Relativistically correct DD and DT neutron spectra. High Energy Density Phys. 11, 30–35 (2014).
Munro, D. H. Interpreting inertial fusion neutron spectra. Nucl. Fusion 56, 036001 (2016).
Gamow, G. Nuclear energy sources and stellar evolution. Phys. Rev. 53, 595–604 (1938).
Moore, A. S. et al. The five line-of-sight neutron time-of-flight (nToF) suite on the National Ignition Facility (NIF). Rev. Sci. Instrum. 92, 023516 (2021).
Jarrott, L. C. et al. Thermal temperature measurements of inertial fusion implosions. Phys. Rev. Lett. 121, 085001 (2018).
Hatarik, R. et al. Analysis of the neutron time-of-flight spectra from inertial confinement fusion experiments. J. Appl. Phys. 118, 184502 (2015).
Gaffney, J. A. et al. Making inertial confinement fusion models more predictive. Phys. Plasmas 26, 082704 (2019).
Clark, D. S. et al. Three-dimensional modeling and hydrodynamic scaling of National Ignition Facility implosions. Phys. Plasmas 26, 050601 (2019).
Ross, J. S. et al. Experiments conducted in the burning plasma regime with inertial fusion implosions. Preprint at https://arxiv.org/abs/2111.04640 (2021).
Rinderknecht, H. G. et al. Azimuthal drive asymmetry in inertial confinement fusion implosions on the National Ignition Facility. Phys. Rev. Lett. 124, 145002 (2020).
Peigney, B. E., Larroche, O. & Tikhonchuk, V. Ion kinetic effects on the ignition and burn of inertial confinement fusion targets: a multi-scale approach. Phys. Plasmas 21, 122709 (2014).
Sadler, J. D. et al. Kinetic simulations of fusion ignition with hot-spot ablator mix. Phys. Rev. E 100, 033206 (2019).
Appelbe, B., Sherlock, M., El-Amiri, O., Walsh, C. & Chittenden, J. Modification of classical electron transport due to collisions between electrons and fast ions. Phys. Plasmas 26, 102704 (2019).
Sherlock, M. & Rose, S. The persistence of Maxwellian D and T distributions during burn in inertial confinement fusion. High Energy Density Phys. 5, 27–30 (2009).
Hartouni, E. P. et al. Optimal choice of multiple line-of-sight measurements determining plasma hotspot velocity at the National Ignition Facility. Rev. Sci. Instrum. 92, 023513 (2021).
Moore, A. S. et al. A fused silica Cherenkov radiator for high precision time-of-flight measurement of DT γ and neutron spectra (invited). Rev. Sci. Instrum. 89, 10I120 (2018).
Spears, B. K. et al. Mode 1 drive asymmetry in inertial confinement fusion implosions on the National Ignition Facility. Phys. Plasmas 21, 042702 (2014).
Hatarik, R. et al. A new neutron time-of-flight detector to measure the MeV neutron spectrum at the National Ignition Facility. Plasma Fusion Res. 9, 4404104 (2014).
Grim, G. P. et al. The 27.3 meter neutron time-of-flight system for the National Ignition Facility. In Penetrating Radiation Systems and Applications XIV (eds. Grim, G. P. & Barber, H. B.) 8854, 70–81 (SPIE, 2013).
Clancy, T. J. et al. Engineering architecture of the neutron time-of-flight (nToF) diagnostic suite at the National Ignition Facility. In Target Diagnostics Physics and Engineering for Inertial Confinement Fusion III (eds. Bell, P. M. & Grim, G. P.) 9211, 92110A (SPIE, 2014).
Caggiano, J. A. et al. Design of a north pole neutron time-of-flight (NTOF) system at NIF. J. Phys.: Conf. Ser. 717, 012087 (2016).
Meaney, K. D. et al. Total fusion yield measurements using deuterium–tritium gamma rays. Phys. Plasmas 28, 102702 (2021).
Crilly, A. et al. Constraints on ion velocity distributions from fusion product spectroscopy. Nucl. Fusion 62, 126015 (2022).
We would like to acknowledge many discussions with our colleagues O. Landen, P. Patel, L. Divol, P. Springer and C. Cerjan. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security for the US Department of Energy, National Nuclear Security Administration, under contract no. DE-AC52- 07NA27344. This Article (LLNL-JRNL-824268) was prepared as an account of work sponsored by an agency of the US government. Neither the US government nor Lawrence Livermore National Security, nor any of their employees make any warranty, expressed or implied, or assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represent that its use would not infringe privately owned rights. The views and opinions of authors expressed herein do not necessarily state or reflect those of the US government or Lawrence Livermore National Security and shall not be used for advertising or product endorsement purposes.
The authors declare no competing interests.
Peer review information
Nature Physics thanks Alessio Morace, Peter Norreys and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Hartouni, E.P., Moore, A.S., Crilly, A.J. et al. Evidence for suprathermal ion distribution in burning plasmas. Nat. Phys. 19, 72–77 (2023). https://doi.org/10.1038/s41567-022-01809-3
This article is cited by
Nature Physics (2023)