Inference of the electron temperature in inertial confinement fusion implosions from the hard X‐ray spectral continuum

Abstract

Using the free-free continuum self-emission spectrum at photon energies above 15 keV is one of the most promising concepts for assessing the electron temperature in inertial confinement fusion (ICF) experiments. However, these photons are due to suprathermal electrons whose mean free path is much larger than the thermal one, making their distribution deviate from Maxwellian in a finite-size hotspot. The first study of the free-free X-ray emission from an ICF implosion is conducted, accounting for the kinetic modifications to the electron distribution. These modifications are found to result in qualitatively new features in the hard X-ray spectral continuum. Inference of the electron temperature as if the emitting electrons are Maxwellian is shown to give a lower value than the actual one. KEYWORDShigh-energy-density, inertial confinement, x-ray diagnostic INTRODUCTIONKnowledge of the temperature of the deuterium-tritium (DT) hotspot is crucial for the success of inertial confinement fusion (ICF) experiments. [1,2] Accurate and reliable temperature measurements constrain theoretical models of the implosions and help identify shortcomings of presently available simulation tools, such as radiation-hydrodynamics codes. [3,4] Under thermonuclear conditions the fuel is in the form of a plasma, and the ion and electron temperatures, T i and T e , should generally be distinguished. The ion temperature can be inferred from the spectra of the fusion reaction products. [5,6] Information about the electron temperature is, in principle, carried by the radiation from the hot imploded plasma as this radiation is facilitated by free electrons scattering over ions. As electrons are much faster, it is their distribution only that governs the emission spectrum, which should then provide the basis for the T e inference.In practice, such an inference is obscured by opacity effects. The photon emitted in the hotspot has to travel through the hot-spot itself as well, as through the remnants of the shell, before being registered by spectrometers surrounding the implosion. With a substantial probability of a reabsorption or scattering event, it is a challenge to retrieve the emitting electron distribution from the measured emission spectrum. The current consensus is therefore that the successful diagnostic should operate using the harder part of the spectrum, that is, photon energies ℏ ≳ 15 keV 15 keV, for which the imploded capsule is transparent. [7][8][9][10] In particular, the newly developed spectrometer at the National Ignition Facility (NIF) will infer the electron temperature from the hard X-ray spectral continuum of 20 keV ≲ ℏ ≲ 30 keV, which is established through the free-free Bremsstrahlung by electrons scattering off the fully ionized D and T ions. [11] At the OMEGA laser facility, an electron temperature diagnostic is also being designed based on the time-resolving streak-camera approach, [12] which will look at multiple energy bands in the range of 10 keV ≲ ℏ ≲ 30 keV in cryogenic and warm ICF implosions.