Electron Hot Spot Temperature Profile Discrepancies between DD and DT implosions from X-ray Spectroscopy at the NIF

A necessary measurable for evaluating capsule implosion in inertial confinement fusion (ICF) is the hot spot plasma temperature. Hot spot temperatures are usually constrained by neutron time-of-flight (nTOF) spectrometers, however, nTOF measurements are susceptible to error from plasma flows [1] and give no information about electron temperatures. It is, therefore, requisite to utilize additional diagnostics to measure hot spot thermal temperature. The high resolution spectrometer dHIRES at the National Ignition Facility (NIF) is one such tool which can measure the electron temperature of implosions [2-4]. The dHIRES is an x-ray Bragg crystal spectrometer which captures Kr Heα, Lyα, and Heβ emission.

In this work, we compare two Kr-doped shots, one using DD fuel and one using DT fuel. Results from nTOF show the shots have the same hot spot conditions within error and the Kr Heα emission collected with dHIRES agrees with this assessment. Paradoxically, the Heβ emission differs in intensity, indicating some discrepancy in plasma conditions. Thus, we use radiative transfer atomic kinetics code Cretin [5] to explain this discrepancy between the ion and electron temperatures for comparable shots. Synthetic spectra generated from spherical hot spots with varying temperature and density profiles will be produced and fit to the data, providing new information on shot-to-shot repeatability and exploring the possibility of electron heating from low-yield DT fusion.

[1] M. Gatu Johnson et al., Phys. Rev. E 94, 021202 (2016).

[2] K. W. Hill et al., Rev. Sci. Instrum. 87, 11E344 (2016).

[3] L. Gao et al., Rev. Sci. Instrum. 89, 10F125 (2018).

[4] L. Gao et al., Phys. Rev. Lett. 128, 185002 (2022).

[5] H. A. Scott, J. Quant. Spectrosc. Radiat. Transf. 71, 698-701 (2001).