Self-generated magnetic fields in the hot spot of direct-drive cryogenic implosions at Omega*

This work reports that Biermann self-generated magnetic fields of ≈200-250 MG and Hall parameters of ≈2 are produced in the stagnation phase of direct-drive cryogenic implosions at OMEGA.  The magnetic fields produce a drop of 2.4% in fusion yield and 1% in ion temperature. A quantitative estimate of the effect of self-generated magnetic fields on yield and ion temperature is essential, since direct measurements of these fields are not available. Reconstructed simulations of the 50 Gbar implosions, with all the stagnation measurements reproduced simultaneously by a combination of mid- and low-mode asymmetries as degradation mechanisms [1], are used to obtain the estimates. The magnetic fields cause a decrease in yield due to the Righi-Leduc heat flow, which exceeds any benefits from heat flow suppression due to magnetization. It is important to note that both direct-drive Omega scale implosions and indirect-drive NIF scale implosions [2] produce similar estimates for the magnetic field strength and both show a decrease in fusion yield with the Right-Leduc transport as the loss mechanism. However, the yield degradation at Omega is small, and lower by ≈5× compared to the indirect-drive ignition-scale NIF estimate.

[1] A. Bose et. al., Phys. Plasmas, 25, 062701 (2018).

[2] C. A. Walsh et. al., Phys. Rev. Lett. 118, 155001 (2017).