Cryogenic targets for optimized laser preheat in MagLIF

In magnetized liner inertial fusion (MagLIF), a magnetized sample of gaseous deuterium fuel is “preheated” and subsequently compressed. Preheat is required to allow for the compression to follow a higher adiabat, thus achieving much higher end-temperatures than starting with cold fuel. This temperature enhancement is critical for achieving fusion conditions. To deposit preheat into the target with a high-energy laser, a film-covered laser-entrance hole (LEH) is required, which can confine the deuterium gas while allowing laser energy to reach the fuel. However, this thin-film window absorbs laser energy and can mix with the fuel, which degrades the fusion yield. Cryogenically cooling the deuterium fuel allows the thickness of the LEH window to be significantly reduced. Simultaneously, the implementation of a larger-diameter laser beam waist in the interaction volume allows for a reduction of the laser intensity. By reducing the overall mass of the heated window material and lowering the intensity on target, the average fraction of laser energy that is deposited in the fuel improves from 65% to 83%, without overshooting the useful depth of a MagLIF target. Informed by previous campaigns, the laser pulse length and spot profile were chosen to reduce the intensity below the threshold for losses from stimulated Brillouin scattering. Although the initial expectation was that the dominating contribution to a preheat increase should be the reduced material in the entrance window, it was found that reducing residual laser-plasma-instabilities is even more important for maximizing laser deposition despite the relatively low intensities. The increase in laser-fuel coupling presented here has contributed to the highest performing MagLIF experiments to date.