Development of a cryogenically cooled platform for the Magnetized Liner Inertial Fusion (MagLIF) Program

Abstract

A cryogenically cooled hardware platform has been developed and commissioned on the Z Facility at Sandia National Laboratories in support of the Magnetized Liner Inertial Fusion (MagLIF) Program. MagLIF is a magneto-inertial fusion concept that employs a magnetically imploded metallic tube (liner) to compress and inertially confine premagnetized and preheated fusion fuel. The fuel is preheated using a ∼2 kJ laser that must pass through a ∼1.5-3.5-μm-thick polyimide "window" at the target's laser entrance hole (LEH). As the terawatt-class laser interacts with the dense window, laser plasma instabilities (LPIs) can develop, which reduce the preheat energy delivered to the fuel, initiate fuel contamination, and degrade target performance. Cryogenically cooled targets increase the parameter space accessible to MagLIF target designs by allowing nearly 10 times thinner windows to be used for any accessible gas density. Thinner LEH windows reduce the deleterious effects of difficult to model LPIs. The Z Facility's cryogenic infrastructure has been significantly altered to enable compatibility with the premagnetization and fuel preheat stages of MagLIF. The MagLIF cryostat brings the liquid helium coolant directly to the target via an electrically resistive conduit. This design maximizes cooling power while allowing rapid diffusion of the axial magnetic field supplied by external Helmholtz-like coils. A variety of techniques have been developed to mitigate the accumulation of ice from vacuum chamber contaminants on the cooled LEH window, as even a few hundred nanometers of ice would impact laser energy coupling to the fuel region. The MagLIF cryostat has demonstrated compatibility with the premagnetization and preheat stages of MagLIF and the ability to cool targets to liquid deuterium temperatures in approximately 5 min.

Constraining preheat energy deposition in MagLIF experiments with multi-frame shadowgraphy

“…For this reason, it is not currently thought that ZBL can exceed 2 kJ deposited into the fuel with its current parameters (maximum 3 kJ on target) and the current target designs. Options for overcoming this limitation include the planned increase to the lasers' available energy and techniques to reduce the foil thickness, including cryogenically cooling the target to reduce the fuel pressure at a given density 24 and the laser-gate concept. 4…”

An overview of magneto-inertial fusion on the Z machine at Sandia National Laboratories

“…Simulations also suggest that ∼72% coupling efficiency (2.7 kJ) to the imploding region is possible [103] when using cryogenically cooled [105] MagLIF targets. Cooling the target to ∼70 K significantly reduces the initial fuel pressure (4-8 bar at room temperature) on the LEH foil required to achieve typical target densities to <1.7 bar, enabling much thinner (500 nm thick) LEH windows and improved energy coupling to the fuel.…”