Abstract
At the Z Facility at Sandia National Laboratories, the magnetized liner inertial fusion (MagLIF) program aims to study the inertial confinement fusion in deuterium-filled gas cells by implementing a three-step process on the fuel: premagnetization, laser preheat, and Z-pinch compression. In the laser preheat stage, the Z-Beamlet laser focuses through a thin polyimide window to enter the gas cell and heat the fusion fuel. However, it is known that the presence of the few μm thick window reduces the amount of laser energy that enters the gas and causes window material to mix into the fuel. These effects are detrimental to achieving fusion; therefore, a windowless target is desired. The Lasergate concept is designed to accomplish this by “cutting” the window and allowing the interior gas pressure to push the window material out of the beam path just before the heating laser arrives. In this work, we present the proof-of-principle experiments to evaluate a laser-cutting approach to Lasergate and explore the subsequent window and gas dynamics. Further, an experimental comparison of gas preheat with and without Lasergate gives clear indications of an energy deposition advantage using the Lasergate concept, as well as other observed and hypothesized benefits. While Lasergate was conceived with MagLIF in mind, the method is applicable to any laser or diagnostic application requiring direct line of sight to the interior of gas cell targets.
Highlights
Studies were performed that explored (1) the threshold fluence required for the cutting laser to reliably implement Lasergate, (2) the dynamics of the laser entrance window (LEW) as it opens for a variety of thicknesses and fill pressures, (3) the gas dynamics inside and outside the gas cell during the Lasergate process, and (4) the impact Lasergate makes on the laser preheat stage of magnetized liner inertial fusion (MagLIF)
Images of the LEW recorded through shadowgraphy and x-ray thermal emission confirm that a windowless target can be made reliably and reproducibly
The absence of LEW material in the heating beam path is expected to dramatically reduce the amount of higher-Z LEW and cushion material mixed with the MagLIF deuterium fuel in the implosion region
Summary
Inertial confinement fusion (ICF) is a topic of strong interest among the scientific community due to its relevance to stockpile stewardship and potential energy production applications. At Sandia National Laboratories, the Z facility enables ICF studies through heating and compression of fusion fuel as demonstrated through the magnetized liner inertial fusion (MagLIF) program. In MagLIF, deuterium gas is contained within a cylindrical beryllium cell (or liner), which is imploded by up to 22 MA of electrical current delivered by the Z-machine. That the D2 scaling would result in much larger heating losses if the spot diameter were enlarged, as would be necessary to heat a larger volume of fuel than just a 1.1 mm central column of the 4.65 mm MagLIF gas cell [see Fig. 1(b)] This enlargement would reduce laser plasma instabilites and thermal self-focusing effects, while increasing the total preheat energy that can be deposited and the final temperature achievable for the imploded fusion fuel.. In addition to the energy loss mechanisms stated above, the LEW can further impede fusion yields by introducing mix between the fuel and window/liner material, allowing greater radiation energy loss from the mixture.22,23 Another possible detriment is the pronounced axial non-uniformity observed in heating blastwaves (see Sec. VI), which may seed magneto-hydrodynamic (MHD) liner instabilities that develop during the deceleration stage of the Z-pinch.. The Lasergate approach presented in this paper can help to quantify LEW effects through comparison to non-Lasergate experiments, and it may be one of the most viable approaches to solving the LEW problem without introducing prohibitive complexity or side-effects
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