Thermomechanical Effects in a Laser IFE First Wall

Laser fusion chamber walls will experience large, pulsed heat loads at frequencies of several hertz. The heating, consisting of x-rays, neutrons, and ions, occurs over a few microseconds and is deposited volumetrically over the first few microns of the wall. For a reasonable chamber radius, the heating will be such that the surface temperature is a significant fraction of the melt temperature of the wall, and significant plasticity can be expected in ductile wall materials. This paper presents results for the transient temperatures and stresses in a tungsten-coated steel first wall for a laser fusion device. Failure analyses are carried out using both fatigue and fracture mechanics methodologies. The simulations predict that surface cracks are expected in the tungsten, but the cracks will arrest before reaching the substrate if the crack spacing is sufficiently small. In addition, the thermal and stress fields are compared for a laser fusion device with several simulation experiments. It is shown that the simulations can reproduce the peak surface temperatures, but the corresponding spatial distributions of the stress and temperature will be shallower than the reactor case. Introduction The chamber walls in inertial fusion energy (IFE) devices will experience a harsh set of thermal loads resulting from deposition of energy from target implosions. Hence, the chamber design must account for these effects to ensure adequate wall life. Early wall design efforts will require detailed thermomechanical analyses including all thermal loads, nonlinear material effects, and, in some cases, phase changes due to melting or vaporization. The analyses presented in this paper have been carried out as part of the High Average Power Laser (HAPL) [1] program. HAPL has focused on solid chamber walls, with the primary candidate being a tungsten-coated steel wall. Hence, the conclusions are specific to this design approach. The heat loads in the HAPL design are delivered to the wall in the form of x-rays, neutrons, and ions, and the majority of their energy is deposited in the first few microns of the wall. In some cases a gas (xenon) is placed in the chamber to absorb some of this heat which is radiated to the wall at a later time, thus effectively spreading the heat load over a longer time and reducing the peak wall temperatures. In all cases, the resulting thermal stresses are expected to be well above the yield stress of the tungsten coating, and thus the mechanical design will require consideration of fatigue and fracture to ensure that the steel wall is protected for a lifetime sufficient to provide an economical design for a commercial power plant. This paper presents an overview of the heat loads expected in a HAPL chamber and a series of thermal and structural analyses for a variety of target yields and chamber sizes. All analyses incorporate temperature-dependent properties and nonlinear material effects (primarily plasticity). Fracture models are also used to assess the likelihood of cracks in the tungsten growing until they reach the steel potentially leading to fracture in the steel or delamination of the tungsten coating. Heat Loads and Properties From a thermomechanical perspective, the thermal loads on an IFE chamber wall can be viewed as resulting from x-rays, ions, and neutrons. In laser IFE, the energy carried by each of these is typically on the order of 1, 29, and 70 percent, respectively, though these energy splits will vary somewhat depending on the target design and yield. The xrays typically arrive to the wall first and deposit their energy within a few nanoseconds. The neutrons arrive after 100-200 nanoseconds and are deposited over less than 20 nanoseconds. Since the neutrons have fairly long mean free paths, only a small fraction of their energy will be captured in the first wall. The ions arrive last, with deposition beginning after about 200 nanoseconds and continuing over a time span of approximately 3 microseconds. Again, these numbers will vary with chamber size and target characteristics. A significant design parameter is gas in the chamber. One can protect the chamber wall by filling the chamber with gas (xenon, for instance) at tens of mTorr pressure (or greater). This gas will stop some of the ions and x-rays before they reach the wall reducing the peak temperature of the first wall surface. The heat absorbed by the gas will then be reradiated to the wall over a longer time period (typically hundreds of microseconds), effectively spreading the heat deposition over a longer time period and reducing the peak heat flux and wall surface temperature. As an example, 10 mTorr xenon gas in a 6.5 meter radius laser IFE chamber will typically stop about 20% of the ion energy and about 9% of the x-ray energy before it reaches the wall. The x-ray energies are as high as 100 keV, with the bulk falling below 10 keV. Hence deposition of the x-ray energy largely occurs within the first micron of the surface. The ions created by the target burn have energies as high as 10 MeV, with the bulk falling below a few MeV. Their energy is typically deposited over a few microns. The so-called debris ions, initially present in the target and accelerated by the burn, have energies as high as 20 MeV, with the bulk falling below 200 keV. Their energy is deposited over about 1 micron. These data are summarized in Table 1. As will be shown later, there is a local peak in the chamber wall temperature as a result of the x-ray fluence, but the global peak temperature occurs near the end of the ion arrival period. Type Fraction of Yield Max energy (keV) Arrival time (ns) Pulse width (ns) x-rays 0.01 100 0 1 neutrons 0.70 160 20 Burn ions 0.12 25,000 200 800 Debris ions 0.17 15,000 1,000 2,750 Table 1: Characteristics of energy deposited in chamber wall as a result of a single laser target implosion