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Inradiationthermodynamics, ahohlraum (German:[ˈhoːlˌʁaʊ̯m]ⓘ; a non-specificGerman word for a "hollow space", "empty room", or "cavity") is a cavity whose walls are inradiative equilibrium with theradiant energy within the cavity. First proposed byGustav Kirchhoff in 1860 and used in the study ofblack-body radiation (hohlraumstrahlung),[1] thisidealized cavity can be approximated in practice by a hollow container of anyopaque material. The radiation escaping through a small perforation in the wall of such a container will be a good approximation ofblack-body radiation at the temperature of the interior of the container.[2] Indeed, a hohlraum can even be constructed from cardboard, as shown by Purcell'sBlack Body Box, a hohlraum demonstrator.[3]
Inspectroscopy, theHohlraum effect occurs when an object achievesthermodynamic equilibrium with an enclosing hohlraum. As a consequence ofKirchhoff’s law, everything optically blends, and the contrast between the walls and the object effectively disappears.[4]
Hohlraums are used inhigh energy density physics (HEDP) andinertial confinement fusion (ICF) experiments to convertlaser energy to thermalX-rays forimploding capsules, heating targets, and generatingthermal radiation waves.[5] They may also be used innuclear weapon designs.
The indirect drive approach toinertial confinement fusion is as follows: thefusion fuel capsule is held inside acylindrical hohlraum. The hohlraum body is manufactured using a high-Z (highatomic number) element, usuallygold oruranium. Inside the hohlraum is a fuel capsule containingdeuterium andtritium (D-T) fuel. A frozen layer of D-T ice adheres to the inside of the fuel capsule.The fuel capsule wall is fabricated using light elements such as plastic,beryllium, or high-densitycarbon, i.e.diamond. The outer portion of the fuel capsule explodes outward when ablated by theX-rays produced by the hohlraum wall upon irradiation by lasers. Due toNewton's third law, the inner portion of the fuel capsule implodes, causing the D-T fuel to be supercompressed, activating afusion reaction.
The radiation source (e.g.,laser) is pointed at the interior of the hohlraum rather than at the fuel capsule itself. The hohlraum absorbs and re-radiates the energy asX-rays, a process known as indirect drive. The advantage of this approach, compared to direct drive, is that high-mode structures from the laser spot are smoothed out when the energy is re-radiated from the hohlraum walls. The disadvantage of this approach is that low-mode asymmetries are more complex to control. It is essential to be able to control both high-mode and low-mode asymmetries to achieve a uniformimplosion.
The hohlraum walls must havesurface roughness less than 1micron, and hence accurate machining is required during fabrication. Any imperfection of the hohlraum wall during fabrication will cause uneven and non-symmetrical compression of the fuel capsule inside the hohlraum duringinertial confinement fusion (ICF). Hence, imperfections are to be carefully avoided, so surface finishing is critical, as during ICF laser shots, due to the intense pressure and temperature, results are highly susceptible to hohlraum texture roughness. The fuel capsule must be preciselyspherical, with texture roughness less than onenanometer, forfusion ignition to start. Otherwise, instability will cause fusion to fizzle. The fuel capsule contains a small fill hole with a diameter of less than 5 microns to inject D-T gas into the capsule.
The X-ray intensity around the capsule must be verysymmetrical to avoidhydrodynamic instabilities duringcompression. Earlier designs had radiators at the ends of the hohlraum, but maintaining adequate X-ray symmetry proved difficult with this geometry. By the end of the 1990s, target physicists developed a new family of designs in which theion beams are absorbed in the hohlraum walls, so that X-rays are radiated from a significant fraction of thesolid angle surrounding the capsule. With a judicious choice of absorbing materials, this arrangement, referred to as a "distributed-radiator" target, gives better X-ray symmetry and target gain in simulations than earlier designs.[6]
The termhohlraum is also used to describe the casing of athermonuclear bomb following theTeller-Ulam design. The casing's purpose is to contain and focus the energy of the primary (fission) stage toimplode the secondary (fusion) stage.
