Editing Inertial confinement fusion (section)

===Challenges===
[[Image:Nif hohlraum.jpg|thumb|right|Mockup of a gold plated [[National Ignition Facility]] (NIF) hohlraum]]The primary challenges with increasing ICF performance are:
* Improving the energy delivered to the target
* Controlling symmetry of the imploding fuel
* Delaying fuel heating until sufficient density is achieved
* Preventing premature mixing of hot and cool fuel by [[hydrodynamic]] instabilities
* Achieving shockwave convergence at the fuel center

In order to focus the shock wave on the center of the target, the target must be made with great precision and [[sphericity]] with tolerances of no more than a few [[micrometres]] over its (inner and outer) surface. The lasers must be precisely targeted in space and time. Beam timing is relatively simple and is solved by using [[Analog delay line|delay line]]s in the beams' optical path to achieve [[1 E-12 s|picosecond]] accuracy. {{Anchor|instability2016-01-29}}The other major issue is so-called "beam-beam" imbalance and beam [[anisotropy]]. These problems are, respectively, where the energy delivered by one beam may be higher or lower than other beams impinging and of "hot spots" within a beam diameter hitting a target which induces uneven compression on the target surface, thereby forming [[Rayleigh-Taylor instabilities]]<ref>{{cite journal|last1=Hayes|first1=A. C.|last2=Jungman|first2=G.|last3=Solem|first3=J. C.|last4=Bradley|first4=P. A.|last5=Rundberg|first5=R. S.|year=2006|title=Prompt beta spectroscopy as a diagnostic for mix in ignited NIF capsules|journal=Modern Physics Letters A|volume=21|issue=13|pages=1029|arxiv = physics/0408057 |bibcode = 2006MPLA...21.1029H |doi = 10.1142/S0217732306020317 |s2cid=119339212}}</ref> in the fuel, prematurely mixing it and reducing heating efficacy at the instant of maximum compression. The [[Richtmyer-Meshkov instability]] is also formed during the process due to shock waves.

[[Image:1995 Nova Laser Implosion of DT hohlraum target.jpg|thumb|right|An Inertial confinement fusion target, which was a foam filled cylindrical target with machined perturbations, being compressed by the Nova Laser. This shot was done in 1995. The image shows the compression of the target, as well as the growth of the Rayleigh-Taylor instabilities.<ref>{{cite journal|title=Measurement of Feedthrough and Instability Growth in Radiation-Driven Cylindrical Implosions |journal=Physical Review Letters |volume=78 |issue=20 |pages=3876–3879 |date=May 1997 |doi=10.1103/PhysRevLett.78.3876 |last1=Hsing |first1=Warren W. |last2=Hoffman |first2=Nelson M. |bibcode=1997PhRvL..78.3876H }}</ref>]]

These problems have been mitigated by beam smoothing techniques and beam energy diagnostics; however, RT instability remains a major issue. Modern [[Cryogenics|cryogenic]] hydrogen ice targets tend to freeze a thin layer of deuterium on the inside of the shell while irradiating it with a low power [[infrared]] laser to smooth its inner surface and monitoring it with a [[microscope]] equipped [[camera]], thereby allowing the layer to be closely monitored.<ref>{{Cite web|url=http://www.lle.rochester.edu/pub/progress/doe_apr02.pdf|archiveurl=https://web.archive.org/web/20090511175948/http://www.lle.rochester.edu/pub/progress/doe_apr02.pdf|url-status=dead|title=Inertial Confinement Fusion Program Activities, April 2002|archivedate=May 11, 2009}}</ref> Cryogenic targets filled with D-T are "self-smoothing" due to the small amount of heat created by tritium decay. This is referred to as "[[beta radiation|beta]]-layering".<ref>{{Cite web|url=http://www.lle.rochester.edu/pub/progress/MarDOE06.pdf|archiveurl=https://web.archive.org/web/20090511180008/http://www.lle.rochester.edu/pub/progress/MarDOE06.pdf|url-status=dead|title=Inertial Confinement Fusion Program Activities, March 2006|archivedate=May 11, 2009}}</ref>

[[Image:Fusion microcapsule.jpg|right|thumb|An inertial confinement [[Nuclear fusion|fusion]] fuel microcapsule (sometimes called a "microballoon") of the size used on the NIF which can be filled with either deuterium and tritium gas or DT ice. The capsule can be either inserted in a hohlraum (as above) and imploded in the '''indirect drive''' mode or irradiated directly with laser energy in the '''direct drive''' configuration. Microcapsules used on previous laser systems were significantly smaller owing to the less powerful irradiation earlier lasers were capable of delivering to the target.]]

In the indirect drive approach,<ref>{{Citation|last1=Lindl |first1=John |last2=Hammel |first2=Bruce |contribution=Recent Advances in Indirect Drive ICF Target Physics |year=2004 |title=20th IAEA Fusion Energy Conference |publisher=Lawrence Livermore National Laboratory |url=http://fire.pppl.gov/iaea04_lindl.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://fire.pppl.gov/iaea04_lindl.pdf |archive-date=2022-10-09 |url-status=live |access-date=August 23, 2014}}</ref> the absorption of thermal x-rays by the target is more efficient than the direct absorption of laser light. However, the hohlraums take up considerable energy to heat, significantly reducing energy transfer efficiency. Most often, indirect drive hohlraum targets are used to simulate [[Nuclear weapon|thermonuclear weapons]] tests due to the fact that the fusion fuel in weapons is also imploded mainly by X-ray radiation.

ICF drivers are evolving. Lasers have scaled up from a few [[joule]]s and kilowatts to megajoules and hundreds of terawatts, using mostly [[Nonlinear optics|frequency doubled or tripled light]] from [[Neodymium#Glass|neodymium glass]] amplifiers.{{Citation needed|date=February 2023}}

[[Heavy ion fusion|Heavy ion beams]] are particularly interesting for commercial generation, as they are easy to create, control, and focus. However, it is difficult to achieve the energy densities required to implode a target efficiently, and most ion-beam systems require the use of a hohlraum surrounding the target to smooth out the irradiation.{{Citation needed|date=February 2023}}