Editing Inertial confinement fusion (section)

==Description==
===Fusion basics===
{{main|Nuclear fusion}}Fusion reactions combine smaller atoms to form larger ones. This occurs when two atoms (or ions, atoms stripped of their electrons) come close enough to each other that the [[nuclear force]] dominates the [[electrostatic force]] that otherwise keeps them apart. Overcoming electrostatic repulsion requires [[kinetic energy]] sufficient to overcome the ''[[Coulomb barrier]]'' or ''fusion barrier''.<ref name=basic>{{cite web |url=https://www.iaea.org/topics/energy/fusion/background |title=Basic fusion physics |website=International Atomic Energy Agency|date=12 October 2016 }}</ref>

Less energy is needed to cause lighter nuclei to fuse, as they have less electrical charge and thus a lower barrier energy. Thus the barrier is lowest for [[hydrogen]]. Conversely, the nuclear force increases with the number of [[nucleon]]s, so [[isotope]]s of hydrogen that contain additional [[neutron]]s reduce the required energy. The easiest fuel is a mixture of <sup>2</sup>H, and <sup>3</sup>H, known as D-T.<ref name=basic/>

The odds of fusion occurring are a function of the fuel density and temperature and the length of time that the density and temperature are maintained. Even under ideal conditions, the chance that a D and T pair fuse is very small. Higher density and longer times allow more encounters among the atoms. This [[Cross section (physics)|cross section]] is further dependent on individual ion energies. This combination, the [[fusion triple product]], must reach the [[Lawson criterion]], to reach ignition.<ref>{{cite web|first=Mark |last=Hoffman |url=http://www.scienceworldreport.com/articles/5763/20130323/lawson-criteria-make-fusion-power-viable-iter.htm |title=What Is The Lawson Criteria, Or How to Make Fusion Power Viable |publisher=Scienceworldreport.com |date=2013-03-23 |access-date=2014-08-23}}</ref>

===Thermonuclear devices===
{{see also|Teller-Ulam design}}

The first ICF devices were the [[hydrogen bomb]]s invented in the early 1950s. A hydrogen bomb consists of two bombs in a single case. The first, the ''primary stage'', is a fission-powered device normally using [[plutonium]]. When it explodes it gives off a burst of thermal X-rays that fill the interior of the bomb casing. These X-rays are absorbed by a special material (like [[Fogbank]]) surrounding the ''secondary stage'', which consists of fusion fuel, sandwiched between a fission fuel [[Sparkplug (nuclear weapons)|sparkplug]] and [[Tamper (nuclear weapon)|tamper]]. The X-rays heat this secondary and initiate further fission. Due to [[Newton's third law]], this causes the fuel inside to be driven inward, compressing and heating it. This causes the fusion fuel to reach the temperature and density where fusion reactions begin.<ref name=sub>{{cite web | url = http://nuclearweaponarchive.org/Nwfaq/Nfaq4.html | title = Section 4.0 Engineering and Design of Nuclear Weapons | last = Sublette | first = Carey | date = 2019-03-19 | website = Nuclear Weapon Archive | language = en | access-date=2021-02-09 | url-status = live | archive-url = https://web.archive.org/web/20210206233652/http://nuclearweaponarchive.org/Nwfaq/Nfaq4.html | archive-date = 2021-02-06 | df = dmy-all }}</ref>{{sfn|Nuckolls|1998|p=1}}

In the case of D-T fuel, most of the energy is released in the form of [[alpha particle]]s and neutrons. Under normal conditions, an alpha can travel about 10&nbsp;mm through the fuel, but in the ultra-dense conditions in the compressed fuel, they can travel about 0.01&nbsp;mm before their electrical charge, interacting with the surrounding plasma, causes them to lose their speed.{{sfn|Keefe|1982|p=10}} This means the majority of the energy released by the alphas is redeposited in the fuel. This transfer of kinetic energy heats the surrounding particles to the energies they need to undergo fusion. This process causes the fusion fuel to burn outward from the center. The electrically neutral neutrons travel longer distances in the fuel mass and do not contribute to this self-heating process. In a bomb, they are instead used to either breed tritium through reactions in a lithium-deuteride fuel, or are used to split additional fissionable fuel surrounding the secondary stage, often part of the bomb casing.<ref name=sub/>

The requirement that the reaction has to be sparked by a fission bomb makes this method impractical for power generation. Not only would the fission triggers be expensive to produce, but the minimum size of such a bomb is large, defined roughly by the [[critical mass (nuclear)|critical mass]] of the [[plutonium]] fuel used. Generally, it seems difficult to build efficient nuclear fusion devices much smaller than about 1 kiloton in yield, and the fusion secondary would add to this yield. This makes it a difficult engineering problem to extract power from the resulting explosions. [[Project PACER]] studied solutions to the engineering issues,{{sfn|Nuckolls|1998|p=1}} but also demonstrated that it was not economically feasible. The cost of the bombs was far greater than the value of the resulting electricity.<ref>{{cite journal |first=F. |last=Long |url=https://books.google.com/books?id=4QsAAAAAMBAJ&pg=PA18 |title=Peaceful Nuclear Explosions |journal=Bulletin of the Atomic Scientists |volume=32 |issue=8 |date=October 1976 |page=18 |doi=10.1080/00963402.1976.11455642 |bibcode=1976BuAtS..32h..18L }}</ref>

===Mechanism of action{{Anchor|ICF mechanism of action}} ===<!-- This section is linked from [[HiPER]]. See [[WP:MOS#Section management]] -->
The energy needed to overcome the Coulomb barrier corresponds to the energy of the average particle in a gas heated to 100 million [[Kelvin|K]]. The [[specific heat]] of hydrogen is about 14 [[Joule]] per gram-K, so considering a 1 milligram fuel pellet, the energy needed to raise the mass as a whole to this temperature is 1.4 megajoules (MJ).{{sfn|Emmett|Nuckolls|Wood|1974|p=24}}

In the more widely developed [[magnetic fusion energy]] (MFE) approach, confinement times are on the order of one second. However, plasmas can be sustained for minutes. In this case the confinement time represents the amount of time it takes for the energy from the reaction to be lost to the environment - through a variety of mechanisms. For a one-second confinement, the density needed to meet the Lawson criterion is about 10<sup>14</sup> particles per cubic centimetre (cc).{{sfn|Emmett|Nuckolls|Wood|1974|p=24}} For comparison, air at sea level has about 2.7 x 10<sup>19</sup> particles/cc, so the MFE approach has been described as "a good vacuum".

Considering a 1 milligram drop of D-T fuel in liquid form, the size is about 1&nbsp;mm and the density is about 4 x 10<sup>20</sup>/cc. Nothing holds the fuel together. Heat created by fusion events causes it to expand at the [[speed of sound]], which leads to a confinement time around 2 x 10<sup>−10</sup> seconds. At liquid density the required confinement time is about 2 x 10<sup>−7</sup>s. In this case only about 0.1 percent of the fuel fuses before the drop blows apart.{{sfn|Emmett|Nuckolls|Wood|1974|p=25}}

The rate of fusion reactions is a function of density, and density can be improved through compression. If the drop is compressed from 1&nbsp;mm to 0.1&nbsp;mm in diameter, the confinement time drops by the same factor of 10, because the particles have less distance to travel before they escape. However, the density, which is the cube of the dimensions, increases by 1,000 times. This means the overall rate of fusion increases 1,000 times while the confinement drops by 10 times, a 100-fold improvement. In this case 10% of the fuel undergoes fusion; 10% of 1&nbsp;mg of fuel produces about 30&nbsp;MJ of energy, 30 times the amount needed to compress it to that density.{{sfn|Emmett|Nuckolls|Wood|1974|pp=25-26}}

The other key concept in ICF is that the entire fuel mass does not have to be raised to 100 million K. In a fusion bomb the reaction continues because the alpha particles released in the interior heat the fuel around it. At liquid density the alphas travel about 10&nbsp;mm and thus their energy escapes the fuel. In the 0.1&nbsp;mm compressed fuel, the alphas have a range of about 0.016&nbsp;mm, meaning that they will stop within the fuel and heat it. In this case a "propagating burn" can be caused by heating only the center of the fuel to the needed temperature. This requires far less energy; calculations suggested 1&nbsp;kJ is enough to reach the compression goal.{{sfn|Emmett|Nuckolls|Wood|1974|p=26}}

Some method is needed to heat the interior to fusion temperatures, and do so while when the fuel is compressed and the density is high enough.{{sfn|Emmett|Nuckolls|Wood|1974|p=26}} In modern ICF devices, the density of the compressed fuel mixture is as much as one-thousand times the density of water, or one-hundred times that of lead, around 1000 g/cm<sup>3</sup>.{{sfn|Malik|2021|p=284}} Much of the work since the 1970s has been on ways to create the central hot-spot that starts off the burning, and dealing with the many practical problems in reaching the desired density.

[[Image:Inertial confinement fusion.svg|thumb|center|600px|Schematic of the stages of inertial confinement fusion using lasers. The blue arrows represent radiation; orange is blowoff; purple is inwardly transported thermal energy.
{{ordered list
| Laser beams or laser-produced X-rays rapidly heat the surface of the fusion target, forming a surrounding plasma envelope.
| Fuel is compressed by the rocket-like blowoff of the hot surface material.
| During the final part of the capsule implosion, the fuel core reaches 20 times the density of lead and ignites at 100,000,000 ˚C.
| Thermonuclear burn spreads rapidly through the compressed fuel, yielding many times the input energy.
}}]]

===Heating concepts===
Early calculations suggested that the amount of energy needed to ignite the fuel was very small, but this does not match subsequent experience.

==== Hot spot ignition ====
[[File:NIF output over 11 years without legend.png|upright=1.5|thumb|alt=Plot of NIF results from 2012 to 2022|Plot of NIF target gain from 2012 to 2022, on a logarithmic scale. Note the 10× increase in gain in 2021 due to the achievement of ignition, followed by the achievement of target gain greater than 1 in 2022.]]

The initial solution to the heating problem involved deliberate "shaping" of the energy delivery. The idea was to use an initial lower-energy pulse to vaporize the capsule and cause compression, and then a very short, very powerful pulse near the end of the compression cycle. The goal is to launch shock waves into the compressed fuel that travel inward to the center. When they reach the center they meet the waves coming in from other sides. This causes a brief period where the density in the center reaches much higher values, over 800&nbsp;g/cm<sup>3</sup>.{{sfn|Pfalzner|2006|p=15}}

The central hot spot ignition concept was the first to suggest ICF was not only a practical route to fusion, but relatively simple. This led to numerous efforts to build working systems in the early 1970s. These experiments revealed unexpected loss mechanisms. Early calculations suggested about 4.5x10<sup>7</sup>&nbsp;J/g would be needed, but modern calculations place it closer to 10<sup>8</sup>&nbsp;J/g. Greater understanding led to complex shaping of the pulse into multiple time intervals.<ref>{{cite web |url=https://nifuserguide.llnl.gov/home/4-laser-system/44-pulse-shape-timing-and-prepulse/441-pulse-shaping |title=Pulse Shaping |website=LLNL}}</ref>

=== Direct vs. indirect drive ===
[[Image:Hohlraum irradiation on NOVA laser.jpg|thumb|right|Indirect drive laser ICF uses a ''[[hohlraum]]'' which is irradiated with laser beam cones from either side on its inner surface to bathe a fusion microcapsule inside with smooth high intensity X-rays. The highest energy X-rays can be seen leaking through the hohlraum, represented here in orange/red.]]In the simplest method of inertial confinement, the fuel is arranged as a sphere. This allows it to be compressed uniformly from all sides. To produce the inward force, the fuel is placed within a thin capsule that absorbs energy from the driver beams, causing the capsule shell to explode outward. The capsule shell is usually made of a lightweight plastic fabricated using [[plasma polymerization]], and the fuel is deposited as a layer on the inside by injecting or diffusing the gaseous fuel into the shell and then freezing it.<ref>Morse, Samuel F. B., editor. "Direct-Drive Target Designs for the National Ignition Facility." LLE Review 79: Quarterly Report, vol. 79, Apr. 1999, pp. 121–130.</ref>

Shining the driver beams directly onto the fuel capsule is known as "direct drive". The implosion process must be extremely uniform in order to avoid asymmetry due to [[Rayleigh–Taylor instability]] and similar effects. For a beam energy of 1&nbsp;MJ, the fuel capsule cannot be larger than about 2&nbsp;mm before these effects disrupt the implosion symmetry. This limits the size of the laser beams to a diameter so narrow that it is difficult to achieve in practice.

On the other hand, "indirect drive" illuminates a small cylinder of heavy metal, often [[gold]] or [[lead]], known as a [[hohlraum]]. The beam energy heats the hohlraum until it emits [[X-ray]]s. These X-rays fill the interior of the hohlraum and heat the capsule. The advantage of indirect drive is that the beams can be larger and less accurate. The disadvantage is that much of the delivered energy is used to heat the hohlraum until it is "X-ray hot", so the end-to-end [[Efficient energy use|energy efficiency]] is much lower than the direct drive method.

Within the direct inertial confinement fusion scheme, there are two alternative approaches: shock ignition and fast ignition. In both cases the compression and heating processes are separated. First, a set of driver lasers compress the fuel up to an optimal point were the plasma is condensed and found in a stagantion state, this is, it has approximately homogenous temperature and density at its core.<ref>{{Cite journal |last=Sunahara |first=A. |last2=Takabe |first2=H. |last3=Mima |first3=K. |date=1999-02-01 |title=2D simulation of hydrodynamic instability in ICF stagnation phase |url=https://linkinghub.elsevier.com/retrieve/pii/S0920379698003627 |journal=Fusion Engineering and Design |volume=44 |issue=1 |pages=163–169 |doi=10.1016/S0920-3796(98)00362-7 |issn=0920-3796}}</ref><ref>{{Cite journal |last=Kodama |first=R. |last2=Norreys |first2=P. A. |last3=Mima |first3=K. |last4=Dangor |first4=A. E. |last5=Evans |first5=R. G. |last6=Fujita |first6=H. |last7=Kitagawa |first7=Y. |last8=Krushelnick |first8=K. |last9=Miyakoshi |first9=T. |last10=Miyanaga |first10=N. |last11=Norimatsu |first11=T. |last12=Rose |first12=S. J. |last13=Shozaki |first13=T. |last14=Shigemori |first14=K. |last15=Sunahara |first15=A. |date=August 2001 |title=Fast heating of ultrahigh-density plasma as a step towards laser fusion ignition |url=https://www.nature.com/articles/35090525 |journal=Nature |language=en |volume=412 |issue=6849 |pages=798–802 |doi=10.1038/35090525 |issn=1476-4687}}</ref><ref>{{Cite journal |last=Betti |first=R. |last2=Zhou |first2=C. |date=2005-11-08 |title=High-density and high-ρR fuel assembly for fast-ignition inertial confinement fusion |url=https://pubs.aip.org/aip/pop/article-abstract/12/11/110702/261440/High-density-and-high-R-fuel-assembly-for-fast?redirectedFrom=fulltext |journal=Physics of Plasmas |volume=12 |issue=11 |pages=110702 |doi=10.1063/1.2127932 |issn=1070-664X}}</ref> Then, another mechanism heates the plasma up to fusion conditions.<ref>{{Cite journal |last=Rodríguez Beltrán |first=Pablo |date=2024 |title=Systematic Study of the Interaction between Ion Beams and Plasmas via Spatial-Temporal Simulations in the context of Nuclear Fusion by Ion Fast Ignition |url=https://accedacris.ulpgc.es/handle/10553/135693}}</ref> 

==== Shock ignition ====
Proposed by C. Zhou and R. Betti,<ref>{{Cite journal |last=Betti |first=R. |last2=Zhou |first2=C. D. |last3=Anderson |first3=K. S. |last4=Perkins |first4=L. J. |last5=Theobald |first5=W. |last6=Solodov |first6=A. A. |date=2007-04-12 |title=Shock Ignition of Thermonuclear Fuel with High Areal Density |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98.155001 |journal=Physical Review Letters |volume=98 |issue=15 |pages=155001 |doi=10.1103/PhysRevLett.98.155001}}</ref> after an early compression phase similar to that of the direct drive approach,  an additional driver is applied (such as a laser, electron beam, or similar pulse). This create a shock wave orders of magnitude stronger. The separation of the compression process from the final heating, where ignition is achieved, offers the advantage of reducing the compression requirements and utilizing more efficient energy deposition mechanisms. Additionally, some theoretical and experimental findings claim that these approach enhances ignition conditions,<ref>{{Cite journal |last=Theobald |first=W. |last2=Betti |first2=R. |last3=Stoeckl |first3=C. |last4=Anderson |first4=K. S. |last5=Delettrez |first5=J. A. |last6=Glebov |first6=V. Yu. |last7=Goncharov |first7=V. N. |last8=Marshall |first8=F. J. |last9=Maywar |first9=D. N. |last10=McCrory |first10=R. L. |last11=Meyerhofer |first11=D. D. |last12=Radha |first12=P. B. |last13=Sangster |first13=T. C. |last14=Seka |first14=W. |last15=Shvarts |first15=D. |date=2008-03-26 |title=Initial experiments on the shock-ignition inertial confinement fusion concepta) |url=https://pubs.aip.org/aip/pop/article-abstract/15/5/056306/1016578/Initial-experiments-on-the-shock-ignition-inertial?redirectedFrom=fulltext |journal=Physics of Plasmas |volume=15 |issue=5 |pages=056306 |doi=10.1063/1.2885197 |issn=1070-664X}}</ref> as demonstrated, for instance, at the [[OMEGA laser]] at the University of Rochester.<ref>{{Cite journal |last=Ribeyre |first=X. |last2=Tikhonchuk |first2=V. T. |last3=Breil |first3=J. |last4=Lafon |first4=M. |last5=Le Bel |first5=E. |date=2011-10-11 |title=Analytic criteria for shock ignition of fusion reactions in a central hot spot |url=https://pubs.aip.org/aip/pop/article-abstract/18/10/102702/317098/Analytic-criteria-for-shock-ignition-of-fusion?redirectedFrom=fulltext |journal=Physics of Plasmas |volume=18 |issue=10 |pages=102702 |doi=10.1063/1.3646743 |issn=1070-664X}}</ref> This increases the efficiency of the process while lowering the overall amount of power required.

==== Fast ignition ====
Fast ignition is a promising alternative for achieving nuclear fusion within the inertial confinement fusion scheme.<ref>{{Cite journal |last=Tabak |first=Max |last2=Hammer |first2=James |last3=Glinsky |first3=Michael E. |last4=Kruer |first4=William L. |last5=Wilks |first5=Scott C. |last6=Woodworth |first6=John |last7=Campbell |first7=E. Michael |last8=Perry |first8=Michael D. |last9=Mason |first9=Rodney J. |date=1994-05-01 |title=Ignition and high gain with ultrapowerful lasers* |url=https://pubs.aip.org/aip/pop/article-abstract/1/5/1626/103434/Ignition-and-high-gain-with-ultrapowerful-lasers?redirectedFrom=fulltext |journal=Physics of Plasmas |volume=1 |issue=5 |pages=1626–1634 |doi=10.1063/1.870664 |issn=1070-664X}}</ref><ref name=":1">{{Cite journal |last=Roth |first=M. |last2=Cowan |first2=T. E. |last3=Key |first3=M. H. |last4=Hatchett |first4=S. P. |last5=Brown |first5=C. |last6=Fountain |first6=W. |last7=Johnson |first7=J. |last8=Pennington |first8=D. M. |last9=Snavely |first9=R. A. |last10=Wilks |first10=S. C. |last11=Yasuike |first11=K. |last12=Ruhl |first12=H. |last13=Pegoraro |first13=F. |last14=Bulanov |first14=S. V. |last15=Campbell |first15=E. M. |date=2001-01-15 |title=Fast Ignition by Intense Laser-Accelerated Proton Beams |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.86.436 |journal=Physical Review Letters |volume=86 |issue=3 |pages=436–439 |doi=10.1103/PhysRevLett.86.436}}</ref><ref name=":2">{{Cite journal |last=Roth |first=M |date=2009-01-01 |title=Review on the current status and prospects of fast ignition in fusion targets driven by intense, laser generated proton beams |url=https://iopscience.iop.org/article/10.1088/0741-3335/51/1/014004 |journal=Plasma Physics and Controlled Fusion |volume=51 |issue=1 |pages=014004 |doi=10.1088/0741-3335/51/1/014004 |issn=0741-3335}}</ref> Similar to the shock ignition scheme, fast ignition divides the fusion process into two distinct steps: compression and heating, each of which can be optimized independently. After the precompression phase, a powerful particle beam is used to provide additional energy directly to the core of the fuel. It is important to note that, in fast ignition, this relies on a separate and rapid heating pulse, while shock ignition primarily employs shock waves to achieve ignition. The beam applied creates a heated volume within the plasma. If any region of such volume is able to ignite the nuclear fusion process, then, the burning will start and spread to the rest of the fuel.  

The two types of fast ignition are the "plasma bore-through" method<ref name=":0">{{Cite journal |author1=Max Tabak |author2=James Hammer |author3=Michael E. Glinsky |author4=William L. Kruer |author5=Scott C. Wilks |author6=John Woodworth |author7=E. Michael Campbell |author8=Michael D. Perry |author9=Rodney J. Mason |date=1994 |title=Ignition and high gain with ultrapowerful lasers |url=https://pubs.aip.org/aip/pop/article-abstract/1/5/1626/103434/Ignition-and-high-gain-with-ultrapowerful-lasers?redirectedFrom=fulltext |journal=Phys. Plasmas |volume=1 |issue=5 |pages=1626–1634 |bibcode=1994PhPl....1.1626T |doi=10.1063/1.870664 |access-date=2023-11-20}}</ref> and the "cone-in-shell" method.<ref>{{Cite journal |author1=P. A. Norreys |author2=R. Allott |author3=R. J. Clarke |author4=J. Collier |author5=D. Neely |author6=S. J. Rose |author7=M. Zepf |author8=M. Santala |author9=A. R. Bell |author10=K. Krushelnick |author11=A. E. Dangor |author12=N. C. Woolsey |author13=R. G. Evans |author14=H. Habara |author15=T. Norimatsu |date=2000 |title=Experimental studies of the advanced fast ignitor scheme |url=https://pubs.aip.org/aip/pop/article-abstract/7/9/3721/264273/Experimental-studies-of-the-advanced-fast-ignitor |journal=Phys. Plasmas |volume=7 |issue=9 |pages=3721–3726 |bibcode=2000PhPl....7.3721N |doi=10.1063/1.1287419 |access-date=2023-11-20 |author16=R. Kodama}}</ref> In plasma bore-through, a preceding laser bores through the outer plasma of the imploding capsule (the corona), before the last beam shot. In the cone-in-shell method, the capsule is mounted on the end of a small high-z (high [[atomic number]]) cone such that the tip of the cone projects into the core. In this second method, when the capsule is imploded, the beam has a clear view of the core and does not use energy to bore through the 'corona' plasma. However, the presence of the cone affects the implosion process in significant ways that are not fully understood. In tests, this approach presents difficulties, because the laser pulse had to reach the center at a precise moment, while the center is obscured by debris and free electrons from the compression pulse.<ref>{{Cite journal |last=Meier |first=W. R. |last2=and Hogan |first2=W. J. |date=2006-04-01 |title=Power Plant and Fusion Chamber Considerations for Fast Ignition |url=https://www.tandfonline.com/doi/abs/10.13182/FST06-A1165 |journal=Fusion Science and Technology |volume=49 |issue=3 |pages=532–541 |doi=10.13182/FST06-A1165 |issn=1536-1055}}</ref> A variation of this cone approach incorporates a small pellet of fuel at the apex of the device, initiating a preliminary pre-explosion that also moves inward towards the larger fuel mass.  

Regarding the power beam, the original proposal for fast ignition involved an electron-based scheme.<ref name=":3">{{Cite journal |last=Tabak |first=Max |last2=Callahan-Miller |first2=Debra |date=1998-05-01 |title=Design of a distributed radiator target for inertial fusion driven from two sides with heavy ion beams |url=https://pubs.aip.org/aip/pop/article-abstract/5/5/1895/1069095/Design-of-a-distributed-radiator-target-for?redirectedFrom=fulltext |journal=Physics of Plasmas |volume=5 |issue=5 |pages=1895–1900 |doi=10.1063/1.872860 |issn=1070-664X}}</ref> However, it was limited by the high electron divergences, kinetic energy constraints and sensitivity.<ref>{{Cite journal |last=Robinson |first=A P L |last2=Zepf |first2=M |last3=Kar |first3=S |last4=Evans |first4=R G |last5=Bellei |first5=C |date=2008-01-21 |title=Radiation pressure acceleration of thin foils with circularly polarized laser pulses |url=https://iopscience.iop.org/article/10.1088/1367-2630/10/1/013021 |journal=New Journal of Physics |volume=10 |issue=1 |pages=013021 |doi=10.1088/1367-2630/10/1/013021 |issn=1367-2630|arxiv=0708.2040 }}</ref><ref>{{Cite journal |last=Green |first=J. S. |last2=Ovchinnikov |first2=V. M. |last3=Evans |first3=R. G. |last4=Akli |first4=K. U. |last5=Azechi |first5=H. |last6=Beg |first6=F. N. |last7=Bellei |first7=C. |last8=Freeman |first8=R. R. |last9=Habara |first9=H. |last10=Heathcote |first10=R. |last11=Key |first11=M. H. |last12=King |first12=J. A. |last13=Lancaster |first13=K. L. |last14=Lopes |first14=N. C. |last15=Ma |first15=T. |date=2008-01-11 |title=Effect of Laser Intensity on Fast-Electron-Beam Divergence in Solid-Density Plasmas |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.100.015003 |journal=Physical Review Letters |volume=100 |issue=1 |pages=015003 |doi=10.1103/PhysRevLett.100.015003}}</ref><ref>{{Cite journal |last=Debayle |first=A. |last2=Honrubia |first2=J. J. |last3=d’Humières |first3=E. |last4=Tikhonchuk |first4=V. T. |date=2010-09-21 |title=Divergence of laser-driven relativistic electron beams |url=https://journals.aps.org/pre/abstract/10.1103/PhysRevE.82.036405 |journal=Physical Review E |volume=82 |issue=3 |pages=036405 |doi=10.1103/PhysRevE.82.036405}}</ref><ref>{{Cite journal |last=Kemp |first=A. J. |last2=Divol |first2=L. |date=2012-11-09 |title=Interaction Physics of Multipicosecond Petawatt Laser Pulses with Overdense Plasma |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.109.195005 |journal=Physical Review Letters |volume=109 |issue=19 |pages=195005 |doi=10.1103/PhysRevLett.109.195005}}</ref> Meanwhile, fast ignition by laser-driven ion beams, offers a much more localized energy deposition, a stiffer ion transport, with the possibility of beam focusing, and a better understood and controlled ion-plasma interaction.<ref name=":3" /><ref name=":1" /><ref name=":2" /><ref>{{Cite journal |last=Tabak |first=M. |last2=Norreys |first2=P. |last3=Tikhonchuk |first3=V.T. |last4=Tanaka |first4=K.A. |date=2014-05-01 |title=Alternative ignition schemes in inertial confinement fusion |url=https://iopscience.iop.org/article/10.1088/0029-5515/54/5/054001 |journal=Nuclear Fusion |volume=54 |issue=5 |pages=054001 |doi=10.1088/0029-5515/54/5/054001 |issn=0029-5515}}</ref> At first, the proposed projectiles of the beam were light ions, such as protons. However, these ions deposit most of their energy at the edge of the fuel, resulting in an asymmetrical geometry of the heated plasma.<ref name=":1" /> Later, heavier projectiles were suggested. Their interaction with the plasma is semi-transparent at the edge, allowing for deposition of most of their energy in the centre of the fuel, which optimises a symmetrical propagation and explosion.<ref>{{Cite journal |last=Gus’kov |first=S. Yu. |last2=Il’in |first2=D. V. |last3=Sherman |first3=V. E. |date=2014-07-01 |title=Spatial distribution of the plasma temperature under ion-beam fast ignition |url=https://link.springer.com/article/10.1134/S1063780X14070034 |journal=Plasma Physics Reports |language=en |volume=40 |issue=7 |pages=572–582 |doi=10.1134/S1063780X14070034 |issn=1562-6938}}</ref> The ion beam used for the final ignition can be optimized, in order to achieve the desired conditions for the plasma and the burning, and to reduce system requirements. 

Currently, several research facilities worldwide are actively experimenting with Fast Ignition nuclear fusion, notably: the High Power Laser Energy Research Facility ([[HiPER|HiPer]]), located across multiple institutions in Europe. [[HiPER|HiPer]] is a proposed £500 million facility by the [[European Union]]. Compared to NIF's 2&nbsp;MJ UV beams, HiPER's driver was planned to be 200&nbsp;kJ and heater 70&nbsp;kJ, although the predicted fusion gains are higher than NIF. It was to employ [[Laser diode|diode lasers]], which convert electricity into laser light with much higher efficiency and run cooler. This allows them to operate at much higher frequencies. HiPER proposed to operate at 1 MJ at 1&nbsp;Hz, or alternately 100 kJ at 10&nbsp;Hz. The project's final update was in 2014. It was expected to offer a higher ''Q'' with a 10x reduction in construction costs times.<ref>{{Cite web |title=60 Project News |url=http://www.hiper-laser.org/News%20and%20events/60projectnews.html |access-date=2021-08-21 |website=Hiper Laser}}</ref> Several other projects are currently underway to explore fast ignition, including upgrades to the [[OMEGA laser]] at the Laboratory for Laser Energetics (LLE) in the University of Rochester and the [[GEKKO XII]] device at the Institute of Laser Engineering (ILE) in Osaka, Japan.<ref>{{Cite journal |last=Kodama |first=R. |last2=Mima |first2=K. |last3=Tanaka |first3=K. A. |last4=Kitagawa |first4=Y. |last5=Fujita |first5=H. |last6=Takahashi |first6=K. |last7=Sunahara |first7=A. |last8=Fujita |first8=K. |last9=Habara |first9=H. |last10=Jitsuno |first10=T. |last11=Sentoku |first11=Y. |last12=Matsushita |first12=T. |last13=Miyakoshi |first13=T. |last14=Miyanaga |first14=N. |last15=Norimatsu |first15=T. |date=2001-05-01 |title=Fast ignitor research at the Institute of Laser Engineering, Osaka University |url=https://pubs.aip.org/aip/pop/article-abstract/8/5/2268/859704/Fast-ignitor-research-at-the-Institute-of-Laser?redirectedFrom=fulltext |journal=Physics of Plasmas |volume=8 |issue=5 |pages=2268–2274 |doi=10.1063/1.1352598 |issn=1070-664X}}</ref> Nonetheless, fast ignition faces its particular challenges, such as achieving an optimal deposition of energy in the target, avoiding unnecessary losses and properly transporting the fast electrons or ions through the plasma without creating divergences or instabilities.<ref>{{Cite web |last=Guo |first=Zekuan |date=2024-02-03 |title=Nuclear Fusion: Overview of Challenges and Recent Progress |url=https://nhsjs.com/2024/nuclear-fusion-overview-of-challenges-and-recent-progress/ |access-date=2025-05-02 |website=NHSJS |language=en-US}}</ref>

=== Polymer fuel capsule fabrication ===
For fuel capsules constructed using glow-discharge polymer (GDP) via [[plasma polymerization]], outer diameters can range from 900&nbsp;μm (typical for the OMEGA laser system at the [[Laboratory for Laser Energetics]]) to 2mm (typical for the [[National Ignition Facility|NIF]] laser at the [[Lawrence Livermore National Laboratory]].
The process for producing GDP capsules begins with a bubble of poly([[α-methylstyrene]]) (PAMS) that serves as a decomposable mandrel. Next, the bubble is coated with GDP to the desired thickness. Finally, the coated bubble is heated in an inert atmosphere. Upon reaching 300&nbsp;°C, the PAMS bubble decomposes into its monomers and diffuses out of the coating, leaving only a hollow sphere of the GDP coating.<ref>K. R. Schultz, J. L. Kaae, W. J. Miller, D. A. Steinman, and R. B. Stephens, "Status of inertial fusion target fabrication in the USA," Fusion Engineering and Design, vol. 44, no. 1, pp. 441–448, 1999, doi: https://doi.org/10.1016/S0920-3796(98)00356-1.</ref>

GDP lends itself to inertial fusion capsules—especially those used in direct-drive configurations—due to its ability to create low-defect, uniform thin films that are permeable to deuterium and tritium fuel. Permeating the fuel into the capsule precludes the need for drilling into the capsule to facilitate fuel injection, reducing the overall fusion target complexity and asymmetry. The rigorous uniformity and sphericity requirements of direct drive fusion experiments result in GDP being favored over other capsule materials. Additionally, the GDP layers can be doped with different elements to provide diagnostic signals or prevent preheating of the fuel.<ref>"2 Technical Background." National Research Council. 2013. Assessment of Inertial Confinement Fusion Targets. Washington, DC: The National Academies Press. doi: 10.17226/18288. National Academies of Sciences, Engineering, and Medicine. 2013. Assessment of Inertial Confinement Fusion Targets. Washington, DC: The National Academies Press. https://doi.org/10.17226/18288.</ref>

===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}}