Editing Inertial confinement fusion (section)

=== 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>