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Nuclear fusion in all its forms

Why lasers are important for nuclear fusion

On September 6th, 2022 |
4 min reading time
Sebastien LePape
Sébastien LePape
Deputy Director of the Laboratory for the Use of Intense Lasers (LULI*) at École Polytechnique (IP Paris)
Key takeaways
  • For 50 years, researchers have been trying to mimic the process of fusion, which occurs in stars, to generate energy.
  • Nuclear fusion happens when two light nuclei, such as hydrogen and its isotopes, fuse to produce a larger, heavier nucleus which releases energy.
  • The Lawrence Livermore National Laboratory (LLNL) in the US recently succeeded in creating a “burning plasma” state at the National Ignition Facility (NIF).
  • Researchers used a set of powerful lasers focused tightly on a millimetre-sized fuel capsule containing tiny pellets of hydrogen isotopes – deuterium and tritium – suspended inside a cylindrical X-ray “furnace” called a hohlraum.
  • This is the first time a system has been developed in which fusion itself provides most of the heat – a key step towards achieving even higher levels of performance.

Nuclear fusion occurs in celes­tial objects, such as the cores of stars, and in ther­monu­clear weapons. It hap­pens when two light nuclei, such as hydro­gen and its iso­topes, fuse to pro­duce a larg­er, heav­ier nucle­us which releas­es ener­gy. For 50 years, researchers have been try­ing to mim­ic this process to gen­er­ate ener­gy and oth­er nation­al defence appli­ca­tions but build­ing a fusion reac­tor that can deliv­er ener­gy in con­trolled way is not easy.

The Lawrence Liv­er­more Nation­al Lab­o­ra­to­ry (LLNL) in the US recent­ly suc­ceed­ed in cre­at­ing a “burn­ing plas­ma” state at the Nation­al Igni­tion Facil­i­ty (NIF). The LLNL is one of the two main lab­o­ra­to­ries in the world work­ing on a tech­nique called iner­tial con­fine­ment fusion (ICF) with high-pow­er lasers1. The oth­er ICF lab­o­ra­to­ry is the CEA’s Laser Mega­joule in France, cur­rent­ly under construction.

Inertial confinement fusion 

To achieve ther­monu­clear fusion in the lab­o­ra­to­ry, a fuel must be heat­ed to incred­i­bly high tem­per­a­tures – close to those in the Sun. At such tem­per­a­tures, the fuel goes from being a sol­id to a “plas­ma”, a state in which fusion reac­tions read­i­ly occur.

Once fusion has tak­en place, more ener­gy must be pro­duced than was put in so that the excess ener­gy can be used in appli­ca­tions such as elec­tric­i­ty generation.

There are two main meth­ods to heat and then con­fine the plas­ma. The first is “mag­net­ic con­fine­ment” in a device called a toka­mak. Here, a super­con­duct­ing ring con­fines the plas­ma at rel­a­tive­ly low-pres­sure den­si­ties, but at very high tem­per­a­tures for long peri­ods of time. The sec­ond is to use high-pow­er lasers: lasers that emit pow­er­ful puls­es of light last­ing just 10 to 20 nanosec­onds that pro­duce between 1 and 2 mega­joules (MJ) of energy.

This sec­ond tech­nique is known as iner­tial con­fine­ment fusion (ICF) and requires high tem­per­a­tures and pres­sures. What is more, once fusion has tak­en place, more ener­gy must be pro­duced than was put in, so that the excess ener­gy can be used in appli­ca­tions such as elec­tric­i­ty gen­er­a­tion. The fusion reac­tion must also be self-sus­tain­ing – a process that is trig­gered by a phe­nom­e­non called “igni­tion”, in which alpha par­ti­cles that are also emit­ted dur­ing fusion release heat to ini­ti­ate new fusion.

At the NIF, researchers used a set of pow­er­ful lasers focused tight­ly on a mil­lime­tre-sized fuel cap­sule con­tain­ing tiny pel­lets of hydro­gen iso­topes – deu­teri­um and tri­tium – sus­pend­ed inside a cylin­dri­cal X‑ray “fur­nace” called a hohlraum. In this type of exper­i­ment, the heat from the X‑rays emit­ted by the fur­nace caus­es the sur­face of the cap­sule to explode, or ablate. Thus, by implod­ing, the sur­face of the cap­sule com­press­es and heats the deu­teri­um-tri­tium fuel until the hydro­gen nuclei fuse into heli­um, releas­ing neu­trons and oth­er forms of energy.

In this type of exper­i­ment, we are talk­ing about a cap­sule that is ini­tial­ly mil­lime­tre-sized. We then con­verge it to a diam­e­ter of about 50 microns to increase both the den­si­ty and the tem­per­a­ture and gen­er­ate the fusion reaction.

Compressing matter at high speed 

Each laser pulse lasts only a few nanosec­onds and the lasers can deliv­er about 1.9 MJ of ener­gy. It is this pow­er­ful blast that caus­es the cap­sule to rapid­ly implode, pro­duc­ing extreme tem­per­a­tures of up to 100 mil­lion degrees Cel­sius. Inside the cen­tral hot spot, where the fusion reac­tions take place, pres­sure den­si­ties are 100 times high­er than atmos­pher­ic pressure.

The shock cre­at­ed by the laser com­press­es mat­ter at such high speeds (about 400 km/s) that it reach­es enor­mous kinet­ic ener­gies. It is only when the com­pres­sion “stalls” that the kinet­ic ener­gy is trans­formed into ther­mal ener­gy, which is also colos­sal. Only an instru­ment such as a high-pow­ered laser has the ener­gy to com­press mat­ter in this way.

This is the first time we have a sys­tem in which fusion itself pro­vides most of the heat – a key step towards achiev­ing even high­er lev­els of per­for­mance. Until now, fusion exper­i­ments pro­duced fusion reac­tions thanks to huge amounts of exter­nal heat to heat the plasma.

Will we see ignition soon?

While the NIF has not yet achieved igni­tion, the researchers have man­aged to pro­duce 1.35 MJ of ener­gy using 1.9 MJ of laser ener­gy, giv­ing a Q (Efusion/Elaser) of 0.7, where igni­tion is defined as a Q of 1. We are thus close to the goal.

It is often said that nuclear fusion will still not be fea­si­ble in 30 years’ time, but new break­throughs in this field sug­gest that – soon­er or lat­er – fusion sci­en­tists will have the last word.

Lasers at École Polytechnique

At École Poly­tech­nique, there are two lasers, a kilo­joule (kJ) nanosec­ond laser called LULI2000 and the Research Infra­struc­ture (IR*) APOLLON, a poten­tial­ly mul­ti-petawatt fem­tosec­ond laser. The for­mer can be used to pro­duce lab­o­ra­to­ry plas­ma con­di­tions close to those asso­ci­at­ed with iner­tial con­fine­ment fusion, while the lat­ter is intend­ed for fun­da­men­tal research in the very high inten­si­ty regime.

While we can­not exper­i­ment with neu­tron cre­ation because we do not have suf­fi­cient input ener­gy or a pow­er­ful enough laser beam, we can pro­duce the con­di­tions nec­es­sary for fusion – high tem­per­a­tures and high plas­ma den­si­ty – to study the physics of iner­tial con­fine­ment. The plas­ma we cre­ate at LULI will allow us to study plas­ma micro­physics and to test the numer­i­cal codes used to design fusion experiments.

LULI oper­ates at micron wave­lengths with a max­i­mum ener­gy of about 1 KJ and gen­er­ates light puls­es that last between 10 to 20 nanosec­onds. The laser emits a pulse approx­i­mate­ly every hour and can be housed into a build­ing at least 80 metres long, which makes it a rel­a­tive­ly large exper­i­men­tal facility.

1https://www.nature.com/articles/s41586-021–04281‑w