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Nuclear fusion in all its forms

ITER and plasma control: where are we?

On September 6th, 2022 |
4 min reading time
Pascale Hennequin
Pascale Hennequin
CNRS Research Director and Head of the "Magnetic Fusion Plasmas" team at the Plasma Physics Laboratory of École Polytechnique (IP Paris)
Key takeaways
  • Nuclear fusion is a potential energy source that does not produce greenhouse gases, nor long-lived fissile or highly radioactive elements.
  • The ITER (International Thermonuclear Experimental Reactor) project is an experimental nuclear fusion reactor born from a long-term international collaboration between 34 countries, but the first plasmas will not be obtained before 2027.
  • The device used, known as a tokamak, must maintain relatively high densities of light ions at enormous temperatures (~100 million °C) for a sufficiently long time using intense magnetic fields.
  • ITER remains essential for the community because it is the only place where it will be possible to test all the problems linked to fusion energy production in an integrated way.
  • The entire community is working to progress the scientific and technical issues that could make fusion energy available in the second half of the century.
  • An increasing number of these advances involve start-ups and private initiatives that undoubtedly signal the growing maturity of the field.

Nuclear fusion, which is being explored in par­tic­u­lar in the frame­work of the ITER (Inter­na­tion­al Ther­monu­clear Exper­i­men­tal Reac­tor) project in Cadarache, is show­ing much promise. It is a sci­en­tif­ic and tech­no­log­i­cal chal­lenge tak­ing place over sev­er­al decades. One of the most impor­tant chal­lenges of fusion is to cre­ate and main­tain plas­ma, a medi­um con­tain­ing ener­get­i­cal­ly charged par­ti­cles, at 150 mil­lion degrees Cel­sius to sus­tain fusion reactions.

Plas­ma is con­sid­ered as the “fourth state of mat­ter” (after sol­id, liq­uid and gaseous states) and is some­times described as a “soup of elec­trons and ions”. The plas­ma state is wide­spread in the uni­verse. It can be observed, for exam­ple, in the auro­ra bore­alis or the ionos­phere: the bom­bard­ment of par­ti­cles from the solar wind where solar radi­a­tion rips elec­trons from atoms or mol­e­cules in the very high atmos­phere, caus­ing ion­i­sa­tion. The ionos­phere is a dilut­ed, par­tial­ly ionised medi­um, in which the elec­trons can be quite ener­getic but in which the ions, atoms or mol­e­cules remain quite cold; the col­lec­tive effects char­ac­ter­is­tic of plas­mas are already at play there.

Anoth­er plas­ma with rad­i­cal­ly dif­fer­ent con­di­tions is that found at the heart of the sun or oth­er stars: ful­ly ionised mat­ter reach­es tem­per­a­tures of the order of ten mil­lion degrees, which allows light ele­ments, such as hydro­gen, to fuse to form heav­ier atoms. The fusion process requires a lot of ener­gy, and it pro­duces even more. It is this process that we hope to repro­duce when we talk about “fusion energy”.

ITER: an experimental reactor

The ITER project involves exper­i­ment­ing with the pro­duc­tion of ener­gy by fus­ing the hydro­gen iso­topes (which have the same charge but dif­fer­ent mass) deu­teri­um and tri­tium to form heli­um. The device used is called a toka­mak (the Russ­ian acronym for “toroidal cham­ber with mag­net­ic coils”!). Devel­oped in the Sovi­et Union in the 1950s and 1960s, it involves mag­net­i­cal­ly con­fin­ing charged par­ti­cles in the plasma.

ITER is an exper­i­men­tal reac­tor and it the result of an unprece­dent­ed inter­na­tion­al col­lab­o­ra­tion (34 coun­tries). It is cur­rent­ly under con­struc­tion in Cadarache, sit­u­at­ed north of Mar­seille, and rep­re­sents the demon­stra­tion stage of fusion after 50 years of research. The ITER core is a giant toka­mak almost 30m high.

ITER, an exper­i­men­tal reac­tor result­ing from an unprece­dent­ed inter­na­tion­al col­lab­o­ra­tion between 34 coun­tries, is the demon­stra­tion stage of nuclear fusion from 50 years of research.

For fusion to occur, the nuclei must col­lide quick­ly enough to over­come their coulom­bic repul­sion and often enough for the process to be self-sus­tain­ing. A toka­mak must there­fore main­tain rel­a­tive­ly high den­si­ties of light ions at enor­mous tem­per­a­tures – around 100 mil­lion degrees – for a suf­fi­cient­ly long time.

In con­trast to the fis­sion of heavy atoms, where the prod­ucts of the reac­tion them­selves cause oth­er reac­tions, there is no medi­a­tor of the fusion reac­tion and there­fore no chain reac­tion. It is pre­cise­ly the absence of the pos­si­bil­i­ty of a run­away reac­tion in a fusion reac­tor that makes fusion much more attrac­tive than fis­sion. More­over, fusion does not pro­duce green­house gas­es, nor long-lived fis­sile or high­ly radioac­tive ele­ments (although the mate­ri­als inside the reac­tor are acti­vat­ed, they have short lives; the fuel for fusion is abun­dant – there is 33g per mof deu­teri­um in water – and it can be eas­i­ly extract­ed by elec­trol­y­sis; tri­tium can be pro­duced in the fusion reac­tor from lithi­um, which is abun­dant on Earth too).

Intense magnetic fields

As it is not pos­si­ble to con­tain a plas­ma at ther­monu­clear con­di­tions (150 mil­lion degrees) in an ordi­nary mate­r­i­al con­tain­er, intense mag­net­ic fields (of the order of 5 to 10 T) are used to iso­late the charged par­ti­cles of the plas­ma from the cham­ber that holds them. The toka­mak con­fig­u­ra­tion, which is the best per­form­ing, com­bines three coil sys­tems to gen­er­ate a torus-shaped mag­net­ic “cage” in which the charged par­ti­cles cir­cu­late while remain­ing confined. 

How can this plas­ma be heat­ed to ini­ti­ate fusion reac­tions? There are sev­er­al options: heat­ing by radiofre­quen­cy (put sim­ply, by microwaves); heat­ing by col­li­sions, by inject­ing hydro­gen ions car­ried at high ener­gy in an accel­er­a­tor; the ques­tion is how to neu­tralise these ener­getic ions so that they can enter the toka­mak (if “noth­ing” exits, noth­ing goes in either).

How can the plas­ma be main­tained at these very high tem­per­a­tures so that the process is sus­tained? The plas­ma must above all remain “sta­ble”: large-scale insta­bil­i­ties lead to plas­ma loss on the con­tain­er walls, in the same way that the large arch­es that devel­op on the sur­face of the sun eject the mat­ter that reach­es us in the form of the solar wind. This is because insta­bil­i­ties and tur­bu­lence can devel­op in very hot plas­ma – a state of mat­ter that is by def­i­n­i­tion not in ther­mo­dy­nam­ic equilibrium. 

The large size of ITER comes from the fact that tur­bu­lent agi­ta­tion increas­es col­li­sion­al dif­fu­sion across the mag­net­ic field lines: vor­tices of dif­fer­ent sizes stir up the mate­r­i­al and mix the hot­ter core with the cold­er edge. More insu­lat­ing “lay­ers” are need­ed to main­tain the heat in the core of the plas­ma. We are work­ing both to devel­op obser­va­tion­al tools, under con­di­tions nev­er before expe­ri­enced, to under­stand and mod­el these phe­nom­e­na and to opti­mise the con­trol of turbulence.

Other containment techniques?

Noth­ing that can be done in your garage… con­trary to what some far-fetched the­o­ries about fusion would have us believe. Oth­er mag­net­ic con­fig­u­ra­tions exist, such as the “steller­a­tor”, devel­oped by the Max Planck Insti­tute in Ger­many, to name just one.

The dif­fer­ence is main­ly in the way the com­plex mag­net­ic struc­ture is pro­duced (by field lines wound on nest­ed cores): in the case of the steller­a­tor, it is the coils, which are extreme­ly com­plex in design, that gen­er­ate the mag­net­ic struc­ture direct­ly and “con­tin­u­ous­ly”; in the case of the toka­mak, it is the com­po­si­tion of the field pro­duced by the coils and the field pro­duced by a cur­rent, which will be main­tained for about ten min­utes in ITER and will there­fore have to be cyclical. 

It is dif­fi­cult to pre­dict the most effi­cient con­fig­u­ra­tion in terms of con­tain­ment qual­i­ty, which is close­ly linked to the size of the device, and in terms of eco­nom­ic viability.

ITER: a very long-term project 

Launched in the mid-1980s, the first plas­mas will not be obtained before 2027. How­ev­er, the entire com­mu­ni­ty is work­ing to progress on the sci­en­tif­ic and tech­ni­cal issues that could make fusion ener­gy avail­able in the sec­ond half of the cen­tu­ry. The Euro­pean toka­mak JET, the largest cur­rent­ly in oper­a­tion, recent­ly broke records for the fusion ener­gy pro­duced (59 MJ in a real­is­tic deu­teri­um-tri­tium tar­get plas­ma). Very intense mag­net­ic fields (20 T) have been obtained with toka­mak-type coils using high-tem­per­a­ture super­con­duc­tors, paving the way for small­er, more effi­cient and there­fore more eco­nom­i­cal devices. An increas­ing num­ber of these advances involve start-ups and pri­vate ini­tia­tives that undoubt­ed­ly sig­nal the grow­ing matu­ri­ty of the field. ITER remains essen­tial for the com­mu­ni­ty because it is the only place where it will be pos­si­ble to test all the prob­lems linked to fusion ener­gy pro­duc­tion in an inte­grat­ed way.

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