ITER and plasma control : where are we ?

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”.

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 m³ of 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).

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.

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.

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.