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.