π Science and technology
Lasers: promising applications for research and beyond

Using X‑rays to test materials without damaging them

Isabelle Dumé, Science journalist
On June 29th, 2022 |
4 min reading time
Cédric Thaury
Cédric Thaury
Research Fellow at LOA* at ENSTA Paris (IP Paris)
Key takeaways
  •  The Applied Optics Laboratory and its spin-off company, SourceLab, are developing a device for using laser-generated plasmas for non-destructive testing of materials by X-ray.
  • Wake plasma accelerators’ have emerged as a promising alternative. These devices use a pulse of energy to create an electric field wave in a plasma.
  • One of the first applications of these plasma accelerators would be in radiography. The precision of these lasers is such that they can probe an object to the nearest 100 microns.
  • The challenge is all the greater because it is a question of penetrating materials and not human bodies. So more power is needed to achieve incredible accuracy.

Laser-gen­er­at­ed plas­mas can be used to accel­er­ate par­ti­cles that can then be used to cre­ate short, bright puls­es of X‑rays and gam­ma rays. The Lab­o­ra­toire d’Op­tique Appliqué (LOA) and its spin-off com­pa­ny Source­Lab are devel­op­ing a device to use these sources for the non-destruc­tive test­ing of mate­ri­als using X‑rays. This cut­ting-edge tech­no­log­i­cal inno­va­tion could enable researchers to detect and size defects in objects with a res­o­lu­tion of a few tens of microns, some­thing that is still inac­ces­si­ble with con­ven­tion­al systems.

An increasingly accessible technique

Par­ti­cle accel­er­a­tors have enabled us to make the most impor­tant dis­cov­er­ies in physics, but they col­lide par­ti­cles at ever high­er ener­gies, push­ing exist­ing tech­nolo­gies to their lim­its. Indeed, accel­er­a­tion facil­i­ties, such as the Large Hadron Col­lid­er at CERN, are get­ting larg­er and larg­er and are becom­ing inac­ces­si­ble to many researchers.

In recent years, “plas­ma wake­field accel­er­a­tors” have emerged as a promis­ing alter­na­tive. These devices use a pulse of ener­gy to cre­ate an elec­tric field wave in a plas­ma (a gas trans­formed into a cloud of elec­trons and ions), rather like the wake in water cre­at­ed by a boat. If a group of par­ti­cles is prop­er­ly syn­chro­nised, it can “surf” on this wave and be accel­er­at­ed much faster than in a con­ven­tion­al accel­er­a­tor. How­ev­er, it is not easy to cre­ate this ener­gy pulse.

One way to do this is to send extreme­ly short and intense laser puls­es into a gas. The front of the pulse, which lasts only a few fem­tosec­onds (10-15 s), imme­di­ate­ly ionis­es the atoms in the gas and is so intense that it push­es the elec­trons out of its path, form­ing an emp­ty cav­i­ty of elec­trons in its wake. Some elec­trons in the wake of the pulse are accel­er­at­ed by the wave of pos­i­tive­ly charged plas­ma ahead of them, just like a surfer on the wave behind the stern of a boat. The elec­trons “surf­ing” on this wake can be accel­er­at­ed to speeds close to the speed of light.

What are these applications?

We study this phe­nom­e­non at LOA. It allows laser-plas­ma accel­er­a­tors to achieve accel­er­a­tion forces up to a thou­sand times greater than those achieved by the most pow­er­ful machines avail­able today. Our lab­o­ra­to­ry is a pio­neer in this field, and we have been work­ing on this sub­ject since the mid-2000s. Togeth­er with oth­er teams around the world, we have devel­oped the tech­nol­o­gy to the point where we cre­at­ed the Laplace Laser Plas­ma Accel­er­a­tion Cen­tre in 2022 to bet­ter under­stand the mech­a­nisms involved and devel­op applications.

The first is radi­og­ra­phy, par­tic­u­lar­ly in the field of non-destruc­tive test­ing, a set of tech­niques that are used to check com­po­nents and iden­ti­fy defects in a mate­r­i­al with­out destroy­ing it. The size of the par­ti­cle pack­ets sup­plied by laser-plas­ma sources is on the order of ten microns, mak­ing it pos­si­ble to probe com­po­nents at high res­o­lu­tion, for exam­ple those impor­tant to the nuclear and aero­nau­ti­cal indus­tries. Ini­tial lab­o­ra­to­ry results indi­cate that it would be pos­si­ble to mon­i­tor the appear­ance of cracks as small as 100 microns in parts such as land­ing gear. This is ten times small­er than the detec­tion lim­its of cur­rent equipment.

So, it is a sim­ple way to see if there’s a crack in a piece of steel, for exam­ple, with­out hav­ing to slice it, which would mean replac­ing a poten­tial­ly very expen­sive part.

What we are inter­est­ed in here is the con­ver­sion of elec­tron beams into a high­ly ener­getic X‑ray beam – using the tech­nique known as brak­ing radi­a­tion. In prac­tice, we send the elec­tron beam into a mil­lime­tre-thick sheet of a fair­ly dense mate­r­i­al, such as tita­ni­um. The elec­trons are slowed down, or braked, and the ener­gy lost by their slow­ing down is con­vert­ed into X‑rays whose max­i­mum ener­gy cor­re­sponds to the ener­gy of the ini­tial elec­trons with a very broad spec­trum. This is a fair­ly sim­ple and rel­a­tive­ly effi­cient method of pro­duc­ing X‑rays from an elec­tron beam.

Materials radiology

The size of the defects we can detect and mon­i­tor is large­ly deter­mined by the size of our accel­er­a­tor source. Laser-plas­ma accel­er­a­tion is a good solu­tion in this respect since we start with a very small source size. This means that the size of the X‑ray beam will also be small. In fact, it is a bit larg­er than the elec­tron beam, because the elec­tron beam has a large spa­tial diver­gence. As the beam diverges, the diam­e­ter of the elec­tron beam on the con­vert­er is larg­er than its size at the plas­ma exit. Typ­i­cal­ly, if you have a micro­met­ric source size, you end up with a beam size of the order of tens of microme­tres, typ­i­cal­ly 30 to 100 microns, which is still a good order of magnitude.

We are a bit like med­ical radi­ol­o­gists, but our “patients” are mate­ri­als. When we pass X‑rays through a mate­r­i­al, they will be less absorbed in the region of a defect. We can image that defect in exact­ly the same way as a radi­ol­o­gist does for bone frac­tures. Since our X‑rays do not pass through the human body, but through pieces of con­crete or steel, this means that we need much high­er ener­gy rays than those used by radiologists.

Références :

  • V. Mal­ka, C. Thau­ry, S. Corde, K. Ta Phuoc and A. Rousse ; Accéléra­teurs à plas­ma laser : principes et appli­ca­tions Reflets de la physique 33, 23–26 (2013)

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