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The laser line

The twenty-two laser lines at the LMJ are located on either side of the experimental chamber in the four laser halls.

22 chaînes laser implantées dans les 4 halls. Crédit CEA

Twenty-two laser lines are installed in the four halls. © CEA

Each laser line consists of eight virtually identical laser beamlines.

A laser line is composed of three elements:

  • The laser source
  • Amplifier section
  • Line end

Une chaîne laser. Crédit CEA

A laser line. © CEA

 

Laser source

The laser source delivers the initial light pulse that will be amplified in the line. Its role is to generate the initial beam and give it its spatio-temporal shape and frequency (color of light). Laser beams are generally circular in shape. However, in the case of the LMJ, the laser beam will be square in order to optimize the structures' small form factor. Measuring 40 × 40 mm upon exiting the source, the beam increases in size to 400 × 400 mm in the amplifier section. It has little energy (of the order of one joule) and lasts a few billionths of a second.

The LMJ laser source comprises:

  • Four optoelectronic sources (one per laser hall) that deliver the first photons (several billionths of a Joule).
  • One hundred and twenty preamplifier modules (PAM – 1 PAM for every 2 beams) that increase the beam energy from several billionths of a joule to one joule.

Amplifier section

The initial pulse emitted by the laser source is greatly amplified in the LMJ (by a factor of approx. 20,000) in order to obtain the energy required for each experiment. This amplification is performed by the amplifier section, which is also referred to as the amplifier chain.
The LMJ facility is designed to accommodate 176 laser beamlines grouped into twenty-two lines of eight beamlines They will be installed in the four laser halls located on either sides of the experimental chamber.

Each laser beam acquires its energy by passing through the amplifier four times. This principle has the advantage of extracting the maximum amount of energy from the amplifiers and significantly reducing the size of the laser halls.

Simply put, the amplifier blocks consist of:
  • Neodymium-doped laser glass plates. Neodymium is the element that gives the glass its pink color.
  • Installation of the neodymium-doped amplification plates. © CEA
    Installation of the neodymium-doped amplification plates. © CEA
  • Xenon flash lamps that provide the light energy. These lamps convert the electrical energy delivered by the capacitors into radiation intended for the optical pump of the laser glasses.
 
Optical processing of LMJ components using the sol-gel process. © CEA
Optical processing of LMJ components using the sol-gel process. © CEA

Dust on the surface of optical components can lead to laser-induced damage. As in the microelectronics industry, it is necessary to maintain a clean environment that is assessed in terms of the number of particles greater than 5 microns per cubic foot (per 28,317 cubic centimeter) of air.
The LMJ will include clean rooms ranging from class 100 (ISO 5) to class 100,000 (ISO 8). One-third of the building is dedicated to air-conditioning and air-handling equipment.

 

Line end

The amplified laser beams are carried to the experimental chamber. Each beam is oriented by a set of six successive mirrors to switch them from a parallel configuration (in the laser halls) to a configuration in which they are distributed all around the test chamber.

Before entering the experimental chamber, each beam passes through a frequency conversion and focusing system.

In the frequency conversion and focusing system:

1/ The light frequency of each beam shifts from infrared (frequency at which the beam can be best amplified) to ultraviolet (frequency that allows the best interaction between the light and the experimental target).


2/ Each beam is focused to shift from a cross-section of 40 × 40 mm to a diameter of a few tenths of a millimeter to allow it to enter the target without losing energy.

Crédit CEA
© CEA
KDP – an exceptional single crystal used for frequency conversion. © CEA
KDP – an exceptional single crystal used for frequency conversion. © CEA

For more information:

MàJ: 18/11/2014
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