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Measurements and diagnostics
The goal of the Megajoule Laser is to reproduce, in the laboratory, conditions similar to those encountered during the operation of a nuclear weapon or in the cores of planets and stars.
In several billionths of a second, matter will be taken to:
- temperatures of several tens of millions of degrees,
- pressures tens of millions of times that of atmospheric pressure,
- densities several hundred times that of solid matter.
During each experiment, many parameters will be measured and compared to those predicted by computations. For example:
- laser energy absorbed by the target,
- temperature reached by the target,
- pressure generated,
- the implosion speed of the microballoon in the case of inertial confinement fusion (ICF),
- degree of symmetry of the implosion,
- density and temperature of the compressed deuterium-tritium mixture,
- and the energy generated by fusion of the deuterium-tritium mixture.
Teams from DAM have developed special diagnostic equipment to obtain these measurements.
- The brevity of the phenomena investigated necessitated the design of ultra-rapid detectors with a temporal resolution on the order of one picosecond (10-12 of a second).
- During the experiments, the diagnostics inside the experimental chamber are put through their paces (electromagnetic interferences, shock waves, intense X-rays and neutron radiation, etc.) to ensure that they are robust enough to withstand these attacks.
- Each experiment requires several months of preparation. A large number of measurements, sometimes as many as several hundred, are necessary so that they can be used to validate the theoretical models and computer codes. To limit the experimental costs, they must function with a high degree of reliability.
There are several types of diagnostics:
- Optical diagnostics are used for the laser energy balance by analyzing the laser energy that is backscattered, transmitted and deflected, and for material state equation measurements by using interferometry to measure shock and material speeds as well as the temperature reached by the material studied.
- X-ray diagnostics consist of imaging equipment (pinholes, X-ray microscopes) to measure plasma hydrodynamics across space and time, and spectrometric devices (broadband or high resolution) to characterize plasma emissivity or absorption and infer its temperature.
- Nuclear diagnostics measure the emissions (primarily gamma radiation and neutrons) produced during deuterium and tritium fusion reactions. It is used to characterize the density and temperature of fuel as well as ICF target gain.