Measuring laser power and energy in laser-induced nuclear fusion systems

The quest for controlled inertial confinement fusion (ICF) using high-powered lasers has been ongoing for more than half a century and continues to be a major goal among the scientific community.

While significant progress has been made in this field in recent years, there continue to be significant challenges ahead, particularly where laser systems are concerned.

Why should you measure your beam during ICF?

One of the main challenges that has plagued nuclear fusion systems since the 1970s is the delivery of laser energy to the target material. This, in turn, has affected our ability to control the symmetry, heating, and hydrodynamic stability of the imploding fuel.

For fusion experiments to be successful, the aim of the lasers needs to be highly accurate, with all beams approaching the target at the same time, at all points.

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In addition to timing, the beams impinging on the target must also possess similar characteristics. One of the greatest obstacles to the achievement of high symmetry and high densities of the imploding target is beam-beam imbalance. This problem occurs in situations where the lasers impacting the target possess different power or energy levels.

Another cause of beam-beam imbalance during ICF is inconsistent power distribution over the beam diameter hitting the target. This can lead to the creation of ‘hot spots’ within the impacted area, which can give rise to uneven compression and Rayleigh-Taylor instabilities.

The difficulty with measuring ICF beams

The lasers used in nuclear fusion have evolved rapidly from the late 1970s. While early lasers were only capable of generating energy levels of only a few joules per pulse, today’s lasers are capable of delivering tens of kilojoules to the target.

As a result, high-energy-density environments can be regularly produced in the laboratory. However, due to the intense power and energy levels used, there are only a few measuring tools that can withstand the rigors of this aggressive and demanding application.  

Choosing the right equipment to measure your ICF beam

The complex and multistage process of nuclear fusion requires various tools that can check different power and energy levels. For seed lasers, low-power detectors with direct PC interfacing can be used for off-site monitoring of the laser characteristics.

However, for end-of-line, high-powered beams used in fusion systems, you need a meter capable of measuring energy levels in the order of kilojoules. In this case, a high-energy calorimeter is one of the few viable solutions available. These devices, developed specifically for high-energy applications such as inertial confinement fusion, measure the pulse energy of high-powered lasers by measuring the increase in temperature of an optical absorber exposed to the beam. The most reliable calorimeters on the market can calibrate lasers with uncertainties within ± 3% with repeatability better than ± 2%.

Laser calorimeters are appropriate for routine measurements, as they have a long cycle time of 20 minutes or more. To measure laser stability in higher repetition rate environments, such as novel fusion power production designs that may require 10 shots per second or more, these devices are not appropriate for measurements of this rapidity. To interface with high-energy beams that require a large beam area in the propagation phase, Gentec-EO has developed the QE195 joulemeter, the largest pyroelectric laser energy meter in the world. With a 195 mm diameter, laser beams in the hundreds of joules can be measured with precision to characterize laser performance and ensure reaction repeatability.

The overall goal of nuclear fusion - thermonuclear ignition - is widely considered to be a significant milestone in the scientific community. The success of this process, however, is heavily dependent on the accuracy of the impinging laser beams. Therefore, if you are a professional involved in this area of research, be sure to verify that you are using the best power and energy meters to achieve optimal results.