ELI Beamlines, a part of the Extreme Light Infrastructure (ELI) pan-European project of scientific excellence in Europe, aims to operate some of the world's most intense laser systems. Lawrence Livermore National Lab (LLNL) was awarded the contract for the L3 laser system from ELI Beamlines, and both teams had come together on development of a high-average-power, high-energy system—a petawatt laser operating at 10 Hz.
Generally, there are two choices for pump technology in a high-energy laser system: laser diodes or flashlamps. ELI made the decision that laser diodes were the best way to advance system technology and were introduced to the Leonardo (Lasertel business unit) team. In this interview, Lucia Koubíková, a Laser Systems Engineer on L3 HAPLS at ELI, shares unique insights from the project.
Why did your team choose to use laser diodes in the project?
Flashlamps have been used previously as an energy source in high-power laser systems, but they lack the energy transfer efficiency that laser diodes can offer.
Flashlamps have inherently significant issues in low efficiency due to their broad emission spectrum used for a narrow gain material absorption, creating excessive residual heat. While the following characteristics of Leonardo laser diodes helped our team to avoid these issues:
- High beam quality—allowing for efficient and consistent delivery of energy
- High-peak-power & rep rate capabilities—allowing for a significant increase in average power
- Size—compactness of the diodes is a crucial advantage in keeping the overall system small.
- Cooling capabilities (water-cooled diodes)—increases overall efficiency while maintaining reduced volume of cooling circuit. Our systems can currently run without overheating in 10° C, offering a significant benefit to facility requirements.
For your type of research, how does reliability of the pump source impact your work?
We need perfection for our research—consistency and reliability. Our team was looking for high-quality diode arrays to ensure this. You can’t afford to have different energies from pulse to pulse. Inconsistent sources run the risk of damaging the gratings in the compressor, the transport optics, or the user components in the experimental chamber at the end of the beamline. Moreover, unstable PW laser pulses would directly cause inconsistent and unrepeatable results of laser-matter interactions that are being conducted in the ELI Beamlines' experimental halls. We went through extensive testing of the laser diodes when the system was with LLNL to ensure that they would meet our requirements. These same tests were repeated at our location and the results were exactly the same. The reliability of these diode arrays is incredible.
What are the different applications that advancements in petawatt lasers could support?
ELI has a range of possible experimental offerings for users. The L3-HAPLS system mainly offers opportunities for plasma physics experiments and particle acceleration experiments. While the first is mostly used for basic research, the latter has potential for medical applications and can drive the development of new cancer treatment devices using particle beams.
We work with researchers in many fields: basic research, plasma physics, particle acceleration, and x-ray imaging—all of which could benefit from higher average powers and rep rates. With our system, we have already made a shift from existing system rep rates, which were quite low at 1 Hz or lower. Our system is now running at 3.3 Hz with the ability to reach 10 Hz, encompassing a wider range of applications by increasing to a higher average fluence of desired particles. The high rep rates are achievable because the use of diode pumping allows for the practical use of laser-matter interactions studied so far, at lower rep rates. The pursuit of high-peak powers (1 PW, 10 PW, etc.) paves the way for discoveries of new interaction processes. However, for their use outside research facilities, high rep rates are crucial—they make it possible to deliver a sufficient dose of radiation to cancerous tissue or to collect enough statistical data during experiments. Thus, the aim of the L3 laser is not only to look for new applications but mainly to introduce existing applications into practice.
In addition to pushing research in these applications, does your decision to use laser diodes impact adoption outside of the lab?
Yes, the compactness of the system will be especially crucial. A medical facility can’t necessarily support the space for a treatment system that is the size of a soccer field. The goal would be for a system to be table sized. Leonardo’s diode modules are quite compact, making a big step in the right direction to achieve this goal. These systems are built to easily scale—adding a new element (array or module), adds more power. This makes it essential that the elements, diode arrays, are small in size to keep the size of the overall system down.
Additionally, the flexibility of the diode modules supports use in different fields. With the current control system, developed by LLNL, it is extraordinarily easy to adjust and program the spatial and temporal profile of the beam and control the pulses coming from each diode array. This is a key feature in order to optimize the gain profile of our main laser. We’ve taken the time to test and improve this process to meet any design parameters required. The versatility of the diodes will support various applications in and outside of the lab. Right now, we have granular manual control over the beam, but we look forward to advancements in automated or algorithm-based programming to improve adjustment of the gain for any application.
What other advancements do you see on the horizon in this space? Are you being asked for anything specific?
We will definitely continue to see the push for more power and increased rep rate. I believe the most significant goal right now would be to reach 1 kHz in a petawatt laser system. The diodes Leonardo offers currently have already increased in power by a factor of 2 since our system was built, and while achieving higher rep rate and power will be a big challenge for diodes, we will likely also need to see new advancements in optics and cooling capabilities to accommodate energy requirements that will ultimately lead to high-average-power exawatt systems.