Next generation laser plasma accelerator

One of the most promising avenues for achieving new target levels of high peak intensity and high average power in an ultrafast laser system is to turn to fiber lasers.

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Professor Almantas Galvanauskas (Co-PI), and the femtosecond laser, Hercules, housed in the Center for Ultrafast Optical Science

Michigan is part of a multi-institution collaboration to develop key laser technology that will enable the design of a high-power, ultra-short-pulse laser system which is expected to enable new low-cost, compact accelerator-based light sources for a wide variety of biological, chemical, materials science, and security applications. The technology may also lead to compact, portable TeV (tera electron volt) linear colliders, and enable the same kind of research now being conducted in conventional accelerators, such as the 17 mile Large Hadron Collider, on a table top.

Femtosecond lasers facilities, such as Hercules at the University of Michigan, have been true “workhorses” in exploring fundamental aspects of laser-matter interactions at very high intensities. But they lack the ability to achieve the simultaneous emission of extremely high peak intensity and high average powers. Hercules, for example, holds the record for highest intensity, but can only operate at low repetition rates, on the order of about 1 beam pulse per second. This is because current short-pulse laser systems rely on fairly large size Ti:Sapphire crystals to achieve high peak beam intensities. For example, the best commercial systems use a 12cm crystal to reach 60 joules. Due to such a large size, heat produced at higher repetition rates cannot be effectively dissipated without distorting output beam.

New technology is needed to increase the pulse repetition rate to levels required for long-term scientific goals. The ultimate goal is to increase the repetition rate of petawatt pulses by 3-4 orders of magnitude. Using laser plasma acceleration (LPA), in 2013 researchers at the Lawrence Berkeley National Laboratory generated 4.25 GeV (giga electron volt) beams powered by 310 terawatt laser pulses (15 J) in a 9 cm long plasma channel. This achievement presented a vision of replacing commercial accelerators with laser accelerators, and reaching TeV (terra electron volt) beams.

“To get a conventional accelerator to this level of acceleration you need 100 meter long structure,” explained Prof. Almantas Galvanauskas. “They were able to do the same thing in a capillary that’s a few centimeters long.”

One of the most promising avenues for achieving new target levels of high peak intensity and high average power in an ultrafast laser system is to turn to fiber lasers. Fiber lasers are efficient, generate quality beams, are able to emit high average power, and can be monolithically integrated.

The current roadblock with fiber lasers, however, is their low repetition rate at high intensities. Prof. Galvanauskas, a recognized leader in the field who has already demonstrated several record-breaking achievements in the performance of fiber lasers, is leading the effort at Michigan to get to a higher repetition rate.

Prof. Galvanauskas plans to achieve 10s of joules with fiber lasers, orders of magnitude improvement over the ~1 millijoule currently possible, by coherently combining and phasing the laser beams both in spatial and time domains. He explained:

“We’re going to take multiple pulses, as much as a thousand pulses in a time domain, split them into parallel channels and then combine them back into one beam – and then we’re going to stack them in time, into one pulse. If we can get 100 to 1,000 pulses, then we can reduce the number of channels to 10 to 100, which is already technologically feasible. What is not feasible yet is how to do it with short pulses on a large scale. But we have an idea for how to do it and have already demonstrated our technique on a small scale.”

In the next couple years, the research team plans to scale up to reach higher numbers and higher energy levels. Achieving this scaling will be the key to their success.

“If we’re successful here –we may actually change the paradigm in lasers. We will no longer use just one aperture and a crystal to achieve high energy, but we will use multiple apertures which we multiplex in space and time, and replace the crystal with multiple integrated circuits. The resulting system will be practical, and have a huge impact on existing applications while leading to many new applications.”

For example, laser plasma accelerators enable research in high energy physics (HEP), which has led to economical MRI machines, and even the WWW. Particle accelerators have also led to improved medical diagnosis, scanners, and advanced computing technology, while advancing general research in industry and the sciences.

This project is a collaboration between Lawrence Berkeley National Laboratory (Wim Leemans, PI), which is leading the project, and Lawrence Livermore National Laboratory (Jay W. Dawson, Co-PI). It is the first project funded under the new government program to expand research in HEP, and is called, A novel coherent combining approach towards high peak and high average power ultrafast lasers.