Light for fusion

© Photo | (c) Science and Technology Facilities Council

Anyone who wants to turn ultra-high-power lasers from a research topic into a research tool needs pump sources that cause as few headaches as the pump modules found in today’s laser tools.

Researchers have finally succeeded in producing more energy from a laser-induced fusion reaction than it consumes during ignition. And for many experiments, laser particle accelerators barely the size of a room may soon replace kilometer-long synchrotrons. These are just two news items, but they show that laser-plasma interactions have the potential to cause a revolution – but only if diode-pumped solid-state lasers delivering pulses in the kilojoule range fire several times per second rather than only once every few hours.

There is more

The company

ingeneric-asphaeren-array

  • Ingeneric GmbH develops and manufactures ultra-precision optics and micro-mechanical components for high-end applications.  The company employs a wide range of technologies for this, ranging from ultra-precision machining and micro-structuring to ultra-precision molding for mass-production environments.

Institutes and projects

  • The DipoleE Projekt: The aim is to develop a laser amplifier that generates multiple pulses per second with pulse energies upwards of one kilojoule. The project is being conducted at the Rutherford Appleton Laboratory’s Central Laser Facility.
  • The Hilase Projekt: The aim of this project is likewise to develop diode-pumped pulsed solid-state lasers with high repetition rates and high pulse energy. These are intended for industrial use and for small and mid-sized research institutes in the ERA – European Research Area.

 Laser fusion

Laserfusion

  • The fusion of hydrogen into helium could be an inexhaustible source of energy – provided fusion ignition could be controlled and run successfully. One promising method is to induce fusion ignition using high-energy plasma generated by laser light. The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in California is currently conducting research on inertial confinement fusion, whereby 192 laser beam deliver 1.8 megajoules of energy to a fuel capsule. In 2014, for the first time, researchers achieved a fusion reaction that produced more energy than it consumed.

 

More pulses, more power

In the UK, the Appleton Rutherford Laboratory’s Dipole project is set on creating just such a laser. In its mature form, the gas-cooled cryogenic YAG laser amplifier will generate pulses with significantly more than one kilojoule of pulse energy at a frequency of up to ten hertz.

Pulses with the desired characteristics, called seed pulses, are run through a pre-amplifier before the light is amplified by two pump sources to the desired pulse energy. As with a modern laser tool, these pump sources are both important and unimportant at the same time: while their performance and light quality significantly influence the output beam, pump sources are simply there to supply energy and should fit in unobtrusively.

DiPOLE laboratory setup

In a vacuum chamber, the pump source achieves a two by two centimeter, extremely homogenous focus point on the laser amplifier’s rectangular YAG:Yb disk.

Pump source of the future

That was the jumping off point when Ingeneric first started developing the highly efficient pump sources for Dipole in 2010. AMTRON came on board as the energy supply partner, with diode stacks initially supplied by JENOPTIK. Since 2013 these have been supplied by TRUMPF, which became Ingeneric’s parent company in spring 2014.

The first version of the pump source was designed for 20 kW of pulse peak power per pump laser with a repetition rate of 10 hertz. Dipole managed to turn heads with it in 2011, when its prototype laser amplifier generated pulses with 10 joules pulse power at a frequency of 1 hertz and an optical-to-optical efficiency of 21 percent – an outstanding performance.

Light on demand

As the Dipole prototypes developed, so too did the pump sources. The current generation can now achieve 30 kW, and Hilase , a parallel project, is producing 250 kW pump sources. In their final version, the sources should be able to provide up to 1000 kW of energy.

The aim was also to develop a plug-and-play solution that is suitable for mass production and, in conjunction with the Dipole laser, very close to reaching industrial maturity.

But the central challenge faced by the Dipole team remains light quality. Their 20 kW source delivers a wavelength of 939.5 nanometers, and at a distance of 60 cm it achieves an intensity of 5 kW/cm² across an extremely homogenous focus spot measuring 2 x 2 cm. This corresponds to the rectangular YAG:Yb disks within which the seed pulses and pump light meet inside the laser amplifier. What connects the systems is this intensity – even more powerful systems achieve an intensity of 5 kW/cm². The focus geometry and working distance are adjusted in accordance with the performance scaling.

Efficient-Pumping-of-inertial-fusion

3-D profile of intensity distribution: the pump beam demonstrates a perfect top hat profile.

The laser light source consists of diode bars with a continuous-wave output of 200 watts. 25 bars share a copper heat sink as one passively cooled vertical stack. In order to further aid cooling and prevent thermal deformation, the distance between bars is 1.7 millimeters, which is an increase over the standard spacing formation. This decouples the individual bars both mechanically and thermally, while also facilitating collimation of the light.

Spacing the bars farther apart allows for the use of lenses with longer focal lengths. This improves the optical characteristics and makes the stacks much less susceptible to axis shifts between bars. Ingeneric’s highest-quality fast-axis collimation lenses ensure that, for bars with a 47 degree angle of aperture in the fast axis, 95 percent of the energy is contained within a divergence of 2.1 mrad.

Each of these base units in itself delivers all the desired properties. They deliver constant optical performance over 15,000 hours of operating service and 1.4 million pulses, while the spectrum remains stable within the given bandwidth. In fact the spectrum moves within a bandwidth of just three nanometers.

The perfect pulse

Energy distribution also exceeds expectations. 75 percent of the pulse energy is concentrated between 937 and 943 nanometers wavelength. Both the physical spacing and the energy density of the light of individual bars fully correspond to the specifications. Meanwhile, each pulse completely builds up and fades away again in under one percent of the pulse duration, with a plateau of constant intensity between the two. In all, the base units achieve an electrical-to-optical efficiency of over 80 percent.

Each of these base units in itself delivers all the desired properties. They deliver constant optical performance over more than 15,000 service hours and 1.4 million pulses.

A 30 kW pump laser in the current generation comprises four base units. Their light is overlaid and the frequency range can be fine-tuned by making small adjustments to the temperature via the current and cooling. Ingeneric’s successful homogenization achieves results that are well above initial expectations, with tolerances of less than five percent.

Series production for research

This pump source architecture fulfills not only the Dipole project’s immediate demands – that is, delivering scalable pump light that meets a certain set of specifications. It also meets the other aim that Ingeneric had for the architecture: to develop a plug-and-play solution that is suitable for mass production and, in conjunction with the Dipole laser, very close to reaching industrial maturity.

Pumpmodul

Two compact Ingeneric pump lasers comprise the pump source for a Dipole laser amplifier.

In the next few years, research on high-power lasers such as Dipole will increasingly turn into research using such lasers – for instance in test reactors for laser-induced fusion. That will certainly call for more than just one laser amplifier with two pump sources.

At the National Ignition Facility in California, for instance, researchers pool the light from 192 lasers. So it is safe to say that there will be a need for standardization and modular solutions to limit the cost of installation and operation. As a comparison, remember that anyone who wanted to use a femtosecond laser for research ten years ago had to build their own. These days you can simply order one. Pump modules are nothing more than interchangeable equipment – which is exactly how it should be.

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