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discovered_01_2016 - Extreme-tuning a Power Laser

WWW.HZDR.DE discovered 01.16 TITLE _TEXT . Frank Grotelüschen EXTREME-TUNING A POWER LASER In order to effectively accelerate particles via laser plasma, you need one thing above all: extremely high-performance lasers. At HZDR, this is DRACO’s job. The acronym stands for "Dresden Laser Acceleration Source". It was launched in 2008 and has since been delivering ultra-short pulses at a power of 100 terawatts (TW). Now, the laser has been fundamentally expanded with additional new components. Thanks to this upgrade, DRACO can now also generate pulses that are ten times stronger, i.e. one petawatt. DRACO consists of several core components: At the center of it all is a compact titan sapphire laser oscillator, which can produce 78 million ultra-short laser pulses per second. Each of these pulses lasts only about 30 femtoseconds and has an energy of a few nanojoules. For the experiments, energy billions of times stronger is required. This is done with optical amplifiers, which are basically additional laser stages that multiply the number of light particles, thus strengthening the energy of the laser pulses. The danger with a high-performance laser is that the amplified light pulses can become so strong that they might damage the optical elements. The experts therefore resort to clever tricks: They stretch the pulses spatially, enlarging their diameter from a few millimeters to ten or even 30 centimeters. This spreads their energy over a larger surface and decreases its intensity. Not until the very end, shortly before the experiment, is the beam re-concentrated onto a spot the size of a micrometer. Yet this spatial enlargement alone is not enough. The pulses also have to be stretched out in time. To do this, the beam is directed through an arrangement of optical gratings. This "expander" stretches the flashes from their original duration of 30 femtoseconds to one nanosecond, which is 30,000 times longer. This distributes the energy over a longer period of time, its intensity decreases. At the end of this expansion chain, the pulses must be compressed back to their original duration of 30 femtoseconds. This happens in large compressor tanks, once again with the help of optical gratings. Originally, DRACO consisted of a front-end area that generated the pulses and boosted their energy up to one joule. After this, an amplifier increased the power of the flashes to six joules. This was followed by a compressor assembly that compressed the pulses back to their original length. As part of the upgrade, another section was added to this set-up: Now, the beam that exits the front-end area is divided by a beam- splitter. The first section is the final stage of the original 100 TW laser. The second, new section consists of a big optical amplifier as well as a large compressor tank. Since the beam has to be stretched to 30 centimeters instead of ten, this tank is considerably larger than the one in the 100-TW section. This new section boosts the pulses to an energy level of about 40 joules – which, at a pulse duration of 30 femtoseconds, means that its power is in the petawatt range. One of the challenges in upgrading DRACO was manufacturing the optical elements. Making the twelve- centimeter titan sapphire disks to boost the laser was a particularly difficult task, because amplifying the energy evenly requires extremely high-quality crystals. It has only been in the last few years that industry has even been capable of growing crystals of the desired size and type. So far, there are only a few such crystals in the world – some of which can be found in Dresden. In addition, the new petawatt section meant the front-end area had to be remodeled as well, because each of the ultra- strong pulses has a sort of vanguard that precedes it. If one were to simply boost the energy of the pulses to the petawatt range, this ‘vanguard’ would be so intense that it would destroy the sample ahead of time. This is why HZDR scientists had to try to weaken the pre-pulse considerably in relation to the actual pulse. They achieved it by adding an additional filter stage that cleans the laser pulses better, keeping the vanguard of pre-pulses small. The experts want to make sure that once remodeling is completed, both sections can be used simultaneously. They take advantage of the fact that both the petawatt and the 100-TW pulses are fed from the same source. With the smart use of delaying techniques, they can make pulses from both sections meet in the experiment. This makes it possible to conduct pump-probe experiments, for example, whereby the pulse from one section interacts with a material sample, which is immediately afterwards illuminated and analyzed by the pulse from the other section. The upgrade is a joint development project by HZDR and the French corporation Amplitude Technologies. Work began in 2011 with the expansion of the lab building. In 2015, the facility was completed, followed by initial tests. The next milestone is planned for August when scientific experiments at DRACO will resume. CONTACT _ Institute of Radiation Physics at HZDR Prof. Ulrich Schramm u.schramm@hzdr.de // The "DRACO" laser is being substantially upgraded in order to produce even stronger pulses.

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