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discovered 02_2012

discovered 02.12 FOCUS WWW.Hzdr.DE Electrons act as the energy transfer media in laser-ion acceleration. The hotter they become in their interaction with the laser, the more energy they are able to transfer to the ions, where the temperature of the electrons increases with increasing power of the laser. It follows that this temperature must be precisely known. Thomas Kluge has now discovered that the conventional, twenty-year-old models for determining the temperature of laser-heated electrons are too inaccurate for the new generation of high-power lasers. Together with colleagues, he has drafted a new theoretical model that considers entire clouds of electrons, taking relativistic effects into account. This model yields lower electron temperatures than previously anticipated. At very high laser intensities – as can be expected at the DRACO laser or petawatt laser PENELOPE at HZDR and other future petawatt lasers, or indeed the existing petawatt laser in Austin, Texas – the previous models quickly reach their limitations. “It all comes down to how the temperature of the electrons increases with laser power,” Thomas Kluge explains. “When it comes to new high-power lasers, relativistic effects play a much bigger role than previously assumed. If we ignore these, then the plans for new laser systems will be based on unrealistic assumptions, meaning the facilities would most likely fall short of their targets.” Einstein and particles Instead of calculating the temperature for only a single electron, as typically taught in the textbooks, Thomas Kluge factored the entire electron cloud heated by the laser into his equations. This is considerably harder to do since it involves finding the correct average energy of all particles in the electron cloud. Adding to the difficulty are relativistic effects because the laser very quickly accelerates electrons to near light speed. Einstein’s theory of relativity states that clocks moving at different velocities tick at different rates. This time dilation effect is negligible for slow-moving particles, but becomes very important as they reach velocities close to that of light – as our laser-heated electrons do, for example. If we imagine that each individual electron in the cloud produced by the laser pulse has its own clock and that it ‘rides the laser wave’ like a surfer riding a sea wave, then, at any given point in time, some electrons would be riding up on top, at the crest of the wave, while others would be riding down in the trough, and others still would be somewhere in between. The surfers on the wave crest experience the greatest acceleration. Given their greater velocity, their clocks LIGHT FIREWORKS: If the beam of a high-performance laser is focused in the air, it creates a filament – a range in which laser light and matter interact in such a way that a new light spectrum is produced. Here, white light, which, along its path of travel, meets dust particles in the air, is seen producing colorful laser light.