discovered_01_2016

WWW.HZDR.DE 06 07 TITLE // THE HZDR RESEARCH MAGAZINE neighboring ELBE accelerator. They are currently trying to synchronize the processes in the vacuum chamber so that the plasma really can accelerate the ELBE electron beam yet further. And what do the Dresden physicists want to do with the fast plasma electrons? "One obvious idea is to produce strong X-radiation with the help of fast electrons," Irman answers. It might then be possible to make the careering electron bundles collide with another laser beam, for example, which would generate extremely bright, short X-ray flashes. Such flashes would be a most valuable research tool for investigating things like the extreme states of matter that exist inside planets and stars and that can already be produced in the lab today, at least very briefly. Fast particles with healing powers Karl Zeil is in the process of exploring one practice-related application: He uses DRACO’s high-intensity laser pulses to speed up protons and other ions – that is, particles that are significantly heavier than electrons. "Fast protons and carbon-ions help to irradiate patients’ tumors effectively and gently," says Zeil. "If we were able to accelerate the particles with laser pulses, future irradiation facilities would be more compact, simpler and cheaper." Many physicians think particle therapy is more effective and has fewer side-effects than conventional radiation therapy using X-ray light. The principle underlying the therapy is that an accelerator speeds up charged particles – usually protons, but also carbon-ions. Then the particles are fired at the tumor: They penetrate deep into the tissue, releasing most of their energy at a certain point – the tumor. This comes much closer to the goal of radiation therapy: maximum dose in the tumor, minimum dose in the healthy tissue surrounding it. But there is a snag: The dimensions of the equipment needed for particle therapy are enormous. Up to now, it has required an efficient accelerator that uses radio waves to get the protons up to speed. Bulky magnets keep the particles on the intended path and lead them to the patient. Not least because it is so complex, there are only very few treatment centers in Germany, such as HIT in Heidelberg and the new University Proton Therapy Dresden (UPTD) at the OncoRay Center for Cancer Research. This is why Zeil and his team are working on a compact and hopefully simpler method: Instead of radio waves, they accelerate the particles with strong laser pulses. In contrast to laser-electron acceleration, however, the ions and protons cannot be kick-started by the bow wave of the laser flash – they are far too heavy and inert. Instead, an indirect effect comes into play: After the laser pulse has transformed the matter into a plasma, it drives the electrons out of the matter into the vacuum and leaves behind positively-charged ions. This generates an extremely strong electric field – so strong that the ions are literally ripped apart and thus accelerated. "For our experiments we don’t use gas like we do for electron acceleration," Karl Zeil explains. "What we use are solid matter targets, such as thin metal foils." The physicist now heads for his workplace – a lab with direct access to the DRACO laser. In the middle of the room there is a voluminous, stainless steel vacuum chamber containing a wide range of optics and components. Zeil points to one element that is reminiscent of a king-sized shaving mirror. "It bundles the laser pulses on the foil," the physicist explains. "The protons are then accelerated vertically onto the reverse side of the foil." PROTON THERAPY: At University Proton Therapy Dresden (UPTD), the charged particles are accelerated in the cyclotron (blue, right) and transported along the beamline where heavy electromagnets (yellow) keep them on track. Part of the beam is diverted into an experimental hall. A second line leads to the gantry (blue, left), a rotatable steel construction with a patient treatment unit in the middle. Diagram: OncoRay

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