Schrödinger Prize 2006 for IPP scientists

Ten times as hot as the interior of the sun / innovative heating for the ITER fusion test reactor

July 21, 2006
What has been functioning in the sun from eternity at just six million degrees centigrade requires great effort on earth: Here it is much more difficult to initiate nuclear fusion, in which hydrogen atoms fuse to form helium, thereby releasing energy. A hydrogen plasma, the fuel for a future fusion power plant, must first be heated to over 100 million degrees. This is accomplished by, for example, injecting fast hydrogen particles into the plasma. Scientists at Max Planck Institute for Plasma Physics (IPP) in Garching have further developed such a heating system for the extreme requirements of the ITER test reactor.

For this achievement the Helmholtz Association has awarded the Erwin Schrödinger Prize 2006 to Dr. Eckehart Speth, Dr. Hans-Dieter Falter, Dr. Peter Franzen, Dr. Ursel Fantz and Dr. Werner Kraus of IPP. This award, endowed with 50,000 euros, is presented annually for outstanding interdisciplinary research.

"The source of negative ions developed by this year's recipients of the prize can essentially meet the high requirements imposed by ITER. No other ion source in the world can compete with this development. The new concept thus has good chances in 2007 of being selected for application in ITER. The prizewinners have admirably succeeded in combining the disciplines of plasma chemistry, surface physics and electrical engineering to achieve a real breakthrough", stated jury member Prof. Dr. Johanna Stachel from the Physics Institute of the University of Heidelberg. The prize is to be presented by Prof. Dr. Jürgen Mlynek at the annual conference of the Helmholtz Association in Berlin on 13. September 2006.

The ITER (Latin for “the way”) international test device, which, it was recently decided, is to be built at Cadarache in France, is the next major step in international fusion research. With 500 megawatts of generated fusion power ITER is to demonstrate for the first time that an energy-producing fusion fire is possible. The objective is to develop a power plant that, like the sun, produces energy from fusion of atomic nuclei. For this purpose the fuel – an ionised low-density hydrogen gas, a "plasma" – has to be confined in a magnetic field cage without wall contact and heated to high temperatures till ignition of the fusion reactions. About half of the heating of the ITER plasma is to be done with "neutral particle heating": Fast hydrogen atoms injected into the plasma release their energy on colliding with the plasma particles. Present devices thus yield a multiple of the sun's temperature at the push of a button.

In order to accelerate hydrogen atoms in these heating devices, they first have to be made tangible to electric forces as charged particles – positive or negative ions. It has been exclusively positively charged particles that have been used hitherto in the heating systems: For this purpose the electrons are removed from neutral hydrogen and the positive charged hydrogen ions are then pumped out and accelerated. Before being injected into the fusion plasma, however, the ion beam must be neutralised again because charged particles would be deflected by the magnetic field of the plasma cage: For this purpose the ions have to pass through a gas curtain. Here the ions regain the missing electron and are injected as fast neutrals into the plasma.

ITER now imposes new requirements on this proved method: For example, in the ITER large-scale device the particles have to be three to four times as fast as hitherto so that they can penetrate deep enough into the plasma. It is therefore no longer possible to work with positively charged ions. This, unfortunately, is because they are all the more difficult to neutralise the faster they are – at the velocities of 9000 kilometres per second needed for ITER it is almost no longer possible. For ITER it is therefore necessary to change to negatively charged ions, which are easy to neutralise at high temperatures as well. They are, however, more difficult to handle than positive ions: The additional electron, which is responsible for the negative charge of the particles, is just loosely bound and is accordingly readily lost again.

In order to produce these fragile objects for ITER, so-called high-frequency plasma sources are particularly suitable. The new ion source was developed at IPP on the basis of preliminary work at the University of Giessen and has been in operation in IPP’s ASDEX Upgrade experiment since 1995 – albeit for positive ions. Since 2002 Dr. Eckehart Speth and co-workers at IPP have been engaged in further developing the new beam source for negative ions. This is being done jointly with the University of Augsburg, where Dr. Ursel Fantz and co-workers are working on sophisticated diagnostic and modelling methods for the joint project.

The new high-frequency plasma source takes its name from a high-frequency wave injected into hydrogen gas, thereby ionising some of the hydrogen atoms. The resulting cold plasma, a mixture of neutral atoms, negative electrons and positive ions flows into the beam source, onto its inner walls and onto a first lattice-shaped electrode. If the surface of the electrode is coated with suitable material, e.g. caesium, electrons can then be captured by the plasma particles, thus producing the required negative hydrogen ions. Since the scientists succeeded in fathoming the complicated dynamics of the caesium distribution on the walls it has been possible to deposit the caesium continuously from a small oven as an ultra-thin coating with about the thickness of an atomic layer.

The negative ions produced in the vicinity of the lattice can now be extracted from the beam source. They are then caught by the electric field of a second lattice, bundled into a beam and accelerated by a third lattice. With its results to date – some of them world records – IPP’s high-frequency source already has good chances of being selected for ITER. For a final assessment it first has to be shown whether the technology can be extended to ITER size. The decision on application in ITER is scheduled for mid-2007. But there are also other fields of application for the new ion source, e.g. in accelerators or for producing large-area plasmas for industrial use.

Isabella Milch

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