Record for ASDEX Upgrade – great prospects for ITER
New plasma state could afford dramatic improvements for ITER operation
The objective of fusion research is to develop a power plant that, like the sun, derives energy from fusion of atomic nuclei. To ignite the fusion fire the fuel, a hydrogen plasma, has to be confined thermally insulated in magnetic fields and heated to temperatures of over 100 million degrees. In this way the JET (Joint European Torus) experiment at Culham, UK, was able in 1997 to generate a fusion power of 16 megawatts for 2 seconds. More than half of the power used for plasma heating was recovered through fusion. The next step is to be taken with the ITER (Latin for “the way”) international test reactor: The aim is to deliver for a lengthy period a fusion power of 500 megawatts, i. e. ten times more than is needed for heating the plasma. ITER was prepared by research scientists from Europe, Japan, Russia, and the USA; China and South Korea have meanwhile joined the project. The site and start of construction are to be decided this year.
Meanwhile, there are good prospects of extending the good results from ASDEX Upgrade to the much larger ITER device: That is, six years ago, when the “Improved H-regime” (improved high-confinement regime) was discovered on ASDEX Upgrade, it was still unclear whether the plasma state was to be regarded as not just a peculiarity of this particular device, albeit with major advantages: The plasma afforded very good thermal insulation with high energy content. In addition, small instabilities at the plasma boundary ensured that the thermal energy from the plasma was uniformly unloaded in small portions to the walls. This afforded the desired gentle power decoupling to prevent mage to the vessel walls. By virtue of these good properties the “Improved H-regime” was distinctly superior to the ordinary H-regime - another IPP discovery - which is envisaged as the basic mode of operation for ITER. The new plasma state accordingly attracted great interest: The higher the energy content and hence the fusion yield of the plasma can be driven, the smaller and hence the cheaper a future fusion power plant will be.
In recent years ASDEX Upgrade has succeeded in implementing the “Improved H-regime” over an ever wider operating range. For this purpose it is indispensable that the plasma current, which in devices of the tokamak type produces part of the magnetic cage, be put on the right track in the plasma. For such “current drive” it has now been possible for some time in ASDEX Upgrade to use neutral-particle heating: By injecting fast hydrogen atoms - actually a heating method - it is likewise possible to generate an electric current and externally control it. Properly started, the current profile formed at the start of the discharge is kept stable throughout the discharge by complex feedback between the plasma and the magnetic field. Up to 50 per cent of the plasma current is then borne by the heating (and a pressure-driven internal current), the rest being conventionally produced by transformer in the plasma.
After the success in ASDEX Upgrade the favourable plasma state was also achieved in the similarly structured DIII-D fusion experiment in San Diego, USA. Last summer IPP scientists finally succeeded in realising the “Improved H-regime” in the large-scale device, JET, in Culham, UK. At the end of April 2004 the topic again featured in Garching’s experimentation schedule, which involved the presence of a visiting scientist from the USA. Even with his method developed on DIII-D the desired plasma state was achieved right away. The result was a new device record for ASDEX Upgrade in energy content of the plasma: 1.5 megajoule*. “Now that the ‘Improved H-regime’ has been achieved in different ways in three devices of different sizes we are confident that this will also be possible in the even larger ITER device”, states Professor Hartmut Zohm, head of IPP’s Experimental Plasma Physics Division 2. “This would at least double the fusion yield expected in ITER”. Comparative experiments are meanwhile in progress worldwide, on ASDEX Upgrade, DIII-D and Japan’s JT-60U.
In absolute terms, this is not much - enough to boil 3.5 litres of ice-cold water - which at a plasma temperature of 100 million degrees may at first seem surprising. However, the energy is distributed over just a small number of plasma particles: With about 7 x 1019 particles per cubic metre vacuum conditions prevail in the ultra-low-density plasma. In fusion collisions the impact of the few particles with one another is all the more intense.