W7-AS has been successfully operated In of the European fusion programme for the high-confinement mode (H-mode), a 1994-1998, where W7-X is proposed as region of improved thermal insulation [Wag­ the major new project to be started in this ner F., Europhys. News 17 (1986) 48] which period. The project has meanwhile received is the most relevant tokamak confinement full Phase I approval and the Phase II pro­ regime. This is the first time that the H-mode cess, the final step in the European assess­ has been achieved outside the tokamak ment procedure, has begun. Members of family. The W7-AS device at Garching has the European Associations have been invi­ also demonstrated ultra-high density opera­ ted to join the IPP in the pioneering enter­ tion and the possibility of utilizing the natural prise to realise the W7-X. islands residing at the edge of the plasma to If this final step is passed successfully divert the fluxes crossing the last closed flux the first plasma can be expected in 2002. surface. Fig. 2 — Bird’s eye view of the city of W7-X will, however, not be built at the Greifswald on the Baltic Sea. Greifswald is IPP, Garching, the site of present German W7-X a university town where plasma physics is stellarator research where the optimization well established in the physics faculty of idea was born. It will most likely be located The W7-X stellarator is an experimental Ernst Moritz Arndt University. The Institute in Greifswald (Fig. 2) following another opti­ device (Fig. 1). It will not only deliver many of Low-Temperature Plasma Physics of the mization process which has to do with the new results important to successful conti­ former east German Academy of Sciences research structure in Germany after reunifi­ nuation of the stellarator line but has also to is also located in Greifswald, and the site cation. The national financial support (55%) demonstrate that its optimization based on of a disused nuclear plant at Lubmin some is being provided under the condition that theoretical concepts is indeed feasible (opti­ 20 kilometres from Greifswald has been W7-X Is treated by the German fusion com­ mization — see insert — became possible proposed for the ITER reactor. munity as a contribution to the development with the introduction of modern computers). of research structures in the eastern part Work carried out so far has demonstrated of Germany. that the various goals of the optimization The IPP has been charged with the task process, which occasionally conflict, can be Status of the W7-X Project of founding an extension at Greifswald and met in a single device. Experimental flexibi­ W7-X was approved by the IPP in 1990 operating the institute within the framework lity Is Incorporated so that stability aspects and received restricted Phase I approval of the Max-Planck Gesellschaft, which has of the optimized magnetic architecture such by EURATOM review panels in 1991. One already given its approval. As “W” stands for as shear (spatial variations of the field-line crucial question remained unanswered at Wendelstein, a prominent mountain in the pitch) and magnetic well (the minimum of that time, namely how W7-X would fit into Bavarian Alps, the device would then be­ magnetic field at the plasma centre) can be the European fusion strategy. It was ans­ come a symbol that fusion is Indeed capable changed. Exhausting of plasma impurities wered In the frame of a detailed assessment of moving mountains. can be studied and modified by operating with a series of edge islands or with a more ergodic edge configuration for the so-called ANOMALOUS TRANSPORT divertor used to define the structure of the magnetic field surfaces at the edge of the European Task Force Formed plasma. Moreover, resonating field lines can be placed in the core of the plasma to simu­ The ubiquitous anomalous transport pro­ the possibility of a European TTF. While late, if necessary, the effect of sawteeth-like cesses observed in magnetically confined there was general support for the idea it was variations of the core parameters on impuri­ plasmas have defied a convincing explana­ recognized that the situation In Europe was ties, or at its edge to simulate a divertor con­ tion. The US Transport Task Force (TTF) very different: a simple replica of the US figuration comprising a single island. Inaugurated in 1989 has been a very suc­ organization was inappropriate. Neverthe­ It will also be interesting to explore the cessful In harnessing US resources to deve­ less, stronger collaboration between existing effect of optimized drift orbits on the mecha­ lop understanding. Working groups com­ programmes would enhance European work. nisms leading to turbulent transport. The prising experimentalists, diagnostic experts, A number of topics, mainly those being characteristics of trapped particles in turbu­ theoreticians, and modellers from participat­ treated by existing collaborations, were sug­ lent transport can be changed by operating ing institutes concentrate on specific topics, gested and individuals offered to take the at various ratios of the minimum to the such as the physics of the transition from the lead in building on these collaborations and maximum field strengths (mirror ratio). It is low-confinement mode (L-mode) to the high- focusing their efforts to clarify important confinement mode (H-mode). The groups issues. The readiness of the theorists to join important to note that the resonance of trap­ then report at the TTF annual meeting. This these ‘‘topical groups” was particularly ped particles is avoided in W7-X because focussed and collaborative mode of working encouraging. Interactions with the US TTF diamagnetic fluid flow and trapped particle has already clarified many facts, although would involve Information exchange rather poloidal precession are of opposite sign. many questions remain unanswered — in­ than active participation in joint working The availability of 10 MW of steady-state deed new ones have arisen ! groups. electron cyclotron resonance heating There is also considerable activity within Additional meetings and costs would be (ECRH) will allow the study of plasmas the European Fusion programme addressing avoided and existing conferences and work­ under reactor-relevant, long mean-free path transport issues. The question of enhanced shops should be used to develop collabora­ conditions, and 20 MW pulsed neutral beam co-operation between the various European tions within the topical groups and with mem­ injection will permit a systematic investiga­ groups involved has naturally arisen; formal bers of the US TTF. However, it was envis­ tion of MHD stability and the β-limit. The links between Europe and the US are an­ aged that significant progress would be ECRH current drive can be used to provide other possibility. reported at successors to the Göteborg a toroidal current of 50-100 kA, which will A discussion, led by Friedrich Wagner meeting, namely the annual meetings of the add further experimental flexibility for inves­ (Institute for Plasma Physics - IPP, Garching) US TTF and a new, biannual joint meeting in tigating MHD and confinement issues while and attended by Ben Carreras, head of the Europe (the first will be held in 1996, possibly allowing stellarator physics to be linked to US TTF, at a workshop organized by Jan in Vienna). The IPP Jülich, IPP Garching and the tokamak world. Welland In Göteborg last June considered Chalmers University have volunteered to Europhys. News 26 (1995) 5 organize several European-US workshops. ted the outcome of the Göteborg discussion sections and approximate analytical solutions Jack Connor (Culham) agreed to co-ordi­ to the European Programme Committee last were verified and the estimated energy gain nate the European TTF and maintain con­ September. General approval was given and of 30 agreed with the results of a Monte- tacts with the US TTF, with help from terms of references suggested. The Euro­ Carlo simulation. Francesco Romanelli (Frascati), Jan Weiland pean collaboration must take full account of On the analysis side, the window of oppor­ (Chalmers University) and Friedrich Wagner. the ITER physics and Expert Group activity to tunity for the amplifier approach is narrowing They would coordinate workshops, dissemi­ avoid duplication; appropriate dissemination as the group rediscovers various technologi­ nate information, organize the biannual Euro­ of knowledge must occur; working structures cal, environmental and commercial re­ pean meeting and, jointly with the European must incorporate all EURATOM Fusion Asso­ straints. A practical system delivering 108 to topic coordinators, propose European contri­ ciations as well as JET; additional travel 109 W would operate in a different regime butions to the US meetings. J.W. Connor and costs must be kept low, the organizational where transmutations modify significantly the B. Carreras belong to the Confinement and overhead minimized and activity concentra­ evolution of cascades (thus allowing the Transport ITER Expert Groups so the results ted at the working level. required breeding of a Th fuel with a long of TTF activity can be fed into the ITER The Topic Coordinators are: edge turbu­ burn-up period). Studies have been extended framework to avoid duplication. lence (Hidalgo, Madrid), core transport and from the original graphite and water-cooled The support of laboratory Directors in turbulence (Romanelli, Frascati), physics of systems based on thermal and epithermal Europe is being sought. As it is hoped that short timescales in transport (Cordey, JET), neutrons to systems involving fast neutrons the initiative will harness the undoubted perturbative transport (Jacchia, Milan), and with lead coolant. This allows a higher gain strength of the European Fusion Programme stellarator-tokamak comparisons (Stroth, which is not flux limited, thus avoiding the to solve the grand challenge of understand­ Garching). need for several accelerators. But it also ing the cause of anomalous transport in mag­ means that one is now discussing larger netic confinement devices, F. Wagner repor- J.W. Connor, Culham Laboratory (1 GW), more exotic reactors working closer to criticality. Fuel regeneration has also be­ come more complicated: established pro­ cesses to separate out long-lived actinides need Improving, and economic considera­ tions suggest storing short-lived fission pro­ ducts while concentrating processing on the long-lived, high-mobility varieties. On the accelerator side, the experiment Elegant Demonstration demonstrated that there is no point in going beyond about 1 GeV proton energies. The Successful analysis so far indicates that useful systems may also involve fairly modest currents (=10 mA, roughly an order-of-magnitude less than for the high-intensity linac-based fission schemes being proposed for various applica­ The sub-critical reactor assembly used for demonstra­ tions of transmutation — see EN, August ting the energy amplifier comprises 270 Al-clad bars of 1995, p. 135). So some existing cyclotron de­ natural uranium immersed in demineralised water with signs may be suitable, notably isochronous a neutron spallation source at the centre. The assem­ ring cyclotrons with C-separated magnets of bly is normally used in Madrid for teaching purposes. the type operating at the Paul Schemer Institute. Overall, the economics of the fast- neutron option appear favourable, but a de­ tailed analysis is needed. It is just over a year since C. Rubbia, Proton Synchrotron beam over a two-week Professor Rubbia aims is to remain an ex­ CERN’s former Director-General, made head­ period. Observed temperature increases of perimentalist at heart by seeking support for lines by proposing an energy amplifier — a the order of a mere 0.01 °C (since the device a pilot facility involving a larger reactor nuclear reactor based on an accelerator-dri­ only generated 1 W to minimise radioactive assembly, fuelled this time with thorium, to ven sub-critical assembly with thorium as the waste production) correlated with measure­ once again verify calculations (a proposal breeder fuel. Neutrons produced by spallation ments of fission fragments. Assumed cross­ along these lines is expected shortly). multiply via fission reactions in a fuel-modera­ tor medium to give an overall energy gain. The main advantages, apart from sub-critical operation, lie in: the use of thorium (inexhaus­ tible on the human scale; little plutonium pro­ duction so reduced risk of nuclear prolifera­ tion); the production of much less transuranic actinide waste than conventional nuclear reactors; mainly existing technologies unlike other hybrid systems for energy production. His group found resources to check calcu­ lations experimentally using well-established but nonetheless elegant techniques. The re­ sults were presented in detail by Professor Rubbia at CERN in early December. A spal­ lation target, placed at the centre of a sub-cri­ tical assembly comprising 3.6 tonnes of natu­ A possible fuel cycle for the energy amplifier. Fuel reprocessing is limited by the efficiency of ral uranium encased in aluminium tubes and separating out the actinide fission products. For cost-effectiveness, medium-lived fission immersed in water, was irradiated by a low- products may be stored, with costly processing reserved for long-lived elements. intensity proton beam produced by CERN’s

6 Europhys. News 26 (1995)