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Posted December 20th 2008
Diagnostics Topical Group
It is widely recognized that improvements in diagnostic capabilities are very important for further understanding of the plasma processes and their control. This is the reason why research in diagnostics has to be a continuous process, both by the refinement and improvement of existing techniques and by the development of new concepts.
Diagnostics can be grouped into three major families, depending on their role: measurements required for machine protection; measurements required to infer physical processes and parameters; and measurements for plasma control.
One intrinsic difficulty in the development of a diagnostic research program is the extremely wide range of parameters which need to be measured. Among the most important ones are the plasma current, density and temperature as well as neutron flux. There are almost one hundred physical parameters that are routinely diagnosed in fusion experiments. These parameters can be deduced by detecting fields, particles and radiation coming from the plasma. The plasma emits particles and radiation with a huge variability: for radiation we have microwaves, infrared, visible and ultraviolet light, soft-X rays, hard-X rays and gamma rays; for particles we have D, T, He neutrals, alphas, impurities and neutrons escaping from the core with energies ranging from keV to MeV or low energy electrons or ions at the plasma edge. In addition the plasma is characterized by strong electromagnetic fields. All these can be detected using a wide range of different techniques.
Tony Donné obtained his PhD degree in 1985 in the field of nuclear physics at the Free University of Amsterdam. He then moved to the FOM Institute for Plasma Physics, where he started as post-doc on the Rutherford scattering diagnostic for TEXTOR. In 1986 he became leader of the TORTUR tokamak group and from 1988 till 1998 he headed the diagnostics division of the Rijnhuizen Tokamak Project. In 1997 he moved to TEXTOR in Germany, first as head of the diagnostics group of FOM at TEXTOR and from 2004 as ‘Chef de Mission’ of the FOM-team at TEXTOR. In 2006 he became deputy head of the fusion research department of FOM Rijnhuizen. Tony Donné has been involved in diagnostics for ITER for a long time. During the ITER EDA phase he coordinated the work in Europe in the field of microwave and far infrared diagnostics. In 1999 he became chair of the ITER Experts Working Group on Diagnostics and later from 2001 till July 2008 was chair of the ITPA Topical Group on Diagnostics. Additionally he chaired the JET Diagnostics Expert Group for EP2. Since 2001 he has been a member of the Science and Technology Advisory Committee (STAC). He has published about 140 papers in refereed journals and approximately 250 conference proceedings.
In some cases the same measurement can be exploited for different purposes such as physics studies, machine protection, control. Therefore the diagnostics are operated with different time resolution, spatial resolution, dynamic range, sensitivity, accuracy etc. optimized for different functions. Nowadays we have hundreds of diagnostics at our disposal. The ambitious goal of controlling the fusion plasma therefore benefits from these continuous diagnostic developments.
Marc Beurskens got his PhD at FOM, the Netherlands. He worked from 1999 to 2003 at JET as a long term FOM secondee and as a UKAEA contractor. After spending 2 years again at FOM, Dr Beurskens returned to JET in 2005 and now works for UKAEA. He has experience on JET in a wide variety of roles such as diagnostician and scientific coordinator. Dr Beurskens was closely involved in the development of the new JET high resolution Thomson scattering system and is now also involved in the design of the ITER LIDAR system. He manages to be active in both the areas of diagnostic development (on JET, the EFDA DTG and the ITPA Topical Group on Diagnostics) and in the study of edge pedestal physics on JET and within the ITPA Topical Group on pedestal and edge physics. He is the author of over 90 scientific publications.
For future burning plasma experiments, like ITER, the demands on diagnostic performance become much more stringent. The difficulties of implementing diagnostics on ITER arise mainly from the conditions under which it has to operate, especially the relatively high levels of neutron flux and fluence. New phenomena which are expected to occur in future experiments (radiation induced conductivity, radiation induced absorption and luminescence in optical materials, activation, transmutations…) can change the physical properties of the component/sensor. Other effects, like tritium retention in the materials, can be a safety issue. Burning plasma experiments are extremely demanding also in terms of engineering of the diagnostic systems: integration on the machine, accessibility and maintenance. For example, due to the high neutron fluences some components are difficult to access either for maintenance or repair.
There are many areas in the field of diagnostics that require further R&D: fast detection of MHD instabilities, fusion product measurements, real time diagnostics for plasma control and radiation resistant detectors are only a few examples among the most crucial ones. Developments consistent with the requirements for DEMO (the first electricity producing fusion power plant which will be built after ITER) represent an excellent opportunity to launch an ambitious program on diagnostic research, stimulating a relevant step forward, in terms of new concepts, extremely advanced performances and highly integrated sets of diagnostics. In order to face all the challenges ahead, a great effort will be required to coordinate the research on diagnostics.
To integrate the research activities of the different laboratories and help EFDA in the definition of research guidelines and future work programs, EFDA has created the Diagnostics Topical Group. The Topical Group is led by Tony Donné (chairman), helped by Andrea Murari and Marc Beurskens (deputies).
The topical group activities include both diagnostics developments for further and more detailed physics studies, in order to improve the knowledge basis; and specific developments aiming at performing the required measurements in the difficult environments posed by future fusion devices such as ITER. In particular, as an example of the activities to be carried out in the framework of this Topical Group, the creation of three expert groups on relevant subjects, common to most of the techniques, is foreseen: Methodologies for feedback control, Calibration techniques, Data and error analysis. The first one is to promote the development of integrated control systems, incorporating sensors, hardware, software and actuators, with a particular emphasis on the algorithm, related to more powerful and complex modelisation of the plasma. The second one arises from the necessity to develop accurate and reliable calibrations for many diagnostics, in particular for future burning plasma experiments, and to standardise these techniques. The third group will face the important issue of improving data analysis capability. Different statistical methods will be considered, after a proper evaluation of the error bars and uncertainties related to the measurements.
Heating and Current Drive Topical Group
Magnetically confined plasma devices for fusion research rely more and more on flexible heating systems able not only to heat the plasma and to increase plasma current but also to condition the plasma parameters with the view of achieving safe, high performance and plasma pulses of long if not continuous duration.
Indeed, passing electric current through the plasma generated by induction as in a tokamak alone is not sufficient to achieve the temperatures required for positive energy balance (an ion temperature of ~ 100 million degrees is needed while the ion temperature reached by inductive current is in the range 10 to 20 million degrees). Different methods have been developed to overcome this difficulty. One of these methods, the injection of neutral particles (NBI heating) allowed the Princeton tokamak PLT (USA) to reach an ion temperature of the order of 70-80 million degrees as early as at the beginning of the eighties. Plasma-wave interaction is another way to transfer energy, leading to an increase in temperature and momentum (causing faster rotation) of the plasma. The principle is to launch electromagnetic waves via antennas or wave guides in such a way as to fulfil inside the plasma some plasma wave interaction conditions. The condition could be the matching of the wave frequency with one of the naturally existing frequencies in the plasma, such as the ion cyclotron frequency or the electron cyclotron frequency, which are both related to the particle gyration around the magnetic field lines, but it could be a match with another frequency like the lower hybrid resonance frequency or the Alfvén resonance frequency.
Even if today the performances of the H&CD (Heating and Current Drive) systems suffice to reach high plasma performance, several issues have still to be considered. For example, in contrast to present experimental devices, all the plasma facing equipments, like the antennas, have to be protected from the neutron flux resulting from fusion reactions. The neutron flux will reach values much higher than in the present JET tokamak (UK) while the power flux, which is today less than 15 MW/m2, will reach 24 MW/m2 in ITER and possibly more in the demonstration reactor DEMO.
Dr Andrea Murari received his B.A. degree in applied electronics and M.S. degree in plasma engineering in 1989 and 1991, respectively, from the University of Padua, where he obtained his PhD degree in nuclear power plants from the Faculty of Electrical Engineering in 1993. After experience in private companies (in the fields of laser diodes and vacuum technology), he has mainly worked on measurement techniques and technologies for nuclear fusion experiments. He has installed various diagnostic systems on several European experiments and, between 1998 and 2002, he was responsible for the support to all the diagnostics of the RFX experiment in Padua. Dr Murari is presently the Leader of the Group on JET Diagnostic and CODAS (Control and Data Acquisition Systems) in the JET Close Support Unit. He is the Scientific Coordinator of all JET diagnostic upgrades for the next framework programme and in January 2008 he was nominated co-chair of the EFDA Topical Group on Diagnostics. He has been a member of the Eiroforum thematic working group on measurements since 2003. In August 2008 he was appointed the coordinator of the European delegation to the ITPA Topical Group on Diagnostics.
Also, due to the greater plasma volume in a reactor, the need for power capabilities will also strongly increase. For instance, ITER is designed in its first phase with 20 MW electron cyclotron, 20 MW ion cyclotron and 33 MW negative ion based neutral beam. This has triggered significant R&D on the three heating systems. A main goal in terms of electron wave heating has recently been achieved thanks to the successful new 170 GHz gyrotron. Major difficulties for the ion cyclotron wave heating system and the lower hybrid system for ITER still have to be solved. One is the coupling of the waves with the plasma in the large gap between the antenna and the plasma; another is the stability of the coupling during large magnetohydrodynamical edge events like the Edge Localized Modes which transport large amount of particles and energy. Such problems are well identified and handled with great attention in the EU and elsewhere. In particular, ITER-like antennas are undergoing tests at Tore Supra, France, and JET.
In 2007 EFDA set up physics activities together with the Heating and Current Drive Topical Group (H&CD-TG) to foster the collaborative activities amongst the Associations with the view of solving the most urgent and important tasks required to insure the success of ITER and to prepare the ground for an efficient and cost effective fusion power plant.
Alain Bécoulet is a former student of the Ecole Normale Supérieure in Paris. “Professeur Agrégé” in Physics since 1986 he finished his PhD work in CEA Cadarache in 1990 on the Hamiltonian approach of the wave-particle interaction in tokamak plasmas and its application to ion cyclotron resonant heating. He then took responsibility for the Ion Cyclotron Physics studies in CEA. His next interest was linked to advanced tokamak studies, when he took the leadership of the JET task force on Advanced Scenarios in 2000-2001. He then became leader of the new European Task Force on Integrated Tokamak Modelling between 2003 and 2006, setting up the overall activity in Europe and the necessary connections with the other ITER partners. He took over the Chairmanship of the European Topical Group on Heating and Current Drive in October 2007. He has been leading the Plasma Heating and Confinement division in the”Institut de Recherche sur la Fusion par confinement Magnétique” at CEA Cadarache since 2004.
The duties of the H&CD-TG are multiple, ranging from advising the EFDA leader, to assisting in the resolution of physics and technology issues, in particular in terms of performance and reliability of the H&CD systems. The TG, in close collaboration with Fusion for Energy (F4E), prepares the EU Fusion Community to support ITER and DEMO H&CD physics in the short term but also in the long term as in the case of the lower hybrid current drive system. The TG also works in close collaboration with the EFDA Integrated Tokamak Modelling Task Force in promoting experiments and modelling activities.
The annual activities of the H&CD-TG are proposed by the TG-Chairman, assisted by the Scientific and Technical Board, including representatives of F4E. The Chairman of the group is Alain Bécoulet.
The main activities of the Heating and Current Drive Topical Group for the years 2008 and 2009 can be summarized as follows:
Burning plasmas, that is plasmas in which fusion reactions are taking place, will be simulated experimentally using heating and current drive tools, eventually including helium injection. The objective of this research is to prepare the path for high power steady state plasmas in reactor relevant conditions. Controlling plasma termination is also important in order to avoid electromagnetic forces which are too strong acting on the device structures. Experiments on plasma termination will be performed using radio-frequency waves.
The way the plasma is generated in a tokamak constrains its later evolution. One reason is that the magnetic field flux consumed during plasma start-up cannot be quickly replaced and must therefore be saved to allow for long pulse operation. Coordinated studies will be performed to determine the best combination of H&CD to be used during the early evolution of the plasma.
Wall conditioning, in particular using the ion cyclotron heating scheme, is needed in order to control and eventually modify the composition of the gas trapped in the plasma facing materials. This is particularly important for avoid- ing impurity flow from the walls to the plasma or for avoiding uncontrolled changes in, for example, hydrogen and helium composition of the plasma during its evolution. Experimental work on this subject is in progress.
Specific studies will also be performed to test different techniques such as gas puffing allowing the radio frequency waves propagating from the antennas to be coupled to the plasma when the distance between the antennas and the plasma is as large as will be the case in ITER.
Technology related activities are needed for making the H&CD systems ready for safe operation of the tokamak. In particular, improvement, qualification, and compatibility of arc detection systems, which will be studied, are important for protecting the heating systems against failure. Other safety operation issues will be considered, for example those related to the overheating of the machine structures.
The mission of the H&CD-TG also extends to ITER upgrades and to DEMO, in particular the possibility of using the lower hybrid current drive system in large reactors.
This structure of the H&CD-TG activities coincides with the proposed organization of future EFDA activities in terms of 7 R&D missions that have to be accomplished for a successful approach to a fusion energy source based on magnetic confinement. These missions include, among others, physics activities related to burning plasmas or long pulse and steady operation but also engineering type of activities necessary to achieve reliable tokamak operation.
As can be concluded from the list of activities, the H&CD systems take an important share in the EFDA workprogramme. Indeed, plasma operation always requires one or more H&CD systems: assisted plasma initiation to save magnetic flux, heating or/and current generation all along the plasma pulse in order to help sustain high performance or assisted termination to avoid loss of control.
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