2013 Nuclear Fusion Prize Acceptance Speech

[摘要] I would like to express gratitude to the IAEA, the journalNuclear Fusionand its board for this acknowledgement of work carried out at the MIT Alcator C-Mod tokamak. I must begin by making it clear that this is in no way an award to an individual.The experiments, data analysis and paper were a true collaborative effort from the C-Mod team. It is a honor to work with them and to accept the award on their behalf. I would also like to thank the US Department of Energy for their support in funding this research.The paper describes the exploration of the 'improved' confinement regime dubbed 'I-mode'. The distinguishing feature of this operational mode is a robust boundary pedestal in temperature with the somewhat surprising lack of any form of density pedestal. Thus the regime exhibits an enhanced energy confinement similar to H-mode, roughly double of L-mode at fixed input power, yet has global fuel and impurity particle transport of L-mode. These features are intriguing from a scientific and practical point of view. On the science side it is extremely useful to obtain such a clear demarcation between the energy and particle transport. For example, soon after its discovery, the I-mode was used to extract the observation that the edge T pedestal is the strongest determinant for intrinsic rotation in work by John Rice, Pat Diamond and colleagues. Recent results regarding core transport by Anne White, Nate Howard and colleagues show that I-mode has intriguing properties with respect to core response of fluctuations and profile stiffness. Mike Churchill's recent Ph. D study on C-Mod shows that I-mode exhibits no strong poloidal impurity asymmetry, unlike H-mode. The I-mode posed an interesting test for the peeling-ballooning-KBM model of the pedestal, the subject of the 2014Nuclear Fusionaward of Phil Snyder, and was examined by John Walk and Jerry Hughes showing that in fact the lack of the density pedestal pushed the I-mode far away from the P-B limit, and thus the limiting transport/stability feature of the I-mode was in a sense self-enforcing for keeping the regime free of ELMs. Also intriguing is that the I-mode exhibits global energy confinement scaling with a very weak power degradation, presumably this arises from a temperature pedestal which is not regulated by stability, it seems I-mode provides fertile ground for studying basic plasma phenomena.From a pragmatic point of view I-mode has now been obtained and studied in ASDEX-Upgrade and DIII-0D, as reported by Amanda Hubbard at this conference. There are interesting similarities to the C-Mod observations, such as a Te pedestal without a density pedestal, but also differences which are being sorted through in details of the edge fluctuations and the operational window to access I-mode. The I-mode is 'found' at power levels between L- and H, and thus suppressing the H-mode transition is a key aspect to maintaining I-mode. This is done basically by operating with the ion grad-B drift pointed away from the primary X-point.This is one of the reasons why intrinsically stationary regimes like I-mode, and others like QH-mode,are an attractive option for burning plasma scenarios without the need for ELM, if accessible and maintainable this requires no additional hardware since the pedestal self-regulates.In addition I-mode is highly attractive because of the L-mode particle confinement: the turbulence-dominated particle transport greatly eases both fuelling requirements and impurity control. Indeed I-mode is highly successful on the all high-Z wall of C-Mod. And in a burning plasma the control of the operating point is primarily through density control, thus one foresees that burn control through densification is very powerful and promising.Recent work has suggested that the power requirement to access I-mode has rather weak B dependence. This may explain why the regime has a relatively wide operating window in the high-field C-Mod and I-mode may be highly applicable to high B ITER and reactors. So while it is relatively early after this paper and the I-mode discovery, we expect continued interesting work in this area.I would also like to point out another feature of theNuclear Fusionpaper from 2010. Approximately one third of the co-authors were students at the time when the paper was written. Indeed, it is unlikely that I-mode would have been discovered without students. A student, Rachael McDermott, was seeking to slow down the confinement transition time in order to capture it with her new charge-exchange spectroscopy diagnostic. The attempt was a 'crazy idea' to use the 'unfavorable' grad-B drift with a very careful set of small power steps just below the H-mode transition. It was in such steps that the I-mode was found and then later expanded to a much wider operating window once it was realized how attractive the I-mode appeared. I believe the fusion community can take two lessons from this.First, it is vital that we continue to support the education of young scientists. Our investments in new devices are for naught if we do not have an extremely talented and trained new generation coming behind us. And to do that means we need to assure student are integrated with access to leading facilities like C-Mod where one third of the session leaders are students. Secondly, and related, small, capable and versatile fusion experiments are both highly appropriate to meet the education mission and to push forward the fusion science because of their ability to take on risk and try new ideas, and to explore unique, but relevant, parts of parameter space such as high magnet field fusion. I urge that we continue to support such facilities in the international fusion portfolio.Thank you again on behalf of the co-authors and the C-Mod team.