Thirteenth International Conference on Plasma Physics
Washington, D.C., 1-6 October 1990
T. MIYAZIMA
Chairman, Fusion Research Council of Japan
Mr. Chairman, Ladies and Gentlemen,
I want to thank the International Atomic Energy Agency for asking me to give this lecture. It is my great honour to be here and to speak to you who share a strong interest and enthusiasm for developing plasma physics and technology with the common goal of attaining thermonuclear energy for the welfare of mankind.
I think that fusion energy is essentially different, in many respects, from traditional energy sources. Fusion is really a very difficult technology, to capture the 'sun' within a man-made cage.
In this lecture, allow me to consider only magnetic confinement fusion. In this case, we have to keep ultra-high temperature plasmas stationary within a limited volume; but energetic plasma is fairly violent, and we have to know its nature thoroughly before we can confine it well. Even in common internal combustion engines, physical and chemical processes taking place within combustion cylinders are also extremely complicated and rapidly changing in time, and we are far from understanding the detailed mechanism of combustion. Consequently the control of chemical products in exhaust gases, for example, is still not easy. However, the phenomena and the relevant materials are now within our reach and can be investigated experimentally well enough to obtain the data necessary for the design of practical engines.
Now, when we recollect the history of fusion research and development, we first imagined a high temperature plasma to be like a gaseous octopus or jellyfish that has no bones or shell and is difficult to catch, especially when it becomes violent at high temperatures. Our only hope, if there was any, was that the electromagnetic field might possibly play a role in the long range order in a plasma and produce a sort of macroscopic order. Our vague idea was first to arrange the electromagnetic field and other boundary conditions to make the confinement macroscopically stable - this step corresponds to providing a hard shell to confine the octopus. The next step was to improve plasma confinement by means of reducing various losses of plasma particles and energy which were thought to occur owing to certain instabilities. We found that the shells were sometimes not strong enough; for instance, a shell could automatically break or change form. Further new difficulties appeared one after another when we approached the regions of breakeven conditions. We tried to find clues to solve the problems through the combined use of plasma physics, diagnostics and technologies. However, the non-linear behaviour of plasmas was often beyond what we had anticipated and, when we tried to push a projecting head under the shells, several hidden legs were likely to appear.
The realization of nuclear fusion encountered difficulties not only because it is an unprecedented technology but also because it is a qualitatively new attempt to confine the totality of a slippery object in a reasonably quiet and stationary manner. We have overcome many of these difficulties successively. What were the secrets of successful cases of our efforts and how should we learn from them to go ahead to arrive at the final goal? I want to consider these questions with respect to two key areas, namely plasma physics and international collaboration.
I hope you will forgive me for basing my observations frequently on research made in Japan or by Japanese researchers. This is only because I have many acquaintances in Japan who are willing to tell me their research stories.
I think you will immediately remember Academician Artsimovich for his outstanding contribution in these two key areas. In 1968, at the IAEA international conference held at Novosibirsk in the USSR, remarkable results were reported indicating that electron temperatures up to 2 keV as estimated by diamagnetic measurements had been obtained in the tokamak T-3. The T-3 programme had been under the strong leadership of Academician Artsimovich. In response to his call, the Culham Laboratory sent the 'Culham Mission', consisting of Dr. Peacock and his group, to confirm the results by using the newly developed laser scattering diagnostics. In this way, the T-3 results confirmed by this new technique received worldwide recognition. This is a good example of success attained through scientists' enthusiasm and by international collaboration. You know, international collaboration was rather exceptional in those days. It is worth noting that international collaboration has been very effective in increasing the reliability of research results, and the 1968 IAEA conference provided a chance to promote international collaboration.
Secondly, let me tell you about another, more recent example. It is the case of efforts towards achieving higher beta in tokamaks. Computational codes of MHD theory were developed in many countries, each in an original way; they were compared with one another and gradually a common understanding was achieved. Around 1980, JAERI's small tokamak JFT-2, with a circular plasma cross-section, gave a value of beta of about 3 % for the first time.
Theory also predicted that the upper bound of beta could be much increased when plasma cross-sections were elongated. This prediction was beautifully verified by the DIII-D experiment with US-Japanese collaboration. The values of beta were about 5% in 1982 and have recently been above 9% in DIII-D, with a D shaped cross section. This is also a story of success in which physics researchers started from personal, original ideas and competitive research work, followed by exchange of information among them, leading to an international common understanding, and finally to major results obtained by joint experiments through international collaboration. I think this is a model of productive R&D.;
Thirdly, I want to tell you how physics was useful in finding reliable linkages among results obtained by facilities of different sizes and types. Let us observe, as an example, the case of current drive, which is thought to be one of the most useful means to attain stationary tokamaks.
In 1966, Dr. Shoichi Yoshikawa and others at the Princeton Plasma Physics Laboratory showed the possibility of driving current by means of electromagnetic wave injection using the C-stellarator. After preliminary studies, the theory of current drive by means of lower hybrid waves was developed and experiments started in tokamaks in many laboratories, namely WT-2, JFT-2, JIPP-II and many other devices throughout the world. A recent result in this area is the realization of large current (2 MA) and long pulse drive (over one hour) at JT-60 and TRIAM-IM (Kyushu University), respectively.
Current drive by means of neutral beam injection also began with theoretical work by Dr. Tihiro Ohkawa (General Atomics), and experiments were performed in JET and TFTR, among others. Current drive for 2.5 s was demonstrated using only NBI drive in the DIII-D facility. Driving efficiency was also studied extensively and was shown to agree with theoretical estimates. In this way, you see how various ideas of current drive were conceived and proved using many tokamaks.
Fourthly, let me consider how the interaction of physics and technology is important. The success of lower hybrid current drive experiments in TRIAM-1 M was brought about by a clever combination of technical and physical ideas. In this case, plasma position was determined using real time sensing, and was controlled and appropriately swept in order to disperse heat load to the plasma facing wall. In this way experimenters succeeded in maintaining a quiet and pure plasma for more than one hour by relatively small (30 kA) current driven by LH waves. Thus we see that progress in plasma physics will reduce the technical burden imposed upon facilities and vice versa.
Now, although plasma physics has made remarkable progress, it is not yet so powerful that the design of plasmas of next phase facilities or reactors can be done relying only upon physics without extrapolation of empirical scaling laws. In order, however, to make design really reliable, it is clearly desirable that the design be done on the basis of natural laws as far as possible. However, the situation is not so easy. First, since many degrees of freedom are still alive in high temperature plasmas, even the most advanced diagnostics are not adequate to measure the physical quantities of plasma with sufficient spatial and temporal precision to completely characterize the plasma. Secondly, although computational physics has made tremendous progress, the plasma is non-linear in principle and we cannot solve the basic equations exactly. What we can do is to find solutions on a case by case basis by giving the physical quantities (e.g. initial and boundary conditions) necessary to characterize the plasma under consideration.
Such being the case, what we have to do from the physics side may be, among other things, to uncover and clarify important factors hidden behind various plasma phenomena, say, self-organization, for example pinches or bootstrap current, disruption, etc., by using the method of analysis and simulation. Most important in this case is the power of insight to point out the most essential factors hidden behind the phenomena. Such insight is originally based upon personal intuition, but it can be polished by training face to face with reality. In this sense, mutual contact and collaboration among theoretical and experimental physicists and engineers are essential.
Central problems to be attacked by this kind of collaboration are as follows: the characterization of H mode and other improved modes, elucidation of various kinds of anomalous transport, further understanding of plasma-wall interaction, establishment of a method of particle and heat removal from the reactor core, etc., and finally one of the most fundamental problems of plasma physics - long pulse stationary operation with good confinement.
I think this is just the occasion to refer to the present activities of the epoch making international collaboration, ITER. Everybody concerned recognizes the important progress made in the confinement database, namely the construction of an internationally evaluated database and efforts to improve it by new experiments and analyses done at various research institutions. Moreover, strong correlations have become evident among a number of variables - confinement, divertor, density limit, beta limit, etc. - each of which had been studied separately. Significant progress has been made and effective procedures established to examine overall consistency. I would like here to stress the importance of individual, original ideas, enthusiasm, collaboration and a consistent physical and engineering approach; these are indispensable to attaining a reliable design and possible future construction.
Finally I want to consider what policy is needed in order to succeed in attaining the goal of fusion energy.
Firstly, independent tests and verifications are extremely important in research work, and no results are believable unless similar results can be reproduced in analogous facilities. You know well that even the existence of four large tokamaks - JET, JT-60, TFTR and T-15 - was not always enough for the needs of ITER design work. How to meet this need for a diversity of machines will be a very important question if we pursue the line of a single large international machine.
Secondly, the question of flexibility is also serious and cannot be bypassed. Namely, when we follow the main line of a standard tokamak project like ITER, I think we have to encourage the birth of various new ideas and always be prepared to respond to them; for instance, the idea of a stationary reactor based upon positive use of bootstrap current, of a microscopically more stable tokamak with low aspect ratio, or of a non-tokamak reactor, which might become more desirable for commercial purposes, and so on. Thus we face a serious problem of how to divide limited human and material resources in a balanced way among a variety of research items. This problem needs to be discussed and a solution has to be found that is understandable to all the people concerned.
Next, I want to discuss the importance of different points of view. You know that the, free activity of researchers is essential in producing new ideas which might have an epoch making effect on the development of science and technology. Therefore, when many people always work together towards the construction of a single large machine, we have to take special care not to kill a free and critical atmosphere.
I would like to remark that, generally speaking, joint international work towards the common goals of mankind gives us a strong belief in its being one of the surest pathways to attaining global understanding. I have personally shared, as a co-chairperson of the US-Japan Co-ordinating Committee, in the important experience of continued collaboration between the USA and Japan in the past twelve years. The collaboration has been successful, as you see, for instance, from the good results of Doublet; but our path has never been easy. Besides the benefit of cost and risk sharing, we shall obtain a precious treasure called mutual understanding, which only arrives after patient efforts to overcome such cultural sources of friction as differences in budgetary systems, industry participation, planning ideas and the innumerable trifles of daily life. I am sure people working together on ITER also share the same impression.
When the mutual understanding obtained by such international collaborative programmes as ITER spreads in many other fields, for example global environmental studies, west to east and north to south, I can truly hope we shall find a glorious entrance to a peaceful world.
