Naka Fusion Institute

 

 

By: M. Amin Rahimian

 

The first factory study tour of the new semester took place on April the first, a cloudy but otherwise splendid spring day. With Sakuras shining in their utmost beauty, we cherished our chances when it came to observing the cherry blossom trees, which appeared every now and then, springing up out of the riverside parks in the Odaiba region and around Tokyo Bay, as the bus rode across the high rises, shopping malls, and office buildings of the central Tokyo. Tokyo's urban scenery however, was soon replaced with the natural beauty of the peaceful countryside and the farmlands, which connect Tokyo-to to the Ibaraki prefecture, producing the overall effect of a pleasant if not a bit long, journey that lasted for about three hours.

The first disruptions to the harmony of the rural sceneries emerged as we entered the Hitachi-naka city perhaps best known for its northern nearby town of Hitachi which is the founding place for one of Japan's leading integrated companies, also named after the city. A quarter or so later we went pass the Tokai Nuclear Power plant, of which we could only see a tall chimney, sending out a white fume high into the sky. Located at the Tokai-mura city, it is said to be Japan's first nuclear power plant, which is still operating after being modernized. With Hirose-sensei, explaining about the power plant as we circled around the high chimney the atmosphere of the visit has already started to take a pretty nuclear color to it. This way, we headed toward the Naka Fusion Institute of the Atomic Energy Agency of Japan, a world-class research center that is determined to pave the way for supplying the "Future Energy of the Earth".

A few minutes later, as I tossed aside the March 2009 issue of IEEE spectrum magazine which as if by good chance had a couple of articles on the present status of the worldwide fusion research, we entered the Naka Fusion Institute. Amongst those articles, of particular interest to me was the news of the opening (seven years after the originally intended date and at a cost almost three-times that of the planed) of National Ignition Facility in California*, which apart from sharing the same initials as well as the worn out cause of "Endless Carbon-Free Power", did not have much else in common with the Naka Fusion Institute. Unlike its Japanese counterpart, the American facility utilizes neither Neutral Beam Injection (NBI) system, nor Radio frequency heating system. Instead it exploits a combination of 192 of the world's strongest lasers to produce the extremely high temperatures necessary for the ignition of the fusion reaction**. Ironically enough, the next article talked about how easily one can actually generate a small-scale fusion reaction, with the experimental facilities set up at a home laboratory***. This way one might be able to sustain a fusion reaction for matters of a fraction of a second, a far less (?) impressive record of course compared to the institute's record of 65 seconds of continuous fusion reaction; however, for an equally slimmer amount of investment in terms of both time and budget.

The bus came to a halt close to a curious metallic statue, which we soon found out to be a section of the vacuum vessel, comprising the fusion reactor.

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Following our arrival at the institute we were seated in a hall, which we soon left to have lunch at the institute's cafeteria. After the lunch we went back to the conference room to listen to a half-hour lecture and presentation briefing us on the Naka Fusion Institute and that day's visit. The presentation also elaborated on the general aspects of the fusion research as well as what was under way in Japan and around the world as part of the ITER (International Thermonuclear Experimental Reactor) project, since its inauguration in 2006.

The basic idea of the fusion reaction, as was explained throughout the lecture, is to have atoms with light nuclei combine, resulting in an atom with a heavier nucleus and fast (high energy) Neutrons, which carry the energy output, resulted from the reaction. The classical example of the fusion reaction happens when a Deuterium and Tritium both isotopes of the natural Hydrogen combine to produce Helium (which we refer to as "ash", highlighting its undesirable nature, generated as a side product of the fusion reaction) and a fast Neutron:

13T + 12D => 24He + 01n (1)

The treatment of Tritium however, necessitates the observation of certain precautionary standards, which makes it unattractive, when it comes to research and experimental activities. Hence, with Deuterium being available in ample amounts from the seawater, the following reaction may instead be utilized:

12D + 12D => 23He + 01n (2)

Nonetheless, due to its energy superiority, the reaction in (1) is the preferred choice for commercial implementations.

To have the atoms combine in either of the ways described above, we should first devoid the nucleus of the electrons which surround it. In other words, the matter should be in the plasma form, which can only be realized at very high temperatures where the atoms are in an agitated state. Moreover, the naked nuclei should be made to collide with each other, which again is challenging, since normally two nuclei having the same electrical charge will repel each other. The collision (Ignition of the fusion reaction) therefore can only happen at extremely high temperatures with the particle speeds surpassing 1000km/s, enough to overcome the repulsion force between equally charged nuclei.

According to what was mentioned, the fundamental challenges faced by the fusion technology include the production and sustainment of an extremely high temperature (100million oC for D+D and 12 million for D+T) necessary to initiate and sustain the fusion reaction as well as containment, support and control of the fusion fuel, which for years has lightened the stars in the sky, and _one day_ may as well light up our houses.

The cornerstone of the facilities employed at the Naka Fusion Institute is the so-called large tokamak device dubbed JT-60. JT-60 which actually stands for Japan Torus, refereeing to its doughnuts like figure, basically operates as the fusion reactor, containing, controlling and supporting the high temperature plasma, fundamental to the fusion reaction.

The device's structure is comprised of three major units: the vacuum vessel, the diverter and the blanket. The first of the three, being a 15m high doughnuts-shaped chamber with an external diameter of about 30m, is the basic skeleton which holds the structure where the fusion reaction is going to take place. By the same token, the vacuum vessel is expected to support heavy components, which constitute the JT-60 structure as a whole, and needs to endure high magnetic forces and pressure during its operation. Another equally significant respect of the vacuum vessel's function is the provision of space-level vacuum within the reactor. This is important not only to ensure the thermal separation between the extremely hot plasma and the vessel's body, but also to prevent any external impurities finding their way into the plasma fuel mixture. The thermal separation provided by the vacuum is crucial since at the high temperatures where the fusion reaction occurs, no solid structure can tolerate a physical contact with the ignited plasma fuel. As such the plasma is always contained at the center of the vacuum vessel using the flow of electrically induced high magnetic fields, which keep the charged plasma particles within the center of the vessel and is used to control the plasma flow. In addition to the magnetic confinement, the internal surface of the vacuum vessel is also covered by carbon tiles (same as those used in the space shuttles), which protect the vacuum vessel from the high temperature plasma.

The primary task of the diverter on the other hand, is to remove the Helium ash from the plasma mixture. The resultant Helium if otherwise left untouched will in time cause the fusion reaction to cease since not only it provides a way for the heat to escape, but also decreases the plasma density. The latter, which is defined in terms of the number of particles per cubic centimeters of the plasma fuel, is a major factor in the Lawson fusion Law, according to which a particular relation should be satisfied between the cooling-time (energy confinement time), plasma density and its temperature, thus setting the conditions for the so-called Break-even, and Self-ignition Conditions. Accordingly, the Helium impurity that is a side product of the fusion process can pose a serious hurdle to the continued ignition of the plasma fuel, and as such should be removed by the diverter.

Last but not least is the blanket, which, covering most of the vacuum vessel's internal surface except for the bottom side where the diverter lies, has three major functionalities. Namely, shielding, heat generation and fusion fuel breeding. In other words, as the fast neutrons carrying the output energy of the fusion reaction are shielded and slowed down following their collision with the blanket, they transfer their energy to it.

The transferred energy in this stage is in the thermal form, which can then be used to heat up a boiler. The boiler's output will in turn operate a steam turbine, thus generating electricity. Moreover, as the fast neutrons collide with the blanket containing Lithium, Tritium is produced according to the bellow equation:

01n + 37Li => 13T + 24He + 01n (3)

This way through exploitation of the blanket technology we can supply the fuel of tritium for the fusion reaction, a process known as breeding.

Following the introductory lecture, we were divided into two groups A and B, and set for the tour, starting from the famous JT-60. The first of the facilities to be visited was the central control room, a large room filled with computer screens, various gages, measurement devices, indicators, and the like. The room also was equipped with a large LCD screen on one of the walls, the principal usage of which was for visual observation of the vacuum vessel.

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Invisible however, as the plasma is, we could only see its computed shape, which had been derived using computer programs. Even in the computed form though, the shape of the plasma was constantly altering, alluding to its unstable nature. This instability poses a further challenge when it comes to the control of the plasma flow, in which the roles played by the plasma shape, as well as its current and position, are of essential significance. The principal tool used for the control of plasma flow is the neutral beam terminal, which as stated in the beginning paragraphs of this article, is one of the two major devices used for the heating of plasma in JT-60. The other heating system, Radiofrequency heating, we were told it comes in two major types, using either 2.4GHz or 130 GHz Frequency bands. The RF heating provides us with the possibility of a diagonal alignment, which would come into contact with a greater proportion of the plasma as it is being heated. Most of the presented material, I should say were based on the records of previous experiments as the facility itself had been shutdown since last August, because it was being prepared for replacement with a newer version of the large tokamak device, the so-called JT-60SA (JT-60 Super Advanced). The principal improvement brought about to the super advanced version stems from the substitution of super conducting material for the copper in the magnetic field coils. The excessive heat generated as a result of the high current passing through the magnetic field coils, is the principal factor, limiting the maximum length of time for which we can sustain ignited plasma.

Finished with the central control room next on the line, was the actual site of the JT-60, which we visited while being briefed on some further details of the operation of the tokamak device. There, we were also told bout institute's world records in achieving an energy multiplication factor of 1.25 in break-even condition. The energy multiplication factor which basically denotes the ratio between the output and input energy for an ignited plasma is a major function of the plasma cooling-time (confinement-time), since an increase in the cooling time translates into lesser needs for the continued heating of the fuel mixture to compensate for the escaped heat, which in-turn will increase the output-input ratio at the ignition state. As the size of facilities increase, tending toward more commercially viable prototypes, the issue of cooling time and heat confinement, will be easier to manage. This is due to the fact that as the size of the ignited plasma increases, cutting the volume-surface ratio, the heat, which can only escape through the plasma surface will be better confined.

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Next we visited JT-60's power and electrical supply facilities, which provided the necessary power for the operation of the rest of the plant. It is worth noting that despite the energy multiplication factor measurements, the output energy at the present stage of the research is not used for power generation. The visited facilities included 3 cubical motor-generator units, as well as the motor generator with flywheel for plasma heating system, used at particle accelerator in the neutral beam injection system. Moreover, since the tokamak JT-60 device is DC operated a whole set of equipment has also been devoted to the DC/AC conversion, coaxial DC feeders, as well as silver and aluminum alloy bars used for DC power transmission. The DC current at transmission lines is measured using circular rings made up of semiconductor material that were placed around the DC transmission lines and are able to sense the resultant magnetic field as current passes through the line. This way these semiconductor rings have been able to measure DC currents as large as 120kA.

As already suggested by the transmission from JT-60 to JT-60 SA, a key technology for the future of the fusion research is said to be the super conducting magnet, which through exploitation of superconductor technology, can overcome the heating problem of the copper magnetic field coils, thus eliminating a major obstacle in sustaining the continued ignition of the plasma fuel. The copper coils which are currently employed in the JT-60 device, include a central coil for plasma ignition and a pair of poloidal and toroidal magnetic field coils used for the plasma flow control. The substitution of super conductors for the copper in the coils will not only decrease the generated heat as the high currents pass through the coils but will also allow us to operate the facilities with a much lower power. This however, will need the deployment of a cryogenic system, which should operate at low temperatures, necessary in order for the super conducting material to function.

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The activities at the ITER Superconducting Magnet Technology Group, which was our next destination following the tour of JT-60, were aimed at demonstrating the feasibility of such a super-conductive cryogenic system for future fusion reactors. To this end, the super-conducting core will be cooled by a flow of liquid Helium. Furthermore, extensive research and development activities should be performed to arrive at the suitable structure for the super conducting material. Some of the materials, for instance, have been produced after one month of continues heating at 360oC. Moreover, special care should be paid at the quality of the ground insulation, and any scratch or insulation breakdown must be avoided. In this regard the techniques developed in the jacketing facilities in Kyushu, as well as the production of ITER's CS (Central Solenoid) insert coil in Japan were pinpointed. It was further highlighted that due to the less than a few millimeters gap between the inner and outer modules no sensors can be installed within the super conducting coil structures. Finally, several samples of the delicately produced coil structures were demonstrated.

The last of the facilities to be visited on that day was the Blanket technology and ITER Tokamak Device Group. As already pointed out, being responsible for shielding, heat generation as well as fusion fuel breeding, blanket is one of the crucial sections in the structure of the Tokamak device. Its diverse range of functionalities necessitates a detailed design of both the physical structure as well as the comprising materials. In particular the blanket structure should be able to withstand heavy radiation and the resultant heat. Consequently, a complex combination of beryllium ceramics, mass of stainless steel, in addition to air or water operated cooling channels have been made use of. Another rather interesting challenge faced by the blanket sector is the maintenance, installment and remote speculation of the blanket tiles in the very high gamma wave environment inside the vacuum vessel. To this end, an ingenious remotely controlled system of advanced robotics in combination with rail paths and complex mechanical schemes, which will facilitate the deployment, transportation, repair and re-installment of the gigantic robotic arms, as well as the heavy blanket tiles, has been devised. The visit was concluded by a demonstration of the movements of the robotic arm, which using its 6 degrees of freedom in movement and supported by the circular rail path, could reach for any particular blanket box inside the vacuum vessel.

In the same way as the previous FSTs, this factory study tour was also concluded with a Q&A session, followed by a group photo with the metallic curvature, which was the first thing to capture our attention upon arrival at the institute. By then however, the statue though not yet totally devoid of the initial sense of peculiarity that it conveyed to its viewers, was known to us all, both in terms of its structure and building materials as well as the role it played in JT-60. Most of the Q&A session was devoted to discussions, which further elaborated upon the technical details of the operation and structure of the materials and devices employed at the fusion research institute. A particular turn of the discussion, which was of particular interest to me, however took place when the guides were asked to comment on how much of the research in the fusion institute can be said to be about physics rather than engineering citing the specific example of LHC where another set of facilities perhaps on a much larger scale, are employed for the sole purpose of unraveling the mysteries behind the structure of matter, and thus humbly refraining from pinpointing any direct implications that the research might actually have, as it probably will, on our day-to-day life. The answer, which highlighted the essential contributions made to the developments of plasma physics research as well as other advanced technologies, shed light on a crucial though less emphasized aspect, regarding the nature of fusion research at it's the present stage.

To be specific, it is worth noting that the scientific beauty of fusion research is indeed undeniable if not awe-inspiring and nobody can argue against the positive effects that such scientific improvements, even in their purest forms, can have on our lives. However, to think of fusion as a reliable solution for tomorrow's energy dilemmas, involves an unjustified amount of scientific optimism, a rather unwise practice at a time when an approaching energy crisis calls for immediate and definite response. Based on what was mentioned as far as the energy crisis is concerned our best hopes seem to be the renewables (wind, solar, and geothermal), most of which have already passed their technical challenges, starting to evolve as mature reliable resources, well-worthy of our slightest if any at all, glimpses of hope.

To recapitulate, with the prospects of a global energy crisis just on the horizon and the grave ramifications that such a crisis can bring about it does not seem wise for us to focus our hopes and more importantly investment policies on a scientific theory, whose success even with a great amount of scientific optimism can only start to affect our lives in a matter of a century or so. Indeed, a day will come when we will have fed all our gas and oil recourses to the insatiable industrial machines and when it does come, which should not be too far from now, in all likeliness there would not be any fusion-based electricity generator to come to our rescue, but what will be and to some extent are already there, are the wind turbines, photocells and geothermal reactors along with all other renewable energy recourses. It is best to wrap up this rather offbeat discussion in the same way as the late William E. Parkins did. In an article titled "Fusion Power: Will it ever come?" in March 2006 issue of Science magazine****, this prominent man of science concluded that, it is time to sell fusion for physics [i.e. science], not power, and indeed if fusion is ever going to bring us any power it would only be after those scientific developments that one might hope fusion research can bring about, but until then it is only unwise to put any credit beyond that of other scientific theories, into fusion.

Notes:

*Fusion Factory Starts Up:
http://spectrum.ieee.org/energy/nuclear/fusion-factory-starts-up

**NiF does not seem to be doing as bad as the previous fusion attempts:
http://news.bbc.co.uk/2/hi/science/nature/8485669.stm?lsf

***Fusion on a Budget:
http://spectrum.ieee.org/energy/nuclear/fusion-on-a-budget

****Fusion will it ever come:
http://sciencemag.org/cgi/content/summary/311/5766/1380

*****ITER, faltering all the way down:
http://www.bbc.co.uk/news/science-environment-16170550
http://news.bbc.co.uk/2/hi/science/nature/8103557.stm