A tokamak is type of a practical fusion reactor that uses a magnetic field to hold plasma in the shape of a torus. Research has been conducted since the mid -50s of the last century, but it has still not been possible to carry out economically profitable thermonucle ar fusion. The fundamental reason for the failure is the completely wrong approach to creating these reactors : in fact, the scientists arbitrarily choose the shape, cross -section, and relative sizes of a vacuum vessel. To implement cost -effective controlle d fusion, it is necessary to change the shape and proportions of the vacuum vessel of tokamaks. Keywords: tokamak; vacuum vessel; plasma; ring current ; magnetic field lines The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore progress depends on the unreasonable man. Bernard Shaw 1. Introduction At very low temperatures, close to absolute zero, all substances are in a solid state. Heating causes a substance to transition from solid to liquid, and then to gas. At sufficiently high temperatures, gas ionization begins due to collisions of very fast moving atoms or molecules. The substance passes into a new state called plasma – a partially or fully ionized gas. A tokamak is type of practical fusion reactor that uses a magnetic field to contain the plasma . Experiments in the tokamaks aim to reproduce the processes occurring in stars, namely, the transformation of the nuclei of two isotopes of hydrogen - deuterium and tritium - into helium nuclei, with the emission of neutrons and the release of enormous energy. The central part of the tokamak is a vacuum vessel inside which hydrogen is converted into plasma using a strong electric current (passed through an external winding) and other external heaters. This vessel has the shape of a toroid (a hollow "donut") and the cross -section of circular shape or form of the letter «D». To capture high -energy neutrons, there is a special wall (blanket) inside the vacuum chamber. However, due to plasma instability, to implement the economically viable fusion is still not possible. Therefore, it is necessary to understand the fundamental reason for failure. 2. Discussion Plasma is an electrical conductor, so current passing through the electromagnetic winding causes a ring current in the plasma [1]. As is known , when current flows along a closed loop, the magnetic field lines take a form very similar to the curves of constant width , as shown in Figure. Figure. Magnetic line s of the ring current In general, these magnetic lines are a set of closed contours of decreasing sizes, placed one inside the other (like nesting dolls). 2 One closed contour from this set is particularly interesting (and is highlighted in bold) because we can easily determine that it has a relatively small inner radius r, about an orde r of magnitude smaller than its external radius R: r/R ~ 10–1. For other magnetic lines located outside (one such closed curve is shown in the figure), the inner radius slowly decreases and the outer radius quickly increases, i.e. the r/R ratio decrea ses rapidly. For other magnetic lines located inside the selected (bold) contour, on the contrary, the inner radius slowly increases and the outer radius quickly decreases, i.e. the r/R ratio increases rapidly. The width (R – r) of the closed magnetic lines decreases and their shape approaches to the shape of a perfect circle. Consequently, the smallest closed contour has the shape of a perfect circle, whose value r is only slightly less than R, i.e. r/R ~ 0.99 (or even greater) . This means that the radius ( ro) of this circle is very small compared to the value of r: ro << r. For example, if r = 0.99 R, then the circle radius ro = (R – r)/2 = 0.01 R/2 = 0.005 R, the ratio r/ro = 198 or r/ro ~ 2×102, i.e. the value of r is approximately two orders of magnitude greater than the radius of a circle. The first tokamaks also had a vacuum vessel with a circular cross -section. In the early 1970s, it was proposed to change the chamber cross -section s o that it was shaped like the letter «D». Several D -shaped plasma machines have been built, for example, the Joint European Torus (JET) in the UK. The D -shape demonstrated an advantage over the normal round section and now this arrangement is considered un iversal. Decades of fusion research have shown that the larger the reactor, the better its indicators. Therefore, a trend has emerged to build ever larger and more powerful installations. Currently, high hopes are placed on the International Thermo nuclear Experimental Reactor (ITER), which is being built in southern of France. ITER will have a D -shaped vacuum vessel of enormous dimensions: its minor radius 2 m, the major radius 6.2 m (the ratio r/R ~ 0.3), its height 11.3 m and is approximately 1.7 times greater than its width. In addition, the inside part of the D -shaped cro ss- section is a straight line [2]. Thus, the generally adopted D -shape is completely inconsistent wi th the shape of the magnetic line (with same ratio r/R) which, as we said, is practically a curve of constant width. Since ITER (in the sense of the vessel shape) is not fundamentally different from other machines with a D -shaped plasma (e.g., JET), then most likely it will not be able to give outstanding results either. As for tokamaks with a circular cross -section (its radius should be written as ro), they could not provide a positive energy output. The reason is that the proportions of their vacuum chamber even more (one might say catastrophically) do not correspond to the relative sizes of the magnetic line in the shape of an ideal circle. For example, these reactors have the ratio r/R ~ 0.1÷0.5 [3], the ratio of the hole radius to the chamber radius is r/ro = 0.2÷2. A magnetic line in the shape of a perfect circle, as we previously showed, has the ratio r/R ~ 0.99 , and the ratio r/ro ~ 2×102. From a practical point of view, the difference is simply huge. This means that the O -shaped vacuum chamber must have the hole radius of at least 200 times the radius of its cross -section. For example, if the section radius ro = 10 cm, then the hole radius r must be at least 20 m. However, there is no need to build the O -shaped tokamaks. 3. Conclusion Generally speaking, the situation with tokamaks looks absurd: - first, the scientists design a vacuum vessel the cross -section of which does not correspond to the shape of the ma gnetic line of the ring current (i.e., in fact, they arbitrarily choose the shape, cross - section, and relative sizes of the vessel); - then, when a tokamak is operating, with the help of external magnetic fields they try to force the magnetic lines not to touch the inner surface (blanket) of the chamber (i.e. they try to shape these magnetic lines the cross -sectional shape of the chamber). To understand what has been said, let’s give an example from ordinary life. When we buy, for example, a hat, we c hoose a thing of the appropriate size . But we’re not trying to change the size of head to fit the first specimen we see. 3 Namely, a similar situation occurs with tokamaks. Using B. Shaw’s phrase, the scientists are trying to adapt the world (nature) to themselves. Naturally, the question arises: how to adapt to nature in this case? The answer is simple. To do this, it is need to fundamentally change the approac h to creating the tokamaks: - first, a specific magnetic line of the ring current is selected and its exact shape and relative dimensions are determined; - then, the sizes of a vacuum chamber are determined, the cross -section of which exactly matches the shape of the selected magnetic line; only after this the actual design begins. If the specified requirement is met, then during operation of an ideal tokamak the situation will be as follows: - the magnetic lines will not touch the inner surface (blan ket) of the vacuum vessel; - the plasma particles (electrons and ions) moving along the magnetic lines will not collide with the inner surface of the vessel, losing their energy. As a result, the current -carrying ring will be stable (possibly even wit hout the use of additional stabilizing magnetic fields). And this is a necessary condition for the relatively quiet occurrence of a fusion reaction with a large energy release. Thus, to carry out an economically profitable controlled f usion , it is nece ssary to change the shape and proportions of the vacuum vessel of tokamaks. If this change is carried out , then we can hope that in foreseeable future thermonuclear energy will be obtained on an industrial scale. Competing Interests The author declare s no competing financial interests . Funding The article had no funding.