[en] A novel tokamak with a new type of toroidal field (TF) coils and a central solenoid (CS) whose stress is much reduced to a theoretical limit determined by the virial theorem is designed and constructed. According to the virial theorem, the best TF coil to produce the strongest magnetic field under the weakest stress requires equal averaged principal stresses in all directions. Therefore, the pitch number of a helical coil is determined to satisfy the uniform stress condition. Moreover, the helical winding is modulated in such a way that poloidal field exists only outside of a torus, because the poloidal field in the torus prevents the breakdown of plasma and causes the torsional force. In the case of the aspect ratio A=2, our optimal coil theoretically increases the magnetic field to 1.4 times larger than the conventional TF coil

[en] Strong-field tokamaks with high plasma current are attractive in raising the plasma performance of devices with small sizes. However, the coil-support structures become more demanding in order to withstand the stronger electromagnetic forces. In this paper, a new concept of a high field tokamak reactor is proposed. High magnetic field is generated using the force-balanced coil (FBC) system, that can reduce the huge electromagnetic force of coils by balancing the poloidal and toroidal magnetic field strengths. The poloidal-toroidal hybrid coil is wound in a multi-helical structure with varying pitch angle windings. A small size superconductor torus with the FBC system has thus been constructed to demonstrate proof of principle. A special operation scenario, owing to FBCs helical tokamak, is also demonstrated by realisation of the tokamak plasma. Future plans are reported. (orig.)

[en] A DT sub-ignited tokamak reactor is investigated as a transmuter of fission products with an average neutron wall loading of 1 MW m^-2 basing on a novel coil concept to generate strong magnetic fields. By the use of the force-balanced coils (FBC) which reduces the net centering force by a factor of ten from that exerted on toroidal magnetic coils of the same dimension, the steady-state tokamak reactor can be designed to achieve the fusion reaction of Q_DT≅3.7. To transmute the fission product ¹²⁹I effectively, a blanket is designed which consists of the neutron multiplier Be, the tritium breeder Li₂O and a neutron reflector C. Using the optimal blanket, the amount of ¹²⁹I is effectively halved in 2 years with quasi-self-consistent condition, Q_NET≅5.3, local TBR>1.7 and nuclear heating rate in superconducting magnets <1 mW cm^-3. (orig.)

[en] A DT sub-ignited tokamak reactor has been investigated that has an average neutron wall loading of 1 MW/m² and is based on a novel coil concept to generate a strong magnetic field. When the aspect ratio of the coil winding is about 1.7, the forces in the major direction are balanced with the winding pitch of two poloidal rotations round the torus. Thus force balanced coils can be designed which consist of twisted loops in a shape like a figure of eight. Although the coil current has a toroidal component, the poloidal component of the produced magnetic field is made to be negligible inside the winding by modulating the winding pitch. The net centering force is reduced by a factor of 10 from the exerted on toroidal magnetic coils of the same dimension. The authors designed a steady state tokamak reactor with force balanced coils. The fusion reaction of Q_DT approx. = 3.7 is maintained with 58 MW of neutral beam power into a plasma with a major radius of 3.64 m, a minor radius of 0.80 m, an elongation of 1.7, a plasma current of 6.0 MA, an H factor of 2 and a toroidal field on the axis of 8.4 T. (author)

[en] Although high field is very favorable for magnetically confined fusion devices such as tokamaks, electromagnetic forces derived from the field become serious. In particular, the large in-plane centering force hinders toroidal field coils from operating at higher fields. Instead of toroidal field coils, force-balanced coils (FBCs), by which this force is drastically reduced, are proposed for the coil system of a tokamak. FBC can also provide enough poloidal flux for plasma breakdown and current induction during the ramp-up phase of the coil current. In this paper, how to design the FBC for tokamaks is discussed. FBCs of ITER-EDA size were designed and compared with conventional toroidal field coil systems in terms of the electromagnetic force and the poloidal field in the vacuum vessel. The construction of a small tokamak with FBCs, for the demonstration of tokamak plasma confinement, was completed and plasmas have been generated in it. (orig.)

[en] Reversal effects of the toroidal field B_t on the principal divertor plasma parameters were investigated under radiative and detached divertor conditions in L mode discharges. The ion flux to the inboard separatrix strike point decreased before a MARFE occurred, irrespective of the ion ∇B drift direction. The local electron temperature, T_e,div, decreased to around 10 eV. The maximum fraction of power radiated in the divertor was comparable between the two directions of B_t. With the power flowing into the two divertor fans being slightly larger on the outboard than on the inboard, a nearly symmetric in-out heat load was observed for the ion ∇B drift away from the target. This was due to the outboard enhanced asymmetries in the particle flux and radiation loss distributions. From the viewpoint of in-out symmetry in the target heat load and T_e,div, operation with the ion ∇B drift away from the target plate is desirable as long as the attached divertor condition is maintained. On the contrary, during the MARFE, although deterioration of the energy confinement as well as the increase in the fuelling efficiency were comparable, for the ion ∇B drift towards the target the plasma did not detach completely, and the strong in-out asymmetry in the particle recycling was relaxed to a relatively symmetric distribution. From the viewpoint of particle exhaust to the divertor, operation with the ion ∇B drift towards the target is favourable. (author). 40 refs, 17 figs

[en] A flywheel motor-generator (MG) for the toroidal field (TF) coils of a small fusion device was developed which utilizes a commercially available squirrel-cage induction motor. Advantages of the MG are comparably-long duration, quick power response, and easy implementation of power control compared with conventional capacitor-type power supply. A 55-kW MG was fabricated, and TF coils of a small fusion device were energized. The duration of the current flat-top was extended to 1 s which is much longer than those of conventional small devices (around 10–100 ms).

No abstract available

[en] A fast reciprocating probe system with a long drive shaft was incorporated into a multi-junction lower hybrid (LH) wave launcher on JT-60U in order to investigate an improved coupling mechanism of the radio frequency wave to the core plasma. The system has been operated reliably over a horizontal scan of 25 cm in 1.5 s using a compact pneumatic cylinder drive and springs. A double probe measurement provided the scrape-off layer plasma profile between the last closed flux surface and the first wall with the spatial resolution of 1-2 mm measured with a laser displacement gauge. The profiles of the electron density n_e and temperature T_e were in good agreement with those obtained with a triple probe method. During the LH wave injection with good coupling to the core plasma, an increase in the local T_e was observed in front of the LH launcher mouth. The local n_e was (7-10)x10¹⁶ m^-3, consistent values needed for the good coupling. copyright 1995 American Institute of Physics

[en] Studies on the loss of alpha particles enhanced by toroidal field (TF) ripple in a low-aspect-ratio tokamak reactor (VECTOR) have been made by using an orbit-following Monte-Carlo code. In actual TF coil systems, the ripple loss of alpha particles is strongly reduced as the aspect ratio becomes low (the power loss ∝ A^8.8 for A≥2.5) and the reduction of the number of TF coils results in a large amount of ripple loss even in a low-aspect-ratio tokamak. To reduce the number of TF coils from 12 to 6, about 40% of coil size enlargement is necessary in VECTOR. Ferrite plates are very effective to reduce ripple losses of alpha particles. By using ferrite plates, the coil size enlargement for N=6 can be relaxed to 15% and the number of coils can be reduced from 12 to 8 without enlargement of coil size in VECTOR. (author)