P2-02 in 20th SOFE

Plasma Production in a Small High Field Force-Balanced Coil Tokamak Based on Virial Theorem

The plasma production and confinement experiments in a novel tokamak device with a new type of toroidal field (TF) coils and a central solenoid (CS) whose stress is reduced to a theoretical limit determined by the virial theorem are presented. Recently, we had developed a tokamak with force-balanced coils (FBCs) [1] which are multi-pole helical hybrid coils combining TF coils and a CS coil. The combination reduces the net electromagnetic force in the major radius direction by canceling the centering force and the hoop force due to the TF-coil and CS-coil currents, respectively. This excellent feature of FBC such as a reduction of the net electromagnetic force and its capability of tokamak operation were demonstrated by the first FBC tokamak "Todoroki-I'' [2], while working stress in coils has not yet been investigated.

Then we have extended the FBC concept using the virial theorem which is the relation between magnetic field energy and working stress in coils and their supporting structure. According to the theorem, high-field coils should have the same averaged principal stresses in all directions, whereas conventional FBC reduces stress in the toroidal direction only. In Ref. 3, we derived the poloidal rotation number of helical coils which satisfy the uniform stress condition, and named the coil as virial-limit coil (VLC). According to Ref. 3, VLC with a circular cross section of aspect ratio A=2 reduces the maximum stress to 60% compared with that of TF coils.

In order to prove the advantage of the VLC concept, we manufactured a small VLC pulsed tokamak "Todoroki-II'' with a major radius of 0.3 m, a minor radius of 0.08 m, toroidal magnetic field strengths of B_T < 1.5 T and plasma currents of I_P < 40 kA.

Since error fields in the vacuum vessel of Todoroki-II by VLCs in Todoroki-II were much reduced by their modulated winding pitch, plasma breakdown became easier than that in Todoroki-I while pre-ionization was still required. In the initial phase of experiments without external vertical field, a plasma pulse length of 0.4 ms and a maximum plasma current of 5 kA were achieved by two-step VLC excitations. External vertical field increased both the plasma pulse length and current to 1 ms and 11 kA, respectively, while they were restricted because of no vertical field control. Using a Cauchy-condition surface (CCS) method [4], the shape and displacement of plasma boundary was reconstructed, and the validity of CCS under large eddy currents was verified.

1) Y. Miura, J. Kondoh, R. Shimada: in Fusion Technology (Proc. 18th Eur. Symp. Karlsruhe, 1994), Vol. 2, Elsevier, Amsterdam (1995) 957-960.
2) S. Tsuji-Iio et. al.: Fusion Energy 1998 (2001 Edition) (Proc. 17th IAEA Fusion Energy Conf. Yokohama, 1998) IAEA, Vienna (2001) FTP-30.
3) H. Tsutsui, K. Nakayama, S. Nomura, R. Shimada, S. Tsuji-Iio: IEEE Trans. Appl. Supercond., 12, No. 1, 644-647, (2002).
4) K. Kurihara: Fusion Eng. Des., 51-52, 1049-1057, (2000).

htsutsui@nr.titech.ac.jp