The Dipole Magnetic Configuration for a Novel Negative Ion Source

The dipole is a magnetic configuration with a purely poloidal magnetic field, which can achieve equilibrium with no induced plasma current, being at the same time intrinsically stable w.r.t. ideal m = 0 and m = 1 modes[1]. At the same time, the absence of a toroidal field makes radial drifts of particles to vanish. To see this, write the toroidal canonical momentum as P_ζ = gρ_∥ − ψ_p, where g is the toroidal magnetic field, ρ_∥ = mv_∥/eB, and ψ_p is the poloidal flux coordinate. In the dipole, g = 0 and the momentum simplifies to P_ζ = −ψ_p, which is energy-independent. As a consequence, in the LDX experiment, a dipole device which was operated jointly by MIT and Columbia University[2], record beta values of ≈ 25% were obtained.

In this paper we propose to use a dipole field for the realization of an efficient negative ion source. To this purpose, we analyze the MHD and neoclassical properties of the dipole plasma source, in order to assess its advantages over the present ion schemes, such as the ”kamaboko” and RF ion sources, which are presently used for the development of the ITER negative neutral ion beam (NNBI)[3].

[1] J. P. Freidberg, Ideal MHD (Cambridge, 2014) pp. 452-458, 2nd ed.

[2] D. T. Garnier, J. Kesner and M. E. Mauel, Phys. Plasmas 6, 3431 (1999).

[3] R. S. Hemsworth et al, New J. Phys. 19, 025005 (2017).