Physics of Heliotron J Confinement

Abstract

This paper reviews the results of an experimental study undertaken in Heliotron J over the past few years to explore the physics design base for a new concept of a helical-axis heliotron. Measurements of electron cyclotron resonance (ECR)/neutral beam injection (NBI)/ion cyclotron range of frequencies (ICRF) heating plasmas have been made for understanding global energy confinement in connection with the international stellarator scaling law (ISS04), spontaneous confinement improvement (L-H transition), confinement improvement based on supersonic molecular beam injection (SMBI), magnetohydrodynamic (MHD) activity, edge plasma characteristics, including rotation of a filamentary turbulence structure, and plasma current control, including the electron cyclotron current drive (ECCD), the energetic-particle driven Alfvén eigenmodes, and related fast ion dynamics. The results are discussed in terms of the rotational transform ι/2π and the bumpiness ε b (or the effective helical ripple ε eff ). Control of these two parameters was experimentally demonstrated to be the key issue in determining the optimum performance of Heliotron J. The result confirms that the helical-axis heliotron provides a unique and high potential for exploiting an alternative and advanced path to future helical systems.

Numerical investigation into the peripheral energetic-particle-driven MHD modes in Heliotron J with free boundary hybrid simulation

“…These toroidally asymmetric magnetic field components can cause the formation of new toroidally asymmetric gap modes [1,2] and additional EP and SAW resonances [3,4]. Clarification of the interactions between SAW and SAW, and between SAW and EPs, are important to clarify the experimental observation and for the development of the mitigation method, such as the effects of the induced positive and negative magnetic shears on the linear stability of the observed EP-driven MHD instabilities [5][6][7] in Heliotron J [8][9][10], a low-shear quasiisodynamic optimized helical axis stellarator/heliotron. The EP-driven MHD instabilities in Heliotron J were numerically investigated with MEGA, a nonlinear MHD-EP hybrid simulation code [11][12][13] for the low beta currentless MHD equilibrium.…”

Magnetohydrodynamic hybrid simulation of Alfvén eigenmodes in Heliotron J, a low shear helical axis stellarator/heliotron

“…Heliotron J [6][7][8] is a low magnetic shear helical axis stellarator/heliotron in Kyoto University with four toroidal equilibrium field periods. The main objectives of Heliotron J are to optimize the compatibility between the reduction of neoclassical transport and the MHD stability with helical coil winding.…”

Non-uniformity of fluctuation characteristics inside an edge magnetic island in Heliotron J

“…Heliotron J, a medium-sized helical-axis heliotron device, boasts a major radius (R) of 1.2 m, an average minor radius (a) ranging from 0.1 to 0.2 m, and a magnetic field strength on the magnetic axis of less than 1.5 T. It is equipped with a unique L/M = 1/4 continuous winding helical coil, where L represents the pole number and M the pitch number, along with two types of toroidal coils and three sets of vertical magnetic field coils. These components together produce a low magnetic shear configuration [18][19][20]. The device's magnetic configuration is adjustable, allowing for the alteration of current ratios in each coil.…”