Core density profile control by energetic ion anisotropy in LHD

Experimental and theoretical studies on thermal and particle transport in fusion plasmas have been intensively carried out because of their impact on the performance of fusion reactors [1]. Controls of density profiles in the LHD were investigated by using tangential and perpendicular neutral beam injectors. The LHD plasmas have not yet achieved a high particle confinement state of particles except for a temporal state of density peaking created by a pellet injection. However, we found that reduced anisotropy of stored energy for energetic ion En_perp/En_para enhances the inward transport, resulting in an electron density peaking successfully in the core plasma of the LHD for the first time. The low En_perp/En_para (< 0.4) regime, where the reference [2] examined the particle transport, results in a flattening profile (dn/dr~0 [/m^4]) for electron and a hollow profile (dn/dr~0 or slightly positive) for impurity carbon, while the new En_perp/En_para regime of 0.47 or higher can access to peaked density profiles (dn/dr~-6 [/m^4]). At the same time, density peaking profiles rise in neutron flux and stored energy. On the other hand, the rise in the anisotropy flattens the density profile, which leads to an increased turbulence amplitude in the spectral range below 500 kHz in the core plasma.

As a pioneering and crucial research result, the operation of energetic ion anisotropy by neutral beam heating has newly demonstrated that the direction of the radial transport of bulk and impurity ions can be controlled. This new finding holds promise for a control knob of nuclear fusion reactors to enhance fusion power output.

[1] M. Maslov et al., Nucl. Fusion 58 (2018) 076022.

[2] K. Ida et al., Phys. Plasmas 16 (2009) 056111.