Two approaches to the reactor-relevant high-beta plasmas with profile control in the large helical device

In order to realize economical fusion reactor, high-beta operation of plasma is required. Volume averaged β ∼ 5% has been achieved in the LHD; the potential of the Heliotron type configuration as a reactor has been thereby demonstrated. This high-beta regime is realized by the increase in the heating power and by the optimization of the magnetic configuration. The heating efficiency of NB in the lower magnetic field is better when the magnetic axis remains unshifted. We adjust the aspect ratio of the plasma so that the Shafranov-shift of the magnetic axis is reduced. The edge MHD instabilities remain. However, the magnitude of them are saturated in a low level and do not make a serious deterioration of the confinement. They are identified as the resistive interchange modes. Therefore, the effect will be reduced when the plasma parameters approach reactor-relevant values. From an MHD point of view, there are many advantages in the peaked profile plasmas; the magnetic well is deeper in the core region from the larger Shafranov-shift and the pressure gradient in the edge region is smaller. A peaked profile is formed in the recovery phase after sequentially injected hydrogen pellets. While the electron density decreases after the pellets, the electron temperature recovers quickly. In this recovery phase, the pressure profile becomes peaked; high-central-beta plasma is formed in this phase. Though the final plasma with peaked pressure is stable, MHD stability is important in the process of the formation. When the vacuum magnetic axis Rax-vac is located inward (e.g., Rax-vac = 3.6 m), sawtooth-like instabilities are activated when the pressure profile is being peaked. The peaking of the plasma is thereby disturbed and the degree of peaking is small. On the contrary, in the outward-shifted cases (Rax-vac > 3.7 m), the achieved electron density is higher; a higher central-beta β0 can be achieved. However, the increase of β0 is limited by the so-called core density collapse events when the magnetic axis position exceeds 4.1 m. The magnetic axis should be then kept between 3.7 m and 4.1 m in order to avoid these two unstable regions. The highest β0 is obtained with Bt = 0.65 T. The central β (∼ 9.9%) is comparable to the value in the highest averaged-beta discharge (Rax-vac = 3.6 m and Bt = 0.425 T) with a higher toroidal magnetic field. (author)