[show abstract][hide abstract] ABSTRACT: A clear transition of edge particle transport was observed for the neutral beam heated plasmas in CHS. The heating power threshold for the transition is about 1 MW for a plasma with 2 × 10 19 m –3 average density. The H α emission drops within 1 msec and the increase of the local edge density at the transition was confirmed by means of YAG Thomson scattering and beam emission spectroscopy. When the heating power is well above the threshold, the transport barrier is maintained for the full duration of NBI heating (100 msec). A clear back transition appears when the heating power is decreased during the discharge.
Journal of Plasma and Fusion Research. 01/2003; 79(10).
[show abstract][hide abstract] ABSTRACT: The edge transport barrier (ETB) has been observed for the two co-injected neutral beam heated helical plasma in CHS. When the heating power exceeds the power threshold of P ˜ 800kW, H_alpha signal showed a clear spontaneous drop followed by the increase of line-averaged density after the second neutral beam injection. The stored plasma energy with the diamagnetic measurement also increases about 20 ms after the transition. The optimization for the magnetic configuration using quadrupole coils is effective for the ETB formation. The increase of the stored energy by the optimization achieves to ˜ 40%,and H-factor (ISS04v03) of ˜ 1.3. The profile characteristics of the electron density and temperature of the ETB formation has been investigated with a multi channel YAG Thomson scattering system. A considerable increase of the edge density by ˜ 50-100% in rho>0.5 has been observed, while the electron temperature in rho>0.7 increases by 30-50%. These results show the increase of the stored energy is mainly caused by the improvement of a particle transport in the edge region.
[show abstract][hide abstract] ABSTRACT: The physics aspects of CHS-qa, a quasi-axisymmetric helical device, which is being designed as the post-CHS device, are discussed. Its main objective is to improve the transport in helical systems by reducing the parallel viscosity and by applying max. J criterion to its design. The parallel viscosity is much reduced compared with that in CHS and the max. J criterion is fulfilled at the region near the edge in the typical CHS-qa configuration. As an experimental device, a variety of knobs for controlling magnetic field configuration are taken into consideration in the design to study transport-related physics subjects: bifurcation phenomena of radial electric fietd and max. J criterion. Other physics requirements which CHS-qa should satisfy as POP experiment, such as MHD characteristics, neoclassical transpon aspects, are also discussed.
[show abstract][hide abstract] ABSTRACT: To clarify the effect of polarization on electron cyclotron heating (ECH) in magnetized plasmas, experiment controlling the polarization of injected EC-waves is carried out in Compact Helical System (CHS). In the experiment, plasmas are generated and sustained only with 106.4 GHz ECH power. Magnetic field at the magnetic axis is 1.9 T so that the wave frequency is second harmonic. The optimum direction of linear polarization for the shortest time-delay of density start-up from the start of power injection and the optimum direction for the highest electron temperature and plasma stored energy during plasma duration show clear difference. The difference is attributed to the CHS magnetic configuration with strong shear and the plasma volume expansion from magnetic axis to the last closed flux surface.
[show abstract][hide abstract] ABSTRACT: The characteristic difference of neoclassical bootstrap current and its effects on MHD equilibria are described for the CHS heliotron/torsatron and the CHS-qa quasi-axisymmetric stellarator. The direction of bootstrap current strongly depends on collisionality in CHS, whereas it does not in CHS-qa because of quasi-axisymmetry. In the CHS configuration, it appears that enhanced bumpy (Bs1) and sideband components of helical ripple (By1) play an important role in reducing the magnetic geometrical factor, which is a key factor in evaluating the value of bootstrap cuffent, and determining its polarity. The bootstrap current in CHS-qa is theoretically predicted to be larger than that in CHS and produces significant effects on the resulting rotational transform and magnetic shear. In the finite B plasmas, the magnetic well becomes deeper in both CHS and CHS-qa and its region is expanded in CHS. The existence of co-flowing bootstrap current makes the magnetic well shallow in comparison with that in currentless equilibrium.
[show abstract][hide abstract] ABSTRACT: The global ideal magnetohydrodynamic (MHD) stability for a proposed compact quasi-axisymmetric stellarator CHS-qa has been investigated taking the effect of bootstrap current into account. Assuming experimentally achievable density and temperature profiles, the stability properties of global low-n modes have been studied by using threedimensional numerical codes based on fixed boundary MHD equilibria including self-consistent bootstrap current for the CHS-qa reference configuration. Consequently it has been shown that values of edge rotational transform play a crucial role in triggering external kink instability. Concerning a lot of other possibilities in experimental practice to change the total parallel current, we have also studied equilibria with increased or decreased parallel current, but fixed profile. The onset of external kink modes depends on rotational transform or current profile, and we found a stable equilibrium in spite of the edge rotational transform above 0.5. The results imply the possibility of stabilizing external kink modes through current and/or pressure profile control in high beta equilibria.
[show abstract][hide abstract] ABSTRACT: Heavy Ion Beam Probe (HIBP) is a very powerful tool that is able to simultaneously detect potential, density, magnetic field and their fluctuations in interior of high temperature plasmas. In CHS, so far, the HIBP has revealed interesting behavior of the plasma interior, such as bifurcation, internal transport barrier, MHD phenomena, by observing potential profile and density fluctuation. Now we are making efforts to make a full use of the other abilities of HIBP, that is, to detect potential fluctuation and magnetic field fluctuation. In order to study potential fluctuation, a new ion source has been developed to increase beam current density. With the new ion source, we have obtained current 3 5 times larger than the previous one. This improvement allows us to detect the potential fluctuation, and also density fluctuation with higher S/N ratio. As for the magnetic field fluctuation estimated from the beam displacement, the interpretation of the signal is quite difficult because of 3D magnetic field configuration of CHS. In the first place, a numerical calculation has been performed in axisymmetric toroidal field i. e., tokamak configuration, in order to obtain a basic perspective of the relationship between the beam displacement and local magnetic field fluctuation. In our presentation, we will describe the resultant progress of HIBP measurements, and the recent results of density and potential fluctuation measurements in the CHS stellarater.
[show abstract][hide abstract] ABSTRACT: The orbit topology and confinement of neutral beam-injected energetic ions are investigated for the current target configuration of the CHS-qa quasi-axisymmetric stellarator. It was shown that tangentially co-injected neutral beam (NB) heating is efficient even at a low magnetic field strength Bt of 0.5 T, whereas the heating efficiency of the counter-injected NB becomes significantly lower as Bt decreases because of the increase of first orbit loss. The energy loss rate increases as the beam injection angle becomes perpendicular, suggesting that the residual non-axisymmetric ripple in the peripheral domain plays a role in enhancing the transport of trapped ions. An interesting observation involves the appearance of the island structure in both the gyro motion following orbit and the guiding center collisionless orbit of counter-moving transit beam ions. It appears under a particular, narrow range of parameters, i.e., energy, pitch angle v///v, normalized minor radius r/a at the launching point and Bt.