No abstract

Three-dimensional spatiotemporal dynamics of detached helium plasma parameters along time, radius, and magnetic field were revealed in the linear device NAGDIS-II. To measure plasma parameters before and after the radial plasma ejection that was enhanced around the volume-recombining region, the conditional averaging technique was applied. The radial ejection was found to correlate with low-frequency changes of plasma-column parameters, which seemed to suppress the axial movement of the recombining region. Moreover, an azimuthal charge separation inside the ejected structure was observed, similar to the typical edge transport phenomenon: blobby plasma transport. The neutral flow effect was suggested as a candidate of the driving force.

To increase the accuracy of a particle, momentum, and energy source terms in the detached helium plasma simulation, rate coefficients with the collisional-radiative model were introduced into the fluid code LINear Divertor Analysis (LINDA). Obtained effective rate coefficients and related source terms were compared with those from the conventional empirical databases. It is shown that a high-density condition in future fusion devices causes larger deviation between the effective and the empirical source terms. One-dimensional detached plasma simulation indicated that the peak amplitude of the plasma density during the rollover is sensitive to the source term difference related to the recombination. This study additionally revealed that the heating effect in the three-body recombination process strongly affects the detached plasma formation and downstream plasma parameters.

Multipoint measurements were carried out by employing a microwave interferometer (MI) and a Langmuir probe (LP) in steady-state detached plasmas in the linear plasma device NAGDIS-II to reveal the structure of fluctuations along the magnetic field. We changed the LP position along the magnetic field while the MI was fixed at an upstream position. In addition, a fast framing camera was used to identify an azimuthal mode number, and the predominant mode number was identified as m = 1. By analyzing correlations between signals observed by the LP and the MI, it was found that a time delay of 10–20 kHz fluctuations gradually decreased toward the downstream direction. The results indicate a decrease in the rotation velocity in the E × B direction, and suggest that the 10–20 kHz fluctuation forms a spiral shape.

Dynamic mode decomposition (DMD) was applied to time-series snapshots of dynamic behavior in detached plasmas with a fast framing camera in the linear plasma device NAGDIS-II. The DMD extracted radial plasma ejection and Er×B rotation structures, which are associated with blob-like plasma structures. Besides, we investigated the influences from neutral gas pressure on the growth rates of the DMD modes. By increasing the neutral gas flow rate in the detached plasma, the growth rate of rotation mode became larger while the frequency decreased. The results indicated that the ejected plasma existed for a longer time in the periphery region. It is likely due to the fact that the ratio of the radial velocity to the rotation velocity of the ejected plasma decreased.

The computer tomography for divertor impurity monitor, which measures plasma emissions in the divertor region, for ITER has been conducted using a ray-tracing technique. We have attempted four different solution methods for the inversion problem and compared the results. The solution methods which minimize errors in logarithmic scale had better performance than the methods which minimize errors in linear scale. This is likely due to the fact that the values in the emission profile vary in a wide range of orders of magnitude. The accuracy of the reconstruction has been investigated by changing discharge conditions and the number of field-of-views used. The deterioration in accuracy was most noticeable when the emission profile was reconstructed using only two field-of-views. In addition, the accuracy deteriorated, making the estimation more challenging, under discharge conditions with low emission intensity because of the wider range of emission intensity under such conditions.

The sawtooth oscillation is one of the important instabilities driven by the plasma current in tokamak plasma. The sawtooth period is a key parameter that characterizes the effect of the sawtooth oscillation on the plasma behavior. For prediction of sawteeth in burning plasma, the sawtooth model in the 1.5-dimensional transport code TOTAL has been extended to include the effects of fast particles and the magnetic shear. By using the newly implemented model, we simulated sawteeth in the presence of the alpha particles as the fast ion in ITER. It is found that the sawtooth periods in DT plasma are longer than those in DD plasma. In DT plasma, the sawtooth period doesn't change monotonically but has a peak as a function of the average central ion temperature or the RF heating power. This is because the mechanisms of triggering sawteeth are different in the low RF heating power region and in the high RF heating power region.

From pulsed plasma experiments focusing on neutral pressure dependence, the impacts of a transition from a low to a high recycling target on the particle load were investigated and discussed in the linear plasma device, Magnum-PSI. Time traces of the target ion flux were mitigated in high neutral pressure cases because of a plasma-neutral interaction. On the other hand, in low neutral-pressure cases, the target ion flux indicated partial suppression in the last part of the pulse. The Langmuir probe, located 200 mm upstream from the target plate, did not exhibit such a suppression. Pulse suppression can be expected from the localized interaction between recycled neutral flux and pulsed plasma in front of the target. The mean-free paths of recycled neutral particles regarding the charge exchange with pulse ions and elastic scattering with background neutral particles were compared. Modeling using a fluid code coupled with a neutral transport code was performed, and it was concluded that dynamic pressure induced by the transient recycled neutral flux caused sufficient momentum loss to stagnate the pulsed plasma toward the target plate.