No abstract

Divertor power load reduction is one crucial issue for magnetically confined fusion reactors. Increased edge radiation can dissipate power before reaching divertor plates. Stable sustainment of such enhanced radiation, i.e. radiative divertor (RD) or detached divertor, is, however, still an open issue. In this paper, we present experimental evidence of edge radiation stabilization effect and potential of divertor power load control with resonant magnetic perturbation (RMP). Figure 1 shows time evolution of peak divertor power load, radiation intensity (P rad ) and line averaged density ( e n ) during density ramp up experiments with and without RMP. RMP has m/n=1/1 mode, which has resonance layer in the edge stochastic region, and creates remnant island. The perturbation strength is kept constant at ≈ 0 / B b r 0.1% throughout the discharge in the case with RMP. The plasma is heated by neutral beam injection (NBI) with ~ 8MW of deposited power in both cases. The divertor power load is estimated with Langmuir probe. The radiation is obtained with photo diode array viewing almost entire plasma at specific toroidal location. Without RMP (gray lines), the radiation intensity gradually increases with increasing density. The rapid increase of radiation intensity at t~3.8 sec with concomitant density rise indicates onset of thermal instability. The instability grows so rapidly that it is difficult to stabilize the density rise, leading to discharge termination. With RMP (black lines), on the other hand, transition to enhanced radiation state occurs at t~3 sec, and it leads to divertor power load reduction by a factor of 3 ~ 10. The RD operation is successfully sustained by gas puff feedback control up to the end of NBI. The results show stabilization effect of RMP on the radiating edge plasma. The enhanced radiation with RMP is due to increased volume of low T e (~10 eV) region caused by temperature flattening at the Opoint. 3D edge transport simulation result, which is consistent with the radiation profile measurement, show that the radiation increases further around X-point of the island (1) , where the code predicts n e > 10 20 m -3 and T e ~ a few eV.The well structured edge radiation with RMP such as the selective cooling around X-point is considered to provide stabilization effect by holding the intense radiation there and thus avoids it penetrating inward. Fig.2 shows dependence of controllability of RMP assisted RD on radial location of the island X-point,

The deuterium operation of the Large Helical Device(LHD) began in March 7, 2017, after long-term preparation and commissioning of apparatuses necessary for execution of the deuterium experiment. A comprehensive set of neutron diagnostics was developed and installed onto LHD through numerous efforts in preparation. Neutron diagnostics play an essential role in both neutron yield management for the radiation safety and extension of energetic-particle physics study in LHD. Neutron flux monitor characterized by fast-response and wide dynamic range capabilities is successfully working. Total neutron emission rate reached 3.3×10 15 (n/s) in the first deuterium campaign of LHD. The highest neutron emission rate was recorded in inward shifted configuration. Neutron yield evaluated by neutron activation system agrees with neutron yield measured with neutron flux monitor. Performance of vertical neutron camera was demonstrated. Neutron emission profile was inwardly shifted in the inwardly shifted configuration whereas it was outwardly shifted in the outwardly configuration. Secondary DT neutrons produced by triton burnup in LHD deuterium plasmas were detected for the first time in stellarator/heliotron devices in the world. Similar to total neutron emission rate, the inward shifted configuration provided highest triton burnup ratio.

In situ calibration of the neutron activation system on the Large Helical Device (LHD) was performed by using an intense Cf neutron source. To simulate a ring-shaped neutron source, we installed a railway inside the LHD vacuum vessel and made a train loaded with theCf source run along a typical magnetic axis position. Three activation capsules loaded with thirty pieces of indium foils stacked with total mass of approximately 18 g were prepared. Each capsule was irradiated over 15 h while the train was circulating. The activation response coefficient (9.4 ± 1.2) × 10 of In(n, n')In reaction obtained from the experiment is in good agreement with results from three-dimensional neutron transport calculations using the Monte Carlo neutron transport simulation code 6. The activation response coefficients of 2.45 MeV birth neutron and secondary 14.1 MeV neutron from deuterium plasma were evaluated from the activation response coefficient obtained in this calibration experiment with results from three-dimensional neutron calculations using the Monte Carlo neutron transport simulation code 6.