Core plasma confinement during detachment transition with RMP application in LHD

Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation

“…Power detachment experiments at LHD were achieved with an increased amount of low-Z impurities at the plasma boundary, either via increased density with carbon released from the plasmafacing components or by additional seeding of nitrogen or neon. This allowed radiating away up to 40% of total input power with asymmetric reduction of the divertor power loads [58]. LHD requires additional magnetic perturbation provided by the external coils to stabilise detachment; otherwise, a strong radiation region penetrates inside the separatrix, and the plasma discharge ends in radiation collapse.…”

Experimental Studies of and Theoretical Models for Detachment in Helical Fusion Devices

“…The strong cooling of the edge plasma by impurity radiation leads to the decrease of the plasma column effective radius a 99 displayed in Figure 3(d). Figure 4 shows the radial profiles of the electron temperature T e ,densityn e ,and pressure in the edge region along the LHD mid-plane, in the attached and detached discharge phases [24]. With the RMP application, a clear flattening of the T e profile due to the magnetic island is observed at the outboard, R = 4.60-4.75 m, in the attached phase.…”

“…It is also interesting to note that with the growing n e , the width of the region with the profile flattening becomes slightly narrower, and at the same time, a region with the flattening appears Radial profiles of the plasma parameters in the LHD edge region: electron temperature (a, d, g), density (b, e, h), and pressure (c, f, i); the panels (a-f) correspond to discharges with RMP, (g-i)-without RMP. The region of the T e flattening caused by the magnetic island is indicated by yellow patch [24]. 5 Experimental Studies of and Theoretical Models for Detachment in Helical Fusion Devices DOI: http://dx.doi.org /10.5772/intechopen.87130 at the inboard, R = 3.1-3.2 m. This is interpreted as a result of the plasma response to the external RMP field.…”

“…The edge impurity radiation profiles are estimated using the T e and n e profiles presented in Figure 4 and by assuming a concentration of carbon of 1% with respect to n e . Figure 5 shows the temporal evolution of the radiation profiles together with L C distributions [24]. Without RMP, Figure 5(b), the impurity radiation starts to peak around the X-point of the divertor leg, at R $ 4.8 m, and later moves gradually radially inward due to the decrease in the temperature as the density increases; the X-point of the divertor leg should be distinguished from that of the magnetic island created by the RMP.…”

“…The calculated L C distribution in the outboard edge region (upper panels); time evolution of the carbon radiation estimated from T e and n e profiles as shown in Figure 4, by assuming 1% carbon concentration and noncoronal cooling rate at n e τ =10 17 m À3 s (lower panels), with (a) and without (b) RMP [24]. the radiation profile measured by the AXUV diagnostics [20].…”