Influence of the dynamic ergodic divertor on transport properties in TEXTOR

Abstract

In this work the influence of external Resonant Magnetic Perturbations (RMPs) on the radial electric field E r in magnetically confined plasmas is investigated by Charge Exchange Recombination Spectroscopy (CXRS) at the Tokamak TEXTOR. For this purpose, an optical diagnostic was developed to perform CXRS with C 6+ ions utilizing a neutral hydrogen diagnostic beam. Two systems are used to measure the poloidal rotation as well as the ion pressure and the toroidal rotation, respectively. The setup allows measurements with 1-2 cm radial resolution and a velocity resolution of typically 1 km/s for v θ and 5 km/s for v φ . For the radial calibration active beam emission spectra (Doppler shift) are used. The wavelength reference is determined via differential Doppler shift spectroscopy and a neon reference line for the poloidal and toroidal system, respectively. The radial electric field, E r , is then deduced from the radial force balance of C 6+ ions. The experiments have been performed at the Tokamak TEXTOR. Here, the RMPs are produced with the Dynamic Ergodic Divertor (DED), a set of 16 helical perturbation coils located at the high field side of TEXTOR. Within this work, the base mode number of perturbations has been m/n = 6/2. With these new observation systems, we have first investigated the influence of external torque from neutral heating beams on plasma rotation and E r . The toroidal rotation in the center is proportional to the net beam torque with an offset counter-current rotation at zero net torque. At the plasma edge, the toroidal rotation is dampened by friction with neutrals and coupled to the counter-current Scrape-Off layer flow by perpendicular viscosity. The poloidal rotation follows the poloidal component of the beam torque in the plasma center (ranging from 3 ± 3 km/s), at the plasma edge it is dominated by drift effects and loss of fast particles of the counter beam. Under influence of the DED, a spin-up of rotation with the rise of the perturbation amplitude causes the E r to become more positive. An increase of +10 kV/m is observed. The toroidal contribution increases all across the minor radius because of the large perpendicular viscosity, while the poloidal contribution increases only locally inside the ergodic zone caused by the RMPs. The ergodic zone causes an electron loss, and subsequently a j × B force driven by the compensating ion return current. In addition, the DED changes the global confinement properties. Depending on the edge safety factor ("field line twist") q a , either increased or decreased particle confinement is observed. In case of the increased particle confinement (IPC) the increase in density (40%) and particle confinement time τ p (30%) is correlated to the connection of field lines at the q=5/2 surface to the DED target, locally changing the transport properties and the E r . Transport is reduced and the E r shear is increased locally at q=5/2 up to 1.5 · 10 5 s −1 , while the E r becomes more positive. The changes in magnetic topology calculated in the vacu...

Resonant Pedestal Pressure Reduction Induced by a Thermal Transport Enhancement due to Stochastic Magnetic Boundary Layers in High Temperature Plasmas

“…In general, a pronounced modulation of p e with dependence on q 95 or the edge safety factor q a for the limiter case was observed in both experiments. For TEXTOR, the p e ðÉ N ; tÞ was measured by means of an ECE system (T e ) and a laser interferometer for (n e ) in m=n ¼ 12=4 base mode [24], similar to the RMP spectrum considered for ITER. Both experiments exhibit the most pronounced impact on p e in the plasma edge region, with strong p e ðq 95 Þ reduction at certain q 95 values.…”

The influence of resonant magnetic perturbations on edge transport in limiter H-mode plasmas in TEXTOR

“…Earlier findings have shown that with n=2 RMPs (6/2 base mode configuration of the DED) the pedestal pressure is steadily reduced with increasing perturbation current in the DED coils down to L-mode levels [4] [5]. At the same time, the ELM size is continuously reduced.…”

“…As part of this programme a limiter H-mode has been developed [4] [5]. The power threshold to access H-mode conditions in TEXTOR is considerably larger than predicted for the H-mode in poloidal divertor tokamaks, which is related to larger convective heat losses out of the confined plasma in limiter configuration compared to divertor configurations (cf.…”

Escape patterns of chaotic magnetic field lines in a tokamak with reversed magnetic shear and an ergodic limiter

“…19,[21][22][23][25][26][27] However, further experiments have revealed that the heat and particle deposition patterns are not actually uniform as previously assumed, but are rather strongly structured, presenting a self-similarity that suggests an underlying fractal structure. [28][29][30][31][32][33][34][35] These structured patterns of the heat and particle deposition turn out to be undesirable from the point of view of controlling plasma-wall interactions, and have been investigated in a series of experiments with the dynamic ergodic divertor. [32][33][34][35] Although the patterns are not uniform, they proved to be manageable through changing the plasma equilibrium or varying the resonant perturbations, such that one can control heat and particle deposition on the tokamak vessel.…”