Non-Diffusive Transport Phenomena in TJ-II

Introduction ‐ The radial propagation of perturbations is an important tool for probing transport properties in a fusion plasma. Indeed, the propagation of cold pulses, induced by either deliberate or spontaneous edge cooling, has been studied in many devices (e.g. JET [1, 2, 3], TFTR [4], TEXT [5], RTP [6, 7], ASDEX [8] and W7-AS [9]). The analysis of such pulses has led to the conclusion that often this phenomenon is not in agreement with standard diffusive model predictions. In this paper, an analysis of the propagation of cold and heat pulses is presented and it is found that this phenomenon is likewise difficult to reconcile with a simple diffusive transport model in a stellarator like TJ-II. Experimental set-up ‐ The TJ-II stellarator [10] is a low-shear Heliac having 4 periods, B0 < 1.2 T, R = 1.5 m and a 0.22 m. Plasmas are bean-shaped. The configuration used in this paper is denoted by "100_40_63", a standard TJ-II configuration with (a)/2 = 1.61 and (0)/2 = 1.51. The plasmas were heated by the Electron Cyclotron Heating system, using one gyrotron with a frequency f = 53.2 GHz and with power P 300 kW (2 nd harmonic, extraordinary mode). The wall was metallic. A multi-channel heterodyne radiometer [11], provided simultaneous measurement of the electron temperature at 8 positions on the high field side, with good temporal resolution and radial resolution of about 1 cm. The radiometer is absolutely calibrated, and the accuracy is reflected by the good agreement found between the Thomson Scattering and ECE temperatures. The high-resolution single-shot Thomson scattering system measures electron temperatures in the range from 50 eV to 4 keV, with a spatial resolution of 2.25 mm and with about 280 data points along the line of sight [12]. Cold-pulse experiments ‐ Cold pulses were generated using fast nitrogen injection. Nitrogen remains confined mainly in the edge region of the plasma [13], so that the initial perturbation is an edge effect, and the penetration of high-Z material into the plasma is small. The response of ECE radiometer signals is shown in Fig. 1. Each temperature trace is initially nearly constant, and then experiences a sudden drop followed by a slow recovery. The line-average density increases slightly after injection of the cold pulse, but typically the perturbation is less than about 7%. The response of the H detector is shown in Fig. 2. The detector picks up the radiation by molecular nitrogen (654 nm, seen through an H interference filter) and the signal amplitude is thus indicative of the amount of nitrogen