Edge cooling experiments with a tracer-encapsulated solid pellet in the Large Helical Device (LHD) show a significant rise of core electron temperature (the maximum rise is around 1 keV) as well as in many tokamaks. This experimental result indicates the possible presence of the nonlocality of electron heat transport in plasmas where turbulence as a cause of anomalous transport is dominated. The nonlocal electron temperature rise in the LHD takes place in almost the same parametric domain (e.g. in a low density) as in the tokamaks. Meanwhile, the experimental results of LHD show some new aspects of nonlocal electron temperature rise, for example the delay of the nonlocal rise of core electron temperature relative to the pellet penetration time increases with the increase in collisionality in the core plasma and the decrease in electron temperature gradient scale length in the outer region of the plasma.
Abstract. The Large Helical Device (LHD) and Wendelstein 7-X (W7-X, under construction) are experiments specially designed to demonstrate long pulse (quasi steady-state) operation, which is an intrinsic property of Stellarators and Heliotrons. Significant progress had been made in establishing high performance plasmas. A crucial point is the increasing impurity confinement at high density observed at several machines (TJ-II, W7-AS, LHD) which can lead to impurity accumulation and early pulse termination by radiation collapse. In addition, theoretical predictions for non-axisymmetric configurations predict the absence of impurity screening by ion temperature gradients in standard ion-root plasmas. Nevertheless, scenarios were found where impurity accumulation was successfully avoided in LHD and W7-AS due to the onset of friction forces in the (high density and low temperature) scrape-off-layer, the generation of magnetic islands at the plasma boundary and to a certain degree also by ELMs, flushing out impurities and reducing the net-impurity influx into the core. In both the W7-AS High Density H-mode (HDH) regime and in the case of application of sufficient ECRH heating power a reduction of impurity core confinement was observed. The exploration of such purification mechanisms is a demanding task for successful steady-state operation. Impurity transport at the plasma edge/SOL was identified to play a major role for the global impurity behaviour in addition to the core confinement.
The first observation of a significant rise of core electron temperature in response to edge cooling in a helical plasma has been made on the Large Helical Device ͓O. Motojima et al., Phys. Plasmas 6, 1843 ͑1999͔͒. When the phenomenon occurs, the electron heat diffusivity in the core region is reduced abruptly without changing local parameters in the region of interest. Therefore the phenomenon can be regarded as a so-called "nonlocal" electron temperature rise observed so far only in many tokamaks. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2131047͔The clarification of electron heat transport in magnetically confined plasmas is still an important issue, since the performance of a probable fusion reactor should be determined by electron heating as a result of the interaction between electrons and alpha particles as a fusion reaction product. In order to promote a better understanding of the electron heat transport, the electron heat transport analysis for both transient and steady state has been carried out diligently in many tokamaks 1-3 and helical systems. [4][5][6] One of the significant issues found in these studies is a "nonlocal transport phenomenon" observed in perturbation experiments on many tokamaks 7-12 and a few helical systems. 4 In particular, a rise of the core electron temperature T e invoked by the rapid cooling of the edge plasma has been observed in various tokamaks with both ohmically heated plasmas and plasmas with an auxiliary heating, such as electron cyclotron heating ͑ECH͒, at a sufficiently low density ͑e.g., Ref. 13͒. The amplitude reversal of the cold pulse propagation in the core plasma cannot be explained even by the model based on the assumption that heat flux has a strong nonlinear dependence on temperature and its gradient. In addition there seem to be no changes in the thermodynamic forces, such as those due to the temperature gradient and/or the density gradient, in the core plasma at the onset of the core T e rise. Consequently, the core T e rise invoked by the edge cooling is considered to result from a nonlocality in the electron heat transport. On the contrary, such a core T e rise in response to the edge cooling has not been observed so far in helical systems.14 Recently, to rationalize the so-called "nonlocal" T e rise, some physics-based transport models including a critical gradient scale length, such as the ion temperature gradient ͑ITG͒ model, have been set up and tested.2,15 It should be noted that despite being dependent on local variables, the ITG-based model shows a nonlocal response to small changes in the profiles as a result of a slight deviation from near marginality. The ITG-based model with strongest stiffness 16 can reproduce some of the same qualitative characteristics observed in carbon laser blow-off experiments in the Texas Experimental Tokamak ͑TEXT͒.13 The magnitude and response time of the core T e rise, however, are still in quantitative disagreement with those predicted by the strongest stiff ITG-based model. 15 Moreover, it is an open ...
In the Large Helical Device ͑LHD͒, various spectroscopic diagnostics have been applied to study the ablation process of an advanced impurity pellet, tracer-encapsulated solid pellet ͑TESPEL͒. The total light emission from the ablation cloud of TESPEL is measured by photomultipliers equipped with individual interference filters, which provide information about the TESPEL penetration depth. The spectra emitted from the TESPEL ablation cloud are measured with a 250 mm Czerny-Turner spectrometer equipped with an intensified charge coupled device detector, which is operated in the fast kinetic mode. This diagnostic allows us to evaluate the temporal evolution of the electron density in the TESPEL ablation cloud. In order to gain information about the spatial distribution of the cloud parameters, a nine image optical system that can simultaneously acquire nine images of the TESPEL ablation cloud has recently been developed. Several images of the TESPEL ablation cloud in different spectral domains will give us the spatial distribution of the TESPEL cloud density and temperature.
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