Transport of Impurity Ions in the Wendelstein 7-AS Stellarator Plasma

Abstract

Abstract. The impact of global plasma parameters on impurity transport in the stellarator W7-AS was investigated by laser blow-off technique. Both, density and heating power were identified to have a strong influence on impurity confinement  τ Ι ~ n e 1.2 /P ECRH 0.8. In spite of stationary conditions at lower densities, an increasing trend for accumulation was observed at plasma densities beyond 5·10 19 m -3 due to reduction of the diffusion coefficients. Up to densities of at least 9·10 19 m -3 , launching of electron cyclotron resonance heating (ECRH) power of 1.2 MW is able to counteract the impurity accumulation by deterioration of the impurity confinement with heating power according to the scaling law as given above. In neutral beam injection (NBI) heated plasmas at densities higher than 1·10 20 m -3 , long confinement times were observed, often accompanied by loss of density control and degradation of plasma energy due to increasing radiation losses. The installation of island divertor allowed a general extension of the range of accessible densities up to 4·10 20 m -3 : beyond a certain power-dependent threshold density (1.5-2.1·10 20 m -3 ), the plasma enters the High Density Hmode (HDH) regime and the impurity confinement time drops to values comparable to the energy confinement time. High density plasmas could be sustained quasi-stationary with a low level of impurity radiation. The favourable impurity behavior goes along with a reduction of the inward impurity convection in the core plasma and possible changes in the edge transport. For the characterization of the general impurity behavior in W7-AS plasmas the usual transport models for axisymmetric devices are not sufficient and additional stellarator specific processes have to be considered.

Overview of core impurity transport in the first divertor operation of Wendelstein 7-X

“…List of fit parameters of the scaling law τ = C × P α ECRH × n β e,0 × a γ × B δ 0 for W7-AS taken from Burhenn et al (1995) and for W7-X.…”

“…The attempt to derive scaling laws allows an extrapolation of the impurity transport in future plasma scenarios. Initially, an existing scaling law from W7-AS (Burhenn et al 1995) as the predecessor of W7-X was examined for purely ECRH plasma scenarios as the ECRH power P ECRH is considered only. That scaling law also takes into account the central electron density n e,0 , minor radius a and the magnetic field strength on axis, B 0 .…”

Observation of the TESPEL-injected impurities behaviour by the PHA system at Wendelstein 7-X

“…Understanding of impurity transport in stellarator plasmas is a key issue for proper optimisation of the device. Possible accumulation of heavy ion impurities may lead to undesired effects like decreased plasma performance or even early pulse termination [6]. It is also known that impurity confinement in plasma is mainly determined by transport mechanisms that occur in plasma.…”