Rotation characteristics of main ions and impurity ions inH-mode tokamak plasma

Abstract

Poloidal and toroidal rotation of the main ions (He2+) and the impurity ions (C6+ and Bs+) in Hmode helium plasmas have been measured via charge exchange recombination spectroscopy in the DIII-D tokamak. It was discovered that the main ion poloidal rotation is in the ion diamagnetic drift direction while the impurity ion rotation is in the electron diamagnetic drift direction, in qualitative agreement with the neoclassical theory. The deduced radial electric field in the edge is of the same negativewell shape regardless of which ion species is used, validating the fundamental nature of the electric field in L-H transition phenomenology.PACS numbers: 52.55.Fa, 52.55.Pi, 52.70.Kz Since its original discovery in ASDEX [1],the H mode has proven to be one of the most robust and ubiquitous modes of improved confinement in toroidal magnetic fusion devices. The physics of the L-mode to H-mode (L-H) transition has attracted a great deal of interest and effort from both the experimental and theoretical communities. Critical reviews of this subject have recently been made by Burrell et al. [2] and Groebner [3]. Although a complete, quantitative theory of the confinement improvement at the L-H transition does not yet exist, combined theoretical and experimental work has merged into a paradigm that a highly sheared EXB flow in the plasma edge can lead to better confinement through decorrelation of the fluctuations, decreased radial correlation lengths, and reduced turbulent transport [4,5]. The experimental observations that the edge impurity ion poloidal rotation [6-9] and the edge radial electric field [6-11] change dramatically and abruptly at the L-H transition have led to several theories which consider how the radial electric field or the (main) ion poloidal rotation changes across the L-H transition. Among these are theories based on bifurcation of the radial electric field [5,12], Stringer spin-up [13,14], turbulent Reynolds stress [15], temperature gradient-induced poloidal rotation [16], and particle and energy confinement bifurcation [17].Only the impurity ion rotation measurements have been available previously and some theoretical models were developed to explain the sudden (~I msec) increase of poloidal rotation (in the electron diamagnetic drift direction) at the L-H transition; these theories implicitly assumed that the main ion rotation and the impurity ion rotation are identical. However, recent neoclassical derivations of rotation velocities predict that the main ion poloidal rotation and the impurity ion poloidal rotation speed could be quite different [18,19]. Although not in H mode, a comparison of the main and the impurity ion poloidal rotation in Ohmic plasmas in TEXT was reported previously [20], which showed little diff'erence between them. Experimental clari6cation of the rotation behavior of the main and impurity iona in H-mode plasmas is important not only as a check of the neoclassical rotation theories, but also as a test of the existing L-H transition theories. However, charge exchange recom...

Transition physics and scaling overview

“…However, within a short time, a few ms in some cases, the dominant contribution to E, comes from the main ion pressure gradient term. [As pointed out at the last H-mode workshop, there can be a difference in the magnitude and direction of the rotation between the main ions and impurity ions, but the infefied E, is the same for both ions (Kim et al 1994).] This suggests a two-stage process for the development of H-mode.…”

Collisionality scaling of main-ion toroidal and poloidal rotation in low torque DIII-D plasmas

“…For intermediate aspect ratios K 1 decreases in magnitude but remains positive in the banana regime. Impurity poloidal flow is generally smaller than the main-ion flow, but is more commonly measured [20,23,28,40,41]. Impurity poloidal flow can be expressed as…”

Experimental test of the radial force balance equation in the compact helical system

“…The measurements of the poloidal and toroidal flow of bulk and impurity ions in Doublet-IIID ͑D-IIID͒ 8 show that the deduced radial electric field derived from bulk ions using a radial force balance equation agrees well with that from impurity ions. 9 Although the agreement observed suggests the validity of the radial force balance equation, it was not conclusive because there was no direct measurement of the radial electric field with HIBP.…”

“…The measurements of the poloidal and toroidal flow of bulk and impurity ions in Doublet-IIID ͑D-IIID͒ 8 show that the deduced radial electric field derived from bulk ions using a radial force balance equation agrees well with that from impurity ions. 9 Although the agreement observed suggests the validity of the radial force balance equation, it was not conclusive because there was no direct measurement of the radial electric field with HIBP.The radial electric field profiles measured with charge exchange spectroscopy ͑CXS͒ are compared with those measured with the HIBP in the Compact Helical System ͑CHS͒.10 The comparison of the two radial electric fields measured with CXS and HIBP is considered to be a test of the radial force balance equation ͑1͒,In CHS, both poloidal and toroidal rotation velocity profiles as well as the ion temperature of fully ionized carbon are measured with charge exchange spectroscopy by using the heating neutral beam ͑NB͒ at the midplane of the vertically elongated poloidal cross section. The charge exchange spectroscopy consists of two sets of 30 channel poloidal array viewing the same plasma radii vertically with the opposite direction ͑viewing downward and viewing upward͒ in order to determine the Doppler shift precisely.…”