Cross-field potential hill arisen eccentrically in toroidal electron cyclotron resonance plasmas in the Low Aspect ratio Torus Experiment device to regulate electron and ion flows from source to boundary

Abstract

We have investigated the electron and ion flows in toroidal electron cyclotron resonance (ECR) plasmas maintained by a 2.45 GHz microwave power around 1 kW under a simple toroidal field in the low aspect ratio torus experiment (LATE) device. We have found that a vertically uniform ridge of electron pressure that also constitutes the source belt of electron impact ionization is formed along just lower field side of the ECR layer and a cross-field potential hill (V S ∼ =30 V while T e ∼ =10 eV), eccentrically shifted toward the corner formed by the top panel and the ECR layer, arises. Combination of the hill-driven E × B drift and the vertical drift due to the field gradient and curvature, being referred to as vacuum toroidal field (VTF) drift, realizes steady flows of electrons and ions from the source to the boundary. In particular, the ions, of which VTF drift velocity is much slower than the electron VTF drift velocity near the source belt, are carried by the E ×B drift around the hill to the vicinity of the top panel, where the ion VTF drift is enhanced on the steep down slope of potential toward the top panel. On the other hand the electron temperature strongly decreases in this area. Thus the carrier of VTF drift current is replaced from the electrons to the ions before the top panel, enabling the current circulation through the top and bottom panels and the vessel (electrons mainly to the bottom and ions mainly to the top) that keeps the charge neutrality very high. A few percent of electrons from the source turn around the hill by 360 degree and reentry the source belt from the high field side as seed electrons for the impact ionization, keeping the discharge stable.

Shift in principal equilibrium current from a vertical to a toroidal one towards the initiation of a closed flux surface in ECR plasmas in the LATE device

“…Here, j bottom array and j top array denote the vertical current densities in the bottom and top panels, respectively, and their signs are defined as positive for the upward current direction, the same direction as the vertical charge separation current. When = B 0 V G both radial profiles are nearly the same, having originated from the VTF drifts of the electrons and ions and reflecting mainly the radial profile of electron pressure [18]. In particular the radial profile of the ion current flowing onto the ion collector j ion collector is the same as j top array except for a reduction factor of 1/6, suggesting that j top array is due to the ion current.…”

“…Here ≡ + p p p p e i e since the electron pressure is much higher than the ion pressure in ECR plasmas. When the vacuum vessel which contains the plasma is made of conducting materials, the entire vertical current circulates via the conducting vessel to maintain the plasma [16][17][18].…”

“…Each of the top and bottom panels, the electrodes of the top and bottom radial electrode arrays, and the vacuum vessel is independently connected to a common point through a current pickup resistor (See figure 3 in [18]). Thus, all the currents flowing from the plasma into the electrodes and the vessel can be monitored.…”

“…Three Langmuir probes, which were all motor-driven both in the vertical direction and in the horizontal direction around the vertical probe axis, were used to measure the profiles of electron temperature, density and space potential [18]. Furthermore, the line-integrated electron densities along two chords on the mid-plane that have a tangency radius of = R 12 t cm and 28 cm, respectively, are measured by 70 GHz interferometers.…”

Spectroscopic Measurements of Impurity Ion Toroidal and Poloidal Flow Velocities and Their Dependence on Vertical Magnetic Field in QUEST Toroidal ECR Plasmas

“…In the LATE spherical tokamak, the two-dimensional spatial distributions of electron temperature (T e ) and density (n e ) and the space potential (V s ) have been measured in a poloidal section using a Langmuir probe [1,2]. In a weak vertical field (B z ), the radial profiles of all parameters peak just outside the ECR layer, and the vertical profiles of n e and V s peak above the midplane [1].…”