Investigation of the edge ion-to-electron temperature ratio in the J-TEXT tokamak

Abstract

The ion (T i) and electron (T e) temperatures at the last closed flux surface (LCFS) during density scanning discharges and in the scrape-off layer (SOL) with a constant plasma density have been measured in the J-TEXT tokamak using a retarding field analyzer and four-tip Langmuir probe, respectively. Both the LCFS T i and T e are found to reduce with increasing central line-averaged density (). The temperature ratio (τ = T i/T e) is about 2 for a low (1.5 → 2.5 × 1019 m−3), decays in a moderate (2.5 → 3.5 × 1019 m−3) and tends to unity at a high (3.5 → 4.2 × 1019 m−3). This suggests that the ions and electrons at the LCFS (even in the SOL) are thermally decoupled at low or moderate and tend to full thermal coupling in the high-density plasmas. In addition, both T i and T e decrease with the radius and T e falls more steeply than T i due to faster parallel energy loss of electrons in the SOL. In contrast, τ rises with the radius, which indicates that the ions and electrons are much more thermally decoupled in the deeper SOL.