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

The ion (Ti) and electron (Te) 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 Ti and Te are found to reduce with increasing central line-averaged density (). The temperature ratio (τ = Ti/Te) 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 Ti and Te decrease with the radius and Te falls more steeply than Ti 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.