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 ( n ˉ e ). The temperature ratio ( τ = T i / T e ) is about 2 for a low n ¯ e (1.5 → 2.5 × 10 19 m −3 ), decays in a moderate n ˉ e (2.5 → 3.5 × 10 19 m −3 ) and tends to unity at a high n ˉ e (3.5 → 4.2 × 10 19 m −3 ). This suggests that the ions and electrons at the LCFS (even in the SOL) are thermally decoupled at low or moderate n ˉ e 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.