Description
In the J-TEXT fall 2025 experiments, it was found that during ohmic discharges under identical parameters such as toroidal field current and similar conditions, helium plasma achieved a higher density limit compared to hydrogen plasma solely through gas puffing, even exceeding the Greenwald density limit. In hydrogen‑helium mixed plasma experiments, it was observed that as the helium concentration increased, the achievable density limit of the plasma progressively rose.
Compared to hydrogen plasma, helium plasma exhibited greater stability and was less prone to density‑limit disruptions. Its magnetohydrodynamic (MHD) behavior also differed from that of hydrogen plasma—for instance, above a certain density threshold, the frequency of the m/n=2/1 small magnetic perturbation exhibited frequency jumps. As the density limit disruption was approached, magnetic islands in helium plasma grew faster. In helium plasma, the MAFEE radiation was more stable with less pronounced poloidal rotation; however, near the density limit disruption, the poloidal motion direction of the high‑field‑side high‑density front was opposite to that observed in hydrogen plasma. Meanwhile, edge probe measurements revealed that under the same core density, helium plasma had higher edge density and temperature, along with a larger particle flux.