Investigation of the possibility of exceeding the Greenwald density limit during ECRH in T-10

Abstract

In T-10 experiments, attempts were made to significantly exceed the Greenwald limit \(\bar n_{Gr} \) during high-power (Pab=750 kW) electron-cyclotron resonance heating (ECRH) and gas puffing. Formally, the density limit \((\bar n_e )_{\lim } \) exceeding the Greenwald limit \(({{(\bar n_e )_{\lim } } \mathord{\left/ {\vphantom {{(\bar n_e )_{\lim } } {\bar n_{Gr} }}} \right. \kern-\nulldelimiterspace} {\bar n_{Gr} }} = 1.8)\) was achieved for qL=8.2. However, as qL decreased, the ratio \({{(\bar n_e )_{\lim } } \mathord{\left/ {\vphantom {{(\bar n_e )_{\lim } } {\bar n_{Gr} }}} \right. \kern-\nulldelimiterspace} {\bar n_{Gr} }}\) also decreased, approaching unity at qL≈3. It was suggested that the “current radius” (i.e., the radius of the magnetic surface enclosing the bulk of the plasma current Ip), rather than the limiter radius, was the parameter governing the value of \((\bar n_e )_{\lim } \). In the ECRH experiments, no substantial degradation of plasma confinement was observed up to \(\bar n_e \sim 0.9(\bar n_e )_{\lim } \) regardless of the ratio \({{(\bar n_e )_{\lim } } \mathord{\left/ {\vphantom {{(\bar n_e )_{\lim } } {\bar n_{Gr} }}} \right. \kern-\nulldelimiterspace} {\bar n_{Gr} }}\). In different scenarios of the density growth up to \((\bar n_e )_{\lim } \), two types of disruptions related to the density limit were observed.