Error field experiments in JET

Error field induced modes have been a significant concern for next step dev ices. Experiments on JET, using the lower saddle coils to simulate such fie lds, have determined the critical scaling dependences of error field sensit ivity in terms of global plasma parameters, under the conditions most relev ant to next step operation. These experiments indicate relatively weak toro idal field scaling for the error field strength required to induce a locked mode, while confirming the linear density scaling observed on other device s. Correction of intrinsic errors improves low density access and assists e nd-of-pulse disruption avoidance. Additional heating studies have shown a s trong effect of q = 2 rotation on the threshold, but a much weaker dependen ce on beta and the H mode transition. Error field modes have also led to ne oclassical tearing modes at appropriate collisionality and beta. Other para meter dependences have been examined: plasma inductance has a strong effect , but plasma configuration (limiter or divertor) is less important. Applica tion of a dimensional scaling constraint enables determination of machine s ize scaling, which is found to be weak. This leads to the relatively tolera ble error field sensitivity expected for devices such as ITER FDR. However, there remain some important questions to be answered about the effects of rotation on error field sensitivity, and for ITER FDR some means of correct ion remains prudent.