In a tokamak plasma the maximum achievable density is limited. A too high density will result in a violent end of a discharge. Two types of density limit disruption can be distinguished: (a) impure and moderately heateddischarges, if the radiative power exceeds the input power, (b) clean, auxiliary heated discharges, where the Greenwald limit is encountered. It has been found that in TEXTOR-94 these two density limits differ by the radiativeinstability in the plasma boundary, which preceeds the disruption. A symmetric radiativemantle and a detachment are observed prior to the first type, while theGreenwald limit has a MARFE precursor. Control of the impurity content, edge and recycling properties prevents the growth of the MARFE and makes it possible to exceed the Greenwald limit in TEXTOR-94 by more than a factor of 2. High densities have been obtained by means of normal gas feed. Maximum central densities of ne(0) = 1.3 × 1020 m-3 have been obtained.The maximum achievable density scales with the input power and plasma current.Non-disruptive discharges, with a stationary (t > 25 τE) density a factor of 1.93 above the Greenwald limit have been produced in L mode. The radiative losses and impurity concentration have been maintained at a relatively low level during the entire high density phase.
Analytical relation between peripheral and central density limit on FTU