PH/S—tokamak's limit as a result of the plasma sheath breakdown

It was noted earlier [1] that high performance regimes of many tokamaks were achieved in the condition of plasma heating power PH limited from above. The exceeding of this limit usually ended as a plasma collapse. The analysis of the high performance regimes of well known tokamaks which operated during the last 50 years has shown that the values of such 'permissible' PH grow approximately linearly with the area S of the first wall surface facing to the plasma. The paper attempts to explain the existence the PH/S limit for high performance tokamak regimes as a consequence of the vacuum breakdown of the plasma sheath in the area of a plasma contact with the vessel wall and unipolar arcs which followed it.