Stability analysis of the TPX toroidal field coil

The energy stability margin of the TPX/TF conductor has been calculated as a response to heat pulses with short initial quench zones and with long initial quench zones. The short IQZs approximate ramp-rate induced heating, and the long IQZs approximate heating from a plasma disruption. These IQZs are centered in the bore inner leg of the double pancake, where the operating field and temperature are maximum. Energy margin stability curves are plotted as a function of current. At the 33.5 kA design current, the minimum calculated stability margin with short IQZs is 390 mJ/cc. The minimum calculated stability margin with long IQZs is 205 mJ/cc. A comparison the stability margin with the available enthalpy (short IQZs) and with the available internal energy (long IQZs) shows that the conductor utilizes the available helium energy well. The energy margin stability curve is generally divided into two regions. The regions can be characterized on the basis of a decrease or an increase in heating after the initiating heat pulse during a marginal quench. The low current well-cooled region exists when the heating rate during the pulse is greater than the Joule heating after the pulse. Since the heating rate decreases, the energy margin is high. The high current ill-cooled region exists when the heating rate during the pulse is less than the Joule heating after the pulse. Since the heating rate increases, the energy margin is low. In the evaluation of a well-cooled or an ill-cooled condition, this Joule heating should be included in the strength of the initiating pulse. When this is done for the TPX/TF conductor, the limiting current approaches the critical current, explaining why the stability curves are entirely in the well-cooled region. The important result is that the conductor does not exhibit a large decrease in energy margin to an ill-cooled region until the current approaches the critical current