STABILITY ANALYSIS OF THE TPX TOROIDAL FIELD-COIL

The energy stability margin of the TPX/TF conductor has been calculate d as a response to heat pulses with short initial quench zones (IQZ= 1 0, 20 cm for 4, 20, 100 ms), and with long initial quench zones (IQZ=2 .28, 4.53 m for 20, 50, 100 ms). The short IQZs approximate ramp-rate induced heating, and the long IQZs approximate heating from a plasma d isruption. These IQZs are centered in the bore inner leg of the double pancake, where the operating field and temperature are maximum. Energ y margin stability curves are plotted as a function of current. The st ability of the 10 cm IQZ differs from that for the 20 cm IQZ by less t han 20%. Similarly the stability of the 2.28 m IQZ differs from that f or the 4.53 m IQZ by less than 20%. However the stability of the short IQZs (10 and 20 cm) is about twice as high as that of the long IQZs ( 2.28 and 4.53 m). The friction in the long IQZs prevents the conversio n of heat to work by helium expansion during the pulse. At the 33.5 kA design current, the minimum calculated stability margin with short IQ Zs is 390 mJ/cc. The minimum calculated stability margin with long IQZ s is 205 mJ/cc. A comparison the stability margin with the available e nthalpy (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 region s. The regions can be characterized on the basis of a decrease or an i ncrease in heating after the initiating heat pulse during a marginal q uench. The low current ''well-cooled'' region exists when the heating rate during the pulse is greater than the Joule heating after the puls e. Since the heating rate decreases, the energy margin is high. The hi gh current ''ill-cooled'' region exists when the heating rate during t he pulse is less than the Joule heating after the pulse. Since the hea ting rate increases, the energy margin is low. The ''limiting current' ' occurs when the heating rate during the pulse is the same as the Jou le heating after the pulse. The TPX/TF conductor, with its high critic al to operating temperature difference (5.8 K at design conditions) an d the high 2.5/1 copper/superconductor ratio, can operate stabily with significant Joule heating during the pulse, even at high currents. In the evaluation of a well-cooled.or an ill-cooled condition, this Joul e heating should be included in the strength of the initiating pulse. When this is done for the TPX/TF conductor, the limiting current appro aches the critical current, explaining why the stability curves are en tirely in the well-cooled region. The important result is that the con ductor does not exhibit a large decrease in energy margin to an ill co oled region until the current approaches the critical current.