Stability of the TESPE superconducting torus magnets

Our laboratory size superconducting compact torus TESPE was tested in a symmetrical 3-coil arrangement of 3.5 MJ stored energy. The magnet system applying a superconductor of Cu/NbTi < 2:1 reached its design current of 7 kA in the first run. From the experimental investigations performed we select here an extensive study of stability over the total range of operating currents corresponding to values of the normal state heat generation per unit cooled surface from 0 to as high as 13.5 W/cm²applying two heaters of different length and position. When a 17 cm long heater was used three stability regions following one to each other with increasing current have been observed. The experimental results are compared with the three stability theories: Maddock´s equal area theory, Wilson´s MPZ/Cold-End theory and Iwasa´s CCM theory. Our experimental results and their analysis have led us to a combination of the existing theories with the normal state heat generation as discriminating parameter giving a complete description of the stability behaviour of bath-cooled magnets.