Test results of "Tokamak-7" superconducting magnet system (SMS) sections

At the Kurchatov Atomic Energy Institute the "Tokamak-7" installation with SMS for the main toroidal field is being built. Design parameters of SMS were published earlier. At this time the testing of parts of SMS is almost complete. Each part represents 1/8 of the magnet system and consists of six double pancakes. The pancakes are wound of multi-channel electroplated strip with parallel niobium-titanium wires. All pancakes are connected in parallel hydraulically and in series electrically. The possibility of additional current in each pancake or group of pancakes is also provided. The test rig comprises a helium refrigerator (600 W at 4.5K), a test cryostat, several power supplies and a diagnostic system. Precooling of the sections takes about 24 hours. During this period, the maximum helium inlet pressure is about 1.6 MPa (abs.). At working conditions the inlet pressure is about 0.2-0.3 MPa (gauge), temperature is in the range 4.5-5.0K, helium flow is about 300 nm³/h. The highest current was equal to 5650A (maximum design current is 6000A), corresponding to a field at the superconductor of about 3.5T. In some cases the central pancakes were excited with up to 900A additional current. As a rule, the superconducting-to-normal transition occurred in the most peripheral pancakes instead of the most magnetically loaded central pancakes and was caused by shielding currents. The latter have maximum value in the peripheral pancakes. The number and intensity of voltage jumps in the normal zone detection circuit have been much lower than in the test of the CMS-0.25 installation.