Superconducting coil

Superconducting coil

MAIN SUPERCONDUCTING SOLENOID COIL

The main superconducting (SC) coil is a one-layer solenoid 3268 mm in diameter and 2940 mm long with a 5 mm winding step and a conductor cross-section of 3.8*3.5 mm. It is inserted in a cylindrical helium cryostat. Total weight is 12 tons.

The conductor comprizes two connected strands STP-8-0.85 with the dimensions 3.8*1.7 mm made of 8 twisted SC Nb-Ti wires soldered with POS-61. The diameter of the wires is 0.85, coefficient of filling is equal to 0.4 and critical current is 500 A ( in 5 T field ) for each of wire.

The SC coil is wound around and welded to the inner surface of a ring helium vessel and is cooled indirectly through a 10 mm stainless steel wall of the vessel. This construction has some advantages:

1. The ring helium vessel is simultaneously a force bandage for the coil.
2. High electrical tensions cannot appear, because the coil is shunted with helium vessel walls ( resistance of 6.*10**-6 Ohm).
3. Small radiation length of the coil.
4. The transition of SC coil to the normal state is not dangerous, since stored magnetic field energy would dissipate uniformly in the coil and in the helium vessel walls, heating them to the temperature not higher then 90 K.

The SC transformer supplies the coil with current of 8 kA, that allows in the stationary regime switch on the coil into the frozen magnetic flux mode. The helium consumption in this mode is approximately 6 l/hour.

The number of turns of the coil is 588, the inductance is equal to 1.2 H. The operational field 2 T is produced with a coil current of 8 kA. That is 50% of critical current for the used SC. The stored field energy is 42 MJ.

The yoke returns the magnetic flux and provides the required uniformity of the field. The coil must be centred between the yoke poles in order to compensate attraction forces. The coil position can be adjusted from the outside of the cryostat by means of axial arms of supporting system, equiped with strain gauges for force measurements.

Since the resistance of helium vessel is only 6.*10**-6 Ohm for 4.5 K, the time required for current input is equal to 100 hours ( helium heat load 200 W ). The total helium consumption during the current input period is approximately 16 m**3. While current is being fed into the coil, a temperature gradient of 0.2 K in the 10 mm wall separating the coil from helium appears. This gradient is tolerable and cannot lead to transition to normal state.

Simulating experiments were carried out with soldered coils, like the one planned to be used. A coil with following parameters was wound on a stainless steel frame and tested : 130 mm diameter, 220 mm long with a resistance of 2.*10**-6 Ohm, 70 turns of SC wire, inductance 3.*10**-4 H. No aging processes and degradations were observed during tests when the velocity of current input changes from 10 A/sec to 500 A/sec. Break-downs occured for current value of 5.6 kA, although a critical current for this coil was equal to 5.2 kA.

COMPENSATING SUPERCONDUCTING SOLENOID COILS

Two compensating coils are situated at the end-caps of the main coil and serve for the compensation to zero of the magnetic field integral on the beam tragectory. Coils 450 mm long are made of SC conductors with diameters of 1.2 and 0.85 mm. The operational field is equal to 6.5 T, current is 385 A, stored energy is 140 kJ. Current is supplied through special inlets. The inner helium surface of cryostats of the compensating coils is connected with storage ring vacuum volume and serves simultaneously as a cryogenic pump.

The cryogenic maintenance system allows:

1. Refrigeration from normal temperature to 80 K with liquid nitrogen.
2. Refrigiration from 80 K to 4.5 K with liquid helium.
3. Cryostating at the level of 4.5 K during thfor 4.5 K.

Last modified on 9 April 2001