Fig. 2. External view of the magnet with oil cooling of the solenoid with the bore diameter being 60 mm

Generation of magnetic fields with higher intensities requires operation of the magnet at high current densities (about 100 A/mm²). At such current densities, only superconducting electromagnetc (cryomagnets) can operate.

A “wet” (liquid-containing) cryomagnet is shown in Fig. 3. A coil with a superconducting multi-wire niobium-titanium (NbTi) enameled cable is placed in a tank filled with liquid helium (LHe). In its turn, the helium tank is placed within a nitrogen shield with a nitrogen tank filled with liquid nitrogen (LN). The helium tank and the nitrogen tank with the shield do not contact each other. The temperature of boiling LHe is 4.2°K, and that of boiling LN, 77°K. The NbTi cable goes into the superconducting state at a temperature of 9.2°K. To prevent heat exchange with the atmosphere, the unit is placed in a vacuum cryostat. The special structures for mechanical suspension of the He tank with the solenoid, coolant filling, and injection of the current to the solenlid have low heat conductivity. In the process of operation, “wet” cryomagnets evaporate coolants and require regular refilling with LHe and LN. The filling is performed in order to maintain the level required for submerging of the superconducting coil in the low-temperature medium and maintain the LN temperature of the shield. The coil is cooled additionally at the cost of evaporation of the gaseous helium, which has a temperature close to the boiling point. The evaporating gas removes heat from the current leads and the suspension as well, which reduces heat gain even in non-optimized structures.

а	б
Fig. 3a, b. Scheme and external view of a “wet” cryomagnet: NbTi-wire coil (1), current leads (2), He tank (3), N tank (4), pipes for He gas collection (5), and vacuum volume (6)

Currently, GYCOM has two models of “wet” cryomagnets with the parameters presented in Table 1. Magnets GCM-4 are used for gyrotrons with an operating frequency in the 28 – 105~GHz range, and GCM-6, for the operating frequency in the 105 – 170 GHz.

The most modern and easy-to-operate device is the superconducting liquid-free (“dry”) magnet cooled by a cryorefrigerator, which has an entirely closed working cycle with gaseous high-purity helium. It consists of a compressor (1), which is connected via low- and high-pressure lines (2) with the two-stage expander (3) (see Fig. 4).

Compressor (1), high- and low-pressure lines (2), expander (3) Fig. 4. Main elements of the cryorefrigerator

Helium is delivered to the expander in the heat exchanger via the high-pressure line. There, in the process of expansion, cold helium removes heat from cryopanels. After that, helium is returned to the compressor via the low-pressure line for further compression and cooling in the compressor. The use of cryorefrigerators requires only an electric power supply system (3-phase mains with a voltage of 380 V at a consumed power of not more than 10 kW) and water cooling (water consumption of 10 l/min).
A two-stage heat exchanger is used to cool the superconducting coil. The first stage (nitrogen cryopanel) has an operating temperature of about 50°K, while the second one (helium cryopanel) has a temperature being close to the temperature of liquid helium, about 4°K. Here, the first stage of the heat exchanger is used to cool the heat shield and remove heat leaks from the current leads and the suspension (at a room temperature of 50°K), and the second stage cools down the superconducting coil directly and removes the residual heat leaks from the lead currents and the suspension (50°K – 40°K). In magnets of this type, the superconducting coil is placed in vacuum, and heat exchange happens only due to the heat transfer from the cryopanel via intermediate elements to the superconductor.
As of now, several foreign companies produce cryorefrigerators, which have the required cooling performance. Using them allows one to eliminate the necessiry of the helium and nitrogen tanks, which simplifies the design of the magnetic system considerably. On the other hand, it becomes necessary to impregnate the solenoid with a special cryogenic compound being a link in the heat transfer chain. A cryogenic compound (cryocompound) is a compound substance having relatively high conductivity, a certain heat expansion factor, and electric insulation properties.
GYCOM has designed, manufactured, and tested a liquid-free cryomagnet (see Fig. 5 and Table 2) with the use of the SRDK-415D cryocooler manufactured by SHI-APD Cryogenics.

	Table 2. Parameters of the liquid-free cryomagnet Maximum magnetic field 2.5 Т Diameter/position of the bore 64 mm / vertical Dimensions (diameter/height) 550 mm / 635 mm Average energy consumption ~ 7 kW Time of cooling to the operating temperature 30 hours
Fig. 5. General view of the liquid-free magnet

Operation of high-frequency gyrotrons with magnetic fields exceeding 2.5 T is ensured by using “dry” cryomagnets purchased from the leading manufacturers, JASTEC (Japan) and CRYOMAGNETICS (USA).