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Main parameters of the cable

SUPPORT AND MOVING STRUCTURE

THANK YOU FOR YOUR ATTENTION!

CRYOGENIC SYSTEM

ЬMPD Solenoidal Magnet

MAIN FEATURES

HEAT EXCHANGER OF THE SC COIL

The cooling method chosen for the MPD magnet is based on the natural convection of liquid helium flow (thermosyphon mode). The coil conductor is cooled indirectly via the thermal contact with the aluminum support cylinder and heat removal through the cylinder to the square aluminum tubes of “rib-cage” type heat exchanger, where vapor-liquid helium mixture circulates. The cross section of a tube is 25x25 mm with a hole of 19 mm. There are 28 parallel branches attached to the external surface of the cylinder by epoxy glue.

The cryogenic system is intended for continuous cooling of the superconducting coil and the thermal shields of the MPD magnet in various modes of its operation. Main components of the cooling system:

1. Cryostat comprising:

  • SC coil cooled by indirect heat conduction to liquid helium circulated through heat exchanger tubes fixed on the outer surface of its support cylinder;
  • Thermal shield cooled by liquid nitrogen flowing through the tubes fixed on its surface;
  • Vacuum casing.

2. Control Dewar, including:

  • Vessel with liquid helium;
  • Current leads;
  • Thermal shield cooled by liquid nitrogen flowing through the tube, fixed on its surface;
  • Valve block (valves, sensors);
  • Vacuum casing.

3. Vacuum tube (Chimney) connecting the cryostat with sc coil and control Dewar;

4. The helium satellite refrigerator (4.5 K) for cooling (heating) and cryostatting the sc coil;

5. Nitrogen re-condenser for cooling (heating) and cryostatting the thermal screens;

6. Transfer lines between the satellite refrigerator, nitrogen re-condenser and control Dewar.

CONTROL DEWAR

CHIMNEY

The control Dewar is a functional unit of the cryogenic system. It is placed between the superconducting coil cryostat and the helium satellite refrigerator. The control Dewar serves to accumulate the liquid helium in the helium bath, maintain the required parameters of the helium and nitrogen flows (flow rate, pressure, temperature), and to provide cooling of the current leads. Current leads cooled by vapors of boiling helium, control valves, temperature and pressure sensors are allocated in the vacuum volume of the control Dewar. Safety valves, sensor connectors, bayonet connectors of the transfer lines are placed on the sidewalls and top plate of the control Dewar housing.

Technical Design Report

The vacuum connecting tube (chimney) connects the vacuum volumes of the cryostat and the control Dewar. Its vacuum jacket of stainless steel encloses superconducting bus bar lines, direct and return helium and nitrogen tubes, measurement cables, and a thermal shield. Superconducting bus bar lines cooled by a direct liquid helium flow are necessary to connect the superconducting coil lead-outs with the vapor-cooled current leads placed in the control Dewar.

The flow (4.34 g/s, 1.3 bar, 4.5 K) comes to the helium vessel of the control Dewar from the helium satellite-refrigerator in the steady-state regime.

SATELLITE REFRIGERATOR

A helium satellite refrigerator with a liquefaction performance of about 150 l/h of liquid helium will be used for cooling (heating) and cryostatting the superconducting winding of the MPD magnet. Application of the satellite refrigerator allows the system to meet two contradictory requirements as high reliability and efficiency. This type of refrigerator contauins heat exchangers and vessel for liquid helium and doesn’t include helium expander. It takes liquid helium from the main refrigerator and compressed gas for its operation. The refrigerator is placed on the top platform of the magnet and connected with the main refrigerator of the NICA collider circuit by transfer lines.

SC Coil

CRYOSTAT OF THE SOLENOID

YOKE

MAGNET AND POLE SUPPORTS

YOKE AND POLE TRANSPORT SYSTEMS

The cryostat consists of

  • A vacuum vessel with a support system for fixating the cryostat to the yoke.
  • A cold mass with a system for suspending it inside the vacuum vessel.
  • A thermal shield. 
  • A control Dewar with a system of valves and gas-cooled current leads. 
  • A connecting tube (chimney).

To transfer the weight load to the foundation, the yoke rests on a support consisting of two cradles joined by box profiles. The total mass of the support is ~93 t. At the assembly site the magnet is assembled on six stationary supports, which are also used later to position it for operation in the accelerator.

The magnet transport system comprises 

  • A rail track (two parallel rails attached to the foundation) ~30 m long
  • Four roller skates under two magnet cradles.
  • Two hydraulic cylinders to move the magnet.

The pole transport system comprises

  • Rail tracks for the poles (two pairs of parallel rails attached to the foundation) ~8.1 m long
  • Four roller skates under each pole platform
  • Two pairs of hydraulic cylinders to move the poles

The cryostat vacuum vessel is made of stainless steel. It consists of an inner and an outer shell 16 mm and 25 mm thick respectively. The weight of the vacuum cryostat vessel is 49.1 t. Under all loads, the change in the outer cryostat dimensions (diametric or axial) must not exceed 2 mm.

Stationary (yellow) and roller (green) supports of the magnet. Shown in red are the shims under the stationary supports (in the figure the magnet rests on the stationary supports).

The conductor preinsulated by dry fiberglass tape is wound onto inside the support cylinder with the larger side of its cross section kept radially directed. Internal winding and indirect conductor cooling simplify the cryostat design and allow the amount of liquid He in the coil to be minimized, thus avoiding the risk of emergency pressure increase in the cryostat.

The yoke comprises two support rings, 24 barrel beams, two poles with trim coils and transportation platforms, and a support structure consisting of two cradles, six stationary supports, and four roller skates. The total mass of the assembled yoke is 727 t. During the operation at the maximum solenoid current without any technological deviations each yoke beam is pressed against the support rings by an axial magnetic force of 116 kN and radial force of 125 kN.

Stationary support of the magnet. At the center of the suppport there is a hydraulic jack. Shown in red is the shim between the support and the baseplate.

The coil is a one-layer solenoid made of a superconducting NbTi cable in the aluminum matrix.

The cold mass is fixed relative to the outer cryostat shell by 2x12 radial and 6 axial tie rods, which are fixed on the thicker parts of the aluminium support cylinder. So many radial rods are needed because the coil must be rigidly fixed in relation to the outer shell of the vacuum vessel and the cylindrical shape of the coil must be maintained under the action of the radial decentering force.

YOKE BARREL

POLE TRIM COILS

The following loads to the radial tie rods are possible:

  • 1. Dead weight of the cold mass G=166.7 kN
  • 2. Operation loads under the normal operation conditions: 
  • weight of the cold mass G=166.7 kN,
  • radial magnetic decentering force PR=9.2 kN, which can be directed along the circumference in any direction. The maximum stress arises in the vertical rodswhen the radial decentering magnetic force is directed downwards and in the inclined rods when the force is directed along the X axis
  • the force caused by thermal stresses during the cooling,
  • external pressure on the cryostat, 0.1 MPa ,
  • tightening force Qz of the threaded rod-nut connection.
  • 3. Dynamical loads as the cryostat is lifted/lowered (ay=1.6 g, ax=0.5 g), which act together with the tightening force Qz=40 kN of the threaded rod-nut connection.

The poles of the magnet are made up of two end caps fit intothe borings of the support rings. Each pole weighs 43.7 t (without the transport platform andthe trim coil). The poles are inserted in the magnet and fixed relative to the yoke support rings by axial stops and radial spacers. The internal recesses of the poles are cone shaped with an angle of 14°.

To correct the magnetic field in the TPC region, there are trim coils fixed in the pole recesses. They are wound using a hollow aluminum conductor 42×42 mm2 in cross section with a hole 27 mm in diameter and an edge round-off radius of 2 mm. The trim coils will be cooled by demineralized return water circulating in a closed loop.

There is no displacement of the cold mass under the action of the thermal load, because of the symmetrical arrangement of the radial tie rods.

The support cylinder serves to limit the azimuthal stresses in the aluminum matrix of the superconducting cable, which result from magnetic pressure, provide indirect cooling of the coil, and fix the coil inside the cryostat against weight load and magnetic decentering forces. The cylinder will be made of the Al 5083 aluminum alloy. The weight of the cylinder is 6950 kg. The cylinder length is 7598 mm; its outer diameter is 5092 mm, the thickness is 18 mm in the central part and 45 mm at the ends, where tie rods are attached for suspending the cylinder on the outer vacuum shell of the cryostat and keeping it stable against the axial shifts.

The barrel of the solenoid yoke consists of two end support rings and 24 beams, which are rigidly connected to the rings and make up a cylindrical barrel-like structure, and a support structure. To have the field of the required quality in the TPC region, the relative displacements of the yoke components under loads should be smaller than 1 mm. This is achieved by choosing an appropriate yoke design, precisely fitting the components together, tightly connecting them, and laying a rail track of appropriate quality for moving the magnet.

Structurally, the trim coils are two-layer pancakes wound from the center, which are fixed in the circular recess. The trim coils lead-outs are at the outer radius. The maximum current density in the trim coil aluminum in the basic mode of operation 3.27 A/mm2. The maximum ampere-turns in the each magnet pole trim coil are 151 kA.

The Poles

Task definition

Presentation outline

TDR of solenoidal magnet

Design description

The superconducting magnet of MPD is intended for providing a highly homogeneous magnetic field of 0.5 T in an aperture 4596 mm in diameter at NICA accelerator complex.

The field inhomogeneity in the TPC region must be less than 0.001

Rated current of the magnet is 1790 A (it corresponds to a field in the aperture of 0.5 T). The maximum magnet field at which the specified value of the integral of the radial component of the induction in the area of TPC Int ≤ 0.775 mm is maintained, and which can be achieved with a maximum level of technological deviations from the optimized geometry of the magnetic system, is 0.57 T.

  • Main principles
  • Task Definition
  • Design Description
  • Overview
  • Detailed Description
  • A cryostat with a superconducting coil and a control Dewar
  • A flux return yoke with two support rings, 24 bars, and two poles with trim coils
  • Magnet support cradles
  • Auxiliary platforms for moving the poles
  • Stationary supports
  • Hydraulic actuators for displacement of the yoke and poles
  • Roller skates for movement of the magnet and its poles.
  • Cryogenic system